Veterinary · Pre-clinical and foundation · also MSVE VAN-211, Microscopic Anatomy
Tissues and organs at microscopic and ultrastructural level, and how the body develops from gametes to organs. Teratology explains many congenital defects met in practice.
In the exams: ICAR AICE-JRF core group 07; ICAR AIEEA-PG 13 (Anatomy)
Veterinary histology is the study of tissues and organs under the microscope; embryology is the study of how the body develops from two gametes into a newborn animal. Histology tells you what normal looks like at the level of cells, so that the abnormal can be recognised in a biopsy, a cytology smear or a post-mortem sample. Embryology explains why organs lie where they do and why certain defects keep appearing in calves, foals, piglets and puppies.
The subject is taught comparatively. The rumen has no glands, the pig lymph node is inside out, the mare's ovary has its cortex on the inside, the bitch ovulates an immature oocyte, and the placentas of the cow, mare, sow and bitch differ in shape, in the layers between the two bloodstreams and in how much antibody can cross. Many of these differences decide clinical practice, such as why a calf or foal must receive colostrum within hours of birth.
Histology and embryology are examined inside the anatomy paper almost everywhere: in the Anatomy share of India's ICAR AIEEA (PG) Veterinary Science paper and core group 07 of the ICAR AICE-JRF/SRF, in the basic-science paper of China's practising-veterinarian examination, in the Anatomy and Histology items of Nepal's NLEV and the Philippine licensure examination, and in the North American BCSE. Questions range from recall (which cell makes surfactant) to applied problems (what caused a lamb's cyclopia). This manual follows the curriculum units, and where textbooks give different numbers or species vary, it says so rather than picking one.
Fixation stops autolysis and hardens tissue. Autolysis is fastest in the gut mucosa, pancreas and kidney, so samples should enter fixative within minutes. The standard fixative is 10% neutral buffered formalin. Commercial formalin is about 37-40% formaldehyde, so 10% formalin (one volume in nine of buffer) is about 4% formaldehyde. Formaldehyde cross-links proteins; it penetrates quickly but fixes slowly, so slices should be no thicker than about 5 mm and immersed in at least ten, ideally about twenty, times their volume of fixative. Bouin's fluid (picric acid, formaldehyde, acetic acid) is used for testis and embryos. For electron microscopy, tissue is fixed in glutaraldehyde and post-fixed in osmium tetroxide, which fixes lipids and adds electron density.
For paraffin sections, tissue is dehydrated in graded ethanol, cleared in xylene, infiltrated with molten wax, embedded and cut on a rotary microtome at about 3-6 micrometres. Frozen sections cut on a cryostat avoid alcohol and xylene, so they preserve lipids and enzyme activity and allow rapid diagnosis. Resin embedding allows semithin sections (about 0.5-1 micrometre, stained with toluidine blue) and ultrathin sections (roughly 50-100 nanometres) for electron microscopy. Bone must be decalcified: acids (formic, nitric) are fast but damage staining and antigens if overused; EDTA is slow but gentle and preferred before immunohistochemistry.
In haematoxylin and eosin (H&E), haematoxylin with an aluminium mordant acts as a basic dye and stains acidic structures blue: chromatin and ribosome-rich cytoplasm (basophilia). Eosin, an acidic dye, stains most cytoplasm, collagen and red cells pink (acidophilia). Nissl substance and plasma-cell cytoplasm are basophilic because of their RNA.
Periodic acid-Schiff (PAS) turns sugars with neighbouring hydroxyl groups into aldehydes that react with Schiff reagent to give magenta: glycogen, neutral mucins, basement membranes and fungal walls. A PAS-positive deposit removed by diastase is glycogen. Alcian blue at pH 2.5 stains acidic mucins; at pH 1.0 only sulphated mucins. Toluidine blue stains mast cell granules purple-red (metachromasia). Masson's trichrome colours collagen blue or green, and van Gieson's collagen red; both assess fibrosis. Verhoeff's stain shows elastic fibres; silver shows reticular fibres. Perls' Prussian blue shows ferric iron (haemosiderin); von Kossa shows calcium deposits; oil red O and Sudan black show lipids in frozen sections; Congo red shows amyloid with apple-green birefringence in polarised light. Luxol fast blue stains myelin, cresyl violet Nissl substance, and Golgi silver impregnation whole neurons. Romanowsky stains (Giemsa, Wright's) are used for blood smears.
Histochemistry localises a substance by a reaction whose coloured product stays where it forms. In the Feulgen reaction, mild acid hydrolysis exposes aldehydes in DNA that react with Schiff reagent, so the colour is specific and proportional to DNA content. Methyl green-pyronin stains DNA green and RNA red.
Histoenzymology demonstrates enzyme activity: the section is incubated with a substrate and the product is trapped as an insoluble coloured precipitate. Because paraffin processing destroys most enzymes, fresh frozen sections are used. Standard examples are alkaline phosphatase (intestinal and proximal-tubule brush borders, osteoblasts), acid phosphatase (lysosomes; tartrate-resistant acid phosphatase marks osteoclasts), succinate dehydrogenase and NADH-tetrazolium reductase (oxidative capacity of muscle fibres), myofibrillar ATPase at different pH (muscle fibre typing), and acetylcholinesterase (cholinergic nerves). In muscle biopsies from dogs and horses, ATPase and oxidative stains distinguish type I from type II fibres and reveal fibre-type grouping after reinnervation. The absence of acetylcholinesterase-positive ganglion cells and nerve bundles in a gut biopsy supports aganglionosis. Controls without substrate confirm that colour comes from the enzyme.
Immunocytochemistry (immunohistochemistry on sections) uses an antibody to locate an antigen. In the direct method the primary antibody is labelled; in indirect methods a labelled secondary antibody against the primary antibody's species amplifies the signal, and avidin-biotin or polymer systems amplify it further. Labels are enzymes such as horseradish peroxidase (with diaminobenzidine giving a brown product) or alkaline phosphatase (useful in pigmented tissue), fluorochromes, or colloidal gold for electron microscopy. Formalin masks many epitopes, so heat-induced or enzymatic antigen retrieval comes first. Each run needs a positive control tissue and a negative control without primary antibody; endogenous peroxidase in red cells and neutrophils must be blocked. Antibodies raised against human or mouse proteins must be validated in each animal species.
Lineage markers include cytokeratins (epithelium), vimentin (mesenchyme), desmin (muscle), glial fibrillary acidic protein (astrocytes), CD3 (T cells), CD79a or CD20 (B cells), von Willebrand factor or CD31 (endothelium), chromogranin A and synaptophysin (neuroendocrine cells) and Ki-67 (proliferation). In situ hybridisation uses a labelled probe to locate a specific DNA or RNA sequence, such as a viral genome.
Total magnification is objective times eyepiece: 40x with 10x gives 400x. Resolution, the smallest distance at which two points appear separate, is d = 0.61 x wavelength / numerical aperture (Rayleigh). With light near 500-550 nm and an oil-immersion objective of NA about 1.3-1.4, d is about 0.2 micrometre, the practical limit. Phase-contrast microscopy shows living unstained cells such as spermatozoa; polarising microscopy shows birefringent structures (collagen, the A bands of striated muscle, crystals, Congo-red amyloid); fluorescence and confocal microscopy detect labelled probes; dark-field shows thin organisms such as leptospires.
The transmission electron microscope (TEM) resolves structures around a nanometre in biological specimens. Ultrathin resin sections, cut with a glass or diamond knife, are contrasted with uranyl acetate and lead citrate; structures that scatter electrons appear dark (electron-dense). The scanning electron microscope (SEM) images surfaces of whole specimens that have been critical-point dried and coated with gold. Freeze-fracture splits membranes to show intramembrane particles. Negative-stain TEM of faeces still identifies enteric viruses such as rotavirus and parvovirus.
Rough endoplasmic reticulum (rER) makes export proteins and is abundant in plasma cells, pancreatic acinar cells and fibroblasts, making them basophilic. Smooth endoplasmic reticulum synthesises lipids and steroids, detoxifies drugs in hepatocytes, and in muscle forms the calcium-storing sarcoplasmic reticulum. The Golgi apparatus packages secretion; in plasma cells it shows as a pale perinuclear 'negative Golgi image'. Mitochondria usually have shelf-like cristae, but steroid-secreting cells (adrenal cortex, Leydig cells, corpus luteum) have tubular cristae together with abundant smooth ER and lipid droplets. Lysosomes contain acid hydrolases; a missing enzyme causes a lysosomal storage disease. Peroxisomes contain catalase. The cytoskeleton has actin microfilaments (about 7 nm), intermediate filaments (about 10 nm; tissue-specific cytokeratin, vimentin, desmin, glial fibrillary acidic protein, neurofilaments) and microtubules (about 25 nm). The nucleolus makes ribosomal RNA and is prominent in active protein-making cells. Cells that make much protein for export, such as plasma cells, therefore show basophilic cytoplasm, a large nucleolus and a pale Golgi zone together.
Microvilli have an actin core and form the striated border of enterocytes and the brush border of the proximal tubule. Stereocilia are long non-motile microvilli (epididymis, inner ear). Motile cilia contain a 9+2 axoneme with dynein arms and arise from a basal body of nine triplets; defective dynein (primary ciliary dyskinesia) causes chronic respiratory infection and immotile sperm.
The junctional complex, from the lumen down, is the tight junction (zonula occludens), which seals the intercellular space and forms barriers such as the blood-testis barrier; the zonula adherens, linked to actin by cadherins; and desmosomes, anchored to intermediate filaments. Gap junctions, each channel made of two connexons of six connexins, couple cardiac and smooth muscle electrically. Hemidesmosomes anchor basal cells to the basal lamina; defects cause epidermolysis bullosa in calves, lambs, foals and dogs. The basement membrane is a basal lamina (type IV collagen, laminin) plus a reticular lamina. Loss of cadherin adhesion is one step in the invasion of carcinoma cells.
Epithelium sits on a basement membrane, is avascular and is named by layers and surface cell shape. Simple squamous epithelium forms endothelium, mesothelium and alveolar lining; simple cuboidal lines thyroid follicles and renal tubules; simple columnar lines the stomach and intestine. Pseudostratified columnar epithelium, every cell touching the basement membrane, lines the airway (ciliated, with goblet cells) and the epididymis (with stereocilia). Stratified squamous epithelium is keratinised in the epidermis and, in ruminants, in the oesophagus and forestomach. Transitional epithelium (urothelium) lines the renal pelvis, ureters and bladder.
Exocrine glands keep a duct; endocrine glands secrete into blood. The goblet cell is a unicellular gland. Multicellular glands are simple or compound and tubular, acinar or tubuloalveolar. Serous cells make watery protein-rich secretion; mucous cells have flattened basal nuclei and foamy cytoplasm; mixed glands may show serous demilunes. Merocrine secretion is by exocytosis; apocrine secretion loses a little apical cytoplasm (milk lipid, the sweat glands of hairy skin in domestic animals); holocrine secretion releases whole cells (sebaceous glands). Myoepithelial cells squeeze secretion out and, in the mammary gland, respond to oxytocin.
Connective tissue cells include fibroblasts, macrophages, mast cells (heparin and histamine granules), plasma cells (eccentric clock-face nucleus, basophilic cytoplasm, pale Golgi area) and adipocytes. Type I collagen forms dermis, tendon and bone; type II cartilage fibrils; type III reticular fibres of lymphoid organs, marrow and liver; type IV basal laminae. Collagen synthesis needs vitamin C. Elastic fibres (elastin on fibrillin microfibrils) are abundant in the ligamentum nuchae of grazing ungulates, elastic arteries and lung. Tissue types are loose (areolar), dense irregular (dermis, capsules), dense regular (tendon), reticular (lymphoid organs, marrow) and mucous (Wharton's jelly of the umbilical cord).
White adipocytes hold one large droplet, giving signet-ring cells in sections. Brown adipose tissue has many droplets and mitochondria with uncoupling protein 1 (thermogenin) that produce heat; it matters for newborn lambs and calves, but newborn piglets have little functional brown fat, one reason they chill easily. Inherited defects of collagen processing cause fragile, hyperextensible skin in cattle, sheep, dogs, cats and horses.
Cartilage is avascular; chondrocytes lie in lacunae, in isogenous groups, within type II collagen and proteoglycan. Hyaline cartilage forms articular surfaces, tracheal rings and the fetal skeleton; elastic cartilage the pinna and epiglottis; fibrocartilage, which lacks a perichondrium, the annulus fibrosus of discs, menisci and pubic symphysis. Articular cartilage heals poorly.
Osteoblasts make osteoid and are rich in alkaline phosphatase; osteocytes sit in lacunae linked by canaliculi; osteoclasts are multinucleated cells of the monocyte-macrophage lineage that resorb bone in Howship's lacunae under a ruffled border, are TRAP-positive and are inhibited by calcitonin. Compact bone is built of osteons with a central Haversian canal joined by transverse Volkmann's canals; woven bone appears first and in callus. Skull flat bones ossify intramembranously; long bones replace a cartilage model (endochondral ossification). Growth plate zones from the epiphysis are reserve, proliferation, hypertrophy, calcification and ossification. Rickets, osteochondrosis in fast-growing pigs, foals and large dogs, and chondrodysplasias are disorders of this sequence.
Mammalian red cells are anucleate. Among common domestic species the dog has the largest red cells, with clear central pallor, and the goat among the smallest; camelids are the only mammals with oval red cells. Horse and cat red cells normally form rouleaux. Birds have oval nucleated red cells and nucleated thrombocytes instead of platelets. Reticulocytes are counted with new methylene blue; the horse does not release them into blood even when regenerating, so marrow must be examined.
In rabbits, birds and reptiles the neutrophil equivalent has eosinophilic granules and is called a heterophil. Equine eosinophils are packed with large orange granules. In adult ruminants lymphocytes normally outnumber neutrophils; in dogs, cats and horses neutrophils predominate. Platelets are fragments of marrow megakaryocytes. Haemopoiesis moves from yolk-sac blood islands to fetal liver and spleen and then to marrow. Red marrow persists in flat bones, vertebrae and the ends of long bones; elsewhere it becomes fatty. Reference intervals must always be species- and laboratory-specific.
Skeletal muscle fibres are multinucleated with peripheral nuclei. The sarcomere runs from Z line to Z line. The A band (myosin, anisotropic) keeps its length during contraction; the I band (actin only) and the H zone (myosin only) shorten. In mammalian skeletal muscle a T tubule and two terminal cisternae form a triad at each A-I junction. Endomysium, perimysium and epimysium wrap fibre, fascicle and muscle. Type I fibres are slow and oxidative, type II fast. Satellite cells allow regeneration.
Cardiac cells are branched with one or two central nuclei, joined by intercalated discs containing fasciae adherentes, desmosomes and gap junctions; T tubules form dyads at the Z lines. Purkinje fibres are large, pale, glycogen-rich conducting cells, conspicuous in ungulates. Cardiac muscle heals by fibrosis. Smooth muscle cells are spindle-shaped with a central nucleus and dense bodies instead of Z lines, and can divide. Cardiac and smooth muscle are both coupled electrically through gap junctions, which lets a wave of contraction spread without separate nerve endings on every cell.
Neurons have Nissl substance (rough ER) in the cell body, absent from the axon hillock. Most are multipolar; bipolar neurons occur in the retina and olfactory epithelium; pseudounipolar neurons form dorsal root and cranial sensory ganglia. A cut axon degenerates distally (Wallerian degeneration) and the cell body shows central chromatolysis.
Central glia are astrocytes (glial fibrillary acidic protein, end-feet on capillaries), oligodendrocytes (each myelinating internodes on several axons), microglia (resident macrophages of yolk-sac origin) and ependymal cells. In peripheral nerves each Schwann cell myelinates one internode; satellite cells surround ganglion neurons. Nodes of Ranvier allow saltatory conduction. Nerves are wrapped by endoneurium, perineurium (a diffusion barrier around each fascicle) and epineurium. Sensory ganglia have round pseudounipolar neurons ringed by satellite cells and no synapses; autonomic ganglia have multipolar neurons and synapses. Chromatolysis after axonal injury is a sign of an attempt at repair; peripheral axons can regrow along their Schwann cell tubes, but central axons rarely regenerate.
The epidermis has a stratum basale, spinosum, granulosum (keratohyalin granules), a stratum lucidum only in thick hairless skin such as footpads and the planum nasale, and a stratum corneum. It also contains melanocytes (from neural crest), Langerhans cells (antigen-presenting) and Merkel cells. Horses and cattle have simple hair follicles; dogs and cats have compound follicles, several hairs emerging from one opening. Sebaceous glands are holocrine. Sweat glands over hairy skin of domestic animals are apocrine and open into follicles; merocrine sweat glands are found in the footpads of dogs and cats. The horse sweats freely; dogs and cats rely little on sweating. The circumanal (hepatoid) glands of dogs are modified sebaceous glands whose adenomas are common in entire males.
In the equine hoof, several hundred primary epidermal laminae, each bearing many secondary laminae, interlock with dermal laminae to suspend the distal phalanx; their failure is laminitis. The bovine claw has primary laminae without secondary laminae. Horn is a keratin sheath over a bony cornual process.
Vessels have an intima, media and adventitia. Elastic arteries (aorta) have many elastic laminae; muscular arteries have a thick smooth-muscle media between internal and external elastic laminae; arterioles control resistance. Capillaries are continuous (muscle, lung, central nervous system), fenestrated (endocrine glands, intestinal villi, choroid plexus, glomerulus) or sinusoidal (liver, spleen, marrow). Veins have thin walls and valves. The ox heart contains two ossa cordis.
The thymus has a dark cortex of thymocytes and a pale medulla with thymic (Hassall's) corpuscles, on a framework of epithelial reticular cells from the third pharyngeal pouch. A typical lymph node receives afferent lymph through the capsule, has a cortex of B-cell follicles, a paracortex of T cells with high endothelial venules, and a medulla draining to the hilus. The pig lymph node is inverted: lymph enters at the hilus, follicles lie centrally and lymph leaves through the capsule. Ruminants have haemal nodes with blood-filled sinuses. Chickens have no true lymph nodes. The spleen has white pulp (periarteriolar T-cell sheaths and B-cell follicles) and red pulp (cords and sinuses); its muscular capsule in the horse and dog stores red cells. The bursa of Fabricius, the B-cell organ of birds, is a dorsal diverticulum of the proctodeum; infectious bursal disease virus destroys it. In young ruminants and pigs the ileal Peyer's patch also acts as a primary B-cell organ.
The nasal respiratory region has pseudostratified ciliated columnar epithelium; the olfactory region has bipolar olfactory neurons, supporting cells and basal cells, with serous olfactory glands. The trachea has C-shaped hyaline cartilages bridged by the trachealis muscle. Bronchi have cartilage plates and glands; bronchioles have neither, and distally contain club (formerly Clara) cells that secrete protective proteins and detoxify inhaled compounds.
Respiratory bronchioles are well developed in dogs and cats but few or absent in cattle, sheep and pigs. Cattle and pigs have complete interlobular septa, which limit collateral ventilation and give bovine pneumonia its lobular pattern; dogs, cats and horses have thin or incomplete septa. Type I pneumocytes cover most of the alveolar surface; type II pneumocytes secrete surfactant from lamellar bodies and replace injured type I cells. Ruminants, pigs, horses and cats, but not dogs or rodents, have pulmonary intravascular macrophages. The avian lung has no alveoli: air flows one way through parabronchi into air capillaries, in cross-current exchange with blood, ventilated by air sacs (nine in the chicken) that do not exchange gas.
Filiform papillae are mechanical and keratinised, large and hooked in the cat. Ruminants have conical and lenticular papillae. Taste buds sit on fungiform, vallate and foliate papillae; foliate papillae are well developed in the horse, pig and rabbit and absent in ruminants. Enamel is made by ameloblasts (ectoderm) and is not renewed; dentine by odontoblasts (neural-crest mesenchyme). Brachydont teeth (carnivores, pigs) stop growing; hypsodont teeth (horse, ruminant cheek teeth) keep erupting. The parotid gland is mainly serous, the mandibular mixed.
The gut wall has mucosa (with muscularis mucosae), submucosa, muscularis and serosa or adventitia. The oesophagus has stratified squamous epithelium, keratinised in ruminants, horses and pigs. Its muscle is skeletal throughout in ruminants and dogs; the caudal part (about a third) is smooth in the horse and cat. Submucosal glands run its whole length in the dog but are limited to the pharyngo-oesophageal junction in horses, ruminants and cats.
Fundic glands contain mucous neck cells; parietal cells (eosinophilic, with canaliculi and many mitochondria), secreting hydrochloric acid and intrinsic factor; chief cells (basophilic), secreting pepsinogen; and enteroendocrine cells. The pig has a non-glandular pars oesophagea, the site of gastric ulcers. The horse has a large non-glandular region separated from the glandular mucosa by the margo plicatus. The ruminant rumen, reticulum and omasum have keratinised stratified squamous epithelium without glands; the rumen has papillae and no muscularis mucosae, the reticulum honeycomb crests, the omasum laminae; the abomasum is glandular.
The small intestine has villi covered by enterocytes and goblet cells, and crypts with stem cells and enteroendocrine cells. Paneth cells occur at crypt bases in the horse and ruminants but are generally absent in the dog, cat and pig. The duodenum is identified by submucosal (Brunner's) glands, whose extent and secretion vary with species; the ileum by Peyer's patches, whose M cells sample antigens. Parvoviruses destroy dividing crypt cells, while rotaviruses and coronaviruses destroy villus-tip enterocytes. The large intestine has no villi and many goblet cells.
The classic liver lobule surrounds a central vein, with portal triads (portal vein branch, hepatic arteriole, bile ductule) at its corners. Pig lobules are completely outlined by connective tissue. In the acinus, zone 1 lies near the portal area and zone 3 near the central vein, the zone most vulnerable to hypoxia and many toxins. Sinusoids contain Kupffer cells; stellate (Ito) cells in the space of Disse store vitamin A. The horse, rat, camelids and most deer lack a gall bladder. The exocrine pancreas has serous acini with centroacinar cells, unique to it; islets contain beta (insulin, most numerous), alpha (glucagon), delta (somatostatin) and PP cells.
Birds have no teeth. The crop, an oesophageal diverticulum lined by stratified squamous epithelium, stores food; in pigeons it produces crop milk under prolactin. The proventriculus (glandular stomach) has compound submucosal glands with a single cell type, the oxynticopeptic cell, secreting both acid and pepsinogen. The ventriculus (gizzard) grinds food under a hard koilin layer secreted by its tubular glands; koilin is a carbohydrate-protein complex, not keratin. The paired caeca carry caecal tonsils, and the cloaca is divided into coprodeum, urodeum and proctodeum, the last carrying the bursa of Fabricius in young birds. The avian liver is relatively large and bilobed, and a gall bladder is present in the chicken but absent in the pigeon and some other species. The intestine has villi but lacks submucosal duodenal glands, and the relatively short tract suits rapid passage of food. Knowing these features helps at post-mortem, where the proventriculus and gizzard are the first organs checked for ulcers, parasites and haemorrhages.
Dogs, cats, horses, sheep and goats have unipyramidal kidneys with a single renal crest; the pig kidney is smooth outside but multipyramidal inside; the ox kidney is externally lobated. The renal corpuscle has fenestrated glomerular endothelium, a thick basement membrane and podocytes whose foot processes leave slit diaphragms. The proximal tubule has tall eosinophilic cells with a brush border; in normal cats its cells contain lipid droplets. The juxtaglomerular apparatus comprises the macula densa of the distal tubule, renin-secreting juxtaglomerular cells in the afferent arteriole, and extraglomerular mesangial (lacis) cells. The avian kidney has both loopless reptilian-type and looped mammalian-type nephrons, excretes uric acid and has a renal portal system. The ureter and bladder are lined by transitional epithelium. Collecting ducts have pale cuboidal to columnar cells with distinct borders. Transitional epithelium has large dome-shaped surface (umbrella) cells whose thickened membrane plaques resist urine and which flatten as the bladder fills.
Seminiferous tubules contain spermatogenic cells and Sertoli cells. Tight junctions between Sertoli cells form the blood-testis barrier, dividing a basal compartment (spermatogonia) from an adluminal compartment (later spermatocytes and spermatids). Sertoli cells nourish germ cells, phagocytose residual bodies, secrete androgen-binding protein and inhibin, and in the fetus anti-Mullerian hormone. Leydig cells between tubules secrete testosterone and are especially abundant in the boar. Efferent ductules have a scalloped lumen; the epididymis has pseudostratified epithelium with stereocilia, spermatozoa maturing in the head and body and stored in the tail. The dog's only accessory sex gland is the prostate (with ampullae); the cat has a prostate and bulbourethral glands but no vesicular glands; the boar has very large vesicular and bulbourethral glands; the bull, ram and stallion have all three. The penis is fibroelastic with a sigmoid flexure in the bull, ram and boar and musculocavernous in the stallion and dog. The ductus deferens has a very thick muscular wall.
The ovary has an outer cortex of follicles and an inner medulla, except in the mare, whose cortex lies inside and which ovulates only at the ovulation fossa. Follicles progress from primordial (flat granulosa cells) to primary (one cuboidal layer), secondary (several layers, zona pellucida forming) and antral follicles with cumulus oophorus, theca interna (androgen under luteinising hormone, aromatised to oestradiol by granulosa) and theca externa. In ruminants large luteal cells come from granulosa and small ones from theca. Fertilisation occurs in the ampulla of the oviduct. Ruminant caruncles are aglandular endometrial prominences; the cow and ewe cervix has annular folds. Vaginal epithelium thickens and keratinises under oestrogen, the basis of vaginal cytology in the bitch.
The mammary gland is a modified apocrine sweat gland, compound tubuloalveolar, releasing protein by exocytosis and lipid by apocrine secretion, with myoepithelial cells. The bovine teat canal is lined by keratinised stratified squamous epithelium whose keratin plug is a key barrier against mastitis. In the eye, the tapetum lucidum is fibrous in horses and ruminants, cellular in carnivores, and absent in pigs.
In the spinal cord, grey matter surrounds an ependyma-lined central canal. The dorsal horn is sensory, the ventral horn holds motor neurons, and a lateral horn of sympathetic preganglionic neurons is present in thoracolumbar segments. The dorsal funiculus carries conscious proprioception. The corticospinal tract is small in domestic animals, and in ungulates few fibres pass beyond the cervical cord, so brainstem (extrapyramidal) motor pathways dominate.
The neocortex has six layers. The cerebellar cortex has three: molecular, Purkinje cell and granular. Purkinje cells are its only, inhibitory, output; their postnatal loss (cerebellar abiotrophy) causes intention tremor and ataxia. The epidural space with fat and veins exists in the vertebral canal but not the skull. Cerebrospinal fluid from the choroid plexuses, whose epithelium forms the blood-CSF barrier, flows from the lateral ventricles to the third ventricle, the mesencephalic aqueduct, the fourth ventricle and out to the subarachnoid space, to be absorbed by arachnoid villi; aqueductal obstruction causes hydrocephalus. The blood-brain barrier (continuous endothelium with tight junctions, astrocyte end-feet) is absent at circumventricular organs such as the area postrema, median eminence, neurohypophysis and pineal gland.
The notochord induces the neural plate, whose folds fuse into the neural tube; the rostral and caudal neuropores close last. The tube forms the prosencephalon (telencephalon and diencephalon), mesencephalon and rhombencephalon (metencephalon and myelencephalon). The sulcus limitans separates the dorsal alar plate (sensory) from the ventral basal plate (motor). Neural crest cells form sensory and autonomic ganglia, the enteric nervous system, Schwann cells, melanocytes, adrenal medullary chromaffin cells, much of the facial skeleton, odontoblasts and the outflow septum of the heart. A single crest defect can therefore combine white coat with gut aganglionosis, as in lethal white foal syndrome. The telencephalon forms the cerebral hemispheres, the diencephalon the thalamus, hypothalamus, retina and neurohypophysis, the metencephalon the pons and cerebellum, and the myelencephalon the medulla oblongata. The lumen of the tube becomes the ventricles and central canal. Failure of the neural tube to close gives spina bifida caudally or cranioschisis rostrally.
The adenohypophysis (pars distalis, intermedia, tuberalis) grows from Rathke's pouch in the roof of the mouth; the neurohypophysis (median eminence, stalk, pars nervosa) grows down from the diencephalon. In the pars distalis, acidophils are somatotrophs and lactotrophs; basophils, PAS-positive, are thyrotrophs, gonadotrophs and corticotrophs; chromophobes stain weakly. Releasing hormones arrive via the hypophyseal portal system from the median eminence. The pars nervosa holds axons from the supraoptic and paraventricular nuclei, whose swellings (Herring bodies) store antidiuretic hormone and oxytocin, and pituicytes. Pars intermedia hyperplasia or adenoma causes pituitary pars intermedia dysfunction in old horses; most canine hyperadrenocorticism is due to pituitary corticotroph tumours. The pineal secretes melatonin in darkness, timing seasonal breeding in short-day (sheep, goats) and long-day (horses) breeders. The thyroid arises from pharyngeal-floor endoderm; follicle cells are tall when active and flat when resting; parafollicular C cells secrete calcitonin and, in birds, lie in separate ultimobranchial bodies. Ectopic thyroid tissue along the path of descent is common in dogs.
Parathyroid III arises from the third pharyngeal pouch and travels with the thymus, usually lying outside the thyroid (external); parathyroid IV, from the fourth pouch, lies on or in the thyroid (internal). The pig has only parathyroid III. Chief cells secrete parathyroid hormone.
The adrenal cortex, from coelomic mesoderm, has a zona glomerulosa (mineralocorticoids; arched cells, called zona arcuata, in horses and carnivores), a zona fasciculata of lipid-laden spongiocytes (glucocorticoids), and a zona reticularis (glucocorticoids and androgens); a zona intermedia is described in the horse and dog. The medulla, from neural crest, contains chromaffin cells that secrete adrenaline and noradrenaline and turn brown with chromium salts. In birds cortical and medullary tissue are intermingled. Endocrine cells of the placenta include the equine endometrial cups (equine chorionic gonadotrophin) and ruminant binucleate trophoblast cells (placental lactogen, pregnancy-associated glycoproteins). Chromaffin cells are modified postganglionic sympathetic neurons innervated by preganglionic fibres. Other endocrine cells include the pancreatic islets and the scattered enteroendocrine cells of the gut.
Primordial germ cells are set aside from the epiblast, appear in the yolk-sac wall near the allantois, and migrate through the hindgut and its mesentery to the genital ridges. Meiosis I separates homologous chromosomes after synapsis and crossing over during a long prophase (leptotene, zygotene, pachytene, diplotene, diakinesis); meiosis II separates chromatids. Diploid numbers commonly asked are cattle 60, goat 60, sheep 54, horse 64, donkey 62, pig 38, dog 78, cat 38 and chicken 78. The mule (63) is almost always sterile because its chromosomes cannot pair at meiosis.
From puberty, spermatogonia divide on the basal lamina; type B spermatogonia form primary spermatocytes, which cross the blood-testis barrier and enter meiosis I. Primary spermatocytes are the largest and, because prophase I is long, the most frequently seen spermatocytes; secondary spermatocytes are short-lived and rarely seen. Each primary spermatocyte yields four spermatids, joined by cytoplasmic bridges. In spermiogenesis, without division, the spermatid passes through the Golgi phase (acrosomal vesicle forms), cap phase, acrosome phase (nucleus condenses and elongates, flagellum and mitochondrial sheath form) and maturation phase (residual body shed and phagocytosed by Sertoli cells); release is spermiation. The same cell associations recur along the tubule in the cycle of the seminiferous epithelium, and spermatogenesis takes about four and a half cycles. In the bull a cycle is about 13.5 days, so spermatogenesis takes about 60 days, with roughly one to two weeks of epididymal transit; values vary between sources. Semen damaged by fever or heat stress is therefore re-examined about two months later.
Oogonia multiply only in fetal life in most domestic species, then enter meiosis and arrest as primary oocytes at the diplotene (dictyate) stage of prophase I inside primordial follicles, so the female is born with a finite, declining store. The preovulatory luteinising hormone surge restarts meiosis. In most domestic species meiosis I finishes just before ovulation, the first polar body is extruded, and a secondary oocyte arrested at metaphase II is ovulated; meiosis II completes only when a spermatozoon enters. The bitch ovulates primary oocytes, which complete meiosis I in the oviduct over about two to three days before they can be fertilised. Each primary oocyte gives one ovum. The zona pellucida, a glycoprotein coat made mainly by the oocyte, forms in growing follicles and later mediates species-specific sperm binding. Oocyte and granulosa cells communicate through transzonal processes and gap junctions, which hold the oocyte in meiotic arrest until the surge. Most oocytes are never ovulated; the follicles around them undergo atresia.
Ejaculated spermatozoa must first undergo capacitation in the female tract: removal of cholesterol and seminal plasma proteins from the membrane, and hyperactivated motility. In several species spermatozoa wait in a reservoir in the oviductal isthmus. Fertilisation occurs in the ampulla near the ampullary-isthmic junction. The spermatozoon crosses the cumulus, binds the zona pellucida and undergoes the acrosome reaction, releasing enzymes to penetrate the zona; its equatorial segment fuses with the oocyte membrane. A calcium wave triggers the cortical reaction, whose granule contents modify the zona (zona reaction) and, with a membrane block, prevent polyspermy. The oocyte completes meiosis II and extrudes the second polar body; the pronuclei replicate DNA and join on the first spindle (syngamy). In the mare only fertilised embryos pass into the uterus, apparently because they secrete prostaglandin E2. Unfertilised oocytes remain in the equine oviduct. In vitro fertilisation reproduces these steps in the laboratory, which is why sperm must be capacitated before insemination of oocytes.
Mammalian eggs have little, evenly spread yolk (isolecithal) and cleave holoblastically; amphibian eggs cleave holoblastically but unequally; bird, reptile and fish eggs are telolecithal and cleave meroblastically, only a disc of cytoplasm dividing (discoidal cleavage) to form a blastoderm. In mammals, cleavage within the zona produces ever smaller blastomeres with no growth. At about 16-32 cells the embryo is a morula; its outer cells compact and pump in fluid, forming the blastocyst, with an outer trophoblast (future fetal placenta), an inner cell mass (embryo proper) and a blastocoele. The blastocyst then hatches from the zona. Embryos usually reach the uterus as morulae or early blastocysts a few days after ovulation; the canine embryo stays unusually long in the oviduct.
Ruminant and pig blastocysts then elongate into long filaments, the pig's approaching a metre within days. The equine conceptus stays spherical inside a glycoprotein capsule and moves through the uterus until fixation at the base of a horn around days 16-17. Maternal recognition of pregnancy uses interferon-tau in ruminants, conceptus oestrogen in the pig, and conceptus mobility in the mare.
The inner cell mass forms a bilaminar disc of epiblast and hypoblast. A primitive streak appears in the caudal epiblast with a primitive (Hensen's) node at its cranial end. Epiblast cells pass through the streak to form endoderm, replacing the hypoblast, and mesoderm; those left on the surface form ectoderm. Cells through the node form the notochord, which induces the neural plate and survives as the nucleus pulposus. Paraxial mesoderm forms somites (sclerotome: vertebrae and ribs; myotome: skeletal muscle; dermatome: dermis); intermediate mesoderm forms the kidneys and genital ducts; lateral plate mesoderm splits into somatic (body wall, somatopleure) and splanchnic layers (gut wall, heart, vessels, blood islands; splanchnopleure).
Ectoderm forms the epidermis, hair, hoof, horn, enamel, lens, inner ear, adenohypophysis and mammary epithelium; neuroectoderm the brain, spinal cord, retina and neurohypophysis. Mesoderm forms muscle, bone, cartilage, blood, heart, kidneys, gonads, adrenal cortex and serous linings. Endoderm forms the epithelium of the gut, liver, pancreas, respiratory tract, thyroid, parathyroids, thymus, auditory tube and most of the bladder.
The amnion (ectoderm and somatic mesoderm) forms by folding in domestic animals and birds, by cavitation in primates. The chorion is trophoblast plus somatic mesoderm. The yolk sac (endoderm and splanchnic mesoderm) forms the first blood islands; its union with the chorion, the choriovitelline placenta, is transient but supports the equine conceptus for several weeks. The allantois grows from the hindgut, stores fetal urine and carries umbilical vessels; the chorioallantois is the definitive placenta of domestic mammals, and the allantoic stalk becomes the urachus. Amniotic plaques in ruminants and hippomanes in equine allantoic fluid are normal. In the bird, the chorioallantoic membrane breathes through the shell and draws calcium from it. Embryonated eggs grow influenza and Newcastle disease viruses in the allantoic cavity, poxviruses on the chorioallantoic membrane, and chlamydiae in the yolk sac. The umbilical cord carries umbilical arteries and veins and the allantoic stalk. In the mare, placental membranes are checked after birth for completeness.
By villus distribution: diffuse in the mare (microcotyledons) and sow (folds with areolae absorbing histotroph); cotyledonary in ruminants, placentomes formed by fetal cotyledons and maternal caruncles, convex in the cow and concave in the ewe and goat, commonly quoted as roughly 70-120 in the cow; zonary in the bitch and queen, with marginal haematomas green in the bitch (uteroverdin) and brown in the queen; discoid in primates and rodents.
By layers between the bloodstreams (Grosser): epitheliochorial in the mare and sow, all six layers kept; synepitheliochorial in ruminants, where binucleate trophoblast cells fuse with uterine epithelium and deliver placental lactogen and pregnancy-associated glycoproteins (the basis of blood and milk pregnancy tests); endotheliochorial in the bitch and queen; haemochorial in primates and rodents. By loss of maternal tissue at birth: non-deciduate (mare, sow and, by most authors, ruminants) or deciduate (carnivores, primates).
Epitheliochorial and synepitheliochorial placentas block maternal immunoglobulin G, so foals, calves, lambs and piglets are born almost without antibody and need colostrum in the first day; puppies and kittens get only a small fraction across the placenta. In the mare, chorionic girdle cells invade the endometrium at about 5-6 weeks to form endometrial cups that secrete equine chorionic gonadotrophin until they regress between about the fourth and fifth months. Retained fetal membranes are commonest in the cow.
Folding of the embryonic disc encloses the primitive gut: foregut, midgut (open to the yolk sac by the vitelline duct) and hindgut. The respiratory tract buds from the ventral foregut. The whole ruminant stomach, forestomach included, develops from the single foregut stomach primordium, so the rumen is gastric, not oesophageal, in origin. The midgut herniates into the umbilical cord, rotates around the cranial mesenteric artery and returns; the cloaca is divided by the urorectal septum into rectum and urogenital sinus.
Pharyngeal arch nerves are: first, mandibular branch of the trigeminal (muscles of mastication); second, facial (facial expression); third, glossopharyngeal; fourth and sixth, vagus (larynx). Pouch derivatives are: first, tympanic cavity and auditory tube (the guttural pouch of the horse is its diverticulum); second, tonsillar fossa; third, thymus and parathyroid III; fourth, parathyroid IV and ultimobranchial body. The palatine shelves fuse to form the secondary palate; failure gives cleft palate.
The heart tube (sinus venosus, atrium, ventricle, bulbus cordis, truncus arteriosus) loops and is septated; the foramen ovale lets blood pass from right to left atrium before birth. Defects include ventricular septal defect and tetralogy of Fallot (pulmonic stenosis, ventricular septal defect, overriding aorta, right ventricular hypertrophy). In mammals the left fourth aortic arch forms the aortic arch (the right in birds); the left sixth forms the ductus arteriosus, around which the left recurrent laryngeal nerve hooks. Patent ductus arteriosus is common in dogs; a patent ductus venosus is one form of intrahepatic portosystemic shunt in large-breed dogs. A persistent right aortic arch forms a vascular ring round the oesophagus, and puppies regurgitate at weaning with the oesophagus dilated cranial to the heart.
Three kidneys form in sequence from intermediate mesoderm. The pronephros is rudimentary. The mesonephros works as a fetal kidney for a time and is large in the pig and sheep; its mesonephric (Wolffian) duct drains to the cloaca. The metanephros, the permanent kidney, arises by mutual induction between the ureteric bud, an outgrowth of the mesonephric duct forming the ureter, pelvis and collecting ducts, and the metanephric blastema, forming the nephrons from glomerulus to distal tubule. Failure of the bud gives renal agenesis; misplaced budding gives ectopic ureters, which open beyond the bladder sphincter and cause continuous dribbling, diagnosed most often in young bitches. Persian and related cats have an inherited polycystic kidney disease. The bladder forms from the urogenital sinus; the allantoic stalk becomes the urachus, and a patent urachus, commonest in foals, leaks urine from the umbilicus. The ends of the mesonephric ducts are absorbed into the dorsal bladder wall as the trigone.
The indifferent gonad forms from coelomic epithelium and mesenchyme of the genital ridge, colonised by primordial germ cells. The SRY gene on the Y chromosome switches on SOX9, and supporting cells become Sertoli cells; without SRY the gonad becomes an ovary. Sertoli cells secrete anti-Mullerian hormone, which makes the paramesonephric (Mullerian) ducts regress; Leydig cell testosterone keeps the mesonephric ducts, which form the epididymis, ductus deferens and vesicular glands; dihydrotestosterone, made by 5-alpha-reductase, masculinises the prostate, penis and scrotum. In the female, the paramesonephric ducts form the oviducts, uterus, cervix and cranial vagina, and the mesonephric ducts regress. Their degree of fusion sets uterine form: duplex in the rabbit, bicornuate in domestic ungulates and carnivores, simplex in primates. Segmental aplasia of the paramesonephric ducts in white Shorthorn heifers is white heifer disease; incomplete fusion gives a double cervix.
Guided by the gubernaculum, the testes descend before birth in ruminants and pigs, around birth in the horse, and in the weeks after birth in dogs and cats. Cryptorchidism, commonest in horses, pigs and dogs, leaves a testis that makes testosterone but no spermatozoa; it has a hereditary component, so affected animals should not be bred.
A congenital anomaly is a defect present at birth. Causes are genetic, environmental (teratogens: infections, toxic plants, drugs, deficiencies or excesses, hyperthermia, radiation) or both, and often none is found. Susceptibility depends on the conceptus's genotype and developmental stage; teratogens act by specific mechanisms; outcomes are death, malformation, growth retardation or functional deficit; and effects rise with dose. Around implantation an insult usually kills the embryo or leaves it undamaged (all or none). During the embryonic period each organ has a critical window for gross malformation. During the fetal period tissues still developing, notably the cerebellum, cerebrum, eye and genitalia, can be damaged. An environmental agent can mimic a genetic defect (a phenocopy), so outbreak investigations examine both. The same agent can cause different defects depending on when it acts, so the first question in a cluster of deformed newborns is when in gestation the damage occurred; the second is what the dams met at that time.
Bovine viral diarrhoea virus infecting the dam in mid-gestation can cause cerebellar hypoplasia, hydranencephaly and eye defects in calves; earlier infection, before fetal immunocompetence, can produce persistently infected calves that shed virus for life. Border disease virus causes hairy shaker lambs with hypomyelination; classical swine fever virus causes congenital tremor. Bluetongue virus, including some attenuated vaccine strains, causes hydranencephaly in lambs. Insect-borne Akabane, Schmallenberg and Cache Valley viruses cause arthrogryposis-hydranencephaly in calves, lambs and kids. Feline panleukopenia virus, in late gestation or the first weeks of life, destroys the external granular layer of the cerebellum, causing non-progressive cerebellar hypoplasia.
Ewes eating Veratrum californicum around day 14 of gestation bear lambs with cyclopia; its alkaloid cyclopamine blocks sonic hedgehog signalling. Lupines containing anagyrine cause crooked calf disease when grazed during roughly the second and third months of gestation; poison hemlock and wild tobacco cause arthrogryposis and cleft palate in piglets and calves by reducing fetal movement. Griseofulvin given to pregnant queens causes cleft palate and brain and eye defects; some benzimidazoles are restricted in early ruminant pregnancy. Confirm any drug use in pregnancy against the product label and your national formulary. Iodine deficiency causes goitre and hairless newborns, and copper deficiency in ewes causes swayback.
Inherited defects are controlled by DNA testing and culling carriers: bovine arthrogryposis multiplex, dwarfism, syndactyly, spider lamb syndrome, cerebellar abiotrophy of Arabian foals and collie eye anomaly. In lethal white foal syndrome, foals of two frame overo parents that are homozygous for an endothelin receptor B mutation are white and die of colic from ileocolonic aganglionosis, because neural crest cells fail to reach the gut. In goats, polled intersex syndrome links the polled trait to XX sex reversal.
Structural defects include spina bifida, hydrocephalus, cleft palate (milk from the nostrils, aspiration pneumonia), atresia ani in piglets and calves, atresia coli in calves, umbilical hernia, and inherited inguinal hernia in pigs. Schistosomus reflexus of cattle, with a dorsally flexed spine and exposed viscera, is a classic cause of dystocia needing fetotomy or caesarean section.
About nine in ten heifers born co-twin to a bull are freemartins, because their placental circulations fuse early: anti-Mullerian hormone and cells from the male reach the female, whose gonads and Mullerian derivatives fail to develop and whose vagina is short. Blood shows XX/XY leucocyte chimerism, detected by karyotype or PCR for Y-chromosome sequences.
For fetal ageing, crown-rump length is converted with species-specific formulas; one taught for cattle is X = 2.5 (Y + 21), X being age in days and Y crown-rump length in centimetres. Such formulas are approximate and vary between sources.
In the first professional year you will spend hours at the microscope with glass slides or a virtual slide box. Learn each tissue by one or two features that no other tissue has: Hassall's corpuscles for thymus, centroacinar cells for pancreas, Brunner's glands for duodenum, Peyer's patches for ileum, the margo plicatus for the horse stomach. Draw what you see and label it; drawing forces you to look. In embryology, learn the sequence of events and then the derivatives of each germ layer, arch and pouch, because examiners at every level ask them. Link every species difference to a clinical consequence, such as placental type and colostrum, so that the facts stay with you into the clinical years.
In practice this subject is in the background of daily decisions. You read histopathology and cytology reports and need to know what 'villus atrophy with crypt necrosis' or 'zone 3 hepatocellular necrosis' implies for the case. You fix biopsy samples properly: thin slices, ten to twenty volumes of formalin, the right fixative for the test the laboratory will run. You recognise congenital defects in newborn animals, decide which are compatible with life, and advise owners and farmers on whether the defect is likely to be inherited, infectious or toxic, and whether the parents should be bred again. You plan colostrum management because you know the placenta has blocked antibody transfer.
After many years the value lies in pattern recognition and investigation. When a herd or flock produces a cluster of deformed newborns you work backwards from the defect to the window of gestation when it was caused, then to what the dams were exposed to at that time: which virus was circulating, which plants were grazed, which drugs were given. You advise breed societies and artificial insemination centres on genetic testing for recessive lethal defects and on culling carriers. You are the colleague who can explain to a farmer why a heifer twin to a bull should be tested before she is reared as a replacement, and to a breeder why two frame overo horses should not be mated.
At research level, histology and embryology meet molecular biology. Developmental work in domestic species covers germ-cell biology and in vitro gamete and embryo production, somatic cell nuclear transfer, embryo sexing, placental immunology and why the synepitheliochorial placenta tolerates a semi-allogeneic fetus, maternal recognition of pregnancy, and the genetics of congenital defects through genome-wide association and whole-genome sequencing. Research histology depends on quantitative methods such as stereology and digital image analysis, validated immunohistochemistry in each species, in situ hybridisation and electron microscopy. A senior researcher also knows the limits of antibodies raised against human or mouse proteins and insists on species validation and proper controls before publishing localisation data.
There are no micrographs yet, and histology cannot be learned without looking at slides; use an atlas or virtual slide collection alongside this text. Species differences are given for the common domestic mammals and the fowl; camelids, buffalo, fish, reptiles and laboratory rodents are covered only in passing. The extent and secretion type of Brunner's glands, the trachealis position and the timing of embryo transport, placentome numbers and testicular descent vary between sources and are given only in broad terms or left out. Durations of spermatogenesis are given only for the bull, as an approximation. The crown-rump-length formula quoted for cattle is one of several in use and gives an estimate only. The manual does not cover neuroanatomy at the level of individual cranial nerve nuclei or detailed tract anatomy, eye and ear histology is summarised briefly, and drug safety in pregnancy is mentioned only by class; confirm any use in pregnant animals against the product label and your national formulary.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.