Sebaceous glands are simple, branched holocrine acinar 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 cornified 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 hypodermis, from which the hair shaft grows. Bundles of smooth muscle cells, called arrector pili muscles, are attached to the hair follicle at one end and the papillary dermis at the other. Contraction of these muscles raise the hairs and dimple the epidermis («goose flesh»). The follicles and associated sebaceous glands are known as pi-losebaceous units.
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
coiled – намотанный
tubular – трубчатый
widely – широко
to distribute – распространять
columnar-like – колоночноподобный
pyramidal – пирамидальный
surface – поверхность
duct portions – части трубочки
corkscrew-shaped – имеющий форму штопора
luminal – люминал
thermal – тепловой
innervation – иннервация
abundant – в изобилии
to facilitate – облегчать
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