Intestine consists to two parts – small and large. The small intestine is divisible into three portions, – duodenum, jejunum and ileum, which gradually pass into one another. Their general structure is similar although each division has certain distinctive features. The intestinal wall consists of a mucosa, submucosa, muscularis and a serosa.
Tunica mucosa
- This may be divided into the following layers:
- Epithelium
- Lamina propria, which has glands and in the small intestines forms villi;
- Muscularis mucosae
- Epithelium
- Surface epithelium is simple columnar with goblet cells distributed among the columnar cells. Columnar cells are narrow and high and reach their greatest height on the villi. Nucleus is oval and is situated in the lower half of the cell. The cytoplasm is finely granular and usually acidophilic. These cells shown striated free border, which appear under light microscope as fine striations in the cytoplasm.
- EM studies revealed that the striated free border is actually made up of fine closely packed cytoplasmic filaments or processes, which are now referred to as microvilli. The microvilli are considered as an adaptation of cell surface for better absorption of digested food material from the intestine.
- Scattered among the columnar cells are mucous secreting goblet cells. They vary in appearance according to the amount of secretion they contain. The cell at the beginning contains a small amount of mucin near its apical border. As secretion increases the mucous gradually displaces the cytoplasm and the nucleus is flattened pushed the nucleus towards the base of the cell.
- The distended cell has the appearance of goblet and the cytoplasm stains faintly basophilic or not at all but can be stained intensely by special mucin stains. The secretion is discharged through the free surface (apocrine) and goblet cells go through cycles of secretory activity.
- Lamina propria
- It has a supporting framework of reticular tissue with elastic fibres and delicate collagenous fibres. There is diffuse infiltration with numerous lymphocytes and in certain places isolated lymph nodules known as solitary glands may be found in the lamina propria. In the small intestine, projections of lamina propria covered by surface epithelium called villi occur throughout. Both in small and large intestines, the lamina propria contains simple tubular glands, called crypts of lieberkuhn or intestinal glands.
- Intestinal Villi
- These structures serve to increase the intestinal surface area available for absorption. They are narrow and elongated projections, measuring on the average of 0.5-1mm in length and 0.2 mm in width. In the center of each villus occur a tubular lymph space known as a lacteal and is lined by endothelium. The reticular spongy stroma of villi contains leucocytes, fat droplets, capillaries, elastic fibres and bundles of smooth muscle fibres. These muscle fibres originate from muscularis mucosae.
- Intestinal glands or Crypts of Lieberkuhn
- These are simple tubular glands found in the lamina propria. They open between the villi and extend to the lamina propria as far as the muscularis mucosae. They are surrounded by reticular fibres and consist of glandular epithelium resting on a thin basement membrane. The epithelium is made-up of columnar cells as the surface epithelium with which it is continuous but the cells of gland are shorter and their striated border becomes less distinct until they finally disappear, in the deeper portions of the gland. Goblet cells are more abundant in the deeper portions than in the surface epithelium.
- In the small intestine the gland fundus contains the specialized granular cells of Paneth. These are serozymogenic having the characteristic striated chromophilic material in the basal region and large refractile acidophilic granules apically. They show activity during digestion. It is believed that these cells may be producing some digestive enzymes but the exact functional significance is not known.
- Enterochromaffin or argentaffin cells also occur among the crypts of the intestinal glands. They occur usually singly and contain specific granules in the basal part of the cell, which are stained by silver and chromium salts. They contain serotonin but their exact functional significance is not known.
- Muscularis mucosae: consist of smooth muscle fibres, which are in two layers perpendicular to one another. They may interweave.
Submucosa
- Consists of loose connective tissue and elastic fibre nets. In contains large blood vessels, lymphatics and nerve plexus in the submucosa.
Tunica muscularis
- This is well developed, made up of plain muscle fibres and consists of thinner outer longitudinal layer and thicker inner circular layer. The two layers are connected by inter-muscular connective tissue.
Species differences
- In the Ox and pig, numerous renal pyramids are present with the formation of minor and major calyces. In the horse, sheep and dog the apices of the pyramids do not appear separate and the inner part of medulla forms the renal crest (results of fusion of papillae in the embryo), which shows numerous openings the area cribrosa.
- The renal crest projects into the renal pelvis the dilated origin ureter. No minor and major calyces are formed. In the cat the cells in the proximal convoluted tubules contain many fat droplets and a similar condition is also terminal portions of the tubule in the dog.
- The Excretory Passages are renal calyces pelvis, ureter, urinary bladder and urethra.
- From the renal papillae urine passes into the renal pelvis of the renal pelvis of the kidney, hence it flows through the ureter into the urinary bladder and leaves the body by way of urethra.
Regional differences in small intestine
- Duodenal Mucosa: The villi are broad spatula shaped and thin. In the epithelium, columnar cells preponderate over goblet cells. Submucosa – Branched tubuloalveolar glands are found in the connective tissue known as Brunner’ glands or Duodenal glands.
- These glands are lined by low columnar cells, which resemble pyloric gland. The nucleus is flattened and is located at the base of the cell and the cytoplasm is faintly basophilic. These glands secrete mucous and the ducts, which are lined by columnar cells, open between the villi or into the crypt of Lieberkuhn.
