Connectivity in the Human Fetal Brain

Abstract

It is generally accepted that complexity of neuronal connections in the human brain underlies complexity of cognitive and behavioral functions. In this paper we explain how developmental cellular events and connectivity patterns form special framework for early functions and establishment of adult circuitry. At the beginning of fetal period distributions of circuitry elements (axons, dendrites and synapses) is strictly bilaminar outside of proliferative zones. The first quantifiable number of synapses occurs after 15 postconceptional weeks (PCW) in the transient subplate zone. This early synapses are substrate of endogenous, sensory non-driven activity. Distribution within the connectivity compartments is random and no modular units were observed. Important phenomenon in development of afferent input is waiting phenomenon: axons wait for several months until they gradually invade target cortical area. These waiting synaptic compartments may be visualized in both in vivo and postmortem images. The bipolar orientation of synapses, with predominance of deep circuitry is basis for cortical surface-positive electric response. The majority of described circuitry is transient. It is only after 23 PCW that sensory driven afferents establish connections in the corresponding cortical target area. At the end of gestation another phenomenon take place and that is a dramatic reorganization of fetal circuitry due to the retraction of axons and gradual disappearance of transient fetal layers. In conclusion, early fetal circuitry shows transient laminar distribution and transient endogenous activity. Sensory driven activity is late phenomenon and further development of connectivity proceeds after dramatic spatial and cellular reorganization of cortex.