Porcine placentae from 18 to 114 days of gestation were investigated histologically, histochemically and immunohistochemically. The placenta was histologically divided into four areas : the chorionic fossa (CF), chorionic ridge (CR), regular areola (RA) and irregular areola (IRA). The IRA was further subdivided into fetal IRA, maternal depression and ruffled area of the maternal depression (RAMD). In histochemical analysis, a reaction product suggestive of acid mucopolysaccharides was detected exclusively in the uterine and chorionic epithelia of the IRA. In immunohistochemical analysis, the localization of IgG and IgM was demonstrated in the endometrial lamina propria mucosae, and the epithelia of the RAMD and fetal IRA. Regional IgG and IgM localization was identical to that of the acid mucopolysaccharides. In the fetal lymphatic tissues, a small number of IgG-containing cells was first noted at 114 days of gestation. In a serological survey, IgG and IgM were demonstrated at a constant level in all fetal sera from days 40 to 114 of gestation, although IgA was negative except for 2 of 23 sera from fetuses. Comparing the relative levels of maternal immunoglobulins (Igs), the fetal Igs were 1/200 in IgG and 1/500 in IgM. These results suggest that there is a possible diaplacental transition of IgG and IgM from the mother into the fetus occurring via epithelia exclusively at the RAMD and the fetal IRA
