neurohypophysis The neurohypophysis (posterior, or neural lobe) of the pituitary is a direct ventral extension of the infundibulum at the base of the hypothalamus. The majority of the gland is composed of the varicose axons of magnocellular neurons of the supraoptic and para ventricular nuclei which leave the nuclei and course together through the internal zone of the median eminence and pituitary stalk into the neural lobe where they branch repeatedly and form ‘nerve endings’ on the fenestrated capillaries. In the neural lobe the varicosities are often very large (‘Herring bodies’; axonal ‘swellings’). Vasopressin and oxytocin are synthesized as precursors in the magnocellular neuron cell bodies, and packaged in ~160 nm diameter dense- cored vesicles (DCV) which are transported along axonal microtubules to the neural lobe where they are released by exocytosis into the perivascular space of the capillaries. The DCV are found throughout the undilated axons, ‘endings’ and ‘swellings’ but the perivascular ‘endings’ are distinguished by the presence of a population of electron- lucent synaptic- like vesicles which contain glutamate the significance of which is unclear. Although the perivascular ‘endings’ may be the principal site of hormone release, exocytosis of DCV can occur from any part of the varicose axons. In some ‘nerve swellings’ there is massive lysosomal destruction of DCV, disposing of aged and effete DCV that have not been released. Intermingled among and surrounding the neurosecretory axons and also contacting the capillaries are pituicyte cells. These cells are characterized by the presence of lipid droplets and of glial acidic fibrillary protein (GFAP) an intermediate filament protein found in astrocytes. Like astrocytes, pituicytes can form gliomas (‘pituicytomas’). Pituicytes express β- adrenergic and opioid receptors. When secretion of vasopressin or oxytocin is particularly active (dehydration; parturition) the extent which pituicytes enclose the neurosecretory axons is much reduced. They are also capable of engulfing and destroying parts of the axons. Classical macrophages are located in the perivascular space of the neurohypophysis. These act to ‘trim off’ any neurosecretory axons that penetrate the peri vascular basal lamina. The ability of neurosecretory axons to grow is also exemplified by what happens after traumatic pituitary stalk rupture. At first the distal segments of the axons die (releasing vasopressin and temporarily reducing urine output) but then, after a period of central diabetes insipidus, the increase in urine output de creases as the proximal parts of the neurosecretory axons grow and make functional contacts with capillaries in the median eminence and upper infundibulum.

Adenohypophysis

The adenohypophysis (anterior pituitary) forms about 80% of the pituitary; it comprises the large pars distalis, the very small pars intermedia, and the pars tuberalis the cells of which extend up around the infundibulum to the tuber cinereum.

The pars distalis is formed from five types of endocrine cells: corticotrophs, gonadotrophs, thyrotrophs, lactotrophs, and somatotrophs all of which synthesize and store hormone in dense- cored secretory vesicles; together with folliculo- stellate (F- S) cells— agranular, ‘non- endocrine’ cells, and the endothelial cells lining the vascular sinusoids. The endocrine cell types are not evenly distributed: somatotrophs and lactotrophs (acidophils) predominate laterally whereas the central zone is rich in basophil, PAS- positive corticotrophs, gonadotrophs, and thyrotrophs. Of the total, somatotrophs form about 50%, lactotrophs 20%, corticotrophs 15%, gonadotrophs 10% and thyrotrophs 5% of the endocrine cells. Somatotrophs, which secrete growth hormone (GH; 191 aa), are ovoid cells of medium size containing abundant spherical DCV of ~300 nm diameter. Somatotroph adenomas contain variable numbers of DCV, but those with few DCV tend to be more aggressive with a worse response to therapy. Lactotrophs secrete prolactin (PRL; 199aa) and exist in a number of different types, depending on the size and morphology of their DCV. Type I lactotrophs contain many large (300– 900 nm) irregularly shaped DCV that are thought to represent greater storage; type II lactotrophs contain smaller (<200 nm) spherical DCV and are thought to be the more actively secreting form (though type I cells can be stimulated to exocytose their large irregular DCV). There are also intermediate forms, and some ‘somatomammotrophs’ produce both GH and prolactin which is not unexpected as both somatotrophs and lactotrophs are derived from a common precursor. Lactotrophs are particularly influenced by oestradiol, and in pregnancy the increased number and size of lactotrophs causes the entire pituitary to enlarge. Some lactotrophs are situated in close relationship with gonadotrophs. Increased prolactin has a marked inhibitory effect on gonadotrophs and prolactinomas are a common cause of infertility. As with somatotroph adenomas, lactotroph adenomas tend to have fewer, smaller DCV. Corticotrophs are medium sized irregular polygonal cells in which the DCV (150– 250 nm) are largely situated at the cell periphery, just beneath the plasmalemma. Corticotrophs synthesize pro- opiomelanocortin (PMC) which is cleaved in corticotrophs to yield adrenocorticotrophic hormone (ACTH; 39aa), β- lipotropin, and endorphins. Most corticotroph adenomas are basophilic (many DCV) or chromophobe (few DCV) monoclonal tumours most of which secrete ACTH and cause Cushing’s disease. When cortisol is excessive, corticotrophs develop Crooke’s hyaline changes caused by an accumulation of microfilaments. Because the adenoma cells remain sensitive to and inhibited by cortisol, they remain small microadenomas. In contrast ‘silent’ adenomas or macroadenomas synthesizing high- mw POMC are aggressive and invasive.

Gonadotrophs are large basophilic cells with extensive cytoplasm; most produce both follicle- stimulating hormone (FSH; 204aa) and luteinizing hormone (LH; 204aa), though in some only one of the gonadotrophins can be detected. Their DCV tend to be scattered throughout the cytoplasm except at the time of the preovulatory gonadotrophin surge, when they are located more marginally. The DCV vary in appearance from small spherical (250 nm) vesicles with uniformly dense cores, to larger, more ovoid vesicles with a dense core and more lucent surround. The relative production of FSH and LH is determined by the frequency of GnRH pulses and the amount of inhibin. Functioning gonadotroph adenomas (mostly macroadenomas) are very rare. They can cause menstrual irregularity and ovarian hyperstimulation in premenopausal females, testicular enlargement in males, and precocious puberty in children. Most gonadotroph adenomas are ‘silent’ or non- functioning and the most common type of macroadenoma. They may be asymptomatic or cause neurological effects via increasing mass. Most commonly they produce FSH or the alpha subunit, but do so very inefficiently.

Thyrotrophs are the least common, the smallest and contain the smallest DCV (100– 150 nm) of any pituitary endocrine cell. They produce thyrotropin (TSH; 201aa).

Folliculo- stellate cells are a heterogenous group of cells that comprise about 5% of anterior lobe cells and whose functions are only recently being revealed. They are characterized by a lack of dense- cored secretory vesicles, by the expression of S100 protein, and by their linkage together to form follicles and a network throughout the anterior pituitary coupled electrically by gap junctions and calcium waves. They produce many signalling molecules with paracrine effects on the endocrine cells including growth factors, cytokines, nitric oxide and annexin 1 (which is implicated in the early inhibitory effect of glucocorticoids on ACTH release). There is evidence that some folliculostellate (FS) cells may act as stem/ progenitor cells for the anterior pituitary.

The pars tuberalis (pars infundibularis) is an upward extension of the anterior lobe along the pituitary stalk.

The pars intermedia is vestigial in human adults, but its scattered cells can produce POMC and these can intrude into the anterior margin of the neurohypophysis (basophilic invasion’)

The pharyngeal pituitary is a small, but consistent embryologic remnant mass of tissue located either in the submucosa of the dorsal wall of the nasopharynx or in the sphenoid (i.e. along the line of Rathke’s pouch contact with the forebrain vesicle). The cells are often chromophobe (few DCV) and inactive, but rarely form adenomas which cause Cushing’s syndrome, acromegaly, hyperthyroidism, or prolactinoma.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة: نهج البلاغة مصدر صافٍ للفكر والمعرفة ومحصِّنٌ من الشبهات

العتبة العباسية المقدسة تطلق محاضرات محفل نهج البلاغة الأسبوعي في ذي قار

العتبتان المقدستان الحسينية والعباسية تدعوان للمشاركة في مهرجان الشموع بنسخته السادسة والعشرين

بالتعاون مع مستشفى الكفيل.. قسم الشؤون الطبّية يواصل إقامة البرنامج التدريبي لملاكاته النسوية

المزيد

الآخبار الصحية

الصحة العالمية تكشف حقيقة انتشار فيروس نيباه خارج الهند

متى يصبح محيط الخصر "مؤشر خطر"؟

دراسة تكشف دور "العوامل الوراثية" في تحديد عمر الإنسان

رائحة كريهة في الجسم.. "علامة خفية" أخطر مما تتصور

المزيد

الآخبار التكنلوجية

مايكروسوفت تكشف عن الجيل الثاني من شرائحها للذكاء الاصطناعي

خرافة "ثمانية أكواب يوميا".. كم من الماء يحتاج جسمك فعليا؟

هل توجد حياة خارج كوكبنا؟.. اكتشاف جديد يثير اهتمام العلماء

اكتشاف الخلايا المسؤولة عن فقدان ذاكرة الطفولة

المزيد

مواضيع ذات صلة

combined Pituitary Hormone Deficiency

Hypothalamic neuroendocrine neurons: endocrine and central effects

Blood Supply of the Hypothalamus and Pituitary

Development of the Hypothalamus and Pituitary

Pituitary Gland

Hypothalamic Neural Connections

Hypothalamic-Pituitary Anatomy

Hypothalamic Control of the Pituitary Gland

الغدة النخامية Pituitary gland

الغدة النخامية Pituitary Gland

الهرمون المضاد لإدرار البول Anti-diuretic Hormone , ADH

هرمون الأوكسي توسين Oxytocin , OT