Mammalian embryogenesis

Mammalian embryogenesis is the process of cell division and cellular differentiation which leads to the development of a mammalian embryo.

From one cell to blastocyst
A mammal develops from a single cell called a zygote, which results from an oocyte (egg) being fertilized by a single sperm. The zygote is surrounded by a strong membrane of glycoproteins called the zona pellucida which the successful sperm has managed to penetrate.

The zygote undergoes cleavage, increasing the number of cells within the zona pellucida. When there are about 4 to 16 cells, the embryo is in the morula stage. When the number of cells reaches 40 to 150, a central, fluid-filled cavity (blastocoel) forms. The zona pellucida begins to degenerate. This stage in the developing embryo, reached after four to six days, is the blastocyst(blastula stage), and lasts approximately until the implantation in the uterus. The outer cells develop into the placenta.

Blastocyst grows and invades


In the blastocyst there are cells which eventually form the placenta, and inner cell mass (also called embryoblast) of undifferentiated cells which are a source of embryonic stem cells. The blastocyst can be thought of as a ball of a (mostly single) layer of trophoblast cells, with inner cell mass attached to this ball's inner wall. The embryo plus its membranes is called the conceptus. By this stage the conceptus is in the uterus. The zona pellucida ultimately disappears.

The trophoblast then differentiates into two distinct layers: the inner is the cytotrophoblast consisting of cuboidal cells that are the source of dividing cells, and the outer is the syncytiotrophoblast.

The cytotrophoblast implants the blastocyst in the endometrium (innermost epithelial lining) of the uterus by forming finger-like projections called villi that make their way into the uterus, and spaces called lacunae that fill up with the mother's blood. This is assisted by hydrolytic enzymes that erode the epithelium. The syncytiotrophoblast also produces human chorionic gonadotropin (hCG), a hormone that "notifies" the mother's body (though not necessarily the mother) that she is pregnant, which prevents menstruation

The villi begin to branch, and contain blood vessels of the fetus that allow gas exchange between mother and child.

Inner cell mass differentiation
While the syncytiotrophoblast starts to penetrate into the wall of the uterus, the inner cell mass (embryoblast) also develops.

The embryoblast forms a bilaminar (two layered) embryo, composed of the epiblast and the hypoblast. The epiblast is adjacent to the trophoblast and made of columnar cells; the hypoblast is closest to the blastocyst cavity, and made of cuboidal cells. The epiblast, now called primitive ectoderm will give rise to all three germ layers of the embryo: ectoderm, mesoderm, and endoderm. The hypoblast, or primitive endoderm, will give rise to extraembryonic structures only, such as the lining of the yolk sac.

Cavity formation
By separating from the trophoblast, the epiblast forms a new cavity, the amniotic cavity. This is lined by the amnionic membrane, with cells that come from the epiblast (called amnioblasts). Some hypoblast cells migrate along the inner cytotrophoblast lining of the blastocoel, secreting an extracellular matrix along the way. These hypoblast cells and extracellular matrix are called Heuser's membrane (or exocoelomic membrane), and the blastocoel is now called the primary yolk sac (or exocoelomic cavity).

Cytotrophoblast cells and cells of Heuser's membrane continue secreting extracellular matrix between them. This matrix is called the extraembryonic reticulum. Cells of the epiblast migrate along the outer edges of this reticulum and form the extraembryonic mesoderm, which makes it difficult to maintain the extraembryonic reticulum. Soon pockets form in the reticulum, which ultimately coalesce to form the chorionic cavity or extraembryonic coelom.

Another layer of cells leaves the hypoblast and migrates along the inside of the primary yolk sac. The primary yolk sac is pushed to the opposite side of the embryo (the abembryonic pole), while a new cavity forms, the secondary or definitive yolk sac. The remnants of the primary yolk sac are called exocoelomic vesicles.