On the pars distalis hormone activities involved in spleen development in chicken embryos.

Spleen development was studied in 11 groups of chicken embryos: (1) intact, /; (2) 'hypophysectomized', H, by partial decapitation, and nine groups of' hypophysectomized' embryos in which each host received a specific type of pars distalis graft; (3) one cephalic region, HCe; (4) one middle region, HM; (5) one caudal region, HCa; (6) the entire pars distalis, HE; (7) two cephalic regions, H2Ce; (8) two middle regions, H2M; (9) two caudal regions, H2Ca\ (10) one cephalic and one caudal region, HCeCa; or (11) one cephalic, one middle and one caudal region, HCeMCa. Embryos were killed at 20-5 days of incubation. Levels of spleen development were assessed by changes in mean weight and histogenesis. Spleen development in 'hypophysectomized' embryos was normal only with cephalic and middle region pars distalis grafts but not with caudal grafts. Grafts of entire partes distales or of two cephalic regions were no more effective than single cephalic regions in correcting development. Double caudal region grafts were as ineffective as single caudal region grafts. Apparently there are qualitative differences between pars distalis regions. Cephalic and middle grafts probably produced sufficient ACTH and TSH activities to stimulate adequate levels of adrenocorticoids and thyroid hormone and perhaps prolactin to repair spleen development. These may be the only hormones important for spleen development, because caudal graft ineffectiveness suggests that STH is unnecessary and may not be involved.