Page:Tumors of the pituitary gland.djvu/21

 Tumors of the Pituitary Gland

the lateral wings of the adult gland and the midline portion becomes the anteromedial mu- coid wedge. By midgestation, the medial cleft becomes a residual lumen and growth of the pars nervosa reverses the convexity of the posterior wall of the cleft to a concave structure. The border between Rathke’s pouch and the pars nervosa becomes indistinct; it consists of rem- nants of the obliterating lumen, a few cystic cavities lined by cuboidal or columnar epithe- lium. This represents the rudimentary pars in- termedia of the human hypophysis.

The pituitary grows rapidly in early fetal life: the mean weight at 10 to 14 weeks of gestation is 3 mg, at 25 to 29 weeks 50 mg, and at term approximately 100 mg (23,24).

The pituitary portal vascular system begins to form before 7 weeks of gestation and by 12 weeks the anterior pituitary and median emi- nence are well vascularized. Portal vessels are recognized at 11.5 to 14 weeks, are well devel- oped by 15 to 16 weeks, and are fully established by 18 to 20 weeks (34,35).

Remnants of the developing adenohypophysis may be deposited along the route followed by Rathke’s pouch. The most common site is the roof of the nasopharynx. This so-called “pharyngeal pituitary” is found in most individuals (20,31) and contains all the hormone-producing cell types found in the normal gland; it is thought to have transsphenoidal vascular connections to the sellar hypophysis to maintain homeostatic feedback mechanisms (21). Ectopic adenohypo- physial tissue has also been described in a supra- sellar location in up to 20 percent of people (27). These ectopic foci are usually of incidental inter- est only, but they may be the site of adenoma formation that can confound the clinical diagno- sis (30) or they may be detected with sophisti- cated imaging techniques and mimic a tumor (22). Salivary gland rests are relatively common if carefully sought, and are thought to be contin- uous with Rathke’s cleft (28,37).

Aplasia of the pituitary is usually associated with severe congenital malformations, and forms part of the Cornelia de Lange syndrome (19). Aplasia or hypoplasia may be associated only with evidence of hypopituitarism, including adrenal and thyroid aplasia or hypoplasia (25, 29,32). Dystopia of the gland is the result of failure of union of the adenohypophysis and neu-

rohypophysis (33). Duplication of the pituitary gland has also been reported, usually in associ- ation with other craniofacial malformations (36).

The hypothalamic nuclei that give rise to the neurohypophysis are divided into four anatomic areas: the preoptic, supraoptic-lateral, tuberal, and mamillary regions (fig. 1-13). Whereas the nuclei are topographically discrete in many spe- cies and may be demarcated in the human fetus, they are poorly defined in the mature human hypothalamus (46). Structure-function correla- tions are difficult because of the cellular hetero- geneity of many hypothalamic nuclei. Any given hypothalamic hormone is often produced in more than one nucleus, and in many cases a single nucleus may express more than one hormone. The physiologic roles of many nuclei remain un- known. Nevertheless, this area is responsible for the production of the neurohypophysial hor- mones, oxytocin and vasopressin, and for the hypophysiotropic hormones that are released into the hypophysial portal vasculature and reg- ulate adenohypophysial function, including growth hormone-releasing hormone (GRH), somatostatin (or somatotropin release-inhibiting hormone [SRIH]), dopamine and other putative prolactin-inhibiting substances, corticotropin-re- leasing hormone (CRH), thyrotropin-releasing hormone (TRH), gonadotropin-releasing hor- mone (GnRH), and numerous other peptides that can affect adenohypophysial function (46).

The most anterior nuclei are the paired medial and lateral nuclei that are associated with auto- nomic function, particularly temperature control and olfaction. The suprachiasmatic nucleus, in the preoptic area dorsal to the optic chiasm and ante- rior to the supraoptic nucleus, is an area that is essential for gonadotropin release and sexual be- havior in lower animals. This sexually dimorphic nucleus, which decreases in volume and cell num- ber with age, is thought to play a role in the sexual differentiation of the brain which, in the absence of male gonadal hormones, remains female, but if. exposed to male gonadal hormones at a critical stage in development, becomes male (64,110). It