UNIVITELINO TWINS: CHARACTERISTICS, HOW THEY ARE FORMED AND TYPES - BIOLOGY - 2025

The identical twins or monozygotic are individuals resulting from a multiple pregnancy. In this type of pregnancy, individuals come from a single fertilized oocyte (zygote) that separates at some stage of development.

It occurs when the zygote separates resulting in two cells or groups of daughter cells that develop independently. The stage of development in which the zygote divides establishes the type of monozygotic twins that will result (monochorionic, dichorionic…).

Source: MultipleParent

Multiple pregnancies occur in many mammals. The female may give birth to more than one litter or offspring per birth (multiparous), or produce only one offspring at each birth (uniparous).

Multiple pregnancies have been used as case studies and models for a variety of purposes, from genetic studies with implications for embryonic development and the development of genetic diseases, to psychological, behavioral and social studies.

Characteristics of univiteline twins

Identical twins come from a single zygote and may or may not share the same placenta and amniotic sac. These individuals are characterized by sharing their genetic makeup so they are of the same sex. They are very similar in their blood groups, fingerprints, and physical appearance.

Although univitelline twins share 100% of their genes, differences caused by hereditary disorders can be detected in them, manifested in only one of them. These individuals may differ by somatic cell mutations, antibody composition, and degree of susceptibility to certain diseases.

Differences in these traits may be due to epigenetic changes. It has been shown that in twin individuals the epigenetic profiles are more similar at an early age, beginning to differ over the years.

These differences are more notable if the twins develop in different environments, suggesting that certain factors such as exposure to tobacco, physical activity, and diet have a significant effect on the epigenetics of these individuals.

Multiple pregnancies in humans

In humans, a multiple pregnancy can be caused by the fertilization of two oocytes by different sperm, which are called dizygotic or fraternal twins.

In this case, the individuals present significant genetic variations, and may be of different sexes, since they develop from a unique combination of gametes with their own genetic variability.

Dizygotic twins look like each other (genetically) just as any pair of siblings born at different births look like.

Monozygotic twins completely share their genes and are always of the same sex, being very similar to each other, also receiving the name of identical twins.

There are rare cases, apparently random, where one of the embryos develops a defect or special condition in one of its cells during an early stage of development, such as the inactivation of certain genes. This causes a member of the twin pair to be born with some genetic disease - for example, muscular dystrophy.

Multiple pregnancies can also result in more than two individuals (from triplets to more than 10 individuals per delivery). In these cases, one of the pairs of individuals may come from a single zygote, while the others come from different zygotes.

Incidence

Of the 100% of cases of multiple pregnancies, only 30% are monozygotic twins. Treatments with ovulation inducing agents, assisted reproductive techniques, and pregnancies in elderly women are factors that increase the incidence of multiple pregnancies of dizygotic twins.

In recent decades, the occurrence of this type of pregnancy has increased significantly due to these factors.

Monozygotic twin pregnancies are not influenced by the factors mentioned above, which explains why their occurrence occurs much less frequently. Only between 2.3 to 4 out of every 1000 pregnancies are identical twins.

Genetic studies

Research on genetic diseases in multiple pregnancies is carried out in certain study animals.

These studies consist of controlling a series of genetic, environmental and physiological factors in one of the twins. In this way, it is possible to compare the development of the tissue or organ involved in the condition and the effect of said diseases on them, both in the affected individual and in the healthy one.

Other studies are carried out in mono and dizygotic twins, where one of the individuals is affected by a specific condition or disease. When the corresponding twin also manifests the condition, it is determined that the disease is concordant for that specific trait.

The study organisms thrive under equivalent environmental conditions. In some cases, a certain trait or condition shows greater concordance in monozygotic twins than in dizygotic twins. This indicates that the disease or trait studied is genetically determined.

The concordance index for a certain trait between identical twins indicates the level of relevance of genetic factors in determining that trait or condition.

How are they formed?

Univitheline twins arise as a consequence of the division of the first blastomeres in the early stages of embryonic development.

In the embryonic development of mammals, there is a stage called cellular commitment, in which cells are "marked" with a path towards a certain cellular differentiation.

In the development of identical twins, cell involvement involves a conditional specification of cells. This mechanism involves the interaction between neighboring cells. In this way, the differentiation of an embryonic cell (blastomere) is conditioned by signals from neighboring cells.

In the early stages of the embryo's development, the cells of the inner cell massif are pluripotent, that is, they can form any cell type of the embryo. It is in this stage where the blastomeres undergo division, originating two embryos that will develop individually.

The events that cause blastomeres to divide in the early stages of development are not yet very clear.

Types

Univitelline twins can be classified according to the relationship between the fetuses and their membranes, relating the moment of development in which the separation occurred with the compartment of the chorionic and amniotic membranes of developing individuals.

Diamniotic and dichorionic cufflinks

The earliest separation occurs in the two-celled period, with two separate zygotes developing and each blastocyst implanting with a separate placenta and chorionic membrane. This process is carried out in the first three days after fertilization.

Although the development of this type of twins is similar to that of bivitelline twins, these individuals continue to share 100% of their genes. When twins are separated by this process, they are considered diamniotic and dichorionic, representing between 20% and 30% of cases.

Separation after the third day from fertilization implies the beginning of vascular communications between the placentas.

Diamniotic and monochorionic twins

Within two weeks from fertilization, zygote separation originates in the early stage of a blastocyst, where the inner cell mass divides into two cell groups within the trophoblastic envelope cavity.

In this case, the embryos share a placenta and a chorionic membrane, but will develop into separate amniotic membranes.

These twins are called diamniotic monochorionics and represent 70% of the cases of univitelline twins.

Monoamniotic and monochorionic twins

The less frequent case is the separation in the bilaminar germ disc stage, before the appearance of the primitive streak. In this case, the amniotic membrane has already formed, so the twins will share the placenta and the chorionic and amniotic sacs.

These twins are called monoamniotic monochorionics and represent only about 1% of cases.

There are cases of univiteline triplets, but they are extremely rare, with an incidence of less than 1 in 7600 pregnancies.

Univetyl twin pregnancy abnormalities

There is a high probability that functional and structural defects occur during a monozygotic twin pregnancy. About 15% of these pregnancies suffer from some abnormality, such as acardia, papyraceous fetus and conjoined twins.

In addition, these types of pregnancies have a higher perinatal mortality rate and a greater propensity for premature delivery. Some studies indicate that only 29% of twin pregnancies reach term by giving birth to two healthy individuals.

Between 5 and 15% of the cases of monochorionic and monoamniotic univithelial twins, the twin transfusion syndrome occurs. This condition causes placental vascular anastomoses to form so that one twin receives more blood flow than the other.

Conjoined twins

When the division of the embryo occurs at an advanced stage of development, the rupture of the primitive nodule and the primitive streak can lead to the formation of conjoined twins, better known as Siamese twins.

These individuals develop united by some body region, where they are vascularly connected, and may or may not share some of their organs. Siamese twins can be classified according to the degree of union and the region by which they are united.

Omphalopagi are the most common type of Siamese and the most likely to be separated. These are united by the umbilical region.

Thoracópagos are united by the anterior region of the thorax and can share the cardiac chambers. Survival of individuals to separation surgery is very low.

The hamstrings are united by the pelvis, being able to present one or two pairs of lower limbs. The bony unions in these Siamese twins make their separation almost impossible.

Pygópagos unite at the sacrum and craniopagi by the head and are the two rarest cases.

In asymmetric Siamese twins, one of the twins is incomplete and totally dependent on its pair (parasitic twin).

References

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