Week Two,Implantation Completed

Week Two, Implantation Completed

The blastocyst finishes implanting into the uterine wall, and through a series of cavitations, defines the amnion and early yolk sac.

Implantation Completed

During implantation, the inner cell mass becomes a bilayered disc known as the embryonic disc. Based on studies of mouse embryos, the upper epiblast layer is thought to form all the tissues of the entire embryo proper that develop into the fetus.

The lower hypoblast layer is thought to contribute only to extraembryonic (or non-fetal) tissues.

At this stage, the amniotic cavity forms above the epiblast, and will later engulf the embryo proper.

Exocoelom Cavity Formation

As embryonic cells proliferate, a series of cavities form reflecting the newly differentiated cell types. The extra-embryonic, or non-fetal tissues, expand to form a second cavity within the blastocyst called the exocoelom. It is contained within a layer of cells known as the primitive

A third cavity, the extraembryonic coelom, will now form around the primitive yolk sac.

Loose, migratory cells (mesenchyme) spill between the inner and outer walls of this extraembryonic coelom.

A portion of these mesenchyme cells will bridge the extraembryonic coelom to form a connecting stalk which later becomes the umbilical cord, connecting the embryo proper to the placenta forming from the trophoblast cells.

Definitive Yolk Sac Formation

Towards the end of the second week, the definitive yolk sac forms. The definitive yolk sac begins as an outpocket of hypoblast cells covered with extraembryonic mesoderm.

It is quickly replaced by the primary yolk sac which is immediately below the embryonic disc.

Week Three, Gastrulation

Once the embryo is fully embedded in the uterine wall, it initiates a process called gastrulation which converts its single epiblast cell layer into three unique cell layers.

Primitive Streak

Epiblast cells converge on the midline of the embryonic disc creating a furrow, the primitive streak marking a visible longitudinal axis of bilateral symmetry around which all embryonic structures and organs will align. From now on, the embryo will have distinct rostral (head) and caudal (tail) regions, as well as left and right sides and dorsal (back) and ventral (front) surfaces.

The rostral tip of the primitive streak ends in a small pit surrounded by a raised circle of cells, the primitive node. The node marks where epiblast cells invaginate into the mesoderm and endoderm layers. Epiblast-derived cells that migrate directly rostrally of the primitive node become the notochordal process. The elongating notochordal tissue contributes cells to the nervous system and spine.

Mid Third Week Gastrulation

With the formation of the notochordal process, the primitive streak gradually shortens, after reaching its maximum length during the middle of the third week. The hypoblast will eventually recede into the extraembryonic yolk sac and the epiblast will differentiate into three germ layers which will create every structure of the embryonic body.

Accompanied by rapid cell proliferation in the epiblast layer, cells converge on the midline of the bilayered embryonic disc.

Migrating cells emerge from the epiblast layer, forming a middle (mesoderm) layer between the epiblast and hypoblast.

At the same time, epiblast-derived cells migrate into the hypoblast layer, replacing the resident hypoblast cells in the area of the embryonic disc.

Late Third Week Gastulation

Gastrulation continues in the late third week as epiblast-derived cells migrate into the mesoderm layer along either side of the primitive streak.

The epiblast layer in the caudal region near the primitive streak gives rise to extraembryonic mesoderm; the middle region on either side of the primitive streak gives rise to the embryonic mesoderm and endoderm; and the rostral region of epiblast gives rise to the nervous system and skin.

After the epiblast cells that contribute to the mesoderm and endoderm have migrated through the primitive streak, the remaining epiblast differentiates into the ectoderm, precursor to the skin and nervous tissue of the neural crest.