NS

Lecture 13: Early Human Development

Foetal Development Timeline

  • 1-4 weeks: Key events in early human development.

  • 35-38 weeks: Full-term foetal development.

Learning Objectives

  1. Describe the major developmental events in week 1 = Fertilisation, cleavage of the zygote, formation of the zygote and blastocyst, implantation

  2. Describe some of the developmental events in week 2-4 = Trophoblast development and embryonic disk, gastrulation, structures derived from the 3 primary germ layers

  3. Understand what stem cells are and why we need them

  4. List the hierarchy of stem cells

  5. Describe one clinical application of stem cells

Embryogenesis Achievements

  • Key Achievements:

    • Patterning: Cells acquire identity in space and time.

    • Axis Definition: Major axes (anterior/posterior and dorsal/ventral) established.

    • Germ Layer Formation: Three germ layers formed (gastrulatoin) and major organ rudiments laid down.

Fertilisation Process

  • Fertilisation Steps:

    1. Path of Sperm Cell:

      • Travels through the corona radiata and zona pellucida to penetrate the oocyte.

    2. Completion of Meiosis II: Oocyte completes meiosis, forming a polar body.

    3. Formation of Pronuclei: Male and female genetic material combine.

Cleavage and Blastocyst Formation

  • Process Overview:

    • Cleavage of the zygote leads to the formation of a morula by day 4, and a blastocyst by day 5 which consists of an inner cell mass (embryoblast- gives rise to embryo itself ) and outer trophoblast (gives rise to extra embryonic tissue). Around day 4/5 the blastocyst cavity is formed.

Implantation Process

  • Timeline of Events Preceding Implantation:

    1. Fertilization occurs 12-24 hours post-ovulation. Occurs in uterine tube

    2. Cleavage is completed about 30 hours later.

    3. Morula forms 3-4 days after fertilization.

    4. Blastocyst forms 5 days after fertilization.

    5. Implantation occurs approximately 6 days following fertilization.

Gastrulation and Germ Layers

  • First sign gastrulation is taking place is formation of primitive streak where the ectoderm cells displace endoderm cells to create the mesoderm - primitve streak moves from connecting stalk to future anterior side of the embryo

  • Gastrulation: Generation of three primary germ layers:

    • Ectoderm:

    • Gives rise to external structures: epidermis, hair follicles, nails, glands.

    • Develops all nervous tissue including brain and spinal cord.

    • Mesoderm:

    • Gives rise to Connective tissues, muscle, cartilage, skeleton, circulatory system.

    • Endoderm:

    • Gives rise to gut lining, respiratory epithelium, liver, and glands.

Development of the Trophoblast

  • Trophoblast Development:

    • Consists of syncytiotrophoblast and cytotrophoblast which establish chorionic villi/fetal placenta to facilitate nutrient exchange with maternal endometrium.

Embryonic Disc

  • Formation of Bilaminar Embryonic Disc: Contains hypoblast and epiblast which are crucial for further layer formation.

Embryonic Folding

  • Tail and head fold into towards each other

  • Connecting stalk and yolk sack will form future umbilical cord

  • Sides of embryo will fold down to form a tube

Stem Cell Characteristics

  • Stem Cells:

    • Defined by their ability to generate differentiated daughter cells and self-renew, function is growth, renewal, repair of tissue, not limited in their replication potential (Unlike differentiated cells)

  • Types of Stem Cells:

    Hirearchy of Stem Cells -

    • Totipotent: Capable of giving rise to all cell types including embryonic tissues.

    • Pluripotent: Derived from blastocyst, can differentiate to almost all cell types in body except extra-embryonic tissue

    • Multipotent: Limited to differentiating into all cell types in a particular tissue/organ

    • Unipotent: Can only produce one cell type, e.g., skin stem cells for renewal.

  • Stem Cell Clinical Use:

    • Developments in gene editing for long-lasting skin grafts.

    • Overview of gene therapy strategies.

Recap of Developmental Stages

  • Key developmental stages include:

    • Fertilization: n + n = 2n.

    • Cleavage and Blastula Formation: Leading to implantation.

    • Gastrulation: Creates ectoderm, mesoderm, and endoderm.

    • Organogenesis: Formation of organs post-gastrulation.

    • Importance of Stem Cells: Essential for cellular replacement, growth, repair.

  • Clinical Relevance of Stem Cells: Long-lasting skin grafts with gene-editing applications.