Page 68 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

50 Veterinary Histology of Domestic Mammals and Birds

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2.24 Differentiation of an exocrine extra-epithelial tubular gland and various types of endocrine glands from
an ectodermal anlage (schematic).

Endocrine glands (glandulae endocrinae) fuse through the plasmalemma to reach receptors located
The cells of endocrine glands are typically surrounded within the cytoplasm. Thereupon, the hormones are able
by a dense capillary network. The glandular secretions, to influence cellular metabolism. Examples of endocrine
known as hormones, are released into the surrounding glands include the pituitary, thyroid and adrenal glands.
connective tissue from which they are typically taken up In certain epithelia, hormonal secretions do not pass
through the wall of a capillary and transported by the into the circulatory system. Instead, after diffusing into the
bloodstream to their target organ. There, they either bind extracellular compartment, they exert their effect directly
to specific receptors on the target cell membrane or dif- on surrounding cells (tissue hormones e.g. neurotensin,

Site of hormone action
Paracrine secretion located immediately
adjacent to site of
hormone synthesis,e. g.
neighbouring epithelial
cells, smooth muscle
cells or vessels

2.25 Endocrine and paracrine modes of cellular communication (schematic).

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