6. Cutaneous appendages
Cutaneous appendages are all derivatives of the epidermis.
Eccrine (merocrine) sweat glands are simple, coiled, tubular glands that are widely distributed over the body. Secretory portions are tightly coiled and consist of a single layer of columnar-like pyramidal cells.
Duct portions, composed of two cuboidal cell layers, are corkscrew-shaped and open onto the epidermal surface. These glands are important in thermal regulation.
Control of the eccrine glands is mainly by the innervation of cholinergic fibers.
Apocrine sweat glands are also simple, coiled, tubular glands but are much less abundant in their distribution than eccrine glands. They can be found in the axillary, ar-eolar, and anal regions.
Secretory portions of these glands are composed of a single layer of cuboidal or columnar cells. They are larger and have a much wider luminal diameter than eccrine sweat glands. Myoepithelial cells surround the secretory cells within the basement membrane and contract to facilitate secretion.
Duct portions are similar to those of eccrine sweat glands but open onto hair follicles instead of onto the epidermal surfaces.
Functions of these glands in humans is not at all clear. Specialized apocrine glands in the ear canal (ceruminous glands) produce a secretion in conjunction with adjacent sebaceous glands to form the protective earwax (cerumen). Control of the apocrine glands is hormonal and via the innervation of adrenergic fibers. These glands do not begin to function until puberty.
Sebaceous glands are simple, branched holocrine aci-nar glands. They usually discharge their secretions onto the hair shaft within hair follicles. These glands are found in the dermis throught the skin, except on the palms and soles.
Secretory portions consist of peripherally located, flattened stem cells that resemble basal keratinocytes. Toward the center of the acini, enlarged differentiated cells are engorged with lipid. Death and fragmentation of cells nearest the duct portion result in the holocrine mechanism of secretion.
Duct portions of sebaceous glands are composed of stratified squamous epithelium that is continuous with the hair cat and epidermal surface.
Functions involve the lubrication of both hairs and corni-fied layers of the skin, as well as resistance to desiccation.
Control of sebaceous glands is hormonal. Enlargement of the acini occurs at puberty.
Hairs are long, filamentous projections consisting of dead keratini-zed epidermal cells. Each hair derives from an epidermal invagination called the hair follicle, which possesses a terminal hair bulb, located in the dermis or hypo-dermis, from which the hair shaft grows. Contraction of smooth muscles raise the hairs and dimple the epidermis («goose flesh»).
Nails, like hair, are a modified stratum corneum of the epidermis. They contain hard keratin that forms in a manner similar to the formation of hair. Cells continually proliferate and keratinize from the stratum basale of the nail matrix.New words
cutaneous – кожный
appendace – покров
tubular – трубчатый
pyramidal – пирамидальный
surface – поверхность
thermal – тепловой
innervation – иннервация
Главная | В избранное | Наш E-MAIL | Добавить материал | Нашёл ошибку | Вверх