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exposed to an identical, if somewhat peculiar, gonadal
hormone environment during development. In fact,
the male side of the brain had a much larger HVc
nucleus, as compared to the female side (Agate et al.,
2003). Thus, the genetic sex of the brain cells plays a
primary role in their differentiation.
diencephalon
A
B
E14 male
E14 female
LEARNING TO REMEMBER
Learning is often portrayed as an extension of
neural development, and there are many similarities,
particularly at the cellular level (Chapter 9). But this
portrait does not capture an important fact: learning
and memory themselves change during the course of
development. Some forms of learning emerge during
a limited period of development, and then disappear.
The filial imprinting of a baby duckling on its mother
occurs during a brief time interval after hatching.
Other forms of learning are robust in young animals
but gradually become less efficient. Humans retain the
capacity to learn new vocabulary words throughout
life, but there is a window of development when we
learn words at a remarkable rate. Still other forms of
learning seem to improve with time, perhaps owing to
the wealth of information already stored in a mature
nervous system. One clear line of evidence demon-
strating the effect of environment on the developing
nervous system comes from rearing rats in an enriched
environment. Developing rats that are housed with
many objects in the cage have almost 25% more
synapses per neuron in the visual cortex (Turner and
Greenough, 1985).
Memory is usually divided into two general types:
recollection of facts (things that can be stated, or
declarative memory) and recollection of skills (things
that can be performed, or procedural memory).
Humans with focal brain injuries are often found to
have specific learning and memory deficits (Milner et
al., 1998). For example, people with extensive damage
to the limbic system are completely unable to recall
new facts, yet they can learn and remember new motor
tasks. Exceptionally rapid learning or memorization
has sometimes been mistaken for intelligence. In fact,
human brilliance is often specialized: a knack for game
theory coupled to rapid learning of spatial patterns
might allow one person to be a champion bridge
player, while a taste for chewing tobacco coupled to
robust motor learning can produce a major league
pitcher. Therefore, it is not too surprising that clinical
measures of learning and memory are often difficult
to reconcile with the broad patterns of human
behavior.
prolactin-stained neurons at 10 DIV
FIGURE 10.21 Development of sexual dimorphism before
gonadal development. A. A transverse section through the adult
mouse brain shows the location of the caudal diencephalon. B. When
rat diencephalic neurons were explanted at embryonic day 14, the
neurons from females embryos had two to three times more pro-
lactin-expressing neurons (red) compared to cultures from E14
males. (Beyer et al., 1992)
gene. Testis development does not occur in these
animals, and the gonadal signal is eliminated (Carruth
et al., 2002). When mesencephalic neurons were
explanted at embryonic day 14, the number of tyrosine
hydroxylase-expressing cells was greater in genetic
males than genetic females, even though both groups
had female gonads. Therefore, it appears that the ver-
tebrate nervous system may also develop some sex-
specific characteristics independent of the gonads. In
fact, the Sry gene is transcribed in the hypothalamus
and midbrain of adult male mice, suggesting that these
cells may be masculinized by a genetic signal.
A particularly compelling example of genetic deter-
mination of brain sexual dimorphism was discovered
in a rare zebra finch gynandromorph (i.e., an animal
that is a mosaic of male and female structures because
some cells are chromosomal females while others are
chromosomal males). In this instance, the animal was
genetically male on one side and genetically female on
the other (Figure 10.22A). Since only females carry a
W chromosome, it was possible to stain for mRNA
encoding a W chromosome gene and to show that
expression was limited to one side of the brain (Figure
10.22B). As expected, one side of the animal developed
a male-like gonad, and the other side developed a
female-like gonad (Figure 10.22C). Thus, the brain was
 
 
 
 
 
 
 
 
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