From Fertilized Egg to Organism

Transforming a Fertilized Egg

• The transformation of a fertilized egg (zygote) into an organism involves three inter-related processes: cell division, cell differentiation, and morphogenesis.

Roles of Cell Division, Differentiation, and Morphogenesis

• Cell division involves successive mitotic divisions of the zygote, creating a large number of cells.
• Cell differentiation is the process by which cells become specialized in structure and function.
• Morphogenesis organizes differentiated cells into tissues and organs in a specific three-dimensional arrangement, giving the organism its shape and structure.

Sources of Development Information

• Cytoplasmic determinants in the egg's cytoplasm.
• Inductive signals from neighboring cells.
• These two sources provide cells with instructions on which genes to express.

Cytoplasmic Determinants

• Maternal substances, such as mRNAs and proteins, encoded by the mother's DNA.
• Include other substances and organelles, unevenly distributed in the cytoplasm of an unfertilized egg.

Function of Cytoplasmic Determinants

• Influence early embryonic development by regulating gene expression during cell differentiation.
• Different daughter cells receive different combinations of cytoplasmic determinants after fertilization, which helps determine their developmental fate.

Uneven Distribution of Cytoplasmic Determinants

• The cytoplasm of an unfertilized egg is inherently not homogeneous; mRNAs, proteins, other substances, and organelles are localized in specific regions by maternal cells.

Induction Defined

• Signals conveyed to an embryonic cell from neighboring cells, causing changes in the target cell.
• Signals involve contact with cell-surface molecules or the binding of secreted growth factors.
• Transmitted within the target cell via cell-surface receptors and signaling pathway proteins, leading to changes in gene expression and cellular changes.

Differentiation Defined

• A less specialized embryonic cell becomes a more specialized cell type with a distinct structure and function.
• Involves expressing a specific set of genes that allow the cell to perform a particular role within the developing organism.

Pattern Formation Defined

• Establishment of the overall three-dimensional organization of an organism, including the arrangement of tissues and organs in their correct spatial locations.
• Involves setting up the major body axes through molecular cues that provide cells with positional information.

Positional Information Defined

• Molecular cues that guide pattern formation in a developing embryo by indicating a cell's location relative to the main body axes and its neighboring cells.
• Come from cytoplasmic determinants and inductive signals, influencing how cells will respond to developmental signals.

Key Vocabulary

• Embryonic Development: transformation of a fertilized egg into an organism.
• Fertilized Egg (Zygote): single cell from the fusion of sperm and egg.
• Cell Division: mitotic divisions increasing cell number during development.
• Cell Differentiation: cells become specialized in structure/function.
• Morphogenesis: processes giving an organism its shape/structure.
• Developmental Information: guides cells by indicating which genes to express.
• Cytoplasmic Determinants: maternal substances in egg cytoplasm that influence early embryonic development.
• Induction: signals from neighboring cells cause changes in gene expression.
• Tissue-Specific Proteins: Proteins found only in a specific cell type.
• Pattern Formation: development of spatial organization of tissues/organs.
• Positional Information: cues controlling pattern formation by indicating a cell's location.
• Morphogen Gradient Hypothesis: Gradients of morphogens establish an embryo's axes.
• Maternal Effect Genes (Egg-Polarity Genes): Genes of the mother, mutant phenotype in the offspring.