Cutaneous Anatomy and Physiology

Anatomy

Three layers: the epidermis, the dermis and the subcutis with fascia.

Epidermis: germinative pool of keratinocytes (KC) and differentiating KC`s. Substructured into the basal membrane, stratum basale (dividing stem cells), stratum spinosum, stratum granulosum and stratum corneum. Melanocytes, Langerhans cells and Merkel cells.

Dermis: fibroblasts producing a rich extracellular matrix (collagen and elastic fibers), adnexal structure associated muscles, dermal dendritic cells, mast cells, Merkel cells, upper and lower vessel pool, apocrine and eccrine glands and different nerve types.

Subcutis: connective tissue and adipose tissue including vessels and immune surveillance cells.

Fascia

Skin glands: Eccrine sweat glands (palms and soles), apocrine sweat glands (axillae, areola and nipples, perianal region and genitalia) produce the sweat, sebaceous gland follicles, hair follicles with terminal hair and vellus hair follicles.

Thermal regulation

Skin vasculature plays a key role in thermal regulation. Glomus cells help to regulate the diameter of the vessels so that the higher or lower amount of blood can heat or cool down the skin. The eccrine glands produce sweat when stimulated by cholinergic nerves (stress reaction, temperature cooling down the skin), which cools the skin physically by evaporation. An anchoring muscle of the hair follicle (musculus arrector pili) can produce some heat by moving the follicle upward.

Physical protection

Epidermis (especially the stratum corneum): provides protection from evaporation of water and physical stress.

Dermis: protects the skin against tears and shear forces and provides elasticity.

Subcutis: fat tissue can absorb and distribute physical harm.

Protection against chemical injury

Stratum corneum: corneocytes (keratinocyte envelopes) and sebum help the skin to resist various kinds of chemical injuries and water.

Photoprotection

Melanocytes produce melanin and transfer them in vesicles to the surrounding keratinocytes. Melanin absorbs ultraviolet (UV)-radiation and thereby protect genetic material. KC produce ROS absorbers and repair enzymes against DNA damage.

Sensory function

The skin is one of the most important sensory organs: nerves and receptors such as nociceptors (pain and itch receptors) and Merkel cells (mechanoreceptors) are responsible for the touch.

Immune function

Most of the lymphocytes in the skin, other leukocytes, mast cells, and macrophages. Epidermis: Langerhans cells (antigen-presenting cells); keratinocytes (epithelial cells with immune properties), dendritic epidermal T lymphocytes, resident memory T cells that may serve as a primitive T-cell immune surveillance system; and immunogenic melanocytes. Dermis: most of the lymphocytes in the skin, other leukocytes, dermal dendritic cells, mast cells and macrophages.

Subcutis: immune surveillance reservoir and progenitor cells for wound healing.

9.1.1 Cutaneous Anatomy and Physiology