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Histology: The study of minute structural details from the very level of tissue to the various structural differentiations produced by it in an organ or body is the scope area of the histology.

ALIMENTARY CANAL

The alimentary canal or digestive tube of a mammal's body is composed of maximum five basic layers. These are from outside inwards-

1. Serous membrane or serosa

2. Muscular coat or muscular layer

3. Sub-mucosa

4. Muscularis mucosae

5. Mucosa

1. Serous membrane-The visceral peritoneum is the outermost covering of the digestive tuba. It is thin and membranous coelomic epithelium. It is made up of single layer of squamous epithelial cells resting upon a thin basement membrane. It is continuous with the mesentery and the peritoneal lining of the body cavity. It is composed of outer layer of mesothelial cell and inner thin layer of connective tissue.

2. Muscular coat: It is composed of smooth muscle fibres arranged in two layers:

(a) Outer longitudinal muscle layer

(b) Inner circular muscle layer.

Inner muscle layer is thick. Between these two layers, is a thin layer of connective tissue containing autonomic ganglionated plexus of Auerbach. When longitudinal muscle fibres contract, affected part of alimentary canal becomes shorter and when circular muscle fibres contract, affected part of alimentary canal becomes longer. Continuous contraction and relaxation of these fibres cause a rhythmic wave like movement in digestive tube. This is called peristalsis. This pushes food backward within the tube. Certain other type of contraction which do not form peristalsis are responsible for segmentary pendular movement of the wall. They help in churning and mixing of the food with the digestive juices.

3. Sub-mucosa: It is thick and made up of dense and elastic fibrous connective tissue. It may contain large blood vessels, lymphatics and nerves. It may also contain glands. It supports sub-mucous plexus of Meissner which is composed of nerve cells and fibres.

4. Muscularis mucosae: Muscularis interna is also muscular consisting of outer longitudinal and inner circular smooth muscle fibre.

5. Mucosa: It is innermost coat of the tube. It is composed of an outer connective tissue and inner mucous membrane. Connective tissue layer is called lamina propria or corium. Corium contains blood capillaries, lacteal vessels, nerves and often a number of small lymph nodules. Innermost lining of gut is called mucous membrane, which is composed of single layer of columnar epithelial cells resting upon basement membrane. It lies next to lamina propria. Some cells of basement membrane are goblet cells which secret mucous. This mucous keeps membrane lubricated and slippery. Absorptive cells bear microvilli at their free end which increases surface of absorption.

Mucosa is variously folded forming numerous pits and glands of different kinds. Mucosa is mainly concerned with secretion and absorption.

OESOPHAGUS

1. It does not have serosa. Instead it is covered by a layer of dense fibrous connective tissue called adventitia externa. The epithelium is highly stratified (about 25 cell layers).

2. Fibres of circular muscular layer are arranged spirally rather than circular.

3. Mucosa does not have digestive glands.

4. Mucosa is folded and contains stratified squamous epithelium.

5. Muscularis mucosae have longitudinal fibres.

6. Upper half of oesophagus contains striped muscle fibres and lower half contains unstriped muscle fibres.

7. Upper part has a layer of oblique muscle bundles.