general embryology

• Embryology


  The study of pre-natal development
• Zygote


  Fertilised egg (single-celled)
• Embryo


  1. 8 weeks
• Foetus


  9 weeks - birth
• Ontogeny


  Stages of development of an individual
• Teratology


  The study of abnormal development
• Phases of embryological development

  1. Phase 1: Fertilisation to 3 weeks in utero

  2. Phase 2: 4-8 weeks in utero

  3. Phase 3: 9 weeks to birth
• Fertilisation


  1. Gamete production via meiosis

  2. Fertilisation - each parent contributes one gamete

  3. Post fertilisation - single-celled zygote is 2n2c and represents the entire information content required to make an organism
• Phase 1 (Weeks 1-3)

  • Predominantly cell proliferation and migration, with a small amount of differentiation
  • Most perturbations in this phase result in embryo loss
  • First cell division (~24 hours) - initiation of cell-cell signalling; spatial orientation/axes of development
  • Normally, cell division proceeds unchecked in the absence of external factors: exponential increase in cell number; increase in size of cell mass a function of cell surface area to volume ratio
  • Cleavage - rapid mitotic division with little net change in embryo size (i.e. cells get smaller) leading to morula then blastocyst
  • Blastocyst composed of external trophoblast (implantation/placentation) and embryoblast (embryo)
  • Differentiation - cells must be competent to receive an external induction signal; competency is transient
  • Day 8 - embryoblast differentiates into bilaminar germ disk composed of ectodermal cells and endodermal cells
  • Day 13 - ectoderm contains amniotic cavity, mesoderm migration forms secondary yolk sac
• Establishment of embryonic axis in Phase 1

  1. Rostral bulge = prochordal plate

  2. Week 3 - bilaminar to trilaminar disc (gastrulation)

  3. Primitive streak formation

  4. Primitive node formation

  5. Notochord formation

  6. Ectodermal migration to streak; invagination laterally and rostrally to form mesoderm; cardiac plate formation

  7. Ectoderm and endoderm now only in contact at prochordal (rostrally) and cecal (caudally) plates
• Phase 2 (Weeks 4-8)

  • Predominantly cellular differentiation and division, and organismal morphogenesis
  • Histo-differentiation and organogenesis
  • Pertubations in this phase can lead to significant congenital abnormalities
  • Ectoderm gives rise to neural crest tissues and nervous system, as well as external epithelium (neurulation)
  • Mesoderm gives rise to a range of internal tissues
  • Endoderm forms the gut
  • Embryo folding; rostro-caudal and lateral axes

  
• Neural development in Phase 2

  1. Neural plate develops rostrally

  2. Neural folds define neural groove - fusion of folds leads to neural tube running rostro-caudally, with intervening mesoderm

  3. Neural 'crest' cells differentiate and migrate from lateral aspect of neural plate in mammals

  4. NC cells further differentiate into a range of developmentally important cells
• Tissues derived from neural crest cells

  • Cranial and sensory ganglia (S2)
  • Sympathetic neurons (S2)
  • Cartilage of the branchial arches
  • Connective tissue in the developing face (ectomesenchyme)
  • All tissues of the teeth and supporting structures except enamel
• Mesoderm development in Phase 2

  1. Midline mesoderm thickens into paraxial mesoderm, separates into somites (vertebrae; discs; muscle; dermis)

  2. Peripheral mesoderm gives rise to intermediate mesoderm (urogenital system) and lateral plate mesoderm (connective tissue of muscles/viscera [mesenchyme]; serous membranes; blood/lymph; cardiovascular/lymphatic systems; spleen; adrenal cortex)
• Embryo folding in Phase 2

  1. Day 21 - Embryo folding initiated - occurs through differential growth rates

  2. Lateral folding results in ectoderm lining all outward-facing surfaces of the embryo - forms surface epithelium

  3. Rostro-caudal folding - head fold gives rise to primitive stomatodeum (oral cavity)

  4. Stomatodeum (lined by ectoderm) separated from gut (endoderm) by buccopharyngeal membrane