Quantitative analysis of somatosensory cortex development in eutherians, with a comparison with metatherians and monotremes

Quantitative comparison of cerebral artery development in metatherians and monotremes with non-human eutherians

A quantitative comparison of the internal diameters of cerebral feeder arteries (internal carotid and vertebral) and the aorta in developing non-human eutherians, metatherians and monotremes has been made, with the aim of determining if there are differences in cerebral arterial flow between the three infraclasses of mammals such as might reflect differences in metabolism of the developing brain. There were no significant differences between eutherians and metatherians in the internal radius of the aorta or the thickness of the aortic wall, but aortic internal radius was significantly smaller in developing monotremes than therians at the < 10 mm body length range. Aortic thickness in the developing monotremes also rose at a slower rate relative to body length than in metatherians or eutherians. The sums of the internal calibres of the internal carotid and vertebral arteries were significantly lower in metatherians as a group and monotremes compared with non-human eutherians at body lengths up to 20 mm and in metatherians at > 20 mm body length. The internal calibre of the internal carotids relative to the sum of all cerebral feeder arteries was also significantly lower in monotremes at < 10 mm body length compared with eutherians. It was noted that dasyurids differed from other metatherians in several measures of cerebral arterial calibre and aortic internal calibre. The findings suggest that: (i) both aortic outflow and cerebral arterial inflow may be lower in developing monotremes than in therians, particularly at small body size (< 20 mm); (ii) cerebral inflow may be lower in some developing metatherians than non-human eutherians; and (iii) dasyurids have unusual features of cerebral arteries possibly related to the extreme immaturity and small size at which they are born. The findings have implications for nutritional sourcing of the developing brain in the three infraclasses of mammals.

Quantitative comparison of cerebral artery development in human embryos with other eutherians

The embryonic and early fetal human brain is known to undergo extraordinary expansion of its cellular population during embryonic and early fetal life, and is critically dependant on a steady supply of nutrients and oxygen for proper brain development. Quantitative analysis of the internal radius of the aorta and cerebral arteries in a range of eutherian mammals has been used to compare arterial flow to the developing human brain with that to the brains of non-human eutherians. Human embryos showed a much steeper rise of internal radius of the aorta with increasing body size than the embryos of non-human eutherians, but the thickness of the aorta rose at the same pace relative to body size in both humans and non-humans, suggesting that aortic pressure is similar in all eutherian embryos of a similar size. The sums of internal radii of both the internal carotids and vertebral arteries of human embryos raised to the fourth power were much lower at embryonic stages (less than 22 mm body length) than in non-human eutherians, were similar between humans and non-humans at 22-30 mm body length, and exceeded the non-humans at body lengths of more than 30 mm. The relative size of the internal calibre of the cerebral feeder arteries (internal carotid and vertebral) to the aorta did not change between embryonic and fetal sizes in either humans or non-humans. The findings suggest that the developing human brain may actually receive less blood flow at embryonic sizes (less than 22 mm body length) than do other mammalian embryos of a similar body size, but that internal carotid and vertebral flow is higher in human fetuses (body length greater than 30 mm) than in developing non-humans of the same body size. Increased flow to the developing human brain relative to non-humans is achieved by simultaneous increases in both aortic and cerebral feeder artery internal calibre.