It is the largest part of the pituitary in all vertebrates except cyclostomes and teleosts and is further distinguished into pars distalis, pars intermedia (also regarded as intermediate lobe) and pars tuberalis.
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It is made up of three types of cells—peripheral basophils, central acidophils and scattered throughout chromophils, these are supposed to be the precursors of the acidophils and basophils.
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No doubt this portion of the pituitary is derived from the different precursor than of neurohypophysis but it does not function in splended isolation from the neurohypophysis, for it is intimately related to the hypothalamus by means of an important vascular connection, the hypophysial portal system, which begins as a capillary plexus in the hypothalamus and ends in the sinusoids of the adenohypophysis.
The direction of flow in these vessels is from the hypothalamus to the adenohypophysis.
The morphological aspects of the communication between adenohypophysis and neurohypophysis have attracted special attention in recent years.
The axon termini of the hypothalamic neurosecretory cells are closely associated in the neurohypophysis with the walls of capillaries in the capillary network of that organ.
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In addition, in mammals, there are capillaries surrounding the neurosecretory axons in the median eminence of the diencephalon, where the axons often show an accumulation of neurosecretory material.
The presumption is that the secretory products of the nerve cells are discharged from the axons and axon termini into the blood of these capillaries.
In the lower vertebrates, some of the neurosecretory axons enter into the tissues of the adenohypophysis as well.
The capillary network of the median eminence and sometimes of the neurohypophysis, communicates directly with the capillary network of the adenohypophysis, usually through a system of portal vessels; that is, the capillaries unite into larger vessels which then again give rise to a capillary network in the adenohypophysis.
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Structurally, this provides an ideal situation for chemical transmission from the hypothalamic neurosecretory cells to the adenohypophysis.
It has been established experimentally that neurohypophysial principles do act on the cells of the adenohypophysis.
The adenohypophysis of the pituitary gland is characteristically glandular in nature and secretes six different hormones which, with one exception, are the trophic hormones.
A trophic hormone stimulates some other endocrine gland as its target organ to secrete its own hormones.
The trophic hormones are the gonadotropins—follicle- stimulating hormone (FSH), luteinizing hormone (LH) in the female or interstitial cell stimulating (ICSH) in the male and luteotropic hormone or lactogenic hormone or prolactin (which stimulate gonads and lactation by the mammary glands respectively) ; adrenocorticotropin (ACTH) (which stimulates the adrenal cortex); thyrotropin or thyroid stimulating hormone (TSH) (which stimulates the thyroid gland).
The sixth hormone, growth stimulating hormone (GSH) also called somatotropin (STH), is not strictly speaking a trophic hormone since it does not require another endocrine gland as its target organ but produces its widespread effects by direct action and is responsible for the growth of the body.
Very recently one more complex of hormones has been reported from the adenohypophysis what are known as lipotropins or adipo- kinetic hormones.
The production of these hormones particularly in mammals and birds, as is now believed, is due to secretion of certain chemical messengers or regulation factors or releasing factors by the neurosecretory cells of the hypothalamus which after passing down into the sinusoids of adenohypophysis through the hypophysial portal system, stimulate the various hormone-producing cells of this gland to discharge their products.
The chemical messengers or releasing factors in turn are secreted in response to various chemical and nervous stimuli. The chemical nature of these neurohumoral releasing factors is not fully known, but they seem to be small polypeptides.
Table 141 lists the regulatory factors that are released by the hypothalamus and stimulate or inhibit the secretion by the hypophysis of specific adenohypophysial hormones.