L7 &8 Histo PDF

L7&8 histo Nucleus It is the largest single membrane-bound compartment in the cell that controls its functions. Site: is usually central but may be eccentric e.g. fat cells. Number: - is usually single, may be; - binucleate e.g. hepatocytes and ce...

L7&8 histo Nucleus It is the largest single membrane-bound compartment in the cell that controls its functions. Site: is usually central but may be eccentric e.g. fat cells. Number: - is usually single, may be; - binucleate e.g. hepatocytes and cells of autonomic ganglia, - multinuclear e.g. osteoclasts, skeletal muscle fibers and syncytiotrophoblast. - anuclear e.g. erythrocytes, some lens fibers and keratinized cells of epidermis, nail and hair. Size: varies between 5-10 m. Shape: - The form of the nucleus depends on the form of cell, e.g. flat in squamous cells, rounded in cubical cells, oval in columnar cells. - However, it may be rod-shaped, or kidney shaped. - The nucleus is usually non lobed but may be lobed e.g. neutrophils and megakaryocytes. 1|Page  L7&8 histo Movement: it may rotate clock or anti-clockwise or even change its site. Appearance: - it is basophilic in reaction, and it appears more refractile than the cytoplasm. - It may be either: 1. Lightly stained (vesicular): The details of its components are well seen inside it. e.g. plasma cell, liver and nerve cells. 2. Deeply stained: The details are not seen e.g. the nucleus of a lymphocyte. Structure: 1- Nuclear envelope. 3- Nucleolus. 2- Chromatin. 4- Nuclear matrix 2|Page  L7&8 histo Packaging of chromatin into the chromosomal structure 3|Page  L7&8 histo LM: - It appears as a basophilic line surrounding the nucleus. - This basophilia is due to the chromatin attached to the inner surface. EM: - two parallel unit membranes separated by a space 30 nm called the perinuclear cisterna. - Polyribosomes adhere to the outer surface of the outer membrane and sometimes continuous with rough endoplasmic reticulum. - Fibrous lamina is closely associated with the internal membrane. Nuclear pore complex (NPC): - The NPC is composed of four protein elements: 1. The scaffold surrounds the pore and supports the other elements. It maintains the pore and simple diffusion channels. 2. The transporter (central hub) is the central protein ring responsible for transporting proteins into and out of the nucleus via receptor-mediated transport. 3. Thick filaments (about 3 nm in diameter) project from the scaffold ring into the cytoplasm and may serve as a staging area prior to protein transport. 4. The basket is projecting from the scaffold ring into the nucleoplasm. It is function in RNA transport. - Function: The NPC permits passive movement across the nuclear envelope via a 9 nm open channel. Most proteins, regardless of size, pass in either direction only by active transport. 4|Page  L7&8 histo It is composed mainly of coiled stands of DNA bound to basic proteins (histones). LM: - It appears as basophilic granules which may be coarse or fine according to their size. - They represent the coiled parts of the chromosomes as the extended parts are very thin to be visible. - They give + ve Feulgen reaction. - Molecular structure: Chromatin is a nucleoprotein formed of: 1. Nucleic acid (DNA). 3. Acidic protein. 2. Basic proteins (histones). EM: - It appears as a mass of fine fibrils. - The fibrils attach to the inner side of the nuclear membrane and to the annuli of the pores. - Types of chromatin: 1. Heterochromatin (condensed or inactive): It appears as aggregations of dense granules distributed within the nucleus as: a) Peripheral or marginal chromatin: lies close to the inner nuclear membrane. b) Nucleolus-associated chromatin: lies around the nucleolus. Nucleosome is the smallest chromatin subunit. It packages genetic material in a condensed form. It can coil further to form solenoids )‫(ملفات لولبية‬ c) Chromatin islands or karyosomes: lie in the nuclear sap in the form of clumps or discrete bodies of irregular size and shape. 2. Euchromatin (extended or active): - It appears as very thin uncoiled threads. - Their genes are active and direct protein synthesis. 3. Perichromatin: - appears as dense granules. 5|Page  L7&8 histo LM: - It appears as one or more rounded basophilic bodies. - It deeply stained due to its content of ribonucleoprotein. - It is Feulgen-ve. EM: - It appears to be formed of: 1. Pars fibrosa: is composed of a fine filamentous material representing newly transcribed precursor rRNA. 2. Pars granulosa: consists of 15-20 nm ribonucleoprotein granules representing maturing ribosomal subunit particles. 3. Pars amorpha: is a structureless proteinaceous material of variable amount and density. 4. Pars chromosoma: which lies in and around the nucleolar parts being attached to the chromatin masses. 5. Temporary components: are protein or lipids. 6|Page  L7&8 histo Nucleolar organizer regions: They are the secondary constriction of the satellited chromosomes 13, 14, 15, 21 and 22. Have the genes coding for rRNA. They stain with silver salts. Nucleolus is formed of: rRNA, proteins, lipids, lipoprotein and DNA/RNA hybrids. Function: It is the site of production of the rRNA. - It is the component that fills the space between the chromatin and the nucleoli in the nucleus. - It is composed mainly of protein; some of which have enzymatic activity, metabolites, nucleic acids, sugars, enzymes and ions. - LM: It appears as a colorless medium. - EM: It appears as fine randomly distributed granular areas. NUCLEAR SKELETON A sponge like proteinaceous structural framework of nucleus visible after extraction of chromatin and karyoplasm. The lamina of the nuclear envelope is part of nucleoskeleton. 7|Page  L7&8 histo General functions of the nucleus: 1. It guides protein synthesis. 2. Ribosomal RNAsynthesis 3. Sharing in the formation of ATP which is considered as a free nucleotidecarrying high energy. 4. Reproduction of the cell (cell division). 5. It is the origin of the RER. 6. Formation of annulate lamellae. 7. Protection of the mRNA from the RN-ases in the nucleus. 8. The karyolymph provides a medium for the movement of the different types of the RNA (m, t and r) towards the nuclear pores. 8|Page