Zebrafish Development

 

Zebrafish Development

Zebrafish development is a rapid process, especially when compared to humans. Zebrafish undergo meroblastic cleavage. This means that only a small part of the embryo, the blastodisc, undergoes cellular division. The yolk stays intact and does not divide. This type of meroblastic cleavage is called discoidal since only the cytoplasm of the blastodisc becomes the embryo. This is unlike humans, as a human embryo undergoes holoblastic cleavage, or it divides completely with no leftover yolk. In zebrafish during this initial cell division the cell cycle is rapid (~15 minutes) and division occur synchronously. This leads to the formation of the blastoderm, or a mass of cells at the animal pole of the developing embryo. Around the tenth cell cycle cell division begins to take longer and become asynchronous as cells begin to transcribe their own genomes instead of relying on maternal mRNA. At this point distinct cell populations can be identified.

Gastrulation begins at 4 hours post fertilization (hpf) when epiboly begins to occur. This is the

spreading of the blastoderm to cover the yolk. When the blastoderm covers about half of the yolk cell a thickening occurs, which is composed of the epiblast and hypoblast. These two intercalate on the future dorsal side to form the embryonic shield, the organizer in zebrafish.

At 10 hpf epiboly is finished. The tailbud is forming at the vegetal pole and the head is forming at the animal pole. The mesoderm is segmenting into somites and neurolation begins. The heart emerges at 24 hours. For more information on zebrafish heart development click here. By 36 hpf most major organs have been formed.

Between 48 to 72 hpf the fish will hatch. 72 hpf marks the end of the embryonic period. They are considered to be in a larval state until about 4 weeks, were they are then reclassified as juveniles. They are sexually mature around 10-12 weeks post-fertilization.

Gestation Periods

Gestation is defined as the period of development between conception and birth. In

mammals, including humans, gestation begins right after the egg is fertilized and ends when the child exits the mother. In humans this process takes about 40 weeks (280 days). Gestation in fish is very different from mammals as they fertilize their eggs outside of their bodies. Like mammals the gestation period begins when the egg is fertilized, but unlike mammals gestation ends with the egg hatches. This is a much faster process than humans, as this process takes about 2-3 days post fertilization. (1)

Environment

Zebrafish live in a very different environment than humans, and because of this deal with very different environmental pressures. Fish can lay eggs outside of their body without a hard coating, like a chicken shell, because of their aqueous environment. Their eggs will not dry out, and unlike in humans and other land mammals, they do not need to worry about removing waste, as the aqueous environment will clear it away. However, because of being out of the body, fish embryos are more likely to be exposed to environmental contaminants than mammals. This is notable for developmental research, as the embryos are easy to observe and manipulate due to being outside of the mother. For more about keeping alive in a laboratory setting click here.