Formation of the initial kidney and mouth opening in larval amphioxus studied with serial blockface scanning electron microscopy (SBSEM).

Background: For early larvae of amphioxus, Kaji et al. (Zool Lett 2:2, 2016) proposed that mesoderm cells are added to the rim of the forming mouth, giving it the quality of a coelomoduct without homology to the oral openings of other animals. They depended in part on non-serial transmission electron microscopic (TEM) sections and could not readily put fine structural details into a broader context. The present study of amphioxus larvae is based largely on serial blockface scanning electron microscopy (SBSEM), a technique revealing TEM-level details within an extensive anatomical volume that can be reconstructed in three dimensions.

Results: In amphioxus larvae shortly before mouth formation, a population of compact mesoderm cells is present at the posterior extremity of the first left somite. As development continues, the more dorsal of these cells give rise to the initial kidney (Hatschek's nephridium), while the more ventral cells become interposed between the ectoderm and endoderm in a localized region where the mouth will soon penetrate. SBSEM reveals that, after the mouth has opened, a majority of these mesoderm cells can still be detected, sandwiched between the ectoderm and endoderm; they are probably myoblasts destined to develop into the perioral muscles.

Conclusions: SBSEM has provided the most accurate and detailed description to date of the tissues at the anterior end of amphioxus larvae. The present study supports the finding of Kaji et al. (2016) that the more dorsal of the cells in the posterior region of the first left somite give rise to the initial kidney. In contrast, the fate of the more ventral cells (called here the oral mesoderm) is less well understood. Although Kaji et al. (2016) implied that all of the oral mesoderm cells joined the rim of the forming mouth, SBSEM reveals that many of them are still present after mouth penetration. Even so, some of those cells go missing during mouth penetration and their fate is unknown. It cannot be ruled out that they were incorporated into the rim of the nascent mouth as proposed by Kaji et al. (2016). On the other hand, they might have degenerated or been shed from the larva during the morphogenetic interaction between the ectoderm and endoderm to form the mouth. The present SBSEM study, like Kaji et al. (2016), is based on static morphological data, and dynamic cell tracer experiments would be needed to decide among these possibilities.