Development of human embryonic and fetal dermal vasculature

This report summarizes recent advances in the understanding of the structure and organization of the microvasculature in developing human skin. Previous observations suggested that the skin contains no blood vessels as late as eight weeks estimated gestational age (EGA). Computer reconstructions, in conjunction with light and transmission electron microscopy (TEM), however, demonstrated that specimens as young as 35-45 d show a level of vascular complexity previously unknown. The computer reconstructions showed that the vasculature was organized in one or two planes parallel to the epidermis. A simple, single plane was evident in specimens 40-50 d EGA, whereas specimens 50-75 d EGA showed two planes. Fewer vessels were continuous throughout the tissue sample in the younger specimens compared with the older specimens. Superior views highlighted the continuities and connections of vessels. The younger specimens showed more discontinuous segments of vessels when compared with the network established in the older specimens. In the earliest specimens examined morphologically (35-40 d), simple, capillarylike vessels were morphologically identifiable in presumptive dermis. The samples studied by TEM revealed detailed structure of the vessel wall including extreme attenuations and projections, plasmalemmal vesicles, and junctions similar to adult endothelial cells. Little or no basal lamina surrounded the vessel. The basal lamina first appeared in the form of amorphous deposits and eventually thickened and became continuous. By the end of the first trimester, the basal lamina still lacked the organization of adult cutaneous arterial and venous segments. These findings suggest that the major vascular organization of the dermis is defined in the first trimester of development.