Developmental Biology Study Notes

Developmental Biology Study Notes

• Why study development?

  • To understand how normal structures develop
  • To understand how abnormalities arise
  • To understand the positioning of adult structures
  • To recognize that defects in one structure can indicate defects in other structures (e.g., Holt-Oram syndrome)
  • To understand the relationship between structure/organ development for accurate diagnosis

• Range of hand abnormalities Different variations in hand structure exist.

• Phenotype and development

  • Mutations in genes required during development impact the development of multiple tissues
  • Understanding the relationships between development and organ systems can aid in diagnosis
  • Key development stages include embryonic development, fetal development, organogenesis, placental function and abnormalities noted at birth

Human Development

• Gestation period:

  • Humans require 9 months (40 weeks) to fully develop.
  • First trimester is critical for body formation.
    • Weeks 1-2: Blastocyst stage (early development)
    • Weeks 3-8: Embryonic stage (organ development)
    • Week 9 onward: Fetal stage (growth and maturation of physical processes)
  • Second trimester = rapid growth of the foetus
  • Third trimester = fat production and other physiological processes

• Fertilization:

  • Occurs in the fallopian tube.
  • Sperm penetrate the egg's protective layers.
  • Fertilization triggers completion of meiosis and establishes the diploid chromosome number, determining the sex of the embryo.
  • Takes approximately 4 days to reach the uterine cavity.

Early Human Development

• Stages of embryonic development:

  • Zygote formation (fertilized egg)
  • Morula (early cell division stage)
  • Blastocyst (ball of cells with a fluid cavity)
  • Inner cell mass gives rise to all cells of the body
  • Various stages of cell divisions Twinning

• Twinning:

  • Identical twins develop from a single fertilized egg dividing into separate structures
  • Fraternal twins develop from two distinct fertilized eggs.

Post-fertilization

• Zygote: The fertilized egg
• Cleavage: Early cell divisions of the zygote
• Blastocyst stage (important for twinning): development continues
• Implantation: The blastocyst embedding itself in the uterus wall
• hCG: Hormone to confirm pregnancy

Blastocyst to Fetus Development

• Blastocyst: Early embryonic stage, which differentiates into a foetus
• Embryonic stage: A period of rapid development
• Foetal stage: Continued growth and refinement
• Stages: Fetal development occurs in three stages (9 days, 5-6 weeks, 3 months for example)

Formation of Embryo/Fetus

• Gastrulation: Cells rearrange to form three germ layers: ectoderm, mesoderm, and endoderm.
• Germ layers: These layers give rise to various tissues and organs (ectoderm to skin and nervous system, mesoderm to muscles and skeleton, and endoderm to digestive system).
• Neurulation & Neurulation: Formaiton of the neural tube and the nervous system.
• Somite formation & Somitogenesis: Formation of blocks of tissue on either side of the neural tube, that eventually develop into bones, muscles, and connective tissues.

19 days to 23 days

• Closure of the neural tube
• Formation of main body axes
• Formation of organs and tissues, notably the heart and ear

Lateral Body Folding

• Folding of the embryo causes changes in shape from a flat disc to a cylindrical structure
• The gut tube is separated from the yolk sac
  • Amniotic cavity completely surrounds the embryo

Folding creates the embryo

• Further development of the embryo occurs through different phases of folding that create the basic shape and structures needed for further development.
• By day 28 of development, the majority of necessary structures have formed.