EXPRESSION OF HOMEOBOX GENES, INCLUDING AN INSULIN PROMOTING FACTOR, IN THE MURINE YOLK-SAC AT THE TIME OF HEMATOPOIETIC INITIATION

The visceral yolk sac (YS), a simple bilayer structure formed during g astrulation, supplies blood cells and intestine-and liver-like functio ns to support embryonic growth. To better understand gene regulation i n extraembryonic tissues, we examined the early murine YS for expressi on of the homeobox family of developmental transcription regulators. W e identified a subset of known homeobox sequences (Hox a1, b1, a9, c9, a7, b7, b8, a10, cdx-1, and PDX-1), as well as two novel homeodomains consisting of a fourth labial class Hox genes and one that matches th e Antennapedia class on the amino acid level. The two most frequently isolated YS Hox genes, a9 and c9, ave initially expressed only in the YS (E7.5) and subsequently expressed in both the embryo and YS (E8.5), Another of the identified genes, PDX-1, is involved in pancreatic dev elopment and insulin regulation. Whereas the rodent YS is known to pro duce insulin from mid to late gestation, YS insulin expression had not been examined earlier in development. We detected insulin mRNA in the YS at both E7.5 and E8.5, prior to expression in the embryo proper or formation of the pancreas. However, other pancreatic products, such a s glucagon, somatostatin, and carboxypeptidase A, are not expressed in the YS. In situ analysis indicates insulin is produced in YS mesothel ial cells and endoderm cells, but not in blood cells. We hypothesize t he early expression of insulin in the YS is required for the expansion of insulin responsive cells including primitive erythroblasts. (C) 19 97 Wiley-Liss, Inc.