Nail fold (NF) is the skin fold surrounding the nail; it is divided into proximal and lateral portions. Since only the proximal NF has been the site of microvascular studies, the adjective is usually dropped. A sagittal section of this fold, which covers the root of the nail, is shown in Figure 1 .
NF has a dorsal epidermis, similar to the usual description of skin anatomy, having epidermal ridges and dermal papillae alternating along the dermo-epidermal junction. The ventral epidermis of the NF is flat, loosely adhering to the nail. Between the two epidermal layers we find the dermal connective tissue with blood vessels, nerves, and other structures, such as sweat glands. In the proximal part of the NF the dermal papillae, containing capillaries, are perpendicular to the skin surface; distally they gradually become slanted and become parallel to the ventral epidermis at the edge of the NF (Figure 1; see also Figure 4b).
The thickness of the NF, the slope of the decreasing distance between the two layers of the epidermis from the proximal NF to its edge, and the length of the NF vary considerably between individuals and also from finger to finger.
Superficial blood vessels in the skin can be visualized by depositing oil on its surface. In an average healthy adult, there is a relatively uniform distribution of capillaries in the NF, as seen by in vivo capillary microscopy (Figure 2, bottom). Only the tips of the capillary loops are visible in the proximal NF, where they are perpendicular to the skin surface. Gradual sloping of dermal papillae in more distal NF allows visualization of capillaries partially from the side and at the edge of the NF they can be seen the best, now horizontal and showing their arterial, apical, and venular portions of the loop.
In about 5 percent of the normal young adult population, a highly visible and extensive subpapillary venous plexus can also be observed below the superficial papillary capillaries. This visible venular plexus is often associated with longer capillary loops, especially at the edge of the NF. The overall length of the NF tends to be longer in these individuals. Small sections of this plexus can sometimes also be seen in other subjects near the edge of the NF.
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