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Ahmed et al. (2008). Pubertal hormones modulate the addition of new cells to sexually
dimorphic brain regions. Nature Neuroscience, 11, pp. 995-997.
Summary by David Guzman, Alicia Witter, Jordy Serwin, and Michelle Greenberg
For Dr. Mills Psyc 310 Class, Spring 2010
Hormones are crucial and the driving force behind sexual differentiation in
species. The fetus, by default, is actually female with a single “X” chromosome. The next
chromosome they inherit from the male, either “Y” or another “X”, will determine how
the fetus responds to the influx of hormones. The gene “SRY” on the “Y” chromosome is
what promotes male sex differentiation and defeminization. Males produce more
testosterone and have higher levels of androgens then females. The influx of hormones
will help develop sex organs, body development, and even the brain as the hormone
receptors react differently depending on the gene.
Male and females brains are specialized in different ways due to hormones in
prenatal development. Males tend to use one large portion of the brain for behavior or
performing tasks. Female’s brain specialization is more lateral and usage of multiple
areas of the brain are put into effect when operating and functioning. A possible reason
for this is females have a larger corpus callosum which is the tissue connecting both
hemispheres of the brain. There is also a difference in the number of nuclei in the
hypothalamus of men and women. And both sexes show greater skill in certain areas such
as women showing great ability in verbal skills and men in visual-spatial tasks.
But new research is showing that the brain is not simply set in stone during
prenatal development. The sexual differentiation process may occur again during puberty,
another time of sexual development. Are large amount of hormones are once again
reintroduced into the body resulting in developing secondary sexual characteristics such
as breast, facial hair, etc. It is possible that the brain continues in its sexual dimorphic
formation through this resurgence of hormones. The belief that our brains are
permanently hardwired for our gender at birth may be challenged. The sexual dimorphic
nature our species might be more of a process.
This study found that mammals indeed go through another process of sexual
dimorphism after birth by an increase of cells and neurons in areas of the brain that are
unique to the gender and in accordance with sexual dimorphisms found in adulthood.
Male and female rats were injected with a cell birth-date marker at three stages in their
lives: pre-puberty, puberty, and mid-puberty. Their brains were dissected 20 days later
when they became adults. Three parts of the brain were to be examined: The
anteroventral petriventricular nucleus of the hypothalamus (AVPV), the sexually
dimorphic area of the preoptic area (SDN), and the medial amygdala (Ahmed 2008).
These areas were chosen as they are sexually dimorphic in that the AVPV is bigger in
females and the SDN and amgydala is bigger in males. Researchers also labeled the cells
in these areas to see their characteristics. The two cell types were neuron specific cells
(NeuN) or glial fibrillary acidic protein (GFAP). GFAP cells are also known as mature
neurons. Finally, researchers removed the gonads of some of the rats to see if their
absence had an effect on cell growth in the sexually dimorphic areas of the brain.
It was found that cell count in these areas increased greatly for the sexually
dimorphic areas in the rats. This correlated to the volume size of each structure, meaning
the cells increased in proportion to the surface area. Females grew more cells in the
AVPV and males in the SDN and amygdala. Researchers found that over half of the
AVPV’s cells expressed NeuN and none expressed GFAP. The medial amgydala
expressed NeuN in a quarter of the cells and even greater amount in GFAP. Neither were
expressed in the SDN at detectable levels. The other cells surrounding them are either
programmed for eventual apoptosis or differentiation in case of functional demand for
neuronal or glial needs (Ahmed 2008). The removal of gonads greatly affected the
sexually dimorphic areas of the brain depending on the gender of the rat. A removal of
the gonads yielded a smaller number of cell growth in the AVPV for females but not for
males. Conversely, a removal of the gonads decreased cell growth in the SDN and medial
amygdale for males but not for females.
These results show that cell growth is necessary in structuring and modeling the
sexually dimorphic areas of the brain during adolescence. It also shows that the gonadal
hormones are needed to achieve this spurt in cell growth. This can be seen as cells
increased in the sexually dimorphic areas of the rats and were not seen after the gonads
were removed during puberty. This might explain the further sex differences that occur in
behavior and development during and after puberty, such as homosexuality. It is
unknown whether cell growth in these areas increases after adulthood. In conclusion,
gonadal hormones are necessary in cell growth for function and development in the
sexually dimorphic areas of the brain of rats during adolescence and emerging adulthood.
Outline
I.
II.
III.
IV.
Hormones
A. Prenatal growth
1. Chromosomes
B. Males versus females
1. Female by default
2. Androgens in males
3. Cause sexual development
Sexual Dimorphic Brains
A. Differences
1. Number of nuclei in hypothalamus
2. Males – Visual spatial skills, localized areas for single task
3. Females – Verbal skills, larger corpus callosum, multiple areas in
brain for single task
New Research
A. Puberty may add to sexual dimorphism
B. Brains might not be permanently hardwired at birth, need boost.
Study
A. Male and female rats were injected with a cell birth-date marker at three
stages in their lives
1. Pre-puberty
2. Puberty
3. Mid-puberty
a. Brains dissected 20 days later when in adulthood.
B. Brain structures
1. Anteroventral petriventricular nucleus of the hypothalamus (AVPV)
a. Larger in females
2. Sexually dimorphic area of the preoptic area (SDN)
a. Larger in males
3. Medial amygdale
a. Larger in males
C. Cell Characteristics
1. Neuron specific cells (NeuN)
2. glial fibrillary acidic protein (GFAP)
D. Removal of Gonads during puberty
V. Results
A. Greater cell growth for females in AVPV
B. Greater cell growth for males in SDN and medial amygdale
C. Half of the AVPV’s cells expressed NeuN and none expressed GFAP
D. The medial amgydala expressed NeuN in a quarter of the cells and even
greater amount in GFAP.
E. Neither were expressed in the SDN at detectable levels
F. The removal of gonads greatly affected the sexually dimorphic areas of the
brain
IV. Conclusions
A. These results show that cell growth is necessary in structuring and modeling
the sexually dimorphic areas of the brain during adolescence.
B. Might explain the further sex differences that occur in behavior and
development during and after puberty.
C. Gonadal hormones are necessary in cell growth for function and development
in the sexually dimorphic areas of the brain of rats during adolescence and
emerging adulthood.
Critical Review:
Strengths
1. Focus on sexually dimorphic areas of the brain. Gonads had a significant impact on
development.
2. Evidence corresponds with behavior and properties we see. Such as influx of hormones
in puberty that we know create secondary sex characteristics might also effect the brain.
3. Evidence is objective and purely scientific.
Weaknesses
1. Subjects were rats.
2. Cannot be performed on humans as death is necessary to view the hypothalamus.
3. More research is needed to see if this relates to humans and their more complex, social
behavior.
True/False
1. Gonadal hormones are needed during puberty to cause further sexual dimorphism in
the brain.
Answer: True
2. The anteroventral petriventricular nucleus of the hypothalamus (AVPV) is smaller in
female rats compared to male rats.
Answer: False
3. The brain does not grow new cells and neurons in sexually dimorphic areas of the
brain
Answer: False
Multiple Choice:
1. Which of the following brain structure is larger in male rats?
A. Anteroventral petriventricular nucleus of the hypothalamus (AVPV)
B. Medial amygdala
C. Corpus Callosum
2. Sexual Dimorphism occurs
A. Only at birth
B. Only at puberty
C. Once at birth and once at puberty
3. Females showed greater cell and neuron growth in
A. Anteroventral petriventricular nucleus of the hypothalamus (AVPV)
B. Medial amygdala
C. Sexually dimorphic area of the preoptic area (SDN)