Immunohistochemical survey of migration of human anterior pituitary cells in developmental, pathological, and clinical aspects: a review.

Developmentally pathological conditions of the anterior pituitary cells include failed separation of the primary pituitary gland into sellar and pharyngeal ones, ectopic migration into the subarachnoid space, and basophil invasion into the posterior lobe although the last is a physiological phenomenon with pathological potentiality in certain circumstances. Pituitary primordium appears at about 4 weeks of gestation. One of the causes of the pituitary gland agenesis may be a formation of the primary hypothalamic ganglionic hamartoma just at the time of occurrence of the pituitary primordium, as analyzed in cases of Pallister-Hall syndrome. A double pituitary in a single individual is a rare malformation. Its pathogenesis is considered as a result of notochordal anomaly. In the 8th gestational week, the primary pituitary gland separates into sellar and pharyngeal parts. The disturbance of this histogenesis results in a rare pituitary malformation, a "pharyngosellar pituitary." Despite the failed separation in this case, differentiation of the pituitary cells proceeds and the hormone production of this malformed pituitary gland can be displayed immunohistochemically. In this case, the distribution of the different hormone producing cells was atypical, particularly in those of gonadotropic hormones and ACTH. Life-long existence of the pharyngeal pituitary is a normal anatomical state in humans. Cell differentiation (hormone production) in the pharyngeal pituitary occurs about 4-10 weeks later than in the sellar pituitary. In pharyngeal pituitary, all kinds of adenohypophyseal hormones are produced. Extracranial pituitary adenomas (with intact sellar pituitary), exclusively found in the nasopharynx, sphenoid sinus, and clivus, may occur from the pharyngeal pituitary while another tumorigenesis can develop from the residual tissue fragment in the craniopharyngeal canal. The "overshoot" of the adenohypophyseal cell migration in the distal part of the sellar pituitary is frequently observed in the leptomeninges of the peri-infundibular or peri-hypothalamic region as ectopic pituitary cell clusters that are apparently independent of the pars tuberalis. It is suggested that these cells, frequently found in "normal" individuals, may be one of the possible origins of the intracranial ectopic pituitary adenomas. However, the reason why a majority of the reported intracranial ectopic pituitary tumors are ACTH-adenomas remains unexplained, since the ectopic cells, found in "normal" individuals, consist of fairly different hormone-producing cells. A further enigmatic phenomenon is a "basophil invasion." ACTH-positive cells invade from the pars intermedia into the posterior lobe of the pituitary. This invasion increases in intensity and frequency according to increase in age. However, the invasion of ACTH cells is observed as early as in the fetal life. The invasive cells display occasionally cell atypia as well as mitotic activity. The origin of extremely rare pituitary adenomas inside the posterior lobe can be explained by the existence and proliferative activity of basophil invasion.