As mentioned earlier, all skin areas excluding the glabrous skin, that is, the palmar (palms) and plantar (soles), possess pilosebaceous units. Although the number of secreting follicles, and consequently sebum output, varies greatly between individuals, the distribution and shape of follicles tend to follow the same pattern over the human body (19). The highest density of sebaceous and vellus follicles is found on the face, especially on the forehead, where there may be as many as 900 glands/ cm2 in some areas (20), but the number varies according to the study: Blume et al. (21) determined a density of 423 follicles/cm2, Pagnoni et al. (22) found 455 follicles/cm2 on the lateral forehead and up to 1220 follicles/cm2 in the nose area, and most recently, Otberg et al. (23) determined a number of 292 follicles/cm2 on the forehead. Nevertheless, there is agreement that sebum output is maximal on the forehead, nose, and chin, the so-called "t-zone" and decreases toward the outer edges of the face (22). Elsewhere on the body, there may be fewer than 100 pilosebaceous units/cm2 (23). The face contains the largest sebaceous glands, but despite this the ducts serving these glands are smallest on the face, particularly, on the forehead and have a significantly smaller pore compared to those found on the back. It has been calculated that the smaller duct creates a five times greater resistance to sebum output on the forehead compared to the back (24). This greater resistance exerted by the small duct on sebum flow, may, in part, explain the prevalence of pilosebaceous diseases on the face, involving high sebum secretion, such as acne.
Other types of sebaceous glands are found in humans distributed over epithelial surfaces and open directly onto the surfaces upon which they secrete. These so-called free sebaceous glands are particularly prevalent in transitional areas between skin and mucosal membranes for example, anogenital region, lips, and meibomian glands in the eyelids. Sebaceous glands are also located in oral mucosa (called Fordyce spots), the digestive and respiratory tract, and the areoles of the nipples.
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