Hamburger-Hamilton Stages - Stages of Development

Stages of Development

Chick embryos can be "staged" according to the different morphological landmarks. Although most organ systems have a stereotypical appearance at each stage, there are a few which particularly lend themselves to use in staging chick development.

• In the very early embryo, the primitive streak is the only visible landmark, and its shape and size is used to stage HH1-6 embryos.
• The nervous system is formed by a process of neurulation. Stages 5–8 May be defined by the formation of a head fold, the neural folds, and their fusion to form the neural tube. The expansion of anterior neural tube to form the brain may also be used to identify later stages.
• Somitogenesis – the progressive segmentation of the paraxial mesoderm – provides a convenient method for staging embryos between stage 6 and 14. Somites form with surprising regularity every 90 minutes. Stage 10 embryos have 10 somites, and as a rule of thumb, the embryo gains 3 somites during each stage (i.e. Stage 11 embryos have 13 somites, Stage 12 embryos have 16, etc.). The authors note, however, that beyond 22 somites (HH14) it is better to rely on other markers.
• Formation of the Branchial arches; which will give rise to the structures of the jaw, pharynx and larynx; begins at HH14 and is used as a marker throughout development.
• The morphology of the limbs, starting with the appearance of wing bud at stage 16, is a useful landmark for staging chick embryos until hatching. Between stages 15 and 35, the appearance of specific structures within the limbs (such as joints and digits); at later stages the length of the toes are used.
• The formation and development of the eyelids, primordial feathers and beak are used in a similar way to stage later development.