The Adenohypophysis (Pars Distalis) Produces Follicle-Stimulating Hormone, Luteinizing Hormone, and ProIactinrAII OfWhich Control Reproductive Processes
The pituitary gland is composed of three parts: an anterior lobe called the adenohypophysis, or pars distalis; an intermediate lobe called the pars intermedia; and a posterior lobe called the neurohypophysis, or pars nervosa.
The lobes are of different embryological origins; the pars distalis is derived from the endoectoderm (derived in turn from a small diverticulum off the dorsal pharynx, called Rathke’s pouch), and the pars intermedia and pars nervosa are derived from neuroectoderm. I he adenohypophysis produces protein hormones that are important for the control of reproduction: two gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), and a third hormone called prolactin; other pituitary hormones include growth hormone (GH), corticotropin (adrenocorticotropic hormone, ACTH), and thyroid- stimulating hormone (TSH). FSH and LH are synergistic in Iolliculogenesis and ovulation in the ovary; FSH plays a more dominant role during the growth of follicles, and LH plays a more dominant role during the final stages of follicle maturation through ovulation. The gonadotropins, as well as TSH, are called glycoproteins because their molecules contain carbohydrate moieties that contribute to their function. Oxytocin, which is released by the neurohypophysis, is a hormone of importance in reproduction.Besides being an important center for the control of reproduction, the hypothalamus regulates appetite and
temperature and integrates the activity of the autonomic nervous system. Because of a common embryological origin, the hypothalamus has a direct connection to the neurohypophysis. This connection is through the neural stalk, which contains axons that originate from neuronal cell bodies located in the hypothalamus. Two sets of neurons within the hypothalamus, the supraoptic and paraventricular nuclei, are responsible for the synthesis of vasopressin and oxytocin, respectively.
These small peptide hormones are coupled to larger peptide molecules, called Iieurophysins, and are transported from the site of synthesis in the hypothalamus (neuronal cell bodies) through axons to the site of storage and eventual release, the neurohypophysis.The connection of the hypothalamus to the adenohypophysis does not involve the direct passage of axons through the neural stalk. A venous portal system connects the median eminence within the hypothalamus to the adenohypophysis. Hypothalamic substances that control the adenohypophysis are carried from the median eminence of the hypothalamus to the pituitary by a venous portal system. For example, gonadotropin-releasing hormone (GnRH), a peptide, is produced in the medial preoptic nucleus, and dopamine, an amino acid, is produced in the arcuate nucleus. Axons transport both substances from the hypothalamus to the median eminence, where they are released into the venous portal system. The synthesis of GnRH, as with oxytocin and vasopressin, involves the production of a larger precursor molecule, with a C- terminal region of 56 amino acids, called GnRH-associated peptide (GAP). Although GAP can stimulate the release of FSH and LH, GnRH is still thought to be the critical hormone for gonadotropin release. An even more important function of GAP may be its ability to inhibit prolactin secretion.