• Day 09
    Stacks Image 16

    The cells of the trophoblast differentiate into two layers: an inner cytotrophoblast which retains distinct cellular boundaries and an outer syncytiotrophoblast which is a syncytium.
    The syncytiotrophoblast displaces endometrial cells at the point of implantation. Apoptosis of the endometrial cells facilitates the process. Endometrial stromal cells near the implantation site assume a polyhedral shape and accumulate lipids and glycogen to become decidual cells. The syncytiotrophoblast ingests these cells as a source of nutrition. A fibrin clot seals the defect in the endometrium.

    The cytotrophoblast continues to be a cuboidal epithelium. Cells derived from this layer fuse with one another and become part of the syncytiotrophoblast. Remember, a syncytium is a mass of cytoplasm that contains many cell nuclei but no distinct cellular boundaries.

    The syncytiotrophoblast, which enlarges by the fusion of cells that enter it from the cytotrophoblast layer continues to erode deeper into the endometrium. Spaces called lacunae begin to develop in the syncytiotrophoblast. Eventually, after the syncytiotrophoblast has eroded the walls of the maternal blood vessels, these lacunae will become filled with maternal blood.

  • Day 12
    Stacks Image 43

    The cytotrophoblast layer of the original trophoblast continues to exist as a lining layer of simple cuboidal epithelial cells. Note that at the embryonic pole (adjacent to the developing embryo) that this layer is a little thicker. It will eventually give rise to columns of cells that will contribute to the primary villi of the placenta.

    The syncytiotrophoblast continues to erode more deeply into the endometrium. As it does so, it erodes the walls of blood vessels in the uterine wall (maternal sinusoids) and blood from these sinusoids begins to percolate throughout the lacunae within the syncytiotrophoblast. This is the beginning of the uteroplacental circulation that is necessary to permit exchange of nutrients and waste materials between the maternal and fetal circulations.

    A loose collection of cells develops between the cytotrophoblast and the outer surface of the primitive yolk sac. This tissue is called extraembryonic mesoderm because it is outside of the developing embryo and is a layer that is in the "middle" of or between two other layers (cytotrophoblast and primitive yolk sac)("meso" means middle). Eventually, clefts will develop within this mesoderm leaving just two layers of the mesodermal cells. One of these layers will adhere itself to the cytotrophoblast layer and will become known as the somatopleuric layer of extraembryonic mesoderm. The other layer will attach itself to the outer surface of the primitive yolk sac and will become known as the splanchnopleuric layer of extraembryonic mesoderm. The space between these layers is called the extraembryonic coelom. Coelom is the generic name given to any cavity that is lined by mesoderm. Since this particular mesoderm lined cavity is located outside of the embryo proper, it is called "extraembryonic"

  • Day 13
    Stacks Image 72

    The cytotrophoblast still consists of a single layer of cuboidal cells. Note that in the region of the developing embryo, they form the central core of what are called primary villi. Thse will become functional villi as this region develops further into the placenta.

    The syncytiotrophoblast has eroded into the maternal blood vessels in the endometrium. Maternal blood now fills the lacunar spaces and is able to provide nourishment to the embryo by way of simple diffusion. Note that the syncytiotrophoblast covers the cytotrophoblast in the region of the primary villi.

    The cavity of the original blastocyst is now lined by a layer of extraembryonic mesoderm that is attached to the cells of the cytotrophoblast (somatopleuric mesoderm). In addition, a layer of extraembryonic mesoderm covers the outside of the secondary yolk sac (splanchnopleuric mesoderm). Because mesoderm makes up the lining of this cavity, we call it a coelom. Because this cavity is outside of the embryo proper, it is the extraembryonic coelom or chorionic cavity. Where the somatopleuric and splanchnopleuric mesodern come together, in the region of the developing embryo, we have the formation of a connecting stalk. You can think of the whole arrangement at two bubbles (amniotic cavity & secondary yolk sac) inside a balloon (chorionic cavity, aka extraembryonic coelom).