Abnormal Sexual Differentation/Development

Sometimes this seemingly simple process of genetic sex determination may fail to occur normally, and the baby is born with an abnormal set of sex chromosomes. The effect of such situation is devastating.

 

 

When an abnormal sperm carrying no sex chromosomes fertilises a normal egg then the baby is born with 'Turner's syndrome' (named after H.H.Turner, in 1938). One in 2,700 babies is born with such a problem. Often undetected a birth, the diagnosis of Turner's syndrome is made at adolescence when the individual fails to attain normal puberty. A typical short stature, a webbed neck, shield-shaped chest with wide-set nipples sets the person apart from others. A karyotupe analysis reveals the presence of only one X. Unfortunately little can be done except for hormone treatment which is recommended for growth of bones and attainment of height.

 

An equally serious problem arises when an individual has an excess of sex chromosomes. Most commonly encountered cases are those that have an X in addition to the normal XY complement. These individuals diagnosed as 'Klinefelter syndromes' (named after J.E. Klinefelter, in 1942) are males with poorly developed testes and enlarged breasts. There are a variety of sex chromosomal abnormalities that can arise during fertilisation and early development. Unusual cases may be mosaic with some cells with one X and others with an extra X. These individuals too fail to attain puberty. Some patients do improve with hormone therapy.

 

Careful studies of sex chromo- somes abnormalities provide some insigths into the role of X and Y in the development of sex. X seems to be important for fertility in both men and women. Genes present on the X control ovarian development and both the X chromosomes are necessary for normal gonadal growth. (Females with XO complement are often sterile). The presence of Y chromosome determines the male features. (Klinefelter with multiple X is a male as the Y chromosome is present). Following these arguments, the role of genes on the X and Y in gonadal development has been worked out to the molecular level. While most genes on the X chromosome are important for functions other than sex determination, some are essential for early gonadal development in men and women. The Y chromosome carries the testis determining genetic information. The presence of Y initiates embryonic testis formation, while its absence (as in normal females) leads to formation of ovaries. Genes that play a role in gonodal development are not restricted to sex chromosomes. Some genes are present on other chromosomes as well. A well-regulated expression of these genes is most important for normal gonadal development.

 

The human embroyo has no distinct sex organ until it is about five-weeks old. Just a little later a set of cells destined to form the gonads can be located as a thickening called the 'gonadal ridge' or 'gonadal crest'. In female embryos these cells eventually form the supporting tissue in the ovary, the granulosa and the thecal cells. Interestingly, the germ cell, the cell that gives rise to the ovum, originates in an entirely different embryonic region, i.e., the gut. These extra embryonic cells, destined to form the ova, migrate, moving through the tissues, probably guided by the chemical cues, to the genetial ridge. On reaching their destination, they settle down and multiply to form the mass of fonadal tissue.