Histological structure of hair, with a brief comparison to other epidermal appendages and epidermis itself

TLDR: It can be shown that the connective tissue beneathactively growing Malpighian layers, e.g., the hair papilla, is highly vascularized, contains many nuclei, and is rich in phosphatase' and metachromatic substance^,^ in contrast to the derma in general.

Abstract: Hair plays a fundamental role in the maintenance of body temperature in mammals. Man, whose hair is no more than a symbol of strength or beauty, uses animal hair, such as fur or wool, to achieve the same end. *The remarkable properties of hair-heat-retaining capacity, durability, and resiliency-are due to its high content of a sulfur-containing protein, keratin.* Keratin is also present in epidermis and epidermal appendages, nails, claws, horns, feathers, elc. In all of these structures, the superficial layers, which are impregnated with keratin, arise from the transformation of the cells of the so-called Malpighian layers. Thus, Malpighian and keratinized layers make up the epidermis (FIGURE l), the hair (FIGURE 2), and the other appendages. The Malpighian layers present in all these structures take the form of stratified squamous epithelium, the basal cells of which are separated from the underlying connective tissue by a thin layer of fairly distinct reticular fibers (as can be seen, for instance, a t the limit between epidermis and derma, FIGURE 3). In areas of low biologic activity, such as the outer root sheath of the hair, the reticular fibers fuse to form a continuous basement membrane (FIGURE 4), whereas in areas of high activity, such as the Malpighian layer at the base of the hair follicle, reticular fibers are scarce or even absent (PIGURE S). These facts suggest a relationship between the thinness of the reticular layer and the intensity of the metabolic exchanges across such a layer from the subjacent connective tissue. The activity of the Malpighian layer is related to that of the subjacent tissue in many other ways. Thus it can be shown that the connective tissue beneathactively growing Malpighian layers, e.g., the hair papilla, is highly vascularized, contains many nuclei (FIGURE 7), and is rich in phosphatase' and metachromatic substance^,^ in contrast to the derma in general. The keratinized layers are formed by cells having a fairly homogeneous appearance and in which most cytological details are no longer recognizable. The keratin content is responsible for their low solubility and high resistance to many chemical agents. On the basis of histological appearance and chemical constitution, two types of keratin, namely, soft and hard, can be distinguished. Soft Keratin. The structures containing soft keratin, typically the epidermis, show a characteristic transition between the Malpighian and keratiniied layers. The cells in the upper part of the Malpighian contain irregular basophilic granules which more or less fill up the cytoplasm. This has led to the convention of subdividing the Malpighian layers into a