<unk>  generally is due to the diminished size of the largest part of the human brain, the cerebral cortex, and the condition can arise during embryonic and fetal development due to insufficient neural stem cell proliferation, impaired or premature neurogenesis, the death of neural stem cells or neurons, or a combination of these factors. Research in animal models such as rodents has found many genes that are required for normal brain growth. For example, the Notch pathway genes regulate the balance between stem cell proliferation and neurogenesis in the stem cell layer known as the ventricular zone, and experimental mutations of many genes can cause<unk>  in mice, similar to human<unk> . Mutations of the abnormal spindle-like<unk> -associated (ASPM) gene are associated with<unk>  in humans and a knockout model has been developed in ferrets that exhibits severe<unk> . In addition, viruses such as cytomegalovirus (CMV) or Zika have been shown to infect and kill the primary stem cell of the brain—the radial glial cell, resulting in the loss of future daughter neurons. The severity of the condition may depend on the timing of infection during pregnancy.<unk>  is a feature common to several different genetic disorders arising from a deficiency in the cellular DNA damage response. Individuals with the following DNA damage response disorders exhibit<unk> : Nijmegen breakage syndrome, ATR-Seckel syndrome, MCPH1-dependent primary<unk>  disorder, xeroderma pigmentosum complementation group A deficiency, Fanconi anemia, ligase 4 deficiency syndrome and Bloom syndrome. These findings suggest that a normal DNA damage response is critical during brain development, perhaps to protect against induction of apoptosis by DNA damage occurring in neurons.
Microcephaly
Otocephaly
Acephaly
Polycephaly
Oxycephaly
Plagiocephaly
Macrocephaly
Brachycephaly