Biology Chapter on Fertilization and Development

Fertilization and Development

• Fertilization is the fusion of sperm and egg (gametes) to form a single cell (zygote).
• The zygote undergoes mitosis to become a blastula and then morphogenetic movements to become a gastrula.
• The dividing cells of a fertilized egg form three distinct embryonic germ layers: ectoderm (outer layer), mesoderm (middle layer), and endoderm (internal layer).

Differential Gene Expression

• Cells with the same genes can express different sets of genes based on their specific functions.
• Different proteins produced due to differential gene expression give rise to different cell types.
• The percentage of the genome expressed in a cell varies, and a portion of the RNA synthesized in each cell is unique.

Genomic Equivalence

• All somatic cells in an organism have the same DNA as the zygote due to mitosis.
• Each somatic cell nucleus has the same number of chromosomes and genes as other somatic nuclei.
• The nucleus of a somatic cell contains all necessary information to generate all cells in the body, as demonstrated by somatic cell nuclear transfer.

Somatic Cell Nuclear Transfer

• Somatic cell nuclear transfer involves transplanting the nucleus of a differentiated cell into an enucleated egg.
• The transplanted nucleus can direct the development of the organism, as shown by the cloning of Dolly the sheep.

Three Postulates of Differential Gene Expression

• The DNA of all differentiated cells is identical.
• Unused genes in differentiated cells are neither destroyed nor mutated.
• Specific gene activation and inactivation define the identity of a cell.

Levels of Control of Gene Expression

• Transcription:
  • RNA polymerase binds to DNA to initiate transcription.
  • Enhancers/activators and silencers/repressors regulate transcription.
  • Chromatin structure (histone modification, DNA methylation/demethylation) influences gene accessibility.
• RNA Processing:
  • mRNA is modified through splicing and transported out of the nucleus.
• Translation:
  • Some mRNA may be degraded or not translated.
  • Differential mRNA stability, selective inhibition/activation, localization, and RNA-induced silencing regulate translation.
  • Cytoplasmic localization controls RNA translation.
• Post-Translational Control:
  • Modifications like cleavage, transport, subunit assembly, ion binding, and covalent modification (phosphorylation, acetylation, glycosylation) control protein activity.

Transcription Factors

• Transcription factors (TFs) are proteins that bind to DNA sequences (cis-acting elements).
• TFs influence RNA polymerase's ability to transcribe genes.
• General TFs are required for RNA polymerase binding to the promoter, while regulatory TFs control the rate of transcription by binding to enhancers or silencers.
• Activators increase transcription, while repressors decrease transcription.
• Different sets of TFs in a cell determine which genes are expressed.

RNA Processing

• RNA processing involves modifications to mRNA before translation.
• miRNA (microRNA) and siRNA (small interfering RNA) are involved in RNA silencing.
• miRNA and siRNA associate with DICER and RISC complexes to regulate mRNA degradation and translation.
• miRNAs can target hundreds of endogenous mRNAs with imprecise matching, allowing for broader regulation.
• siRNAs have more precise targeting, specifically targeting specific sequences.
• miRNA plays a role in the degradation of maternal mRNAs during gastrulation, allowing for the expression of zygotic genes.

Early Stages of Embryonic Development

• After fertilization, a diploid zygote nucleus is formed via the fusion of parental gamete nuclei.
• Nine rounds of nuclear divisions result in a multinucleated syncytium (single cell with many nuclei within a common cytoplasm).
• About 2.5 hours after fertilization, nuclei migrate to the periphery of the egg and undergo four further divisions.
• Pole cells (precursors of germ cells) form at the posterior pole.
• Around 3 hours after fertilization, nuclei become enclosed in membranes, forming a single layer of cells over the embryo surface, known as the syncytial blastoderm.
• About 10 hours after fertilization, segmentation pattern is clearly established: T1-T3 for thoracic segments, A1-A8 for abdominal segments, Acron for head-forming region.

Maternal Effect Genes

• Genes expressed in the mother that direct the early development of the offspring.
• Encode transcription factors, receptors, and proteins that regulate the expression of zygotic genes.

Egg Chamber of Drosophila

• Oogonium undergoes four rounds of mitotic division with incomplete cytokinesis, producing 16 interconnected germline cells (15 nurse cells and 1 oocyte).
• Nurse cells produce mRNAs and proteins that are transported to the developing oocyte.
• Posterior follicle cells send signals back to the oocyte to organize its microtubules.

Maternal Protein Gradient

• Initiates the formation of the anterior-posterior axis.
• Maternal effect: The offspring's phenotype for a trait is controlled by nuclear gene products in the unfertilized egg.
• Two maternal mRNAs / morphogens (determine cell fate by concentration) initiate formation of the A-P axis:
  • Bicoid mRNAs: head/anterior morphogen actively transported along microtubules to the anterior end of the oocyte by Dynein protein.
  • Nanos mRNAs: tail/posterior morphogen diffuse passively to the posterior pole of the oocyte and anchored by proteins.

Terminal Genes

• Encode proteins that generate the unsegmented extremities.
  • Acron: terminal portion of the head.
  • Telson: extreme posterior end (tail).

Zygotic Genes

• Genes transcribed in the embryonic nuclei formed after fertilization.
• Differential transcription is regulated by the distribution of maternal effect proteins.

Segmentation Genes

• Hunchback and caudal mRNAs are regulated by bicoid and nanos proteins:
  • Hunchback mRNA: Expression activated by bicoid protein, translation inhibited by nanos protein.
  • Caudal mRNA: Translation inhibited by bicoid protein; activates zygotic genes important in the formation of the abdomen.

Homeotic Genes (Hox)

• Control the identity of segments and their development into various structures.
• Different Hox genes are expressed in different regions of the embryo, defining the identity of each segment.
• Mutations in Hox genes can cause dramatic transformations in body structures.

Progressive Restriction of Cell Fate

• As development proceeds, the number of potential fates for each cell decreases.
• This is primarily regulated by the expression of different genes at different times and places.
• This process ensures the proper development of body plan.

Hedgehog (Hh) Signaling Pathway

• Hh proteins are paracrine factors involved in regulating Gli, a transcription factor.
• The Hh pathway occurs in the primary cilium of vertebrate cells and plays a crucial role in signaling and development.
• Cholesterol is essential for proper Hh secretion. If cholesterol levels are low, Hh diffuses too quickly before binding to its receptor.

Examples of Hh proteins and their Functions

• Sonic hedgehog (shh): Involved in motor neuron development, feather pattern formation, and various other developmental processes.

Developmental Events Where Hh Proteins Are Active:

• Limb patterning
• Neural differentiation and pathfinding
• Retinal and pancreas development
• Cranial morphogenesis
• Feather formation in the chick embryo
• Hair formation in mammals
• Skin cancer (Hh pathway misregulation)

Wnt/β-catenin Signaling Pathway:

• Regulates cell division, shape, and migration.
• When Wnt is present:
  • Wnt binds to its receptors (Frizzled and LRP5/6), forming a multimeric complex.
  • This complex binds to axin and GSK3, preventing them from degrading β-catenin.
  • Accumulated β-catenin enters the nucleus and activates Wnt-responsive genes.

Fibroblast Growth Factor (FGF) Family:

• FGFs are paracrine factors activating the receptor tyrosine kinase (RTK) pathway.
• RTKs have an extracellular domain that binds to ligands and an intracellular domain with kinase activity.

Types of FGFs and Their Functions:

• FGF1 (acidic FGF): Regeneration and tissue homeostasis.
• FGF2 (basic FGF): Blood vessel formation.
• FGF7 (keratinocyte growth factor): Skin development.
• FGF8: Segmentation, limb development, and lens induction.

Notch Signaling Pathway:

• Regulates cell fate decisions in development.

Pathway When Hh Is Not Present:

• Patched (Hh receptor) is not activated, inhibiting the GAP protein, which keeps Ras active for a longer time.
• Smoothened is inhibited, leading to the retention of the Gli/Cubitus interruptus transcription factor in the cytoplasm.
• Gli is cleaved and acts as a repressor, inhibiting transcription.

Pathway When Hh Is Present:

• Hh binds to Patched and is endocytosed, degrading Patched and allowing Smoothened to become active.
• Active Smoothened inactivates cleavage proteins, releasing Gli from microtubules.
• Gli enters the nucleus and acts as an activator, promoting transcription.

Explore the process of fertilization and the developmental stages that follow, including the formation of the zygote and differentiation of germ layers. Understand how differential gene expression leads to the variety of cell types despite genomic equivalence in somatic cells.