embryology memorize!

• week 1: fertilization and implantation


  1. Ovum & sperm meet in uterine tube -> zygote -> forms morula (through mitosis & cleavage) -> blastocyst (increase in cell size w/ no change in cell number

  2. consists of 2 types of cells - trophoblast & embryoblast). Implantation on 7th day
  3. Trophoblast: helps embryo to attach to the uterus wall; embryo's contribution to placenta
  4. Embryoblast: becomes the embryo
• Week 2: bilaminar embryonic disc


  1. Blastocyst proliferates, forming the bilaminar embryonic disc

  2. Cells receive external induction signals to differentiate into epiblast or hypoblast cell

  3. Epiblast, primary ectoderm & dorsal columnar cells -> amniotic cavity

  4. Hypoblast, primary endoderm & ventral cuboidal cells -> yolk sac

  5. Disc is suspended between these two cavities

  6. Prochordal plate: thickened area where the epiblast & hypoblast meet --> first visual sign of embryonic axis development
• Week 3

  1. Cells of the amniotic cavity differentiate & burrow to form the primitive streak
  2. Epiblast cells (soon to be ectoderm) move towards the hypoblast (soon to be endoderm) to form the mesoderm
  3. Neurulation: ectoderm cells -> neural plate -> invaginates inwards to form neural groove & folds -> neural folds undergo fusion to become neural tube -> neural tube later becomes brain & spinal cord
  4. NCC: develop from neuroectoderm -> epithelial-mesenchymal transformation; migrates from the crests of neural folds to join the mesoderm --> mesenchyme --> ectomesenchyme
  5. Mesoderm -> somites
• Week 4: formation of stomadeum, forebrain, tongue


  1. Rostrocaudal axis folding -> primitive stomadeum
  2. Neural tube expansion -> primitive forebrain
  3. Tongue: from local proliferation of mesenchyme in the floor of the mouth (tuberculum impar & lingual swellings at 1st branchial arch, fuse with hypobranchial eminence)
• Week 5: olfactory placode & nasal process development


  1. Olfactory placode: localised thickenings within ectoderm of frontal prominence
  2. Nasal process: proliferation from mesenchyme around olfactory placode
  3. Lateral nasal process
  4. Medial nasal process
• Week 6: rudimentary mandible formation


  1. Meckel's cartilage acts as a scaffold for intramembranous ossification; it is then resorbed
  2. Ossification occurs where IAN divides into incisive & mental branch
• Week 7-9: 2º palate formation


  1. Nasal septum grows downwards from frontonasal process
  2. Palatal shelves (from Mx processes): grow downwards -> then ascend -> fusion
  3. By W12: final palate is formed
• Formation of lips


  1. Week 5: formation of lower lip (apparent fusion of Md processes)
  2. Week 7: formation of upper lip
  3. Middle portion (+ philtrum, primary palate, anterior maxilla): medial nasal processes + frontonasal processes
  4. Lateral aspect: ant. aspect of the Mx process + lateral aspect of medial nasal process