Chapter 4 - Organization of the mammalian hypothalamus-pituitary axes

Abstract

The mammalian adenohypophysis produces tropic hormones and consists of a pars distalis, a pars intermedia, and a pars tuberalis. The pars distalis produces growth hormone (GH), prolactin (PRL), gonadotropic hormones (follicle-stimulating hormone [FSH], luteinizing hormone [LH]), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), and endorphins (EOPs). The posterior portion of the adenohypophysis is the pars intermedia, which is responsible for synthesis of melanocyte-stimulating hormone (α-MSH) and endorphins. The pars tuberalis contains some stainable cell types and secretes tuberalin that stimulates PRL as well as TSH release. The neurohypophysis forms from the infundibulum and consists of an anterior neurohemal area, the median eminence, and a more posterior neurohemal structure, the pars nervosa. The median eminence stores the hypothalamic releasing hormones (releasing hormones [RHs] and release-inhibiting hormones [RIHs]) produced in hypothalamic nuclei that regulate tropic hormone release from the adenohypophysis. The blood portal system connects the median eminence to the adenohypophysis and brings RHs and RIHs to the tropic hormone-producing cells. The pars nervosa is responsible for storage of nonapeptide neurohormones (usually arginine vasopressin [AVP] and oxytocin [OXY]) produced in the hypothalamus until they are released into the general circulation. There are three categories of tropic hormones based on chemical structure. Category 1 includes the glycoproteins, LH, FSH, and TSH. Category 2 includes the large peptides, GH and PRL. Category 3 includes the very similar smaller peptides, ACTH, α-MSH, and lipotropin. The neurosecretory nuclei controlling tropic hormone release are generally located in the preoptic area or the ventral region of the hypothalamus. Aminergic and peptidergic neurons terminate in the median eminence where they regulate release of RHs and RIHs into the portal circulation. These neurosecretory neurons are innervated by regular neurons (e.g., aminergic, cholinergic, serotonergic, and/or peptidergic) that influence their release. Neurosecretory neurons in the supraoptic nucleus and paraventricular nucleus produce the nonapeptides AVP and OXY, which are stored in the pars nervosa. The major secretory product of the pineal gland or epiphysis is melatonin although peptides such as arginine vasotocin may be secreted as well. The pineal receives innervation from several sources. In mammals, the most important pathway is the sympathetic innervation of pinealocytes that is controlled by two retinal pathways both passing through the superior cervical ganglion.