Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Integumentary System
Skin Appendages

Hair and Nails are classified as Skin derivatives and are primarily of ectodermal origin.

Hairs are keratinous appendages of the skin. They are developed to varying degrees in areas covered with thin skin and are completely absent from the palms and soles.

A hair consists of a central medulla composed of soft keratin, a cuticle, and a cortex composed of hard keratin (Fig. 5.3). The pigment that determines hair color is located within the cortex. The cuticle is formed by thin scales whose free edges point upward (Fig. 5.4). These scales interlock with the oppositely directed, downward-pointing scales of the ROOT sheath, thereby securing the hair in place. The epithelium of the hair follicle is surrounded by a Connective Tissue sheath known as the dermal root sheath (or hair sac). Arrector pili Muscles attach to these hair sacs; these smooth muscles are innervated by the Autonomic Nervous system. By contracting during stress or hypothermia, these muscles cause the hair to stand upright (piloerection). Furthermore, this contraction squeezes the secretion of the Sebaceous Glands—located between the Muscle and the hair sac—into the hair follicle and subsequently onto the skin surface. Muscle contraction also produces "goosebumps" by pulling the areas of skin situated directly above their attachment sites (near the papillary layer of the dermis) inward.

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Fig. 5.3. Structure of a hair:

A — longitudinal section and B — cross section:

1 — sebaceous gland; 2 — arrector pili muscle; 3 — internal root sheath; 4 — matrix; 5 — connective tissue papilla; 6 — hair follicle (hair sac); 7 — medulla; 8 — cortex; 9 — hair cuticle; 10 — external root sheath; 11 — epidermis

Fig. 5.4. Hair (scanning electron micrograph): 1 — hair; 2 — skin epidermis

Nails are specialized areas of modified cornified epidermal layer. Like hair, they grow continuously. Nails are homologous to animal claws and are unique to primates and humans. They serve to protect the highly sensitive fingertips. Thanks to nails, the tips of human fingers receive underlying soft-tissue support and acquire The ability to grasp and firmly hold small objects, which is essential for delicate manual labor.

Hair development begins during the third month of Embryogenesis. Epithelial Cell cords begin to invaginate into the dermis, giving rise to hair follicles (Fig. 5.5). The deepest part of the cord transforms into the germinative matrix (bulb) of the hair follicle and initiates hair formation. A connective tissue papilla containing Blood capillaries grows into the matrix. A lumen appears within the epithelial cord itself, converting it into the external root sheath. Like the skin surface, this sheath is lined with Stratified Epithelium, but its layer becomes thinner deep within the follicle. At the Base of the follicle, the external root sheath surrounds and merges with the hair matrix. Cell Division and proliferation occur near the connective tissue papilla, and the outermost Cells migrate upward through the lumen of the external root sheath. Gradually moving away from the papilla—which serves as their nutritional source—they transform into keratin within the keratogenous zone. Hair keratin differs in composition from the soft keratin covering the skin surface and is referred to as hard keratin. The cuticle and cortex of the hair are built from hard keratin. Some of the proliferating cells differentiate into the internal root sheath and subsequently into soft keratin, which surrounds the deep portion of the hair. Melanocytes are interspersed among the matrix cells to provide hair pigmentation. In old age, these melanocytes disappear, and air bubbles appear within the hair structure, giving it a white color.

By the fifth to sixth month, the fetal skin is covered with fine lanugo hairs, which by the time of birth persist and thicken only in the Regions of the eyelashes, eyebrows, and scalp. No new hair follicles are formed after birth. Hair growth is hormonally regulated and intensifies significantly during Puberty, particularly in males.

Fig. 5.5. Diagram of successive stages (I–IV) of hair follicle development (after Ham and Cormack):

1 — epidermis; 2 — dermis; 3 — hair; 4 — developing sebaceous gland; 5 — external root sheath; 6 — hair follicle bulb; 7 — connective tissue papilla

Nails begin to develop toward the end of the third month of prenatal life. The epithelium covering the dorsal surface of the distal phalanx of each digit invaginates along a curved line lying at a slight angle to the surface. The resulting plate soon splits, forming the nail groove. Deep within this groove, cells proliferate intensively and transform into the nail matrix (Fig. 5.6). Matrix cells proliferate, and the upper ones transform into nail substance—hard keratin. The forming nail is pushed out of the groove and grows toward the distal end of the phalanx while remaining firmly attached to the matrix. The epithelium over which the nail slides lacks a stratum corneum and is called the nail bed. Along each side of the nail, the skin forms a lateral nail groove, while at its proximal border, it overlaps the nail and fuses with it to form the eponychium. Beneath the nail bed, the dermis forms ridges and contains numerous Blood Vessels that show through the nail, giving it a pink hue.

Fig. 5.6. Structure of a nail:

A — external view; B — cross section and C — longitudinal section: 1 — distal border of the nail; 2 — lateral nail groove; 3 — eponychium; 4 — proximal border of the nail; 5 — lunula; 6 — nail plate; 7 — nail matrix