DIGESTIVE TRACT

Department of Histology, Cytology and Embryology Tatiana Globa

State University of Medicine and Pharmacy “Nicolae Testemitanu”

BASIC PLAN OF THE DIGESTIVE BASIC PLAN OF THE DIGESTIVE TUBETUBEFour functional layers:

Tunica mucosa: This layer is composed of epithelium, connective tissue and muscle. These tissues can usually be found in distinct layers as follows: lamina epithelialis mucosae: consists only of epithelium lamina propria mucosae: consists of either loose areolar or reticular

connective tissue lamina muscularis mucosae: consists of smooth muscle

Tunica submucosa: consists of loose connective tissue, nerves, blood vessels, and glands in some organs

Tunica muscularis: consists of at least two layers, an inner circular and an outer longitudinal with parasympathetic ganglia located between the layers

Tunica adventitia or tunica serosa: consists of loose connective tissue.

Esophagus: The esophagus connects the

oral cavity with the stomach allowing and aiding in the movement of food particles to the stomach.

It is a muscular tube having the layers described above for the typical tubular organ.

In the esophagus the layers are specialized for the function of further fragmenting food particles.

Layers of the esophagus Tunica mucosa:

epithelium: consists of stratified squamous epithelium that can be highly folded in an empty organ;

lamina propria: consists of loose connective tissue, contains esophageal cardiac glands that are simple branched tubular glands, they produce mucus, mucin, chlorides and some biologically active substances.

lamina muscularis mucosae: consists of longitudinally oriented smooth muscle fibers that form 1 layer (can be 2)

Tunica submucosa: consists of loose connective tissue that is very elastic allowing for expansion when food is present; contains esophageal glands proper; they are compound tubuloalveolar glands, which produce mucous.

Layers of the esophagus Tunica muscularis: consists of smooth and/or skeletal

muscle;

Proximal end – skeletal muscle cellsMiddle region – skeletal plus smooth muscleDistal end – smooth muscle cells

inner circular layerouter longitudinal layer

Tunica adventitia/serosa: consist of typical loose connective tissue that blends into the connective tissue of surrounding tissues. Serosa: only at distal end that enters peritoneal cavity.

Esophageal wallEsophageal wall

STRATIFIED SQUAMOUS EPITHELIUM

LAMINA PROPRIA

LAMINA MUSCULARIS

Esophageal glands proper

(submucosal, tubuloalveolar

glands; acidic mucus

secretion)

Esophageal cardiac glands not shown Present in lower portion; tubular mucosal glandsproduce mucus with neutral pH

Gastro-Esophageal junction

Comparative characteristic of the wall structure of esophagus and stomach

Esophagus Stomach

Mucosa

Epithelium

Stratified squamous nonkeratinized Simple columnar glandular

Lamina propria of mucosa

Contains esophageal cardiac glands that are simple branched tubular glands. They produce mucus, mucin, chlorides and some biologically active substances.

Contains gastric glands that are simple branched tubular. Differ 3 groups of glands: 1. cardiac glands in the cardiac region2. pyloric glands in the pyloric region3. fundic or gastric glands in the fundic region.

Muscularis mucosae

Consists of longitudinally oriented smooth muscle fibers that form 1 layer (can be 2)

Consists of 3 layers of smooth muscle:Inner – circularMiddle – longitudinalOuter – circular

Submucosa

Contains esophageal glands proper. They are compound tubuloalveolar glands, which produce mucous.

Glands are absent.

Muscularis externa

In the upper one-third – is striated muscle.In the middle one-third – is striated and smooth muscle.In the distal third – is smooth as in rest of the digestive tract.It forms 2 layers: inner – circular; outer – longitudinal

Present only smooth muscle that forms 3 layers:Inner – obligue;Middle – circular;Outer – longitudinal.Between the muscle layers is present Auerbach’s plexus

Superficial tunica

In the thoracic cavity is adventitia.After entering the abdominal cavity is serosa.

Serosa is present.

STOMACHSTOMACH

STOMACHSTOMACH Structure - 4 Regions

Cardia Fundus Body Pylorus

Functions: Continue digestion of carbohydrates started in mouth Add acidic fluid Transform food into chyme (mechanical & chemical

breakdown) – Promote initial digestion of proteins (via pepsin) and

triglycerides (via lipase)

STOMACHSTOMACH Cardiac region – surrounds the cardiac

orifice Fundus – dome-shaped region beneath the

diaphragm Body – midportion of the stomach Pyloric region – made up of the antrum and

canal which terminates at the pylorus

The pylorus is continuous with the duodenum through the pyloric sphincter

The inner surface of the stomach is irregular. There are:

• Rugae – are longitudinally oriented folds

• Gastric (mamillated) areas – are bulging irregular areas

• Gastric pits – funnel-shaped depressions. Gastric glands empty into the bottom of the gastric pits

GASTRIC PITSGASTRIC PITS

Layers of the TUNICA MUCOSA

Epithelium: consists of simple columnar epithelium that forms branched, tubular glands; organized into gastric pits that open onto the lumen and gastric glands that empty into the base of the gastric pits

Lamina propria: consists of loose areolar connective tissue that in the glandular stomach is minimal between gastric glands and difficult to see in sections; highly vascular containing many blood and lymphatic capillaries

Lamina muscularis mucosae: consists of several layers of smooth muscle oriented both longitudinally and circularly; usually not very thick

STOMACHSTOMACH Tunica submucosa: typical loose connective tissue

contains submucosal plexuses also known as Meissner's plexuses

Tunica muscularis: 3 layers of smooth muscle Outer longitudinal Middle circular Inner oblique

between the muscle layers is located the myenteric or Auerbach's plexus

Tunica serosa: small amount of loose connective tissue with overlying simple squamous epithelium or mesothelium

Epithelium simple

columnar glandular

of the stomach mucosae

Gastric glands simple tubular

few branched

Glands of the Stomach Fundus and Body Mucous neck cells – found dispersed between the parietal cells; secrets a

mucus that is thinner than that secreted by the surface mucous cells; mucus protects other glandular cells from action of proteases and HCl.

Parietal cells (oxyntic cells) – found throughout the gastric gland; round cells that contain distinct eosinophilic (pink) cytoplasm and round, prominent nucleus; Secrete HCl and intrinsic factor, needed for absorption of vitamin B12 in the ileum.

Chief cells (zymogenic cells) – found mostly near the base of the gastric glands; very basophilic (purple) containing basally positioned nucleus and prominent basophilic apical cytoplasm filled with many ribosomes; secrete pepsinogen, which is activated to pepsin by HCl in the stomach. Pepsin is an enzyme which is able to break down proteins.

Endocrine cells – difficult to distinguish by conventional light microscopy; Several types are present; some secrete gastrin, glucagon and somatostatin, histamine, endorphins, serotonin, cholecystokinin (CCK) among other hormones.

Undifferentiated cells – located primarily in the neck region; difficult to identify in routine H&E sections; undergo mitosis to form more cells then differentiate into the other cell types present in the gland

Chief cells (basophilic)

Chief cell

Pepsinogen on the apical

part of the cell

Parietal cells (red color)

Parietal cell

Intracellular canalicular system

Mitochondria

Endocrine cells

Type:G-cells – gastrin – is the principal agent for stimulating the secretion of HCl and pepsinogen

D-cells produce somatostatin. They inhibit G-cells

EC-cells produce serotonin and substance Р which increase stomach activity.

ECL-cells produce histamine which regulate gastric secretion

Cardiac glandsCardiac glands

Cardiac glands are tubular, occasionally branched glands (similar to the cardiac glands of the esophagus), which contain mainly mucus-producing cells. A few of the secretory cells characteristic for the corpus-fundic glands (chief and parietal cells) may be present.

Pyloric glandsPyloric glands

Pyloric glands are more coiled than corpus-fundic glands, and they may be more branched. The lumen is relatively wide. A few parietal cells may be present but chief cells are usually absent.

Shallow gastricpits, with simple orbranched tubularglands

Shallow gastric pits, with branchedtubular glands

Deep gastric pits,with branchedtubular glands

SMALL INTESTINE Is the longest component of the digestive tract Is divided into three anatomic segments:

duodenum, jejunum, and ileum Functionally, it is the principal site for the

digestion of food & for absorption of the products of digestion

Endocrine Secretion

The bile duct and main pancreatic duct: Join the duodenum at the hepatopancreatic ampulla Are controlled by the sphincter of Oddi

SMALL INTESTINESMALL INTESTINE Specializations of intestinal

surface Plicae circularesPlicae circulares: deep circular folds of the mucosa and submucosa, most abundant in jejunum

VilliVilli – finger-like projections of the mucosa that extend into the intestinal lumen (epithelium plus lamina

propria) Intestinal cryptsIntestinal crypts (glands of Lieberkühn) – are

simple tubular glands between villi MicrovilliMicrovilli – numerous projections of apical plasma

membranes of absorptive mucosal cells and give the apical region of the cell a striated appearance, called striated border (brush border)

Plicae circularis

Crypts

Villi

Villi• Finger-like

projections of mucosa

• Contain:- fenestrated

capillary network- a central, blind-

ending lymphatic capillary (lacteal)

- few smooth muscle cells derived from muscularis mucosae

- myofibroblasts

• Are covered by intestinal epithelium – simple columnar

SMALL INTESTINESMALL INTESTINE Layers of the Small Intestine

Tunica mucosa:

EpitheliumEpithelium - simple columnar Lamina propriaLamina propria - loose connective tissue rich in blood and

lymphatic vessels present in the core of the villi and between crypts Lamina muscularis mucosaeLamina muscularis mucosae - thin layer of smooth muscle located

at the base of the crypts

Tunica submucosa: This layer blends with the lamina propria and is typical. In the duodenum it has coiled branched glands known as Brunner's glands, the ducts of which open into the base of the crypts.

Tunica muscularis: typical consisting of an inner circular layer and an outer longitudinal layer

Tunica serosa: typical

The epithelium of the villusThe epithelium of the villus

EnterocytesEnterocytes (absorptive cells)

Goblet cellsGoblet cells – unicellular mucin-secreting glands, increase in number from the proximal to the distal small intestine

Enteroendocrine cellsEnteroendocrine cells resemble those described in the stomach

Epithelium lining the small intestine – simple columnar

The epithelium of the cryptThe epithelium of the crypt Enterocytes (absorptive cells)Enterocytes (absorptive cells) Goblet cellsGoblet cells

Enteroendocrine cellsEnteroendocrine cells Paneth cellsPaneth cells – are found in the bases of

the glands. They have a basophilic basal cytoplasm & large, intensely acidophilic apical secretory granules. These granules contain: the antibacterial enzyme lysozyme (digests the cell walls of certain groups of bacteria), glycoproteins, an arginine-rich protein & zinc. The antibacterial action & the phagocytosis of certain bacteria & protozoa by Paneth cells suggest that they have a role in regulating the normal bacterial flora of the small intestine.

Undifferentiated cellsUndifferentiated cells

Goblet cell

Gut-Associated Lymphatic Gut-Associated Lymphatic TissueTissue Lymphatic nodules Lymphocytes Macrophages Plasma cells Eosinophils

GALT serves as an immunologic barrier

Regional variations in the Regional variations in the small intestine:small intestine:

DUODENUMDUODENUM

presence of Brunner's glands in the submucosa - compound tubuloalveolar branched glands, mixed glands

presence of chyme in the small intestine induces cells of Brunner's glands to secrete alkaline mucus that neutralizes gastric acid and pepsin and further promotes digestion

no plicae circulares

Regional variations in the Regional variations in the small intestine:small intestine:

JEJUNUMJEJUNUM

no glands in the submucosa longest villi of all three regions no lymphoid nodules

Regional variations in the Regional variations in the small intestine:small intestine:

ILEUMILEUM

permanent aggregated lymphoid nodules in the submucosa

shortest villi highest number of goblet cells

LARGE INTESTINELARGE INTESTINE Regions

Cecum – Appendix Colon

Ascending Transverse Descending

Rectum Anal canal

FunctionsFunctions: Reabsorption of electrolytes & water Formation of waste B vitamins & vitamin K synthesized

LARGE INTESTINELARGE INTESTINEUnlike the small intestine, there are no plicae circulares or villi

in the large intestine so the surface of the tunica mucosa is more uniform and flatter than that of the small intestine.

Tunica mucosaTunica mucosa: epithelium - simple columnar epithelium that forms straight tubular

glands (crypts)

lamina propria- loose connective tissue that contains numerous blood and lymphatic vessels, collagen, lymphocytes and plasma cells

lamina muscualris mucosae- present beneath the base of the crypts and prominent; undergoes rhythmic contractions

Tunica submucosaTunica submucosa: typical, contains Peyer’s patches which are aggregations of solitary follicles or groups of lymph nodules. Each pach contains from 10 to 70 nodules

The epithelium of the cryptThe epithelium of the crypt

Goblet cellsGoblet cells – are more numerous than in the small intestine

Enterocytes Enterocytes (absorptive cells) - few

Enteroendocrine cellsEnteroendocrine cells Undifferentiated cellsUndifferentiated cells

Colon cryptsColon crypts

LARGE INTESTINELARGE INTESTINE Tunica muscularisTunica muscularis: inner circular and outer

longitudinal layers; outer longitudinal layer is organized into three separate bands known as taenia coli; movement of more solid waste to the rectum

Tunica serosaTunica serosa is typical.

Commensal bacteria reside in the large intestine and play a role in the continued digestion of food.

AppendixAppendix

Colorectal ZoneColorectal Zone(simple columnar epithelium, crypts)

Anal TransitionAnal TransitionZoneZone

(stratified columnar/cuboidal

epithelium)

Squamous ZoneSquamous Zone

Pectinate line