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THE HORMONE‐SYNTHESIZING TROPHOBLASTIC CELL IN VITRO: A MODEL FOR CANCER RESEARCH AND PLACENTAL HORMONE SYNTHESIS

The human trophoblast, the hormonally functional cell of the placenta, differentiates in the first-order cleavage of the fertilized human ovum immediately following conception.1 Hormonal function from the placental trophoblast can be detected in maternal serum by human chorionic gonadotropin (HCG)2 and human placental lactogen (HPL) assays shortly following the 58-cell stage of the human ovum. Thus, trophoblastic cellular differentiation and placental hormone synthesis are among the earliest morphological and biochemical differentiations to occur in the human. This cell type has not been established in vitro. Choriocarcinoma is a unique malignancy of trophoblastic cells which follows normal pregnancy, spontaneous abortion, or hydatiform mole. This tumor closely parallels the normal placenta in hormone function and cellular metabolism, with the exception that the limited invasiveness of the normal placenta during its implantation is far exceeded by that of the malignant tumor, which, proceeding unrestricted, culminates in the death of the patient within a period of less than 1 year.4 A morphological similarity can be observed among the trophoblastic cells of the human blastocyst less than 92 hours after conception (FIGURE l ) , the invading normal placenta of the first trimester (FIGURE 2), and the rapidly proliferating choriocarcinoma (FIGURE 3 ) .