None membranous cell organelles

📘 Non-Membranous Cell Organelles and Cell Membrane Specializations

🖇 I. Ribosomes

📌 Definition
Small, uniformly sized ribonucleo-protein particles (12–15 nm).
Formed in nucleolus (pars granulosa of nucleolonema) → located in cytoplasm.

📌 Light Microscopy (LM)
Too small to be seen directly.
Presence indicated by basophilic cytoplasm in cells usually acidophilic. (Exam Point)

📌 Electron Microscopy (EM)
Composed of large and small subunits.
Not associated with membranes.

📌 Structure

• Small subunit binds to mRNA.
• Large subunit + tRNA form polypeptide chain.
• May attach to RER membrane.

📌 Types

1. Attached ribosomes – Bound to RER; for protein secretion. (Exam Point)
2. Free ribosomes – Suspended in cytosol; in immature/embryonic/stem cells. (Exam Point)
3. Polyribosomes (Polysomes) – Multiple ribosomes on one mRNA strand in spiral/rosette form.

📌 Functions

• Free ribosomes → synthesize intracellular proteins.
• Attached ribosomes → synthesize extracellular proteins. (Exam Point)

🖇 II. Centriole

📌 Definition
Self-replicating organelles duplicating before cell division. (Exam Point)

📌 Site
Near nucleus in centrosome region.
Two centrioles (diplosome) in non-dividing cell.

📌 LM
Appear as minute rods or granules.

📌 EM

• Centrioles lie perpendicular to each other. (Exam Point)
• Wall: 9 triplets of fused microtubules (A, B, C). (Exam Point)

📌 Functions
a. Form Microtubule Organizing Center (MTOC) → essential for cell division & flagella formation. (Exam Point)
b. After division → determine nucleus position & daughter cell organization.
c. Form basal bodies for flagella/cilia. (Exam Point)
d. Dysfunction → centrosome-related cancer development. (Exam Point)

🚨 Clinical Note
Centrosome dysfunction contributes to oncogenesis.

🖇 III. Cytoskeleton

📌 Definition
Structural framework of cytoplasm → supports shape, movement, intracellular transport, endocytosis, and junction stability.

📌 Components

1. Microfilaments
2. Microtubules
3. Micro-trabecular lattice (includes intermediate filaments)

🟠 1. Microfilaments

📌 Function
Responsible for contractility.

📌 Thick Filaments (Myosin)

• Diameter: ~12 nm.
• 18 subtypes.
• Myosin II → in muscle.
• Myosin V → in most cells, interacts with actin for vesicle/organelle movement.

📌 Thin Filaments (Actin)

• Diameter: ~5 nm, beneath plasma lemma.
• Forms:
• • G-actin → globular, free in cytoplasm.
  • F-actin → filamentous, polymerized. (Exam Point)
• Myosin + actin = myofilaments.

📌 Intermediate Filaments

• Structural/support role.
• Rope-like, diameter 8–10 nm (intermediate between actin & microtubules). (Exam Point)
• Five Types (by immunocytochemistry): (Exam Point)
• 1. Keratin – epithelial cells.
  2. Vimentin – fibroblasts.
  3. Desmin – muscle cells.
  4. Neurofilaments – nerve cells.
  5. Glial filaments – glial cells (CNS support).

🟣 2. Microtubules

📌 Structure

• Straight/slightly curved, non-branching tubules.
• Diameter: 20–25 nm; wall 5 nm thick.
• Wall → 13 protofilaments in helix.
• Center: electron-lucent, appears hollow. (Exam Point)

📌 Composition
Tubulin protein (α and β forms).

📌 Dynamics
Exist in dynamic equilibrium with soluble tubulin reserve → rapid formation/disassembly.

📌 Functions (Exam Point)

1. Form mitotic spindle in dividing cells.
2. Major parts of centrioles, cilia, flagella in interphase cells.
3. In platelets → maintain shape.
4. In nerve cells → intracellular (axonal) transport.

🖇 Cell Membrane Specialization (Apical Cell Surface)

📌 General Concept
Epithelial cells have apical projections to increase surface area or enable motility.
Types: Microvilli, Stereocilia, Cilia

🟠 1. Microvilli

📌 Definition
Finger-like projections from apical surface.

📌 LM
Appear as apical striations (brush border). (Exam Point)

📌 EM

• Outer plasma membrane + core of actin filaments.
• Filaments continue with actin of cell cortex & insert into terminal web. (Exam Point)

📌 Function

• Non-motile processes.
• Greatly increase surface area → active absorption. (Exam Point)

📌 Sites (Exam Point)

• Intestine.
• Proximal convoluted tubules (kidney).

🟣 2. Stereocilia

📌 Definition
Very long microvilli (15–20 µm); not true cilia. (Exam Point)

📌 LM
Hair-like apical processes.

📌 EM
Core → actin filaments bound together.

📌 Sites (Exam Point)

• Inner ear hair cells.
• Epididymis.
• Vas deferens.

📌 Function
Aid absorption in male genital tract.

🟠 3. Cilia

📌 Definition
Motile, minute, hair-like projections on free surface.

📌 LM
Appear as apical acidophilic striations.

📌 EM (Axoneme Structure)

• Outer covering → plasma membrane extension.
• Inner core → 9+2 arrangement:
• • 9 peripheral doublet microtubules
  • 2 central singlet microtubules (Exam Point)
• Anchored in basal body (centriole-like).

📌 Functions (Exam Point)

• Move secretions in trachea/bronchi toward pharynx.
• Move ova in uterine tube & spermatozoa in male tract.
• Some non-motile cilia act as sensory receptors (bent by external stimuli).