Histology deals with the study of minute structural details starting from tissue level to organ level.
Different organs which are studied in histology are :
Alimentary canal
This is also called digestive organ. It starts with the mouth and ends into the anus. It is composed of five basic layers.
1. Seroaus membrane
2. Muscular layer
3. Sub-mucose
4. Muscularis Nucosae
5. Mucosa
1. Serous membrane : It is the outermost covering of the digestive tube. It is thin and continuous.
2. Muscular coat : It is made up of smooth muscle fibers. This coat help in churning and mixing of food with the digestive juices.
3. Sub-mucose : Composed of dense and elastic fibrous connective tissue. It contains large blood vessels, lymphatics and nerves. It also contains glands.
4. Muscularis mucosae : It is composed of outer and inner circular smooth muscle fibre.
5. Mucosa : Mucosa is the innermost coat of the organ. It is mainly concerned with the secretion and absorption of juices. It does the job of lubrication.
Mouth and buccal cavity
(i) The mouth is the anterior opening bounded by the upper and lower lips.
(ii) It leads into a space called mouth cavity or buccal cavity. This cavity contains teeth and tongue.
(iii) The teeth are of four types viz., incisors, canines, premolars and molars. In an adult man there are 32 teeth.
Pharynx : The pharynx is a short but wide tube. It opens into the oesophagus and serves as a common passage for both food and air.
4. Oesophagus : The oesophagus is a muscular tube 25 cm in length extending from the pharynx to the stomach.
5. Stomach
(i) The stomach is a bag-like structure, placed transversely in the body cavity.
(ii) It is J-shaped and contains regions called, the cardiac, the fundus, the body and the pyloric.
(iii) The stomach is provided with the anterior cardiac sphincter and the posterior pyloric sphincter.
(iv) Muscular coat is thickest in all parts of alimentary canal.
(v) In empty stomach its walls are folded.
(vi) It has gastric glands.
6. Small intestine
(i) The small intestine is a coiled tube about 7 metres long.
(ii) It is divisible into three distinct parts viz., duodenum, jejunum and ileum.
(iii) Duodenum is the C-shaped anterior part of the small intestine followed by jejunum.
(iv) Walls of duodenum are thicker than ileum.
7. Large intestine : (i) The large intestine is wider than the small intestine. It is about 1.5 metres long and is divisible into two distinct parts viz. colon and rectum.
(ii) At the junction between the small intestine and the large intestine, there lies a blind pouch called caecum which ends into a small projection called appendix.
(iii) The colon shows four parts, viz., ascending colon, transverse colon, descending colon and sigmoid colon.
(iv) The colon leads to the rectum which is about 12 cm long.
(v) The rectum opens to the outside through an opening called anus.
8. Associated Glands: There are three digestive glands, known as salivary glands, liver and pancreas.
(1) Salivary Glands :
(i) There are three pairs of salivary glands in man. These are parotid, submaxillary and sublingual.
(ii) The salivary glands open into the mouth cavity and secrete about 1000 to 1500 ml saliva in a day used in the digestion of food.
(iii) The saliva contains an enzyme called ‘ptyalin’ which converts food starch into maltose.
(2) Liver :
(i) Liver is the largest gland in our body and weighs about 1.5 Kg.
(ii) It is reddish-brown in colour and is divisible into a large right lobe and a small left lobe.
(iii) In between the two lobes of the liver lies a gall bladder.
(iv ) The bile produced by the liver is stored in the gall bladder.
(v) The hepatic duct from the liver and the cystic duct from the gall bladder join to form the common bile duct.
Functions of liver :
1. The function of liver is to secrete bile. It is a complex greenish alkaline fluid which is composed of bile salts, bile pigments, cholesterol and lecithin. Bile salts are carbonate, glycocholate and taurocholate of sodium. Bile pigments are bilirubin and biliverdin. Bile salts help in emulsification of fat.
2. Liver is centre of glycogenesis, gluconeognesis and glycogenolysis.
3. Deamination of various amino acids is performed in liver.
4. It makes urea. Liver cells help in elimination of certain other excretory waste products.
5. Liver detoxifies certain harmful chemicals e.g. prussic acid.
6. Embryonic liver is haemopoetic and in adult it destroys dead R.B.C. Haemoglobin is decomposed in the liver.
7. Certain metals like copper etc. are stored in liver.
8. Liver makes food proteins like prothrombin and fibrinogen.
9. Heparin and certain enzymes are also synthesised in liver.
10. Liver synthesises vitamin A and stores vitamin A, D, B12 etc.
11. It is the main organ of the lymph formation.
12. It is called blood reservoir because enormous blood is stored in the liver sinusoids.
13. Bile salts help in the absorption of fat soluble vitamin like vitamin K.
14. Body heat is regulated by liver.
15. Liver detoxifies ammonia and changes it into urea.
KIDNEY
1. There are two kidneys and each is bean-shaped in structure. Each kidney is 11 cm long, 6 cm broad and 3 cm thick in man.
2. Left kidney is little longer and narrower than the right one.
3. These are retro-peritoneal organs.
4. Foetal kidney is 12 lobuled and in adult lobules fuse and it becomes smooth.
5. Right kidney is lower than left kidney.
6. The kidney is concave which is called hilus.
7. Hilus leads into renal sinus within the kidney.
8. Renal pelvis has 2-3 manor calyces and they in turn into 7-13 minor calyces.
9. Nipple like projection from the wall of the renal sinus are renal papillae and represent the apices of renal pyramids.
10. Renal Capsules of kidney made up of three layers. These are from outside to inside. These are :
(i) Renal fascia (fascia of gerota).
(ii) Perirenal fat or perinephric fat.
(iii) Fibrous capsule made up of white fibres with a few yellow and smooth muscles.
11. Each kidney is made up of one to three millions of uriniferous tubules or nephrons. These are functional units of kidney.
PANCREAS
Pancreas is party enocrine and partly endocrine. These are double glands. Exocrine part secretes digestive pancreateic juices and endocrine part secretes hormone like insulin to control sugar level in the blood. It is a soft lobulated and elongated gland.
(1) Exocrine part :The exocrine part of pancreas is a serous gland which is made up of tubular acini lined by pyramidal cells. These acini are embedded in the connective tissue having blood vessels, nerve fibres, smooth muscle fibres and the mast cells.
(2) Endocrine part : It is made of the islets of Langerhans distributed as small isolated mass throughout the pancreas. Adult human pancreas contains about 2,00,000 to 17,00,000 islets. They are yellowish brown in colour.
Functions of pancreas
(1) Digestive—Pancreatic juice contains many digestive enzymes which play a vital role in digestion of proteins and lipids. Some enzymes are—
(a) Trypsin—Protein digesting enzyme.
(b) Amylase—Carbohydrate digesting enzyme.
(c) Lipase—Lipid digesting enzyme.
(2) Pancreatic juice provides appropriate alkaline mediu, (pH=8) for the proper activity of pancreatic enzymes.
LUNGS
These constitute a pair of respiratory organs situated in the pleural cavity separated by mediastinum. Each lung is conical in shape, black to brown in colour and spongy in texture. Right lung is heavier than left.
The right lung is divided into three parts by two fissures. These three parts are superior, middle and inferior lobes. The left lung is divided by only oblique fissure into two parts. These are upper lobe and lower lobe. The tongue shaped projection of left lobe below the cardiac notch is called lingula which represents the middle lobe morphologically.
Each lung is covered by a capsule formed by an outer serous coat of the simple squamous epithelium. This is also called mesothelium.
Bronchial Tree : The trachea divides into two principle or primary bronchi, one for each lung. The right bronchus is wider and the left bronchus is narrow and more oblique.
Each bronchus enters lungs through hilus and divides into secondary or lobar bronchi, one for each lobe of lungs.
Respiratory unit—This unit is a small part of lung aerated by respiratory bronchioles. It consists of—
1. Respiratory bronchiolels
2. Alveolar ducts
3. Atrium
4. Air saccules
5. Pulmonary alveoili.
The alveoli is responsible for gaseous exchange. They are lined by flattened epithelium.
TESTES
Testis are a pair of male reproductive organ homologous to ovary of female. Testis is oval in shape and in adult they are of about 10-15 grams each. The left testis is slightly lower than right. The failure of descend results in tests being retained somewhere in the route of descend. This condition is called as cryptorchidism. e.g. testes in elephant.
OVARIES
Ovaries are a pair of female reproductive organ lying in the pelvis. They are almond shaped and greyish pink in colour. Ovary is having almost four components—
(1) Germinal epithelim—It is the outermost covering of a single layer of cuboidal cells.
(2) Tunica albuginea—It is a thin layer of eosinophillic collagenous connective tissue.
(3) Stroma—Stroma cells are spindle shaped. It is composed of connective tissue and smooth muscle fibres.
(4) Vesicular follicles—These are also called Graafian follicles. They develop from parent germinal epithelium.
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