Influence of the entoderm on mesodermal expansion in the area vasculosa of the chick

Abstract

ABSTRACT To determine whether expansion of the splanchnic mesoderm of the area vasculosa is influenced by the entodermal substratum on which it occurs, entoderm was separated from a small area of splanchnic mesoderm. The splanchnic mesoderm then contracted and thickened, decreasing to 7 % of its original area in 16 h. By then entoderm had reattached to most of it, and it expanded, reaching 11 % of its original area by 24 h. It was concluded that attachment to entoderm may be required for expansion of the splanchnic mesoderm, but the small amount of expansion obtained made this conclusion tentative. For technical reasons subsequent investigation was done on mesodermal transplants, which attached to the host’s entoderm in 6 h, by which time they had contracted to 15% of their original area. They then expanded, reaching 30% by 16 h and 49% by 24 h. The onset of their expansion was also accompanied by the formation of connexions between their blood vessels and those of the host, and by the resumption of blood flow in them. To see whether their expansion was due to resumption of blood flow or to attachment to entoderm, other transplants were made in which the middle one-third was separated from the host’s entoderm by a piece of Millipore filter. This portion failed to expand although it became connected to the host’s blood vessels and flow of blood resumed in it, while the two lateral thirds, which regained attachment to entoderm, expanded. Transplants were also rotated so that their splanchnic mesoderm attached to ectoderm instead of entoderm. These transplants also formed connexions with the host’s vessels and blood flow resumed in them, but they expanded only slightly compared to non-rotated controls, in which the splanchnic mesoderm attached to entoderm. It was concluded that while flow of blood undoubtedly promotes splanchnic mesodermal expansion as others have shown, attachment of the splanchnic mesoderm to entoderm is also important, and without it the promotive effect of blood flow does not occur. Evidence was also obtained that attachment to entoderm maintains the thinness of the splanchnic mesoderm, and that a vascular growth stimulus may be produced by the unvascularized entoderm distal to the mesoderm.