Joffe, Tracey Helene; (2002) Fetal and infant encephalisation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis examines the relationship between growth, nutritional status, body composition and encephalisation in healthy fetuses and infants up to one year-of-age. It also compares human fetal brain and body growth patterns to those of baboons, rhesus monkeys and common marmosets, and assesses whether sex differences in encephalisation are present in these species during early life. A longitudinal in vivo study was undertaken where ultrasound measures of fetal biometry and anthropometric measures of infant biometry provided the basis for quantifying encephalisation and growth. Head circumference in early life was used as a proxy for brain size. In humans, skinfold thickness measures provided an index of nutritional status. The impact of maternal size, nutritional status and life history parameters were also considered in light of human offspring encephalisation, and the relationship between placenta weight and placental notching and offspring encephalisation was assessed. The effects on offspring encephalisation of maternal smoking and alcohol use during pregnancy were also examined and the implications of encephalisation for maternal-fetal conflict theory were considered. The relationship between the relative size of the brain and that of the other major organs was examined using fetal autopsy organ weight measures. The results demonstrated that intraspecific variation in encephalisation was marked in early life. At this time encephalisation was phenotypically flexible, with encephalisation SD scores undergoing regression to the mean over time. Upward or downward centile shifting in encephalisation related to previous head circumference size and growth. When catch up growth in head circumference occurred, corresponding catch down growth in body length occurred, characterising a trade-off in periodic increased growth between the brain and body. Encephalised neonates tended to be generally well-nourished with high fat and lean tissue deposits and relatively large non-brain organs. Human mothers who produced encephalised offspring had relatively large placentas with few placental notches. Maternal nutritional status further explained a significant but small amount of the variation in offspring encephalisation. Maternal smoking was shown to correlate with decreased neonatal encephalisation and maternal alcohol use was shown to correlate with decreased head circumference growth. Human fetal and infant males had relatively larger head circumferences than females (after controlling for body length differences between the sexes), with the degree of sexual dimorphism increasing over time. Non-human primate fetal encephalisation sexual dimorphism was also present but of a very low magnitude. The metabolic costs associated with human fetal and infant encephalisation sexual dimorphism were calculated to be low and did not represent a major additional energetic burden to the fetus or mother. Human fetal brain growth differed from that of baboons, rhesus monkeys and marmosets in being extended during both the hyperplasic and hypertrophic growth periods. Like the non-human primates, fetal brain growth began to slow in utero, but this occurred later in gestation in humans. Data from the literature showed that humans, cetaceans and pinnipeds differed from vertebrates in general in their increased body fatness which was associated with increased encephalisation. The findings of this thesis are also discussed within the context of evolutionary biology.

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