L38 Cellular Aspects of Development

Page 1: Introduction to Patterns in Cell Contacts

• Are patterns encoded in cell-to-cell contacts?

  • Simple rules can govern complex structures.

  • Example: Cell division can maximize red-white contact while minimizing red-green contact.

  • Implies a strategy-based approach to cellular arrangement and positioning of cells.

Page 2: Cellular Development and Polarity

• Questioning Polarity

  • Exploration of how cellular polarity influences plant morphology.

  • Key questions:

    • What is polarity?

    • Role in development.

    • Initiation and maintenance of polarity.

Page 3: Understanding Polarity

• Definition and Importance

  • Polarity relates to having directional attributes at opposing ends.

  • Fundamental property observed both internally and externally.

  • Associated with pattern formation in living organisms.

  • Generally self-perpetuating nature in biological contexts.

Page 4: Origins and Determinants of Polarity

• Polar Development in Fucus

  • Ideal model organism: Fucus (Brown Alga) developmental process.

  • Germination transforms single unpolarized cell into polarized cells: rhizoid and thallus.

  • Factors that induce germination include:

    • Light (creates shade on rhizoid's dark side)

    • Heat (causes rhizoid formation towards warm side)

    • Osmotic gradient (rhizoid directed towards water)

    • pH and salt conditions (favoring alkaline conditions)

    • Fertilization (inducing rhizoid at the entry point)

Page 5: Timeline for Polar Development

• Key Development Events

  • t=0: Fertilization triggers calcium 'wave'.

  • t~1h: Calcium flux initiates at the future rhizoid end.

Page 6: Progression of Polar Development

• Sequence of Events

  • t~4h: Organizing actin to direct secretion towards future rhizoid.

  • t~6-10h: Establishment of polarity; visible rhizoid tip with fluorescently-tagged actin.

  • t~18-24h: Cell division via mitosis occurs.

Page 7: Mechanisms of Polarity

• Role of Actin

  • Actin plays a vital role in fixing the axis of polarity

  • function can be inhibited by toxins such as cytochalasin B.

  • Cytoplasmic 'threads' are influenced by actin structures.

Page 8: Calcium as a Regulatory Factor

• Role of mysosin

  • Actin interacts with myosin, facilitating directional force and aiding cellular motility.

  • Calcium ions (Ca2+) serve as crucial regulators of cellular polarity.

  • Electron microscopy reveals actin decorated with myosin ATPase.

Page 9: Actin-Myosin Relationship in Cellular Motility

• Muscle and Cell Functionality

  • In muscle tissue, the actin-myosin interaction drives contraction.

  • In single cells, actin and myosin drive cyclosis (cytoplasmic streaming)

  • Example: Cyclosis in tobacco with GFP-tagged Golgi (5x speed)

Page 10: Actin Dynamics

• Continuous State of Change

  • Actin exists in equilibrium between globular (G-) and filamentous (F-) forms.

  • Polymerization results in filamentous networks.

  • 'Treadmilling' of G- and F- actin generates force without myosin.

Page 11: Control of Polymerization and Movement

• Role of Ca2+ Gradients

  • Cells maintain gradients of [Ca2+] to support cellular organization.

  • Calcium ions (Ca2+) are essential in regulating polymerization.

  • Example: Time-lapse of pollen growth highlighting tip-directed [Ca2+] gradients.

Page 12: Coordination of Polarity

• Essentials for Polarity Initiation

  • Ca²⁺ dependent recruiting of actin

  • Polarity requires coordinated recruitment of actin and directed transport/secretion of cell materials.

Page 13: Polarity in Multicellular Plants

• Influence of Stem Cells

  • Root structure arises from stem cells (meristem) divisions that initiate cell files (cell lineage)

  • Example: Utilization of Arabidopsis enhancer-trap GFP lines to study polarity.

Page 14: Sustaining Polarity

• Maintaining polarity in multicellular organs requires coordinated cell division and growth.

  • Cell files are maintained by controlled (limited) cell divisions.

Page 15: Polarity Control in Plant Cells

• Auxin and Hormonal Gradients

  • Apical-basal polarity is maintained by auxin hormone gradients.

  • Auxin gradients result from directed transport between cells

  • Directed transport of auxin involves both influx and efflux carriers through cell membranes.

  • Auxin triggers changes in cellular behaviour and Ca2+ concentration.

Page 16: Effects of Suppressing Auxin Transport

• Pin Mutants

  • Disturbances in auxin transport via pin proteins negatively impact polarity, altering shoot form and rooting orientation.

  • pin proteins transport auxin out of cells (efflux carriers)

  • Pin proteins serve as efflux carriers, regulating auxin movement.

Page 17: Localization of Pin Proteins

• Distribution in Cells

  • Pin proteins are distributed basally.

  • PIN1 localization in plasma membranes exhibits a polar distribution.

  • Investigations using immunolocalization of PIN1 in Arabidopsis seedling root tips.

Page 18: Impacts of Disturbed Pin Distribution

• Mutations and Structural Changes

  • Mutational changes in Pin protein distribution lead to alterations in plant structure.

  • wt (wild type) is un-mutated, mp and gn are mutated causing impaired auxin transport and absent root formation

Page 19: Dynamic Nature of Pin Distribution

• Impact of Cell Structure Changes

  • Structural changes in cells affect Pin distribution dynamically.

  • Protoplast formation alters the Pin protein distribution pattern significantly.

Page 20: Summary of Key Concepts

• Polarity Development

  • Arises from signals, both external and internal.

  • plant tissue structures arises from cell lineages with defines cell fates originating from established cell polarity

  • cellular organisation and polarity in tissues is maintained by signals passed between cells