Immunocytochemical study of the development of vasotocin/mesotocin in the hypothalamo-hypophysial system of the chick embryo.

The hypothalamo-hypophysial system of the chick embryo has been studied with a monoclonal antibody which cross-reacts with arginine vasotocin and mesotocin, using thick (100 micron) sections in conjunction with a peroxidase-conjugated rabbit anti-mouse antibody. Although weakly stained perikarya occur occasionally in the tuberal region on embryonic days 6 and 7, the most consistent immunostaining of perikarya is found in the periventricular region of the caudal midhypothalamus at the level of the optic chiasm after embryonic day 8 1/2. Synthesis of peptides, therefore, takes place while the cells are close to their site of origin. Between embryonic days 9 and 10, beaded axons run along the anterior median eminence closely apposed to the adenohypophysis, thereby forming the anlage of the zona externa. The axons of the hypothalamo-neurohypophysial tract surround the neural lobe between embryonic days 11 and 12. The caudal to rostral wave of neuronal maturation that occurs during development appears to be due to a progressive differentiation of the periventricular zone, as well as the migration of perikarya. The early periventricular perikarya at embryonic day 8 1/2 send processes rostrally in a wing-shaped formation that extends both dorso- and ventrolaterally. From embryonic days 10 to 12, perikarya can be observed in the wing-like extensions, apparently migrating to rostral levels. The dorsolateral pathway gives rise at its midportion to the lateral cell group, whereas those perikarya migrating more laterally form the anlage of the external supraoptic nucleus. The ventrolateral wing-shaped extension of perikarya appears to be directed toward the ventral group and those lateral perikarya continuous with it. The location of mature neuronal cell groups is well established by embryonic day 17.