Ultrastructure of the human dermal microcirculation: the horizontal plexus of the papillary dermis.

Electron microscopy was used to define the ultrastructure of the various segments of the human cutaneous microcirculation in normal forearem skin. The organization of the vessels in the horizontal plexus of the papillary dermis was reconstructed from 1-mum plasticembedded sections. Ultrathin sections were taken at 10- to 20-mum intervals over a distance of 450 mum. Arterioles were followed through the capillary bed to their venous connections. Terminal arterioles, arterial and venous capillaries, and postcapillary venules were identified on the basis of size, cellular composition of their walls, and their relationship to the other segments of the microvascular bed. The arterial segments were characterized by a homogeneous basement membrane and the venous segments by a multilaminated basement membrane. The elastic lamina in the arterioles was a discontinuous layer which gradually disappeared from the arteriolar wall to form an external sheath just before the arteriole connected with the arterial capillary segment. The vascular walls varied from 1 to 5 mum in all of the segments of the microvascular bed. Criteria are proposed for identifying the various segments of the microcirculation so that their roles in dermatoses and vascular malformations can be evaluated.