Glands are organs that produce and secrete specific substances that are important for the activities of certain body systems. Some glands are separate organs, others are part of some organs. Most glands develop from epithelium .
There are exocrine glands with external secretion (see. Sweat glands , salivary glands , etc.), secreting a secret to the surface of the skin or in the body cavity (gastrointestinal tract), and endocrine glands secreting into the blood or lymph. (see Pituitary, Pancreas, Thyroid , etc.). According to the type of secretion (see), the glands are divided into merocrine (form a secret without disturbing the cytoplasm of the secretory cell), apocrine (forming a secret with partial destruction of the cytoplasm) and holocrine ( secretion accompanied by the death of secretory cells).
The glands are single-celled (for example, goblet epithelial cells of the intestinal mucosa and respiratory tract) and multicellular. Multicellular glands are simple and complex, branched and unbranched, alveolar, tubular and alveolar-tubular (Fig.). In these glands distinguish secretory, or terminal, department and excretory duct. If the excretory duct does not branch, the gland is called idle; a gland with a branching duct (if at the same time in each of its branches opens but several end sections) is classified as complex.
The secret of the glands can be protein, slimy, mixed, greasy. Therefore, the glands are also divided into protein (serous), mucous, mixed and sebaceous.
The structure of multicellular exocrine glands: 1 - a simple unbranched tubular gland; 2 - simple unbranched alveolar gland; 3 - a simple tubular gland with a branched end; 4 - simple alveolar gland with branched end; 5 - complex alveolar-tubular gland.