Figure 2. Expression of FN mRNA and protein in developing embryos.

A-B1. FN mRNA staining. A. FN mRNA is expressed in the LPM in a one somite (s) embryo. Note undetectable levels of FN mRNA in the embryonic midline in A-B. B Expression of FN mRNA by cells around the node (no, arrow). Transverse section at the level of the node (arrow) is shown in B1. Note expression of FN mRNA by the presumptive crown cells (arrow) and endoderm (arrowhead). The section is skewed toward one side creating impression of asymmetric FN mRNA distribution. C. FN protein staining. 3D reconstruction of confocal sections through the embryo. FN staining in 2-3 somite (s) control embryo shows the domain of FN protein (green) is extended posteriorly at the left side of the node (red dotted line). Boxed area is expanded in panel C1. Filled arrows point to the extent of FN staining on the left and right sides of the node. Dashed lines indicate orthogonal confocal slices shown in C3 and C4. C1. Expanded view of the node. C2. An optical z slice in xy plane, close to the center of the embryo (in the dorsal-toventral direction). Dashed double headed arrows outline the region of the node on the right devoid of FN (C1-C2). C3. Orthogonal view through the notochord FN (open arrow) is localized between the notochordal plate and the floor plate (fp). C4. Orthogonal view through the node shows that FN protein localizes to the basal surface of the ventral node (open arrow) between the ventral and the dorsal node. Note that there is a single layer of FN protein at the basal surface of the node and the notochordal plate (spanning 1-2 confocal planes), while FN protein is present throughout the embryonic mesoderm, spanning the majority of the confocal planes through the embryo. Therefore, FN protein appears undetectable in the whole mount view in panels C-C1. Scale bars are 100 μm, except in C1-2, the scale bars are 50 μm.