Digestive system

The main components of the digestive tract are:

a. Oral cavityb. Esophagusc. Stomachd. Small intestinee. Large intestinef. Rectum and anus

The entire digestive tract can be considered as a hollow tube surrounded by a wall composed of four main layers:

1. Mucosa 2. Submucosa3. Muscularis externa4. Serosa or adventitia

1. Mucosa- The mucosa consists of:

a. Epithelium (lining of the lumen)i. Forms a selective barrier between the external environment (lumen) and the

bodyii. May contain goblet cells that secrete mucus for lubrication

iii. Endocrine cells: are common in the epithelium and produce polypeptide hormones, which play a role in the regulation of the digestive process

b. Lamina propria (loose connective tissue)i. Below epithelium; consists of loose CT with an abundant blood supply

ii. Lymphatic nodules, lymphocytes and plasma cells are commoniii. Forms first line of immunological defense against bacterial and viral invasion

c. Muscularis mucosae (thin layer of smooth muscle cells)i. Causes local muscular contractions in the mucosa

2. Submucosa- The submucosa consists of dense CT and Meissner’s nerve plexus3. Muscularis- 2 sublayers ofsmooth muscle cells: inner circular layer & outer longitudinal layer)

o Involved in the peristaltic movements of the intestineo Responsible for the propelling and mixing of the foodo Auerbach’s (mysenteric) nerve plexus: found between the 2 muscle layers

Generates movements in the digestive tract (rhythmic peristaltic contractions) The neurons of the plexus can be visualized by silver impregnation techniques

Plexuses consist of aggregates of nerve cells in the form of small parasympathetic ganglia

4. Serosa or adventitia- Serosa:Has thin layer of loose CT covered by simple squamous epithelium (mesothelium) - Serosa: Present in parts of the intestinal tract that are present in the peritoneal cavity- Adventitia: outer layer of loose CT that holds in place the regions in the digestive tract that are

not present in the cavity.

I. ESOPHAGUS:a. Identification points:

i. Mucosa lined by stratified squamous non-keratinized epitheliumii. Submucosa containing esophageal glands

iii. Muscularis externa containing inner circular and outer longitudinal muscle layer- A straight muscular tube connecting the oral cavity to the stomach- Contains 4 basic layers common to the rest of the digestive tract- Mucosa: innermost

o Lined by stratified squamous nonkeratinized epithelium which is resting on the basement membrane

o Surface epithelium may contain few specialized cells belonging to APUD (amine precursor uptake and decarboxylation) cell system

- Lamina propria: o Beneath the epitheliumo A layer of loose CT; folded into many finger like projections called papillae

Papillae: help in firm attachment of surface epithelium to the underlying lamina propria

o In the upper and lower one-third of the esophagus, lamina propria may contain few small lymphoid aggregations and tubule-alveolar mucous glands

- Muscularis mucosa: o has thin layer of bundles of longitudinal muscle fibers o Development:

absent or poorly developed in the upper one-third of the esophagus, moderately developed in the middle one-third and very distinct in the lower one-third

- submucosa:o loose areolar CT rich in CT fibers, cells and blood vesselso characterized by the presence of numerous compound tubule alveolar mucous glands

also called esophageal glands- muscularis externa:

o beneath submucosao consists of inner layer of circular muscles and outer layer of longitudinal muscle fiberso muscle types:

upper one third: striated muscle middle one third: mixed striated and smooth muscle

lower one third: purely smooth muscleo it is assumed that the muscle fibers present in the lower end of esophagus may form

cardio-esophageal sphincter but anatomically it is not accepted- adventitia

o outermost protective layero dense irregular CT

II. STOMACHa. Identification points: Fundus

i. Mucosa lined by simple columnar epitheliumii. Numerous chief and parietal cells in lamina propria

iii. Muscularis externa containing inner oblique, middle circular and outer longitudinal muscle layer

- muscular bag like structure which stores the food temporarily for few hours- 3 major parts: fundus, body and pylorus- Histologically speaking, it is divided into 2

o Fundus and bodyo Pylorus

FUNDUS

- Mucosa:o Fundic stomach is folded into many rugae which disappear on expansiono Lined with simple columnar epithelium which are resting on basemento Most of the places it dips into the underlying lamina propria forming walls of depression

known as gastric pitso Gastric pits: shallow in fundic part of stomach and form abouth one fourth of the total

thickness of mucosa- Lamina propria:

o Rich in fundic glands consisting of following types of cells:1. Stem cells:

a. these are active, undifferentiated cells found at the base of lamina propria which gives rise to all other types of cells

2. Enteroendocrine cells: a. special type of cells found just above stem cells b. AKA argentaffin cells (stained by silver salts)c. Belong to APUD cell system d. Produce some local hormones like somatostatin and gastrin

3. Chief cells:a. Small cuboidal cells which lie upon the enteroendocrine cellsb. Very large in number with basophilic cytoplasm and few zymogen granulesc. Referred to as zymogenic cellsd. Known as peptic cells: they produce protein digestive enzyme like pepsin

4. Oxyntic cells:a. Large pyramidal or polyhedral cells with central nucleus found lying

between mucous neck cells and chief cells

b. Present in the upper half of the lamina propria and are easily identified by its strong bright eosinophilic cytoplasm—hence oxyntic cells

c. AKA parietal cells as they lie against the basement membrane of surface epithelium

5. Mucous neck cells:a. Low columnar cells present near neck of the gastric glandsb. Large cells with flat nucleus at its base and whole cell is accumulated by acid

mucousc. They open into gastric pits and secrete acid mucous

- Muscularis mucosa:o Has thin layer of inner circular and outer longitudinal muscle fibers extending into the

lamina propria here and there- Submucosa

o Made up of Loose areolar CTo Rich in CT fibers and cells with many blood vessels in ito Also contains Meissner’s plexus of nerve fibers in it

- Muscularis externa:o Made up of inner oblique, middle circular and outer longitudinal muscle layers o Between circular and longitudinal muscle layer, few mysenteric (Auerbach’s) plexus of

nerve fibers are seen- Serosa:

o Outermost layer made up of few CT cells and fibers covered by mesothelium of the visceral peritoneum

PYLORUS

- Gastric pitso Longer and wider than the pits of the fundus or body

- Gastric glandso Shorter and more coiledo Consists almost entirely of mucus secreting cellso No parietal or zymogen cells o Also secrete lysozyme

- Endocrine cellso Include cells secreting gastrin which stimulates acid secretion by the parietal cells

- Musculariso Composed of 3 layers of smooth muscle (not always easily distinguishable in histo

sections) External longitudinal layer Middle circular layer Internal oblique layer

o Pyloric sphincter: Controls discharge of stomach contents to the duodenum Consists of an enlarged middle layer of smooth muscle

III. SMALL INTESTINE

- Main site of absorption of digested food- Specialized for the completion of the digestion process and the subsequent absorption of the

digested product- Overall length: 5 meters- 3 main segments:

o Duodenumo Jejunum o Ileum

- Characteristic features of the small intestine include:a. Intestinal villi:

i. Finger-like projections into the lumen (consisting of surface epithelium and underlying lamina propria)

ii. Lining epithelium: simple columnar heterogenous epithelium with goblet cells Apical surface (absorptive epithelial cells): has a “brush border”—

resulting from an orderly arrangement of closely-packed microvilli Microvilli

a. In the transmission electron microscope, have a central core of actin filaments

b. Main function: increase the surface area available for absorption

c. The absorptive cells have oval nuclei, typically in the basal half of the cells

iii. Lamina propria: Formed form loose CT Contains blood vessels, nerves, large lymphatic vessels (site of

absorption of lipids) and cells of the immune system (usually lymphatic nodules

b. Intestinal glands i. Simple tubular glands that open to the intestinal lumen between the base of the

villiii. Sometimes called Crypts of Lieberkuhn

iii. Secretory cells or Paneth Cells: With large acidophilic granules are found at the base of the intestinal

glands Function not fully understood but know to secrete lysozyme

(antibacterial properties)c. Valves of Kerckring:

i. Lining of the small intestine has permanent folds known as Valves of Kerckring or plicae circulares

ii. Most prominent in the jejunumiii. Seen macroscopically in transverse sections, consist of mucosa and submucosa

DUODENUM

A. Identification points:

i. Numerous villiii. Crypts of lieburkuhn present

iii. Brunner’s glands in submucosa- Shortest segment of small intestine (25 cm)- Divided into 4 parts:

o Superioro Descending o Horizontal o Ascending

- The transverse section of duodenum shows the following different layers:o Mucosa:

Innermost layer of duodenum lined by simple columnar epithelium with fine microvilli at its tip

Few goblet cells are also present in surface epithelium Mucosa is in large folds or finger-like projections called villi (helps in increasing

the surface area of absorption Typical villus is lined by surface epithelium and lamina propria projecting into it.

It contains loose areolar CT and a central core of lymphatics called Lacteal Just below the mucosa there exists lamina propria containing simple tubular

intestinal glands which open into the crypts of villi and are also called Crypts of Lieburkuhn

Cells in the crypts of Lieburkuhn:a) Stem cells: undifferentiated cells undergoing continuous mitotic cell

division which migrates, eventually replaces the old surface eputhelium thus forming enterocytes

b) Goblet cells: secretes mucousc) Entero-endocrine cells: also called argentaffin cells; belongs to APUD

cell system and secrete local hormones (secretin, cholesystokinin pancreozymin

d) Paneth cells: AKA zymogenic cells (rich in zinc and zymogen granules in it); produce digestive enzymes and lysozymes

o Muscularis Mucosa: Seen below lamina propria and made up of thin layer of disrupted circular

muscle fiberso Submucosa:

It contains loose areolar CT with numerous mucous secreting duodenal glands called Brunner’s gland

They secrete an alkaline fluid in bicarbonate Combined secretion of Brunner’s gland and intestinal gland is called succus

entericus.o Muscularis externa:

Muscular coat made up of inner circular and outer longitudinal muscle fibers in it

In between the two muscle layers few parasympathetic ganglionic cells of mysenteric plexus can be seen

o Serosa: It is the outermost layer made up of connective tissue cells, adipocytes and few

blood vessels Also blended by visceral peritoneum

JEJUNUM

A. Identification points:i. Tall villi lined by simple columnar epithelium with goblet cells

ii. Absence of Brunner’s glands in submucosaiii. Absence of Peyer’s patches

- Histological features of jejunum are almost same as that of duodenal except in few places where jejunum shows:

o Mucosal villi are rich in mucous secreting goblet cells and along with submucosa they form prominent circular folds called plicae circularis or valves of kerkring

o Small region of mucosa cpntaining loose areolar CT and absence of Brunner’s gland

ILEUM

A. Identification points:i. Small villi rich in goblet cells

ii. Crypts of Lieburkuhn presentiii. Mucosa containing Peyer’s patches

- Mucosao Lined by simple columnar epithelium and villi are very smallo Lamina propria contains few intestinal glands and numerous aggregations of lympatic

modules called peyer’s patches. o Muscularis mucosa is disrupted

- Submucosa o Very thin and smallo Made up of loose areolar CT o Also contains some lymphoid tissue which is migrated from lamina propria

- Muscularis externao Made up of thin layer of inner circular and outer longitudinal muscle fibers

- Serosao Contains connective tissue cells with a layer of visceral peritoneum

IV. LARGE INTESTINEa. Identification points:

i. Mucosa with folds but no villiii. Numerous goblet cells

iii. Prominent taenia coli- Mucosa:

o Lined by simple columnar epithelium rich in in mucous secreting goblet cellso Does not contain villi

o Below mucosa, there are numerous intestinal glands (crypts of lieberkuhn) distributed within the lamina propria, however, it does not contain paneth cells

- Submucosa o Made up of loose areolar CT and fibers with numerous adipocytes in it

- Muscularis externao Consists of inner circular and outer longitudinal muscle layero In certain regions of colon, longitudinal muscle layers becomes very thick resulting in

formation of three bands like structures called taenia coli- Serosa

o It is said to be incomplete in posterior aspect of ascending and descending colono Appendices epiploicae—many peritoneal pockets filled with adipose tissue