DIGESTIVE SYSTEM part 2

Joanna Sekita-Krzak MD PhDDepartment of Histology and Embryology

PARIETAL (oxyntic) CELLS

FUNCTION

The main functions of oxyntic cells

include:

the secretion of hydrochloric acid

(hydrogen ions, protons) and

the secretion of intrinsic factor.

PARIETAL (oxyntic) CELLS

FUNCTION

Secretion of intrinsic factor

Intrinsic factor plays very important function. It is

necessary for the normal absorption of vitamin B12

in digestive system.

Vitamin B12 is necessary for the normal maturation

of red blood cells.

The lack of intrinsic factor (for example in atrophic

gastritis that destroys the stomach mucosa) leads

to pernicious anemia that is the result of vitamin

B12 deficiency.

GASTROENDOCRINE (APUD, DNES)

CELLSGastroendocrine cells are also reffered to as APUD cells (amine precursor uptake and decarboxylation), or

DNES cells (they are members of the diffuse neuroendocrine system cells).

They are scattered among chief cells.

They possess cytoplasmic granules that contain hormones .

Hormones are secreted into the blood and regulate the activity of the gastrointestinal tract.

So, these cells have an endocrine function in the digestive system.

Endocrine cells are found not only in the stomach but also in the mucosa of duodenum, small and large intestine.

Lack of intrinsic factor

Normal peripheral

blood smear pernicious anemia

vitamin B12 deficiency

Peripheral blood smear in

iron deficiency anemia

showing microcytic,

hypochromic red blood

cells

GASTROENDOCRINE (APUD, DNES)

CELLSGastroendocrine cells are also reffered to as APUD cells (amine precursor uptake and decarboxylation), or

DNES cells (they are members of the diffuse neuroendocrine system cells).

They are scattered among chief cells.

They possess cytoplasmic granules that contain hormones .

Hormones are secreted into the blood and regulate the activity of the gastrointestinal tract.

So, these cells have an endocrine function in the digestive system.

Endocrine cells are found not only in the stomach but also in the mucosa of duodenum, small and large intestine.

GASTROENDOCRINE (APUD) CELLS

Gastroendocrine cells are divided into several types depending on the kind of hormone they secrete.

For example

⚫ A-cells located in the stomach produce glucagon,

⚫ G-cells located in the pylorus and duodenum secrete gastrin that strongly stimulates the production of hydrochloric acid.

⚫ D-cells located in the pylorus and duodenum secrete somatostatin. This hormone inhibits other endocrine cells of the digestive tract and is used in clinic for the inhibition of secretion in digestive system for example in patients with gastrointestinal haemorrhage.

PYLORUS

The mucous membrane of the

pylorus has very deep gastric

pits.

The lamina propria comprises pyloric glands which open into the bottom of gastic pits.

They are simple or branched tubular glands They secretemucus but

some oxyntic and gastroendocrine cells are found.

PYLORUS

PYLORUS - G cellsGastroendocrine cells located in thepylorus (G-cells) secrete gastrin- thehormone that

⚫ stimulates the parietal cells to producehydrochloric acid,

⚫ to a smaller degree, it also stimulates thechief cells to produce pepsinogen.

G-cells are intercalated among the mucouscells of pyloric glands.

Gastrin secretion is stimulated by:

⚫ vagus nerve (under the influence of sight, smell, taste or tought of food),

⚫ by the presence of protein foods in thestomach and in duodenum.

Gastrin is released from G-cells into theblood and with the blood is carried to thegastric glands where it stimulates theparietal and chief cells.

STOMACHMUCOUS MEMBRANE

Under the lamina

propria of the mucous

membrane there is

muscularis mucosae

that separates the

mucosa from the

underlying submucosa.

STOMACHSUBMUCOUS MEMBRANE

The submucosa is

composed of dense

connective tissue that

is infiltrated by

lymphocytes,

macrophages and

mast cells.

It contains nerves,

blood and lympatic

vessels.

STOMACHMUSCULARIS EXTERNA

The muscularis externa

is composed of smooth

muscle cells arranged in

three layers:

⚫ the external layer is

longitudinal,

⚫ the middle layer is

circular and

⚫ the internal layer is

oblique.

At the pylorus the middle

layer is thick and forms

the pyloric sphincter.

STOMACHSEROSA

The muscularis externa is

covered by serosa that

consists of a thin layer of

connective tissue covered

by mesothelium (simple

squamous epithelium) and

constitutes the most

external layer of the

stomach wall.

SMALL INTESTINE

The small intestine is the site of

terminal food digestion,

nutrient absorption and

endocrine secretion.

The small intestine consists of

three parts:

⚫ the duodenum about 25 cm long

⚫ the jejunum about 4 m long

⚫ the ileum about 2 m long.

SMALL INTESTINESTRUCTURE

The wall of the small intestine consists of the usual four layers:

the mucous membrane,

the submucous membrane,

the muscularis externa and

the serosa.

Small intestine

SMALL INTESTINEMUCOUS MEMBRANE

The mucous membrane is thrown into the numerous circular, permanent folds called the plicae circulares.

These permanent folds extend one-half or two-thirds of the way around the lumen. They effectively increase the surface area.

INTESTINAL VILLI

The free surface of the mucous membrane has small, fingerlike projections called intestinal villi.

Each villus measures about 0,5 to 1,5 mm long.

In the duodenum the villi are long and broad, they become shorter and thinner in the distal part of the small intestine.

INTESTINA VILLI

The villi are covered by simple columnar epithelium.Each villus has a core of connective tissue containing an artery, a capillary network, a vein and lymphatic vessel.

Narrow strands of smooth muscle derived from the muscularis mucosae extend into each villus and by their contraction aid the circulation of lymph through lymphatic vessel.

INTESTINAL VILLI

epithelium

The epithelium covering

the villi is a simple

columnar epithelium and it

contains three types of

cells:

absorptive cells

mucus-secreting goblet

cells and

enteroendocrine cells

(DNES cells).

ABSORPTIVE CELLSAre tall columnar cells.

They have oval nuclei located in

the basal half of the cell.

At the apex of each cell is a layer

called the brush border. This

border is a layer of densely

packed microvilli.

Each microvillus is a cylindrical

protrusion of the apical

cytoplasm covered with cell

membrane and a core of actin

filaments.

Microvilli greatly increase the

surface area of contact between

the intestinal mucosa and food

(absorptive surface) about 20

folds.

ABSORPTIVE CELLS

The outer surface of microvilli is

bounded with several enzymes which

are very important in the terminal

digestion of food. These are:

⚫ Disaccharidases: sucrase, maltase and

lactase – enzymes which hydrolyze

disaccharides into monosaccharides;

⚫ Lipase – an enzyme that hydrolyzes fats

into glycerol and fatty acids and

⚫ Dipeptidases which hydrolyze dipeptides

into amino acids.

Deficiences of these enzymes lead to

digestive disturbances

ABSORPTIVE CELLS

functionThe most important function of the columnar absorptive cells is the absorption of nutrient molecules.

Amino acids and monosaccharides cross the apical plasma membrane by facilitated diffusion, whereas the products of lipid hydrolisis (fatty acids and monoglycerides) cross passively and they are collected in the SER where they are resynthesized into triglycerides and then assembled into chylomicrons

GOBLET CELLS

The goblet cells are scattered in the epithelium between the absorptive cells.

They sectrete mucus.

The main function of this mucus is to protect and lubricate the lining of the intestine.

ENTEROENDOCRINE CELLS

The enteroendocrine cells of small intestine belong

to DNES cells (APUD cells).

The enteroendocrine cells of small intestine secrete

hormones such as:

⚫ GIP

⚫ serotonin,

⚫ secretin,

⚫ cholecystokinin,

⚫ motylin and

⚫ urogastrone.

These hormones are secreted into the bloodstream through

the basal surface of enteroendocrine cells.

These cells are small and are scattered between the bases

of columnar absorptive cells.

GIP Glucose-dependent insulinotropic peptide

Gastric inhibitory peptide

K-cells located in the small intestine

secrete GIP glucose-dependent

insulinotropic peptide that stimulates

insulin secretion.

ENTEROENDOCRINE CELLS

EC cells secrete Serotonin increases

gut motility.

S cells secrete Secretin stimulates

bicarbonate secretion by pancreas and

biliary tract.

I cells secrete Cholecystokinin

stimulates release of pancreatic

enzymes and contraction of gall

bladder (with release of bile)

Mo cells secrete Motylin increases gut

motility.

Urogastrone produced by Brunner

glands of the duodenum inhibits

gastric HCl secretion and enhances

epithelial cell division.

INTESTINAL GLANDSThe lamina propria of the mucous membrane consists of loose connective tissue and contains intestinal glands which are known as the crypts of Lieberkühn.

These glands are tubular in shape and extend through the entire thickness of mucous membrane almost to the muscularis mucosae.

Intestinal glands open between the bases of the villi.

Intestinal glands are lined with the simple columnar epithelium.

INTESTINAL GLANDS

The epithelium covering the

upper half of the intestinal

glands is identical to that

covering the villi.

In the lower half of the glands

two new types of cells are

found:

⚫ Paneth’s cells and

⚫ stem cells.

STEM CELLS

Stem cells (regenerative cells)

are continuously undergoing

mitosis to replace the cells

that are exfoliated from the

tips of the villi.

They can differentiate into

absorptive cells, Goblet cells

and enteroendocrine cells.

They are located in the basal

half of the crypts of

Lieberkühn.

PANETH’S CELLS

Paneth’s cells are found in

small groups at the bottom

of the intestinal glands.

They stain deep pink in

H+E stained slides.

They contain lysozyme –

an enzyme that digests the

bacterial wall and playes

an important role in

controlling the intestinal

flora.

Peyer’s patches and M-cells

The lamina propria of small intestine has numerous lymphatic nodules which form Peyer’s patches.

The epithelium covering Peyer’s patches has special type of cells which are called M-cells (Microfold cells).

Microphold cells can endocytose antigens and transport them to the underlying lymphoid cells (macrophages and lymphocytes) in underlying nodules. So, they help to initiate immune response.

Peyer’s patches and M-cells

The lamina propria of small intestine has numerous lymphatic nodules which form Peyer’s patches.

The epithelium covering Peyer’s patches has special type of cells which are called M-cells (Microfold cells).

Microphold cells can endocytose antigens and transport them to the underlying lymphoid cells (macrophages and lymphocytes) in underlying nodules. So, they help to initiate immune response.

SMALL INTESTINE MUSCULARIS MUCOSAE

The muscularis

mucosae consists of

a thin layer of

smooth muscle cells

and is the deepest

layer of mucous

membrane

separating it from the

underlying

submucosa.

SMALL INTESTINE SUBMUCOUS MEMBRANE

The submucosa consists

of dense connective

tissue and contains

Meissner’s plexus.

DUODENUMSUBMUCOUS MEMBRANE

In the duodenum submucosa

contains compound tubular

glands called Brunner glands (or

doudenal glands).

⚫ These glands are mucous glands.

⚫ They empty into the intestinal

glands.

⚫ They secrete mucus that is

alkaline (pH 8-9) and

urogastrone.

⚫ The former helps to increase the

pH of the acidic chyme comming

in from the stomach.

⚫ This increase of pH also helps

pancreatic enzymes in becoming

active to allow further digestion.

⚫ These glands help in protecting

the epithelium by neutralizing the

acidic chyme.

SMALL INTESTINE MUSCULARIS EXTERNA

The muscularis externa of the small intestine consists of an inner circular layer and outer longitudinal layer of smooth muscle.

Between the muscle layers the Auerbach’s plexus of autonomic nerves is located.

SMALL INTESTINE SEROSA

The serosa is the

outermost coat of the

small intestine and

consists of a thin

layer of loose

connective tissue

covered by a simple

squamous

epithelium.

LARGE INTESTINE

The large intestine is

divided into the:

⚫ cecum,

⚫ appendix,

⚫ colon

⚫ ascending,

⚫ transverse and

⚫ descending,

⚫ rectum and

⚫ anal canal.

LARGE INTESTINE

STRUCTUREThe structure of the large intestine shows the usual four layers that are characteristic of the alimentary canal:

the mucous membrane,

the submucous membrane,

the muscularis externa and

the serosa.

LARGE INTESTINE

STRUCTUREThe structure of the large intestine differs from

that of the small intestine in many respects:

There are no permanent folds of mucous membrane (no

plicae circulares)

There are no villi on the surface of the mucous

membrane.

The mucous membrane has long intestinal glands (crypts

of Lieberkühn) that have no Paneth’s cells but more

Goblet cells. Goblet cells constitute 60-70% of all

epithelial cells.

Although numerous solitary lymphatic nodules are

present there are no Peyer’s patches.

The muscularis externa differs from it counterpart in the

small intestine by the arrangement of its outer,

longitudinal layer into three thick bands called teniae

coli.

The serosa is arranged differently in the different parts of

the large intestine. It forms a complete covering of

peritoneum for the cecum.

It covers the anterior surface and sides of the ascending

and descending colon.

LARGE INTESTINEepithelium

The cecum and colon are lined with

simple columnar epithelium with

numerous

⚫ goblet cells,

⚫ surface absorptive cells, and

⚫ occasional endocrine cells

The rectum is lined with simple

columnar epithelium similar to that

of the colon.

The anal canal is lined with

⚫ simple columnar epithelium

proximal to the anal valves,

⚫ stratified nonkeratinized squamous

epithelium distal to the anal valves

and

⚫ stratified squamous keratinized

epithelium at the anus.

LARGE INTESTINE

FUNCTIONSFunctions:⚫ the large intestine functions

primarily in the absorption of electrolytes, fluids and gases.

⚫ It houses bacteria that produce ⚫ vitamin B12 (necessary for

hemopoiesis )and

⚫ vitamin K necessary for coagulation.

⚫ The large intestine produces abundant mucus, which lubricates its lining and facilitates the passage and elimination of feces.

APPENDIXIs a wormlike diverticulum of the cecum.

Contains a large amount of lymphoid

tissue.

The appendix has a usual four-layered

structure of the alimentary canal.

⚫ The mucous membrane is identical to that

of the large intestine, possessing shallow

tubular crypts of Lieberkühn. It does not

form villi.

The lamina propria contains numerous

lymphoid nodules and lymphoid cells. In

many areas lymphoid tissue extends

through the muscularis mucosae into the

submucosa.

⚫ The submucosa of the appendix is

composed of connective tissue containing

lymphoid nodules

⚫ The muscularis externa of the appendix is

composed of an inner circular and an outer

longitudinal layers of smooth muscle.

⚫ The serosa completely surrounds the

appendix.

Fotos and diagrams in presentation from:

Color Textbook of Histology by Leslie P. Gartner, James L. Hiatt, fourth edition 2017; Basic

Histology by L. Carlos Junqueira, 11th edition, 2005; Wheater’s Functional Histology by B. Young,

J.W. Heath 4th ed.