Zygote

Zygote is the cell resulting from the fertilization of a secondary oocyte by a sperm. The secondary oocyte finishes its second meiotic division immediately after the entry of the sperm and becomes ovum.

Fertilization occurs in the ampullary part of fallopian tube so zygote is formed in the ampullary part of fallopian tube.   Immediately after its formation it starts: 1.    Moving towards uterus 2.    Multiplying mitotically

Cleavage is a mitotic type of cell division where the daughter cells are smaller than the parent cells. The number of cells increases but the size of the daughter cells and the cell-mass does not increase.

Zygote immediately after its formation, in the ampullary part, is guided medially through the uterine tube toward the uterine cavity. As the zygote, passes through the uterine tube, it undergoes mitotic cell divisions known as cleavage.

This phase of development begins with the first mitotic division of the zygote and ends with formation of blastocyst. It extends for 6 to 7 days or a week.

The 1st division of zygote gives rise to two daughter cells The 1st division of zygote gives rise to two daughter cells. They are called blastomeres. Each blastomere is half the size of parent cell. They develop about 30 hours after fertilization. Subsequent divisions follow one another, forming progressively smaller blastomeres.

After three or four divisions, the zygote, looks like a mulberry and is known as morula (L. Morus, mulberry). It is a solid mass of 12 to 16 cells (blastomeres). This stage is reached about three days after fertilization and the mass of cells is about to enter the uterus.

At this time, the morula consists of a group of centrally located cells, the inner cell mass, and a surrounding layer, the outer cell mass.

The inner cell mass will give rise to the tissues of the embryo proper, while the outer cell mass forms the trophoblast which later contributes in the formation of placenta and other membranes. Note also the degeneration process of zona pellucida.

Morula enters uterine cavity about 4th day after fertilization Morula enters uterine cavity about 4th day after fertilization. As the morula enters the uterine cavity fluid begins to penetrate through the zona pellucida. Spaces appear between the inner cell mass and outer cell mass.

The fluid-filled spaces soon fuse to form a single large space or cavity, pushing the whole inner cell mass to one side. The conceptus is now called blastocyst and the cavity is known as blastocyst cavity or blastocele.

During the formation of blastocele the cells of outer cell mass becomes flattened. So the outer cell mass is now a single layer of flattened cells forming the wall of blastocyst and it is now given the name trophoblast (Gr. Trophe, nutrition), which will form placenta and associated membranes.

The cells of the inner cell mass are attached to one side of trophoblast and project into the blastocyst cavity. They are now referred to as embryoblast, which will form the embryo.

The side where the embryoblast is present is called embryonic pole of the blastocyst. The other side obviously will be abembryonic pole.

The zygote lies free in the uterine secretions for about two days. During this time the zona pellucida degenerates and disappears. The blastocyst hatches out and this is called hatching of blastocyst.

At about 6th day after fertilization the embryonic pole of the blastocyst becomes attached to the endometrium and start penetrating into it.

Zona pellucida Zona pellucida is created around primary oocyte when primordial follicle is transformed into primary follicle. So Zona Pellucida is not present in primordial follicle while primary follicle is characterized by Zona Pellucida.

In each menstrual cycle 12 to 15 primordial follicles are selected for maturation yet only one is guided to full maturity. As any primordial follicle starts maturing its follicular cells and perhaps primary oocyte, secrete a layer of glycoproteins on the surface of oocyte, thus forming zona pellucida.

This Zona Pellucida is present in all follicles (primary follicles, secondary follicles, tertiary follicles and Gra’afian follicles) except primordial follicles. Zona Pellucida surrounds primary oocyte and itself Zona Pellucida is surrounded by granulosa cells.

Just before ovulation the primary oocyte is converted into secondary oocyte and it is always the secondary oocyte, which ovulates. Now the Zona Pellucida contains secondary oocyte and first polar body.

After ovulation the granulosa cells surrounding the secondary oocyte and zona pellucida are given the name corona radiata cells.

During fertilization the sperms after passing through corona radiata, become attached to Zona Pellucida. The enzymes released from the acrosome (esterases, acrosin, and neuraminidase) causes lysis of Zona Pellucida, thereby forming a path for the sperm through Zona Pellucida.

Once the sperm penetrates Zona Pellucida a change in the properties of Zona Pellucida occurs that makes it impermeable to other sperms. This is called zona reaction. This zona reaction is believed to result from the action of lysosomal enzymes released by cortical granules near the plasma membrane of oocyte.

After fertilization Zona Pellucida contains ovum with two polar bodies. Zona pellucida remains present around zygote and morula at different stages of development.

Shortly after the morula enters the uterus (about 4 days after fertilization), fluid starts appearing within the morula between the blastomeres. This fluid passes from the uterine cavity through the zona pellucida to form spaces.

As the spaces coalesce and enlarge the morula is converted into blastocyst. These uterine secretions start destroying the zona pellucida.

After the blastocyst has floated in the uterine secretions for about two days, the zona pellucida gradually degenerates, ruptures and disappears. The blastocyst hatches out and this is called hatching of blastocyst.

It is important to note that the life of zona pellucida is 15 days if there is no fertilization.   The life of zona pellucida is 20 days if wife becomes pregnant.