The epidermal appendages include the nails, hair and glands (i) (glandulae cutis). They arise from invaginations of the epidermis into the dermis.
The Nails, Hair and Glandulae cutis
The nails are horny plates firmly attached to the nail bed. They are about 0.5 mm thick and consist of the front free edge - the body of the nail - and the nail matrix, which is embedded in the proximal nail fold.
The hair is divided into the protruding hair shaft and the hair root. The root thickens at the end to become a bulb (bulbus), which together with the underlying dermal hair papillae is responsible for the nourishment, development and growth of hair.
A dermal sheath of connective tissue surrounds the whole hair root and together these form a unit known as the hair follicle. The sebaceous glands (i) open into the infundibular part of the hair follicle.
The glands (i) of the skin (glandulae cutis) include the sweat, scent, sebaceous and milk glands (i). The sebaceous glands (i) are nearly always connected to hair follicles, which deliver the lipid-containing secretion (i) to the surface through their funnel shaped openings. The size of the sebaceous glands (i) and therefore the amount of sebum itself differ according to body region. The glands (i) found on the face, for example, are bigger than those found on the arms or legs. An important influencing factor in sebaceous gland activity is the androgens.
Sebaceous and sweat glands (i) are exocrine glands (i) (exo = outer, external/Gk.), which means they deliver their secretion (i) directly to a surface such as the skin. In the case of the sebaceous glands (i) this occurs with complete disintegration of lipid-rich cells. They are continually replaced through division of the basal cells (holocrine glands (i)). In the case of apocrine glands (i) like the mammary glands (i) or the sweat glands (i) of the axilla (underarm) only the outer parts of the cell body are lost with the secretion (i). The cells of the eccrine glands (i) like the small sweat glands (i) of the skin show no loss of cytoplasm after the secretion (i) process.
Together with the sweat glands (i), the sebaceous glands (i) provide vital substances that - along with the epidermal lipids - form the hydrolipid film.
Schematic diagram of the follicle. The hair follicle and sebaceous gland form a structural and functional unit.
1 Hair shaft
2 Sebaceous gland
4 Hair papillae
5 Sweat gland