Hypophysis

__OBSAH__ náhled|200 px|Hypofýza The pituitary gland (pituitary gland, lat. glandula pituitaria) is the central organ of the endocrine system. It is located behind the chiasma opticum, in the recess of the wedge bones called the Turkish saddle (sella turcica), in its deepest part, the fossa hypophysialis. It is suspended on the peduncle (infusion), which protrudes from the hypothalamus at the base of the diencephalus. It is largely superior to all other glands of the endocrine system.

It consists of two compartments, the anterior adenohypophysis and the posterior neurohypophysis. These compartments differ not only in their function, but also in embryonic origin. Adenohypophysis originates as Rathke's bulge from a primitive oral cavity – stomodea. It is noticeable already in the third week of embryonic development. The neurohypophysis differentiates from the outgrowth of the diencephalus. Thus, the adenohypophysis is of ectoderm origin, while the neurohypophysis is of neuroectoderm origin.

Adenohypophysis
náhled|300px| Hormony hypofýzy It consists of pars distalis, pars tuberalis and pars intermedia.

Pars distalis
Pars distalis is composed of beamed epithelium, sparse collagen connective tissue and blood sinusoids. It contains chromophobic and chromophilic cells. Chromophilic cells stain intensively due to the number of cytoplasmic secretory granules where hormones are stored. Chromophobic cells stain little and represent either degraded chromophilic cells or follicular cells that provide support to chromophilic cells. Chromophilic cells are further divided into acidophilic cells (stained intensively with eosin) and basophilic cells (stained with hematoxylin).

Acidophilic cells produce simple proteins and predominate in the beamed epithelium, there are two main types of cells:


 * somatotropic cells − somatotropin (STH, growth hormone);
 * mammotropic cells − prolactin (PRL).

Basophilic cells secrete glycoproteins – they are PAS positive:


 * thyrotropic cells − thyrotropic hormone (TSH);
 * gonadotropic cells − follicle-stimulating hormone (FSH), luteinizing hormone (LH);
 * corticotropic cells − adrenocorticotropic hormone (ACTH).

Pars tuberalis
Funnel-shaped upper protuberance of pars distalis surrounding the upper scape of the infundibula. It contains mainly gonadotropic cells, they are chromophobic.

Pars intermedia
Pars intermedia is the section between the posterior and anterior lobes of the pituitary gland. There are variously sized closed follicles, so-called. Rathke's cysts, which are lined with single-layered epithelium and contain a colloid. There are also beams of chromophobic cells and also basophilic melanotropic cells secreting melanocytes stimulating hormone (MSH).

Neurohypophysis
The neurohypophysis has two basic structural components – the pars nervosa and the infundibulum. Pars nervosa consists of axons of neurosecretory cells of the nuclei of the hypothalamus, pituicytes and fenestrated capillaries. It does not have the structure of a gland, does not synthesize any hormones.

Neurosecretory cells, which are found mainly in the nucleus supraopticus and nucleus paraventricularis, send unmyelienized axons to the neurohypophysis. This connection together forms the hypothalamohypophysary tract (tractus hypothalamohypophysialis). Axons produce large dilatations filled with basophilic neurosecretory granules, which are called Herring bodies. The contents of the granules are synthesized in neurosecretory neurons and released into the vicinity of the capillaries in the neurohypophysis. Granules store oxytocin, which is produced in the nucleus paraventricularis and antidiuretic hormone (ADH, vasopressin) from the nucleus supraopticus. Pituicytes are glial cells that fall between macroglia that surround unmylienized axons, providing them with support.

Blood supply
Blood comes to the pituitary gland through '''aa. hypophysiales superiores et inferiores''', branches of a. carotis interna and drains through the pituitary portal veins. To the adenohypophysis comes a. hypophysialis superior, which in front of it branches around the eminentia mediana hypothalami in the primary capillary plexus. It is then collected in the pituitary portal vein, which leads the blood to the secondary capillary plexus running through the adenohypophysis. This venous blood is collected into the vv. hypophysiales, which flows into the sinus cavernosus and through it into the v. jugularis interna. This phenomenon, when there is an insertion of the capillary plexus between the vessels of the same type, is called the rete mirabile, in this case it is the rete mirabile venosum (the plexus is inserted between the veins).

This arrangement serves to the fact that hormones affecting the activity of the pituitary gland, released by neurosecretory neurons of the hypothalamus into the primary capillary plexus, reach the place of action - the adenohypophysis - through the blood.

To the neurohypophysis comes a. hypophysialis inferior, which also goes to the vv. hypophysiales.

Activity regulation
Both stimulation and inhibition of the release of hormones from the pituitary gland are controlled from the hypothalamus by hormones that are synthesized in the bodies of secretory neurons of the hypothalamus and released by unmyelinated axons into the primary capillary plexus.

Stimulating hormones include:


 * corticoliberin - corticotropin-releasing hormone (CRH, corticotropine-releasing hormone);
 * gonadoliberin - gonadotropin-releasing hormone (GnRH, gonadotropine-releasing hormone);
 * thyreoliberin - thyreotropin-releasing hormone (TRH, thyrotropine-releasing hormone).

Inhibitory hormones:


 * somatostatin inhibits somatotropin output;
 * dopamine inhibits the release of prolactin.

Related articles

 * Vyšetření funkce hypofýzy
 * Hypofyzární adenom
 * Hypothalamo-hypofyzární systém
 * Hypothalamus