There are three types of follicle on the face and it is the sebaceous follicle which is  the  one most  frequently involved in acne. The sebaceous glands, although larger in acne  subjects, are  similar in  structure  to  those of control subjects. The sebaceous gland consists of three types of cells: the more basally situated undifferentiated cells; the differentiating cells; and the mature sebumcontaining cells which eventually disintegrate, liberating  the  sebum which finds its way through the sebaceous duct  into the pilosebaceous canal. In the sebaceous  follicle  the  pilary   part  of the  pilosebaceous canal is small  and it is the so-called  infrainfundibular  part of the canal which is the primary site of involvement in the formation of endogenous  acne lesions. The earliest lesion is probably the microcomedo.

  The precise stimulus to the formation of the microcomedo is not known, certain fractions of sebum such as squalene and linoleic acids  may be important. Alternatively, the pilosebaceous duct may be under hormonal influences. Whatever the mechanism, the cornified cells  of  the  acne-prone  pilosebaceous duct  is  more coherent. Thus  eventually the  microcomedo evolves into a closed comedo and possibly into an open comedo. During the process of comedo formation the sebaceous gland itself may  sometimes undergo partial atrophy.

  Inflammatory  changes represent initially the diffusion of molecular substances through the duct wall into the dermis. Later partial or complete rupture of the wall may occur.  The type of lesion clinically will depend upon the level within the dermis of the initial inflammatory reaction and on the patient's cellular and humoral response.