Page 741 - Atlas of Histology with Functional Correlations

SECTION 1 Hormones and Pituitary Gland

The endocrine system consists of cells, tissues, and organs that synthesize and

secrete hormones that are then released into the interstitial connective tissue
from which they pass into the blood or lymph circulation. As a result, endocrine
cells, tissues, glands, and organs are called ductless glands because there are no
excretory ducts for the hormone release. Furthermore, the cells in most
endocrine tissues and organs are arranged into cords and clumps and surrounded

by a capillary network for more efficient transport of the hormones.

Hormones produced by endocrine cells include polypeptides, proteins,
steroids, amino acid derivatives, and catecholamines. Because hormones act at a
distance from the site of their release, the circulatory system delivers them to

their target organs. Here, they influence the structure and the programmed
function of the target organ cells by binding to and interacting with specific
hormone receptors.

Hormone receptors are located on the cell membrane, in the cytoplasm, or
in the nucleus of target cells. These receptors are specific for certain hormones.

Nonsteroid receptors for protein and peptide hormones are usually located on
cell surfaces because these hormones do not penetrate the cell membrane. The
interaction with the hormone produces molecules called second messengers,

which is cyclic adenosine monophosphate (cAMP) for most hormones. cAMP
then activates a specific sequence of enzymes and cellular events in the
cytoplasm and/or nucleus for a hormone-specific response.

Other receptors are intracellular, are localized in the nucleus, and are
activated by hormones that penetrate the cellular and nuclear membranes.

Steroid and thyroid hormones are lipid soluble and cross these membranes. Once
inside the target cells, these steroid hormones combine with specific receptor
molecules. The resulting hormone–receptor complex enters the nucleus and
binds to a particular DNA sequence that either activates or inhibits specific

genes. The activated genes then initiate the synthesis of messenger RNA that
enters the cytoplasm to initiate production of hormone-specific proteins and
induce cellular changes specifically associated with the particular hormone. The
hormones that combine with the intracellular receptors do not use the second

messenger. Instead, they directly influence gene expression of the affected cell.

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