4. each human cell contains 46 chromosomes, occurring in pairs
23 pairs of chromosomes
22 autosomes (determines e.g., hair color)
23rd pair are the sex chromosomes
male: XY
female: XX
Chromosomal sex
Germ cells:
male: sperm - contains “X” or “Y”
female: ovum - contains an “X”
“Y” chromosome contains less genetic
material than “X” chromosome
6. Gonadal
Differentiation
Undifferentiated gonad develops as a
‘germinal ridge’ off of the
protokidney
The gonad at this stage (pre 7-weeks)
has a medullary and cortical region
(inner and outer)(testes develop
during the 7th week of gestation,
ovaries develop during the 13th to 16th
week)
Without TDF input, the cortical
region will develop into an ovary, and
the medullary region will regress
SRY causes development of the
medullary region into a testis, and
development proceeds from there,
resulting from the hormones secreted
by the gonads
8. Ovaries: fairly quiescent during embryogenesis. They secrete very low levels of
estrogens, but don’t produce significant quantities of hormones until puberty
to repeat: in the absence of hormones from a testis (differentiated because of
SRY), morphological, neural, and behavioral development is female
Testes: produce testosterone (from the leydig cells) and proteins (mullerian
inhibiting hormone MIH from the sertoli cells)
hormonal sex
12. The progenitors for the duct systems are present for both male and females
embryologically
In contrast to gonadal development, where one gonad becomes either male or female.
mullerian vs wolffian
1) With no hormones, the mullerian duct develops, and the wolffian duct undergoes
programmed cell death (regresses and degenerates)
1) This causes formation of female internal morphology
2) Mullerian system becomes fallopian tubes, uterus, and cervix
2) With testes present:
1) Leydig cells produce testosterone, cause development of wolffian ducts (turn
off programmed cell death)
1) Wolffian ducts become vas deferens, seminal vessicles, and prostate gland
2) Sertoli cells produce Mullerian inhibitory hormone(MIH), cause regression of
mullerian ducts
1) If the sertoli cells fail to produce MIH, a male can have both male and
female internal structures
Mullerian and Wolffian Duct Systems
13. External
Genitalia
Development of male external genitalia is
dependent on dihydrotestosterone (DHT), an
androgen
-The skin of the genitalia contains the
enzyme necessary to convert testosterone to
DHT (5-alpha reductase)
-The testis produces androgens, and in the
genital area, that is converted to DHT, causing
masculinization of the external genitalia
(fusion of the genital swelling)
Both males and females have this enzyme (5-
alpha reductase) in the developing genitalia, but
females have no T to convert at this time, so
no DHT produced
-If female is accidentally producing T, she can
have masculinized genitalia at birth
- If male does not produce the enzyme,
external genitalia will be female.
14. Male:
Testosterone secreted into the blood reaches the brain
testosterone converted to estradiol and dihydrotestosterone
in the brain
estradiol masculinizes the brain
Female:
alpha-fetoprotein binds to estradiol
prevents estradiol from entering the brain
protects female brains from being masculinized by estradiol
Brain Differentiation