Linked Regularities in the development and evolution of mammalian brains
Finlay and Darlington 1995.
This study deal mainly with brain development and evolution among different mammalian taxa. They test two hypothesis: 1) “Developmental constraint hypothesis” which predict that the size of any brain structure can be predicted, 2) “adaptation hypothesis” predict the opposite, were brain region are variable among different taxa.
The first part explore the relation between brain size and ten brain division, they found that the neocortex expand quickly in comparison with other brain part as brain size increase. Moreover, 96.29% of the brains size variation can be explain by increase in neocortex. If olfactory bulb is also taken into account 99.19% of the brain size variation can be explain by these two regions. The second part of the paper explore the importance of embryo developing time over brain structure size. The general idea is that the longer neurogenesis is delay, the more precursor cells can be formed and the larger the structure that result. The number of precursor is correlated with neuron number. They found that indeed larger brain size increase with late birth dates, increasing the number of precursor pool.
Why the neocortex increase faster than other brain areas? May be because increasing the capacity in complex functions such as sensory perception and spatial reasoning are more important than for expample vision abilities?
If the number of precursor is higly influence by the developing time, one my expect that animals like wales and ungulates may have high number of neurons. This data is not not available in this paper, but do we think that this is the case when compare to primates with shorter gestation time?
This study deal mainly with brain development and evolution among different mammalian taxa. They test two hypothesis: 1) “Developmental constraint hypothesis” which predict that the size of any brain structure can be predicted, 2) “adaptation hypothesis” predict the opposite, were brain region are variable among different taxa.
The first part explore the relation between brain size and ten brain division, they found that the neocortex expand quickly in comparison with other brain part as brain size increase. Moreover, 96.29% of the brains size variation can be explain by increase in neocortex. If olfactory bulb is also taken into account 99.19% of the brain size variation can be explain by these two regions. The second part of the paper explore the importance of embryo developing time over brain structure size. The general idea is that the longer neurogenesis is delay, the more precursor cells can be formed and the larger the structure that result. The number of precursor is correlated with neuron number. They found that indeed larger brain size increase with late birth dates, increasing the number of precursor pool.
Why the neocortex increase faster than other brain areas? May be because increasing the capacity in complex functions such as sensory perception and spatial reasoning are more important than for expample vision abilities?
If the number of precursor is higly influence by the developing time, one my expect that animals like wales and ungulates may have high number of neurons. This data is not not available in this paper, but do we think that this is the case when compare to primates with shorter gestation time?
posted by Gustavo at 11:15 AM
3 Comments:
Barb’s paper uses published data of brain sizes and brain neurogenesis in 131 mammal species to examine the size changes in neural structures attributable to constraints on brain development versus changes attributable to structurally specific adaptation. The data show that the structures of brain regions are highly predictable by brain size, that the vast majority of the variance of structure size across species is attributable to variation in brain size. Also, the peak day of neurogenesis corresponds with brain neural structures across taxa.
What stands out to me is the weaker association between olfactory bulb size and brain size, suggesting that the constraints on the size of the olfactory region are lower relative to other sensory regions? How might olfactory sensory equipment in the brain be more independent than other regions?
Also, the article mentions that granule cells (which were not included in the model) are responsible for protracted neurogenesis, which is functionally what occurs in many cases where there is specific adaptation of brain structures for a particular behavior. This would be very interesting to examine in more detail (how do developmental processes using cell types influence how constrained structures are).
I’m a little confused by the final conclusions of the paper, the answer to the question: what does a conserved nonlinear relationship across mammals mean? Why would crude neurogenesis changes for the brain as a whole be the most “readily available” variation for making changes in particular structures?
In the Finlay, Darlington paper the relationship was studied between sizes of particular brain regions to overall brain size in 131 mammals. There was found to be a high positive correlation between overall brain size and increased size of all regions of the brain except for the main olfactory bulb. Finlay and Darlington also studied the effect that the duration of neurogenesis has on brain structure size in seven mammals. Their findings suggest that brain size increases as a result of the delay of the peak day of neurogenesis, or “birthday”, after the first 7 post conceptual days. They also found that brain structures that grow disproportionately large in comparison to overall brain size have later “bithdates”. This all makes sense since a delayed “birthday” gives more time for precursor cells to divide.
The Kaskan study found that all cortical areas appear to increase in size proportionately with the exception of V1 to S1 and A1. It also showed that the size of the cortical areas is best predicted by total neocortex size. The cortical areas increasing in size together suggests that many of their functions may be dependent upon each otheror that one gene is responsible for all of their growth. Another finding of the Kaskan paper was that nocturnal species did not have any corresponding increase in the size in any of the structures of the neocortex when compared to diurnal species. Even though the nocturnal species had an increasing number of rods, there was no increase in the size of the visual cortex. This finding was particularly odd since one would expect cortical structures to increase in size with the corresponding senses that are more acute in nocturnal animals.
Finlay and Darlington find that across over 100 species of mammals, the sizes of brain components could be predicted from absolute brain size. Previously, Memory, song, language, etc. have been linked to the cell numbers and volumes of particular brain areas. Two prevailing hypothesises dominate arguments about scaling species and structure sizes: the developmental constraint hypothesis, and the adaptation hypothesis. With the exception of the neocortex, all other structures and their volumes (Cerebellum, basal forebrain and medulla, etc.) were highly predictable. Further, the authors see the rate and duration of cell division as the limiting factor in controlling brain size. In the rhesus monkey (Macaca mulatta), they find the largest brain and most "corticalization" as well as the longest gestational period. We see now that brain evolution partially rests on cell generation period.
Chenn and Walsh over-expressed beta-catenin in neural precursor cells to create enormous cortical features in mice. They find that the protein is influential in how cells proliferate in nervous system development. As evidence, In situ hybridization was performed, ultimately revealing that expression of beta-catenin increased neural precursor cell population in the transgenic animals. Two key points: 1) "small alterations in the fraction of cell divisions that expand the progenitor pool can result in large chages in the final size of the brain" 2) "Beta-catenin activation functions in neural precursors to influence the decision to re-enter the cell cycle instead of differentiate."
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