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
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
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
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