Staging Embryos in Development and the Embryonic Body Plan

Chick and Mouse Embryo Staging Series

In recent years, the external characteristics of laboratory animal species have been available and shown within staging schemes. Computing power now permits the manipulation of external images, sectional information and three-dimensional (3D) representations of internal structures in embryos. Databases of developmental information of laboratory animals have been collated in collaborative projects by those involved in experimental embryology. Chick development was described by Hamburger and Hamilton as a series of 46 stages over the 20-day incubation period. Photographs and movies of all stages of chick development are available on an online database, e-Chick Atlas.

For the mouse similar staging systems have been developed by Theiler based on the Streeter staging of human embryos (see later) and continued by Kaufman. Kaufman noted the care with which specimens needed to be prepared and sectioned and the level of experience necessary for the interpretation of serial sections in order to understand the complexity of spatial and temporal development of body systems and internal organs. The use of magnetic resonance imaging (MRI) to obtain images of both external features and sectional anatomy is available online ( emouseatlas.org ) based on Kaufman’s original images. MRI studies of mouse embryos are also providing a further way of seeing digital images through any plane.

Both chick and mouse databases now display the expression of a range of genes within the developing organs and tissues linked to sections and 3D reconstructions of embryos.