In this article we will discuss about the histological peculiarities of various organs. The various organs whose histological peculiarities has been discussed are: 1. Tongue 2. Spleen 3. Pancreas 4. Lung 5. Kidney 6. Adrenal 7. Testis 8. Ovary 9. Artery 10. Vein 11. Thyroid 12. Liver 13. Gastro-Intestinal Tract 14. Stomach 15. Small Intestine.
1. Tongue:
Epithelium is of stratified squamous type and often forms lingual papillae.
Papillae are of three types:
(a) Filiform—most numerous, without taste bud,
(b) Fungiform—resembles a mushroom, with taste bud,
(c) Circumvallete—largest papilla, lodged within a trench.
Loose areolar connective tissue forms the core of the papillae. Taste buds contain two types of cells gustatory and sub-tentacular. Bundles of stratified muscle fibres interlace in all directions.
2. Spleen:
Spleen is a lymphatic organ. It plays many functions:
(i) Destroys old RBC,
(ii) Stores excess of RBC,
(iii) Manufactures RBC,
(iv) Produces antibodies and
(v) Destroys harmful microbes.
A sectional view shows the presence of a connective tissue capsule which extends inside the gland as the trabeculae. It is divided into cortex and medulla. The medulla contains white pulp (rounded aggregations of splenic corpuscles) and red pulp (rest of the portion with red texture due to the presence of RBC).
3. Pancreas:
Mixed type of gland consisting of two altogether different glandular entities:
(a) Multiple alveolar type (Exocrine):
Each alveolus is walled by truncated pyramidal cells. Nuclei in these cells are situated towards the basal membrane. The alveolar cells have zymogen granules and deeply stained basophilic bodies towards the basement membrane. Presence of some flattened cells in the lumen of alveolus (centro-acinar cells) is the characteristic of pancreatic alveolus.
(b) Islets of Langerhans (Endocrine):
Islets include three types of cells, α (alpha), β (beta) and ϒ (gamma) cells.
The gamma cells are regarded to be the precursors of α (alpha) and β (beta) cells. In human islets a fourth type of cell—the δ (delta) cells have been described. The β cells produce the hormone—Insulin. This fact is experimentally attested by causing Alloxan diabetes (treatment of Alloxan, a derivative of Urea, which causes degeneration of β cells).
The islets are special areas in between the pancreatic acini and consist of irregular clumps of cells, capillaries and reticular tissues. Islets are not encapsulated but remain separated by reticular fibres.
4. Lung:
It is formed by round, transparent and irregular air cells or alveoli. Each alveolus is lined by large irregular and flattened squamous epithelial cells. Desquamated dust cells may be seen. Bronchioles and lymphoid tissue may be present.
5. Kidney:
It is formed of an outer cortex and an inner medulla. Cortex is externally covered by a fibro-muscular capsule.
(a) Cortex:
It shows (i) Tuft of blood vessel forming thousands of Renal corpuscles or Malpighian corpuscles encircling network of blood vessels—the glomerulus and (ii) excretory canals.
(b) Medulla:
The medulla includes uriniferous tubules and connective tissues. The structure of epithelial cells varies in the different length of the tubules flattened (at the capsule), granulated cubical (convoluted tubule) and non-granulated cubical (collecting tubule).
6. Adrenal:
Adrenal or suprarenal is an endocrine gland situated above each kidney. A sectional view reveals the presence of an outer cortex and an inner medulla. The cortex is encapasulated by a connective tissue capsule.
The cortex is subdivided into three zones, viz.:
(i) Zona glomerulosa,
(ii) Zona fasciculata and
(iii) Zona reticularis.
The cortex produces a hormone called the cortin. The medulla is composed of chromaffin cells which secrete a hormone named the adrenalin.
7. Testis:
Tunica albugenia and vasculosa form the outer capsule of the testis. Germinal epithelium surrounds the seminiferous tubules.
Each tubule contains a basement membrane on which are arranged from periphery to the lumen:
(1) Spermatogonia— cubical cells with oval and compact nuclei,
(2) Spermatocytes—large cells with dividing nucleus,
(3) Spermatids—small, simple cells with round nuclei, and
(4) Spermatozoa— elongated cells in the lumen.
Associated with the germinal epithelium are the Sertoli cells which are tall and irregular in outline. These cells extend from the basement membrane to the lumen. Interstitial or Leydig cells occur in the angular spaces between seminiferous tubules.
8. Ovary:
The outer germinal epithelium is columnar or cuboidal. Inner stroma or fibrous layer is divisible into a peripheral part and central part. Immature Graafian follicles are present in the peripheral stroma as large spherical cells with prominent round nuclei. Mature follicles remain embedded within the central part of the stroma. The corpus luteum as a mass of yellowish stained tissue and vascular fibrous tissue is present.
The structure of a mature Graafian follicle:
(a) Theca folliculi,
(b) Membrana propria,
(c) Membrana granulosa,
(d) Discus proligerous,
(e) Zona pellucida,
(f) Vitelline membrane containing vitellus and
(g) Germinal vesicle containing the germinal spot.
9. Artery:
Presence of three distinct coats. The outer coat (tunica adventitia) is composed of connective tissue having numerous elastic fibres. The middle coat (tunica media) is thick and made up of network of elastic fibres and circularly arranged plain muscle fibres. The innermost coat (tunica intima) contains highly refractile and elastic membrane of Henle, sub-endothelial layer and endothelial layer.
10. Vein:
The three coats are not so marked as in artery. The outer coat is well developed. Instead of muscular and elastic tissues, there are white fibrous tissues in the middle coat. In the inner coat the endothelial cells are elongated, and refractile membrane of Henle is usually absent.
11. Thyroid:
From the outer fibrous tissue layer arise the trabeculae. There is presence of numerous alveoli which are either spherical or oval. Each alveolus has a lining of cubical epithelium and contains colloidal material in its lumen. Some alveoli may be empty (Fig. 6.3C).
12. Liver:
Units of liver are liver lobules. Each lobule is surrounded by connective tissue materials containing collagen. A particular lobule is composed of polyhedral cells (often binucleate) radiating from a central canal—the hepatic vein.
Associated with one liver lobule, a portal canal occurs which is composed of a trinity of:
(a) Portal artery,
(b) Portal vein, and
(c) Bile duct.
All these are surrounded by connective tissue materials forming Glisson’s capsule.
13. Gastro-Intestinal Tract:
The wall is composed of four principal layers:
(i) Tunica adventitia or serosa,
(ii) Tunica muscularis,
(iii) Tunica submucosae and
(iv) Tunica mucosae.
The serosa is made up of loose connective tissue. The tunica muscularis has smooth muscles oriented in circular and longitudinal fashion. The tunica sub-mucosae consists of collagenous fibres with blood vessels and plexuses (Meissner’s plexus) of autonomic nerve fibres. The tunica mucosae has three components surface epithelium, lamina proparia and muscularis mucosae.
14. Stomach:
In addition to these typical features, the mucous membrane is relatively thick with numerous small simple tubular cardiac glands (Fig. 6.3A). There is no gland in sub-mucosa except the pyloric part. The muscularis layer has, in addition to circular and longitudinal muscles, obliquely disposed muscles. The mucous membrane comprises of surface epithelium, parietal or oxyntic cells, mucous neck cells and zymogenic cells.
15. Small Intestine:
Like all other portions of the gastro-intestinal tract, the small intestine also contains usual four coats from outside inward, namely, serous coat, muscle layer, sub-mucosa and mucosa. The peculiarities are chiefly confined to the mucous or the innermost layer.
They are the mucous membrane, lined by columnar epithelium, is thrown into finger-like projections called villi- Lymphoid nodules, called ‘Peyer’s patches’ occur in the mucous membrane; presence of few goblet cells in the epithelium of the villi.
The peculiarities of the different regions of the gastro-intestinal tract is given in Table 6.1:
Table 6.1: Comparative histological picture of the different regions of the alimentary canal:
