Research | Salamander Development

Ensatina eschscholtzii is a salamander that lives in the Reed Canyon. Its embryos undergo a fascinating variation of amphibian development—they develop via an embryonic disk. In the typical amphibian egg, all cells are incorporated into the embryonic body. In contrast, eggs of disk-forming species develop a disk that forms the body and an external yolk sac that is consumed during development. This pattern superficially resembles that of reptiles and birds; in the amniotes, however, the disk forms as a consequence of incomplete cleavage at the top of an uncleaved yolk and gastrulation then occurs within the disk. In amphibians, cleavage is complete and the disk forms late as a consequence of gastrulation.

The processes of gastrulation and embryonic disk formation in the amphibian species are not understood, although beautiful work has been done on the frog Gastrotheca riobambae (del Pino and Elinson, Nature 306: 589–591, 1983) and the salamanders Aneides flavipunctatus and Ensatina eschscholtzii (Collazo, Ph.D. dissertation, University of California, Berkeley, 1990). Neither is it known if any of the mechanisms underlying early development of Xenopus operate in the disk-forming species. By studying Ensatina development, we hope to learn about the diversity of solutions to accommodating high yolk content, and we will begin to identify which mechanisms in typical amphibian development are conserved or immutable and which are modifiable. This may inform our thinking about the origin of the amniote egg, which was an important evolutionary innovation.

Fig. 1

Ensatina adults. Left, dorsal view. Right, female laying eggs. In the lab, they typically oviposit in this upside-down position. Photos courtesy of Jason Oakes ‘00.

Fig. 2

Still from a realtime movie of an Ensatina pharyngula, about a month old. Note enormous yolk sac with vitelline vessels. The embryos regularly rock back and forth at this stage. Development to hatching is said to take about 6 months. View movie.

Fig. 3

A, Living Ensatina neurula viewed through jelly coats, showing embryonic disk as a white field of raised cells on a background of yellowish yolky cells. Disk is outlined with dots and neural folds are indicated with asterisks; anterior at 12 o’clock. About day 17. The egg is ~5mm in diameter (cf. Xenopus, ~1.2mm).

B, Section of blastula stained with DAPI to show blastocoel roof nuclei; animal pole at 12 o’clock.

C, Transverse section of neurula stained to show endodermal crests (e) ventral to prospective notochord region (n) and neural plate (np). Note continuity between left crest and notochord (blue arrowhead), indicating origin of some notochord cells from the archenteron. This is the typical salamander pattern.  Photos courtesy of Jason Oakes ‘00.