The Pituitary Gland And Hypothalamus Notes

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.

1. 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:

2. The intermediate lobe (pars intermedia)

The pars intermedia is located in the residual lumen of Rathke’s pouch and extends into the pars nervosa. It consists of a series of small cystic cavities filled with colloid. Its cells include basophils and chromophobes. In horses, they are called melanotropes.

Melanotropes convert POMC into MSH. This is done via the enzyme prohormone convertase II, which cleaves POMC into αMSH and corticotropin-like intermediate lob peptide (CLIP). Prohormone convertas I is also present in the pars intermedialis, yield ACTH from POMC. This is different to in the pars distalis, where only prohormone convertase I is present.

3. The posterior lobe (neurohypophysis)

The neurohypophysis is controlled via axons from supraoptic and paraventricular nuclei in the hypothalamus. These form supraopticohypophyseal and...

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