Pilar cysts (trichelemmal cysts) are cysts arising from the outer root sheath of a hair follicle(the trichelemma).
These may be
- sporadic or may be
- autosomal dominantly inherited.
They can be
- solitary or
Once formed, they may
a) remain non-growing,
b) grow larger,
c) get inflamed or infected and go on to form an abscess,
d) occasionally extrude their contents to form a soft, cutaneous 'horn' (these horns may presumably get dislodged by activities such as combing the hair etc).
e) the cyst may burst open and remain open('marsupialized'). If the cyst cavity remails exposed, its lining eventually fuses with the surrounding epidermis to form a depression or 'crypt'. This may then lead to complete healing;
f) very rarely, malignant transformation may occur (proliferating pilar tumors).
Situations d & e described above might explain the disappearance of the cyst in your case.
MB BS, MS, FRCS(Edinburgh)