Stem Cells and Cell Differentiation

Introduction

• All specialized cells (muscle, nerve, skin, red blood cells, etc.) originate from unspecialized stem cells.
• Stem cells are pluripotent, meaning they can differentiate into any somatic cell type.

The Role of DNA

• Each cell contains DNA in its nucleus (except red blood cells which lack nuclei).
• DNA acts as a library containing all genetic instructions.
• Genes = books in this library; they provide specific instructions to make proteins.
• Proteins determine the cell's appearance and function through gene expression.
• Gene expression: turning on/off specific genes.

Differentiation Process

1. Gene Expression: Specialized cells express specific sets of genes while turning off others.

   • Muscle cells express muscle-specific genes, developing contractile proteins, and a specific shape.
   • Neurons express neuron-specific genes, developing dendrites and elongated shapes.

2. **Pluripotency Limitations: **

   • Once a stem cell differentiates into a specialized cell, it cannot revert to a stem cell or change into another cell type.

Determining Gene Expression

• Cells decide what genes to express based on internal and external cues.

Mechanisms of Differentiation

Asymmetric Segregation of Cellular Determinants

• Transcription Factors: Proteins that activate certain genes.
  • Initially present in the zygote's cytoplasm and lead to gene expression changes in daughter cells.
  • Transcription factors are unevenly distributed after cell division, causing different daughter cells to have different determinants.

Inductive Signaling (Induction)

• One group of cells induces another group to differentiate using signals.
• Methods of Signal Transmission:
  • Diffusion: Signals released from one group diffuse to another, binding to receptors.
  • Direct Contact: Surface proteins on cells contact each other.
  • Gap Junctions: Connected cells send signals through connexons.
• Induction plays a key role in forming various body parts (e.g., limbs, ears, eyes).

Summary

• Differentiation is driven by gene expression changes prompted by transcription factors and inductive signals.
• Understanding cell differentiation helps us comprehend development and specialization in organisms.