The skin, or integument, consists of two main layers: the epidermis, composed of stratified squamous epithelium that is keratinized, and the dermis, composed of connective tissue. Under the dermis is a layer of loose connective tissue called the hypodermis, which is also generally referred to as the subcutaneous tissue or, by gross anatomists, as the superficial fascia. Typically, the hypodermis contains large amounts of adipose tissue, particularly in an adequately nourished individual. The epidermis gives rise to nails, hairs, sebaceous glands, and sweat glands. On the palms of the hands and soles of the feet, the epider-mis has an outer keratinized layer that is substantially thicker than that over the other parts of the body. Accordingly, the skin over the palms and soles is referred to as thick skin, in contrast to the skin over other parts of the body, which is referred to as thin skin. There are no hairs in thick skin. In addition, the interface between the epidermis and the dermis is more complex in thick skin than in thin skin. The fingerlike projections of the dermis into the base of the epidermis, the dermal papillae, are much longer and more closely spaced in thick skin. This provides greater resistance to frictional forces acting on this skin.
Thick skin, human, H&E ×45.
In this sample of thick skin, the epidermis (Ep) is at the top; the remainder of the field consists of dermis, in which a large number of sweat glands (SW) can be observed. Although the layers of the epidermis are examined more advantageously at higher magnification, it is easy to see, even at this relatively low magnification, that about half of the thickness of the epidermis consists of a distinctive surface layer that stains more lightly than the remainder of the epidermis. This is the keratinized layer. The dome-shaped surface contours represent a cross section through the minute ridges on the surface of thick skin that produce the characteristic fingerprints of an individual.
In addition to sweat glands, the dermis displays blood vessels (BV) and adipose tissue (AT). The ducts of the sweat glands (D) extend from the glands to the epidermis. One of the ducts is shown as it enters the epidermis at the bottom of an epithelial ridge. It will pass through the epidermis in a spiral course to open onto the skin surface.
Thin skin, human, H&E ×60. Often, as in this tissue sample, the hair follicles and the glands, both sebaceous and sweat, extend beyond the dermis (De) and into the hypodermis.
A sample of thin skin is shown here to compare with the thick Note the blood vessels (BV) and adipose tissue (AT) in the hypodermis.
skin in the above figure. In addition to sweat glands, thin skin contains hair follicles (HF) and their associated sebaceous glands (SGI). Each sebaceous gland opens into a hair follicle.
Epidermis, skin, human, H&E ×320; inset ×640.