Re: Is development of the nipple as fetus, the differentiating point for m/f?

Dear Rachel,

There are two different aspects to the determination of the gender of the 
fetus. The earlier one occurs at fertilization. Here the spermatozoa that 
bear X chromosomes will determine femaleness whereas spermatozoa possessing 
a Y chromosome determines maleness. This is known as the "primary sex 
determination."

Prior to the seventh week of development (i.e., seven weeks after 
fertilization) the male and female gonads appear identical and are referred 
to as the "indifferent gonads." But shortly thereafter the Y 
chromosome begins to induce the formation of testicles with cells known as 
Leydig cells that manufacture the male sex hormones (androgens). This 
hormone then does two things: 

1. It induces the differentiation of the mesonephric duct into the male 
genital tract and

2. induces the differentiation of another cell type in the testes, known as 
Sertoli cells. 

Sertoli cells manufacture a hormone known as "anti Mullerian hormone," 
which suppresses the development of the Mullerian duct. Since the Mullerian 
duct differentiates into the female reproductive tract.

As far as the nipple is concerned it does not have anything to do with sex 
determination. Although the nipples begin to develop around the sixth to 
seventh week after fertilization, they remain submerged and do not 
become really evident until after birth. Additionally, the nipple of a 
prepubescent male is identical with that of a prepubescent female and they 
do not change their characteristics until after puberty, when the female's 
nipples become enlarged. And, just as an additional piece of information, 
the mammary gland may be thought of as a part of skin; essentially it is a 
modified sweat gland. 

I hope this helps.

Leslie P. Gartner, Ph.D.
Associate Professor of Anatomy
Dental School
University of Maryland
Baltimore, Maryland