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Background |
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WHAT IS CHROMOSOMAL MOSAICISM? | |||
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Chromosomal mosaicism is when different cells within an individual, who has developed from a single fertilized egg, have a different chromosomal makeup. Most commonly there will be some cells with a typical number of chromosomes (46 chromosomes) and other cells with an altered number or structure of chromosomes. The most common kind of chromosomal mosaicism found at prenatal diagnosis involves trisomy, where the abnormal cells contain 47 chromosomes. Down syndrome mosaicism is an example of trisomy mosaicism. These individuals have some cells with the typical number of chromosomes (46) and some cells with an extra chromosome 21, for a total of 47 chromosomes. Mosaicism may exist for all kinds of chromosome abnormalities (monosomy, triploidy, structural changes, etc). Although more rare, there may even be mosaicism where both different cell types are abnormal in structure or number, and there are no normal cells involved. | |||
| The cells with abnormal chromosomes may be found in mutiple tissues, or in just one tissue. Consider the example of trisomy 21 mosaicism or Down syndrome mosaicism. In one individual, if we look at a blood sample we may find some cells with 47 chromosomes and some with 46 chromosomes. If we look at a skin sample we may also see some cells with the abnormal number of chromosomes (47) and some with the normal number of chromosomes (46). This is illustrated in diagram a. On the other hand if we look at another individual, it is possible that in one tissue, say skin, we see mostly cells with 46 chromosomes, but in another tissue, say the intestine, we may see mostly cells with an extra chromosome 21, for a total of 47 chromosomes. This is illustrated in diagram b.
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| a) Normal and abnormal cells are found in most tissues | |||
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Blood cells |
Skin cells
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| b) Normal and abnormal cells are confined to specific tissues | |||
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Skin cells |
Intestine cells
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Changes in the number or structure of chromosomes in different cells of the body can have variable impacts on the proper functioning of the human body. This will be explored in detail throughout this website. In general, as we would expect, individuals who are mosaic for a chromosome change tend to have a less severe form of the condition seen. Chromosomal mosaicism describes a group of disorders. Any chromosomal change, in number or structure, can be present in a mosaic form. However, trisomy mosaicism is the most common kind ascertained during prenatal diagnosis.
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| How does trisomy mosaicism occur? | |||
| All of the cells in our body come from a single cell, the fertilized egg or zygote. In order for the zygote to develop into a baby, this single cell must grow and divide. Before cells can divide each chromosome must make an identical copy of itself. At cell division each chromosome and its identical copy pull apart into two separate cells. Now the resulting cells, also called daughter cells, have the same chromosome make-up as the original cell. The two new cells will repeat this process. In each cell, the chromosomes will duplicate and divide into two new cells. The result, is 4 cells identical to the first original cell. This process of cell division is called mitosis. Sometimes a mistake can occur when the chromosomes are separating into the two daughter cells. An extra chromosome may travel into the wrong cell or a chromosome may get lost in the separation of the cells. The result would be two daughter cells with different chromosomal make-up. | |||
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| In this illustration, the green cell represents a cell with an abnormal chromosome make-up. All cells that come from the green cell will share the same chromosome change. We say that all cells originating from that cell are in the same cell line. The baby that develops from this embryo will have some cells in his/her body which have the typical number of chromosomes and some that have the chromosome change.
Trisomy mosaicism can occur in one of two ways:
In both cases the result is a baby with two different cell lines, one cell line with 46 chromosomes and one with 47 chromosomes. Tell me more about how trisomy mosaicism occurs. (more scientific details) The effect that mosaicism can have on the development and overall health of an individual is extremely variable. We will now delve into the different ways in which chromosomal mosaicism can be diagnosed and what factors are involved in determining the clinical presentation of these conditions.
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