Principles of Medical Embryology

Key terminology

  • Gamete: mature haploid male or female germ cell which is able to unite with another of the opposite sex in sexual reproduction to form a zygote
  • Embryonic period: weeks 3 - 8 in humans
  • Teratogen: cause irreversible, deleterious structural malformations in fetuses
  • Mutagen: cause changes (mutations) in the genetic material of cells

Phases of embryogenesis

1. Gametogenesis

  • Formation of gametes
  • Spermatogenesis: formation of male gametes
    • Occurs at puberty and continues throughout life
  • Oogenesis: formation of female gametes
    • Primary oocytes begin meiosis by weeks 28-30 but arrest in prophase until puberty

2. Fertilisation

  • Union of gametes - fusion of sperm and oocyte to form a zygote

3. Cleavage

  • Period of rapid mitotic cell division with no increase in size
  • Formation of morula then blastocyst (days 1-4)

4. Gastrulation

  • Formation of the germ layers - ectoderm, mesoderm, endoderm
  • Body axis established

5. Embryonic folding

  • Formation of the body plan

6. Organogenesis

  • Formation of organs and organ systems
  • Basis in place by end of embryonic period, development continues through fetal period

7. Fetal period

  • From week 9 until birth
  • Growth, weight gain
  • Tissues mature and become functional
  • Overt sexual differentiation
  • Bone laid down; connections made in CNS

Primary processes of embryonic development

  • Cell division
  • Differentiation
  • Cell attachment
  • Apoptosis
  • Induction
  • Cell migration

Secondary processes of embryonic development

  • Axis formation
  • Folding/rotation

Factors resposible for the regulation of embryonic development

  • Genetic
  • Epigenetic
  • Environmental influences