Clinical significance Chromosomal abnormalities Chromosomal abnormalities can occur during fertilization impacting the development of the male reproductive system. The genotype of the male consists of a Y chromosome paired with an X chromosome. Female sex is determined by the absence of a Y chromosome. Some individuals are male who have the XX male syndrome and androgen insensitivity syndrome. This occurs when one X chromosome contains a segment of the Y chromosome, which was inserted into the X chromosome of the father's sperm. Rarely females are born with the XY genotype. They are found to be missing the same portion of the Y chromosome it was inserted into the chromosome of XX males. The gene for sexual differentiation in humans, called the testis determining factor (TDF), is located on the short arm of the Y chromosome. The presence or absence of the Y chromosome determines whether the embryo will have testes or ovaries. An abnormal number of sex chromosomes (aneuploidy) may can occur. This includes Turner's syndrome - a single X chromosome is present, Klinefelter's syndrome - two X chromosomes and a Y chromosome are present, XYY syndrome and XXYY syndrome. Other less common chromosomal arrangements include: triple X syndrome, 48, XXXX, and 49, XXXXX. The observable, visual differences become apparent between male or the female reproductive organs are not seen initially. Maturation continues as the medial aspect of each mesonephros grows to form the genital ridge. The genital ridge continues to grow behind the developing peritoneal membrane. By week six, string-like cell congregations called primitive sex cords form within the enlarging genital ridge. Externally, a swelling called the genital tubercle appears above the cloacal membrane. External distinctions are not observed even by the eighth week of pre-embryonic development. This is the indifferent stage during which the gonads are relatively large and have an outer cortex of primitive sex cords and an inner medulla. Specialized primordial germ cells are forming and migrating from the yolk sac to the embryonic gonads during week eight and nine. These are the spermatogonia in the developing male. Before seven weeks after fertilization, the gonads have the potential to become either testes or ovaries. Reproductive sex organs for both male and female are derived from the same embryonic tissues and are considered homologous tissues or organs. After the testes have differentiated, male sex hormones, called androgens, are secreted from interstitial cells (cells of Leydig). The major