Sequence of events in sex differentiation

Sex differentiation proceeds in a cascadelike
manner, with a series of temporally regulated
successive steps at different levels of differentiation.
After the primordial germ cells migrate
into the undifferentiated gonads, early embryonic
testes develop under the influence of
testis-determining factor (TDF) if a Y chromosome
is present. TDF is identical with the Yspecific
sequences of the SRY region (see
p. 386). During normal male differentiation, the
further development of the müllerian ducts is
suppressed by the müllerian inhibitor factor.
Testosterone can exert its effect only in the
presence of an appropriate intracellular receptor
(androgen receptor TFM, see p. 390).
When a Y chromosome is not present or when
the SRY region is missing or altered by mutation,
testes are not formed. In this case the
wolffian ducts cease to develop. In the absence
of testes, ovaries develop from the undifferentiated
gonads; the wolffian ducts degenerate;
and the müllerian ducts differentiate into
uterine tubes, uterus, and the upper vagina.
Testosterone also has an effect on the central
nervous system (“brain imprinting”). It is assumed
that this is required for the psychosexual
orientation apparent later in life. When testosterone
is absent or ineffective due to a receptor
defect, gender orientation is female.
In the majority of genetically determined disorders
of sexual differentiation, gonadal and
genital sex do not correspond (pseudohermaphroditism).
In true hermaphroditism,
where the gonads consist of both testicular and
ovarian tissues, male and female structures
exist side by side.