Development of the Nervous System
The nervous system, derived from ectoderm, begins with embryonic disk formation. The neural tube develops bulges, bends, and crevices that form mature brain structures and ventricles. Three major bulges appear by approximately day 28 of gestation: the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). At approximately day 36, the posterior (caudal) portion of the forebrain develops into the diencephalon; the anterior part develops into the telencephalon (eventually cerebral hemispheres). The cerebral cortex has a specific outline by 6 months but develops sulci and gyri only in the 3 months before birth. The developing brain is affected, especially in the first trimester, to injuries caused by various chemicals such as drugs. Various neurotransmitters and growth hormones play critical roles in development of normal CNS function and restoration of function after injury. Efforts aimed to identify these substances and design drugs that will facilitate or enhance their actions are ongoing.