Week 2

Trophoblast

Trophoblast forms two layers:

  • Cytotrophoblast as said previously is the inner layer of cuboidal, epithelial like cells.
  • Syncytiotrophoblast is an amorphous mass of protoplasm that begins to invade the uterine endometrium. It is derived from the fusion of cells in the cytotrophoblast.

Embryoblast

Embryoblast forms a two layered disc (bilaminar germ disc)

  • Epiblast is the more dorsal mass. It will give rise to all three germ layers of the trilaminar embryonic disc in the third week.
  • Hypoblast is the layer of cells located ventral to the epiblast and which will contribute to the formation of the primary yolk sac.

Cavities

Two cavities form during the second week of development:

  • Amniotic cavity develops dorsal to the epiblast.
  • Primary yolk sac develops ventral to the hypoblast.

Mesoderm

Extraembryonic mesoderm forms two layers during the second week of development:

  • Somatic layer of extraembryonic mesoderm develops adjacent to the cytotrophoblast. Together they form the somatopleure.
  • Splanchnic layer of extraembryonic mesoderm develops adjacent to the hypoblast. Together they form the splanchnopleure.

Week 2 - Day by Day

  • Day 09
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    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.

    The epiblast continues as a layer of columnar epithelial cells. A cleft begins to develop just dorsal to these cells. Further development of the epiblast in week three will result in the production of three primary germ layers from which all of the organs and tissues of the embryo will develop.

    Cells from the hypoblast surround the primitive yolk sac forming what is called the exocoelomic (Heuser's) membrane. Note that at this stage, the wall of the primitive yolk sac is in contact with the cells of the cytotrophoblast layer. Soon, a loose aggregate of new cells will appear between these to layers to form what is known as the extraembryonic mesoderm.

    The primitive amniotic cavity appears as a cleft in the cells above the epiblast.

    The primitive yolk sac is lined by cells that are derived from the hypoblast.

  • Day 12
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    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.

    By day 12, the cells of the embryoblast have differentiated into two layers. The dorsal or topmost layer is termed the epiblast. It consists of a single layer of columnar cells.

    By day 12, the cells of the embryoblast have differentiated into two layers. The ventral of lowermost layer is called the hypoblast. It consists of a single layer of cuboidal shaped cells. Cells that are derived from this layer surround and line a cavity known as the primitive yolk sac.

    A cleft appears in the cells above the epiblast. This is the beginning of the development of the amnion and the cavity which it surrounds, the amniotic cavity.

    The primitive yolk sac is lined by cells derived from the hypoblast layer. These cells constitute what is known as the exocoelomic membrane or Heuser's Membrane.

    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
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    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 bilaminar embryonic disc is very prominent. The upper layer is still termed the epiblast. In the next week, the cells in the epiblast layer will differentiate into the three germ layers (ectoderm, mesoderm and endoderm) of the embryo that are responsible for building all of the organs and tissues.

    The bilaminar embryonic disc is very prominent. The lower layer is the hypoblast. Cells derived from this layer proliferated to form a new cavity that displaced the primitive yolk sac to become the definitive or secondary yolk sac.

    A cleft which developed in the cells just dorsal to the epiblast continues to enlarge to become the amniotic cavity. Later, the amniotic cavity will fill with fluid and completely surround and protect the growing fetus.

    The original yolk sac, lined by cells derived from the hypoblast, is displaced by a second set of cells that originate from the epiblast and which become the endoderm. The displaced primitive yolk sac is known now a an exocoelomic cyst. The new sac is known as the definitive or secondary yolk sac. Certain cells in the wall of the yolk sac differentiate into angiogenic cell clusters that will eventually contribute to the cardiovascular system. Other cells in the wall of the yolk sac will differentiate into primordial germ cells that will migrate into the body of the embryo to seed the developing gonads. The yolk sac itself will eventually become incorporated into the developing digestive tract.

    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).