The Integumentary System
The integumentary system is comprised of skin, the largest organ in the body by both weight and surface area. It is mainly responsible for providing protection against a wide variety of external threats, moisture control, and thermoregulation. Moreover, the skin is considered to be the largest sensory organ, containing receptors for touch, pressure, pain, and temperature. Skin derivatives include hair, nails, and several types of sebaceous and sweat glands.
The skin (also known as the integument, or the cutaneous layer) consists of three distinct regions: the epidermis, the dermis, and the hypodermis.
The superficial epidermis is an avascular epithelial layer of ectodermal origin. It is lined mainly by keratinized stratified squamous epithelium, composed of cells called keratinocytes. Other epidermal cell types include the pigment producing melanocytes, the antigen presenting Langerhans (dendritic) cells, and the tactile Merkel cells.
The thickness of the epidermal layer distinguishes between the thick skin found on palms and soles, and the thin skin found on the rest of the body.
The inferior dermis is a vascular epithelial layer of mesodermal connective tissue that’s separated from the epidermis by a distinct layer of basement membrane. Its superficial surface is irregular and has many projections of dermal papillae that interlock with the epidermal ridges. This region of skin is known as the papillary layer of the dermis.
The deeper layer of the dermis is called the reticular layer. It is thicker, characterized by the dense irregular tissue of type I collagen fibers, and has less cells than the papillary layer. There’s no distinct layer separating the reticular and papillary layers of the dermis.
Meissner’s (tactile) corpuscles are located closer to the skin surface of the dermal papillae. They are responsible for detecting light touch. Pacinian (lamellated) corpuscles on the other hand are located deep in the connective tissue of the dermis, and are responsible for detecting coarse touch, pressure, and vibrations.
Beneath the dermis lies the hypodermis or subcutaneous layer, a layer of loose connective tissue and adipocytes. It contains a lot of blood vessels, lymph vessels, and nerves, as well as numerous sensory receptors. The hypodermis corresponds to the superficial fascia seen in gross anatomy.
Hair is an elongated keratinized structure derived from invaginations in the epidermal epithelium called hair follicles. All skin has at least some amount of hair except in the palms, soles, lips, glans penis, clitoris, and labia minora.
Hair follicles have a terminal dilatation termed the hair bulb. The highly vascularized papillary layer of the dermis inserts into the base of the hair bulb and sustains the hair follicle. Hair color is produced by the activity of melanocytes present between the papilla and the epithelial cells of the hair root.
Nails are hard flexible plates of keratin present on the dorsal surface of each distal phalanx. Nail plates arise from the nail matrix extending from the proximal region of the nail, the nail root, and rest on nail beds formed of stratified squamous epithelium. The distal end of the nail plate is free of nail beds, and is usually worn away or cut off.
Sebaceous glands are branched acinar glands embedded in the dermis over most areas of the body, except for the thick skin of palms and soles. Several acini converge to form a short duct that usually empties in the upper portion of hair follicles. In hairless regions, sebaceous ducts open directly in the epidermal surface.
Acini are comprised of a basal layer of flattened undifferentiated epithelial cells present on the basal lamina. Sebocytes grow, accumulate secretions, and disintegrate, releasing an oily product called sebum, which contains a complex mixture of lipids. Sebaceous gland secretions greatly increase during puberty.
Sweat glands are epithelial derivatives embedded in the dermis throughout the body. There are two types: eccrine and apocrine.
Eccrine sweat glands occur everywhere in the skin, particularly on the palms, soles, forehead, and axillae. They assist in thermoregulation by producing sweat and opening onto the skin surface. Eccrine glands can be divided into a secretory and duct portion. The secretory portion is comprised of three types of cells: clear cells that produce sweat, dark cells that produce mucous, and myoepithelial cells that discharge secretions into the duct.
Apocrine sweat glands are confined to the axillary and perineal skin regions. They have similar architecture to eccrine glands, but with a much larger lumen. Apocrine glands have a possible role in producing odor.