http://www.thegreatonwardpress.com/9975/07/index24article2.html
The human animal, Homo sapiens or wise man, like all other multicellular animals, propagates the next generation by sexual reproduction. In the fertilization process, whether through intercourse or through in vitro insemination, an ovum and a sperm cell become a unicelleluar organism. A unique moment of human history is represented in the single cell, the zygote.…
The number of cells making up the zygote increases exponentially. It is important to understand that all cells in each generation contain the same DNA, and thus have an equal potential to become virtually any type of specialized cell. The single factor that will determine the fate of each new healthy cell is its local external environment of chemicals and energies. This dynamic context determines the internal metabolic processes of each cell and what it will become in its mature existence.
In humans, the zygote cleaves into two daughter cells at about 30 hours after fertilization. A sphere of a dozen or more cells forms by 3 days, and by the end of week 1 there is an embryo composed of thousands of cells. In the third week of development, the embryo begins to divide into a distinctive three-layer or trilaminar structure. The three germ cell layers are called mesoderm, endoderm, and ectoderm. Cells of the mesoderm will produce connective tissues, striated muscle cells, blood vessels, blood cells, bone marrow cells, and the tissues of the excretory and reproductive organs; endoderm cells will divide and differentiate into epithelia of the respiratory system, glandular cells, and the pancreas and liver; and the ectoderm layer will produce outer epithelial tissues and the cells that make up the nervous system.
Early in the third week of human development, a thick band of cells appears along the midline of the embryo. This primitive streak begins at one end and elongates toward the other. The formation of this structure provides the landmark by which a three-dimensional (3D) morphological coordinate system for the growing embryo may be defined. The origin of this streak is at the caudal (rear or posterior) end, and its growth is toward the cranial (head or anterior) end. Thus, the right and left sides and the dorsal (top) and ventral (bottom) surfaces of the embryo may now be identified. The primitive streak is eventually replaced by a tubular column of cells called the notochord, which also migrates from the caudal region toward the expanding cranial end. The vertebral column eventually forms in the notochord region. The mesodermal cells of the notochord become the defining bony structures of the midline axis: the cranium, the vertebrae, the ribs, and the sternum.
The central nervous system, which comprises the brain and spinal chord, arises out of the neuroectoderm, also called the neural plate, a region of cells that runs parallel and dorsal to the notochord. By the end of the third week, the entire length of the neural plate has folded into the neural tube, whose ends close to form a protected space inside the larger embryo. The interior space of the neural tube becomes the fluid-filled ventricular system of the brain and spinal cord central canal. The neural crest cells, situated along the outer length of the neural tube, give rise to most of the peripheral nervous system, which extends throughout the body. All the progenitors of the cells that make up the central nervous system—the neurons and glia cells—exist in the inner walls of the neural tube, a region called the ventricular zone. Thus, nerve and glia cells are intimately related, from their origins in primordial cells of the ventricular zone to their final destinies as intertwined functional systems within the brain and spinal cord.
The developing spinal cord becomes anatomically divided into a dorsal and a ventral region, and groups of nerve and glia cells called the dorsal horn nuclear groups and the ventral horn nuclei develop in the respective regions. The dorsal region will receive sensory information from the peripheral nervous system and send it toward the brain (afferent flow), and the ventral nuclei will transmit nerve impulses from the brain to all areas of the body (efferent flow). Clusters of neural crest cells that lie alongside the spinal cord differentiate to become the spinal ganglia (dorsal root ganglia) and the ganglia of the sympathetic nervous system. It is through these peripheral systems of cells that we experience the external and internal sensory worlds, send the information to our brains, and generate our behaviors and memories. All that we are, and can be, must relate to the basic patterns of afferent and efferent activities of the nervous system.
During the fourth developmental week, three primary brain vesicles form out of the neural tube: the forebrain, the midbrain, and the hindbrain. The forebrain further divides into two distinct regions, the telencephalon and the diencephalon, and the hindbrain partly divides into the pons and the medula. Simultaneously, all other human body systems—respiratory, cardiovascular, musculoskeletal, endocrine, reproductive, and so on—are developing from the mesodermal and endodermal germ cells. In just 4 weeks of development, genetic expression and epigenetic influences have shaped the protected embryo into a basic three-dimensional design that reveals its future developmental course. Through massive waves of cell division, migration, and differentiation, the embryo has developed a definite rostral-caudal organization, in addition to dorsal, ventral, and midline axial orientations.
— Richard M. Pico,
Consciousness in Four Dimensions -
Biological Relativity and the Origins of Thought, Chapter 4 – Development and Systems of Neurons
Filed under: natural philosophy
