Histology Final 2023

Penis & Urethra

* lined by urothelium (transitional epithelium) : stratified squamous epithelium near the external opening


* surrounded by vascular erectile tissue, the corpus spongiosum → the corpora cavernosa are additional erectile tissue surrounded by the dense CT of the tunica albuginea
* the prepuce is lined by stratified squamous epithelium: can have sebaceous & sweat glands
* visceral prepuce lines the distal free portion of the penis → fornix is the deep refection point between the two

placentation

* Nutrient/gas exchange between the mother and the embryo
* Horses & dogs, a temporary choriovitelline placenta is formed by fusion of the yolk sac and the chorion
* The definitive placenta of eutherian (“placental”) mammals is the chorioallantoic placenta

Reproductive embryology

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* Sertoli cells start producing Müllerian inhibitory substance which causes regression of the paramesonephric ducts and interstitial cells start producing testosterone
* Paramesonephric ducts develop to form the uterine tubes, uterus, cervix and cranial vagina and external genitali
* The mesonephric ducts regress, leaving mainly vestigial structures

Lacrimal System

* the lacrimal system consists of secretory components and a draining system → nasolacrimal duct


* secretory components include: orbital lacrimal gland, nictitating gland, accessory lacrimal glands and conjunctival goblet cells
* the orbital lacrimal gland is a tubuloacinar gland that can be serous, mixed or mucus

Mammary Gland

* specialized sweat glands
* Main function: providing the neonate with nourishment and passive immunity
* Multiple lobules which contain tubuloalveolar glands and intralobular ducts
* Secretory alveoli: cuboidal cells when inactive and columnar when active + are surrounded by contractile myoepithelial cells
* Secretion
* Milk flows from the lobules → interlobular ducts → lactiferous ducts → gland & teat sinuses
* Larger ducts have stratified cuboidal to columnar epithelium lined by keratinized stratified squamous epithelium

Inner Ear

* Bony labyrinth: interconnected spaces in the temporal bone which contains perilymph and the membranous labyrinth
* it has 3 components
* Semicircular canals, vestibule & cochlea
* Epithelial-lined ducts & sacs, filled with endolymph
* Membranous labyrinth compartments contain areas with mechanosensory hair cells:
* The spiral organ of the cochlea for hearing
* the maculae and ampullae cristae for assessing head position / acceleration / balance

Iris

* most anterior part of the uvea


* Thin, contractile diaphragm with a central pupil which can be round (dog), oval (horse, cow) or a vertical slit (cat)
* Loose CT stroma with variable number of melanocytes, a smooth muscle sphincter
* Posterior surface is covered by two layers of epithelium
* Superficial layer of pigmented cells
* Basal layer of partially pigmented myoepithelial cells
* Anterior surface is not covered by epithelium
* Horses, ruminants & pigs have corpora nigra: outgrowths of the stroma and pigmented epithelium

Testes

* Surrounded by the tunica albuginea, a dense CT capsule, with the visceral layer of tunica vaginalis lying on the outer surface
* CT septa extend from the tunica albuginea extend to divide the testes into lobules
* Testicular lobules contain highly convoluted seminiferous tubules which are lined by:
* spermatogenic cells & supporting Sertoli cells; interstitial cells found between the tubules
* spermatogenesis includes: spermatogonia give rise to primary spermatocytes → secondary spermatocytes → early spermatids→ late spermatids and released into the lumen as spermatozoa
* Blood-testis barrier is formed by tight junctions, between Sertoli cells: subdivides epithelium into basal and adluminal regions
* Elongate myoid cells surround the seminiferous tubules and their contractions help propel fluid & sperm
* Polygonal interstitial cells are present between the seminiferous tubules: they produce testosterone
* The intratesticular ducts carry sperm from the seminiferous tubules to the epididymis and lined with simple squamous to cuboidal or columnar epithelium
* Seminiferous tubules join straight tubules which connect to the efferent ductules are lined by a simple columnar or pseudostratified epithelium, are ciliated

The Eye

* Many animals have a reflective tapetum lucidum
* carnivores have a cellular tapetum
* horses & ruminants have a fibrous tapetum densely packed collagen fibers/fibroblasts
* Two types of photoreceptor cells for detecting light
* Optic disc is the blind spot since there are no photoreceptors
* Lamina cribrosa refers to the sieve-like sclera where nerve fibers pass through
* Dual blood supply, ie the photoreceptors in the outer retina supplied by choroidal blood vessels and the remaining retinal layers

secondary (antral) follicles

* Fluid-filled spaces develop between the granulosa cells.
* Form a large C-shaped space called the follicular antrum
* the mesenchymal cells immediately around the follicle form a theca : outer covering
* More fibrous outer layer -endocrine cells

Lens

* Transparent, avascular biconvex disc; cornea it refracts light on the retina held by the zonular fibers
* Lens capsule; a basement membrane secreted by the subcapsular lens epithelial cells: lens fibers that are remnants of cells and cytoplasm with transparent proteins: lens fibers lack organelles & orderly arrangement
* Postnatal life relies on aqueous humor for nutrition/removal of metabolic waste
* maintained by a Na/K pump in the subcapsular anterior epithelium; the lens is one of the most dehydrated parts of the body
* the lenticular cells rely almost entirely on cytoplasmic anaerobic glycolysis: energy production: No organelles, including mitochondria

Biliary Tract

* Hepatocytes secrete water and bile components into bile canaliculi between adjacent hepatocytes in the hepatic plates
* Bile flows from the canaliculi into bile ductules into bile ducts in the portal triads
* All ducts after the canaliculi are lined by simple cuboidal or columnar cells called cholangiocytes : carry bile from the separate liver lobules to the cystic duct then to gallbladder for temporary storage
* gallbladder empties into duodenum via common bile duct

Conjunctiva

* the conjunctiva is a continuous transparent mucous membrane
* Palpebral– lines the inner surface of the eyelids
* Bulbar– covers the exposed, anterior portion of the sclera, from limbus to fornix
* Fornix – junction between the bulbar and palpebral conjunctivas
* 3rd eyelid – continuous with bulbar & palpebral conjunctivas
* Non-keratinized stratified squamus/columnar epithelium with goblet cells

Retinal Pigment Epithelium

* Single layer of cuboidal cells that lies between the neural retina & the choroid
* Tapetal region: little to no pigment present: numerous melanin pigment granules which absorb scattered light
* Provides nourishment & supportive functions for the photoreceptors form part of the blood-retina barrier

Embryology Ears

* The ear can be divided into 3 main parts:
* the external, middle and internal ear
* the inner ear arises the ectoderm adjacent to the developing hindbrain
* ectodermal thickening → invagination to form an otic vesicle → growth and morphogenesis into auditory and vestibular portions
* Middle ear: first and second pharyngeal arches - the tympanic cavity / auditory tube arise from endoderm outpocketing of the first pharyngeal pouch
* External ears: mesenchyme in the first and second pharyngeal arch that surrounds pharyngeal cleft;
* External auditory meatus arises from an inward expansion of ectodermal epithelium from the first pharyngeal cleft

External ear

* Consists of the pinna and an external auditory meatus: sound waves to the tympanic membrane
* The lining of the external auditory meatus is a continuation of the surface skin
* Cerumen (ear wax) is combined secretion of the sebaceous & ceruminous glands
* Outer regions: cartilage and bone
* Tympanic membrane consists of fibroelastic CT with stratified squamous epithelium
* Simple squamous & cuboidal epithelium on the inner surface

tertiary follicles

Cumulus oophorus: ovary in which multiple granulosa cells enlarge around a developing oocyte

Corona radiata = the granulosa cells that surround the zona pellucida & remain with the oocyte at ovulation

Large fluid-filled space called the antrum, and two layers of theca (interna and externa). The secondary oocyte develops a thick covering known as the zona pellucida

Liver

* Largest visceral organ in the body
* Receives cardiac output: portal vein and the hepatic artery: oxygen & nutrients are carried to the liver by both the hepatic artery and the portal vein
* Functional unit of the liver is the acinus
* Hepatocytes near portal triads receive the highest concentrations of oxygen, nutrients and hormones
* Bile flows in the opposite direction: from the canaliculi into bile ductules, then into bile ducts
* Hepatocytes are arranged in radiating plates (cords) separated by sinusoids
* Sinusoids are lined by sinusoidal / discontinuous endothelial cells with Kupffer cells
* Perisinusoidal space separates hepatocytes from the sinusoids and contains scattered hepatic stellate cells
* Bile which has several functions, eg:
* bile acids - necessary for digestion of dietary fats
* excretory route - for various metabolites and
* drugs buffers - neutralize acid pH from the stomach

Neural (Sensory) Retina

* Neural retina lies between the vitreous humor and the RPE
* Several interconnected neuronal subtypes as well as supportive glial cells
* Consists of nine distinct layers; from the outer region toward the inner region,
* Rod and cone layer - the outer segments of the rod & cone (photoreceptor cells 2)
* Outer limiting membrane - formed by junctional complexes holding rod/cones
* Outer nuclear layer - cell bodies and nuclei of the photosensitive
* Outer plexiform layer - contains fibers & synapses of the bipolar neurons
* Inner nuclear layer - contains cell bodies of several types of bipolar neurons
* Inner plexiform layer - contains fibers & synapses of ganglion cells & bipolar neurons
* Ganglion cell layer - contains cell bodies of the ganglion cells
* Nerve fiber layer - contains ganglionic cell axons,
* Inner limiting membrane - a basement membrane covered by expanded processes of Müller cells

Exocrine pancreas

* CT septa extend from the outer capsule to divide the pancreas into numerous lobules
* Acini are formed by acinar cells which are pyramid shaped with a basophilic base
* Aapical cytoplasm has digestive enzymes


* Lobules: cuboidal epithelium
* Blood circulation exocrine pancreas is from 2 sources:
* insuloacinar portal system - venules
* acinar vascular system – an independent arterial system also supplies acini
* Pacinian corpuscles are commonly found within the CT of the pancreas, detect pressure and vibration when compressed


* Exocrine pancreatic secretions include electrolytes & digestive enzymes, proteins & carbohydrates
* Digestive enzymes are stored and released as proenzymes which require activation
* If activated prematurely causes serious damage
* Pancreatic secretion is controlled mainly by 2 hormones: secretin and cholecystokinin - neuroendrocrine cells

Ruminant forestomachs

* Mucosal lining of all the ruminant forestomachs is keratinized
* Rumen papillae are projections of the tunica mucosa: they have a core of lamina propria connective tissue with no muscularis mucosae
* Reticulum has a honeycomb-like arrangement with a muscularis mucosae
* Omasum has numerous large folds; each fold has a central strand of muscularis externa

Tubular portion of salivary glands

* Submucosa denser: connective tissue with larger blood lymph vessels, submucosal autonomic nerve plexus, +/- glands & lymphoid tissue
* Muscularis: smooth muscle blood/lymph vessels & myenteric autonomic nerve plexus
* Serosa / adventitia
* Abdominal cavity: the serosa is a thin layer of loose CT with an outer covering of mesothelium part of the visceral peritoneum of the mestentery
* Esophagus: has an adventitia = a layer of connective tissue which blends with the surrounding connective tissue of the neck or mediastinum in the thorax

Digestive embryology

* the endoderm is lying dorsally over the yolk sac
* the primitive gut is formed through folding of the embryonic disc
* foregut extends to the stomodeum
* the midgut communicates with the yolk sac
* the hindgut communicates with the allantois in the cloaca
* The foregut gives rise to the pharynx, the respiratory system, the esophagus, the stomach as well as the liver and pancreas
* The midgut forms most of the intestine
* the hindgut gives rise to caudal parts of the intestine
* Epithelial lining of the digestive tract: endoderm
* Derived from the neural crest

Oral cavity epithelium

* stratified squamous epithelium
* keratinized in many areas

buccal papilla

* Cheeks
* Ruminants
* keratinized conical projections

Hard palate

* separates the oral and nasal cavities
* core of bone
* oral surface is lined by keratinized epithelium

Soft palate

* separates the oropharynx from nasopharynx
* has a core of skeletal muscle
* oral surface is lined by non-keratinized epithelium

Tongue

* Large core of skeletal muscle
* mucosa and papillae

brachydont teeth

* carnivores and swine
* the enamel covers the tooth above the gumline
* stop growing after fully erupted

hypsodont teeth

* herbivores
* Enamel covers the body of tooth and extends under the gumline
* 1 or 2 infundibula
* The teeth continue to grow for some years after they come into wear

Embryonic development of the Teeth

* teeth are derived from the tooth germ which is an aggregate of cells derived from the ectoderm and **derived from the neural crest**
* the tooth germ consists of: the enamel organ, the dental papilla and the dental sac
* ameloblasts, derived from the inner enamel epithelium, produce enamel and odonoblasts, **derived from the immediately adjacent dental papillae, produce dentin**

enamel

* highly mineralized
* produced by ameloblasts

dentin

* bone-like
* secreted by odontoblasts

Esophagus

* muscular tube transports swallowed material from the pharynx to the stomach
* Muscularis externa is skeletal proximally and smooth muscle distally

Stomach/abomasum

* **The mucosa lined by stratified squamous epithelium**
* The glandular mucosa has gastric pits connected to gastric glands via neck region
* Fundic glands are a mix of parietal cells and chief cells
* Cardiac and pyloric glands
* mucus cells are found on the surface, gastric pit and neck region
* enteroendocrine cells secrete hormones; ex gastrin, which stimulates secretion of HCl & pepsinogen
* stem cells: neck region
* **cats have subglandularis, which lies is between the base of the gastric glands and the thick muscularis mucosa**

Small intestine

* Villi: luminal leaf-like projections composed of cores of lamina propria: simple columnar epithelium of enterocytes and interspersed goblet cells
* Crypts : simple tubular GI glands; simple columnar enterocytes, goblet cells + Paneth cells, enteroendocrine cells & stem cells
* small and large intestine are regenerated continuously throughout adulthood from stem cells that reside near the base of crypts

Duodenum

* short proximal segment
* submucosal mucus glands

Jejunum

* the large middle segment
* attached by the mesentery
* scattered GALT

Ileum

* the short distal segment
* which connects with cecum and large intestine
* abundant GALT

enterocytes

* tall columnar cells with basally located nucleus
* prominent apical brush border
* For transocytosis of sugars and amino acids
* **Lipid subunits are taken up by enterocytes → converted to triglycerides → taken up by lacteals of the villus**

Goblet cells

lubricates & protects the intestinal lining

Paneth cells

* located in crypts
* large, eosinophilic secretory granules in the cytoplasm
* innate immunity

Enteroendocrine cells

* in lining of the crypts and villi
* They sample intestinal content
* triggers the release peptides/hormones

M cells

* located in FAE layer in GALT
* have apical microfolds and pocket-like invaginations **of basal membrane** that contains immune cells
* specialized for the uptake and transepithelial transport of microbes

Small intestine GALT

* diffuse lymphoid tissue
* solitary/aggregated lymphoid nodules

**in mucosa / submucosa**

Small intestine Lamina propria

* loose CT in the core of villi and between crypts
* contains blood vessels, lymphatics, smooth muscle, nerves

Small intestine submucosa

* loose in most species
* moderately dense in carnivores
* large blood/lymphatic vessels
* has a nerve plexus

Small intestine muscularis externa

* internal circular layer
* external longitudinal layer
* myenteric nerve plexus between layers

Small intestine serosa

* covered by mesothelium
* continuous supporting mesentery

Large intestine Mucosa

* no villi
* straight tubular glands which are lined by large numbers of goblet cells, absorptive epithelial cells, and a few scattered enteroendocrine cells

Proliferating stem cells:

* are present in the base of the glands
* for continuous renewal

Large intestine GALT

* diffuse type in the lamina propria
* solitary type in the mucosa/submucosa

Anal canal

* an abrupt change to stratified squamous epithelium
* nonkeratinized in the proximal portion
* keratinized in the distal portion

Glands of the anal sacs

* Many carnivores have paired anal sacs
* lined by keratinized epithelium
* Sacs store secretions of surrounding glands
* apocrine tubular in dogs
* apocrine and sebaceous in cats

Circumanal Glands

* Subcutis around the around the anus of dogs
* Upper portion is sebaceous
* Deeper regions are non-sebaceous

Pancreas Embryology

exocrine acini and the endocrine islets of Langerhans develop from endodermal buds of the caudal end of the foregut

Pancreas

* two lobes that diverge near the pylorus
* right lobe that lies against the stomach
* left lobe along the duodenum
* most dogs have 2 pancreatic ducts
* most of cats have a single pancreatic duct that fuses with the common bile duct

Endocrine pancreas

* Islets of Langerhans
* small clusters of endocrine cells that are scattered throughout the exocrine acinar pancreas

Alpha cells (\~20%)

* produce glucagon
* increase in ECF by target cell catabolism

Beta cells (\~60-70%)

* produce insulin
* decrease in ECF by target cell anabolism/storage

Delta cells (\~5%)

* produce somatostatin
* inhibits release of insulin, glucagon and GI peptides

F or PP cells (\~10%)

* make pancreatic polypeptide
* inhibits intestinal motility
* stimulates secretion of GI enzymes

Liver embryology

* Liver & extrahepatic biliary tract originate from an endodermal bud on the ventral region of the duodenum
* Hepatocytes and cholangiocytes are derived from bipotent liver progenitors, the hepatoblasts
* **Other liver cells are derived from the mesoderm**

Portal triads

* bile ducts - drain bile from hepatocytes
* hepatic artery- supply oxygen-rich blood to sinusoids
* portal vein - supplies nutrient-rich blood from the GI to sinusoids
* the limiting plate is the first row of hepatocytes adjacent to the portal tract connective tissue

Hepatic acinus is divided into three zones

* Zone 1- surrounds the portal triads
* Zone 2 is the midlobular area
* Zone 3 zone surrounds the central vein

Kupffer cells

* phagocytosis
* derived from macrophage progenitors in the embryonic yolk sac / fetal liver
* long-lived
* self-renewing

Hepatic stellate cells

* Located in the space of Disuse
* have lipid droplets for storage of vitamin A
* Maintain & remodel the delicate sinusoidal ECM
* hepatic fibrosis

Gallbladder

* Absent in some species of mammals and some birds
* Bile goes from the cystic duct into the gallbladder for temporary storage and concentration
* epithelial cells can pump water & ions from the bile
* lined by columnar epithelium

Embryology of the eye

* neuroepithelium forms the retina, ciliary body, iris, & optic nerves
* surface ectoderm forms the lens, corneal epithelium and eyelid
* extracellular mesenchyme forms the sclera, corneal endothelium & stroma, blood vessels, muscles and vitreous

fibrous tunic

outer layer consisting of the cornea & sclera

vascular tunic

middle layer consisting of the iris, ciliary body & choroid

retina

inner layer consisting of a neural layer & pigmented layer

Cornea

* avascular
* highly innervated
* transparent
* nonkeratinized stratified squamous epithelium
* endothelium is cuboidal/squamous cells
* corneal stroma is thick and consists collagen fibers and flattened fibroblasts-like cells
* Descemet’s membrane is the thick basement membrane on the corneal endothelium
* ion pumps to keep the stroma relatively dehydrated & transparent

Sclera

* attachment of the extrinsic eye muscles
* Dense well-vascularized CT
* scattered melanocytes
* Limbus: the border between the cornea and the sclera

Choroid

* Between the sclera & retina
* joins with the ciliary body anteriorly
* Nourishes retina
* Loose, well-vascularized CT

Ciliary body

* Extends from the anterior end of the choroid to the base of the iris
* rests on the sclera just posterior to the limbus

ciliary muscle

smooth muscle affecting the shape of the lens

ciliary processes

* radially arranged ridges
* basal layer of pigmented cells and a surface layer of nonpigmented cells
* produce the aqueous humor by modifying fluid from the stromal microvasculature

**ciliary zonule**

**fibers extend from the surface epithelium of the ciliary processes and attach to lens capsule, near its equator**

Retina

* Innermost layer of the wall of the eye
* Photosensitive region extends from posterior aspect of the ciliary region to the optic disc

Eyelid

* pliable skin
* composed of an epidermis and a thin dermis
* skeletal muscles
* a layer of dense fibrous CT (tarsal plate) gives structural rigidity

Glands of Moll

modified sweat glands that open onto the base of eyelashes

glands of Zeis

sebaceous glands that open into the eyelashes

Meibomian (tarsal) glands

* large multilobular sebaceous glands
* embedded in the tarsal plate
* secrete meibum

Nictitating membrane (Third eyelid)

* a protective fold of conjunctiva
* supported by a plate of cartilage

Middle Ear

ossicles in the tympanic cavity provide a bridge between the tympanic membrane and temporal bone

Ovaries

* Produces oocytes and sex hormones
* has a dense CT capsule with a single layer of surface squamous or cuboidal epithelium
* the outer cortex contains follicles
* inner medulla consists of vascular CT

primordial follicles

* immature follicles
* the most numerous follicles in the ovary.
* primary oocyte surrounded by a single layer of squamous follicular epithelial cells

primary follicles

* develop in response to hormonal stimulation.
* Enlarging primary oocyte surround early by cuboidal follicular cells
* Becomes surrounded by an acidophilic protective glycoprotein layer, the zona pellucida.

Uterine tube

* Funnel-shaped infundibulum next to the ovary
* in the infundibulum & ampulla regions mucosa is ciliated columnar epithelium with fewer secretory cells that provide a nutritive mucus for sperm & zygotes
* the muscularis is well developed with a thick circular and a thin longitudinal layer
* a thin serosa is continuous with the visceral peritoneum

Uterus

* domestic mammals have a
* bicornuate uterus
* with two uterine horns a uterine body
* a single cervix.

the endometrium

* often simple cuboidal/columnar
* endometrial glands extend into lamina propria
* ruminants have multiple small non-glandular regions called caruncles

myometrium

* has a inner circular layer and a outer longitudinal layer
* vascular layer between the two

perimetrium

* outer layer is continuous with the visceral peritoneum
* loose CT containing blood & lymphatic vessels with an overlying mesothelium.
* cervix typically has surface folds and is lined by mucus secreting columnar epithelium
* Dogs are an exception as the cervix is lined by stratified squamous epithelium

Gravid Uterus / Placentation

* amnion surrounds and cushions the embryo
* the yolk sac balloons out from the embryonic midgut
* the allantois an outgrowth hindgut stores embryonic waste
* the chorion is the outermost of the fetal membranes

implantation

* in rodents & primates
* newly hatched blastocyst attaches to the endometrial epithelium and penetrates into the endometrial CT
* in domestic mammals
* the embryo remains attached to the endometrial surface throughout gestation
* except in carnivores, placentation is non-invasive

Placental shape and area of contact with the endometrium

* diffuse: almost all of the chorioallantoic membrane
* cotyledonary: multiple, discrete areas of CAM
* zonary: a belt-like band of CAM
* discoid: a single discoid shaped CAM

Tissue layers that remain between fetal and maternal blood

* epitheliochorial: all maternal layers are retained, eg horses swine
* synepitheliochorial: all layers retained with fusion of fetal and maternal epithelium
* endotheliochorial: endometrial epithelium & connective tissue are lost
* hemochorial: direct contact of chorionic epithelium with maternal blood

Vagina

* most mammals: lined by stratified squamous epithelium
* cow: lined by stratified columnar epithelium with interspersed goblet cells
* carnivores: keratinized during estrus

Vulva

* includes the vestibule labia & clitoris
* lined by stratified squamous epithelium
* mucosal glands are found in the vestibule
* the outer surface of the labia has integument with well-developed sebaceous and apocrine glands