The commonly seen skin appendages are:
- Hair follicles
- Sebaceous glands
- Apocrine sweat glands
- Eccrine sweat glands
Hair follicles are found in most regions of skin, except the glans penis/clitoris, labia minora, lip and palms/soles. The density and size of hairs is highly variable, as is their orientation in the skin:
- Asiatic hair follicles are arranged perpendicular to the skin surface
- Caucasian hair follicles are arranged obliquely to the skin surface
- African hair follicles are arranged almost parallel to the skin surface
Hair is generated by specialised keratinocytes, and the hair bulb has regions analogous to the stratum basale and stratum spinosum of the epidermis. The hair bulb is richly supplied by capillaries and nerves. As the keratinocytes ascend, they become fully keratinised although this process differs somewhat to the differentiation seen in the epidermis. This alteration is important for the different structure and appearance of hair compared to skin. Hair colour is produced by melanin granules deposited as cells differentiate by nearby melanocytes.
Sebaceous glands are holocrine; the cells themselves are secreted after undergoing autophagy. Sebaceous glands are found in all areas of the body except the glans; they are in highest quantity in the face. They are usually associated with hair follicles; in regions with no hair they empty directly on the skin surface. The gland is of an acinar type, and the basal layer of cells continuously divides. These cells gradually pass further from the basement membrane, fill with fat droplets and eventually lose their nuclei and cell membrane.
Eccrine Sweat Glands
Eccrine sweat glands are found in nearly all body regions and are important in thermoregulation. They contain a coiled secretory portion and a long, spiraled duct. The secretory portion contains a columnar epithelium with surrounding myoepithelial cells. The columnar cells are mostly 'clear cells' and transport fluid from interstitial fluid into the lumen of the gland. The ducts are lined by bilayered columnar epithelium which is important in the re-absorption of sodium.
Apocrine Sweat Glands
Apocrine glands are only found in the axilla and perineal regions. The secretory lumen is much larger and is lined by pink cuboidal cells with abundant granules. The ducts are similar to eccrine glands. Apocrine glands typically empty into hair follicles.
Nails are a complex keratinised structure also formed by keratinocytes. The nail plate (hard portion covering the distal ends of digits) is generated by keratinocyte differentiation in the nail matrix, which is covered by a small fold of thin skin. The nail plate lies on the nail bed, a variant of the epidermis that only consists of a stratum basale and stratum spinosum. The nail plate is continuously pushed distally by the proliferation of cells in the nail matrix.