Anatomy & Physiology

Anatomy & Physiology Study Guide

Stratum Corneum At the exposed surface of both thick skin and thin skin is the stratum corneum. It contains 15 to 30 layers of keratinized cells. Desmosomes tightly connect the dead cells in this layer. The connections are so secure that these cells are shed sheets or groups rather than individually. It takes 15 to 30 days for a cell to move from the stratum germinativum to the stratum corneum. The cells remain for an additional two weeks before they are shed or washed away. This arrangement places the deeper portions of the epithelium beneath a protective layer of dead cells. The stratum corneum is water resistant but not waterproof. Water from interstitial fluids slowly penetrates the surface, to be evaporated into the surrounding air. The human body will lose roughly about one pint of water in this way each day. The process is called insensible perspiration because you are unable to see or feel the water loss. In contrast, you are usually very aware of the sensible perspiration produced by active sweat glands. The Dermis The dermis is called the “true skin” because it contains blood vessels and nerves that support the skin layers. It lies between the epidermis and the subcutaneous layer. The dermis has two major components: a superficial papillary layer and a deeper reticular layer. The papillary layer contains the capillaries, lymphatics, and sensory neurons that supply the surface of the skin. The reticular layer consists of dense irregular connective tissue containing both collagen and elastic fibers. Both the reticular and papillary layers of the dermis contain networks of blood vessels, lymphatic vessels, and nerve fibers. Arteries to the skin form the cutaneous plexus and the papillary plexus in the hypodermis and the papillary dermis, respectively. Skin is organized such that its collagen and elastic fiber arrangement will resist forces applied from normal movement. The resulting pattern of fiber bundles establishes lines of cleavage of the skin. Lines of cleavage are of clinical importance: a cut parallel to a cleavage line will usually remain closed and heal with little scarring, whereas a cut at right angles to a cleavage line will be pulled open as severed elastic fibers recoil and will result in greater scarring. For these reasons, surgeons choose to make clean incisions parallel to the lines of cleavage. The Hypodermis The hypodermis is the tissue beneath the dermis that connects it to underlying tissues. The connective tissue fibers of the reticular layer are extensively interwoven with those of the hypodermis, or subcutaneous layer. The boundary between the two is indistinct. Although the hypodermis is not part of the integument, it is important in stabilizing the position of the skin in relation to underlying tissues, while permitting independent movement. The hypodermis consists of areolar and adipose tissues and is quite elastic.

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