Ultrastructure of the human dermal microcirculation. II. The capillary loops of the dermal papillae.

Electron microscopy was used to define the ultrastructure of the capillary loops in the dermal papillae of normal, eczematous, and psoriatic skin. Capillary loops were reconstructed from 1-mum plastic-embedded sections. Ultrathin sections were taken at 4- to 6-mum intervals for correlation with the thick sections. The capillary loops of normal forearm and elbow skin could be divided into two segments: an intrapapillary and extrapapillary portion. The intrapapillary portion had the ultrastructural characteristics of an arterial capillary--homogeneous-appearing basement membrane without bridged fenestrations. The ascending limb in the extrapapillary portion was also an arterial capillary and the descending limb in the same portion had venous characteristics--multilayered basement membrane. The intrapapillary arterial loop developed venous characteristics abruptly after the vessel left the dermal papilla proper. The capillary loops in acute nummular eczema were identical in ultrastructure to those of normal skin. Bridged fenestrations were found in one capillary loop in eczema but not in normal skin. By contrast, the intrapapillary capillary loops in psoriasis were venous capillaries which were characterized by bridge fenestrations and multilayered basement membrane. The ultrastructural features of the capillary loops and those of the microcirculatory segments in the horizontal dermal plexus should provide sufficient criteria for one to evaluate the nature of vascular abnormalities that occur in the upper dermis.