Introduction to embryology -2024

Introduction to Embryology and Gastrulation

  • Dr. Cilwyn Shalitha Braganza, Newcastle University

  • Focus on key processes of early human development, germ layers, and basic embryological terms.

Learning Outcomes

Essentials of Medical Practice Case 2

  • Understand early human development processes.

  • Know the development and fates of the ectodermal, endodermal, and mesodermal germ layers.

  • Familiarize with basic embryological terminology.

Links to Medical Cases

  • Paul Tan: Echocardiogram showed systolic dysfunction and a bicuspid aortic valve. Turbulent flow heard through the valve indicates potential narrowing/leaking over time.

  • Future Insights: Case 14 (year 2) discusses embryology in more detail.

Key Processes of Early Human Development

Timeline of Early Development

  • DAY 0: Ovulation - Female and Male gametes.

  • DAY 1: Fertilization - Zygote formation.

  • DAY 2: Cleavage - Progression from 2-cell to 8-cell stage.

  • DAY 3: Formation of the morula.

  • DAY 4-6: Differentiation of inner and outer cell masses; development of early and late blastocyst.

  • DAY 7: Implantation into the uterine wall.

Fertilization Process

Definition

  • Occurs when spermatozoa undergo capacitation and the acrosomal reaction to fuse with an oocyte, resulting in a diploid nucleus (zygote).

    • Capacitation: Prepares sperm for fertilization by increasing motility.

    • Acrosomal Reaction: Enables penetration through the zona pellucida to bind the oocyte.

Events of Fertilization

  1. Phase 1: Penetration through the corona radiata.

  2. Phase 2: Sperm penetrates zona pellucida.

  3. Phase 3: Penetration of oocyte membrane, resulting in membrane fusion and loss of spermatozoa's outer membrane.

Results of Fertilization

  • Completion of the second meiotic division, restoring diploid chromosome number.

  • Determination of genetic sex (XX or XY) and unique genetic combinations.

  • Activation of metabolic processes in the oocyte.

Pre-Implantation Stages of Human Development

  • Cleavage Stage: Progression from 2-cell to morula and blastocyst formations over 1-2 days.

  • 5-7 Days: Hatching and implantation of the blastocyst into the endometrium.

Blastocyst Structure

  • Comprises blastomeres, with specialization into:

    • Inner Cell Mass (Embryoblast): Forms the fetus.

    • Trophoblast Cells: Contribute to placenta formation.

    • Blastocoele: The fluid-filled cavity.

Development of the Bilaminar and Trilaminar Embryo

Bilaminar Embryo

  • Composed of epiblast and hypoblast layers.

  • Epiblast cells give rise to all tissues of the embryo.

Gastrulation Process

  • Transition from bilaminar to trilaminar embryo (week 3) forming:

    • Ectoderm

    • Mesoderm

    • Endoderm

  • Epiblast cells migrate into the hypoblast, forming germ layers through involution.

Fate of the Germ Layers

Endoderm

  • Forms visceral organs such as:

    • Gut tube and associated organs (liver, pancreas).

    • Respiratory structures (trachea, lungs).

    • Urinary and auditory systems.

Mesoderm

  • Develops into:

    • Supporting tissues (bone, cartilage, tendons).

    • Muscular system and visceral organs (heart, blood vessels).

    • Kidneys and reproductive system.

Ectoderm

  • Originates from epiblast and gives rise to:

    • Nervous system and sensory organs (including skin).

    • Neural crest cells contributing to structures like facial bones and cardiac tissue (defects can result in conditions like Treacher Collins Syndrome).

Key Points Summary

  • The bilaminar embryo transforms into trilaminar through gastrulation, establishing three primary germ layers.

  • Germ layers are pivotal for both embryonic and extra-embryonic development.

  • Gastrulation involves intricate movements and signaling that dictate embryonic development.

Basic Embryological Terms

  1. Blastocyst: Hollow structure pre-implantation.

  2. Capacitation: Sperm preparation phase.

  3. Compaction: Formation of tight junctions in morula stage.

  4. Ectoderm: Germ layer for nervous system and skin.

  5. Embryogenesis: Period of organ formation (weeks 3-8).

  6. Endoderm: Forms gut structures.

  7. Endometrium: Uterine lining, sheds during menstruation.

  8. Epiblast: Dorsal cell layer of bilaminar disc, gives rise to embryo.

  9. Extraembryonic: Tissues outside the embryo (e.g., membranes).

  10. Germ Layers: Ectoderm, mesoderm, and endoderm.

  11. Hypoblast: Contributes to yolk sac, non-embryonic.

  12. Induction: Cell communication altering developmental fate.

  13. Inner Cell Mass: Origin of the embryo from blastocyst.

  14. Mesoderm: Forms connective tissues and organs.

  15. Neurulation: Neural tube formation from the neural plate.

  16. Notochord: Essential for neural plate and spinal structure development.

  17. Vitelline Duct: Connects yolk sac to midgut.

  18. Yolk Sac: Origin of first blood cells, attached to midgut.