Learn & Review: CSIR Recall Express 3.0 | Developmental Biology | Unit 5 | Jyoti Kumari | CSIR

Summary of the Lecture on Developmental Biology

This lecture provides a comprehensive overview of key concepts in developmental biology, covering stem cells, cell commitment, induction, morphogenesis, and regeneration.

1. Stem Cells

• Definition: Stem cells are characterized by their high self-renewal property (ability to divide and produce identical cells) and their potential to differentiate into specialized cell types.
• Types of Stem Cells based on Potential:
  • Totipotent: Can form a complete embryo and extraembryonic tissues (e.g., zygote, early blastomeres).
  • Pluripotent: Can form all cell types of the embryo but not extraembryonic tissues (e.g., inner cell mass (ICM)).
  • Multipotent: Can form a limited subset of cell types within a specific lineage (e.g., hematopoietic stem cells, mesenchymal stem cells).
  • Unipotent: Can only form one specific cell type (e.g., spermatogonial stem cells forming sperm).
• Stem Cell Niche: The microenvironment that maintains stem cell properties, providing signals that prevent differentiation. Examples include the "hub" in testes and "germaria" in ovaries.
• Regulation of Stem Cells:
  • Extracellular: Physical mechanisms (extracellular matrix as adhesion/structure factors) and chemical signals (endocrine, paracrine).
  • Intracellular: Cytoplasmic determinants (unequal distribution of molecules during division) and transcription factors (e.g., OSKM factors maintaining pluripotency). Epigenetic regulation (histone modification) also plays a role.
• Induced Pluripotent Stem Cells (iPSCs): Somatic cells reprogrammed to a pluripotent state by expressing specific transcription factors (e.g., OSKM).

2. Interaction, Morphogenesis, and Specification

• Inductive Interaction: Involves an inducer (signaling cell/tissue) and a responder (receiving cell/tissue). The responder must have competence (ability to respond to the signal).
• Stages of Commitment:
  • Specification: Cell fate is specified but can be reversed. Can be conditional (depends on cell-cell interactions) or autonomous (depends on intrinsic cytoplasmic determinants).
  • Determination: Cell fate is fixed and irreversible.
• Types of Development:
  • Regulative Development (Conditional Specification): Cell fate depends on interactions; fate can change; potential is greater than prospective fate; compensation for cell loss is possible (seen in vertebrates).
  • Mosaic Development (Autonomous Specification): Cell fate depends on intrinsic determinants; fate is fixed; no compensation for cell loss (seen in C. elegans).
• Types of Interactions:
  • Instructive Interaction: Requires a specific instruction for development.
  • Permissive Interaction: Requires a suitable environment for a pre-determined fate.
• Epithelial-Mesenchymal Interaction:
  • Induction: Mesenchyme often induces epithelial tissues.
  • Region-Specific Induction: The origin of the mesenchyme determines the fate of the epithelium (e.g., limb mesenchyme inducing limb structures).
  • Gene-Specific Induction: The genetic information within the tissue determines its fate, regardless of the surrounding environment (e.g., frog gastrula transplanted into newt, forming frog mouth).
• Morphogens: Diffusible biochemical molecules that determine cell fate based on their concentration gradient. They act as sources and receivers. Examples include FGF, Wnt, BMP, Shh, Retinoic Acid, etc.
• C. elegans Development:
  • Sex Determination: Hermaphrodites (XX) can self-fertilize or cross-fertilize.
  • Cleavage: Holoblastic, asymmetric cleavage producing founder cells and stem cells.
  • Vulva Development: Involves inductive signaling (e.g., LIN-3 morphogen from anchor cell acting on LET-23 receptor in vulval precursor cells) and lateral inhibition (LIN-12/Notch signaling) to specify cell fates.
  • Axis Formation: Determined by sperm entry point (ventral) and P granules (posterior).
• Eye Field Development (Induction):
  • Optic Vesicle (Inducer): Secretes BMP signaling molecules.
  • Head Ectoderm (Responder): Competent due to Pax6 expression, influenced by Otx2 and Rx1.
  • Signaling Pathways: BMP signaling activates Gβ signaling, leading to Sox activation. FGF8 signaling activates MAPK pathway. These pathways lead to thickening of the ectoderm to form the optic cup and lens.
  • Mutations: Pax6 mutations lead to eye loss or reduction. Sonic hedgehog (Shh) mutations can cause cyclopia.

3. Limb Development

• Formation: Limb buds form from lateral plate mesoderm.
• Domains:
  • Apical Ectodermal Ridge (AER): Secretes FGFs, essential for distal outgrowth.
  • Zone of Polarizing Activity (ZPA): Located at the posterior edge, secretes Shh, determines anterior-posterior axis.
  • Progress Zone (PZ): Mesenchyme under the AER, involved in distal development.
• Genes Involved:
  • Tbx genes: Tbx5 for forelimb, Tbx4 for hindlimb.
  • Hox genes: Specify identity along the proximal-distal axis (e.g., HoxA/D genes).
  • Wnt signaling: Involved in limb initiation and patterning.
• Patterning:
  • Anterior-Posterior Axis: Determined by Shh gradient from the ZPA.
  • Proximal-Distal Axis: Determined by the duration of FGF signaling from the AER.
  • Dorsal-Ventral Axis: Determined by Wnt7a (dorsal) and BMP signaling (ventral).
• Mutation Cases:
  • Tbx5 mutations lead to Holt-Oram syndrome (forelimb and heart defects).
  • Shh mutations can cause syndactyly (fused digits).
  • Wnt signaling defects can disrupt limb formation.
• Limb Segmentation: Proximal-distal axis is patterned by Hox genes.
  • Stylopod (e.g., humerus, femur): Associated with HoxA/D 9-10.
  • Zygopod (e.g., radius/ulna, tibia/fibula): Associated with HoxA/D 11.
  • Autopod (e.g., wrist/fingers, ankle/toes): Associated with HoxA/D 12-13.
• Signaling Gradients: FGFs (distal), Shh (posterior), Wnts (dorsal), BMPs (ventral).

4. Regeneration

• Definition: The process of regrowing lost or damaged body parts. Varies greatly across species.
• Mechanisms:
  • Stem Cell-Mediated Regeneration: Involves resident stem cells that proliferate and differentiate (e.g., planarian neoblasts, hair follicle regeneration).
  • Epimorphosis: Dedifferentiation of cells at the wound site to form a blastema, followed by redifferentiation and growth (e.g., amphibian limb regeneration). Requires nerve supply.
  • Compensatory Regeneration: Hypertrophy (enlargement) of remaining tissue to compensate for loss (e.g., mammalian liver regeneration).
  • Morphallaxis: Remodeling of existing tissue to restore form, without significant cell proliferation (e.g., Hydra).
• Hydra Regeneration: Highly regenerative, capable of regenerating entire body from small fragments. Involves head-inducing factors (hypostome, Wnt signaling) and foot-inducing factors (basal disk, BMP signaling).
• Planarian Regeneration: Possesses neoblasts (pluripotent stem cells) capable of regenerating the entire organism from small pieces. Axis formation is regulated by morphogen gradients (e.g., Wnt for posterior, BMP for anterior).

5. Fertilization and Early Development (Sea Urchin)

• External Fertilization: Occurs in the ocean.
• Sperm-Egg Interaction:
  • Chemotaxis: Sperm are attracted to the egg by species-specific molecules.
  • Binding: Sperm binds to the egg's extracellular matrix (jelly layer, vitelline membrane).
  • Acrosome Reaction: Release of enzymes (e.g., acrosin) from the acrosome to digest the egg's outer layers. Formation of the acrosomal process.
  • Fusion: Sperm fuses with the egg plasma membrane.
• Polyspermy Prevention:
  • Fast Block: Electrical block due to membrane depolarization (influx of Na+ ions). Transient.
  • Slow Block: Mechanical block involving the cortical reaction (release of cortical granule contents) which modifies the vitelline membrane, creating the fertilization envelope and inactivating sperm receptors.
• Cortical Reaction: Mediated by calcium release and activation of PLCζ and Na+/H+ exchange.
• Gray Crescent Formation: Occurs after sperm entry and cortical rotation, marking the future dorsal side.
• Cleavage: Holoblastic, radial cleavage. Animal pole cells (micromeres) are smaller and divide faster; vegetal pole cells (macromeres) are larger.
• Mid-Blastula Transition (MBT): Transition from maternal gene expression to zygotic gene activation. Cells gain motility and gastrulation begins.
• Gastrulation: Formation of three germ layers (ectoderm, mesoderm, endoderm) through cell movements like invagination, involution, ingression, delamination, and epiboly.
  • Ectoderm: Forms from animal pole cells.
  • Endoderm: Forms from vegetal pole cells (influenced by VegT mRNA).
  • Mesoderm: Forms from the marginal zone (influenced by signals from vegetal pole cells).

6. Sex Determination in Mammals and Drosophila

• Mammals:
  • Default Pathway: Female development (XX).
  • Male Determination: Presence of the Y chromosome and the SRY gene initiates testis development. SRY activates SOX9, which drives testis formation. Sertoli cells produce anti-Müllerian hormone (AMH), and Leydig cells produce testosterone.
  • Female Determination: Absence of SRY allows ovary development. Wnt4 and R-spondin signaling pathways are involved.
• Drosophila:
  • Sex Determination: Ratio of X chromosomes to autosomes (X:A ratio).
  • Ratio 1: Female development (X:A = 1). Sex lethal (Sxl) gene is activated, leading to female-specific splicing of downstream genes (e.g., transformer (tra)) and development of female structures.
  • Ratio 0.5: Male development (X:A = 0.5). Sxl is off, leading to male-specific splicing and development of male structures.
  • Intersex: Intermediate X:A ratios result in intersex phenotypes.

7. Teratogenesis

• Definition: The process by which congenital defects are induced by teratogens (environmental agents).
• Critical Period: Most sensitive during organogenesis (weeks 3-8 of gestation).
• Types of Teratogens: Chemical (alcohol, thalidomide, lead), infectious agents (rubella, Zika virus), radiation, maternal metabolic conditions (diabetes), endocrine disruptors (BPA).
• Endocrine Disruptors: Interfere with hormone signaling, potentially causing effects later in life.

8. Cleavage Patterns

• Holoblastic Cleavage: Complete cleavage of the entire egg. Occurs in eggs with little or moderate yolk.
  • Radial: Cells divide perpendicular to each other (e.g., sea urchins, amphibians).
  • Spiral: Cells divide at an angle (e.g., mollusks, annelids).
  • Bilateral: Cleavage plane is bilateral (e.g., tunicates).
  • Rotational: First cleavage is vertical, second is horizontal (e.g., mammals).
• Meroblastic Cleavage: Incomplete cleavage. Occurs in eggs with large amounts of yolk.
  • Discoidal: Cleavage occurs only in a small disc of cytoplasm at the animal pole (e.g., birds, reptiles, fish).
  • Superficial: Cleavage occurs only in the cytoplasm surrounding the yolk, not the yolk itself (e.g., insects).

9. Gene Hierarchy in Drosophila Development

• Maternal Genes: Establish the initial polarity and gradients (e.g., bicoid, nanos).
• Gap Genes: Define broad regions of the embryo (e.g., hunchback, kruppel).
• Pair-Rule Genes: Define patterns within segments (e.g., even-skipped, fushi tarazu).
• Segment Polarity Genes: Define anterior-posterior polarity within each segment (e.g., wingless, hedgehog).
• Homeotic Genes (Hox Genes): Specify the identity of each segment (e.g., Antennapedia complex, Bithorax complex).

10. Axis Formation in Drosophila

• Anterior-Posterior Axis: Determined by maternal gradients of bicoid (anterior) and nanos (posterior).
• Dorsal-Ventral Axis: Determined by dorsal (dorsal) and cactus (ventral) proteins, regulated by Toll signaling pathway.
• Segment Polarity: Established by signaling between adjacent cells (e.g., wingless/hedgehog signaling).

11. Morphogenesis and Patterning

• Morphogenesis: The biological process that causes an organism to develop its shape.
• Patterning: The process by which cells acquire specific fates and positions within the developing organism.
• Signaling Pathways: Wnt, BMP, FGF, Hedgehog, Notch signaling pathways play crucial roles in cell-cell communication and fate determination.

This summary provides a structured overview of the lecture's content, highlighting key concepts, examples, and mechanisms discussed.