But neuroscientists are increasingly interested in how the flood of testosterone and estrogen unleashed by puberty plays a role in driving fundamental and permanent changes in brain anatomy.
Studies in animals and humans indicate that during adolescence, these hormones bind to receptors on brain cells, altering their activity and contributing to the formation of brain circuitry. For example, there is evidence that when they reach the brain, hormones secreted by the gonads help stimulate the growth of new axons , the wire-like extensions of neurons that enable them to send signals to other neurons.
Also, recent studies in both animals and humans show that during adolescence, estrogen and testosterone help stimulate and shape the production of white matter. White matter consists of support cells glia and axons coated in a fatty material called myelin. Glia make myelin, which grows and wraps around axons to protect them and speed up neural transmission.
This white matter growth, called myelination, is critical to the cognitive advances of adolescence as it gives axons the ability to transmit information about times faster than unmyelinated axons. Scientists also have recently learned that myelination can alter the timing and coordination of signaling in the brain, which may allow the brain to engage in more complex mental processes.
The hormone-driven increase in white matter is particularly important in helping the brain mature because it dramatically improves the flow of information between various parts of the brain, says Jay Giedd , a professor of psychiatry at the University of California, San Diego, who uses brain imaging to study adolescent brain development. While the adolescent brain is adding and strengthening some connections, it is also trimming others, and estrogen and testosterone are believed to play a role in this process, called synaptic pruning.
This process eliminates synapses , the points of contact between neurons at which information is sent from one neuron to another. Evidence for synaptic pruning comes from studies that have found an increase in synapses early in life followed by a pronounced and steady decline in adolescence that then levels off in adulthood.
Weak connections are pruned away, while the stronger ones remain and are further strengthened by the addition of myelin. Studies in rodents and non-human primates have revealed that specific regions of the brain contain an especially large number of receptors for estrogen and testosterone. Imaging studies have found, for example, that axons are sprouting and white matter is growing in the amygdala , an area involved in emotions.
Right next door to the amygdala is the hippocampus , an area important for memory. Finally, there is the prefrontal cortex, an area of the brain that acts like a central processing facility, sorting through and synthesizing inputs from other parts of the brain. Imaging studies show that this area undergoes rapid changes during adolescence that are associated with an increased capacity for planning, memory retention, patience, and emotional control.
There are two main ways that hormones can influence your brain cells [ 1 ]. First, hormones can influence how the brain is organized, and these are changes that take some time to occur.
Changes in brain organization can include changes in the number of cells, or changes in the size and shape of dendrites or axons. Testosterone, for example, influences the development of new cells in a brain region called the medial amygdala.
Because boys make more testosterone during puberty, this region becomes bigger in boys than girls [ 2 ]. This was found in animal research, but studies on humans that looked at hormone levels and the size of the amygdala suggest it works the same in humans. Second, a hormone can influence the way that brain cells become activated in response to a situation or environment.
Hormones might help or prevent a cell from exchanging signals with other cells. This can also lead to long-term changes in brain cells. For example, the levels of testosterone in mice and humans increase during a competition or fight. One study showed that mice who win a fight develop more receptors for testosterone in brain regions that are important for reward and social behavior [ 3 ]. These new receptors might also change the behavior of the mouse in the next fight. This shows a process where experiences, like winning a fight, and hormones work together to shape brain development.
This process is especially important during puberty, when the hormone levels are higher than during childhood and the brain is still developing. There is still a lot we do not know about how hormones influence the organization and actions of brain cells in humans. We do know that these effects are different in some ways between boys and girls, and between regions of the brain.
Researchers are just starting to figure out how the hormone-related changes in the brain are important for behavior and learning, so there are a lot of unanswered questions. Children can learn certain things better than teenagers or adults can. For example, young children are particularly good at learning new languages.
It becomes much harder to learn a second language after a person is 9—11 years old. This is probably because of changes in the way the brain processes speech and other language information.
One study looked at the role of puberty in these changes. The second effect is via the reorganization of sensory and association regions of the brain, including visual cortex [Nunez et al.
This results in altered sensory associations, e. These effects are necessary for establishing strong motivation to seek out reproductive opportunities. It is possible that adrenarchel hormones DHEA and DHEAS begin to exert similar effects on brain and behavior prior to the onset of gonadarche, but these effects are poorly understood. Advancing understanding in these areas will require careful attention at two levels: conceptually and methodologically. Conceptually, this will require the development and refinement of models of adolescent brain development that address specific aspects of pubertal maturation e.
Methodologically, it will require studies that are designed with the selection of samples and measures of puberty that permit testing of these specific hypotheses. Because age and pubertal maturation are often correlated and age is easily measured with great precision and validity, while puberty is often estimated with rough categorical measures that are not easily validated , there is a need for studies with designs that explicitly disentangle puberty and age effects e.
These goals raise a number of issues regarding how to measure specific aspects of pubertal maturation in human studies. The most appropriate measure of puberty will therefore depend in part on the specific research question in each study. A commonly used measure of puberty is Tanner Stage. Tanner staging categorizes individuals along an ordinal puberty scale from 1 to 5, on the basis of pubic hair and breast development in females, and pubic hair and genital development in males [Tanner, ; Tanner and Whitehouse, ].
Tanner staging by physical exam should be carried out by a trained clinician. There are several limitations to Tanner staging. Despite these limitations, Tanner staging has historically been considered the gold standard for puberty measurement [Dorn, ]. Hormone assays may be increasingly useful for measuring pubertal stage in the future; however, at the present time it is unclear how hormone measurements should be combined with or used in conjunction with other measures such as Tanner stages [see Shirtcliffe et al.
There are also other practical issues regarding hormonal measures, including cost, subject burden, and the fact that levels of different puberty hormones fluctuate in monthly and circadian cycles. At a conceptual level, for example, some neurobehavioral changes at puberty may be the direct result of increasing hormone levels on specific neural systems during adolescent brain development and thus best quantified by hormone measures while other neurobehavioral changes may reflect more complex influences e.
Tanner staging by physical examination by a qualified clinician can raise practical issues regarding appropriateness and convenience. That is, Tanner staging can be part of a normal physical health exam and therefore should not be associated with any stigma or ethical concerns beyond a normal physical health check. However, the cost clinician time, special room and equipment for a physical exam, and explaining the procedures to the adolescent and family can make this impractical for many research studies.
As such, the PDS measures a composite puberty score that includes the effects of adrenal and growth hormones, as well as gonadal hormones. The PDS can be used with caution to estimate Tanner stage when a physical examination is not possible. In summary, researchers should give ample consideration to which aspect of puberty is most relevant to their research question and select their measures of puberty and overall design of the study accordingly. The advent of noninvasive brain imaging techniques, in particular magnetic resonance imaging MRI , has enabled investigation of the development of the living human brain.
Developmental changes that have been delineated using MRI include alterations in the amount of gray and white matter, and changes in white matter microstructure.
Blakemore, for review]. Gray matter is composed of the cell bodies, dendrites and nonmyelinated axons of neurons, as well as glial cells and capillaries. Therefore, and based on evidence from histological samples [e.
Giedd et al. An early paper by Giedd et al. Shaw et al. In Giedd et al. The ages at which these peaks in gray matter volume were observed correspond to the sexually dimorphic ages of gonadarche onset, which suggests possible interactions between puberty hormones and grey matter development.
Other MRI studies have shown the gradual emergence of sexual dimorphisms across puberty, with increases in amygdala volume during puberty in males only, and increases in hippocampus volume in females only [Lenroot et al. Thus, it is possible that neuroanatomical development in certain brain regions is more tightly linked to puberty than it is in other brain regions.
However, no direct measures of puberty were acquired in these studies. In recent years, a number of adolescent MRI studies have investigated in more detail the relationships among structural brain development, gender, and puberty. An adolescent structural MRI study by Peper et al. For example, Neufang et al. Irrespective of gender, there was a positive relationship between pubertal measures Tanner stage and testosterone and gray matter volume in the amygdala, and a negative relationship between these measures and hippocampal volume.
All of these findings are preliminary and require replication, but they represent an important first step in this new area of research. Many MRI studies show a steady linear increase in global white matter volume between childhood and adolescence, with this increase slowing and stabilizing into adulthood [Giedd et al.
Perrin et al. In addition to changes in white matter volume, studies have shown concurrent changes in white matter microstructure. The researchers took images of the brain activity while the adolescent volunteers were lying still in an MRI scanner.
These images were corrected for age and then were analyzed in a way that measures how strongly brain regions communicate with one another known as "functional connectivity". The values of the functional connectivity of these regions were correlated with the level of maturity at puberty. Monique Ernst continued, "This is a first, because in the past, brain scans of adolescents have mostly been measured against a relatively wide chronological age, whereas, here, we were able to measure brain changes directly against their puberty status.
We found that for an equivalent change in puberty status, the functional connectivity in these specific brain areas increased in boys by an average of around 6. Specifically, these brain areas are within the medial prefrontal cortex and the parietal cortex.
The next critical next step is to examine the role of these brain connectivities in the development of depression as these adolescents get older, using a longitudinal design. While depression is more prevalent in females it still does occur in males too frequently, while the formal diagnostic criteria are identical.
However, the neural pathways to depression in males and females might be partly different, as increasing number of gene-environment interaction studies has shown different interactions in boys and girls, on occasion quite opposite to one other.
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