FETAL BRAIN DEVELOPMENT AND THE PRETERM INFANT 2
NORMAL FETAL BRAIN DEVELOPMENT
Embryonic brain development begins a few weeks after conception through the processes known as gastrulation and neurulation (Gilbert, 2000). During gastrulation the embryo changes from a simple group of cells to a multi-layered organism. Three germ layers are formed during this process: the endoderm, mesoderm, and ectoderm (Gilbert, 2000). The endoderm is the most inner layer and forms the lining of internal organs. The mesoderm is the middle layer which forms the skeletal, muscle, and circulatory systems. The ectoderm is the outer most layer which forms the skin, brain and nervous system.
During the process of neurulation the ectoderm, the outermost germ layer, goes through a thickening process which in turn forms the neural plate. With changes in cell attachment and shape, the plate beings to rise and fold eventually with both sides meeting in the middle creating a tube (Gilbert, 2000). This tube pulls away from the ectoderm forming what is termed as the neural tube. “By 27 days, the tube is fully closed and has already begun its transformation into the brain and spinal cord of the embryo” (Zero to three). Failure of the neural tube to close can result in the cerebral cortex not being able to be formed or in spina bifida.
Cells called neural progenitor cells, which formed and differentiated in the ectoderm during gastrulation, begin forming the brain, hind brain and the spinal column (Jernigan & Stiles, 2010). The brain has become quite complex now including three primary brain vesicles (the prosencephalon, mesencephalon, and hombencephalon) and additional secondary brain vesicles (the telencephalon, diencephalon, metencephalon, and myelencephalon) (Jernigan & Stiles, 2010). The first neurons begin forming in the spinal cord allowing the fetus to make movements. These changes all occur within the eighth gestational week of the pregnancy.
The fetal stage, which consists of gestational week nine through birth, is defined
by significant changes in the brain structures. “The human brain begins as a smooth, “lissencephalic” structure and gradually develops the characteristic mature pattern of gyral and sulcal folding” (Jernigan & Stiles, 2010). The brain splits into two cerebal hemisphers as the first fissure develops. Neurons, which began developing in the embryonic stage, begin to form gray matter in the hindbrain, spinal column, cerebellum, midbrain structures, and the neocortex (Jernigan & Stiles, 2010). Neurons in the cortex begin to differentiate and take on properties which will allow for the formation the neural pathways and communication network of the brain. This network will form the white matter of the brain which will continue to develop well into adolescence. “Much of brain development in the fetal period centers around the processes of neuron production, migration and differentiation” (Jernigan & Stiles, 2010).
Towards the beginning of the...