Cell Biology: Non-Membranous Organelles

Non-Membranous Organelles

• Ribosomes are non-membranous organelles involved in protein synthesis.
• Under light microscopy (LM), ribosomes appear basophilic due to rRNA.
• Ribosomes are composed of rRNA and proteins.
• Electron microscopy (EM) shows ribosomes as electron-dense granules with two subunits (small and large).
• They can be free in the cytoplasm or attached to the endoplasmic reticulum.
• Free ribosomes synthesize proteins for the cell.
• Attached ribosomes synthesize proteins for secretion.

Cytoplasmic Filaments

• Cytoplasmic filaments are non-membranous thread-like structures in cells.
• Subclasses include thin filaments, thick filaments, and intermediate filaments.
• Thin filaments, also called microfilaments, are composed of actin and interact with myosin.
• They are involved in cell movement (microvilli), cell division (cleavage furrow), muscle contraction, and blood clot retraction.
• Thick filaments are composed of myosin and are thicker than thin filaments (variable diameter of 12-16 nm).
• They are associated with actin filaments in muscle cells to form myofibrils.
• Intermediate filaments provide structural support and are present in sites of adhesion between cells (epithelial cells).
• Different types of intermediate filaments include desmin, tonofilament, vimentin, neurofilaments, and glial filaments

Microtubules

• Microtubules are pipe-like structures throughout the cytoplasm with a uniform diameter.
• Under EM, they appear as hollow tubules composed of 13 protofilaments, made from tubulin dimers.
• Microtubules form the cytoskeleton and support cell shape.
• They are crucial in cell division (mitotic spindle).
• They are involved in transporting materials within the cell, and forming cilia and flagella.
• Cytotoxic drugs like colchicine and vinblastine prevent microtubule assembly, used in treating certain malignancies.

Centrosome and Centrioles

• The centrosome is a specialized area of cytoplasm containing two centrioles at right angles.
• Centrioles are non-membranous organelles essential for cell division.
• In non-dividing cells, centrioles are positioned near the nucleus/Golgi apparatus
  • Light microscopy (LM) cannot directly show centrioles, while Iron-hematoxylin staining can identify them as dark blue granules.
• Under EM, centrioles appear as short cylinders, with walls comprised of 27 microtubules arranged in 9 bundles (composed of three microtubules each).
• Centrioles play a major role in cell division (spindle formation) and formation of basal bodies (cilia and flagella).
• EM of centrioles can display 9 triplets of microtubules and two central singlets or 9 doublets and no central singlets.

Cilia

• Cilia are hair-like processes projecting from the cell surface, present in epithelial cells (respiratory, female reproductive tract)
• Under light microscopy, they may appear as numerous short hair-like structures.
• Cilia's EM structure is an axoneme surrounded by a cell membrane, with nine peripheral microtubule doublets and two central singlets.
• Cilia move fluids or mucus over cell surfaces.
• Special ciliary types are stereocilia (non-motile, e.g., epididymis), having light-receiving roles (e.g., in retinal rods and cones).

Flagella

• Flagella are long, whip-like cytoplasmic processes primarily on spermatozoa for motility.
• Structurally similar to cilia, having nine peripheral microtubule doublets and a central singlet.
• Their movement propels the cell.

Additional Notes

• Five key types of intermediate filaments have been identified using special techniques.