Appendages of the skin - part II

PART I

Glands

Sebaceous glands

These are associated with all hairs and hair follicles, there being between one  and four associated with each hair. They may also exist where there is no hair, such as the corner of the mouth and adjacent mucosa, the lips, the areola and the nipple, opening directly onto the skin surface. However, they are absent from the skin of the palm and sole and the dorsum of the distal segments of the digits. The glands vary in size between 0.2 and 2.0mm in diameter. The cells of the glands are continuously destroyed in the production of the oily secretions, known as sebum. This mode of secretion production is known as holocrine secretion.

Inflammation and accumulation of secretion within the sebaceous glands give rise to acne. If plugging of the outlet is permanent, a sebaceous cyst may be formed in the duct and follicles. These may become so enlarged that they require surgical removal. Sebaceous glands do not appear to be under nervous control.

Sweat glands

These have a wide distribution throughout the body(figure below), being more numerous on its exposed parts, especially on the palms, soles and flexor surfaces of the digits. Here the ducts open onto the summits of the epidermal ridges. Each gland has a long tube extending into the subcutaneous tissue, where it becomes coiled forming the secretory body of the gland(figure b). The glands produce sweat, which is a clear fluid without any cellular elements, for secretion(eccrine secretion). The production of sweat is important in temperature regulation, as its evaporation from the skin surface promotes heat loss. These eccrine sweat glands are quently, any disturbance in the sympathetic system will result in a dry warm skin(anhydrosis) either locally or extensively.

In the axilla, groin and around the anus there are large modified sweat glands, being between 3 and 5mm in diameter and lying deeply in the subcutaneous layer. Their ducts may be associated directly onto the skin surface. The secretions of the glands include some disintegration products of the gland cells(apocrine secretions). The odour associated with these glands is not from the secretion itself, but is due to bacterial invasion and contamination from the skin. Pigment granules associated with axillary glands produce a slight coloration of the secretion. The apocrine glands vary with sexual development, enlarging at puberty. In females they show cyclical changes associated with the menstrual cycle.

The glands which open at the margins of the eyelid(ciliary glands) are modified, uncoiled sweat glands, as are the glands of the external auditory meatus(ceruminous glands). The cells of these latter glands contain a yellowish pigment which colours the wax secretion(cerumin).

Mammary gland(breast)

The mammary glands are modified sweat glands, being accessory to the reproductive function in females, secreting milk(lactation) for the nourishment of the infant. In children prior to puberty and the adult male, the glands are rudimentary and functionless.

Blood supply and lymphatic drainage of the skin

The arterial supply of the skin is derived from vessels in the subcutaneous connective tissue layer, which form a network at the boundary between the dermis and subcutaneous tissue(figure b). Branches from the network supply the fat, sweat glands and deep parts of the hair follicles. Branches within the dermis form a subpapillary plexus. The epidermis is avascular. Abundant arteriovenous anastomoses occur within the skin. Lymphatics of the skin begin in the dermal papillae as networks or blind outgrowths which form a dense mesh of lymphatic capillaries in the papillary layer. Larger lymphatic vessels pass deeply to the boundary between dermis and subcutaneous tissue to accompany the arteries as they pass centrally.

Nerves of the skin

The nerves of the skin are of two types, afferent somatic fibres mediating pain, touch, pressure, heat and cold(general sensations), and efferent autonomic(sympathetic) fibres supplying blood vessels, arrector pilorum and sweat glands. The sensory(afferent) endings have several forms. Free nerve endings extend between cells of the basal layer of the epidermis, terminating around the adjacent to hair follicles. They are repetitive to general tactile sensation as well as painful stimuli. Enclosed tactile corpuscules lie in the dermal papillae, being sensitive to touch. Pacinian corpuscules(figure b) exist in the subcutaneous tissue, being particularly plentiful along the sides of the digits, and act as pressure receptors. Specific endings for heat and cold have been described, although general agreement as to their identity has not been reached.