Keratoacanthoma ( KA) is an epithelial neoplasm occurring in sun -exposed skin of the elder-ly.Two striking features of KA are its clinical behavior with spontaneous regression after rapid growth and its nos -ological position on the border between benignity and malignancy .The regression of KA may be associated with the hair follicle cycle,apoptosis and immune function of organism .This article summarizes physiological mecha-nisms such as Wnt/retinoic acid signalling cross -talk,the function of p53,bcl-2,bcl-xL and bak in apopto-sis,and the mechanism of action of several immune -related cells,that drives KA regression .It can provide theo-retical basis for us to select the method as well as time of the treatment in KA .

Objective To explore the expressions and significances of p 53、bcl-2、Ki-67 and apopto-sis in keratoacanthoma(KA)and well differentiated squamous cell carcinoma (WDSCC).Methods The expres-sions of p53、bcl-2、Ki-67 were detected by immunohistochemical technique in 44 cases of KA and 20 cases of WDSCC.Apoptosis was identified by using terminal deoxynucleotidyl transfers ( TdT) -mediated dUTP -biotin nick and labelling(TUNEL)method.Results The positive expressive rates of p53 in proliferative、mature and re-gressive of KA were 22.23%、26.18% and 6.52%, which was lower than the positive rate of WDSCC (41.82%).Significant differences were found between every period of KA with WDSCC in p 53 expression;The intensity of expression and pattern were similar in KA of Ki -67 and p53.There were a positive correlation be-tween p53 and Ki-67 expression rate(r=0.986,P<0.001).The bcl-2 staining showed weak expression in 1 case in WDSCC and 2 cases in KA ,but only a few positive tumor cells was limited to basal cell layer in KA .The average apoptosis rate in KA was 21.72%,which was apparently higher than in WDSCC (9.925%).There were a negative correlation between the apoptosis rate and proliferation rate of Ki -67(r=-0.824,P<0.001).Con-clusion The proliferation and apoptosis in KA can coexist .However ,the apoptosis will occupy a dominance posi-tion in regressive phase ,which results in the regression of tumor .The expressions of p53、Ki-67 and apoptosis would be a certain significance in differentiated KA and WDSCC .