During the process of implantation, the trophoblast differentiates into cytotrophoblast and syncytiotrophoblast layers. The syncytiotrophoblast layer grows larger and invades deeper, surrounding and eroding maternal vessels, forming lacunae filled with maternal blood. Interstitial implantation is almost complete when extensions of cytotrophoblasts, known as primary villi, form and become covered by a layer of syncytiotrophoblast. After interstitial implantation is complete, extraembryonic mesoderm develops from the epithelial layers (amnion, primary yolk sac) and forms an inner layer of the villi, known as secondary villi. Eventually, the mesoderm will give rise to umbilical blood vessels within the core of the villus, forming tertiary villi.

As implantation and placentation progress, syncytiotrophoblasts take on the functions of phagocytosis and bidirectional placental transfer. They also become highly steroidogenic, producing progesterone at sufficient levels to maintain pregnancy independently of the corpus luteum. In addition to progesterone, syncytiotrophoblasts produce other hormones and enzymes that modify hormones.

There is a maternal response to implantation known as decidualization. This response involves the transformation of the endometrial stroma, causing enlargement of stromal cells into lipid- and glycogen-filled decidual cells. The transformed endometrium, referred to as the decidua, forms an epithelial-like sheet with adhesive junctions that prevent migration of the implanting embryo. The decidua also secretes factors like tissue inhibitors of metalloproteinases (TIMPs) that regulate the activity of syncytiotrophoblast-derived hydrolytic enzymes in the endometrial matrix. This regulated invasion allows for proper implantation, without involving the myometrium.
Extraembryonic Primary Secondary Extraembryonic Remains of primary mesoderm yolk sac yolk sac forming coelom yolk sac •Fig. 44.33 A,Beginningofimplantation.Thetrophoblasthasdifferentiatedintocytotrophoblastand syncytiotrophoblast layers. B, As the syncytiotrophoblast layer increases in size and invades deeper, this layer begins to surround and erode maternal vessels, forming lacunae filled with maternal blood. C, Interstitial implantation is almost complete. Extensions of cytotrophoblasts have formed that will becomecoveredbyalayerofsyncytiotrophoblast.Atthispoint,theyarecalled“primaryvilli”.D,Interstitial implantationiscomplete.Extraembryonicmesodermalhasdevelopedfromtheepitheliallayers(amnion, primaryyolksac),andwillformaninnerlayerofthevilli,forming“secondaryvilli”.Ultimately,themesoderm willgiverise toumbilicalbloodvesselswithinthecoreofthe villus,therebyformingtertiaryvilli.(From CarlsonBM.Human Embryology and Developmental Biology.Philadelphia:Mosby;2004.) the corpus luteum and thus progesterone secretion. Syn-As implantation and placentation progress, syncytiotrocytiotrophoblasts also become highly steroidogenic. By 10 phoblasts take on the important functions of phagocytosis weeks the syncytiotrophoblasts acquire the ability to make (during histiotropic nutrition) and bidirectional placental progesterone at sufficient levels to maintain pregnancy transfer of gases, nutrients, and wastes. Exchange across independently of a corpus luteum. Syncytiotrophoblasts the syncytiotrophoblasts involves diffusion (e.g., gases), produce several other hormones as well as enzymes that facilitated transport (e.g., GLUT1-mediated transfer of modify hormones. glucose), active transport (e.g., amino acids by specific transporters), and pinocytosis/transcytosis (e.g., of irontransferrin complexes).
There is also a maternal response to implantation that involves transformation of the endometrial stroma. This response, called decidualization, involves an enlargement of stromal cells as they become lipid-and glycogen-filled decidual cells (at this time the endometrium is referred to as the decidua). The decidua forms an epithelial-like sheet with adhesive junctions that inhibit migration of the implanting embryo. The decidua also secretes factors such as tissue inhibitors of metalloproteinases (TIMPs) that moderate the activity of syncytiotrophoblast-derived hydrolytic enzymes in the endometrial matrix. Consequently, decidualization allows regulated invasion during implantation. Normally the implanting embryo and placenta do not extend to and involve the myometrium.