How fast does a fertilized egg divide

The process of fertilization occurs in several steps and the interruption of any of them can lead to failure. At the beginning of the process, the sperm undergoes a series of changes, as freshly ejaculated sperm is unable or poorly able to fertilize.

The fertilized egg, known as a zygote, then moves toward the uterus, a journey that can take up to a week to complete until implantation occurs. Through fertilization, the egg is activated to begin its developmental process progressing through meiosis II , and the haploid nuclei of the two gametes come together to form the genome of a new diploid organism.

Nondisjunction during the completion of meiosis or problems with early cell division in the zygote to blastula stages can lead to problems with implantation and pregnancy failure. The process of cleavage is the step of embryogenesis where the zygote divides to produce a cluster of cells known as the morula.

Cell division with no significant growth that produces a cluster of cells that is the same size as the original zygote, is called cleavage. At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula. The different cells derived from cleavage up to the blastula stage are called blastomeres. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic total or meroblastic partial.

Cell cleavage : Early development is characterized by cleavage of the zygote, which refers to cell divisions that are not associated with significant growth of the embryo. Holoblastic cleavage occurs in animals with little yolk in their eggs. These species, such as humans and other mammals, receive nourishment as embryos from the mother via the placenta or milk after birth. On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk, such as birds and oviparous reptiles although some viviparous reptiles also exist.

Since cleavage is impeded by the vegetal pole, there is a very uneven distribution and size of cells. Cells are more numerous and smaller at the animal pole of the zygote than at the vegetal pole. In holoblastic eggs, the first cleavage always occurs along the vegetal—animal axis of the egg, and the second cleavage is perpendicular to the first. From here, the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage in various organisms.

The end of cleavage is known as the midblastula transition and coincides with the onset of zygotic transcription. In amniotes, the cells of the morula are at first closely aggregated. However, they quickly become arranged into an outer or peripheral layer, the trophoblast, and an inner cell mass. The trophoblast does not contribute to the formation of the embryo proper; the embryo develops from the inner cell mass. Fluid collects between the trophoblast and the greater part of the inner cell mass, and thus the morula, is converted into the blastodermic vesicle also called the blastocyst or blastula.

The inner cell mass remains in contact with the trophoblast at one pole of the ovum. This is named the embryonic pole, since it indicates the location where the future embryo will develop.

In the case of monozygotic twins derived from one zygote , a zygote divides into two separate cells embryos at the first cleavage division. Monozygotic twins can also develop from two inner cell masses. A rare occurrence is the division of a single inner cells mass giving rise to twins. However, if one inner cell mass divides incompletely, the result is conjoined twins. Dizygotic twins is the development of two embryos from two different zygotes.

The blastocyst forms early in embryonic development and has two layers that form the embryo and placenta. In humans, the blastocyst is formed approximatelyy five days after fertilization.

This stage is preceded by the morula. The morula is a solid ball of about 16 undifferentiated, spherical cells. As cell division continues in the morula, the blastomeres change their shape and tightly align themselves against each other.

This is called compaction and is likely mediated by cell surface adhesion glycoproteins. The blastocyst, though only the size of a pinhead, is actually composed of hundreds of cells. During the critically important process of implantation, the blastocyst must attach itself to the lining of the uterus or the pregnancy will not survive. Updated by: David C. Editorial team. Cell division. The egg is swept into the funnel-shaped end of one of the fallopian tubes. At ovulation, the mucus in the cervix becomes more fluid and more elastic, allowing sperm to enter the uterus rapidly.

Within 5 minutes, sperm may move from the vagina, through the cervix into the uterus, and to the funnel-shaped end of a fallopian tube—the usual site of fertilization. The cells lining the fallopian tube facilitate fertilization. If fertilization does not occur, the egg moves down the fallopian tube to the uterus, where it degenerates, and passes through the uterus with the next menstrual period.

If a sperm penetrates the egg, fertilization results. Tiny hairlike cilia lining the fallopian tube propel the fertilized egg zygote through the tube toward the uterus.

The cells of the zygote divide repeatedly as the zygote moves down the fallopian tube to the uterus. The zygote enters the uterus in 3 to 5 days. In the uterus, the cells continue to divide, becoming a hollow ball of cells called a blastocyst. The blastocyst implants in the wall of the uterus about 6 days after fertilization.

If more than one egg is released and fertilized, the pregnancy involves more than one fetus, usually two twins. Because the genetic material in each egg and in each sperm is slightly different, each fertilized egg is different.

The resulting twins are thus fraternal twins. Identical twins result when one fertilized egg separates into two embryos after it has begun to divide.

Because one egg was fertilized by one sperm, the genetic material in the two embryos is the same. Once a month, an egg is released from an ovary into a fallopian tube. After sexual intercourse, sperm move from the vagina through the cervix and uterus to the fallopian tubes, where one sperm fertilizes the egg. The fertilized egg zygote divides repeatedly as it moves down the fallopian tube to the uterus. First, the zygote becomes a solid ball of cells.

Then it becomes a hollow ball of cells called a blastocyst. Inside the uterus, the blastocyst implants in the wall of the uterus, where it develops into an embryo attached to a placenta and surrounded by fluid-filled membranes. About 6 days after fertilization, the blastocyst attaches to the lining of the uterus, usually near the top. This process, called implantation, is completed by day 9 or The wall of the blastocyst is one cell thick except in one area, where it is three to four cells thick.

The inner cells in the thickened area develop into the embryo, and the outer cells burrow into the wall of the uterus and develop into the placenta. The placenta produces several hormones that help maintain the pregnancy. For example, the placenta produces human chorionic gonadotropin, which prevents the ovaries from releasing eggs and stimulates the ovaries to produce estrogen and progesterone continuously.

The placenta also carries oxygen and nutrients from mother to fetus and waste materials from fetus to mother. Some of the cells from the placenta develop into an outer layer of membranes chorion around the developing blastocyst.

Other cells develop into an inner layer of membranes amnion , which form the amniotic sac. When the sac is formed by about day 10 to 12 , the blastocyst is considered an embryo.