PURPOSE: Incomplete obliteration of the processus vaginalis (PV) in children with inguinal hernia or hydrocele has recently been proposed to relate to smooth muscle cell (SMC) persistence. The aim of this study was to evaluate the diversity and differentiation of smooth muscle phenotypes in sacs associated with inguinal hernia and hydrocele through the expression of alpha-smooth muscle actin (SMA), h-caldesmon, desmin, and vimentin.METHODS: Sacs associated with male hernia (n = 22), female hernia (n = 8), and hydrocele (n = 10) were immunohistochemically evaluated using monoclonal antibodies against SMA, h-caldesmon, desmin, and vimentin. Peritoneal samples (male, 4; female, 3) and obliterated PV (male, 3) obtained from age-matched patients served as controls. Expressions according to the groups were compared through chi-squared test, and P values less than 0.05 were considered to be statistically significant.RESULTS: Immunohistochemistry did not shown the presence of SMCs in control samples. The expression of SMA, desmin, and h-caldesmon did not differ among sacs obtained from patients with inguinal hernia and hydrocele. However, strong expression of vimentin in SMCs within sacs obtained from patients with hydrocele in comparison with sacs from male patients with inguinal hernia were observed.CONCLUSIONS: Our results indicate that sacs from patients with inguinal hernias and especially from male inguinal hernias have fully differentiated SMCs. On the other hand SMCs in sacs obtained from boys with hydrocele are in an intermediate state of differentiation-dedifferentiation. This phenotypic modulation may represent attempted apoptosis of SMCs, since sacs more sensitive to apoptosis appeared to have more dedifferentiated SMCs. It also probably depicts the differing influence of sympathetic and parasympathetic tonuses during the descent of the testis and the obliteration of PV.