Sebaceous glands in man are distributed over all skin areas with the exception of the palms and soles and only sparsely on the dorsal surfaces of the hands and feet. These glands vary in size with their location  and are largest and most numerous on the face, especially the forehead, and the upper back. On the scalp, forehead, cheeks and chin it is usually  stated that there are between 400 and 900 glands per cm2;1 elsewhere there are said to be fewer than 100 glands per cm2. The earlier observations were obtained by the indirect method of staining inert material, such as absorbent material, with a lipophilic stain.  Direct microscopy using a surface microscope at a magnification of X 50 confirms these indirect observations.2 It is  however naive to assume that one lipid droplet represents one sebaceous follicle, for phase-contrast  microscopy will reveal many  lipid droplets  emerging from each duct. It is therefore perhaps rather surprising that the direct and  indirect means of assessment should give comparable data. Direct microscopy methods have shown  about 400 pilosebaceous ducts per  cm2 on the forehead and 160 per cm2 on the back.2 This lower number of pilosebaceous units on the  back may explain to some extent the greater  frequency of acne on the forehead.

Most sebaceous glands are intimately related to the development  of the  hair follicle1 and  usually  vary inversely in size  with the hair  follicles. These two structures are often referred to  collectively as  pilosebaceous units    and  when  very large  as sebaceous follicles. However, at a number of sites the sebaceous glands open directly onto the surface. Examples of such glands are  the  meibomian glands of the eyelid. 'Free' sebaceous glands are also found on the mucocutaneous surface  of the  female genitalia, the areola of the nipples and ectopic sites such as the uterine cervix.3 'Free' sebaceous glands in the glabrous Vermillion border of the lip and in the mouth are often visible to the naked eye. These  are seen as pale yellow bodies which  vary in  size and are  usually 2 to  3 mm in diameter, but may  coalesce attaining a size of 15  mm; they are known as  Fordyce spots. It is interesting that man has more sebaceous glands than any other mammal,  but   large glands  with  many similarities to sebaceous  glands and not associated with hair follicles occur in many  mammals.  For example, rodents have flask-shaped preputial glands which open  by single ducts alongside the urethra; the  inguinal glands of the rabbit  consist  of solid masses of sebaceous  material. Guinea pigs have a large supracaudal  gland;4 shrews have abdominal glands; lemurs have  large 'brachial glands', one on the ventral surface of each shoulder;1 and marsupials have dorsal and cloacal glands.

  It is worthy of note that sebaceous and similar glands are found predominantly in those areas which 'sniff and are 'sniffed'. This  may give a clue to the possible role  of  sebum as  a   pheromone.  Although these sebaceous-like glands of lower-order animals have been used to study sebaceous gland biochemistry and physiology in experimental  animals, some caution is needed in extrapolating these results to human sebaceous gland function.

  Although the pilosebaceous unit is a functional entity it is not unreasonable to divide the subsequent parts of this chapter into two sections.  Firstly, the histology of the sebaceous glands in health and disease, and secondly,  the  pilosebaceous  canal and duct  in  health  and disease, will be discussed.