The aggregation of glandular epithelium into a definite structure for the purpose of carrying on secretion or excretion is known as gland.
Classification of Glands
• Typically, glands are classified into two major groups:
– Exocrine glands
– Endocrine glands
• Exocrine glands secrete their products onto a surface directly or through epithelial ducts or tubes that are connected to a surface. Exocrine glands are classified as:
• Unicellular glands.
– The secretory component consists of single cells distributed among other nonsecretory cells.
– For example, goblet cell. Goblet cells are located in the surface lining and glands of the intestines and in certain passages of the respiratory tract.
• Multicellular glands.
– Multicellular glands are composed of more than one cell.
– They exhibit varying degree of complexity.
– They are further subclassified according to the arrangement of the secretory cells (Parenchyma) and the presence or absence of branching of the duct elements.
• They secrete their products into the connective tissue where they enter the bloodstream to reach their target cells.
• The products of endocrine glands are called hormones.
• In some epithelia, individual cells secrete a substance that does not reach the bloodstream but affects other cells within the same epithelium.
• Such secretory activity is referred to as paracrine.
CLASSIFICATION OF EXOCRINE GLANDS
• When a gland consists of a single secretory passage or a single opening system of secretory passages into an unbranched duct, it is called a simple gland. Simple glands are further classified into:
– Tubular glands- straight, coiled, branched.
– Tubuloalveolar (tubuloacinar)
– Alveolar (acinar, saccular)
• When a gland contains a duct system that is elaborate and branched, it is called a compound gland. Compound glands are further subdivided into:
• The secretory cells of exocrine glands associated with the various body tubes for example, the alimentary canal, respiratory passages and urogenital system are often described as being serous, mucous or both.
• Mucous secretions are viscous and slimy.
• Goblet cells, secretory cells of the sublingual salivary glands and surface cells of the stomach are examples of mucous-secreting cells.
.• Mucinogen granules are water soluble and lost during routine tissue preparation and that is why the cytoplasm of mucous cells appears to be empty in H & E staining.
• nucleus is usually flattened against the base of the cell by accumulated secretory product.
• Serous secretions are watery.
• In serous cells, the nucleus is typically round or oval.
• The apical cytoplasm is often intensely stained with eosin.
• The perinuclear cytoplasm often appears basophilic because of an extensive rough endoplasmic reticulum, a characteristic of protein-synthesizing cells.
• Serous cell-containing acini are found in the parotid gland and pancreas.
• Acini of some glands, such as the submandibular gland, contain both mucous and serous cells.
MECHANISMS OF SECRETION
Cells of exocrine glands exhibit different mechanisms of secretion.
• There are three basic release mechanisms for secretory products:
• This is the most common mechanism of secretion and is found in pancreatic acinar cells.
• The secretory product is delivered in membrane-bounded vesicles to the apical surface of the cell.
• Vesicles fuse with the plasma membrane and extrude their contents by exocytosis.
• The secretory product is released in the apical portion of the cell, surrounded by a thin layer of cytoplasm within an envelope of plasma membrane.
• This mechanism of secretion is found in the lactating mammary gland, apocrine glands of the skin, ciliary (Molls) glands of the eyelid and the seruminous glands of the external auditory meatus.
• The secretory product accumulates within the maturing cell, which simultaneously undergoes programmed cell death.
• Both secretory products and cell debris are discharged into the lumen of the gland.
• This mechanism is found in sebaceous glands of skin and the tarsal (Meibomian) glands of the eyelid.