Zebrafish - Blastula Period
The duration of this period is from 2.25 to 5.25 hours
- Transition from 128 cells to 50% epiboly
- It is called the Blastula period as the blastodisc begins to look ball like here until the onset of Gastrulation
- The orientation of the cleavage plains is indeterminate and the cells do not sit in an organized manner any more
- Embryo enters Mid-blastula Transition (MBT) where the yolk layer forms (YSL) which occurs when the cells that are closest to the yolk fuse to it and the epiboly begins
“Epiboly, beginning in the late blastula (Solnica-Krezel and Driever, 1994), is the thinning and spreading of both the Yoke Syncytial Layer (YSL) and the blastodisc over the yolk cell, as you might model by pulling a knitted ski cap over your head” – Stages of Embryonic Development of the Zebrafish, Kimmel et al.
During the Blastula Period the embryo changes shape from spherical to oblong. The egg goes through mid-blastula transition (MBT), which is when zygotic gene transcription occurs and the yoke syncytial layer (YSL) is formed when the cells closest too it fuse together. The beginning of epiboly begins during this period. This type of cell movement is where all embryonic cells move so that the ectoderm, or outer layer is on the outside of the forming embryo; however note here that the ectoderm does not completely form until the Gastrula Period.
- Student animal model - zebrafish project contributed page 2009.