These abnormalities could include a small defect in chromosomal arrangement such as deletions, isochromosomes and balanced X chromosome-autosomal translocations. However, complete deletion of one X (Turner’s syndrome) has also been recorded (16).
Complete or near-complete absence of one X chromosome is the most common chromosomal defect in humans (17). This condition leads to ovarian dysgenesis characterized by prienorrhea, short stature and characteristic phenotypic features. Two functioning X chromosomes are necessary for normal ovarian function (18). In the presence of only one X chromosome, ovarian follicles degenerate from birth onwards. This is most likely caused by lack of diploid dosage of one or more vital genes and accelerated follicular atresia. Histological data indicate that oogenesis proceeds normally in these individuals until diplotene oocytes begin to be incorporated into follicles. There is a subsequent block in the production of complete follicles manifesting as follicular atresia. In 80% of cases, the paternally derived is lost (19).
However, the identification of genes or critical regions responsible for individual Turner’s syndrome features has turned out to be problematic. Cytogenetic data indicate the reduced dosages of genes on the short arms of the X (Xp) and Y (Yp) chromosomes (2. 6Mb Xp-Yp pseudoautosomal region) tends to be associated with short stature and somatic anomalies (20, 21), whereas deletions on the long arm of the X (Xq) chromosome tends to be associated with ovarian failure but no somatic anomalies (22, 23). The degrees of ovarian dysfunction and the extent of the somatic anomalies are variable.
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