Physiology of glands

Secretory processes in exocrine glands

Exocrine glands that produce mucus usually package their secretory granules initially as mucinogens which later form mucins that are released outside the cell. This phenomenon of packing for export is carried out by the agency of Golgi complex. Cells that produce enzymes and other protein substances usually produce zymogen granules, which are released outside the cell. The process of release of secretory granules differ from gland to gland and there are three major types.

1. Holocrine glands, which release their products by the disintegration of the whole cell. This occurs in goblet cells.


2. Apocrine glands. The exposed portion of the glands (cell) ruptures and this is usually the apical portion. The cell membrane of the apical part of the cell ruptures and releases its contained granules to the outside. Examples include sweat glands of the armpit and genital region. The apocrine sweat glands produce thick viscid secretion unlike the commoner eccrine sweat glands that produce simple fluid. The characteristic odor in the regions mentioned for apocrine glands is caused by the break down of viscid materials produced in those regions, due to bacterial activity.

3. Eccrine (holocrine) glands. These cells release their stored products without damage to the cell. This is done by either by exocytosis or active transport across the cell membrane. All sweat glands in the body aside from the armpit and the genital region are eccrine glands.

There are large organ endocrine glands such as the adrenal gland or ovary and also single cell endocrine glands such as argentaffin cells of the gastrointestinal tact. These are also called APUD or gastroenteropancreatic endocrine cells. The modes of secretion of the endocrine cell glands can be utilized in their classification as follows

Secretory processes in endocrine cells

Synaptocrinia- secretion in this mode is made into synaptic cleft between one cell and another. This is the mode of secretion in the endocrine neurons of the posterior pituitary gland and also in olfactory and gustatory cells and many paraneurons.

Paracrinia. This is transfer of hormone from cell of secretion into intercellular spaces wider than a normal synaptic clefts, i.e. more than 200 nm. The secretion affects vicinal cells.

Endocrinia. This is the classical mode of secretion of large endocrine organs. Their secretion is passed into major blood vessels because they are ductless. Examples include ovary, testis, parathyroid gland.

Lymphocrinia. Hormonal secretion can be passed into lymphatic vessels

Ventriculocrinia. Hormonal secretions can be passed into the ventricles of the brain . A good example is the pinealocytic secretion into the 3rd ventricle of the brain

Two main processes must be present before a secretion can be said to be endocrine

  • Endocrine secretion is basal. Exocrine secretion is apical in all cells

  • Endocrine secretion is aimed at surrounding cells or blood and their glands are therefore ductless.

There are three types of endocrine organs

  • Organs which are entirely endocrine such as the pituitary glands, pineal glands, parathyroid gland, thyroid gland and suprarenal gland.

  • Endocrine tissue in exocrine organs such as islets of Langerhans in pancreas, follicles of the ovary and interstitial cells of the testis

  • Scattered cells which may also be called APUD cells or paraneurons.

There are four types of hormones

  • Amino acid derivatives e.g. epinephrine, thyroxine etc

  • Peptides such as enkephalins, somatostatin

  • Proteins such as insulin

  • Steroids such as progesterone, cortisol etc.

Peptides are the largest group of hormones. They may be neuropeptides, brain-gut peptides, APUD peptides, opioid peptides etc most of which are in neural tissues where they perform neuromodulation and single cell endocrine glands.


    


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