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Here is a structured summary of Cytology Lecture 6 – Cell Signaling, Tissue Organization & Cell Turnover (34 slides, Geomedi 2026, Dr. Giorgi Berishvili):
Part I: Cell Signaling
Why cells signal
Cells must communicate to survive, grow, and function - both as single-celled organisms and as part of multicellular tissues. Signaling involves three steps: transmission (signal molecule released), reception (receptor on target cell binds it), and interpretation (intracellular response alters cell behavior).
Types of signaling
| Type | Mechanism |
|---|---|
| Endocrine | Hormones travel via bloodstream (systemic) |
| Paracrine/Autocrine | Local mediators act on nearby/same cells |
| Neurotransmission | Neurotransmitters at synapses |
| Direct contact | Cell surface signaling molecules |
Receptors
- Cell surface receptors - for signals that cannot cross the membrane
- Intracellular receptors - for lipophilic molecules (e.g., steroids) that enter the cell; act as transcription factors in the nucleus
Three classes of cell surface receptors
- Ion channel-coupled - change membrane permeability to ions
- G-protein-coupled receptors (GPCRs) - activate trimeric G proteins
- Enzyme-coupled receptors - act as or associate with enzymes
Intracellular signaling pathways
Signal cascades relay, amplify, or suppress signals through positive/negative feedback loops. Multiple pathways interact to produce combined responses.
Part II: Tissue Organization
Basic tissues
Cells organize into 4 basic tissue types: Nervous, Muscular, Epithelial, and Connective. Cells are held together by cell-cell adhesions and the extracellular matrix (ECM).
Extracellular Matrix (ECM)
- A network of proteins and polysaccharides filling the space between cells
- Functions: structural support, cell adhesion (via integrins), cell migration, growth factor binding, barrier function, wound healing
- Abnormal ECM remodeling can lead to fibrosis or cancer metastasis
Key ECM components
- Collagen - triple helix synthesized by fibroblasts; provides tensile strength; cells attach via integrin-fibronectin linkage
- Proteoglycans - protein core + glycosaminoglycans (GAGs) like hyaluronan; attract water, form a hydrated gel, resist compression; bones are hardened by calcium phosphate impregnation
- Basal lamina (basement membrane) - 50-200 nm sheet of type IV collagen + laminin; underlies epithelia, surrounds nerves and muscle; thickening of glomerular basement membrane can cause kidney failure
Cell junctions
Tissue-specific junctions vary in strength:
| Tissue | Dominant Junctions |
|---|---|
| Epithelial | Tight, adherens, desmosomes, gap junctions |
| Muscle | Desmosomes, gap junctions, adherens |
| Nervous | Synapses, gap junctions, CAMs |
| Connective | ECM-integrin interactions |
| Immune | Selectins, integrins (variable/transient) |
Part III: Cell Turnover
Cell turnover overview
Continuous process of cell renewal and replacement through proliferation, differentiation, and death to maintain tissue homeostasis.
Cell death: Necrosis vs. Apoptosis
| Necrosis | Apoptosis | |
|---|---|---|
| Cause | Injury | Programmed (PCD) |
| Process | Swelling and bursting | Condensation and shrinkage |
| Effect on neighbors | Harmful (releases contents) | Safe (phagocytes engulf fragments) |
| Content fate | Lost | Recycled |
~5 × 10¹¹ blood cells are cleared by apoptosis daily in humans; ~half of neurons are eliminated by PCD during development.
Apoptosis mechanism - Caspases
- Driven by a cascade of proteolytic reactions involving caspases (cysteine-containing aspartate proteases)
- Caspases cleave hundreds of target proteins, activating some and inactivating others
Intrinsic (mitochondrial) pathway
DNA damage or oxidative stress → cytochrome c release from mitochondria → apoptosome formation → procaspases activated → protein cleavage + DNA fragmentation
Bcl-2 family regulates intrinsic pathway - balance between pro- and anti-apoptotic members determines cell fate.
Extrinsic pathway
Fas receptor on cell surface activated by Fas ligand on immune cells; also induced by TGF-β and TNF; can amplify through the intrinsic pathway.
Survival signals
- Survival factors - suppress apoptosis
- Mitogens - stimulate cell division (proliferation)
- Growth factors - stimulate cell growth (increase in mass)
- Survival is NOT the default state - it must be continuously signaled.
Tissue renewal rates
| Tissue | Renewal Time |
|---|---|
| Gut epithelium | 3-6 days |
| Epidermis | ~2 months |
| Red blood cells | 120 days |
| Bone | ~10 years |
| Neurons | Lifetime (most) |
Stem cells
- Terminally differentiated cells are replenished by progenitor cells, which arise from stem cells via asymmetric division (one SC + one progenitor)
- Intestinal epithelium - stem cells at crypt base → progenitors in crypt → differentiated cells on villi → shed into lumen
- Hematopoietic SCs - multipotent; reside in bone marrow niche; at least 20 colony-stimulating factors/hematopoietic growth factors drive differentiation
- Embryonic stem cells (ESCs) - pluripotent; can become any cell type with appropriate growth factor cocktails
- Induced pluripotent stem cells (iPSCs) - adult mature cells reprogrammed to a pluripotent ES-like state by activating transcription regulators; key to regenerative medicine via cell-replacement therapy
Key exam points to remember:
- Three steps of cell signaling: transmission, reception, interpretation
- Three classes of cell surface receptors (ion channel, GPCR, enzyme-coupled)
- ECM components: collagen, proteoglycans, basal lamina
- Apoptosis vs. necrosis differences
- Intrinsic (mitochondrial) vs. extrinsic apoptotic pathways; role of Bcl-2
- Caspases as the executioners of apoptosis
- Stem cell hierarchy: SC → progenitor → differentiated cell
- iPSCs and their therapeutic potential
Give me MCQ type questions from this pdf
Here are 30 MCQ-style exam questions based on Cytology Lecture 6:
Part I: Cell Signaling
1. Which of the following correctly describes the sequence of cell signaling?
- A) Reception → Transmission → Interpretation
- B) Transmission → Interpretation → Reception
- C) Transmission → Reception → Interpretation ✓
- D) Interpretation → Reception → Transmission
2. A hormone is released from the pancreas and travels through the bloodstream to affect liver cells. This is an example of:
- A) Paracrine signaling
- B) Autocrine signaling
- C) Neurotransmission
- D) Endocrine signaling ✓
3. A cell releases a signal molecule that acts back on itself. This is called:
- A) Endocrine signaling
- B) Autocrine signaling ✓
- C) Paracrine signaling
- D) Direct contact signaling
4. Which type of receptor changes membrane permeability to ions upon activation?
- A) G-protein-coupled receptor
- B) Enzyme-coupled receptor
- C) Ion channel-coupled receptor ✓
- D) Intracellular receptor
5. A lipophilic signaling molecule crosses the plasma membrane and binds to a receptor that directly activates gene transcription. This receptor is best classified as:
- A) GPCR
- B) Ion channel-coupled receptor
- C) Enzyme-coupled receptor
- D) Nuclear (intracellular) receptor ✓
6. G-protein-coupled receptors activate which of the following?
- A) Ion channels directly
- B) Trimeric GTP-binding proteins (G proteins) ✓
- C) Nuclear transcription factors
- D) Caspases
7. Intracellular signaling pathways that relay and modulate extracellular signals often involve:
- A) Direct membrane-to-membrane contact only
- B) Only positive feedback mechanisms
- C) Positive and negative feedback mechanisms ✓
- D) Signal degradation without amplification
8. Which statement about intracellular signaling is TRUE?
- A) Each signal follows a single, independent pathway
- B) Pathways never interact with each other
- C) Various pathways interact to amplify, suppress, or distribute signals ✓
- D) Only extracellular signals can interact
Part II: Tissue Organization
9. Which are the four basic tissue types in multicellular organisms?
- A) Epithelial, Connective, Vascular, Nervous
- B) Nervous, Muscular, Epithelial, Connective ✓
- C) Muscular, Lymphoid, Epithelial, Bone
- D) Connective, Glandular, Nervous, Muscular
10. The extracellular matrix (ECM) anchors cells via which molecules?
- A) Cadherins
- B) Gap junctions
- C) Integrins ✓
- D) Tight junction proteins
11. Abnormal ECM remodeling is associated with which conditions?
- A) Apoptosis and necrosis
- B) Stem cell differentiation failure
- C) Fibrosis and cancer metastasis ✓
- D) Ion channel dysfunction
12. Collagen triple helix is synthesized by which cells?
- A) Osteoclasts
- B) Epithelial cells
- C) Fibroblasts ✓
- D) Macrophages
13. Proteoglycans resist compression because they:
- A) Form covalent cross-links with collagen
- B) Activate integrin signaling
- C) Attract water and form a hydrated gel ✓
- D) Harden bone by calcium phosphate deposition
14. The glycosaminoglycan (GAG) that is a key component of proteoglycans is:
- A) Fibronectin
- B) Laminin
- C) Collagen type IV
- D) Hyaluronan ✓
15. The basal lamina (basement membrane) is composed of:
- A) Collagen type I and fibronectin
- B) Elastin and laminin
- C) Collagen type IV and laminin ✓
- D) Proteoglycans and fibrillin
16. Abnormal thickening of basement membranes surrounding the glomeruli leads to:
- A) Liver cirrhosis
- B) Myocardial infarction
- C) Kidney failure ✓
- D) Neurodegeneration
17. Which tissue type has the lowest adhesion strength and relies mainly on ECM-integrin interactions?
- A) Epithelial
- B) Muscle
- C) Nervous
- D) Connective ✓
18. Which cell junctions are found in epithelial tissue but NOT listed as dominant in connective tissue?
- A) Gap junctions
- B) ECM-integrin interactions
- C) Tight junctions and desmosomes ✓
- D) Synapses
Part III: Cell Turnover
19. Which form of cell death is considered harmful to surrounding cells because it releases cellular contents?
- A) Apoptosis
- B) Autophagy
- C) Necrosis ✓
- D) Anoikis
20. Approximately how many blood cells are cleared daily in humans by apoptosis?
- A) 5 × 10⁸
- B) 5 × 10⁹
- C) 5 × 10¹⁰
- D) 5 × 10¹¹ ✓
21. The proteases that drive apoptosis by cleaving hundreds of target proteins are:
- A) Matrix metalloproteinases
- B) Serine proteases
- C) Caspases ✓
- D) Ubiquitin ligases
22. In the intrinsic apoptotic pathway, what is released from mitochondria to trigger caspase activation?
- A) Bcl-2
- B) TNF
- C) Fas ligand
- D) Cytochrome c ✓
23. The complex in which procaspases are activated during intrinsic apoptosis is called the:
- A) Proteasome
- B) Inflammasome
- C) Apoptosome ✓
- D) Spliceosome
24. Cell fate in the intrinsic apoptotic pathway is determined by the balance between pro- and anti-apoptotic members of which protein family?
- A) Caspase family
- B) Bcl-2 family ✓
- C) TNF receptor family
- D) Integrin family
25. Which of the following factors does NOT induce apoptosis via the extrinsic pathway?
- A) Fas ligand
- B) TGF-β
- C) TNF
- D) Cytochrome c ✓
(Cytochrome c is part of the intrinsic pathway)
26. Which of the following correctly describes survival signals?
- A) Survival factors stimulate cell division; mitogens suppress apoptosis
- B) Growth factors stimulate cell division; mitogens stimulate cell growth
- C) Survival factors suppress apoptosis; mitogens stimulate cell division ✓
- D) Growth factors eliminate damaged cells
27. Which tissue has the fastest renewal rate?
- A) Epidermis (~2 months)
- B) Red blood cells (120 days)
- C) Gut epithelial cells (3-6 days) ✓
- D) Bone (~10 years)
28. In intestinal epithelium, where are stem cells located?
- A) On top of the villi
- B) In the middle of the crypt
- C) On the surface facing the gut lumen
- D) At the bottom of the crypt ✓
29. Hematopoietic stem cells are described as multipotent because they:
- A) Can give rise to any cell type in the body
- B) Can only produce red blood cells
- C) Can give rise to a large but limited range of differentiated blood cell types ✓
- D) Divide symmetrically to produce identical daughter cells
30. Induced pluripotent stem cells (iPSCs) differ from embryonic stem cells (ESCs) in that:
- A) iPSCs cannot differentiate in vitro
- B) iPSCs are less useful for regenerative medicine
- C) ESCs are derived from reprogrammed adult cells
- D) iPSCs are derived from reprogrammed mature adult cells ✓
Quick scoring guide: 27-30 correct = Excellent | 22-26 = Good | 18-21 = Review needed | <18 = Revisit the lecture
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