Foundations of Biology 2

Categorizing Cells Based on Origin, Growth Characteristics, and Capacity for Self-Renewal

  • Cell Types Classification:

    • Cells can be categorized based on their origin, proliferation, differentiation, lifespan, and self-renewal capacities.

  • Types of Cells:

    • Primary Cells:

      • Origin: Isolated from tissues or organs.

      • Proliferation: Finite (limited divisions).

      • Differentiation: Retain tissue-specific functions.

      • Lifespan: Limited, leading to senescence or apoptosis.

      • Characteristics: Retain natural functions but can divide a limited number of times.

    • Immortalized Cells:

      • Origin: Derived from primary cells with genetic modification.

      • Proliferation: Unlimited, can divide indefinitely.

      • Differentiation: Lose some tissue-specific functions.

      • Lifespan: Indefinite, capable of proliferating forever but may acquire altered phenotypes.

    • Stem Cells:

      • Origin: Include embryonic, adult, or induced pluripotent stem cells (iPSCs).

      • Proliferation: Unlimited due to self-renewal capabilities.

      • Differentiation: Can differentiate into multiple cell types.

      • Lifespan: Indefinite if undifferentiated.

      • Characteristics: Exhibit self-renewal and pluripotency.

Primary Cell Lines

  • History: The first primary human cell cultures were established in 1907 by Ross Granville Harrison, who cultured nerve cells from frog embryos in a lymph medium.

    • Significance: Demonstrated the survival and growth of animal cells outside the body, laying the groundwork for tissue culture.

  • Definition of Primary Cell Lines: Primary cell lines are directly derived from excised tissues and cultures, either as explant cultures or following dissociation into a single-cell suspension via enzyme digestion.

  • Characterization:

    • Primary cells have not been passaged; once passaged, they become a cell line and are no longer considered primary.

    • Initial cultures are heterogeneous but become dominated by fibroblasts over time.

    • During their limited lifespan, primary cells retain many differentiated characteristics of the original cells in vivo.

Challenges of Primary Cell Isolation and Culture

  • Labor-Intensive Preparation: The process of preparing primary cultures is complex and labor-intensive.

  • Limited Lifespan: Primary cells can be maintained in vitro only for a limited period.

  • Accessibility Challenges:

    • Issues relating to donor tissue supply.

    • Difficulty in the isolation/purification of cells.

    • Consistency and quality assurance problems.

    • Risks of contamination are high due to extensive tissue handling and multiple processing steps.

      • Donor tissues may harbor microbial contaminants (bacteria, fungi, mycoplasma, viruses).

  • Data Comparability Issues: Variability arises from different laboratories' reagents and procedures used for isolation and culture.

Introduction to Immortalization

  • Replicative Senescence:

    • Each cell division progressively shortens telomeres—repetitive DNA–protein structures at chromosome ends.

    • A critically short telomere is perceived as DNA damage, activating checkpoints that permanently arrest the cell cycle, halting further divisions.

    • Further, culture stress and lack of native microenvironment may lead to cell death.

  • HeLa Cells:

    • The first continuous (immortal) human cell line established in 1951 from cervical cancer tissue sourced from Henrietta Lacks.

      • Significance: First human cells capable of indefinite division in culture, facilitating numerous breakthroughs in biology and medicine.

  • Characteristics of Continuous Immortalized Cell Lines:

    • Comprised of a single cell type propagated serially, either for a limited number of divisions (~30) or indefinitely.

Molecular Basis of TERT Immortalization

  • TERT Function:

    • TERT stands for telomerase reverse transcriptase, which is the catalytic subunit of telomerase.

    • Definition: Telomerase is an enzyme that adds repetitive DNA sequences (in humans, “TTAGGG”) to chromosome ends, known as telomeres.

  • Functions of Telomeres:

    • Protect DNA during cell division.

    • Prevent chromosomes from adhering to one another.

    • Ensure accurate DNA copying.

  • Importance of Telomeres:

    • Each normal somatic cell division results in telomere shortening, eventually triggering senescence or apoptosis.

    • This phenomenon is referred to as the Hayflick limit, a natural barrier to unlimited cell division.

  • How TERT Expression Bypasses Senescence:

    1. Telomere Elongation: TERT adds new repeats to telomeres with each division cycle.

    2. Avoiding DNA Damage Response: Shortened telomeres are misidentified as DNA breaks.

    3. Continuous Proliferation: Cells can divide indefinitely without entering senescence.

Methods of Cellular Immortalization

  • Immortalization Definition: Cellular immortalization is the process by which normal cells evade senescence and gain indefinite proliferative potential.

  • Mechanisms of Immortalization:

    • Artificial Activation:

      • Involves engineered changes such as introducing hTERT to maintain telomeres or modifying cell cycle regulators like p53 or Rb to prevent growth arrest.

    • Endogenous Activation:

      • Pluripotency transcription factors (OCT4, SOX2, KLF4, c-MYC) can reactivate telomerase, remodel chromatin, and induce pluripotency gene expression, restoring a stem cell-like state.

    • Viral Oncogenes Activation:

      • Viral proteins from SV40, HPV, or adenoviruses can inactivate key tumor suppressors (p53 and Rb), thereby bypassing normal cell cycle checkpoints enabling immortal growth.

Comparison between Immortalized Primary Cells and Continuous Cell Lines

  • Immortalized Primary Cells:

    • Primary cell lines senesce after a finite number of divisions.

    • Maintaining immortalized cells requires establishing Master and Working banks for long-term viability.

  • Continuous Cell Lines:

    • These can generally be propagated indefinitely as they are transformed into tumor cells.

    • More abundant and available, but they exhibit reduced in vivo characteristics.

  • Comparison Table:

    Property

    Primary Cells

    Continuous Cell Lines


    Lifespan and Cell Proliferation

    Finite; limited cell doublings

    Infinite, if handled properly


    Consistency

    Variability between donors

    Minimal variability


    Genetic Integrity

    Retains in vivo tissue genetic makeup

    Subject to genetic drift


    Biological Relevance

    Mimics in vivo physiology closely

    May drift in relevance over time


    Ease of Use

    Requires optimized conditions

    Well-established protocols exist


    Time and Expense

    More time, less abundance of cells

    Less time, more abundance of cells

    • Implications of Immortalization:

    • Immortalizing cells bypasses only the barrier of telomere shortening; other checkpoints like DNA damage response and tumor suppressor pathways remain intact.

    • Immortalized normal cells eventually undergo senescence, while transformed tumor cells can divide indefinitely, having disabled senescence pathways.

    • Tumor cells may derive from clinical cancers or induced transformation via viral oncogenes or chemical methods.

Aspects of Tumor Cells' Immortalization

  • Why Normal Immortalized Cells Senesce:

    • The only barrier bypassed during immortalization is telomere shortening; other checkpoints remain active, leading to eventual senescence in immortalized normal cells.

  • Tumor Cells' Immortalization Needs:

    • Some proliferative tumor cells coexist with senescent cells or depend on environmental signals; typically, only subsets (cancer stem cells) renew effectively (maintaining telomere integrity).

    • Transformation occurs through inactivation of tumor suppressors or DNA repair pathways.

  • Crisis in Primary Tumor Cells:

    • Primary tumor cells tend to go into crisis in vitro due to higher oxidative stress, continued telomere shortening, DNA damage accumulation, and potential tumor suppressor pathway activation.

Stem Cells

  • Definition of Stem Cells: Unspecialized cells with exceptional self-renewal capability, capable of mitotic division to replenish various cell types throughout life.

  • Key Mechanisms: Stem cells propagate indefinitely by:

    • Maintaining telomere length.

    • Efficient DNA repair mechanisms.

    • Preserving stemness programs (undifferentiated state).

    • Resisting stress-induced senescence.

  • Differentiation Potential:

    • After division, each stem cell may remain a stem cell or differentiate into specialized cells (muscle, blood, neural).

    • Differentiation into organ-specific types can be induced under special conditions.

  • Types of Stem Cells:

    • Embryonic Stem Cells: Derived from embryos.

    • Somatic (Adult) Stem Cells: Undifferentiated cells residing in tissues/organs alongside differentiated cells.

    • Induced Pluripotent Stem Cells (iPSCs): Reprogrammed adult cells (e.g., skin or blood) to a pluripotent state.

Differences Between Stem Cells and Somatic Cells

  • Adult Stem Cells:

    • Present in minimal quantities, located in a specific area called the ‘stem cell niche’.

    • Inactive or non-dividing until activated by internal/external signals (e.g., tissue injury).

    • Can follow standard differentiation pathways to yield specialized cells within their tissue.

  • Examples of Stem Cell Differentiation:

    • Hematopoietic Stem Cells: Differentiate into all blood cells, including red blood cells, B and T lymphocytes, neutrophils, basophiles, eosinophils, monocytes, NK cells, and macrophages.

    • Neural Stem Cells: Found in the brain, differentiating into neurons, astrocytes, and oligodendrocytes.

    • Epithelial Stem Cells: Located in the GI tract lining, differentiating into absorptive, goblet, and enteroendocrine cells.

    • Skin Stem Cells: Comprise both epidermal stem cells (differentiating into keratinocytes) and follicular stem cells (differentiating into follicular cells and keratinocytes).

    • Human Mesenchymal Stem Cells (MSCs): Found in bone marrow; self-renewing multipotent stem cells giving rise to connective tissues (bone, cartilage, muscle).

  • Transdifferentiation: Phenomenon where adult stem cells from one tissue differentiate into specialized types of another tissue (e.g., brain stem cells producing blood cells).

Stem Cell-Based Therapies

  • Clinical Implications:

    • The ability of stem cells to differentiate into specialized cell types suggests potential for cell-based therapies to replace damaged somatic cells.

  • Applications:

    • Cardiac progenitor cells or iPSC-derived cardiomyocytes for myocardial tissue repair post-heart attacks.

    • Neural stem cells for replacing lost neurons/glial cells due to stroke and spinal cord injuries.

    • Retinal pigment epithelial cells from embryonic stem cells for macular degeneration treatments.

    • Pancreatic β-like cells from stem cells developed to restore insulin production in diabetes.

    • Mesenchymal stem cells (MSCs) investigated for their immunomodulatory roles in rheumatoid arthritis and other autoimmune disorders.

  • Case Study: The successful treatment of Ryder Baker, an 11-year-old boy with Fanconi anemia, exemplifies the clinical potential of stem cells.

Types of Stem Cells Classified by Developmental Potential

  1. Totipotent Stem Cells:

    • Definition: Can give rise to all cell types of an organism, including embryonic cells and extraembryonic tissues.

    • Examples: Zygote or early blastomeres post-fertilization.

    • Key Feature: Capable of forming an entire organism.

  2. Pluripotent Stem Cells:

    • Definition: Can differentiate into any cell of the three germ layers but cannot form extraembryonic tissues.

    • Examples: Embryonic stem cells from the inner cell mass of the blastocyst or iPSCs (adult cells reverted to pluripotency).

    • Key Feature: Can form any body cell except a whole organism.

  3. Multipotent Stem Cells:

    • Definition: Can differentiate into multiple cell types within a specific lineage or tissue.

    • Examples:

      • Hematopoietic stem cells → produce red blood cells, white blood cells, platelets.

      • Mesenchymal stem cells → give rise to bone, cartilage, and fat cells.

    • Key Feature: Limited to specific tissue types, not all body cells.

  4. Adult Stem Cells: Often multipotent, found in developed tissues, and not derived from embryos.

Germ Layers

  • Definition: Primary cell layers formed during early embryonic development leading to all tissues and organs.

  • Formation: Occurs during gastrulation post-blastula stage.

  • Types of Germ Layers:

    • Ectoderm:

      • Location: Outer layer.

      • Forms: Skin (epidermis), hair, nails, nervous system, eye lens and cornea, enamel of teeth, various glands.

    • Mesoderm:

      • Location: Middle layer.

      • Forms: Muscles, bones, cartilage, connective tissue, heart, blood vessels, blood cells, kidneys, gonads, lymphatic system.

    • Endoderm:

      • Location: Inner layer.

      • Forms: Lining of the gut and respiratory tract, liver, pancreas, thyroid, parathyroid, lungs, bladder, epithelial organs.