Learning objectives
- Reproduce the importance of the X, Y chromosome and SRY gene in sex differences
- Describe how estradial and testosterone are made
- Reproduce the anatomical differences between the male and female brain at the level of the hypothalamus
o Differences at the level of nucle...
Lecture 5 – Havekes 3,4
Chapter 17 (text related to fi gures in power point: page 581-585, 596-598, 599, 600,
602, 603, 608-612)
Arti cle Dick Swaab: only the fi gures discussed in lecture.
Chromosomal differences between sexes
X-chromosome 1500 genes and Y-chromosome 200 genes (less diverse functions).
Males are hemizygous for these genes (only a single copy).
In the first six weeks, males and females are sexually
undifferentiated, after six weeks different things can happen:
- Males:
o SRY gene encodes a transcription factor called
the testes determining factor (TDF)
o TDF causes testes development —>
testosterone
o Testosterone causes the development of the
Wollfian duct into internal male reproductive
system.
- Females:
o No Y-chromosome, no SRY gene —> no
testosterone
o Development of Müllerian duct into female
internal reproductive system
o Degeneration of Wollfian duct
Hormones
Hormones regulate physiological processes. Sex hormones are steroids and they are released by
endocrine glands. The endocrine glands are regulated by pituitary gland.
Brain —> pituitary —> endocrine glands —> sex hormones
The structure of hormones affects its function. The male hormone is testosterone and the female
hormone is estradiol. However, estradiol is extremely important for masculinisation during early
development.
Males:
- Testes release androgen
- Testosterone increase at puberty leads to development of secondary sex characteristics
Females:
- Ovaries secrete estradiol (estrogen) and progesteron (progestin)
- Estrogen increase at puberty leads to maturation of female reproductive system and
development of breasts.
The blood concentration of sex hormones vary, however in males the levels fluctuate rapidly each
day but in females they fluctuate in a 28-day cycle (in rodents every 4-5 days).
Female is the “default sex”, as long as there is no testosterone present, female genitalia and brain
will develop.
, Subregions in the brain that differ between sexes
The level of estradiol receptors suggests that these brain regions differ between sexes.
The main subregions that differ between sexes are in the hypothalamus.
Subregions that differ between sexes are:
- Sexually Dimorphic Nucleus of the pre optic area (SDN-POA = INAH1)
o Involved in various basic behaviours such as sex emotion and hunger
o More cells in males
o Gradually more androgen receptors in males
- SupraOptic Nucleus (SON) and ParaVentricular Nucleus (PVN)
o More androgen receptors in males
- SupraChiasmaticus Nucleus (SCN)
o Gradually more androgen receptors in males
- Bed Nucleus of Stria Terminalis (BNST)
o No different in androgen receptors
o VIP is a hormone that stimulates the secretion and inhibits the absorption of sodium,
chloride, potassium, and water within the small intestine and increase bowel
motility. The amount of neurotransmitters is way higher in males that females. It is
the same in male homo- and heterosexuals but less in transsexuals that are male but
feel female.
- Interstitial Nuclei of the Anterior Hypothalamus (INAH3)
NOTE that androgen receptors bind testosterone.
Large nuclei/more volume: Region is larger, likely has a more prominent/dominant role in regulating
behavior
More receptors: Region that is more sensitive to exposure to neurotransmitters (i.e. more androgen
receptors-> more sensitive to testosterone)
More neurotransmitter staining: Region contains more of the specific transmitter and can more
strongly influence those regions that have receptors sensitive for the specific neurotransmitter.
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