Expression of medium and heavy chain neurofilaments in the developing human auditory cortex

Neurofilament medium (NF-M) and heavy (NF-H) chain proteins have been used as markers for maturity in the developing brain since their accumulation in axons leads to an increase in conduction velocity. Earlier studies have demonstrated immunoreactivity of neurofilaments in Layer I of the human auditory cortex at 22 gestation weeks (GW), whereas that in other layers developed between 1 and 12 postnatal years, suggesting a gradual increase in the processing of sounds. However, third trimester fetuses and infants are fairly sophisticated in their ability to discern different aspects of complex sounds. Given these contradictory findings, we decided to study the expression of neurofilaments in human auditory cortex between 15 GW and adulthood. We found that mRNA and protein for both NF-M and NF-H were present in the presumptive human auditory cortex in the second trimester and during the postnatal period (1 year--adulthood). Axons in all layers of the auditory cortex were immunoreactive for neurofilaments by 25 GW and the density of the neurofilament-rich plexus in the cortical wall became adult-like during the first postnatal year in humans (9 postnatal months). Our results suggest that in terms of neurofilament expression, axons within the preterm human auditory cortex may be more mature than previously thought.