Glands

Histology 2016-2017
Department of Anatomy &Histology: Dr.Rajaa Ali
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GLANDS
A gland is an organ of secretion made of specialised secretory cells derived from surface epithelium on which it opens.

GENERAL FEATURES
_ Glands can be present as discrete organs or in the layers of viscera.
_ The secretory cells of the glands form functional units called secretory end pieces, which are either fl ask shaped (acini) or cylindrical in shape (tubules).
_ It is epithelial in origin.
_ The fl uid secreted by the gland contains enzymes, hormones, mucus or fat.
_ The rate of secretion is modulated by nervous and hormonal infl uences.
_ The secretory end pieces of some exocrine glands are surrounded by star-shaped contractile cells that lie between the secretory cells and the basement membrane. They are called myoepithelial cells as they share characteristics of both epithelial and muscle cells.

DEVELOPMENT (FIG. 1)
_ Glands arise as invagination of the epithelium into the underlying vascular connective tissue.
_ The distal part of the invagination forms the glandular portion or secretory end piece, which is functionally an active portion.
_ The proximal part forms the excretory duct which opens on the surface of the epithelium from which it is developed.
_ Some cells get detached from the epithelial surface and form ductless glands or endocrine glands.

CLASSIFICATION OF GLANDS
A variety of criteria can be used for classifying glands. These are as follows:
A. Based on the site of secretion:
1. Exocrine gland – secretes its products onto a surface through ducts, e.g. salivary glands.
2. Endocrine gland – secretes its products into the bloodstream, e.g. thyroid glands.
3. Paracrine gland – secretes its products into the local extracellular space affecting the surrounding cells, e.g. enteroendocrine cells of gastrointestinal tract (GIT).
B. Based on the number of cells
1. Unicellular gland – composed of a single cell, e.g. goblet cells in the respiratory and intestinal tracts.
2. Multicellular gland – composed of many cells, e.g. all glands other than goblet cells.
C. Based on the number of ducts and shape of secretory end piece
1. Simple gland – has one duct.
2. Compound gland – has minor and major ducts.