6.3: Secondary Structures of the Skin
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Secondary Structures of the Skin
Above: Illustrated diagram of the skin and underlying hypodermis with all of its associated secondary structures including hair, sweat glands, and sebaceous glands.
Hair is a keratinous filament growing out of the epidermis. It is primarily made of dead, keratinized cells. Strands of hair originate in an epidermal penetration of the dermis called the hair follicle. The hair shaft is the part of the hair not anchored to the follicle, and much of this is exposed at the skin’s surface. The rest of the hair, which is anchored in the follicle, lies below the surface of the skin and is referred to as the hair root. The hair root ends deep in the dermis at the hair bulb, and includes a layer of mitotically active basal cells called the hair matrix. The hair bulb surrounds the hair papilla, which is made of connective tissue and contains blood capillaries and nerve endings from the dermis.
Above: Illustration of a hair and associated structures and tissues.
Hair fibers have a structure consisting of several layers, starting from the outside:
- cuticle: outer covering, which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles
- cortex: which contains the keratin bundles in cell structures that remain roughly rod-like. The highly structural and organized cortex, or second of three layers of the hair, is the primary source of mechanical strength and water uptake. The cortex contains melanin, which colors the fiber based on the number, distribution and types of melanin granules. The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. People with straight hair have round hair fibers. Oval and other shaped fibers are generally more wavy or curly.
- medulla: a disorganized and open area at the fiber's center. The innermost region of the hair.