As they move, they change their local environment and come into contact with new cells. During gastrulation the cells produced during cleavage begin to move relative to each other. Gastrulation refers to a time of great flux in animal embryos. Read more details about Cleavage here, focus on any section you find interesting and especially figure 8.5. The timing of the MBT and the sorting of maternal proteins and RNAs into different cleavage-stage cells varies widely among different animals. If the drugs are applied to later-stage embryos development halts, suggesting that at this point zygotically derived transcripts are necessary for development and cell division. If these drugs are applied to an early cleavage-stage embryo cleavages proceed normally, suggesting that developmental processes and cell division at this early stage are using maternally-derived factors already present in the embryo. One interesting experiment that shows the extent of this transition is to block transcription using the drugs alpha-amanitin or actinomycin D - two inhibitors of RNA polymerase action. The MBT demarcates the transition between using mostly maternal cell and fate determinants (RNAs and protein) to the embryo producing its own RNAs and proteins. Sometimes this is called the "mid-blastula transition" or MBT. Conditional specification typically begins to occur towards the end of cleavage stages when cell division slows down and cells begin to communicate more extensively with each other. If morphogens are asymmetrically distributed in the egg, then these cells can differentiate via autonomous specification even at early stages. Early cleavage stages are different from later cell divisions in that they quickly produce many very similar looking cells. This ball may be hollow inside, have a hollow region, or be fairly solid. During cleavage a single cell embryo rapidly divides to form a ball of cells, called a blastula. Frog Gastrulation and the Dorsal lip of the BlastoporeĬleavage stages of embryogenesis are typically very similar across animals.
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