Chapter 7 Recent estimates suggest that there are about 100 billion neurons in the human brain. Neurons  each develop thousands of synapses, and in the end the brain contains a staggering 100 trillion  synaptic connections! The fertilized egg, or zygote , contains complete genetic blueprints for the new person in its 46 chromosomes. A period of rapid cell division follows, and by the end of a week, the cell mass, now known as a(n) embryo , shows three distinct cell layers. The nervous system will develop from the ectoderm . By the end of the 8th week of embryonic life, rudiments of all major organ systems are apparent, and the head is about 50 % of the total mass of the embryo. After 10 weeks, the embryo is called a(n) fetus . The production of nerve cells is called neurogenesis . Although neurons themselves do not divide, the cells that become neurons (reproduce) through a process called mitosis and form a closely packed zone of cells called the ventricular zone Cells in the ventricular layer of the neural tube undergo division, or mitosis, giving rise to daughter cells that will eventually leave this layer and go on to form neurons and glial cells. Neurons themselves do not divide, and very few are added after birth In contrast to the situation with invertebrates such as C. elegans, vertebrate neuronal development is regulated by cell – cell interactions The increasing weight of the brain during postnatal development has traditionally been attributed to increases in the size of neurons, the branching of dendrites , the birth of glial cells, and the wrapping of myelin around axons. However, we now know that neurogenesis also occurs in humans through adulthood . Before becoming neurons proper, newly born nerve cells move to new positions. Cells tend to move away from the ventricular layer where they were born, creeping along spokes formed by radial glial cells. Alternatively, some neurons move so that they are perpendicular to these glial cells, while still others form a rostral stream to supply the olfactory bulbs . Molecules making up one important specialized family of molecules involved in the guidance of migrating cells and growing cell processes are the cell adhesion molecules (CAMs). In the adult, these molecules may also guide regenerating axons after trauma. Newly born cells, having migrated to their new positions, do not resemble neurons until they begin to express the genes that code for the specialized proteins that make up a neuron. This process is referred to as cell differentiation The process of induction, whereby some spinal cord cells become motoneurons, is a consequence of the release of a protein messenger called Sonic hedgehog from a structure called the notochord . Growth cones at the tips of axons and dendrites give rise to two kinds of outgrowths—the threadlike filopodium and the sheetlike lamellipodium . These outgrowths adhere t