The diffusion tensor imaging (DTI) parameter FA (Increases with the maturation of white matter) was applied to indirectly reflect the changes of brain white matter microstructure in this study. The results showed that the FA of CST, OR, AR and PTR in both preterm and full-term neonates increased with postnatal age. Consistently, (Rose, S. E et al., 2008) has demonstrated that the FA value of neonatal white matter increased with age after birth, indicating the gradual development of neonatal brain. However, the correlation between the OR, AR and PTR in preterm infants and postnatal age was lower than that of term infants, as well as the growth rate. Some research about preterm neonates have shown that compared with full-term infants, the FA values of many white matter in the brain are significantly reduced in preterm and term-equivalent infants. Moreover, there also exist abnormalities in white matter volume and microstructure, such as OR (Chang, L., Akazawa, K et al.,2016). This may be interpreted as delayed brain maturation, oligodendrocyte or axon damage in premature infants. So far, FA value increases more significantly in more mature white matter has been identified (Rose, S. E et al., 2008). Consequently, preterm infants presented significantly lower growth rate in certain brain regions compared with more mature full-term infants. The result above indicates that the maturity of OR, AR and PTR and the development rate of neonatal period of premature infants are significantly lower than that of full-term infants.
Notably, despite the FA of CST in preterm infants at birth is lower than term infants, the correlation between FA in preterm infants and postnatal age and its growth rate is significantly higher than that of term infants. Experimental results have shown that the CST in the brains of late-developing mice catches up with the CST in the brains of early-developing rats two days after birth, and this trend is defined as "catch-up" mode of CST (Canty and Murphy 2008). In addition, some research reported that the FA value of CST in preterm infants at term-equivalent age was higher compared with term neonates (Ment and Hirtz et al., 2009). One explanation for this unexpected result is that premature infants receive extrauterine environment stimulation earlier than full-term infants, and extrauterine environment factors can accelerate the maturation of brain tissue, leading to the trend of accelerated brain development in certain brain regions of healthy premature infants (Lebel and Deoni, 2018). Therefore, it is possible that CST in preterm infants brain may also have a "catch-up" developmental pattern, indicating a trend of catching up with more mature full-term infants in the neonatal period.
We also performed linear analysis of the correlation between active muscle tone (including motor abilities such as limbs and head movements) and neurobehavior (including visual and auditory reflection) with postnatal age (Bao XL and Yu RJ et al., 1991; Dubowitz LMS and Dubowitz V et al., 1999). The results showed that preterm infants had significantly lower active muscle tone than full-term infants, while the correlation between active muscle tone and postnatal age was higher than that of full-term infants, besides, there was also a slight trend of "catching up" to full-term infants. The corticospinal tract is an important white matter connection between the motor cortex and the spinal cord, and also carries the main information between the higher cortical structures and the dominant muscles, which plays a crucial role in autonomous movement (Holodny and Andrei I et al., 2005). Therefore, CST and active muscle tone showed the similar trend with postnatal age. The active muscle tone of preterm infants was weaker than that of full-term infants at birth, but it showed a slight "catch-up" pattern with postnatal age. Due to the critical role of CST pathway in motor function, this finding may suggest that certain intervention would be expected to work well in improving the preterm motor delay or abnormalities in the newborn period. Adversely, the correlation between behavior score of preterm infants and postnatal age was lower than that of term infants, and the increase rate with postnatal age was significantly lower than term infants. This is similar to the trend of FA values of AR and OR with postnatal age. The development of AR and OR in neonates is mainly related to auditory and visual functions (Bassi and Laura et al., 2008), indicating that the visual and auditory functions of premature infants are weaker than those of full-term infants. This point probably expressed that newborns with delayed white matter maturation lead to inferior brain function in corresponding brain region.
Some limitations remain in this study. First, the population is small. Larger sample size, especially for is desired to detail the changes of neonatal WM maturation with the varying postnatal age, and thus determine a more specified close to birth period for each WM. Second, despite taking Dubowitz neurological assessment as reference, efficiency of our neurobehavioral assessment for preterm neonates should be further evidenced by large surveys. Additionally, a longitudinal study targeting early brain development is needed to validate the motor outcomes in children.