Basic anatomy and histology of the pilosebaceous duct in health and disease This topic was also described but some repetition is  necessary.

Histological investigation of the pilosebaceous canal shows that most of the duct consists of an epidermis-like structure undergoing keratinization; the keratinized structures occupy the more central part of the duct. The upper fifth of the duct is identical to the contiguous epidermis but the lower four-fifths, the so-called infrainfundibulum , is vastly different. In contrast to the overlying  epidermis and the epithelium of a terminal hair follicle the infrainfundibulum has an inconspicuous granular layer and glycogen is often present in excess.

The horny cell layers are only a few cells thick and soon desquamate into the central part of the duct to form a heterogeneous mass with the sebum  and bacteria.   The primary  site of abnormality in the developing comedo is at the level of the infrainfundibulum.

The granular  layer becomes more easily defined, the horny cells more compact and less readily separable.5 Similar changes occur also in the sebaceous ducts which join the sebaceous gland to the infrainfundibular  part of the follicular duct.  Thus the horny cells distend the pilosebaceous canal, producing at first a microcomedo and then a clinically obvious lesion. However, it is not known why some lesions are closed (whitehead) or open (blackhead) - but it needs to be emphasized that the whitehead is by far the most frequently  seen clinical  noninflamed lesion. As the lesion enlarges, the sebaceous gland may atrophy but sebum continues to be produced and is secreted into the impacted mass of corneocytes which also contain many bacteria and hairs.

In mature comedones  there are well-defined duct-like structures through which sebum can pass.

 

Acne lesions do not usually occur in a follicle bearing a terminal hair. The hair acts as a wick allowing sebum to drain  from  the pilosebaceous canal, whereas the sebaceous follicle has only small ineffective hairs. Grant emphasized this point clinically, demonstrating that a factor which allows comedo formation in  acne is the absence of an effective hair penetrating the mouth of the follicle.

Comedones may contain several small hairs, and their presence could in part account for the inflammatory reaction in late-stage inflammation since hair keratin is a potent natural irritant.