AXOGENESIS IN THE HUMAN FETAL AUDITORY-SYSTEM, DEMONSTRATED BY NEUROFILAMENT IMMUNOHISTOCHEMISTRY

Morphological, electrophysiological and behavioral evidence indicates that the onset of rapid, synchronized conduction of auditory impulses occurs in the human brainstem during the 28th-29th fetal weeks. This i mplies that axonal connections in the brainstem auditory pathway are g enerated prior to this time. In order to investigate the sequence of a xogenic events in the human brainstem pathway, we employed immunohisto chemical techniques and an antibody to neurofilament protein. Immunost aining for axonal neurofilaments in an age-graded series of fetal brai ns demonstrates that a small number of cochlear nerve axons have invad ed the ventral cochlear nucleus by the 16th fetal week. By this same t ime point, a limited number of trapezoid body-lateral lemniscus axons have reached the superior olivary complex and inferior colliculus. Bet ween gestational weeks 16 and 26, there is marked expansion and collat eralization of the ascending pathway from cochlear nerve to inferior c olliculus. By week 26, ascending axons have begun to form plexuses of terminal neuropil within all of the brainstem auditory nuclei. Beginni ng in week 22, there is development of commissural axons (dorsal commi ssure of the lateral lemniscus and commissure of the inferior collicul us) and descending projections (descending collicular axons and olivoc ochlear bundle). This early establishment of a mature pattern of axona l connections presumably forms the basis for the appearance of myelin, acousticomotor reflexes and recordable brainstem responses by fetal w eek 29.