The Nucleus

Large and either spherical, or ovoid, the nucleus lies centrally located in the soma in most healthy neurons, (Figs. 20 and 21E-21I). The nucleoplasm in large, synthetically active neurons is euchromatic (lacking dense chromatin particles), amorphous, and pale. The chromosomes in these postmitotic cells no longer replicate DNA and double themselves but engage only in gene expression as partly uncoiled, indistinct, sprawling threads. Such a configuration makes the genome easily available for transcription and reflects the constant interplay between the nucleus and cytoplasm. In small neurons, the nuclear contents are concentrated and heterochromatic clumps are seen. In humans, the sex chromatin (condensed, genetically inactive X chromosome of the female) lies near the inner surface of the bilaminar nuclear envelope. In some mammals (mouse, rat, cat), it is satellited to the nucleolus (Fig. 23). In some neurons, the nucleus is shaped irregularly, with deep infoldings of the nuclear envelope.

1. Nuclear Envelope

The nuclear envelope displays numerous nuclear pores (Fig. 23). Seemingly closed by a diaphragm, each pore is a complex of eight radial spokes and a central granule that provides a 10-nm channel. The pores permit two-way traffic of molecules of less than 10 nm by passive diffusion. Because outgoing ribonucleopro-teins and incoming newly synthesized nuclear proteins require a 20-nm opening, such molecules are thought to have a signal sequence of amino acids that binds to the nuclear envelope and somehow triggers opening of the channel.

2. Nucleolus

The nucleolus in neuronal nuclei is prominent (Figs. 21E-21I), and there may be more than one. As in most cells, it is a rounded structure, deeply stained by basic dyes, but unreactive to the Feulgen reaction for DNA. It transcribes ribosomal RNA. Newly synthesized ribosomal subunits, together with messenger RNA (mRNA) assembled in the nucleus, then pass to the cytoplasm through the nuclear pores, traveling along actin filaments within the nucleus and microtubules once outside it.

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