Embryology: Second Week of Development

Second Week of Development Overview

• By the seventh day post-fertilization, the blastocyst begins penetrating the endometrial wall, continuing the implantation process.
• Concurrent changes occur in both the trophoblast and the inner cell mass, also known as the embryoblast.

Primary Villi Formation

• Primary villi are formed during the second week of development.
• The blastocyst is enveloped by the trophoblast, which differentiates into two layers: the inner cytotrophoblast and the outer syncytiotrophoblast.
• The syncytiotrophoblast releases enzymes that erode the decidua, which facilitates invasion and increases in size to enclose the blastocyst.
• Lacunae, or spaces, appear initially near the embryonic pole and merge to form larger spaces.
• The blastocyst stage with lacunae is referred to as the lacunar stage.
• Trabeculae, composed of the syncytiotrophoblast, form between adjacent lacunae.
• Cytotrophoblastic cells pierce the trabeculae, migrating into their core.
• A primary villus is composed of a cytotrophoblast core surrounded by the syncytiotrophoblast layer.

Bilaminar Germ Disc Formation

• The inner cell mass or embryoblast rearranges to form a flattened cell layer, which becomes the hypoblast.
• Following hypoblast formation, the remaining embryoblast cells become columnar, forming the epiblast.
• The formation of the hypoblast precedes that of the epiblast.

Amniotic Cavity and Primary Yolk Sac Formation

• A small cavity appears within the epiblast layer.
• The cavity enlarges, splitting the epiblast cells.
• Some epiblast cells line the floor, while others, known as amnioblasts, line the roof of the cavity, forming the amniotic cavity.
• Amnioblast cells originate from the epiblast layer splitting.
• The hypoblast cells produce a new generation of cells that migrate along the inside of the cytotrophoblast, forming the Heuser's membrane.
• The blastocoel cavity is now lined by the Heuser’s membrane and the hypoblast cells, thus transforming into the primary yolk sac, also known as the primary umbilical vesicle.
• During bilaminar germ disc formation, a cavity appears inside the epiblast cells, forming the amniotic cavity. Simultaneously, the hypoblast also forms new cells which create the primary yolk sac.

Extraembryonic Mesoderm Formation

• Extraembryonic mesoderm, crucial for embryonic nutrition but not forming adult body structures, appears.
• The primary yolk sac wall secretes cells that occupy the space between the Heuser's membrane and the cytotrophoblast.
• These cells form a new cell mass, known as the extraembryonic mesoderm, which surrounds the bilaminar germ disc with its cavities.
• The extraembryonic mesoderm separates the cytotrophoblast from the yolk sac, amniotic cavity, and germ disc.
• The source of extraembryonic mesoderm cells is primarily the wall of the yolk sac, derived from hypoblast cells.

Extraembryonic Mesoderm Cavity and Layers

• Cavities form within the extraembryonic mesoderm, coalescing to create a larger cavity.
• This cavity divides the extraembryonic mesoderm into two layers: the splanchnopleuric and somatopleuric extraembryonic mesoderm.
• The layer surrounding the primary yolk sac is known as splanchnopleuric extraembryonic mesoderm.
• The layer surrounding the amniotic cavity and lining the cytotrophoblastic layer is known as somatopleuric extraembryonic mesoderm.
• The chorion consists of the cytotrophoblastic cell layer and the somatopleuric extraembryonic mesoderm.

Connecting Stock

• A connecting stock of extraembryonic mesoderm connects the developing germ disc to the outer cytotrophoblastic shell.
• The connecting stock is a critical structure through which vessels develop, nourishing the germ disc. It is a precursor to the umbilical cord.

Primary to Secondary Yolk Sac Conversion

• Hypoblast cells produce an additional cell layer that lines a smaller cavity within the primary yolk sac, forming the secondary yolk sac.
• The primary yolk sac divides into the smaller secondary yolk sac and a larger portion which pinches off to create the exocoelomic cyst.
• The exocoelomic cyst floats within the chorionic cavity.

Prechordal Plate Formation

• At one end of the hypoblast layer, some cells differentiate and become columnar, forming the prechordal (or prochordal) plate.
• The prechordal plate arises from the hypoblast cell layer.
• The location of the prechordal plate determines the embryo's cranial end.

Key Changes During Week Two

• Two layers form from the trophoblast: cytotrophoblast and syncytiotrophoblast.
• Two layers form from the embryoblast: hypoblast and epiblast.
• Two layers of extraembryonic mesoderm form.
• Two cavities form: the yolk sac and the amniotic cavity.
• Two types of yolk sacs form: primary and secondary.
• The primary yolk sac divides into two structures: the secondary yolk sac and the exocoelomic cyst.