IMMUNOCYTOCHEMICAL LOCALIZATION OF CYTOCHROMES P450 17-ALPHA-HYDROXYLASE AND AROMATASE IN EMBRYONIC-CELL LAYERS OF ELONGATING PORCINE BLASTOCYSTS

Localized expression of cytochromes P450 17 alpha-hydroxylase (P450(c1 7)) and aromatase (P450(arom)) was investigated in embryonic cell laye rs of elongating porcine blastocysts by immunocytochemistry. Blastocys ts were flushed from the uterus on day 12 of pregnancy, fixed in paraf ormaldehyde, embedded in paraffin, sectioned, and stained using immuno gold- and peroxidase-based techniques. Staining for both P450(c17) and P450(arom) was intense in spherical 7- to 10-mm blastocysts, but was absent in earlier stage 2- to 4-mm blastocysts and less intense or abs ent in later stage 20-mm and filamentous embryos. Cytochrome P450(c17) was limited to the trophoblast of all blastocysts expressing the enzy me, and in spherical 7- to 10-mm blastocysts, essentially all cells of the trophoblast layer stained positively for P450(c17). However, as e longation became apparent in 10-mm blastocysts, the cells of the troph oblast became flattened, and the expression of P450(c17) declined part icularly in those trophoblast cells adjacent to the embryonic disc whe re mesoderm outgrowth was occurring. In fact, two distinct populations of trophoblast cells became obvious: one that maintained P450(c17) ex pression, and one that did not. Moreover, those trophoblast cells expr essing P450(c17) were less flattened than neighboring cells in which P 450(c17) expression was absent. These two morphologically and function ally distinct trophoblastic cell populations were most obvious in area s furthest from the embryonic disc. Cytochrome P450(arom) was expresse d in the trophoblast as well as the hypoblast under the embryonic disc . Neither P450(c17) nor P450(arom) appeared to be expressed in the emb ryonic disc or the mesoderm of the expanding blastocyst. These functio nal and structural changes in the embryonic cell layers of the elongat ing conceptus may be associated with the transient synthesis and secre tion of estrogen that occur at the time of maternal recognition of pre gnancy in the pig.