How do zygotes reproduce

In humans, the zygote stage is on Day 1 of week one post-fertilization until the cell will cleave into two new cells. In humans, the embryo stage is the first eight weeks post-fertilization. An embryo is a living form consisting of many cells as a result of a zygote that underwent a series of mitosis and will soon develop a set of tubes.

When does a zygote become an embryo? In humans, at week one post-fertilization, the cells undergo extensive and rapid growth. As they continue to divide, they eventually form a solid mass of cells, called a morula. This mass of cells is not going to be a full solid sphere but a sphere with distinct layers i.

The inner cell mass will differentiate into cells that will later define an embryo. The trophoblast , in turn, will give rise to cells that will become the structures essential during the uterine wall implantation and the developmental growth of the embryo to the fetus in the uterus.

Thus, the zygote not only forms the embryo but it will also be the source of the subsequent outer fetal membranes i. Because the cells divide fast, with no time to grow, the morula tends to have the same size as the zygote. See the diagram below. From one-celled, it will soon become multicellular, as each cell will undergo mitosis. This stage is crucial because the dividing cells are more sensitive to the effects of mutagens.

Thus, there is a higher risk of genetic errors mutation during this stage. As a result, women are advised not to take any unprescribed medicine during pregnancy.

The cells have innate mechanisms to repair DNA errors, however, they may not be enough when the extent of the damage becomes overwhelming. The fetus is in the final development stage. In humans, the fetal stage begins in the 9th week post-fertilization up to the last week prior to the birth of the baby. This is the period of growth and organ development.

In humans, the zygote is the first cell stage of pregnancy. It is located first in the fallopian tube and moves towards the uterus. As the zygote travels, it divides to give rise to cells that will also undergo mitosis. Soon, the zygote will transition into an embryo that will be implanted in the uterus. There, the embryo will develop further into a fetus.

For more info and facts on human development, read the Developmental Biology Tutorials. These tutorials describe how a living thing grows and attains maturity. In many fungi and protists, the fusion of the nuclei of haploid cells produces a diploid zygote, called zygospore.

In fungi, the zygospores are found in the zygosporangia. The zygote displays inequality in number and alignment of NPBs. The latter are aligned in one PN and scattered in the other one with respect to the PN junction. A clear cortical zone with some inclusion bodies immediately below the area can be noted. It was replaced on Day 3 eight cell cleavage embryo but failed to implant. PNs are juxtaposed and centralized and the polar bodies are located parallel to the longitudinal axis through the PNs.

Observed 16 h after IVF. Diagram showing the plane through the polar bodies which is tangential to the contrasting plane through the longitudinal axis of the PNs. PNs are juxtaposed and centralized; polar bodies the first polar body is fragmented, the second is intact are located tangential to the longitudinal axis through the PNs. PNs are juxtaposed and centralized; polar bodies the first polar body is fragmented, while the second is intact are located tangential to the longitudinal axis through the PNs.

PNs, juxtaposed and centralized, are tangential to the plane through the polar bodies. It was transferred on Day 3 seven cells along with two other embryos but failed to implant. Zygote observed at NPBs are scattered in one PN and aligned in the other. The oocyte is irregular in shape and the PVS is enlarged. It failed to implant following transfer. Polar bodies are highly dysmorphic. Note the presence of small vacuoles in the cytoplasm. The failure of PNs to be juxtaposed and centrally positioned within the cytoplasm or having non-aligned polar bodies with respect to PNs at fertilization check could result in altered development, i.

Large angles between polar bodies Figs — have been suggested to be predictors of poor embryo development Gianaroli et al. This effect could be due to sub-optimal orientation of PNs Figs and generating a great degree of cytoplasmic turbulence, which could facilitate uneven cleavage or fragmentation Garello et al.

A clear cortical area and coarse granularity of the cytoplasm can be observed. It is possible to observe a clear cortical zone and some dark inclusion bodies. It was transferred and failed to implant. PNs are juxtaposed and centralized and the polar bodies form a great angle with respect to the longitudinal axis through the PNs. A clear cortical zone can be observed in the cytoplasm and the ZP is thick and very dense.

Polar bodies form a great angle with respect to the longitudinal axis through the PNs. It was transferred but implantation outcome is unknown. A zygote of irregular shape Polar bodies form a right angle: one is aligned with the longitudinal axis through the PNs and the other is aligned with the meridional axis. In human oocytes, the aster from the sperm centrosome organizes the microtubules, which control the abutment and apposition of PNs Figs — , and direct the formation of polar axes at syngamy by setting the plane of the first division.

The subsequent movements and rotations favour the distribution of the mitochondria and chromatin alignment, which are essential for correct development. It was transferred on Day 3 along with two other embryos and the patient delivered two healthy baby girls and one healthy baby boy.

It was transferred but the result is unknown. NPBs are aligned, but are different in size. There is debris in the PVS as well as fragmentation of one polar body presumably the first polar body , which is significantly separated from the other polar body presumably the second polar body. It was transferred but the outcome is unknown. PNs are juxtaposed in the cytoplasm, which has a clear cortical zone.

NPBs are small and aligned in one PN and scattered in the other. A refractile body is visible at the 11 o'clock position in this view. Failed progression to apposition and syngamy Figs — mostly depends on sperm centrosome activity. Observations of zygotes with separated PNs at fertilization check during subsequent development Figs — demonstrate severe delay or arrest in development Fig.

This condition is most frequently associated with pathological spermatozoa, particularly from epididymal or testicular samples Fig. NPBs are of different size, aligned in one PN and scattered in the other.

The cytoplasm is very granular. PNs are not juxtaposed, different in size and NPBs are symmetrically distributed. The cytoplasm appears slightly granular and the PVS is enlarged. A zygote generated by ICSI and observed at 16, It was discarded. The polar body had been biopsied. PNs are widely separated and contain scattered small NPBs. The cytoplasm is normal in colour but displays two vacuole-like structures which are evident in b.

It failed to cleave during further development. The position of PNs has a relevant effect on the first cleavage plane that normally goes through the pronuclear axis Scott, In the majority of zygotes with centrally positioned PNs Fig. Due to the dynamics of PN movements within the cytoplasm, their orientation on the polar axis varies depending on the progression of rotation Figs and towards the final state which determines the first cleavage plane Fig.

The PN longitudinal axis is parallel to the polar bodies. A clear cortical zone, the halo, appears in the cytoplasm and there is a slightly enlarged PVS. The PN longitudinal axis is almost tangential to the plane through the polar bodies.

The ZP is thick and brush-like. PNs are juxtaposed and centralized; polar bodies are aligned tangential to the longitudinal axis through the PNs. In cases of peripherally positioned PNs Figs and , cleavage occurs according to the pronuclear axis and results frequently in abnormal morphology Fig.

Nevertheless implantation can occur Fig. NPBs are different in number and distribution. One polar body presumably the first polar body is fragmented and the other is intact. NPBs are aligned and different in number. NPBs are small in size and scattered in both PNs. The derived embryo was highly fragmented with uneven blastomeres and was discarded.

The oocyte has a clear cortical zone in the cytoplasm. It was not transferred because of arrested development. The fertilized oocyte is spherical and shows two PNs peripherally located and partly overlapping in this view. NPBs are small and scattered in both PNs. At juxtaposition, nuclear membranes break down a few hours prior to initiation of the first cleavage division Figs and Syngamy occurs Fig.

Further development resulted in cleavage to 4 cell and arrest on Day 3. The observation was performed 15 h after ICSI. The NPBs are no longer distinct. The astral centrosome containing two centrioles splits and relocates to opposite poles of a bipolar spindle to establish bipolarization that controls cell division. The centrioles take a pivotal position on spindle poles, while chromosomes organize on the equator of the metaphase plate.

Anaphase and telophase ensue completing the first mitotic division. In contrast to some animal species, membrane fusion of PNs Fig. The observation was performed 16 h after ICSI. The PN membranes are indistinct, particularly between the PNs where they have broken down, looking like PN membrane fusion. Polar bodies had been previously biopsied. As PNs form after fertilization, there is polarized distribution in the chromatin into the furrow between them Van Blerkom et al.

As NPBs are attached to the chromatin, they should also polarize or align with it implying that, if there is correct chromatin polarization, the NPBs will appear polarized as well Figs — Due to the dynamics of this event, some zygotes show symmetry in appearance of the PNs, but a delay in the alignment of the chromatin into the furrow, or onto the mitotic plate Figs — The polar bodies are fragmented and overlapping in this view. NPBs differ in number and size between PNs.

The polar bodies are overlapped in this view. It was discarded due to subsequent abnormal cleavage. The cytoplasm appears granular. Polar bodies are fragmented; there is debris in the PVS. A zygote generated by IVF at During the progression of the cell cycle, NPBs change in number, size and distribution Fig.

In humans, each gamete has half the number of the total 46 chromosomes that the body requires. The 23 chromosomes within a gamete are referred to as a haploid. When egg and sperm cells combine in fertilisation, they merge the two sets of chromosomes, ending up with 46 chromosomes in total.

The maternal chromosomes from the egg cell and the paternal chromosomes from the sperm cell pair up. The resultant cell is called a zygote. It is diploid as it has two copies of every chromosome - one came from the sperm cell and one came from the egg cell. The zygote will mature into an embryo. It has DNA from both the mother and the father so will have a mixture of characteristics from both parents. In this way, sexual reproduction introduces variation into a species.

The first case can be discerned by formation of a total of three pronuclei, whereas the second process will remain undetected, because it involves a female and a single but diploid male pronucleus.

Digynic triploidy after intracytoplasmic sperm injection is characterized by nonextrusion of the second polar body and formation of three pronuclei.

Digyny can also result from the fertilization of diploid giant oocytes. Depending on how maturation of these gametes proceeds, three or only two pronuclei will be observed.