Histology Exam 2: Gi Tract

Front 1

Describe the tunica mucosa of the esophagus.

Back 1

- epithelium is stratified squamous; uncornified in carnivores, slightly cornified in pigs/horses, heavily cornified in ruminants

- lamina propria (has collagen and elastic fibers) has denser connective tissue than tunica submucosa!

- lamina muscularis mucosae: longitudinally oriented smooth muscle

Front 2

Describe the tunica submucosa of the esophagus.

Back 2

- loose connective tissue

- longitudinally oriented arteries, veins, lymphatic vessels, nerves

- seromucous glands (called esophageal glands)

Front 3

Describe the tunica muscularis of the esophagus.

Back 3

- inner and outer layer

- cranial portion of esophagus: intermixing of skeletal or smooth muscle fiber layers

- transition into inner circular and outer longitudinal muscle layer in caudal portion

- near stomach, inner circular muscle layer thickens to become the cardiac sphincter (very prominent in horses)

- in ruminants, skeletal muscle extends from esophagus into wall of reticular sulcus

Front 4

Describe the outermost tissue layer of the esophagus.

Back 4

- cervical and cranial mediastinal portion of esophagus are covered by adventitia (loose CT, blood vessels, nerves)

- caudal mediastinal portion covered by serosa (made of mesothelial cells of mediastinal pleura)

- in horses/carnivores, esophagus extends beyond diaphragm into abdominal cavity, is covered by serosa (its mesothelium from visceral peritoneum of abdominal cavity)

Front 5

Describe the esophagus-stomach junction.

Back 5

- in carnivores, abrupt change from stratified squamous epithelium to simple columnar at cardiac region

- in some species, esophageal glands extend for short distance into submucosa of stomach

Front 6

What are the species differences regarding the lining of the stomach?

Back 6

- carnivores/primates: glandular epithelium

- herbivorous species: variable amounts of stratified squamous epithelium

Front 7

Describe non-glandular portions of the stomach.

Back 7

- present in ruminants, horses, adult pigs, rodents

- stratified squamous, may be cornified

- tunica mucosa has all 3 layers

- in ruminants, lines entire forestomach; in horses, ~50% of gastric mucosa, terminates at margo plicatus ("folded margin")

Front 8

Describe the epithelial cells of the gastric mucosal surface (glandular mucosa).

Back 8

- simple columnar, mucin-producing cells called surface mucous cells

- each cell has large apical cup of mucinogen granules (cup looks empty because mucinogen is water-soluble, so lost during processing)

- when preserved by alcohol fixation and stained with PAS or Toluidine blue, granules are very blue

- nucleus and Golgi apparatus below the mucous cup

- basal portion of cell slightly basophilic (rER)

Front 9

Describe gastric pits.

Back 9

- wide (compared to lumens of branched tubular cardiac, pyloric, or fundic glands)

- at base of pit, lumen narrows to form isthmus, which opens into gastric glands below

- gastric glands occupy most of lamina propria (few CT fibers and cells)

- surface epithelial cells have rapid turnover, are replaced by lining cells from gastric pit

Front 10

Describe cardiac glands.

Back 10

- branched coiled tubular glands that open into gastric pits

- short body, wide lumen

- mucous secretory epithelium: cuboidal, nuclei located basally

- mucus secretion protects esophageal epithelium from gastric reflux

Front 11

Describe pyloric glands.

Back 11

- branched, coiled, short

- deeper gastric pits

- mucus-secreting cells: slightly basophilic, flat nuclei at base of cell

Front 12

Describe fundic glands.

Back 12

- present throughout entire mucosa of glandular stomach except for cardia and pylorus

- isthmus is site of cell replication for both surface mucosa and fundic glands (cells either migrate up the gastric pit to the stomach surface or down to maintain population of fundic gland epithelium)

- several glands open into one gastric pit

- glands have narrow, long neck and short, wide base (aka fundic segment)

- base usually divides into 2-3 branches that coil slightly near muscularis mucosae

- four cell types (see next question)

Front 13

Describe the four cell types of fundic glands.

Back 13

Mucus neck cells:
- in neck region, interspersed with parietal cells
- shorter than surface mucus cells, have less mucinogen

Chief cells:
- in deeper region of gland
- protein-secreting cell: rER (basophilic), apical cytoplasm with zymogen granules (eosinophilic)
- secrete pepsinogen (turned into pepsin upon contact with acidic gastric juice)

Parietal (oxynic) cells:
- in neck and deeper regions
- large, triangular (apex towards lumen, base on basement membrane), often multinucleated
- eosinophilic
- secretes HCl, intrinsic factor (to bind Vit B12)

Enteroendocrine (argentaffin) cells:
- all levels of gland, but more prevalent in basal region
- small cells that rest on basement membrane, do not always reach lumen
- sample contents of lumen then release appropriate hormones (e.g. gastrin)

Front 14

What is the subglandular layer of the stomach?

Back 14

- carnivores

- in mucosa of fundic gland area, interposed between lamina propria and muscularis mucosae

- two layers (strata):
- stratum compactum: most visible; dense sheet of collagen fibers, acellular
- stratum granulosum: fibroblasts

- function thought to be adaptation for rough diet of carnivores (bone shards)

Front 15

Describe the gastric submucosa.

Back 15

- same regardless of type of epithelium in tunica mucosa above it

- connective tissue, adipose tissue, blood vessels, Meissner's plexus (aka submucosal plexus; nerve fibers and ganglion cells)

Front 16

What does Meissner's plexus innervate?

Back 16

(Meissner's plexus aka submucosal plexus)

- innervates vessels of submucosa and smooth muscle of the muscularis mucosae

Front 17

Describe the tunica muscularis of the stomach.

Back 17

- aka muscularis externa

- randomly organized smooth muscle

- said to have inner oblique layer, middle circular layer, outer longitudinal layer, but not easily distinguished

- Auerbach's plexus (myenteric plexus): unmyelinated nerves, ganglia; between middle and outer layers

Front 18

Describe the gastric serosa.

Back 18

- mesothelial cells continuous with parietal peritoneum of abdominal cavity

- similar to that on small intestine, cecum, colon

Front 19

Name and describe function of the compartments of the ruminant stomach.

Back 19

- forestomach (rumen, reticulum, omasum) and abomasum

- forestomach breaks down fiber (with help of flora) and absorbs nutrients:
- rumen is fermentation vat where microflora produce VFAs that are absorbed by rumen mucosa into blood stream
- reticulum and omasum: mechanically reduce ingesta into small particles

- abomasum: enzymatic digestion

Front 20

Describe the rumen of the ruminant stomach.

Back 20

- tunica mucosa: cornified stratified squamous epithelium; characterized by tongue-shaped papillae

- mucosa has 3 functions: protection (stratum corneum protects against rough fibrous ingesta), metabolism (break down VFAs), absorption

Front 21

Describe the reticulum of the ruminant stomach.

Back 21

- cornified stratified squamous epithelium (identical to rumen)

- mucosal surface has interconnecting folds called reticular crests (honeycomb appearance): taller primary crests separate mucosal surface into reticular cells, which are subdivided into smaller compartments by shorter secondary crests

- sides of each crest have vertical ridges

- mucosal surface on and between crests are covered with reticular papillae that project into lumen

Front 22

Describe the omasum of the ruminant stomach.

Back 22

- lumen is filled with 100+ interdigitating folds called laminae that arise from the internal surface of the greater curvature and sides

- omasal contents pressed into thin layers between the laminae (spaces are called interlaminar recesses) and reduced to fine pulp by horny omasal papillae on the mucosal surface

- cornified stratified squamous epithelium; non-glandular lamina propria with dense subepithelial capillary network; muscularis mucosae

- thin tunica submucosa

- tunica muscularis: thin outer longitudinal layer, thick inner circular layer of smooth muscle

Front 23

Describe the abomasum of the ruminant stomach.

Back 23

- same glandular regions as other species

Front 24

Describe general components of camelid stomach.

Back 24

- C1: analogous to rumen of ruminants

- C2: ~ reticulum

- C3: proximal portion analogous to omasum; remainder analogous to abomasum (has gastric glands)

Front 25

Describe C1 of the camelid stomach.

Back 25

- mostly non-glandular stratified squamous; some glandular simple columnar (with tubular glands)

- no papillae; surface is smooth

- divided into cranioventral and caudodorsal sacs; ventral portion of sacs contains saccules lined with simple columnar mucus secreting cells

Front 26

Describe C2 of the camelid stomach.

Back 26

- mostly glandular simple columnar, except for small dorsal patch (part of esophageal groove) of stratified squamous

- latticework of pillars (studded with papillae) that enclose saccules
(as opposed to reticular crests in ruminants)

Front 27

Describe C3 of the camelid stomach.

Back 27

- proximal 2/3: non-glandular, has low longitudinal pleats

- distal 1/3: simple single-cell epithelium of gastric glands

Front 28

What structural features of the small intestine facilitate absorption?

Back 28

(facilitate absorptive efficiency by increasing mucosal surface area)

- plicae circulares: semi-circular mucosal folds in upper 1/2-2/3 of small intestine; permanent in ruminants but disappear when carnivore intestine is distended

- villi: finger-like projections of mucosal surface containing extension of lamina propria in their core; lined with absorptive enterocytes; long and slender in carnivores, short and wide in ruminants

- microvilli: on apical surface of columnar cells that make up most of mucosal surface

Front 29

What are the crypts of Liberkuhn?

Back 29

- intestinal or mucosal glands

- open between bases of villi, penetrate into mucosa almost to muscularis mucosae

Front 30

Describe the 5 types of cells in the epithelial surface of the small intestine.

Back 30

1) Enterocytes
- microvilli (striated apical border)
- tall columnar
- absorption, secretion of enzymes

2) Goblet cells
- unicellular glands that secrete mucus

3) Paneth cells
- in base or crypt region of intestinal glands
- basophilic; supranuclear Golgi apparatus; large eosinophilic granules in apical region
- granules contain lysozyme digest bacterial cell walls), defensins
- important for innate immunity of mucosal surface; regulate bacterial flora

4) Enteroendocrine cells
- produce paracrine and endocrine hormones that regulate GI motility and physiology (e.g. somatostatin, histamine)

5) M (microfold) cells
- antigen-presenting cells: epithelial cells with microfolds instead of microvilli on apical surface
- cover submucosal lymphatic nodules (Peyer's patches, aka GALT)

Front 31

Describe the tunica submucosa of the small intestine.

Back 31

- loose connective tissue (but denser than lamina propria)

- blood/lymph vessels and Meissner's plexus (submucosal plexus)

- tubuloalveolar glands (called submucosal glands): their ducts open into crypt region of intestinal glands
- mucus-secreting in dogs/ruminants, serous in pigs/horses, seromucous in cats
- do not extend full length of duodenum in sheep/goats/carnivores; extend into jejunum of horses/cattle/pigs

- Peyer's patches (aggregates of lymphoid patches): on anti-mesenteric part; can be found anywhere in domestica animal, but more numerous in distal jejunum and ileum

Front 32

Describe the tunica muscularis of the small intestine.

Back 32

- inner circular and outer longitudinal smooth muscle

- Auerbach's plexus (myenteric plexus) between two layers

Front 33

Describe the outer covering of the small intestine.

Back 33

- serosa is uniform: single-cell mesothelium layer with its basement membrane over a layer of loose connective tissue

Front 34

Describe the cecum and colon.

Back 34

- cannot histologically differentiate

- absorb water/vitamins/electrolytes, secrete mucus

- mucosa of cecum has straight, tubular glands (crypts of Liberkuhn) lined with simple columnar epithelium

- horses/rabbits have large cecum (bacterial fermentation reservoir), carnivores have small cecum, some mustelids (ferret, mink, otter) have no cecum

Front 35

Describe the rectum.

Back 35

- surface of lumen has mucin-producing simple columnar epithelium that invaginates into lamina propria as simple tubular rectal glands; rich in goblet cells

Front 36

Describe the recto-anal junction (aka ano-rectal line).

Back 36

- abrupt transition from simple columnar epithelium to non-cornified stratified squamous

Front 37

Describe the anal canal.

Back 37

- mucosa is smooth and lacks glands in ruminants/horses

- 3 distinct zones in carnivores/pigs:
1) Columnar zone
- contains longitudinal folds called anal columns, lined with non-cornified stratified squamous epithelium
- anal glands (modified tubuloalveolar apocrine sweat glands that secrete lipids in carnivores and mucus in pigs) in propria-submucosa

2) Intermediate zone
- narrow strip with non-cornified stratified squamous and anal glands in the propria-submucosa

3) Cutaneous zone
- cornified stratified squamous
- in carnivores, ducts from anal sacs (bilateral invaginations of anal mucosa) open at junction between intermediate/cutaneous zones
- in dogs, mucosa of the outermost part (near junction w/ skin) contains circumanal glands, which are modified sebaceous glands

Front 38

Describe the anus (anal mucocutaneous junction).

Back 38

- mucocutaneous junction like lips

- covered on outside by skin (cornified stratified squamous epithelium) and on inside by non-cornified stratified squamous mucosal epithelium

Front 39

Describe the avian crop.

Back 39

- saclike diverticulum of esophagus

- storage organ where food is moistened/softened by mucus secretions of esophageal glands

- histologically similar to esophagus, except glands are restricted to area near junction with esophagus

- in some birds, crop glands secrete crop milk (influenced by prolactin), which nourishes nestlings until they can eat on their own

Front 40

Describe the avian proventriculus.

Back 40

- "glandular stomach"

- mucosa has papillae with plicae of varying heights

- plicae are covered with simple columnar epithelium arranged around a central duct emerging from the proventricular glands (submucosal glands)

- duct opens into proventriculus at apex of each papilla

- proventricular glands lined with simple cuboidal or low columnar; only one cell type, oxynticopeptic cell (produces HCl and pepsinogen)

- lamina propria is loose CT
- has muscularis mucosae

- tunica muscularis has 3 layers

- outside covered by serosa

Front 41

Describe avian ventriculus (gizzard).

Back 41

- highly muscular; grinds up ingesta (already softened by proventriculus)

- lining is called cuticle or koilin membrane, which resembles stratum corneum but is actually a secretory product of mucosal glands of ventriculus

- surface epithelium is simple columnar

- lamina propria and tunica submucosa are both loose CT

Front 42

Describe the avian cecum.

Back 42

- paired cecae that open into digestive tract at junction of ileum and colon

- more developed in herbivorous birds than carnivorous

- near base is cecal tonsil: nodular mass of lymphoid tissue that occupies much of lamina propria and submucosa

Front 43

Describe avian colon and rectum.

Back 43

- simple columnar epithelium

- rectum empties into the coprodeum of the cloaca

Front 44

Describe the avian cloaca.

Back 44

- divided by transverse folds into 3 histologically similar parts: coprodeum, urodeum, proctodeum

- mucosal epithelium is simple columnar, has villi

- rectum opens into coprodeum

- paired ureters and ductus deferens enter urodeum

- cloacal bursa (bursa of Fabricius) opens into proctodeum