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External granular cell layer bobbling: a distinct histomorphological feature of the developing human cerebellum.

INTRODUCTION
The external granular layer (EGL) of the developing human cerebellum is detectable until an age of ~ 1 year. It has been described as a thin, evenly calibrated layer of germinal cells. We have repeatedly observed a distinct discontinuous bobbled configuration of the EGL (external granular layer bobbling = EGLB) in human infantile autopsy brains. AIM, MATERIALS AND METHODS: We investigated 106 human fetal and infantile postmortem brains (range of gestational week at birth: 14 - term; range of postpartal age: 0 - 500 days) for presence of EGLB and correlated it with gestational/postpartal age, gender, developmental stage of cerebellar cortex, medical history and neuropathological findings.


RESULTS
EGLB was detectable in 38/106 (35.8%) cases. EGLB presents as focal series of uniform knob-like protrusions of the EGL. In the notches between individual knobs, capillaries penetrate from the primitive leptomeningeal vascular plexus into the molecular layer. We found EGLB predominantly in depths of fissures of cerebellar hemispheres, vermis and/or tonsils. Presence of EGLB was statistically significantly more common in liveborn cases who died after gestational week 25 and cases with higher maturity grade of the cerebellar cortex, respectively. There was no gender difference. EGLB was not associated with medical history or neuropathological findings.


CONCLUSIONS
EGLB is a distinct histomorphological feature of the developing cerebellum, which is predominantly found in infants. Our data indicate that EGLB is a physiological phenomenon occuring during cerebellar development at a certain gestational age, although we cannot exclude that it represents an artifact related to tissue fixation. In any case, recognition of this recurring feature is relevant for the practicing neuropathologist and should not be interpreted as a cerebellar migration disorder.