B. Follicle and Hair Structure
Hair follicles extend from the surface deep into the dermis or hypodermis. The follicle's broad base, or hair bulb, consists of a cap of rapidly dividing epithelial cells (the germinal matrix) overlying a dermal papilla that harbors the nerve and blood supply. Cells from the germinal matrix keratinize, forming the concentric layers of the hair shaft as they move toward the surface. Near the surface, distinct layers can be seen ensheathing the canal that contains the hair shaft. Integumentary system
HAIR
nHair occurs only in thin skin; its color, size, shape, and distribution vary according to race, age, sex, and body region. The structures in skin that form hairs and maintain their growth are called hair follicles.
nA. Follicle and Hair Development:
1. Follicles. Early in the third month of human development, local epidermal thickenings form at the sites of future hairs: first on the eyebrows, chin, and upper lip and then over the rest of the thin skin. Cells at the base of each thickening invade the dermis, and a small dermal papilla invades the leading edge of the epidermal downgrowth. Interactions between the papilla and the invaginating epidermis induce the differentiation of the hair follicle. Hair begins to form in the hair bulb at the base of the hair follicle as a result of the keratinization of the bulb's epithelial cells. These cells are pushed toward the surface by the mitosis in the germinal matrix (hair bulb epithelium). Some epithelial cells in the walls of the developing follicle divide, forming bulges that differentiate into sebaceous glands. integumentary system
2. Hairs. By the fifth or sixth month of gestation, the fetus is covered by fine hairs (lanugo). Just before birth, most of the lanugo is shed, except for the scalp, eyebrows, and eyelashes. A few months after birth, the remaining lanugo has been replaced by coarser mature terminal hairs; the rest of the body is covered with a coat of fine short hairs, called vellus. At puberty, coarse terminal hairs replace the vellus in specific body areas. In males, terminal hairs develop in the axilla and pubic region, on the face, and, to some extent, over the rest of the body. In females, they develop mainly in the axilla and pubic regions.
nA. Follicle and Hair Development:
1. Follicles. Early in the third month of human development, local epidermal thickenings form at the sites of future hairs: first on the eyebrows, chin, and upper lip and then over the rest of the thin skin. Cells at the base of each thickening invade the dermis, and a small dermal papilla invades the leading edge of the epidermal downgrowth. Interactions between the papilla and the invaginating epidermis induce the differentiation of the hair follicle. Hair begins to form in the hair bulb at the base of the hair follicle as a result of the keratinization of the bulb's epithelial cells. These cells are pushed toward the surface by the mitosis in the germinal matrix (hair bulb epithelium). Some epithelial cells in the walls of the developing follicle divide, forming bulges that differentiate into sebaceous glands. integumentary system
2. Hairs. By the fifth or sixth month of gestation, the fetus is covered by fine hairs (lanugo). Just before birth, most of the lanugo is shed, except for the scalp, eyebrows, and eyelashes. A few months after birth, the remaining lanugo has been replaced by coarser mature terminal hairs; the rest of the body is covered with a coat of fine short hairs, called vellus. At puberty, coarse terminal hairs replace the vellus in specific body areas. In males, terminal hairs develop in the axilla and pubic region, on the face, and, to some extent, over the rest of the body. In females, they develop mainly in the axilla and pubic regions.
DERMIS
nThe dermis, which contains the hair follicles (found only in thin skin) and sebaceous and sweat glands, consists of 2 layers of vascular connective tissue that blend at their common border.
nA. Papillary Layer: This layer of loose connective tissue, rich in elastic fibers, lies directly beneath the epidermal basement membrane. Its projections--dermal papillae-interdigitate with the epidermal ridges, increasing the area of contact. Special collagen fibers, anchoring fibrils, extend from this layer into the epidermal basal lamina to reinforce the dermal-epidermal junction. The papillary layer contains immunoprotective cells, a rich capillary net work, and abundant free nerve endings, some of which penetrate the epidermis. The tips of many dermal papillae contain encapsulated touch receptors called Meissner's corpuscles.
nB. Reticular Layer: Beneath the papillary layer is a thicker layer of dense irregular connective tissue. Also richly vascularized, this layer contains many arteriovenous anastomoses, or shunts, that control the amount of blood reaching the papillary capillaries and thus aid in regulating heat loss and blood pressure. The reticular layer also contains a rich supply of nerves in both free and encapsulated endings leg, Pacinian corpuscles).
nA. Papillary Layer: This layer of loose connective tissue, rich in elastic fibers, lies directly beneath the epidermal basement membrane. Its projections--dermal papillae-interdigitate with the epidermal ridges, increasing the area of contact. Special collagen fibers, anchoring fibrils, extend from this layer into the epidermal basal lamina to reinforce the dermal-epidermal junction. The papillary layer contains immunoprotective cells, a rich capillary net work, and abundant free nerve endings, some of which penetrate the epidermis. The tips of many dermal papillae contain encapsulated touch receptors called Meissner's corpuscles.
nB. Reticular Layer: Beneath the papillary layer is a thicker layer of dense irregular connective tissue. Also richly vascularized, this layer contains many arteriovenous anastomoses, or shunts, that control the amount of blood reaching the papillary capillaries and thus aid in regulating heat loss and blood pressure. The reticular layer also contains a rich supply of nerves in both free and encapsulated endings leg, Pacinian corpuscles).
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D. Merkel's cells
D. Merkel's cells: Scattered in the stratum basale, these cells are most numerous in thick skin. They resemble basal keratinocytes but have a clearer cytoplasm containing many small dense granules. Free nerve endings form a disklike expansion (Merkel's disk) that covers the basal surface of each Merkel's cell. This arrangement suggest that the cells function as sensory mechanoreceptors, but other evidence suggests that they may have DNES-related functions
C. Langerhans' cells
C. Langerhans' cells: These star-shaped cells lack tonofilaments and occur mainly in the stratum spinosum (400-1000 cells/mm2 of skin surface). They stain selectively with gold chloride and contain numerous rodlike or racket-shaped cytoplasmic granules (Birbeck's granules). They are thought to be antigen-presenting cells that process and present to the lymphocytes any antigenic material that penetrates the skin's surface. Of mesodermal origin, they arise in bone marrow and may belong to the mononuclear phagocyte system. Langerhans' cells also occur in oral and vaginal epithelia as well as in the thymus.
B. Pigmentation System
B. Pigmentation System: Skin color is conferred mainly by the pigments melanin and carotene, the thickness of the epidermis, the number of dermal blood vessels, and the color of the blood in those vessels.
1. Melanins contribute to skin, eye, and hair color. Synthesized by melanocytes, they include the dark brown pigment eumelanin, found in the epidermis, iris, and brown and black hair; and the cysteine-rich pigment pheomelanin, found in red hair. 2. Melanocytes derive from the neural crest and migrate into the epidermis during em bryogenesis. Although they are scattered among the keratinocytes of the stratum basale, they are not attached to them by desmosomes. They have round cell bodies, central nuclei, and long cytoplasmic processes that pass between the cells of the strata basale and spinosum and terminate in small indentations on the keratinocyte surfaces. Melanocytes make up 10-25% of this layer's cells but do not participate in keratinization.
1. Melanins contribute to skin, eye, and hair color. Synthesized by melanocytes, they include the dark brown pigment eumelanin, found in the epidermis, iris, and brown and black hair; and the cysteine-rich pigment pheomelanin, found in red hair. 2. Melanocytes derive from the neural crest and migrate into the epidermis during em bryogenesis. Although they are scattered among the keratinocytes of the stratum basale, they are not attached to them by desmosomes. They have round cell bodies, central nuclei, and long cytoplasmic processes that pass between the cells of the strata basale and spinosum and terminate in small indentations on the keratinocyte surfaces. Melanocytes make up 10-25% of this layer's cells but do not participate in keratinization.
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