Non-membranous Organelles Quiz

Non-membranous Organelles

• Non-membranous organelles don't have a membrane surrounding them.
• They are comprised of:
  • Ribosomes
  • Microtubules
  • Centrioles
  • Cilia and flagella
  • Cell Filaments

Ribosomes

• Small, non-membranous, dynamic organelles found free or attached to the rough endoplasmic reticulum (rER).
• Can be identified at the light microscope (LM) level by the basophilia of the cytoplasm.
  • This basophilia can be:
    • Diffuse: Spread throughout the cytoplasm
    • Localized: Contained within specific areas
• Can be identified at the electron microscope (EM) level by their electron-dense granules.
  • These granules appear free in the cytoplasm or attached to rER.
  • Polyribosomes: Ribosomes attached together by mRNA, forming whorl-like figures.

Microtubules

• Non-membranous organelles present in all cells, particularly where stiffness, shape, or oriented motion is required.
• Found in: blood platelets, cilia, flagella, mitotic spindle, etc.
• Identified at the LM level as filaments after using immunofluorescent staining techniques.
• Identified at the EM level as long, straight, cylindrical, non-branching structures (longitudinal section).
• The wall of a microtubule is composed of 13 protofilaments (transverse section).
• Microtubule diameter remains constant.
• Functions:
  • Cytoskeleton of the cell
  • Formation of the mitotic spindle
  • Formation of centrioles and the axoneme of cilia and flagella
  • Movement of organelles and vesicles in the cytoplasm

Centrioles

• Non-membranous organoids present near the nucleus and Golgi apparatus in all cells except red blood cells and nerve cells (which don't divide).
• Identified at the EM level as a short cylinder with a wall composed of 9 triplets of microtubules.
  • Each triplet is formed from 3 microtubules (a, b, and c).
    • Microtubule a is formed of 13 protofilaments, while b and c are formed of 10 protofilaments.

Cilia

• Elongated, motile structures that extend from the free cell surface.
• Present in ciliated epithelium.
• Identified at the LM level as short, fine, hair-like processes arising from the free cell surface.
• Identified at the EM level as outgrowths of the cells covered by a plasma membrane.
• Each cilium is comprised of:
  • Basal body: A centriole that has migrated to near the cell surface.
  • Shaft (axoneme): Formed of two single microtubules in the center and nine paired (doublets) microtubulues arranged circularly around the periphery.
  • Rootlets: Extend from the deep aspect of the basal body.
  • Basal foot: Attached laterally to the basal body.
• Functions:
  • Spreading a thin film of fluid or mucus across a surface.
  • Modified to receive stimuli (light stimuli in rods and cones of the retina).

Flagella

• Resemble cilia but are longer and usually present as a single structure.

Cell Filaments

• Classified based on diameter:
  • Thick Filaments: 10-15 nm, such as myosin filaments.
  • Intermediate filaments: 8-10 nm, such as neurofilaments.
  • Thin filaments: 6-8 nm, such as actin filaments.

Cell Inclusions

• Temporary constituents of the cell.
• Can be:
  • Nutritive substances: Proteins (secretory granules), carbohydrates (glycogen), and fats (lipid droplets).
  • Pigments: Endogenous (melanin, hemoglobin, hemoglobin derivatives such as hemosiderin, lipofuscin) and exogenous (carbon, tattoo particles).
  • Crystalloids: E.g., present in Sertoli cells.

Learning Objectives

• Enumerate non-membranous organelles.
• Identify the histological structure of ribosomes at the LM and EM levels.
• Describe the histological structure of microtubules at the EM level.
• Identify the histological structure of centrioles at the EM level.
• Describe the histological structure of cilia at the LM and EM levels.
• Identify the histological structure of microvilli at the LM and EM levels.
• Identify different types of cell inclusions.