Dolphin brains, for example, are much more finely grooved than similarly sized human brains. The second explanation posited that brains fold both as a result of an increasing number of neurons and also as a way to allow this neural growth. The 19th-century neuroanatomist Franz Gall hypothesized that brains folded to allow a much larger cortical surface to fit inside the space of the skull. The alternative would be a cortex that did not fold but expanded like a balloon, an inefficient use of the cramped quarters of the head.
Again, paradoxes abounded. Human cortices have three times the number of neurons than elephant cortices, yet human brains are half the mass and far less folded. Baboon and pig cortices display equivalent amounts of folding, yet baboons have 10 times as many neurons as pigs do. These outliers seemed to suggest that different species possess different mechanisms to control cortical folding—that is, each species has its own way of growing a brain.
The Science study both disproves those hypotheses and reconciles anomalies like the manatee with a simple physical law. Whether or not a brain folds, Herculano-Houzel says, is pure physics. To model brain folding Herculano-Houzel had graphed a power function derived from the product of cortical surface area and the square root of cortical thickness.
Mota had recognized that the same model that predicted the degree of brain convolution also explained the crumpling of paper balls. To see their results you can run essentially the same experiment as the experimenters conducted using four sheets of paper.
First, take one sheet, crumple it hard, and set it down. The paper should expand slightly as it releases some energy but eventually it will settle in a crumpled form. Using MRI imaging of the fetal brain at twenty-two weeks, the team 3D-printed a replica, which was then used to form a silicon mold. Using this, the physicists then created imitation brains out of a gel material, which were then covered with another gel that could swell by absorbing a solvent such as hexane.
After being exposed to the solvent for sixteen minutes, the brain model developed convolutions and folds that greatly resembled those of real brains. In addition, the stages in the development of these shapes were similar to those observed via MRI imaging. Although the results already supported the hypothesis presented in , the researchers still wanted to improve on their simulations.
This process has certain limitations. First of all, the observations reveal a notable asymmetry between the two hemispheres of the imitation brain.
Why is this? In other words, they have the potential to become many different types of cells. The cells continue to divide until they "choose" to become a specific type of cell.
In the engineered mice with more beta-catenin, many of the young cells chose to stay in the cell cycle and keep dividing longer than usual. Eventually, they chose to become mature neurons, but because they divided for a longer period of time, there were many more of them. This increased number of neurons resulted in a larger brain volume and in an increased surface area. Image courtesy of Comparative Mammalian Brain Collections.
Altering genes that regulate the cell cycle can increase brain volume without significant changes in brain surface area. More From Reference. The History of Independence Day in the U. What Are Disadvantages of Playing Sports? What Are the Different Types of Microcomputers?
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