Anatomy & Physiology

Anatomy & Physiology Study Guide The center of the hair matrix possesses basal cells that divide, producing daughter cells that are pushed toward the surface. Daughter cells closest to the center of the matrix form the medulla, or core, of the hair. Daughter cells farther from the center of the hair matrix form the cortex, an intermediate layer. Those at the edges of the hair matrix form the cuticle, which will be the surface of the hair. As cell divisions continue at the hair matrix, the daughter cells are pushed toward the surface of the skin, and the hair gets longer. The medulla contains a flexible soft keratin; the cortex and cuticle contain thick layers of hard keratin, which gives the hair its stiffness. The cuticle consists of overlapping layers of dead, flattened heavily keratinized cells. The epithelial cells of the follicle walls are organized into several concentric layers: the internal root sheath, the external root sheath, and the glassy membrane. The internal root sheath surrounds the hair root and the deeper portion of the shaft. It is produced by the cells at the periphery of the hair matrix. The external root sheath extends from the skin surface to the hair matrix. The glassy membrane is a thickened clear layer wrapped in a dense connective tissue sheath. Types of Hairs Hair first appears after approximately three months of embryonic development. These hairs, collectively known as lanugo, are extremely fine and unpigmented. Most lanugo hairs shed before birth. They are replaced by one of two types of hairs in the adult integument: vellus hairs or terminal hairs. Vellus hairs are the fine “peach fuzz” hairs located over much of the body surface. Terminal hairs are heavy, more deeply pigmented, and sometimes curly. The hairs of the head, including the eyebrows and eyelashes, are terminal hairs that are present throughout life. Hair C lor Variations in hair color reflect differences in the pigment produced by melanocytes at the hair papilla. Different forms of melanin give a dark brown, yellow-brown, or red color to the hair. These structural and biochemical characteristics are genetically determined. Hormonal and environmental factors influence the condition hair. As pigment production decreases with age, hair color lightens. White hair results from the combination of a lack of pigment and the presence of air bubbles in the medulla of the hair shaft. Sebaceous glands and sweat glands are exocrine glands found in the skin. Nails Keratinized epidermal cells protect the tips of fingers and toes by forming nails. The nail body, the visible portion of the nail, covers an area of epidermis called the nail bed. The nail body is bound on either side by lateral nail grooves and lateral nail folds and sits recessed deep to the surrounding epithelium. Nail production occurs at the nail root, an epidermal fold not visible from the surface. The deepest portion of the nail root lies very close to the bone of the fingertip. A portion of the stratum corneum of the nail root extends over the exposed nail and forms the cuticle. The characteristic pink color of the nail comes from underlying blood vessels. Near the root, a pale crescent known as the lunula is sometimes seen due to vessels being obscured.

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