Module D: Developmental Biology - Lecture HS1: Patterning and Cytoplasmic Determinantsk

Overview of Developmental Biology

Definition of Developmental Biology: This field is the study of how a fertilized egg (a single cell) divides and generates a complete animal composed of millions of cells.
Model Organisms in Research: Scientists typically study development using various model organisms, including: - $Xenopus$ (Frog): Typical size approximately $1\,mm$ . - Chick: Typical size approximately $1\,mm$ . - Mouse: Typical size approximately $1\,mm$ . - Zebrafish: Typical size approximately $10\,mm$ . - Evolutionary Context: Historically cited by Karl Ernst von Baer in 1828.
Crucial Inquiries in the Field: - How do cells initially become different from each other? - How is the body plan established? - What controls changes in cell type? - How is growth and differentiation maintained? - How are genes differentially regulated? - How do genes direct the developmental programme?

Course Structure for Module D: Developmental Biology

Lecture 1: Patterning of multicellular organisms – cytoplasmic determinants ( $C.\,elegans$ and $drosophila$ ).
Lecture 2: Patterning of multicellular organisms – positional information (mouse and human).
Lecture 3: Establishing the body plan – Gastrulation (frog and sea urchin).
Lecture 4: Epithelial-mesenchymal transitions.
Lecture 5: Stem cells.

Key Learning Objectives for Patterning and Cytoplasmic Determinants

Process Variation: Comprehend that different organisms utilize quite different processes to generate initial cell diversity during early cleavage.
Organismal Patterning: Describe how early embryos of the nematode ( $Caenorhabditis\,elegans$ ) and the fruitfly ( $Drosophila\,melanogaster$ ) are patterned.
Asymmetric Cell Division: Illustrate the importance of the orientation of the spindle axis and the distribution of cytoplasmic components in asymmetric cell division.
Cell Fate Determination: Identify important cell-cell interactions that help to determine cell fate during development.

Caenorhabditis elegans as a Discovery Tool

Biological Advantages: - Complete Development: The entire cell lineage is known. - Small Genome: Consists of approximately $100 \times 10^6\,bp$ (in contrast to the human genome of $3 \times 10^9\,bp$ ). - Genetic Conservation: Up to $80\%$ of genes are similar to humans. - System Rudiments: Contains the rudiments of all major biological systems. - Efficiency: Features a fast development cycle and is inexpensive to maintain.
Early Developmental Stages and Timing: - $P_0$ (Zygote): The initial single cell containing pro-nuclei. - First Division: Division into the anterior $AB$ cell and the posterior $P_1$ cell. - Second Division: The $AB$ cell divides into $ABa$ (anterior) and $ABp$ (posterior). The $P_1$ cell divides into $EMS$ and $P_2$ . - 28-cell stage: Reached at approximately $1.5\,hours$ .

Cleavage Divisions and Maternal Contributions

Cleavage Definition: This process involves the egg cytoplasm being packaged into smaller and smaller cells known as blastomeres.
Genetic Independence: Early cleavage divisions do not depend on new gene activity from the zygotic DNA.
Maternal Products: Maternal mRNA and proteins are deposited in the egg and utilized for early development.
Zygotic Activation: The timing for the activation of zygotic DNA varies significantly between different organisms.
Cell Growth vs. Cleavage: - Normal Cell Division: Includes cell growth between divisions to maintain constant cell size. - Cleavage Division: Cell division occurs without cell growth, resulting in a progressive reduction in cell size.

Cell Lineage and Fate in C. elegans

Lineage Definition: The progressive determination of cells with a subsequent restriction in developmental potential and differentiation into specialized cell types.
Specific Lineage Paths: - $AB$ Lineage: Gives rise to Neurons and Hypodermis. - $MS$ Lineage: Gives rise to Body muscle and Pharyngeal muscle. - $E$ Lineage: Gives rise to the Gut. - $C$ Lineage: Gives rise to Body muscle, Hypodermis, and Neurons. - $D$ Lineage: Gives rise to Body muscle. - $P_4$ Lineage: Gives rise to Germ cells.
Organizational Dependency: The organization of the nematode body depends on the specific position of early blastomeres. For example, reversing the positions of $ABa$ blastomeres results in reversed symmetry.

Mechanics of Asymmetric Cell Division and PAR Proteins

Cytoplasmic Determinants: Cells become different by inheriting different cytoplasmic determinants through asymmetric cell division.
PAR (Partitioning) Proteins: These proteins localize around the plasma membrane after fertilization. - Function: They organize the distribution of other molecules in the cytoplasm and the orientation of the mitotic spindle. - Types: $PAR-3$ , $PAR-6$ , $PAR-1$ , and $PAR-2$ .
Centrosome Orientation: - In normal development, $PAR-3$ and $PAR-6$ are localized to the anterior, while $PAR-2$ and $PAR-5$ are localized to the posterior. - Fact: $PAR-3$ inhibits centrosome rotation. - Wild-type: Centrosome rotation occurs in the posterior $P_1$ cell, but is inhibited by $PAR-3$ in the anterior $AB$ cell. - Mutant Scenarios: - No $PAR-3$ : Rotation occurs in both cells. - Over-expression of $PAR-3$ : Rotation is inhibited in both cells.

Cell-Cell Interactions in C. elegans Development

Induction Mechanisms: Differentiation can be induced through specific molecular signals. - $P_2$ Signal: The $P_2$ cell produces a signal ( $Apx-1$ ). - $ABp$ Interaction: The $ABp$ cell receives the $Apx-1$ signal via its receptor, inducing differentiation. - $ABa$ Interaction: Although $ABa$ has the same receptor, it does not receive the $Apx-1$ signal because it is not in physical contact with the $P_2$ cell.
Signaling Molecules involved: - $Apx-1$ : The signal ligand from $P_2$ . - $Glp-1$ : The receptor on $AB$ cells. - $Pop-1$ : Features a gradient in the $EMS$ cell descendants ( $E$ and $MS$ ) resulting from asymmetric division.

Summary of C. elegans Patterning Principles

Polarized Distribution: Polarized distribution of cytoplasmic determinants in the fertilized egg results in asymmetric cell division.
Interaction Potential: "Different" cells created by asymmetric division can now interact with each other.
Further Differentiation: These cell interactions result in further cell differentiation.
Polarity Feedback: Cell interactions can set up a polarized distribution of molecules in receptive cells, leading to further asymmetric cell division in a recursive process.

Drosophila melanogaster Development and the Syncytium

Syncytial Growth: Unlike $C.\,elegans$ , $Drosophila$ undergoes nuclear division without cytokinesis during early development. - Karyokinesis: Nuclear division. - Cytokinesis: Division of the cytoplasm. - Definition: A syncytium consists of many nuclei within a common cytoplasm.
Timing and Cellularization: - Stage 1 (0 min): Fertilized egg. - Stages 10-13: Nuclei migrate to the egg surface. - Cellularization (approx. 2 hours): Cell membranes form around each nucleus, facilitated by microtubules and actin filaments.

Antero-Posterior Patterning in Drosophila

mRNA Gradients: Differential localization of maternal mRNAs at each end of the egg sets up the pattern.
Bicoid Protein Gradient: - Bicoid ( $bcd$ ) is localized to the anterior. - Post-translation, Bicoid protein forms a gradient through the syncytium. - Zygotic Activation: Bicoid acts as a transcription factor to regulate the expression of the zygotic Hunchback ( $hb$ ) gene.
Binding Affinities: - The $hunchback$ promoter contains both high-affinity and low-affinity Bicoid-binding sites. - This results in a sharp Hunchback protein expression pattern in the anterior of the embryo.
Segmental Body Plan: Progressive regional regulation of patterns of gene expression organizes the final segmental body plan of the fly.