The integument or skin is the largest organ in the body. It is composed of two layers and contains many different types of appendages, including hair, nails, and several kinds of glands. The layers of the skin include: (1) an outer layer of stratified squamous keratinized epithelium, the epidermis and (2) an inner layer of dense irregular connective tissue, the dermis. These two layers sit upon the subcutaneous tissue, which is composed of loose connective tissue, adipose tissue, and dense connective tissue (Skin). Skin is classified into two types based on the thickness of the epidermis and keratin layer. Thick skin is found on the palms of the hands and soles of the feet (Thick Skin). Thin skin covers the rest of the body (Thin Skin).
The epidermis of thick skin follows the contours of the dermal ridges, producing the epidermal ridges of the fingerprint. The dermal ridges penetrate into the epidermis as true papillae, and are separated by epithelial downgrowths called interpapillary pegs (Thick Skin 1). Five layers of cells or cell products are found in the epidermis: (1) stratum germinativum, columnar basal stem cells; (2) stratum spinosum, polyhedral cells with "spiny" projections: (3) stratum granulosum, diamond shaped cells containing keratohyalin granules; (4) stratum lucidum, a clear, homogenous line composed of eleidin, a keratohyalin transformation product (not always seen); and (5) stratum corneum, the keratin filled squames (Thick Skin 2).
The dermis is composed of two layers: (1) the papillary dermis closest to the epithelium, is composed of less dense connective tissue and is vascularized with capillary networks penetrating the papillae and (2) the underlying reticular dermis composed of avascular, dense irregular connective tissue (Dermis).
Beneath the dermis, a layer composed of adipose and loose/dense connective tissues make of the subcutaneous layer. Numerous structures are found in this layer. The secretory portion of the eccrine sweat glands are found here, with their ducts penetrating the dermis to enter the epidermis through the interpapillary pegs (Subcutaneum). Also, sensory structures (pacinian corpuscles), nerve bundles, blood vessels, and the bases of hair follicles are found in the subcutaneous layer.
The epidermis differs from that of thick skin in having thinner stratum spinosum, granulosum, and corneum, and lacks the stratum lucidum (Thin Skin 1). The dermis is not arranged in ridges, but does project into the epidermis as true papillae. However, no epidermal ridges are produced (Thin Skin 2). The pigment of the skin is produced by melanocytes, which take up residence in the basal layer (stratum germinativum) and produce melanin or pigment granules (Melanocytes).
Hair follicles are formed when the epidermis grows down into the dermis. The deepest portion of the hair follicle is composed only of stratum germinativum cells and is called the germinal matrix of the follicle. It sits upon a papillae of connective tissue. It is from this portion that the hair shaft is produced. The epidermal down-growth between the germinal matrix and the skin surface, forms a canal, the external root sheath (Hair 1). Several sebaceous glands are found associated with the hair follicle, usually along the upper one-third of the external root sheath. Also in this area are the arrector pili muscles (Hair 2). The sebaceous gland cells secrete via rupture, releasing oil. Thus the gland maintains a stem cell population and a gradient from immature to mature, oil filled cells is observed (Hair 3). The growth of the hair shaft from the germinal matrix begins by forming the layers of keratin producing cells called the internal root sheath (Hair 4). The hair shaft, composed of three keratinized layers travels within the external root sheath towards the surface of the skin (Hair 5).
