Unit I

Nucleus: It is made up of deoxyribonucleic acid and ribonucleic acids so it takes basic (Hematoxylin)stain and always appears violet or blue under light microscope . It may be oval, round, spherical or flat inshape and may be central or eccentric in location within the cell. Most of the cells in the human body arenucleated (exception-RBC) and a single nucleus is present. Normally, striated muscles and osteoclastshow multinucleated appearance and hepatocytes and transitional epithelium may show binucleated cells.Depending upon the physical state of the chromosomes in an interphase cell, nucleus is of following types:Euchromatic: The chromatin matter is granular in appearance. The chromosomes are coiled at places butuncoiled at other. The uncoiled portion is so thin that it is invisible under light microscope. The uncoilingindicates that RNA synthesis is going on and this is a feature of working cell or active cell or normal cell,hence the name euchromatic. Nucleolus is usually seen clearly within the granular chromatin.Heterochromatic: The chromatin matter is in a condensed or coiled form. The nucleus is more uniformlyviolet after staining. In resting cell the RNA synthesis is very slow in the nucleus so the uncoiled part of thechromosome is negligible. Nucleolus is usually not seen as it is hidden within the dark condensed chromatin.Vesicular: The chromatin matter is very much extended or uncoiled; as a result it can not be seen under lightmicroscope. Nucleolus is the only structure seen within the nucleus. Prominent nucleolus surrounded byclear unstained white zone within the nucleus gives it an owl’s eye appearance. Extended chromatin indicatesthat rapid RNA synthesis is going on showing that the cell is very active e.g. neurons.Pyknotic: Nucleus is shrunken and chromatin matter is very much condensed so that on staining the nucleardetails are not visible. The dark violet and small sized nucleus indicates that the cells is degenerating, dyingor showing some pathological process e.g. epidermis of skin.Nucleus (E/M structure): It shows a nuclear membrane as two parallel lines each of 70Aº thickness with250 Aº distance between the two. A total thickness of the membrane is 40nm and it shows nuclear poreseach about 100-200 nm in size. Through the pore, the pore complex extends for a short distance into thecytoplasm and nuclear sap. Nuclear pores are closed by diaphragm. Within the chromatin material nucleolusis present which is metabolically active site in the nucleus. Nuclear organizers are the parts of acrocentricchromosomes with satellites (No. 13, 14, 15, 21, 22) which are associated with the formation of nucleoli.Thus a cell can have maximum 10 nucleoli but because of plane of section all cannot be seen at one time.Notes:9