Abstract
Microcephaly or reduced head circumference results from a multitude of abnormal developmental processes affecting brain growth and/or leading to brain atrophy. Autosomal recessive primary microcephaly (MCPH) is the prototype of isolated primary (congenital) microcephaly, affecting predominantly the cerebral cortex. For MCPH, an accelerating number of mutated genes emerge annually, and they are involved in crucial steps of neurogenesis. In this review article, we provide a deeper look into the microcephalic MCPH brain. We explore cytoarchitecture focusing on the cerebral cortex and discuss diverse processes occurring at the level of neural progenitors, early generated and mature neurons, and glial cells. We aim to thereby give an overview of current knowledge in MCPH phenotype and normal brain growth.
Highlights
INTRODUCTIONIs clinically defined by a significant reduction of the occipito-frontal head circumference (OFC) of more than two (microcephaly) or three (severe microcephaly) SDs below the mean for a given sex, age, and ethnicity (von der Hagen et al, 2014)
Is clinically defined by a significant reduction of the occipito-frontal head circumference (OFC) of more than two or three SDs below the mean for a given sex, age, and ethnicity
The journey during brain growth and development is impeded at specific points with crucial steps (Figure 2)
Summary
Is clinically defined by a significant reduction of the occipito-frontal head circumference (OFC) of more than two (microcephaly) or three (severe microcephaly) SDs below the mean for a given sex, age, and ethnicity (von der Hagen et al, 2014). The recent identification of epidemic infections with the Zika virus as a cause for primary microcephaly has highlighted this rare condition as a key topic in neuroscience to understand normal brain development (Kleber de Oliveira et al, 2016; Subramanian et al, 2019). Disruption of one or more of these functions during cortical neurogenesis adversely affects neuronal progenitor proliferation, differentiation, and survival leading to a severe reduction in the total number of generated neurons reflected by the microcephaly phenotype. The RGC nuclei migrate toward the basal side of the developing cortex during G1 phase and remain there during S phase before they migrate apically during G2 phase and proceed with M-phase once they reach the ventricular surface (Kosodo et al, 2011; Miyata et al, 2014) This pattern of migration during early neurogenesis requires functional microtubules and actin filaments (Götz and Huttner, 2005). INM shows differences between species and might affect the total number of the generated neurons and thence the brain size (Okamoto et al, 2014)
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