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Life cycle in Zebrafish

The life cycle of a zebrafish is extremely fast. It normally takes about 3 months for the zebrafish to become an adult. It starts out as an embryo, which all offspring is considered embryos until they are born or hatch. [null (Parichy, D. M., Elizondo, M. R., Mills, M. G., Gordon, T. N., & Engeszer, R. E ,2009)] The hatching of the embryo normally occurs around 48-72 hours after fertilization. The larva isn’t considered an embryo anymore, but it’s not considered a juvenile yet. (Parichy, D. M., Elizondo, M. R., Mills, M. G., Gordon, T. N., & Engeszer, R. E,2009) The juvenile state normally lasts about 4-12 weeks after fertilization, it just depends on the living situations. This state is where the fish has all the adult characteristics, except it is still not sexually mature. (Parichy, D. M., Elizondo, M. R., Mills, M. G., Gordon, T. N., & Engeszer, R. E ,2009) The Zebrafish can’t reproduce at this stage. The adult stage is when the fish can breed or reproduce. Metamorphosis is the step where a juvenile is formed into an adult fish. They lose some larval features during this stage. (Parichy, D. M., Elizondo, M. R., Mills, M. G., Gordon, T. N., & Engeszer, R. E ,2009) The post-embryonic development is basically any growing that occurs after embryogenesis. You can see in figure 1 all the individual steps of life cycle in the Zebrafish.

Cleavage in Zebrafish

Cleavage in zebrafish happens in the blastodisc. (Gilbert,1970) The divisions don’t divide the egg entirely (Gilbert,1970). The cytoplasm of the blastodisc becomes the embryo which is called discoidal. (Gilbert,1970) Figure 2 shows the process of cleavage in many different steps. Cleavage happens every 15 minutes and the first 5 divisions are vertical and creates 32 cells. (Jonatan M.W. Slack, 2013) The actin is activated and changes the embryo shape from spherical to pear shaped. The first 12 divisions form a mound of cells in the animal pole in the yolk cell. (Gilbert,1970) These cells make up the blastoderm. During the 10th cell division the zygotic gene transcription begins, and you can see three distinct cell populations.

The Yolk syncytial layer (YSL) is found during the 9th or 10th division, there are two parts of the yolk syncytial, which is external and internal. The internal is formed from the nuclei going under the blastoderm. (Gilbert,1970) The external is formed in front of the blastoderm. The enveloping layer is the outer cells of the blastoderm. The enveloping layer will later transform into the periderm which is just protection for the cell. The cells in between the enveloping layer and the yolk syncytial layer are called the deep cells. (Gilbert,1970) The blastoderm is set on what it will become before gastrulation happens.

Refences

Parichy, David M., et al. Developmental Dynamics: An Official Publication of the American Association of Anatomists, U.S. National Library of Medicine, Dec. 2009

Gilbert, Scott F. “Early Development in Fish.” ''Developmental Biology. 6th Edition.'', U.S. National Library of Medicine, 1 Jan. 1970

Slack, J. M. W. Essential Developmental Biology. 3rd ed., Wiley, 2013.