It is generally accepted that nerve cells do not divide. However, recently it has been shown that the adult brain retains some cells that exhibit the potential to regenerate. In certain regions of the brain such as olfactory bulb and dentate gyrus of the hippocampus, these neural stem cells are able to divide and generate new neurons. They are characterized by prolonged expression of a 240-kDa intermediate filament protein nestin, which is used to identify these cells by histochemical methods. Neural stem cells are also able to migrate to sites of injury and differentiate into new nerve
FIGURE 12.4 • Nerve cell bodies. a. This photomicrograph shows a region of the ventral (anterior) horn of a human spinal cord stained with toluidine blue. Typical features of the nerve cell bodies visible in this image include large, spherical, pale-stained nuclei with a single prominent nucleolus and abundant Nissl bodies within the cytoplasm of the nerve cell body. Most of the small nuclei belong to neuroglial cells. The remainder of the field consists of nerve fibers and cytoplasm of cental neuroglial cells. 640. b. Electron micrograph of a nerve cell body. The cytoplasm is occupied by aggregates of free ribosomes and profiles of rough-surfaced endoplasmic reticulum (rER) that constitute the Nissl bodies of light microscopy. The Golgi apparatus (G) appears as isolated areas containing profiles of flattened sacs and vesicles. Other characteristic organelles include mitochondria (M) and lysosomes (L). The neurofilaments and neurotubules are difficult to discern at this relatively low magnification. 15,000.
cells. Research studies on the animal model demonstrate that newly generated cells mature into functional neurons in the adult mammalian brain. These findings may lead to therapeutic strategies that use neural cells to replace nerve cells lost or damaged by neurodegenerative disorders such as Alzheimer and Parkinson diseases.
Dendrites are receptor processes that receive stimuli from other neurons or from the external environment.
The main function of dendrites is to receive information from other neurons or from the external environment and carry that information to the cell body. Generally, dendrites are located in the vicinity of the cell body. They have a greater diameter than axons, are unmyelinated, are usually tapered, and form extensive arborizations called dendritic trees. Dendritic trees significantly increase the receptor surface area of a neuron. Many neuron types are characterized by the extent and shape of their dendritic trees (see Fig. 12.2). In general, the contents of the perinuclear cytoplasm of the cell body and cytoplasm of dendrites are similar, with the exception of the Golgi apparatus. Other organelles characteristic of the cell body, including ribosomes and rER, are found in the dendrites, especially in the base of the dendrites.
Axons are effector processes that transmit stimuli to other neurons or effector cells.
The main function of the axon is to convey information away from the cell body to another neuron or to an effector cell, such as a muscle cell. Each neuron has only one axon, and it may be extremely long. Axons that originate from neurons in the motor nuclei of the CNS (Golgi type I neurons) may travel more than a meter to reach their effector targets, skeletal muscle. In contrast, interneurons of the CNS (Golgi type II neurons) have very short axons. Although an axon may give rise to a recurrent branch near the cell body (i.e., one that turns back toward the cell body) and to other collateral branches, the branching of the axon is most extensive in the vicinity of its targets.
The axon originates from the axon hillock. The axon hillock usually lacks large cytoplasmic organelles such as Nissl  FOLDER 12.1 Clinical Correlation: Parkinson’s Disease