Ultrastructural studies on the developing neurohypophysis of the Pacific Treefrog, Hyla regilla

Abstract

The differentiation of the hypothalamo-hypophysial neurosecretory system of the Pacific Treefrog Hyla regilla has been examined by correlated light and electron microscopy. Growing neurosecretory axons reach the diencephalic floor in close proximity to the “migrating” adenohypophysis at embryonic stage 23, approximately at the time of hatching. These axons, which already contain typical elementary neurosecretory granules and small vesicles, then grow caudad into the thin-walled infundibulum, reaching the site of the future neurohypophysis by larval stage 1. Not until active outgrowth of the nerve fibers is completed can the neurosecretory material within them be visualized with the light microscope. A striking synchrony exists between the rate of axon growth and the rate of adenohypophysial “migration.” Axons, usually containing small vesicles and small dense granules, penetrate the pars intermedia extensively, beginning with embryonic stage 24. Vascularization of the neurohypophysial area coincides with the arrival of neurosecretory axons at the site of the pars nervosa, just prior to larval stage I. As soon as these events have occurred, the pars nervosa is a miniature but ultrastructurally complete version of the adult neurohemal organ. Further development of the pars nervosa involves an increase in the number and size of axons, in number and possibly size of elementary neurosecretory granules, and in number of glial cells and capillaries. The median eminence differs from the pars nervosa in ultrastructure. Its axon terminals contain smaller neurosecretory granules (1000 A as opposed to 1500 A) and a greater abundance of small vesicles; the difference is reflected in the lesser intensity of paraldehyde fuchsin staining in the median eminence as compared to the pars nervosa. The characteristic terminals do not appear in large numbers in the median eminence until just before metamorphic climax. The observations are discussed in terms of the ontogeny of neurohypophysial function and the possibility of inductive relationships between various parts of the hypophysis.