Early embryonic development is a highly organized process involving the differentiation of cells into specialized layers. This process unfolds with precision, following initial instructions to build a complex structure.
Formation from the Inner Cell Mass
Following fertilization, a series of cell divisions leads to a structure known as a morula. The cells of the morula reorganize to form a cavity, creating what is called a blastocyst. The blastocyst has two main parts: an outer cell mass that will contribute to the placenta, and an inner cell mass (ICM), or embryoblast, which is the source of the embryo itself.
During the second week of development, the cells within the ICM segregate and differentiate. This process forms a two-layered structure called the bilaminar embryonic disc. The upper layer of columnar cells is the epiblast, while the lower layer of smaller, cuboidal cells is the hypoblast.
This separation establishes the embryo’s fundamental layout. The epiblast is positioned dorsal to the hypoblast, creating a dorsal-ventral axis. The epiblast is adjacent to the future amniotic cavity, while the hypoblast is next to a cavity called the blastocoel.
The Role of the Hypoblast
The hypoblast plays a supportive part in early development but does not contribute to the formation of the actual embryo. Its primary role is to form structures outside the embryo proper. Its cells migrate to line the blastocoel, creating the primary yolk sac, which provides initial nourishment and gives rise to the extraembryonic endoderm.
The hypoblast also functions as a signaling center, releasing molecular signals that guide the development of the overlying epiblast. These signals help determine the body’s axes and establish bilateral symmetry. For instance, signals from the hypoblast are responsible for inducing the formation of the primitive streak in the epiblast.
After its signaling and structural roles are complete, the hypoblast is displaced and replaced by a new layer of cells derived from the epiblast. Its function is therefore transient but directive, setting the stage for the epiblast to build the embryo.
The Epiblast and Gastrulation
The epiblast is the source of all tissues in the developing embryo. During the third week of development, the epiblast undergoes gastrulation, a process of cellular rearrangement that converts the two-layered disc into a three-layered structure. Gastrulation is initiated by the primitive streak.
The primitive streak is a groove that appears on the epiblast’s surface. Epiblast cells migrate toward this streak, detach, and move inward in a process called invagination. This inward movement of cells creates new cell layers beneath the existing epiblast.
The first wave of migrating epiblast cells moves through the primitive streak and displaces the hypoblast cells, forming a new bottom layer called the endoderm. A subsequent wave of epiblast cells migrates through the streak and positions itself between the newly formed endoderm and the remaining epiblast. This middle layer becomes the mesoderm. The epiblast cells that do not migrate and remain on the surface become the ectoderm. This process results in the formation of a trilaminar embryonic disc composed of three primary germ layers.
The Three Primary Germ Layers
The three primary germ layers that form from the epiblast—ectoderm, mesoderm, and endoderm—are each destined to form specific tissues and organs.
The ectoderm, the outermost layer, gives rise to the nervous system, including the brain and spinal cord. It also forms the outer layer of the skin, hair, and nails.
The mesoderm, or middle layer, develops into a wide array of structures, including the skeletal system, muscles, and the circulatory system (heart, blood vessels, and blood cells). The kidneys are also derived from mesodermal tissue.
The endoderm, the innermost layer, forms the linings of the digestive and respiratory tracts. It also gives rise to major glands such as the liver and pancreas.