In Ovo Electroporation for Targeting the Somitic Mesoderm

The somite is a transient structure present in early vertebrate embryos, giving rise to a variety of essential tissues including skeletal muscles, dermis, axial bones and blood vessels. The term “somite” refers to a tissue of spherical structure that forms by pinching off from the continuous tissue called presomitic mesoderm (PSM, also called segmental plate in avian embryos). The PSM is recognized as a pair of longitudinal stripes along the midline of the body. Thus, each somite forms at the anterior end of PSM, and this process recurs periodically in time and space, gener ating the segmented pattern of the body along the antero-posterior axis.

Soon after the invention of the in ovo electroporation technique that was originally applied to the neural tube of chicken embryos (Funahashi et al., 1999; Momose et al., 1999; Nakamura et al., 2004), somites were also challenged for electroporation-mediated transgenesis. However, as long as the PSM was targeted, transgenesis was not successful for unknown reasons. Several years ago, we achieved somitic transgenesis by targeting PSM precursors (presumptive somitic cells) of earlier embryos, the cells residing in the epiblast of the anterior primitive streak (Nakaya et al., 2004; Sato et al., 2002). When development proceeds, these cells ingress and migrate anteriorly beneath the ectoderm to form the PSM on either side of the neural tube (Fig. 5.1). We will describe the methods of electroporation-mediated transgenesis of early somite/PSM by taking advantage of the dynamic morphogenetic movement of the presumptive somitic cells.