Histology/Histological features of small intestine are discussed below:
i) The histology of small intestine has three different regions, these are: Duodenum, Jejunum and ileum.
ii) The duodenum is the first section of the small intestine and is also the shortest part of the small intestine. It is the only region of the small intestine where most of the chemical reactions related to digestion using enzymes takes place.
iii) The jejunum is the middle section of the small intestine and is responsible for absorbing nutrients from digested food into the bloodstream. This region of the small intestine has a large number of finger-like projections called villi which increases their surface area to facilitate better absorption.
iv) The ileum is the last section of the small intestine and is the longest part of the small intestine. It is responsible for absorbing the remaining nutrients (particularly vitamin B12, bile salts and fats) that are not absorbed by duodenum and jejunum region of the small intestine.
v) Histologically the small intestine has four different layers of tissues, the mucosa, submucosa, muscularis and serosa layer.
vi) The serosa is the outermost layer of the small intestine. It is a smooth membrane consisting of a thin layer of cells that secrete serous fluid, and a thin layer of connective tissue. Serous fluid is a lubricating fluid which reduces friction from the movement of the muscularis.
vii) The muscularis is a muscular region which is adjacent to the submucosa membrane. It is responsible for gut movement, or peristalsis. It usually has two distinct layers of smooth muscle, these are: circular and longitudinal layer.
viii) The submucosa is the layer which is made of dense, irregular connective tissue or loose connective tissue that supports the mucosa layer, as well as joins the mucosa layer to the bulk of underlying smooth muscles.
ix) The mucosa layer is the innermost layer of tissue in the small intestines, and is a mucous membrane that secretes digestive enzymes and hormones. Intestinal villi are part of the mucosa layer of the small intestine.
Histological features of stomach discussed below:
i) Histologically stomach has 3 different regions which are: cardiac, fundus and pyloric regions. The cardiac region of the stomach contains mucous secreting gland and is very close to the oesophagus (or food pipe). The fundus region (or the body) is the largest part of the stomach which contains the gastric glands. Pyloric region secrets hormone gastrin and mucous which protects the inner lining of the stomach by coating it.
ii) The wall of the stomach consists of four different layers of tissue, these are: the serosa, muscularis externa, submucosa and mucosa.
iii) The gastric mucosa is the innermost layer of the stomach wall. It is formed by a layer of surface epithelium, which contains a large number of gastric glands and gastric pits and an underlying lamina propria and muscularis mucosae.
iv) Deep to the gastric mucosa layer there is a thick layer of connective tissue known as the gastric submucosa. This layer is durable, yet flexible and mobile. Apart from its rich vasculature and lymphatics, this layer also holds the submucosal (Meissner’s) plexus. The nerve fibres of this plexus contains parasympathetic innervation to the blood vessels and smooth muscle of the stomach wall.
v) The mucularis externa layer of stomach wall has three different layers of muscle: an inner oblique layer, a middle circular layer and an external longitudinal layer. The contraction of these muscle layers help to mechanically break up the food.
vi) Gastric serosais the outermost layer of the stomach wall. It is formed by a layer of simple squamous epithelium, known as mesothelium, and a thin layer of underlying connective tissue.
Histological features of liver are discussed below:
It is the functional component of the liver which is made up of the hepatocytes that filter blood to remove toxins. It is the connective tissue that supports the liver and creates a framework for the hepatocytes to grow. In patients with liver disorders, it is the part of the liver parenchyma which gets damaged or does not function properly.
It is a continuation of the Glisson’s capsule present in the surrounding. It consists of vessels and connective tissues. This capsule is also covered by a layer of mesothelium which arises from the peritoneum covering the liver. The connective tissue of the stroma is of type III collagen (reticulin), which creates a meshwork that provides integrity for the hepatocytes and sinusoids.
Sinusoids are the capillaries that travel between the hepatocytes of the liver.
iv) Spaces of Disse (perisinusoidal spaces)
Spaces of Disse are the regions located between the hepatocytes and the sinusoids of the liver. This layer is filled with blood plasma and chylomicron that passes through the wall of sinusoid.
v) Hepatic lobules
These are the structural units of liver. The liver parenchyma consists of thousands of hepatic lobules. These are roughly arranged in hexagonal shape of irregular plates or cords of hepatocyte which radiates outward from a central vein.
vi) Portal lobules
It is the territory of liver tissue which is centered around a portal triad and is drawn by joining the central veins of three adjacent lobules.
vii) Hepatic acinus
It is a diamond shaped area of liver parenchyma. It has numerous branches that arises at right angle from the blood vessels of portal area, these terminal vessels form backbone of the liver acinus. The acinus can be divided into three different zones based on the gradient of blood supply, these are: zone 1,2 and 3. Zone 1 is present around the vascular backbone and is well oxygenated. Zone 2 is the intermediate zone and is moderately oxygenated. Zone 3 is close to the central vein and is least oxygenated. This zone is most susceptible to anoxic injury.
Histological features of lungs are discussed below:
i) The bronchus of mammalian lung is divided into three main parts, these are: (a) principle bronchus, (b) secondary bronchus (or lobar bronchus) and (c) tertiary bronchus (segmental bronchus).
ii) The principal bronchus is same as that of trachea (also called wind pipe) which is again made of four different layers – Mucosa, submucosa, cartilaginous and muscular layer and adventitia.
iii) The secondary or Lobar bronchus is made up of irregular hyaline cartilage and pseudo stratified ciliated columnar tissues.
iv) The tertiary or Segmental bronchus is made up of columnar epithelium and patches of cartilage.
v) Their epithelium contains reduced number of Goblet cells. Whereas their submucosal layer contains reduced number of Glands.
vi) The bronchioles are the smaller branches of bronchial airways of the respiratory tract. They contain the terminal bronchioles, and finally the respiratory bronchioles. The terminal bronchioles contain columnar epithelium, smooth muscle and clara cells. They doesn’t contain any cartilage. While the respiratory bronchioles contain cuboidal epithelium. There are no mucus gland present in them. They are lined by simple ciliated epithelium or columnar epithelium.
vii) The alveolar duct or Alveolar sac present in the respiratory bronchioles are lined by simple squamous epithelium and are surrounded by network of capillaries.
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