Digestive System | Histology | Anjani Mishra

Histology (Microscopic Anatomy)
Systematic Histology

Digestive system 

             Written By Anjani Mishra

General plan of the alimentary tract:

The alimentary tract in general is consisting of following four layers:

(1) Mucosa- It is sub-divided into three layers;
  (a) Lamina epithelia- Consisting of epithelium (either simple columnar epithelium or stratified         squamous epithelium
  (b) Lamina propria- Consisting of loose connective tissue.
  (c) Lamina muscularis mucosa- Consisting of 1-3 layers of smooth muscles.

(2) Submucosa- Consisting of loose connective tissue, blood vessels, nerve plexuses, and sometime serous and mucous gland as well as lymphatic nodule.

(3) Tunica muscularis/ Muscularis externa- is consisting of varying layers of inner circular and outer longitudinal layer of striated muscle (as in esophagus) or smooth muscles (from stomach to large intestine).

(4) Adventitia/Serosa- The outer layer of a part of an alimentary tract when covered only by connective tissue is called adventitia, as in pharynx and esophagus, in contrast, when a part of an alimentary tract is covered by both connective tissue and mesothelial layer of the visceral peritoneum is called serosa (from stomach to large intestines). 

Esophagus

The wall of esophagus consists of four classical layers;

1. Mucosa: It is the innermost layer of esophagus. It consist of three layers, namely;

a) Lamina epithelia- It is lined with stratified squamous epithelial cell (non-keratinized) in dog, cat and man, but keratinized in herbivores.

b) Lamina propria- It is composed of fine collagen with some elastic fiber.

c) Lamina muscularis mucosa- It is composed of longitudinally arranged smooth muscle fiber. These fibres are indistinct or absent in the initial portion, but present in the caudal half.

Ruminant, horse and cat- Isolated bundles in cranial half and more/less continuous sheets in caudal half.

Dog and cat- Lacking entirely in cranial half and muscle bundles appear in caudal half, but form a continuous layer in the vicinity of the stomach. 

Fig; Esophagus, l.s., pig

 1. Adipose tissue
 2. Adventitia
 4. Lamina propria

 5. Mucous gland
 6. Muscularis externa, inner circular
 8. Muscularis externa, outer oblique
 9. Musularis mucosae
13. Stratified squamous epithelium, keratinized
14. Sub-mucosa

2. Sub-mucosa: It is composed of loose C.T. with large blood vessels and nerves. It contains numerous mucous glands, specially at the cranial part in most of the animal except dog. The loose structure of the sub-mucosa permits the formation of mucosal folds, which in turn allow for great distension of the esophagus.

About mucous gland:

Ruminant, horse and cat- Pharyngoesophageal junction only.

Pig- Fades out about the middle of the esophagus. Contains lymph nodules.

Dog- Found through out the length of esophagus.

3. Muscularis externa: It is composed of striated muscle with some smooth muscle fibers at first and crosses each other obliquely, then take a spiral course. Gradually an inner circular and an outer  longitudinal layers becomes distinct.

In all animal inner circular muscle becomes thicker towards the stomach. In horse outer longitudinal layer becomes thicker caudally.

4. Serosa/fibrosa: It is the outer most layer of esophagus, composed of C.T. rich in elastic fiber, blood vessels and nerves in the cervical region. In thoracic and abdominal part, the tunica muscularis/muscularis externa is invested by serous coat.

Rumen

The wall of rumen consists of four classical layers;

1. Mucosa:

i) L. epithelia­- It is lined with stratified squamous epithelium (kernatinized) with many layers of cells and have some similarities with that of skin.

•    Stratum corneum- It is made up of squamous cells of variable thickness (1 to 15 cell thick)
•    Stratum granulosum- It is composed of thick flattened cells (1 to 3 cell thick)
•    Stratum spinosum- It is made up of slightly thick, large polyhedral cells (1 to 10 cell thick)
•    Stratum basale- It consists of columnar cells placed on a basement membrane.

ii) L. propria- It is composed of a compact network of collagen and elastic fibers

iii) L. m. mucosa- Absent

2. Sub-mucosa:

It is usually composed of loose C.T. with large blood vessels and nerves and blends into the lamina propria without any sharp line of demarcation. It condenses somewhat near the muscular tunic.

3. Muscularis externa:

It is composed of inner circular and outer longitudinal smooth muscle fibre.

4. Serosa/Fibrosa:

It bridges the ruminal groove and is covered by mesothelium. Below the serosa, sub-serosa is present which is specially thick and contains collagen and elastic fibers, large blood vessels, nerves and some amount of fat.

 Fig: Rumen , sheep

3. Connective tissue band

4. Lamina propria

8. Startified squamous epithelium, keratinized

9. Sub-mucosa

 Fig: Reticulum, sheep

1. Conical papilla

4. Muscularis externa

5. Muscularis mucosae

6. Startified squamous epithelium, keratinized

7. Sub-mucosa

 Fig: Omasum, sheep

3. Lamina propria

4. Muscularis externa 

5. Muscularis mucosae

6. Startified squamous epithelium, keratinized

7. Sub-mucosa

Comparison:

Characters	Rumen	Reticulum	Omasum	Abomasum
Surface epithelium	Str.sq.epi. (keratinized)	Same	Same	Sim.col.epi. (non-ciliated)
Mucosal fold	Tongue shape and small	Leaf shape and bigger	Blunt shape and biggest	Irregularly blunt shape
Lamina muscularis mucosa	Absent	May be present or absent	Present & enter into the L. propria	Present & extend into the vicinity of surface epithelium
Lamina propria (bands of smooth muscle fiber)	Absent	Present	Absent	Absent, but reticular and elastic fibers are present
Chief cell & parietal cell	Absent	Absent	Absent	Present
Adipose cells & lymphatic nodule in sub-mucosa	Absent	Absent	Absent	Present

Abomasum (true stomach)

The wall of stomach composed of all the usual layer of alimentary tract. Such as;

1. Mucosa:

i) L. epithelia- It is lined with single layer of simple columnar epithelial cells (non-ciliated)

ii) L. propria- The lamina propria (glandularis) contains gastric glands, supported by a delicate framework of reticular C.T. predominantly with some elastic fiber. In addition, there are scattered lymphatic nodules.

Gastric gland-They are of 3 types, namely;

a) Cardiac gland:

These glands are found in the cardiac region of the stomach near its junction with the esophagus. These are branched tubulo-alveolar glands which produce mucuos.

b) Fundic gland:

These are the main gastric gland and are found in the fundus or body of the stomach. They are less branched tubular glands.

The secretory epithelium consists of 3 types of cells.

Mucous neck cells- Large columnar cells with oval shape nucleus. Secretes mucous that protects the surface epithelium from the action of acid and enzyme.

Chief cells- Cuboidal to columnar with round nucleus contain zymogen granules. Secretes proteolytic enzyme, pepsinogen which form pepsin on hydrolysis.

Parietal cells- Outer portion is rounded & inner portion is pyramidal with round nucleus. Longer than chief cell. Secretes hydrochloric acid (Hcl), activates pepsin and renin, aids directly in the digestion of protein, acts as an antiseptic and bactericide, & also hydrolyse sucrose.

 

 Fig: Fundic gland region, abomasum, cow 

 1. Chief cell 

 3. Globular leukocyte

 4. Lamina propria

10.  Parietel cell

 c) Pyloric gland:

These glands are found in the pyloric region of the stomach. These are simple coiled tubular glands. Some cells of these glands secrete gastrin (called G cells) which stimulates the secretion of Hcl in fundic glands.

iii) Lamina muscularis mucosa:

it consists of thin layer of smooth muscle fibre which extend into the propria to the vicinity of the surface epithelium.

2. Sub-mucosa: It is composed of loose C.T. and many elastic fiber nets. It often contains numerous adipose cells, as well as larger vessels and nerves. Besides this, lymphatic nodules are also found rarely, which is surrounded by a C.T. capsule.

3. Muscularis externa: It is composed of an inner circular and outer longitudinal smooth muscle fibre.

4. Serosa: Consists of a layer of loosely arranged C.T. which is covered by mesothelium

 Fig: Abomasum, pyloric gland region, goat

4. Gastic furrow

5. Gastric pit

8. Muscularis muscosae

9. Pyloric gland

12.Sub-mucoa 

Histology of the small intestine

The small intestine is composed of duodenum, jejunum and ileum. The intestinal tract at the same time act as an excretory and absorptive organ, where the important process of enzymatic digestion occurs. Intestinal digestion, or reduction of food to an absorbable form, begins when the content from the stomach are acted on by pancreatic juice, bile and intestinal secretions, and it continues through out the length of the small intestine.

The digestive and absorptive functions of the small intestine are facilitated by several structures, eg; Villi and microvilli. The intestinal movement also helps in digestion by mixing and propelling the ingesta.

The wall of small intestine consists of four layers, namely;

1. Mucosa (Tunica mucosa)

2. Sub-mucosa (Tunica sub-mucosa)

3. Muscularis externa (Tunica muscularis)

4. Serosa/fibrosa (Tunica externa)

1. Mucosa:- It consists of;

i) L. epithelia- It is lined by Sim. col. epi. cell with numerous goblet cells. The number of goblet cells decreases at the tip of the villi, and the density of the goblet cells increases gradually in the jejunum and ileum.

ii) L. propria- It consists of loose C.T. with reticular & elastic fiber, blood & lymph vessels, gland & lymph nodules, leukocytes and smooth muscle bundles. Isolated and diffused lymphocytes are present in duodenum and jejunum.

iii) L. m. mucosa- It consists of bundles of smooth muscle fibres arranged in two sheets perpendicular to one another as inner circular and outer longitudinal. This layer is thin and incomplete in all animals except dog.

2. Sub-mucosa:-

• It consists of loose C.T. and elastic fiber nets. It contains fat cells, lymph nodules, nerves and vessels.
• This layer contains Brunner's Glands (Duodenal gland) in duodenum only which opens into the base of the intestinal mucosal glands (Glands of Lieberkuhn). 
• Isolatory lymphatic nodules are present in duodenum & jejunum, where as aggregated lymphatic nodules (Peyer's patches) are present in ileum.

                   It is serous in - horse and pig

                   Mucous in - ruminant and dog

                   Sero-mucous in - cat

3. Muscularis externa:

It consists of an inner circular and an outer thinner longitudinal layer of smooth muscle fiber.

4. Serosa:-

It consists of a thin layer of C.T. composed of collagen and elastic fibres covered by mesothelium.

GLANDS OF SMALL INTESTINE

1) Intestinal gland (Glands of Lieberkuhn)

·         Simple tubular glands

·         Location - found in L. propria from pylorus to anus

·         Function - furnish mucuos and several enzymes acts on CHO, fat & peptides

·                                       Also produce enterokinase that activates trypsinogen of pancreatic juice.

                                 

2) Duodenal gland (Brunner's gland/sub-mucosal gland)

Branch tubulo-alveolar gland

Location - Their lobules are usually located in the sub-mucosa, but many project into the L. propria    or even lie entirely within it.

Function - Mucous Secretion as well as serous that contains amylase and interokinase

                                                                       

CELLS OF SMALL INTESTINE

1) Simple columnar cells- Actively participate in the absorptive process.

2) Goblet cells-They usually occur singly between the columnar cells. Their nucleus is located          basally. Goblet cells are more plentiful in the large intestine.

3) Paneth cells- 

•  They are pyramidal shape with spherical acidophilic granules.
•  Absent in carnivores and swine
•  Found in small intestine and ceacum
•  They produce peptidase and lysozyme

Fig; Duodenum, Pyloric gland region, Dog

  3. Central lacteal

  6. Goblet cell

  7. Intestinal absorptive cells

12. Striated border

14. Surface mucous cell

 

Fig: Jejunum, dog

 4. Crypt of  Lieberkuhn

11. Muscularis externa, inner circular

15. Sub-mucosa

16. Villus 

Fig: Ileum, x.s., cat

 3. Goblet cell

 8. Muscularis externa, inner circular

 9. Muscularis externa, outer longitudinal

11. Serosa

12. Sub-mucosa

13. Villus

Comparison

Small intestine

S.N.	Character	Duodenum	Jejunum	Ileum
1	Mucosal fold	Finger Shape	Leaf like	Comb Shape
2	Goblet cell	Few	Numerous	Numerous
3	Brunner's gland	Present	Absent	Absent
4	Peyer's patches	Absent (Scattered lymphatic nodule)	Absent (Scattered lymphatic nodule)	Present
5	Tunica muscularis	Thick	Thickest	Thin

 

 Fig: Cecum, dog

 1. Crypt of  Lieberkuhn

 3. Goblet cell

 8. Muscularis externa, inner circular

10. Muscularis mucosae

12. Sub-mucosa

 

Fig:Colon, x.s., horse

 7. Mucosa 

 9. Muscularis externa, inner circular 

11. Serosa

14. Sub-mucosa

15. Taenia coli

 

Fig: Rectum, x.s., cat

 6. Crypt of lieberkuhn

10. Goblet cell

15. Muscularis mucosae

18. Sub-mucosa

Large intestine

S.N.	Character	Ceacum	Colon	Rectum
1	Villi	Absent	Absent	Absent
2	Goblet cell	More than ileum	More than ceocum	More than colon
3	Lymphatic nodule	Present	More than ceacum	Absent
4	Mucous membrane	Thin	Thickest	Thickest
5	Tunica muscularis	Thin	Thickest	Thickest

 

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