Embryonic Development

Topic 4: Embryonic Development, Maternal to Zygotic Genome Transition, Gastrulation, and Embryonic Cell Migration.
- Key focus areas include:
- How cells migrate during development.
- The formation of the nervous system and body segments.

Development timeline from fertilization to the formation of organ structures: 4mm in size by the end of this period.

4 Weeks Developmental Mark:
- Measurement of 22 cm noted for significant growth.
Embryonic stages represented:
- D1, D2, D3, D4, D5, D6: Represents successive days post-fertilization.

Rapid Cell Division: Key phases in cell cycle during early embryonic development.
- G1 Phase:
- Associated with cell hypertrophy and entry into the cell cycle.
- Variable duration, critical checkpoint once completed.
- S Phase:
- DNA Synthesis occurs, whereby DNA content doubles from 2n to 4n.
- G2 Phase:
- Involves checking DNA integrity and preparing for division.
- Includes checkpoints for completion.
- M Phase:
- Process of division into two daughter cells, with checkpoints.
- G0 Phase:
- A resting phase where cells do not proliferate.

This transition is critical to early embryonic development.
- Transcriptional Changes: Maternal mRNAs packaged within the oocyte are utilized after fertilization but diminish over the first week.
- Zygotic Genome Activation:
- Activation leads to the expression of genes vital for embryonic development and cell fate determination.

RNA derived from initial maternal sources undergoes a transition to zygotic/embryonic RNA.
- DNA Methylation: This epigenetic marking sees a reduction in early embryos, alongside an increase in zygotic transcription.

Post-Fertilization Methylation Dynamics:
- Majority of methylation marks are eliminated post-fertilization.
- Paternal Genome: Affected by active demethylation.
- Maternal Genome: Experiences passive loss of methylation.
- Newly acquired tissue-specific DNA marks occur as cells differentiate.

Corresponds to a loss of totipotency, leading to a bias in cell fate.
Stages of development should be noted:
- Totipotency: Capability of a cell to differentiate into any cell type.
- Zygote to Blastocyst progression showcases this bias.

Mesoderm forms during the process of gastrulation.
- Defined orientations: Dorsal, Ventral, Head end, Tail end.
- Regulation: Growth factors such as FGF and TGF-b play a crucial role.
- Primitive Streak: Establishes the overall body plan and axes.

Includes various movements of cell sheets:
- Invagination: Cells fold inward, creating a pocket.
- Ingression: Individual cells move into the interior of the embryo.
- Involution: A rolling movement of cell sheets over the edge of a blastopore.

Epiboly: Expansion of one cell sheet over others.
Intercalation: Cells intercalate, resulting in layer thinning and extension.
Convergent extension: Cells converge and extend, elongating the body axis.

Sequential developments from implantation to organ formation:
- Rapid cell division, neural tube formation, and somite development highlight key advancements.
- Neural Tube Closure: Critical to the formation of the central nervous system.
- Muscle and Bone Development: Tissue growth is evident by heart activity and blood circulation.

Neural Tube: Formed from the folding of the ectoderm, eventual structures include brain and spinal cord.
Neural Crest Cells:
- Migrate to form components of the peripheral nervous system, adrenal medulla, and melanocytes.

As neurulation occurs, mesoderm segments into somites:
- Somites are mesodermal blocks arranged along both sides of the neural tube, forming in pairs.
- Timeline of Somite Formation:
- Sequential formation from head to tail occurring at different gestational ages: 1 week (mouse), 2 weeks (pig), 3 weeks (human).
- Resulting tissues: Includes vertebrae cartilage, bones, muscles, and tendons.

Detailed understanding of subsequent structure formation:
Somite relation to structures:
- Neural tube, optic vesicle, notochord contribute to segmentation.
- Each stage of development correlates with somite number and position.

Stages of somite formation in various species (e.g., chick, mouse) are illustrated.
- Notable milestones include neural tube closure and developing somatic structures.

Limb Bud Development: Occurs post-neural tube closure, described in stages:
- Stage 12-13: emergence of fore and hind limbs with mesoderm covered by ectoderm.

Importance of the neural tube formation emphasized through specifics:
- Anencephaly: Caused by non-closure of the cranial tube.
- Spina Bifida: Involves failure to close the caudal tube.
- Risk factors: Folic acid deficiency or Vitamin A overdose.

Growth factors influence anterior-posterior Hox gene expression:
- Notable factors, such as Fibroblast Growth Factor (FGF), regulate the mesoderm in the primitive streak, leading to posterior Hox expression.
- Retinoic Acid: Modulates growth factors and impacts anomalies like spina bifida through its presence or receptor diversity.

Hox Genes: Encompasses a large family characterized by homeobox structures critical for developmental biology.
- Functions across species from Drosophila to mammals.
- Regulated by growth factors and retinoic acid, specifying segment identity.

Functional attributes categorized by gene clusters and positions on chromosomes where they retain conservation.
Changes in Hox gene expression align with developmental timing and body axis specification.

Limb buds expand resulting in fully developed fore and hind limbs by specified embryonic days (E10.5F, E12 K, E14).

Blastocyst to Embryo and Fetal Development:
- Programmed by maternal RNA, alongside interactions of cell-cell and locally diffusing growth factors.
- Development is underpinned by local and circulating hormones influences as well as cell activity.