Page 41 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

The cell (cellula) 23

Information held within the nucleus is carried by messen- metabolic activity. This structural diversity is determined
VetBooks.ir ger ribonucleic acid (mRNA). by genetic, chemical, physical and functional factors.
The nucleus is the functional centre of the cell, direct-
ing information exchange and metabolism and regulating Number, size, shape and position
the processes of cell differentiation and maturation. While each cell (except mature mammalian erythrocytes)
The nucleus is delimited by a double membrane termed typically contains one nucleus, cells containing two nuclei
the nuclear envelope (Figures 1.33 and 1.34). This segrega- are regularly found in liver cells, cells lining the glandu-
tion of genetic material from the cytoplasm distinguishes lar gastric mucosa, sweat glands and in the epithelium of
animal eukaryotic cells from prokaryotic cells (bacte- the urinary passages. Some cell types, including striated
ria), in which the nuclear envelope is absent. The nuclear muscle cells and osteoclasts, are multinucleated. Cells
envelope separates the processes of DNA replication and containing multiple nuclei are also distinguished by a large
RNA synthesis (transcription) within the nucleus from amount of cytoplasm (giant cells).
ribosomal protein synthesis (translation) in the cytoplasm. The volume of the nucleus is directly proportional to
Chemical signals that bind with specific receptors at the the volume of the cell (nuclear/cell volume). Each cell
surface of the cell are transported by intracellular carrier population has a characteristic nuclear/cell volume that
substances to the nuclear envelope, through which they pass lies within a certain range. The size of the nucleus is deter-
into the nucleoplasm. In this way, extracellular stimuli such mined by the number and size of the chromosomes. An
as nerve impulses and hormonal signals are able to influence increase in the number of chromosomes (e.g. in endomi-
nuclear coordination and regulation of cellular processes. tosis) is associated with a corresponding rise in the volume
The nucleus exists only as a discrete organelle in cells that of the nucleus. Enlargement of the nucleus can also result
are not undergoing division (see ‘Mitotic (M) phase’). from increased uptake of fluids or metabolites (functional
During this period, it is also referred to as the intermitotic nuclear growth). The size of the nucleus can further be
or interphase nucleus. From a functional perspective, this influenced by daily rhythms (e.g. nutrient intake), age
is termed the metabolic nucleus or nucleus of synthe- and sex. The nucleus of spermatozoa is notably small and

sis. During mitosis, the nuclear matrix incorporating the dense, its metabolic activity being very low.
chromosomes (nucleoplasm) mixes with the cytosol. The shape of the nucleus also varies between cells,
The structure of the nucleus varies between cell generally conforming to the shape of the cytoplasm. In
populations. Differences include chromatin distribution, spherical, cuboidal and polygonal cells, the nucleus is usu-
protein, nucleic acid and water content, enzyme types and ally spheroid. The nucleus of columnar cells assumes an

Inner and outer nuclear
membrane

Heterochromatin

Nucleolus
Nuclear pore
Euchromatin
Euchromatin
Nucleolus
Pars granulosa

Pars fibrosa

Heterochromatin

Nuclear pore
Plasmalemma

1.33 Electron microscope image of a reticular cell with two nucleoli, euchromatin and peripherally located,
dense heterochromatin (x4000).

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