History:
Van Benden (1887) first discovered centrosome. T. Boveri (1888) coined the term centrosome.
Occurrence:
Centrosome mostly present in all animal cells, motile cells of many green algae, bryophytes, pteridophytes and cycads. Centrosome is absent in prokaryotic cells, amoebae, some algae, higher fungi, higher gymnosperms and in all angiosperms.
Structure:
Centrosome or cell centre is a non- membranous organelle usually composed of a pair of centrioles (diplosome) embedded in the amorphous pericentriolar materials called centrosphere or kinoplasm. Around centrosphere are found rays radiating in all directions and called aster or astrosphere. Usually the two centrioles present at light angle to each other in inverted “T” or “±” manner, But centrosome with multiple centrioles present in giant cells of bone marrow (Fig. 3.43).
Under optical microscope the centrioles appear as two minute darkly stained granules. But, under EM centrioles appear as two short hollow micro-cylinders with cartwheel like organization. A centriole is about 0.3 – 0.5/µm long and 0.15/µm in diameter. Each centriole consists of a whorl of nine peripheral triplet fibrils tilted at an angle of 40°. The center is without any fibril. This type of arrangements called 9 + 0 (Fig. 3.44).
Each triplet fibril consists of three microtubules or sub-fibres designated from inside to outside as A, B & C. The three microtubules are joined in such that A sub-fiber is a complete microtubule having 13 protofilaments while B & C sub-fibers are incomplete and share 2-3 protofilaments with A sub-fiber. The adjacent triplets are interconnected at intervals byprotenous C-Alinkers.
The centre of the centriole has a protenous rod called hub (- 2.5 nm diameter) from which a protenous spokes radiate and connect with A sub-fibers through X-thickenings. In between X-thickenings, Y- thickening present which connect both to X thickening and C-A linkers by connectives.
The centrioles are surrounded by one or more series of dense, amorphous, protoplasmic spheres called massules or pericentriolar satellites. Their position changes with the different stages of cell.
Functions:
1. New centrioles arise from the massules of pre-existing centrioles in the G2 phase of cell cycle. Therefore, centrioles are called self-replicating organelles.
2. They help in the organization of spindle fibres and astral rays during cell division. Therefore, they are called microtubule-organizing centres (MTOCs). However, the centrioles are not essential for this purpose. The cells of higher plants lack centrioles and still form a spindle.
3. They act as basal bodies at the base of cilia and flagella.
4. The distal centriole of a sperm gives rise to the axonema or axial filament of sperm tail.