keratoacanthoma ( ka ) is a relatively common benign , proliferative lesion , which involves cutaneous tissues and less frequently the mucosal surfaces . kas are frequently found on sun - exposed areas suggesting that actinic rays among other etiologic factors are largely responsible for the lesion . it is believed that cutaneous lesions arise from hair follicles , while the occurrence of ka on mucosal surfaces suggests its possible origin from ectopic sebaceous glands or surface epithelium . ka is a self - limiting lesion that undergoes spontaneous regression and is known to heal by scarring . here , we report the case of a ka presenting on the lower lip with an attempt to throw light on its pathogenesis . a 45-year - old female was referred for evaluation of an asymptomatic nodule on the lower lip of 4-week duration . clinical examination revealed a well - defined , nontender , ulcerated , nodular growth measuring approximately 1 cm  1 cm on the lower lip vermilion [ figure 1 ] . there was no report of local trauma , and the patient presented no parafunctional habits . clinical image shows noduloulcerative growth with keratin plugging seen on left vermilion of the lower lip an excisional biopsy was performed under local anesthesia . histopathological examination of h- and e - stained section revealed a hyperkeratotic , hyperplastic epithelium with parakeratin plugging in some areas . the underlying connective tissue stroma showed multiple areas of epithelial tissue entrapment with cystic degeneration in the center and keratinization [ figures 2 and 3 ] . the epithelial proliferations in the connective tissue did not exhibit pas positive glycogen granules though [ figure 4 ] . histopathological image shows hyperkeratotic epithelium with parakeratin plugging overlying connective tissue stroma with epithelial proliferations superficially in the stroma ( h&e , 40 ) histopathological image shows epithelial tissue entrapment with cystic degeneration in the center and keratinization ( h&e , 100 ) . schiff - stained section showing negative staining for glycogen granules ( h&e , 100 ) . inset ( pas , 400 ) the lesion healed with no scarring over 5 weeks , but the patient was lost to follow - up later . ka is a benign , rapidly proliferating epithelial growth seen in older age groups ( between the ages of 45 and 69 years ) with a male preponderance . it arises on the sun - exposed parts of the body chiefly face , neck and forearms , especially in light - skinned individuals , as a self - limiting proliferation of the hair follicle apparatus called follicular infundibulum . the etiologic factors implicated include irritation in the form of actinic rays , trauma , tars , virus , immunocompromised status and genetic factors ( which are commonly responsible for squamous cell carcinoma [ scc ] too ) . ka typically presents as a dome - shaped lesion with a central crater filled with keratin . this condition is thought to represent a proliferation of the infundibular portion of hair follicles rather than epidermis . in the normal hair follicle , thus , it is suggested that ka represents a temporary derangement of the normal regulatory apoptotic mechanism in the follicles . tunel results in a study have shown that these aberrant cells do undergo complete apoptosis but by another mechanism . it is believed that ka cells can differentiate into cells that are similar to outer root sheath cells of the hair follicle , further supporting the possibility that cutaneous kas arise from hair follicles . furthermore , it is seen that ka follows a cycle similar to the life cycle of a hair follicle , i.e. , cycles of proliferation ( anagen ) , regression ( catagen ) and quiescence ( telogen ) . ka has been reported to have three clinical stages : ( i ) proliferative , ( ii ) mature and ( iii ) involutional or resolving . ghadially has classified ka as type i , bud shaped if it is of upper follicular origin ; type ii or dome shaped and type iii or berry shaped if the lesion is of lower follicular origin . apart from solitary and multiple forms , the various other types of morphologic and syndromic forms of ka are : agglomerate ka , ka centrifugum marginatum , giant ka , subungual ka , mucous membrane ka ( including the intraoral variety ) , multiple kas of the fergusson smith types , multiple persistent kas , generalized eruptive kas of grzybowski , kas in torre syndrome , ka in xeroderma pigmentosum and kas in nevus sebaceous of jodassohn . ka of the lip is usually a solitary , circular ulcerated nodule with central keratin plug that occurs at the vermilion border of the lip . occurrence of solitary ka on vermilion portion of lips is now widely recognized , and it is debatable as there are no hair follicles in oral mucosa . some report its possible origin from surface epithelium , whereas others suggest that since oral mucosa lacks hair follicles , the ka may develop from ectopic sebaceous glands . pas stain with or without diastase is a special stain demonstrating cytoplasmic glycogen content and stromal hyalinized basement membrane in cutaneous adnexal lesions . the outer two epidermal cell layers of hair bulb ( from which ka is thought to occur ) consist of large , glycogen - rich cells which can be diagnosed with pas stain , thus helping in the determination of cells of origin of the lesion . to understand the histogenesis of mucous membrane ka occurring on the lip staining for pas was negative in the present case , reinforcing that origin of kas of lip could be from ectopic sebaceous glands rather than from a hair follicle . the epithelial entrapments in the connective tissue , typifying epithelial proliferation in the stroma displayed cystic spaces with keratinization . previously , cystic appearance in ka has been reported in two recurrent cases , attributing it to a proliferative signal being switched in a remnant of the infundibular cyst focus , resulting in the cystic appearance . this left us to deliberate whether the lesion in our case was a recurrent one with the patient having overlooked its occurrence the first time or was it a de novo lesion displaying cystic appearance . however , in a perplexing case , histopathology is the mainstay in differentiating between the two lesions . advanced studies directed at differentiating ka and scc have shown that expression of cell proliferation markers , telomerase activity and apoptotic and cell adhesion marker are markedly more in scc , categorizing ka and scc as two distinct entities . different treatment modalities proposed for ka are cryotherapy , electrodessication , irradiation , 5-fluorouracil , curettage , radium implantation , cautery , hydrocortisone and antibiotic ointments , podophyllin and topical photodynamic therapy which have been tested to varying extents . however , complete excision has proved to be the treatment of choice as it facilitates healing without scarring and also allows histopathological assessment of the complete lesion . ka is a common benign cutaneous lesion which occasionally presents itself on the mucous membrane . it is known to originate from follicular infundibulum , but mucosa is devoid of hair follicles . this report of ka of the lip is an attempt to clarify this unsolved query . through pas staining , we concluded that ka of oral mucosa in all probabilities arises from ectopic sebaceous glands and not hair infundibulum . furthermore , ka has striking clinical familiarity with scc , and a certain way of differentiating them is by histopathology . the mainstay in treatment is complete excision as it facilitates healing without scarring and also allows histopathological evaluation of the lesion