Trophoblast

In the first days of pregnancy, what exists is not yet an embryo with a recognizable shape: it is a small ball of cells called a blastocyst. The cells that form its outer layer have a name of their own, trophoblasts, and that name comes from a root meaning to feed or nourish. The name alone tells you what they are for.

At the end of the first week the blastocyst reaches the wall of the uterus, attaches, and embeds itself in the lining, and it does so through those very outer cells. Something striking happens there: when implantation succeeds, the most superficial trophoblast cells fuse with one another and stop being separate cells, becoming a single body with many nuclei, the syncytiotrophoblast. That body digests cells of the endometrium, which is the lining of the uterus, and in that way it anchors the blastocyst firmly, like a root working its way into soil. And the trophoblast also sends out a chemical message, human chorionic gonadotropin, or hCG, which tells the corpus luteum, a structure in the ovary, to stay alive and keep producing progesterone and estrogen.