Physiology of Pituitary gland
Pituitary gland is divided into two distinct portions, the anterior pituitary or adenohypophysis and posterior pituitary or neurohypophysis or pars nervosa.
The pituitary is connected to the brain by means of the pituitary stalk. The adenohypophysis consists of three portions, viz., the pars distalis, pars tuberalis and the pars intermedia.
The pars distalis is the largest of the three parts and secrets the tropic hormones. The pars tuberalis does not have any endocrine function.
The pars intermedia produce melanocyte-stimulating hormone (MSH) and regulate the pigmentation in the body.
Adenohypophysis (Anterior pituitary)
Adenohypophysis contains two types of cells – chromophobes and chromophils. The chromophobes are the inactive precursor or a resting type of cells from which the active (hormone producing) cells known as chromophils arise.
Chromophils are further divided into acidophils and basophils. The acidophils and basophils include five types that produce six hormones-
- Somatotrophs- Growth Hormone
- Lactotrophs/ mammotrophs- Prolactin
- Thyrotrophs- TSH
- Corticotrophs- ACTH & β-LPH
- Gonadotrophs- LH & FSH
Neurohypophysis (Posterior pituitary)
The neurohypophysis is composed of pituicytes and fine unmyelinated nerve fibres arising from the supraoptic and paraventricular nuclei of the hypothalamus and terminating in the neural lobe of the pituitary.
Neurohypophyseal hormones are the secretions of neuro‑secretory cells of the hypothalamic nuclei which move along the axons of the hypothalamo‑hypophyseal tracts to the neuro-hypophysis, stored until the proper signal arises to cause their release into the circulation.
Two octapeptide hormones are released from the neurohypophysis
Antidiuertic hormone (ADH) or vasopressin produced from the supraoptic nucleus of hypothalamus
Oxytocinproduced from the hypothalamic paraventricular nucleus.
These two hormones after their synthesis are complexed with a polypeptide known as neurophysin and transported as Hering bodies within the axons to the neurohypophysis. Neurophysin I and II aid in transport of oxytocin and ADH respectively.
Vascular supply to pituitary gland
In general the inferior or posterior hypophyseal arteries supply the neural lobe.
The superior or anterior hypophyseal arteries from the internal carotid supply the anterior lobe of the pituitary.
The vessels go to the median eminence of hypothalamus and break into primary plexus in the infundibulum near the lower portion of the hypothalamus. Venous blood from these plexus is collected in two series of parallel veins which pass through the pituitary stalk ending in pars distalis. This constitutes the hypothalamo‑ hypophyseal portal system.
Regulating factors from the hypothalamus enter into these portal system and goes to adeno-hypophysis
The hormones of the adenohypophysis diffuse into veins and reach the target cells.