Question

When during development, an embryo begins to show distinct male or female characteristics?

Answer 1

The biological sex of an individual determined at the time of conception and it depends upon the type of sperm (either Y chromosome/ X chromosome carrying sperm) fused with the egg. When an egg fertilizes with the Y chromosome carrying sperm then that fertilized zygote (XY) developed into a male. If egg fertilizes with X chromosome containing sperm then fertilized zygote (XX) developed into a female.

The embryo is called as a fetus from the eighth week of conception and sex differentiation starts during the 6th week of pregnancy before the developing embryo is even old enough to be considered a fetus. During sex differentiation, the fetus gains either male or female characteristic features and it is initiated and controlled by the steroidal hormones of gonads.

Both Wolffian ducts and Müllerian ducts are developed in embryos at the sixth week of conception. The male reproductive system is developed from Wolffian ducts and female reproductive system developed from Müllerian ducts. Under proper chemical instructions, these two ducts either develop into male or female reproductive systems. As the embryo development proceeds presence or absence of the sex-determining region of the Y chromosome (SRY gene) one pair of ducts develop and another degenerate. At the end sex differentiation stage, the fetus posses’ only one pair of these ducts. The gonads are histologically distinguished at the 6-8 week of gestation.

In males SRY gene transcribed and SRY protein promotes the development of gonads into testis and embryonic gonads start producing an anti-Mullerian hormone which causes degeneration of Müllerian ducts and also promotes development of vas deferens and the seminal vesicles. But in females have two X chromosomes without any SRY gene which do not give any signals to male sex organs to develop so Wolffian ducts are degenerated and the Müllerian ducts persist which form the female genitalia. At the ninth week of embryonic development, differentiation of external genitalia starts and completely differentiated by the twelfth week.