Abstract (for Chapter 2)
Objective: The aim was to test the hypothesis that the order is a significant phylogenetic
grouping in terms of quantifiable gyrification indices. Method: The gyrification index
(GI) was measured from serial sections of the brain of twenty five different mammalian
species, representing the different orders i.e. primates, carnivores, artiodactyls and
rodents. Image J analysis was used to measure the contours of the cerebral cortex and the
GI was calculated using three different methods of analysis i.e. complete vs outer; gyral
vs sulcal and outer vs inner surface contours. The measurements were then computed
against the brain weights of each species within the order. Results: An increasing GI
correlates with an increasing brain weight in all the mammalian orders. Each order has its
own specific allometric patterns that are significantly different from the other orders
examined. The artiodactyls were the mammals with the most gyrencephalic brains, these
species being significantly more gyrencephalic than all other mammals when species of
similar brain weights are compared. The North American beaver has an atypically
lissencephalic brain for its size, differing from the trend for increased gyrencephaly found
in the other rodent species examined. Conclusions: Our results show definite trends and
patterns specific to each order. So it would seem that the order is a significant
phylogenetic grouping in terms of this neural parameter, from which we can predict with
a reasonable degree of certainty, the GI of any species of a particular order, if we know
the brain weight.
Abstract (for Chapter 3)
The mammalian order has proven to be a significant phylogenetic grouping in terms of
gyrification from which we can predict with a reasonable degree of certainty, the GI of
any species of a particular order, if we know the brain weight. We have attempted in the
present study to identify potential causes for gyrification at the class level by
investigating relationships at the level of the order. It appears that clues to the extent and
pattern of gyrification in the different mammalian orders might be related to the bones
that constitute the braincase. The external surface areas of the bones of the cranial vault
of seventeen different mammalian species were measured using a microscribe digitiser.
These values were plotted against brain weight from which we could then calculate
residual values, determining if there was more or less external cranial vault area than
expected for the size of the brain. These residuals were then plotted against the
gyrification indices determined in a previous study for the species examined. Results
indicated that for the primates and artiodactyls the skull may potentially be considered as
a limiting factor on the expansion of the cerebral cortex; however, the carnivore and
rodent orders show conflicting results which suggest that the relative surface area of the
skull appears to have no effect on the quantitative extent of gyrencephaly. These
inconclusive findings suggest that causes contributing to the quantitative extent of
gyrification across mammals may be multifactorial, and more parameters may need to be
included in the analysis to arrive at an answer.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5886 |
| Date | 10 December 2008 |
| Creators | Pillay, Praneshri |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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