Page 42 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition
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24  Veterinary Histology of Domestic Mammals and Birds




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                   1.34  Nucleus with adjacent rough endoplasmic reticulum, and ribosomal protein synthesis (schematic).



                   ovoid, elliptical form, aligned with the longitudinal axis   The direct connection between the compartments of
                   of the cell. Flattened endothelial cells have a similarly  the nucleus and rER facilitates the regulatory functions
                   compressed nucleus. Depending on the age of the cell the  of the nucleus. Components of the ER can be used for
                   nucleus may appear rod-shaped or segmented (e.g. in gran-  synthesis of the nuclear envelope – a regular occurrence
                   ulocytes); in mature spermatozoa it is flat and bilaterally  during mitosis.
                   compressed. Within particular cell populations, the shape   The electron-dense  fibrous  lamina  on the internal
                   of the nucleus is generally consistent and can be used for  surface of the inner membrane has a dual function. In
                   identifying cell types.                        addition to its structural role in shaping the nucleus and
                      The  location  of the nucleus within  the  cytoplasm  stabilising the nuclear pores (see below), specific regions of
                   exhibits considerable variation. In free or cuboidal cells,  the lamina regulate the interaction between chromosomes
                   it is usually located centrally. Within fat cells, the para-  (chromatin) and the inner nuclear membrane. The fibrous
                   plasmic inclusions (lipid droplets) displace the nucleus to  lamina may also control the breakdown of the nuclear
                   the periphery. In glandular epithelial cells, the position of  membranes during mitosis.
                   the nucleus is influenced by the type of secretory product   The nuclear envelope is interrupted at regular intervals
                   (peripheral in cells producing mucous secretions, more  by nuclear pores (pori nucleares) (diameter 50–80 nm;
                   central in serous cells). The nucleus of skeletal striated  Figures 1.33 and 1.34). The pores are formed by fusion
                   muscle cells is exclusively located adjacent to the plasma-  of the inner and outer membranes and are surrounded
                   lemma (subplasmalemmal), while that of smooth muscle  on both sides of the nuclear envelope by a ring of gran-
                   cells and cardiac myocytes lies in the centre of the cyto-  ular material. Together, the pore and its associated ring
                   plasm. The functional state of the cell may also influence  are termed the nuclear pore complex. The nuclear pore
                   the location of the nucleus.                   complex contains a central opening through which large
                                                                  proteins are actively transported. Additional water-filled
                   Nuclear envelope (nucleolemma)                 channels allow for the transport of small water-soluble
                   The nucleus is surrounded by a nuclear envelope consist-  molecules.
                   ing of two concentric unit membranes (Figures 1.33 and   The number and size of the nuclear pores reflected
                   1.34). The inner membrane (membrana nuclearis interna)  the activity of the nucleus. The permeability of the pores
                   encloses the nucleoplasm. A nuclear lamina (lamina fibrosa)  allows the  exchange  of substances, particularly RNA
                   composed of fine filaments is usually present on the internal  molecules, between the nucleus and the cytosol.
                   aspect of the inner membrane. The outer membrane (mem-
                   brana nuclearis externa) is lined with ribosomes and is  Nucleoplasm (nucleoplasma)
                   continuous with the rough endoplasmic reticulum. Between  The nucleoplasm (Figures 1.33 and 1.34) is variably defined
                   the two membranes is a perinuclear space (perinuclear cis-  as the entirety of the contents of the nuclear envelope,
                   tern, cisterna nucleolemmae) of variable width (20–60 nm)  including matrix, genetic material (chromatin, chro-
                   that opens into the interior of the rER.       mosomes) and nucleoli, or as just the matrix itself. The









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