Histological Significance Of The Hypothalamus And Pituitary Gland In Endocrinology

The acidophil cells (eosinophilic cells) which form about 50% of the total number of epithelial cells are situated in the posterolateral aspect of the gland. They secrete growth hormone (GH) and prolactin (PRL). The basophils secrete follicle stimulating hormone (FSH), luteinizing hormone (LH), thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH) and melanocyte stimulating hormone (MSH). The chromophobes were considered to be nonfunctional till recently, bu present evidence suggests that they are either precursors of secretory cells or they are cells which are depleted of their secretions. Based on electron microscopic findings, the cells of the pituitary can be classified as somatotrophs, lactotrophs, thyrotrophs, gonadotrophs, corticotrophs and melanotrophs.

Lactottrophs form 15- 20% of the total number of cells and they are distributed throughout the gland. Their number increases during pregnancy. Corticotrophs form less than 10% of cells and are situated anteromedially. Gonadotrophs which form 10% of cells are mainly distributed in the lateral wings, and thyrotrophs are found mainly in the inferomedial and superior parts of the glands.

Hypothalamic hormones that have been identified are thyrotropin releasing hormone (TRH), gonadotropin releasing hormone (GnRH) and growth hormone release inhibiting hormone (GHRIH), somatostatin (SS). These have been isolated and chemically characterized. Other hormones which possible exist, but have not been identified, include corticotrophin releasing factor (CRF), prolactin releasing and inhibiting factors (PRF and PIF) and growth hormone releasing factor (GRF).

Antidiuretic hormone (ADH) or Vasopressin: This is stored in the posterior pituitary. The term ‘vasopressin’ denotes its ability to raise the blood pressure of experimental animals in pharmacological doses. The synthesis and release of vasopressin are regulated by osmotic and non-osmotic stimuli. Under basal conditions, osmotic mechanisms predominate over non-osmotic mechanisms. Changes in serum osmolality by 1 to 2% lead to alteration in the volume of the osmoreceptors in the anterior hypothalamus and this triggers the supraoptic nucleus. Howver, under conditions of stress, volume receptors in the atrium and pulmonary veins are also activated. When there is a fall in blood pressure, receptors in the carotid and aortic sinuses are stimulated and they augment the secretion of vasopressin. Other factors which stimulate vasopressin secretion are psychological and physical stresses. Once triggered, the non-osmotic mechanisms are capable of stimulating the synthesis and secretion of vasopressin.

Vasopressin exerts its main effect on the distal convoluted tubules and collecting tubules of the kidney and increases the reabsorption of water. The exact mechanism of action is not clear. The hormone activates the adenylate cyclase system on the cells lining the tubules and make them more permeable to water by increasing the size and number of pores. In addition, vasopressin stimulates the smooth muscles of the arterioles in the mesentery, coronary circulation and skin. Intestinal smooth muschles is also stimulated to activity. The main source of vasopressin for therapeutic source is synthetic and the preparation is 1-deamino-8-arginine vasopressin (DDAVP_. It is used mainly for the treatment of diabetes insipidus. In other conditions like hemophilia, DDAVP is finding increasing use.

Oxytocin: This hormone enhances uterine contraction during labour and also in postpartum period. This helps in delivery and postpartum hemostasis. Oxytocin also enhances the release of milk from the breast during suckling.

The production of oxytocin is controlled mainly by stimuli arising from the breast during suckling. The impulses reach the hypothalamus through the spinal cord and stimulate the production of oxytocin. Distension of the female genital tract stimulates oxytocin-production whereas stress and emotional disturbances inhibit it. Oxytocin is used mainly to induce labour and stimulate uterine contraction after delivery.