Vessels and Nerves: Week 7
In the skin, all blood vessels develop first as capillaries. They are thought to be derived from dermal mesenchymal cells which form single-layered endothelial tubes. Studies show that the tubes are discontinuous, implying that they form directly from existing cells in the dermis. The first network, a horizontal plexus in the superficial dermis, is formed at about 7 weeks of gestation. This vascular plexus is the precursor to the subpapillary vascular plexus which will partly define the demarcation between the papillary and reticular dermis.
Nerves develop closely with the vessels. Large subcutaneous nerves are present by the 7th week of gestation. These large nerves arborize terminate in the dermis immediately beneath the dermal-epidermal junction during the next week of gestation.
This image is a transmission electron micrograph of a dermal capillary showing endothelial cells arranged in a tubular formation. Note the red blood cells (dark homogeneous masses) in the lumen. The cell at the bottom right of the capillary makes no contribution to the vascular lumen. It is
probably a pericyte.