Hatching in teleostean fishes: Fine structural changes in the egg envelope during enzymatic breakdown in vivo and in vitro

Abstract

During the hatching process distinct ultrastructural changes take place in the egg envelopes of three oviparous teleosts, the pike Esox lucius L., the zebrafish Brachydanio rerio, and the annual fish Nothobranchius korthausae. Under natural conditions the zona radiata interna (ZRI) of N. korthausae, having a relatively thick egg envelope, is entirely digested by hatching enzyme, whereas in the pike and the zebrafish, having thin envelopes, a decrease in thickness of only 30 and 15%, respectively, is observed. In all three species, the extent to which the ZRI is affected in vitro by hatching enzyme or Pronase depends on incubation time, temperature, and enzyme concentration. Total breakdown of the ZRI is possible in all cases, while the zona radiata externa remains intact. In vivo this layer is affected, possibly by microbial digestion. These results demonstrate that major differences exist between enzymatic breakdown in vivo and in vitro and indicate that in species having thin egg envelopes the digestion in vivo of the ZRI is incomplete, whereas it is complete in species with thick envelopes.