Exocrine and Endocrine glands
What is a Gland?
An organised collection of secretory epithelial cells. Most glands are formed during development by proliferation of epithelial cells so that they project into the underlying connective tissue. Some glands retain their continuity with the surface via a duct and are known as EXOCRINE GLANDS. Other glands lose this direct continuity with the surface when their ducts degenerate during development. These glands are known as ENDOCRINE glands.
This is the parotid gland, a type of salivary gland. Can you identify the secretory acini and the ducts in this typical exocrine gland. The intensity of staining should tell you that the secretory cells of this gland are serous. This gland secretes thin watery secretions, which are rich in enzymes and antibodies, and the ducts open onto surface of oral mucosa.
Exocrine glands have ducts - and they secrete onto a surface: examples of exocrine glands are: sebaceous and sweat glands (in the skin), salivary glands (oral), Brunner's glands. So, we have covered their basic structure and function in tissue types, and we have looked at several examples of exocrine glands in other topics.
Exocrine glands can be Unicellular - Goblet cells, or Multicellular - and the basis of their classification was covered in the topic on epithelia.
Hormones co-ordinate, integrate and regulate interdependent processes around the body. These hormones are secreted by the endocrine system.
Endocrine glands do not have ducts. Their secretions (hormones) are secreted into the blood stream. Because of this, the hormones can act over long distances, and reach any organ in the body to co-ordinate activity. Often there is a specific 'target' organ that the hormone acts on. This long range activity is also often called neuroendocrine - as it is somewhat analogous to the co-ordinating activity of neurones.Some short range endocrine activity also occurs in the digestive system - and this is known as paracrine activity - for example enteroendocrine cells of the gut respond to activity by secreting peptides of monoamines that act locally.
The secretory cells of endocrine glands are therefore always found in close proximity to a capillary bed, and have a rich network of blood vessels.
The signalling molecules released - hormones, are usually released by exocytosis, by the secretory cells, into the interstitial spaces and pass through fenestrated cpaillaries to enter the blood stream and move to target organs. The target organs will have specific receptors for the hormone, and can respond when the hormone binds. This means you should know the gland, hormone, target organ/cells and response to the hormone.
How the endocrine glands are classified
Discrete Endocrine Glands - these include the pituitary (hypophysis), thyroid, parathyroid, adrenal and pineal glands. Endocrine component of Glands with both an Endocrine and an Exocrine Function. These include the kidney, pancreas and gonads. And finally, there is a Diffuse Neuroendocrine system, which includes APUD cells.
This topic only covers the Discrete Endocrine Glands; - pituitary, thyroid parathyroid and adrenal glands. Other endocrine glands are covered in their respective topics.
The pineal gland - a small gland 6-8mm long is found in the brain, close to the hypothalamus, and is a photoreceptor organ, which is stimulated by information received via the retina, that secretes the hormone melatonin, which appears to regulate the circadian rhythms of the body. Secretions of this hormone at night, cause a hypnotic effect. Its structure is not covered here.