Skin Structure Skin is made of 3 layers: 1)epidermis –stratified squamous epithelium cells –constantly replaced from inside out –entirely from ectoderm 2)dermis –“true skin” 3)hypodermis – subcutaneous layer –last to develop –immigration of cells from other body regions

Epidermis squamous cells stacked sequentially by levels of maturity older cells on the outside innermost cells are metabolically active and constantly undergoing mitosis = stratum basale

The stratum basale and any newly dividing cells immediately above the stratum basale = stratum germinativum stratum – layer basale – innermost portion germinativum – part that produces other layers

The cells of the epidermis start out from the germinativum as cuboidal or columnar cells, then flattened as they migrate to the surface. it takes days for cells to reach the outermost skin from the germinativum mitosis in the germinativum occurs 2 ways:

1)cytokinesis in the horizontal plane → an outer and inner cell –new skin layers –stimulated by vitamin A –accelerated by continuous pressure or rubbing 2)cytokinesis in the vertical plane → growth of skin –response to growth and stretching

Cells of the stratum germinativum are located on ridges called dermal papillae These projections of the underlying dermis are bound to the epidermis cells by a thin layer of fibrous connective tissue = basement layer

Interspersed in and on top of the stratum germinativum are the melanocytes in a layer called the Malpighian layer These cells secrete melanin – a black to brown colored chemical made of amino acid building blocks Some people take the amino acid tyrosine in an attempt to enhance melanin production during tanning no proof of effectiveness

As germinativum cells move up they form the next layer called the stratum spinosum (prickly layer) many desmosomes on the outer surface of these cells desmosomes mixed in with these cells are Langerhans cells Langerhans cells Langerhans cells

Stratum spinosum cells give rise to the next layer of epidermis called the stratum granulosum –named for granules in the cytoplasm accumulation of yellowish sulfur-rich protein = keratin cells = keratocytes keratin tough protein gives strength to skin

Keratocytes produce a substance that acts as a waterproofing to prevent the free flow of water into and out of the skin

Stratum granulosum cells flatten out into a layer of waterproof cells = stratum compactum –dying cells filled with keratin In thick skin areas, stratum compactum → stratum lucidum –breakable layer

The uppermost layer of skin = stratum corneum flattened dead cells shed during desquamation

Dermis thick layer of connective tissue attached to the stratum germinativum majority of dermis is dense, irregular connective issue the dermis also contains loose connective connective tissue called areolar connective tissue

areolar connective tissue used throughout the body to bind blood vessels, membranes, muscles, nerves, and skin to other structures extensive meshwork of protein fibers secreted by fibroblasts fibropblasts body growth connective tissue maintenance wound healing

Subcutaneous Layer inner most layer of skin thickness and makeup varies throughout the body and form person to person very thin or absent in the areas of the: –eyelids –penis –scrotum –nipples

The subcutaneous layer is responsible for increases in female hips and breasts after puberty The subcutaneous layer differs from the dermis in many ways

1)composed of loosely arranged elastic fibers that anchor the skin to underlying fascia fascia –sometimes the subcutaneous layer is directly attached to bone and joint surfaces 2)contains large amounts of adipose tissue (fat cells) –thickness of the adipose tissue = indirect indicator of body fat 3)contains large blood vessels that branch out into capillaries into the dermis

lymph vessels are found along with the blood vessels (part of the immune system) the subcutaneous layer is still susceptible to infection = fasciitis fasciitis deep nerves are located in the subcutaneous layer

desmosome – cellular junction

Langerhan cells – immune system cells that are important in fighting skin infections and healing injured skin

fascia – a sheet of fibrous tissue that covers muscle, skull bones, and some organs

fasciitis – an irritation of the subcutaneous layer; can also result from injuries that leave bone fragments or other material in skin that is prone to pressure and stretching