A method called high-resolution episcopic microscopy (HREM) produces remarkably detailed images of embryos, allowing one to visualize fine structures such as the whisker pores on the snout of a 14.5-day-old mouse embryo. This month’s issue of Cold Spring Harbor Protocols features a protocol describing the first step in the HREM procedure: embedding embryos in preparation for imaging. It is written by Tim Mohun and Wolfgang Weninger, who developed the HREM technique.
In HREM, embryos are embedded in plastic that contains fluorescent dyes. Sequential images of the block face (i.e., “episcopic” images) are captured before sections are removed from the block with a microtome. This eliminates the need to align images from individual sections; furthermore, there are no distortions due to sectioning, section stretching, or section mounting. A typical HREM volume data set consists of about 1000-4000 block-face images. These images are put together using computer software to create a 3D model of the internal and external structures of the embryo.
In the video below, Mohun provides an overview of the HREM method.
HREM is useful for studying large, opaque embryos (e.g., those from birds and mammals). It can be used to illustrate the impact that certain mutations have on embryo anatomy. Mohun and Weninger employ the technique in their studies of heart and cardiovascular system development.
The authors have also developed a related technique, episcopic fluorescence image capturing (EFIC). In this month’s issue of Cold Spring Harbor Protocols, they also provide an overview of both methods, as well as additional step-by-step protocols for embedding embryos for EFIC and generating volume data. The complete table of contents for this month’s issue is available here.