Developmental Biology 3 - Differentiation

The discussion is centered on how embryos develop from a single cell into complex organisms.
John Gurdon’s quote emphasizes that embryos, such as eggs, inherently know how to develop, transforming from one cell into an entire organism without external instructions.

Genome Encoding: All cells in a multicellular organism contain the entire genome, yet they express different sets of genes leading to diverse cell types.
Developmental Potential: The one-cell embryo has the potential to form all tissue types; however, as development proceeds, cells undergo lineage restrictions, reducing their developmental potential while increasing complexity.

Cells decide which genes to express based on their genome and signals from their surroundings.
Key mechanisms discussed include:
- Autonomous Specification: Cells know their fate beforehand (e.g., through asymmetric cell division).
- Inductive Signals: Cells influence each other to determine their fate through signaling.
- Differentiation: Final stages where cells acquire their specific functions (e.g., neurons vs. muscle cells).

Early Theories:
- Preformation: Organisms are pre-formed and merely grow larger.
- Epigenesis (Aristotle): Organs and tissues develop progressively from basic units, gaining complexity over time.
Important Figures:
- William Harvey: First to understand fertilization and development, supporting the theory of epigenesis.
- Edwin Conklin: Demonstrated localized determinants in embryos, discovering asymmetric cell division.

Concept: Differentiating cells inherit unequal distribution of cytoplasmic determinants (e.g., specific proteins and mRNA).
Key Examples:
- Determinants like macho-1 in sea urchins lead to muscle identity through asymmetric segregation of cytoplasm.
- C. elegans as a model organism where asymmetric cell division is prominent and studied in detail.
Par Proteins: These proteins are crucial for the correct localization of determinants in C. elegans embryos, facilitating differential gene expression.

Inductive Signals: Interaction between cells leads to the induction of cell types (e.g., mesoderm cells induced by signals from endoderm).
- Important experiments demonstrate this signal exchange during embryo development (e.g., work by Peter Koop on frog embryos).
The ability of a cell to respond to signal is known as competence.

Definition: Morphogens are signaling molecules that form gradients to regulate cell differentiation.
Example: Sonic hedgehog (Shh) mediates the patterning of the neural tube by different concentrations leading to varied neuronal identities.
- Lewis Wolpert’s French Flag model explains how cells interpret morphogen gradients and activate specific genes based on their position relative to the source of the morphogen.

Development revolves around the complex interplay of genetic, cellular, and environmental factors.
Upcoming discussions will focus on the molecular details regulating gene expression during development.