Anatomy of the Skin (An Introduction)

Anatomy of the Skin An Introduction

Skin is an integumentary system at the interface between the human organism and its environment. The boundary limits of skin are found at its transition to mucosal surfaces of the respiratory, alimentary, and urogenital systems; at the conjunctival epithelium of the eye; at the ductal epithelium of the lacrimal and mammary ducts; and at the tympanic membrane of the ear.

Anatomically, skin is organized into an outer layer of epidermis covering a deeper layer of dermis, which is further subdivided into papillary dermis and reticular dermis. The epidermal epithelium gives rise during fetal development to the skin appendages, namely the hair follicles and associated sebaceous glands (pilosebaceous units), eccrine and apocrine sweat glands, and nails. Beneath the dermis is the hypodermis or subcutaneous fat layer (the panniculus adiposus). Connections between skin and its underlying hypodermis include ligaments, nerves, blood vessels, and lymphatic vessels.

The gross appearance of skin varies between individuals, within an individual by anatomic location at any point in time and within an individual over a lifetime. Variation is found with respect to texture, tone, distribution and amount of pigmentation, and expression of hair. For example, scrotal skin is very thin, with readily visible hair roots, specialized sebaceous glands, and few or no elastic fibers, but considerably greater laxity compared with skin covering the trunk. By contrast, the glabrous skin of the palms and soles is tightly fixed to the underlying fascia, lacks hair follicles and sebaceous glands, and contains sweat pores opening into ostia located along prominent friction ridges. The epidermis is considerably thicker.

The overall thickness of skin across the body is determined by the thickness of the several layers of epidermis and the thickness of the underlying dermis. Typical thickness in a young adult ranges from the eyelids (epidermis ~50 μm, dermis ~1,000 μm) to the back (epidermis ~40 μm, dermis ~5,000 μm), to the palms of the hands (epidermis ~600 μm). The thick- ness of the outermost cornified layer (the stra- tum corneum) is greatest in areas of callous but also varies among flexor forearm, thigh, back and abdomen across a range of ~13μm (flexor forearm) to ~8μm (abdomen). Published values for these measurements differ considerably, likely as a result of differences in location of sampling, methods of visualization, and methods of preservation in the case of biopsy material.

The structure of the skin is established through the orchestrated arrival, arrangement, and differentiation of a broad array of cell lineages during embryogenesis and fetal development. When it is fully developed, approximately 20 different types of cells are found in association with a complex extracellular matrix that provides both strength and flexibility.

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