Structure of the Skin Epidermal growth and repair Turnover or regeneration time refers to time required for epidermal cells to form in the stratum basale and migrate to the skin surface—about 35 days Shortened turnover time will increase the thickness of the stratum corneum and result in callus formation Normally 10% to 12% of all cells in stratum basale enter mitosis daily Each group of 8 to 10 basal cells in mitosis with their vertical columns of migrating keratinocytes is called an epidermal proliferating unit, or EPU

Structure of the Skin Dermal-epidermal junction A definite basement membrane, specialized fibrous elements, and a polysaccharide gel serve to “glue” the epidermis to the dermis below Functions of the junction “Glue” the two layers together Provide mechanical support for the epidermis Partial barrier to the passage of some cells and large molecules

Structure of the Skin Dermis (corium) Sometimes called “true skin” Much thicker than the epidermis and lies beneath it Gives strength to the skin Serves as a reservoir area for storage of water and electrolytes Dermis on the ventral surface and over the appendages is generally thinner than on the dorsal surface Somatic sensory receptors process information such as pain, pressure, touch, and temperature

Structure of the Skin Dermis (cont.) Contains various structures: Arrector pili muscles and hair follicles (Figure 6-1) Sensory receptors (Figure 6-3) Sweat and sebaceous glands Blood vessels Rich vascular supply plays a critical role in temperature regulation

Structure of the Skin Dermis (cont.) Papillary layer Composed of dermal papillae that project into the epidermis Contains fine collagenous and elastic fibers Contains the dermal-epidermal junction Forms a unique pattern that gives individual fingerprints

Structure of the Skin Dermis (cont.) Reticular layer Contains dense, interlacing white collagenous fibers and elastic fibers to make the skin tough yet stretchable When processed from animal skin, produces leather

Structure of the Skin Dermal growth and repair The dermis does not continually shed and regenerate itself as does the epidermis Wound healing Fibroblasts begin forming an unusually dense mass of new connective fibers If not replaced by normal tissue, this mass remains a scar Cleavage lines (Figure 6-4) Patterns formed by the collagenous fibers of the reticular layer of the dermis Also called Langer’s lines

Structure of the Skin Hypodermis Also called subcutaneous layer or superficial fascia Deep to the dermis, forming connection between the skin and other structures Not part of the skin

Skin Cancer Three major types of skin cancer Basal cell carcinoma Most common Begins in stratum basale Rarely metastasize Squamous cell carcinoma Also arises in epidermis Sun-exposed areas Not likely to mestastisize Malignant melanoma Most deadly Sometimes develops from a pigmented nevus (mole) ABCD rule: assymetry, border, color, diameter

Quick Check What is the name of the gluelike layer separating the dermis from the epidermis? Which layer of the dermis forms the bumps that produce ridges on the palms and toes? Which layer is vascular: the epidermis or dermis?