Fig. 3

Gill slit opening in sterlet sturgeon between stage 36 and 39. A–D SEM pictures showing lateral view of the head surface of sturgeon embryos as shown on corresponding sections in E–P. E–P Cross sections of successive stages of gill slit opening as illustrated in histological sections for pouch 2 (E–H, opercular gill slit, between hyoid and first branchial arch) and lineage tracing with CDCFDA in the corresponding stages (pouch 2, I–L, and pouch 3, M–P). First, a deep cleft is formed on the head surface (E, F, arrow), concomitant with expansion of the periderm (I, J, M, N). The distal tip of the arch becomes completely separated from the head surface (G), and periderm covers all exposed surfaces (K, O). The pharyngeal cavity, first not more than a slit-like opening in the endoderm, widens and expands into the already opened pouch (H, L, P), eventually forming an open communication between the pharyngeal cavity (asterisks in G, H, L, P) and the external environment. These events are repeated for each prospective gill slit and proceed in an anterior-to-posterior manner. The cavity marked by ‘X’ in E, F, M, J and K is a preparation artifact. Q–S Cartoons representing a horizontal view of the head of a newly hatched sturgeon (Q) with cross sections at the level indicated, and comparable to those shown in H, L and P (R corresponding to H, L, and S corresponding to P). b brain, en endoderm, n notochord, op opercular flap, ov otic vesicle, p pharyngeal cavity, P2–P4 pouches 2–4, y yolk. Scale bars in E–P = 100 µm