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The Pituitary Gland and Hypothalamus
1. The pituitary gland The pituitary gland, also known as the hypophysis is located as an appendage of the brain. It is suspended below the hypothalamus by a narrow, fragile stalk and is received into a depression called the hypophysial fossa or sella turcica.
The cavernous sinus, a large venous channel, is located on either side of the hypophysis. The optic chiasm is directly rostral to the hypophysis and the sphenoid bone is located ventrally. The hypophysis is divided into three lobes. The neurohypophysis or posterior lobe has a neural function, and is formed by a downgrowth of the hypothalamus. It consists of the pars nervosa. The adenohypophysis or anterior lobe has a glandular function, and is formed by an outgrowth of the ectoderm of the roof of the developing mouth, known as Rathke's pouch. The adenohpyophysis includes the pars tuberalis and pars distalis. It contains a vestigial flattened space called the cleft of the hypophysis. This separates the pars distalis and pars intermedia. N.B the cleft of the hypohpysis is absent in horses. The pars intermedia is also known as the intermediate lobe.
a) The anterior lobe (adenohypophysis) The adenohypophysis is controlled by releasing factors from the hypothalamus. These releasing factors reach the adenophypophysis via the hypophyseal portal system, which is a system of venous channels allowing direct passage between the hypothalamus and the pituitary gland. These releasing factors cause the adenohypophysis to produce hormones. The adenohypophysis secretes two types of hormone - trophic hormones and direct-action hormones. Trophic hormones stimulate a further endocrine gland. Their secretion is controlled by negative feedback, where the product of the downstream endocrine gland limits the release of the original stimulating hormone. Trophic hormones include thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle stimulating hormone (FSH) and luteinizing hormone (LH). Their secretion is stimulating by releasing factors from the hypothalamus, such as gonadotropin releasing hormone (GnRH) and corticotropin releasing hormone (CRH). Direct-action hormones act directly on non-endocrine tissues. As there is no target gland, release of these hormones is controlled by a balance of releasing and inhibitory factors from the hypothalamus. For example, growth hormone (GH) secretion is controlled by stimulatory somatocrinin or growth hormone releasing hormone (GHRH) and inhibitory somatostatin (SS). Each hormone is produced by a different cell type within the pars distalis. There are five cell types altogether: somatotropes, lactotropes, corticotropes, thyrotropes and gonadotropes. Their distribution and products are summarised below: Cell type
Percent of pars distalis by mass
Action of hormone
Growth hormone (GH)
Stimulated by GHRH.
Also called somatotropin
Inhibited by somatostatin.
IGF-1 secretion by liver. Protein synthesis,
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