![]() The other nuclear components such as the chromatic reticulum and nucleolus remain suspended in the nucleoplasm. The nucleoplasm seems to contain granules of various sizes and densities and is yet of unknown composition and function.ĭuring nuclear division, the nucleoplasm is of course continuous with cytoplasmic matrix. The undifferentiated protoplasm or ground substance present inside the nuclear envelope is called nuclear sap or nucleoplasm or karyolymph. There is no well -defined nuclear envelope in the prokaryotic cells. It disappears at certain stage of nuclear division and reappears on the completion of the division process. The nuclear membrane can easily be observed in the metabolic nucleus. Different annuli in the same nuclear envelope can exist in an expanded or open configuration. A relaxed annulus would contain a temporary central opening large enough to allow passage of resolvable particles as ribosomes. In unstretched annuli a thin membrane is present in the centre. Annulus shows contraction and relaxation. Somewhat more specifically, 8 spheroid particles have been found to be arranged around the circumference of each annulus in the nuclear envelope of amoeba (Fig. The structure of each annulus is eight-partite. Annuli have an outer diameter of 1200 A and inner diameter between 0 and 500 Å (Fig. The middle of the annulus in many but not all nuclei is also covered by a thin single membrane, which evidently checks the free passage of ions or other molecules across the envelope. One of the best established features of annulus ultrastructure is that inner and outer nuclear membranes come together in the margin of each annulus. On the outer extreme the nuclear envelope is not completely closed to passage of large particles, but this maintains a very sensitive control over which particles pass and in which direction. This clearly indicates that there is no existence of permanent openings of about 800 A diam. Some authors have suggested that typical nuclear envelopes are penetrated by permanent pores of500-800 A diam., but some kinds of protein molecules only 100 Å or less in diameter are unable to enter the nucleus from the cytoplasm. That is not simply a pore but a directional mechanism of transport. It allows the passage of materials but only through a restricted region. Thus, it seems preferable to use the term ‘annulus’, in precise sense, signifying a thickened, electron dense ring of material set into the nuclear envelope and functioning by some unknown mechanism to control the passage of particles in and out of the nucleus. Since they almost do not represent permanent openings in the nuclear envelope, the term ‘pore’ and ‘pore complex’ are misleading. ![]() The annulus is a thickened ring surrounding a central pore. Some authors call the entire annular structure a ‘pore’ some others call the entire structure an annulus and still some others use both terms. Annuli can be observed only with electron microscope. 8.2).Īfzelius (1963) found that the nuclear envelope in luminescent marine dinoflagellate noctiluca has a single membrane only which is 150 Å thick and completely lacking annuli. The presence of annuli is the most characteristic structural feature of the nuclear envelope (Fig. The nuclear envelope possesses special circular structures, the annuli (singular-annulus) through which certain materials can pass. It is not known whether these layers have any special function other than mechanical support (Fig. In many vertebrate cells there is a layer of densely packed fibres just inside the inner nuclear membrane forming a third concentric sheath of uniform thickness (about 300 Å). The inner nuclear membrane is free from ribosomes and sometimes it is associated with chromatin. Sometimes it becomes out-folded to give rise to endoplasmic reticulum. Outer nuclear membrane is rough surfaced due to attached ribosomes. The space between the two unit membranes is known as perinuclear space or perinuclear cisterna. The outer and inner membranes are separated by a clear space of 100 to 150 Å. According to Du Praw, each membrane is about 80-100 Å thick. ![]() Electron microscopy has revealed that it is composed of two concentric unit membranes, an outer membrane and an inner membrane of lipoproteins (Fig. It is found in the nuclei of all the eukaryotic cells. ![]() The nuclear membrane or karyotheca is the outer boundary of the nucleus.
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