UCHIDA, K; MURAKAMI, Y; KURAKU, S; KURATANI, S; CDB, Riken, Kobe, Japan; CDB, Riken, Kobe, Japan; CDB, Riken, Kobe, Japan; CDB, Riken, Kobe, Japan: Development of the adenohypophysis in the lamprey: Evolution of epigenetic patterning programs in organogenesis

In gnathostomes, the adenohypophysis is believed to develop through hierarchically organized epigenetic interactions based primarily on the topographical relationships between tissues. FGF and BMP signaling molecules derived from hypothalamic and adenohypophysial anlage are cooperatively involved in the pituitary organogenesis. From a comparison of developmental processes and gene expression patterns of pituitary-related genes between lamprey (Lethenteron japonicum) and gnathostomes, we speculate on the evolutionary pathway of the vertebrate adenohypophysis. In the lamprey, the adenohypophysis was derived from the nasohypophysial placode (NHP) that developed anterior to the oral ectoderm. The NHP was identified by the expression of LjPitxA, before actual histogenesis, but it was initially distant from the future hypothalamic region. Subsequently, the NHP expressed both LjFgf8/17 and LjBmp2/4a gene transcripts, whereas no signaling molecules could be identified in the hypothalamic anlage. The NHP grew caudally to establish a de novo contact with the hypothalamic region by the mid-pharyngula stage, and gave rise to both the adenohypophysis and an unpaired nasal organ. Thus, the topographical relationship between the NHP and the hypothalamic region is established secondarily in the lamprey, unlike gnathostomes. Comparing the developmental pattern of the adenohypophysial homologue in amphioxus, we hypothesize that a modification of the regulation of the growth factor encoding gene may have been involved above heterochrony and heterotopy.