Tuesday, June 20, 2017

Zebrafish Development: From a Few Cells to an Embryo

Zebrafish are a great model organism used to study numerous biological processes.  Some of the benefits of the zebrafish model are that they mature within a short period of time, they are optically transparent, develop outside of the mother’s body, and are vertebrates.  Because of these, and other reasons, zebrafish are used in a wide variety of applications including drug discovery, developmental biology, and molecular genetics. Zebrafish are easy to analyze in early development, as the embryo is relatively easy to manipulate and visualize.

Zebrafish embryonic development: from 16 cells to 1 day

The video above is a great example of this.  In the video, the development of a zebrafish embryo is highlighted, progressing from a few cells to a 1 day old embryo where the heart is beginning to beat and the blood is beginning to flow. This video was provided from a Cytation 5, with images taken every 2 minutes for 24 hours. The very first panel of the video shows an embryo with about 16 cells sitting on top of the yolk sac.  The embryo is surrounded by a protective membrane called a chorion.  For the first week of its life, the embryo will get all the nutrients it needs from the yolk sac. After the initial division, the cells of an embryo divide every 15 minutes.  Therefore, the cell number quickly progresses from 16 to 128 in one hour.  The cell mass forms a ball of cells on the yolk sac and this point marks the beginning of the blastula period.  During this part of development, the cells change from a high piled cell mound to a cup-shaped multilayer. This multilayer of cells begins to thin and spread over the yolk sac, and the embryo changes shape from spherical to oblong.

16 cell zebrafish embryo
16 cell zebrafish embryo
The next stage of development is gastrulation. Gastrulation starts at 50% epiboly, or when the cell layer is covering about half of the yolk sac. This is the stage when the primary germ layers (endoderm, mesoderm, and ectoderm) are formed. The gastrula period is over when epiboly is complete (i.e. when the cell layer is covering the whole yolk sac) and the tail bud has formed marking the posterior of the embryo. 
Zebrafish embryo during segmentation period
Zebrafish embryo during segmentation period
The next portion of development is my favorite; it is known as the segmentation period. This is when the embryo starts to look like something more than a bundle of cells and the eye and somites begin to appear.  Somites are blocks of mesoderm that form along the anterior-posterior axis of the embryo that will eventually give rise to skeletal muscle, vertebrae, and skin.  Halfway through the video you can see the eye in the upper right hand corner of the embryo, the tail in the bottom right hand side of the embryos, and the somites forming along the body. After the somites appear, you will notice that the embryo body begins to lengthen and eventually the embryo begins to move for the first time.  By the end of the video pigmentation has started to develop on the eye and in the skin and the embryo is at the beginning of the pharyngula period. The heart is just beginning to beat, the blood is beginning to circulate and the embryo is quite active. In just 24 hours the zebrafish embryo goes from a few cells to a live, moving, organism!

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By: BioTek Instruments, Sarah Beckman, PhD., Microscopist

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