Embryology

  • Embryologys Overview

    • Sensitive to developmental defects primarily between weeks 3-8 (embryonic period)
    • 23 embryonic stages from fertilization to week 8
    • Preimplantation development
    • Implantation and placentation
    • Basic body plan establishment and tissue primordia

  • Foetal Stage

    • Occurs from week 9 to term
    • Involves:
    • Continued growth and differentiation of tissues and organs
    • Increased foetal size and weight

  • Gametogenesis

    • Meiosis in gonads produces haploid gametes:
    • Testes produce sperm (spermatozoa)
    • Ovaries produce eggs (oocytes)
    • Oocyte maturation is hormonally regulated by the menstrual cycle
    • Oocytes are released approximately 14 days after the last menstrual period (LMP)

  • Endometrial Changes

    • Occur in the endometrium (site of implantation), particularly in the two superficial layers
    • Endometriosis: Endometrial tissue growing outside the uterus due to hormonal influence

  • Embryonic Development

    • Week 1: Preimplantation Development
    • Day 1: Fertilization forms the zygote (diploid cell with male and female pronuclei)
      • Zygote undergoes cleavage, producing smaller cells called blastomeres without a change in overall size
    • Day 4: Formation of the morula (16-32 blastomeres)
    • Day 5: Morula transforms into a blastocyst
      • Comprises an inner cell mass (embryoblasts) and an outer cell mass (trophoblasts)
      • Contains a blastocyst cavity (blastocoel)

  • Day 6-7:

    • The blastocyst hatches from the zona pellucida (protective glycoprotein layer) for implantation in the uterine wall

  • Week 2: Implantation

    • Formation of primordial germ cells begins
    • Day 9: Blastocyst attaches and embeds in endometrial wall
    • Day 10:
      • Inner cell mass forms a bilaminar embryo: epiblast (outer) and hypoblast (inner)
      • Amniotic cavity forms in the epiblast
      • Hypoblast forms the exocoelomic membrane (Heuser’s membrane)
      • Primary yolk sac is established
      • Trophoblasts differentiate into cytotrophoblasts and syncytiotrophoblasts (future placenta)

  • Week 3: Gastrulation and Folding

    • Basic body plan is established
    • All edges fold to form a C-shaped embryo
    • Day 15: Formation of the primitive streak which defines body axes: craniocaudal, ventral/dorsal, left/right
    • Day 16:
      • Gastrulation occurs, transforming the bilaminar embryo into a trilaminar embryo
      • Epiblast cells migrate towards the primitive streak, forming the endoderm
      • Remaining epiblast cells form the ectoderm
      • Intermediate invaginated cells become the mesoderm

  • Endoderm:

    • Closest layer of the embryonic disc
    • Forms the primitive gut tube during folding
    • Develops into the respiratory tract, gastrointestinal tract, and accessory organs

  • Mesoderm Functions:

    1. Notochord: Guides development through signaling
    2. Paraxial mesoderm:
      • Somitogenesis (segmentation into somites)
      • Forms axial skeleton, skeletal muscles, trunk dermis
    3. Intermediate mesoderm:
      • Forms kidneys (metanephros)
      • Regresses pronephros and forms reproductive system (mesonephros)
    4. Lateral plate mesoderm:
      • Connective tissues, heart, blood vessels, smooth muscles, and coelomic linings
      • Forms pericardial, pleural, and peritoneal cavities

  • Ectoderm Functions:

    1. Midplate ectoderm: Forms CNS and PNS
    2. Lateral/Surface ectoderm: Forms surface epidermis
    3. Ectodermal placodes: Thick ectoderm regions for sensory organs (e.g. inner ear)
    4. Neural crest cells: Forms parts of PNS and other structures
      • Hirschsprung’s disease: Failure of intestine innervation

  • Neurulation:

    • Process of neural tube formation, separating skin from neural tissue
    1. Lateral edges of the neural plate elevate to form neural folds and a groove
    2. Neural folds fuse
    3. Formation of the neural tube which will develop into CNS