Integumentary system

Integumentary system The skin, also referred to as the cutaneous membrane covers the external surface of the body and is the largest In adults, the skin covers an area of about 2 square meters (22 square feet) weighs 4.5–5 kg (10–11 lb), about 7 percent of total body weight

The superficial, thinner portion, which is composed of epithelial tissue, is the epidermis (ep-i-DERM-is; epi- above;Dermis skin). It is avascular. For this reason, if you scratch the epidermis there is no bleeding. The deeper, thicker, connective tissue portion is the dermis. It is vascular and a cut in the dermis produces bleeding. Deep to the dermis, but not part of the skin, is the Subcutaneous layer, or hypodermis (hypo-below), which consists of areolar and adipose tissues. The skin and subcutaneous layer form the integument.

The epidermis is composed of keratinized stratified squamous epithelium It contains four principal types of cells: keratinocytes, melanocytes, Langerhans cells, and Merkel Melanin (MEL-a-nin) is a yellow-red or brown-black pigment that contributes to skin color and absorbs damaging ultraviolet (UV) light. Langerhans cells (LANG-er-hans), or intraepidermal macrophage cells, arise from red bone marrow and migrate to the epidermis

They participate in immune responses mounted against microbes that invade the skin. Merkel cells (with their associated tactile discs) are scattered among the keratinocytes of the basal layer The Merkel cells (MER-kel), or tactile epithelial cells, are the least numerous of the epidermal cells. Merkel cells are located in the deepest layer of the epidermis, where they contact the flattened process of a sensory neuron (nerve cell), a structure called a tactile (Merkel) disc Merkel cells and tactile discs detect touch sensations

Several distinct layers of keratinocytes in various stages of development form the epidermis In most regions of the body the epidermis has four strata or layers— stratum basale, stratum spinosum, stratum granulosum, and a thin stratum corneum. This is called thin skin

Structure of the Skin 1. The integumentary system consists of organs such as the skin and hair, and other structures such as nails. 2. The skin is the largest organ of the body in surface area and weight. The principal parts of the skin are the epidermis (superficial) and dermis (deep). 3. The subcutaneous layer (hypodermis) is deep to the dermis and not part of the skin. It anchors the dermis to underlying tissues and organs .

5. The epidermal layers, from deepest to most superficial, are the stratum basale (undergoes cell division and produces all other layers) stratum spinosum (provides strength and flexibility) stratum granulosum(contains keratin and lamellar granules) stratum lucidum (present only in palms and soles), and stratum corneum (sloughs off dead skin) Stem cells in the stratum basale undergo continuous cell division, producing keratinocytes for the other layers.

Epidermal ridges provide the basis for fingerprints and footprints. The color of skin is due to melanin, carotene, and hemoglobin. In tattooing, a pigment is deposited with a needle in the dermis. Body piercing is the insertion of jewelry through an artificial opening The dermis consists of dense irregular connective tissue containing collagen and elastic fibers. It is divided into papillary and reticular regions. The papillary region contains thin collagen and fine elastic fibers, dermal papillae, and Meissner corpuscles. The reticular region contains bundles of thick collagen and some coarse elastic fibers, adipose tissue, hair follicles, nerves, sebaceous (oil) glands, and ducts of sudoriferous (sweat) glands.

Accessory structures of the skin—hair, skin glands, and nails—develop from the embryonic epidermis. Associated with each hair follicle is a sebaceous oil gland, an arrector pili muscle, and a hair root plexus. New hairs develop from division of hair matrix cells in the bulb; hair replacement and growth occur in a cyclic pattern consisting of growth, regression, and resting stages. Hairs offer a limited amount of protection—from the sun, heat loss, and entry of foreign particles into the eyes, nose, and ears. They also function in sensing light touch.

Sebaceous (oil) glands are usually connected to hair follicles; they are absent from the palms and soles. Sebaceous glands produce sebum, which moistens hairs and waterproofs the skin. Clogged sebaceous glands may produce acne. There are two types of sudoriferous (sweat) glands: eccrine and apocrine. Eccrine sweat glands have an extensive distribution; their ducts terminate at pores at the surface of the epidermis. Eccrine sweat glands are involved in thermoregulation and waste removal and are stimulated during emotional stress. Apocrine sweat glands are limited to the skin of the axillae, groin, and areolae; their ducts open into hair follicles.

Ceruminous glands are modified sudoriferous glands that secrete cerumen (ear wax). They are found in the external auditory canal (ear canal). Nails are hard, dead, keratinized epidermal cells over the dorsal surfaces of the distal portions of thedigits. Cell division of the matrix cells produces new nails.