ABSTRACT
When a small mass of hypomeric mesoderm is cultured in vitro within an ectodermal ball (M series) the splanchnic portion is collected in the centre and some haematopoiesis (erythropoiesis) occurs. In explants from post-stage 18 explants, cultured beyond two weeks, granulopoiesis was found in a differentiating reticulo-endothelium.
When a group of endoderm cells were added to the explanted hypomere (M+E series) the younger postgastrula stages (stages 13–18) now demonstrated a histogenesis similar to the older stages in the M series (above), while the poststage 18 explants undergo a similar histogenesis but more rapidly than in the previous series. Some of these older-stage explants gave evidence Of axial orientation in the splanchnic mesoderm.
When the mass of hypomere was increased (2M series) a large amount of peritoneum and endothelium was produced in all explants along with the usual erythropoietic area. No reticulo-endothelium or granulopoiesis appeared. The explants from the older donors (stages 17–20) frequently indicated an axial pattern in the splanchnic mesoderm.
When groups of endoderm cells were added to the increased mass of hypomere (2M+E series) the early postgastrula explants (stages 13–16) evidenced an axial pattern for the first time in the endothelial-haematopoietic histogenesis of the splanchnic mesoderm. Again, no reticulo-endothelium with granulopoiesis was observed.
Analysis of mitotic counts in the various explants indicated that the endoderm was influencing differentiation rather than growth (i.e. cell-division). Similarly, the increase in mass of hypomere appeared to influence differentiation and also to stabilize growth.
The experimental results are considered to further indicate the synergistic role of the endoderm in the histogenesis of the splanchnic hypomere.
