NAILS

nThese plates of highly keratinized cells are analogous to, but harder than, the stratum corneum.
A. Nail Development: The formation of the nails is similar to that of hair, but involves produc ing plates rather than cylinders. At the end of the third month of embryonic development, a narrow plate of epidermis on the dorsal surface of the terminal phalanges invades the underlying dermis of each finger and toe. This invasion continues proximally, forming a furrow called the nail groove. Epithelial cells beneath the groove proliferate to form the nail matrix, whose composition and function are similar to those of the hair's germinal matrix. Proliferation in the nail matrix pushes the upper cells toward the surface. These cells differentiate, becoming highly keratinized to form the nail plate. The plate is gradually pushed out of the groove by further cell proliferation and differentiation in the nail matrix. The growing plate slides distally on the dorsal surface of the digit. The epidermis over which it slides becomes the nail bed.

B. Nail Complex Structure: The nail plate (or nail) consists of 2 parts: the nail body (the visible part of the nail) and the nail root--(the part hidden in the nail groove). The nail and its supporting structure are surrounded by papillary dermis. The nail matrix is a thickened region of epidermis containing proliferating cells in the layer that directly contacts the dermis, and keratinizing cells between this basal layer and the nail plate. The nail matrix surrounds the root and extends beyond the nail groove. The nail bed lies beneath the nail body, distal to the nail matrix. It consists of only the deeper epidermal strata, for which the nail serves as a stratum corneum. The eponychium (or cuticle) is a thick keratinized layer extending from the upper surface of the nail groove over the most proximal part of the nail body. The hyponychium is a local thickening of the stratum corneum underlying the free (distal) end of the tail. The lunula is the whitish, opaque, crescent-shaped region on the proximal nail body, adjacent to the nail groove. Its distal border corresponds roughly to the underlying nail matrix

Apocrine Sweat Glands

nB. Apocrine Sweat Glands
n1. Distribution. Less numerous than the eccrine type, these glands occur mainly in the axilla, pubic and anal regions, and the areolae of the breasts. 2. Structure. Apocrine sweat glands are also simple coiled tubular glands, but are generally larger than eccrine glands. a. Ducts. These coiled ducts are lined with low cuboidal epithelium and open into hair follicles . b. Secretory portions. Coiled and embedded in the dermis, each has a wide lumen lined by cuboidal to columnar cells. Myoepithelial cells are present between the secretory cells and the basal lamina.
n3. Secretory Product. Apocrine sweat is a viscous, odorless fluid that, once secreted, acquires a distinctive odor as a result ofbacterial degradation. The term apocrine derives from early evidence that the secretory cells of these glands released their apical cytoplasm along with the secretory product. Recent evidence, however, argues against apical shedding. Therefore, although the secretory products ofapocrine and eccnne sweat glands do differ, their mode of secretion-merocrine-is similar.

SWEAT GLANDS

nTwo types of sweat glands, eccrine (or merocrine) and apocrine, occur in human skin. Both develop as epidermal invaginations into the dermis, and they differ mainly in their size, distribution, and secretory products.
nA. Eccrine Sweat Glands:
n1. Distribution. The most numerous sweat glands in humans, these average about 3 million per individual. They occur over most of the body, except for the glans penis, glans clitoridis, and the vermiliion border of the lips. They are most abundant in thick skin, such as the palms, where there are about 3000 per square inch. 2. Structure. They are simply coiled tubular glands. a. Ducts. The slightly coiled ducts are lined with simple to stratified cuboidal epithelium; their lining cells are smaller than those in the secretory portions and stain darker. Each duct opens directly onto the skin surface. b. Secretory portions. These highly coiled parts of the sweat glands are located in deep reticular dermis or shallow hypodermis. Surrounding connective tissue condenses to form a sheath around the basal lamina, and there are numerous myoepithelial cells between the basal lamina and the secretory cells. The secretions are released via ex ocytosis (merocrine secretion). Secretory cells are larger and stain lighter than the duct lining cells. Two secretory cell types are seen. Dark (mucoid) cells are pyramidal and line most of the gland's secretory portion; their bases do not reach the basal lamina. They contain rodlike mitochondria, a well-developed Golgi complex, RER, many free ribo somes, and dark glycoprotein-containing granules. Clear cells are also pyramidal. They lack secretory granules, contain abundant glycogen, and surround the inner layer of dark cells. Their basal plasma membranes, which do contact the basal lamina, are highly infolded, suggesting a role in ion and water transport.3. Secretory product. Eccrine sweat is a watery secretion whose main components (besides water) include NaCI, urea, ammonia, and uric acid. The glands thus assist in excreting by-products of protein metabolism. In addition, evaporation of water from the skin surface reduces body temperature by cooling the blood in the papillary capillaries.

SEBACEOUS GLANDS

A. Structure and Location: These exocrine glands occur in all thin skin, most often in association with hair follicles into which their ducts empty, but are most numerous in the skin of the face, forehead, and scalp. In hairless skin, they open directly onto the surface. Their acinar secretory portions contain many large lipid-filled cells that appear pale-staining and foamy.

B. Function: The acinar cells of sebaceous glands fill with lipid droplets containing a mixture of triglycerides, waxes, squalene, and cholesterol and its esters. Their nuclei become pyknotic, and the cells eventually burst, releasing their contents and other cell debris (together termed sebum) into the ducts. The entire cell is shed, a type of secretion known as bolocrine secretion. The oily sebum moves through the ducts and into the hair follicle. It covers the hair and moves out onto the surface. Here, it lubricates the skin and may have some antibacterial or antifungal effects. The secretory activity of these glands, which begin functioning at puberty, is continuous and is increased by androgens.

Hair Growth

Hair growth is not continuous but cycles through repeated growing and resting phases. In the growing phase, the proliferation and differentiation of cells in the germinal matrix cause the hair to elongate. In the resting phase, the germinal matrix becomes inactive and may atrophy. The hair detaches from the bulb, moving upward as the external root sheath retracts toward the surface. Eventually, the hair is shed. During the next growing phase, the lower part of the external root sheath grows downward again, either forming a new germinal matrix over the old papilla or stimulating formation of a new papilla. The bulb re-forms, and the next phase of the cycle--proliferation in the matrix and renewed hair growth--begins. Hair growth cycles do not occur synchronously over the entire body surface. Rather, they occur in patches, a pattern called growth in mosaic. Several hormones, especially androgens, influence the pattern of terminal hair distribution and growth rate.

Keratinization of Hair

Although both the hair and the epidermis contain keratin, there are differences in their keratinization. For example, the keratin of the hair's cortex and cuticle is harder than that of the epidermis; keratinized hair cells remain tightly attached to one another, whereas those of skin are continuously sloughed; keratinization of the hair is intermittent and is restricted to the bulb, whereas that of skin is continuous and occurs over the entire surface; and keratinized cells of the epidermis are identical, whereas those in hairs differ in structure and function depending on their position in the hair.

Associated structures

Found near the neck of the root sheath, sebaceous glands always accompany hairs. They empty their secretions via a short duct into the follicular canal. Arrector pill muscles are small bundles of smooth muscle fibers that originate in the papillary dermis and extend obliquely toward the hair follicle to insert into the follicle's connective tissue sheath below the sebaceous glands. When they contract, these muscles cause the hairs to stand upright, giving the appearance of gooseflesh. Their contraction also compresses the sebaceous glands, pushing their secretions into the neck of the follicular canal and out onto the surface of the skin.

Root sheaths

The concentric sheaths surrounding the hair shaft are more clearly distin- guished in the area between the bulb and the skin surface. a. Internal root sheath. The layer closest to the hair shaft, it extends only from the bulb to the level of the sebaceous gland ducts. At this point the soft keratin-filled cells are shed into the follicular canal. There are 3 component layers: the cuticle of the internal root sheath is a layer of flat cells separated from the hair shaft cuticle only by the follicular canal; the middle layer is Huxley's layer, comprising one to 3 layers of low cuboidal cells; the outermost layer is Henle's layer, a translucent layer of flattened to cuboidal cells resembling the epidermal stratum lucidum. b. External root sheath. This surrounds the internal root sheath and is continuous with the epidermis. Above the level of the sebaceous glands, it includes all the epidermal layers. Below this level, it retains only the granulosum, spinosum, and basale. The granulosum is also lost near the follicle's base, where the spinosum and basale become continuous with the layers of the germinal matrix. c. Glassy membrane. This is the thickened basal lamina underlying the stratum basale of the external root sheath and separating it from the surrounding connective tissue sheath. d. Connective tissue sheath. A layer of condensed connective tissue, this surrounds the entire follicle, including the bulb. It extends along the follicle to the surface, where it blends into the looser papillary dermis

Hair shaft layers

These 3 concentric layers are formed by the germinal matrix. The cell borders are indistinct, however, and cross sections through hair follicles near the skin surface often do not show the cellular nature of these layers. In addition, the hair itself may be dislodged from the canal during tissue processing, leaving only the open space (follicular canal) originally occupied by the shaft. The medulla forms the shaft's thin central core. It is composed of poorly keratinized and often vacuolated cells. The cortex surrounds the medulla and is composed of several layers of well-keratinized polygonal cells. The cuticle is the shaft's outermost layer. Within the bulb, its cells are cuboidal; farther up the shaft they become tall columnar, fill with keratin, and finally change their orientation to become a few layers of flattened, highly keratinized cells. These cells form the hard, shinglelike cuticle that covers the hair's outer surface.

1.Germinal matrix

This cluster of epithelial cells capping the dermal papilla can be divided into 4 indistinct zones that are arranged concentrically around the papilla. The zone closest to the papilla resembles the stratum basale of the epidermis in both structure and function. It contains both columnar epidermal cells and the melanocytes that give the hair its color. This germinal layer gives rise to the poorly keratinized cells of the medulla of the hair shaft and to the cells in the other 3 zones of the germinal matrix. Around the base of the bulb, this layer is continuous with the external root sheath that surrounds the entire bulb and shaft; near the surface, it is continuous with the stratum basale. Cells in the next layer form the cuticle. The most peripheral layer of the germinal matrix forms the poorly keratinized cells of the internal root sheath.

Follicle and Hair Structure

nHair 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.

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