Removing Embryos from Their Chorions

(Source: M. Westerfield from Zebrafish Book 5th Edition)

Some observations of zebrafish development can be made directly through the chorion, however for most procedures it is better to remove the chorion.  When raised in Embryo Medium at 28.5°C, zebrafish develop normally outside their chorions.  Chorions can be removed easily with sharp forceps (Dumont No. 5) by gently making a tear in the chorion and turning it upside down so that the embryo falls out.  To remove the chorions from young embryos (up to 15 h) care must be taken when opening the chorion because these embryos are easily damaged. 

Pretreatment with a dilute solution of Pronase (2 mg/ml in Embryo Medium, 1 minute, 28.5°C) makes the chorions brittle and easier to remove.  Older embryos require longer in the Pronase.  Be sure to rinse the Pronase treated embryos thoroughly at least 3 or 4 times to remove all the enzyme.  Embryos usually fall out of their chorions with gentle swishing during the rinse. 

Embryos removed from their chorions can be transferred from one container to another by gently sucking them up with a fire-polished Pasteur pipette or by gentle pouring.  Small Petri dishes (35 mm diameter) are adequate for holding up to 25 embryos during the first few days of development.  The embryos can be brought to the center of the dish for viewing by gently swirling the medium with a circular motion.