Embryonic Development Stages

• Developmental biology examines how organisms grow and develop.
• Embryonic development is key in numerous organisms' life cycles.
• A single cell (zygote) transforms into a multicellular embryo via complex changes.
• These changes are cell division, cell differentiation, and morphogenesis.
• Orchestration relies on intricate genetic and molecular mechanisms.

Stages of Embryonic Development

• Fertilization starts the process by fusing sperm and egg, forming a zygote.
• Cleavage involves rapid mitotic cell divisions without growth.
• Cytoplasm is partitioned into smaller cells called blastomeres.
• Blastomeres form a hollow sphere called the blastula.
• Blastula features a fluid-filled cavity known as the blastocoel.
• Gastrulation is a critical stage involving cell movements and rearrangements.
• The three primary germ layers are formed: ectoderm, mesoderm, and endoderm.
• In chordates, neurulation involves the ectoderm folding inward to form the neural tube.
• The neural tube is a precursor to the central nervous system.
• Organogenesis involves germ layer interaction and differentiation.
• Organs and tissues of the body are formed in organogenesis.

Cell Differentiation

• Cell differentiation is cells specializing in structure and function.
• This is crucial for building complex multicellular organisms.
• Differentiation is managed by differential gene expression.
• Specific genes are activated or repressed in certain cells.
• Distinct protein sets are produced because of this.
• Cell fate determination involves cells committing to a developmental pathway.
• Determination occurs through autonomous, conditional, or syncytial specification.
• Autonomous specification is cell fate determined by cytoplasmic determinants.
• These determinants are inherited from the mother cell.
• Conditional specification is cell fate determined by neighboring cell interactions.
• Syncytial specification is cell fate determined by nuclei position within syncytium, especially in insects.
• Signaling pathways are crucial in cell differentiation.
• The Wnt pathway is vital for cell fate and tissue polarity.
• The Hedgehog pathway handles pattern formation and cell differentiation.
• The TGF-β pathway regulates cell growth, differentiation, and apoptosis.
• The Notch pathway is critical for cell fate decisions during development.

Genetic Regulation

• Genetic regulation manages the timing and spatial patterns of gene expression in development.
• Transcription factors, regulatory DNA sequences, and epigenetic modifications achieve this.
• Transcription factors bind to enhancers or silencers, either promoting or repressing gene transcription.
• Homeotic (Hox) genes are transcription factors vital for specifying body plans in animals.
• Mutations in Hox genes dramatically transform body segment identity.
• MicroRNAs (miRNAs) are small, non-coding RNAs modulate gene expression.
• miRNAs bind to messenger RNAs (mRNAs), inhibiting translation or promoting mRNA degradation.
• Epigenetic modifications like DNA methylation and histone modification alter gene expression without changing the DNA sequence.

Organogenesis

• Organogenesis is organ formation, where germ layers interact and differentiate into specific tissues.
• Ectoderm forms the epidermis, nervous system, and sensory organs.
• Mesoderm forms muscles, bones, blood, heart, kidneys, and reproductive organs.
• Endoderm forms the lining of the digestive tract, respiratory system, plus the liver and pancreas.
• Cell-cell interactions and signaling pathways coordinate organ development.
• Vertebrate limb formation involves ectoderm and mesoderm interactions.
• The apical ectodermal ridge (AER) is a specialized ectoderm promoting limb bud outgrowth.
• The zone of polarizing activity (ZPA) is a mesoderm specifying the anterior-posterior axis of the limb.
• Heart development involves cell movements and signaling, forming heart chambers and vessels.
• Errors in organogenesis can lead to congenital birth defects.