Stereological investigation and expression of calcium-binding proteins in developing human inferior colliculus

The mammalian inferior colliculus (IC) is a major relay nucleus in the auditory pathway. Prenatal development of the human IC has been inadequately studied. The present study reports the morphometric development and maturation of the human IC using unbiased stereology, in 18 aborted fetuses of various gestational ages (12-29 weeks) and two babies aged 40 postnatal days (PND) and 5 months (that died of postoperative complications). It also demonstrates the functional maturation of the IC by examining the expression of calcium-binding proteins - parvalbumin (PV) and calbindin (CB). There was a significant increase in the total number of neurons and glia from 18 weeks of gestation (WG). The glia and neuron volume increased significantly from 16 WG to 22 WG, respectively. The total volume of IC also increased significantly from 18 WG onwards. On the other hand, the number and volume of undifferentiated cell bodies across all ages decreased significantly. Expression of CB was concentrated in the dorsal cortex while that of PV was mainly confined to the central nucleus of the IC, possibly indicating an early segregation of parallel processing of information in the auditory pathways. Intense staining for CB in the soma and dendrites appeared earlier than that of the PV. The morphological maturation appeared to overlap the onset of functional maturation suggesting an activity-dependent mechanism in the development of IC.