Glands:

Glands:

Glandular epithelium are specialized for secretory activity.
A cell, tissue or organ which secretes a useful chemical material is known as gland.
Glands are made up of cuboidal epithelial cells which are more secretory.
All glands arise as folding of epithelia. The golgi body in gland cells are larger and more secretory.
Most of the glands of body are merocrine types. It originate from all three germinal layers. (ecto, meso and endoderm).  
Liver is the largest gland of the body and lined by glandular epithelium.

Types of glands:

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  • Unicellular gland : It  consist of unicellular gland cells which are called as goblet cells or chalice cells. They secrete mucous and found in mucosa of intestine and stomach. Mucous lubricates the food for easy peristalsis. Their life span is about 2–3 days.
  • Multicellular gland : It consist of many cells and are generally located in underlying connective tissue e.g. gastric and intestinal glands.
  • Exocrine gland : These are those glands which discharge their secretory products into ducts. It is also called ducted glands or glands of external secretion.  e.g. Salivary glands, Mammary glands and Tear glands.
  • Endocrine gland : It is often called ductless gland, because they discharge their secretory products (hormones) directly into the blood. e.g. Pituitary gland, thyroid, parathyroid and adrenal glands.
  • Heterocrine gland : These are those glands which are partly endocrine and partly exocrine in function. e.g. Pancreas.
  • Structural classification of exocrine glands : Multicellular exocrine glands are classified by structure, using the shape of their ducts and the complexity (branching) of their ducts system as distinguishing characteristics. Shape include tubular and alveolar (Sac like). Simple exocrine glands e.g. intestinal glands, mammalian sweat glands, cutaneous glands of frog etc. have only one duct leading to surface. Compound  exocrine glands have two or more ducts e.g. liver, salivary glands etc.
  • On the basis of mode of secretion, glands can be:
     
  • Holocrine glands: In holocrine glands (e.g., sebaceous gland), the product of secretion is shed with the whole cell leading to its destruction.
     
  • Merocrine glands: When the secretory granules leave the cell by exocytosis with no loss of other cellular material, the glands are called merocine glands (e.g., the Pancreas).
     
  • Apocrine glands: In aporcrine glands (e.g., mammary gland and axillary sweat glands), only the apical portion of the cytoplasm is discharged along with the secretory product.
    
    
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