Ontogenesis of adrenocorticotropin-related peptide determinants in the hypothalamus and pituitary gland of the rat.

The time of appearance of the antigenic determinants of ACTH-related peptides (16K fragment, ACTH, gamma-lipoprotein, and beta-endorphin) was determined immunohistochemically in the brain and pituitary gland of the fetal rat. Major antigenic determinants of the pro-ACTH/endorphin precursor first appear in basally located cells of the hypothalamus on day 12 of gestation. Long axonal fibers are evident at day 13. Laterally located cells give rise to lateral, superficial fibers, whereas medially located cells give rise to periventricular fibers. The antigenic determinants appear simultaneously in the anterior lobe of the pituitary gland on day 16 and in the intermediate lobe on day 17. The intensity of immunostaining with anti-16K fragment antiserum is stronger than with the other antisera in the brain but is equal with other antisera in the pituitary. Neuronal perikarya always stain less intensely than pituitary cells. It is concluded that 1) ACTH-related antigenic determinants are produced within the central nervous system, and 2) cells containing ACTH-related antigenic determinants in pituitary gland and brain develop independently of one another.