Neuron cell bodies basically have the same cytoplasmic components as other types of secretory cells. There is a notable exception, however. Mature neurons lack centrioles, which prevents them from dividing mitotically. Because they are difficult to replace, neurons are intended to last a lifetime. The cell's large nucleus and nucleolus are the most prominent cell body structures. Surrounding the nucleus are numerous stacks of rough endoplasmic reticulum (= rER) and groups of free ribosomes. Because they stain well with basic dyes, the stacks of rER and free ribosomes are often referred to as chromatophilic substance or Nissl bodies. These structures synthesize most of the cell's peptides and proteins. The molecules made by the free ribosomes directly enter the cytoplasm. In comparison, those synthesized by the rER are put into transport vesicles. After budding off the rER, the transport vesicles fuse with nearby Golgi bodies (= Golgi apparatus), where the enclosed peptides and proteins are chemically addressed, sorted, and packaged. The altered molecules are then placed in secretory vesicles, which pinch off the Golgi bodies. Pipe-like microtubules move the secretory vesicles down the axon to the synaptic knobs. The energy for this and other cellular activities is supplied by ATP (= adensosine triphosphate) made by mitochondria. Narrow strands of protein, called neurofilaments (or intermediate filaments), move and support the shape of the cell body and other parts of the neuron.