The ontogeny of adhesion molecules expressed on the vascular endothelium of the developing human skin

One of the important functions of adhesion molecules is to regulate the trafficking of lymphocytes and other leucocytes between the different organs and tissues of the body. These molecules are expressed on both the endothelial cells and the leucocytes, enabling them to adhere to one another and ultimately lead to extravasation of the leucocytes from the circulation into the surrounding tissue. P and E-selectin promote 'rolling' of leucocytes along the blood vessel walls, whereas ICAM-1 and VCAM-1 mediate subsequent firm adhesion, thus committing the leucocytes to extravasation. We have investigated the expression of the above endothelial adhesion molecules in relation to the developing dermal vasculature of fetal skin using histology and immunocytochemistry. This study showed that already at 11 wk of gestation some dermal vessels expressed P-selectin and ICAM-1. However, by 18 wk these molecules were identified on a significant number of vessels, including small capillaries supplying the forming dermal pegs. In contrast, E-selectin and VCAM-1 molecules were rarely seen in all specimens examined. Our results show that even at 11 wk of gestation, the fetal skin has a mechanism in place for circulating leucocytes to extravasate and provide primitive immunosurveillance. Furthermore, the similarities between the distribution of P-selectin and ICAM-1 in the 18 wk fetal skin and in the normal adult skin were striking. These findings may shed light on our understanding of how the fetus detects and reacts to infections and may, in the future, lead to advances in the management of some intrauterine infections.