Brain

This entry describes the formation of the brain through seven developmental stages and describes various factors that can influence the process of development. Of the three germ layers (endoderm, mesoderm, ectoderm) that form during gastrulation (the process of forming the trilaminar layer from the single-layered blastula) in the embryo, the brain arises from the neural crest of the ectoderm. This simple cell layer provides the origin for the human brain, deemed the most complex object in the universe. The brain is composed of two main classes of cells: neurons and glia. Each of these major classes of cells is made up of many cell subtypes. Neurons are the cells that process and share information acquired from sensory receptors to produce behavior.

The three principal types of neurons are sensory neurons, motor neurons, and association neurons or interneurons that integrate incoming information. Glial cells support neurons in their function. Four types of glial cells are found in the brain:

• 1.
  Ependymal cells that make cerebrospinal fluid
• 2.
  Astrocytes that provide nutritive and support function for neurons
• 3.
  Oligodendroglia that form myelin around axons in the brain
• 4.
  Microglial cells that are principally defensive cells with phagocytotic (engulfing and digesting) function

At maturity, the human brain weighs about 3 pounds and is made up of approximately 85 billion neurons and about an equal number of nonneural cells or glia. Building a mature brain is a lengthy process that begins about 3 weeks postconception and continues well into adulthood. In fact, the human brain is endowed with a phenomenal capacity for reorganization and change (plasticity), and the plasticity process is active until death. As such, brain development could be considered to proceed over the life course.

The instructions governing the development of the brain are contained in the genes found in the DNA. These instructions are derived from the DNA of an individual’s parents, but they are by no means autonomous and are heavily modified by experience through epigenetic mechanisms. Epigenetics is the process by which chemical substitutions are added or removed from the DNA by experience to alter gene expression in individuals. Remarkably, epigenetic changes can be transferred from parents to their offspring, and research has demonstrated that such changes may even be [Page 276]transferred through multiple generations. Both brain development and plasticity are heavily modified by experience.