Cell Biology: Non-Membranous Organelles

Study Notes

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 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 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.

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 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 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.