DNA in the male pronucleus is packed very tightly with protamines to make it compact enough to fit inside a sperm. These protamines are replaced by histones inside the egg, unpacking the DNA. Afterwards the male and female pronuclei fuse and the egg completes its second meiotic division, resulting in a second polar body.
The zygote undergoes a number of ordinary mitotic divisions that increase the number of cells in the zygote but not its overall size. Each cycle of division takes about 24 hours. The individual cells are known as blastomeres.
At the cell stage the embryo is known as a morula L. The inner cell mass will eventually become the embryo and fetus , while the outer cell mass will eventually become part of the placenta.
The cells on the outside of the morula form tight intercellular junctions and express ion channels to create an impermeable barrier. A fluid-filled cavity forms inside the morula. This cavity is known as the blastocyst cavity or blastocoele , and the morula is now called a blastula or blastocyst.
The inner cell mass is now known as the embryoblast and the outer cell mass becomes the trophoblast. The blastula sheds its zona pellucida. This is required for implantation to occur.
One function of the zona pellucida is to prevent premature implantation. The embryo attaches to and invades into the maternal endometrium.
The trophoblast differentiates into the cytotrophoblast and the syncytiotrophoblast. Every new cell is thus only half as large as the cell from which it derives. The name of this stage comes from its resemblance to a mulberry, since it really looks like a collection of spherical cells.
From two-cell state to young morula 96 hours. The cleavage divisions of the individual cells do not have to happen synchronously and thus phase shiftings can occur. List of the chapters Next page. Early development Implantation The trophoblast The embryoblast. Chronology of the appearance of human embryonic structures:.
Trophoblast Amnion Umbilicus Extraembryonic coelom Allantois. The embryo arises from the inner cells thus also called the embryoblast. The outer cells form the first source for the placental membranes and have been given the name trophoblast.
On the 4th day of development 3 the morula, which now consists of roughly 30 cells, begins with assimilating fluid that is initially secreted by the blastomeres and fills the intercellular space. Under the influence of the hydrostatic pressure of this fluid, a hollow space arises in the morula, the so-called blastocyst cavity. In this stage the embryo is called a blastocyst 3.
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