Life’s First Steps: The Embryoblast

What is an Embryoblast?

Once mammalian fertilization occurs, a blastocyst begins to form, initiating embryonic development. This spherical structure contains two groups of cells: the trophectoderm and the embryoblast, also known as the inner cell mass. Simply put, the trophectoderm aids in embryonic implantation into the uterus, while the embryoblast gives rise to the embryo. The embryoblast forms to one side within the blastocyst and is surrounded by the while the trophectoderm’s single-cell layer.

The embryoblast is formed around the second week of gestation after various stages: fertilization and cleavage, blastocyte development, implantation, and differentiation.

Fertilization and Cleavage

It all starts when a sperm cell fertilizes an egg cell (oocyte), and a zygote forms. Within the fallopian tube, the zygote quickly undergoes cleavage, a sequence of recurrent cell divisions.

A sphere of blastomere cells forms as the cells within the zygote multiply through cell divisions. Around three days after fertilization in humans, the cell divisions create about 12 to 32 blastomeres, and the zygote differentiates into a morula.

By this time, the morula enters the uterus as its cells divide. The cell divisions give rise to the internal group of cells that differentiate into the embryoblast and the external group of cells that become the trophectoderm.

Blastocyte Development

About eight days after fertilization, an amniotic cavity, the blastocoele, starts forming as the two groups of cells differentiate. The cellular sphere now contains a blastocoele, a fluid-filled space; thus, the sphere is now called a blastocyst. At the same time, the embryoblast undergoes two cellular divisions, which give rise to two cell layers: the epiblast and the hypoblast.

The epiblast (primitive ectoderm) originates from the embryoblast cells closer to the trophectoderm layer. Some cells from the epiblast differentiate into a layer of cells called amnioblasts which release amniotic fluid that fills and widens the cavity. The hypoblast (primitive endoderm) originates from embryoblast cells exposed to the blastocyst’s fluid cavity. Later in development, the hypoblast will become the endoderm, while the epiblast will become the ectoderm. Lastly, the trophoblast cells give rise to the placenta and are essential for early development processes like placenta implantation and the endocrine support of pregnancy.

Implantation and Differentiation

The blastocyst should be implanted, or attached to the uterine endometrium, for the embryoblast to differentiate into the embryo. This step occurs 8–10 days after fertilization of the oocyte.

Three main stages occur in the implantation process: adplantation, adhesion, and embedding. Implantation begins with the adplantation stage, during which the blastocyst loosely attaches to the uterus. The blastocyst then “rolls” to the site of implantation, where adhesion begins. During this step, it firmly attaches itself to the endometrium. Uterine trophoblast invasion and embryonic embedding are the last stages of implantation. The placenta continues to develop after the blastocyst is embedded to support fetal and embryonic development.

The blastocyst cells continue to develop while embedded in the uterus. By the third week after fertilization, the embryoblast cells within the blastocyst sphere start differentiating into an embryo. Gastrulation then gives rise to the three embryonic germ layers: the ectoderm, the mesoderm, and the endoderm.

The epiblast becomes the embryonic ectoderm, while the hypoblast becomes the embryonic endoderm. Then, cells between the epiblast and the embryonic endoderm form the intraembryonic mesoderm. The trophoblast cell layer differentiates into the cytotrophoblast, syncytiotrophoblast, trophoblastic column, and extravillous trophoblast cells.

Clinical Significance

The embryoblast is of great interest, apart from being a critical component of development, because it contains stem cells that give rise to all other cells through differentiation. Stem cells have many clinical implications, including the following:

• New drugs: Before using new treatments on patients, the drug’s safety and effectiveness are tested on stem cells that differentiate into the targeted tissue.
• New therapies: Stem cells replace other cells damaged or affected by a disease.
• Increase understanding of diseases: Stem cell differentiation is studied to understand how diseases and conditions develop.

Summary The embryoblast is a mass of cells that forms in the earliest stages of embryonic development within the blastocyst. The steps involve oocyte fertilization, differentiation of the zygote into the morula, and differentiation into a blastocyst which gives rise to the embryoblast.

The embryoblast contains two layers: the epiblast, which becomes the ectoderm, and the hypoblast, which becomes the endoderm. The blastocyst structure also has a group of cells called the trophectoderm, which aids in the process of implantation of the blastocyst carrying the developing embryo into the uterus. The trophectoderm also generates the trophoblast cells, which give rise to the placenta and support early development.