Early Growth
Early growth begins with cleavage, the process of becoming multicellular and then follows with gastrulation, the process of becoming multilayered. Different organisms complete these processes in different amounts of time. For some organisms, such as amphibians, their growth strategy is to store some yolk in their oocyte, but more importantly produce large amounts of ribosomes, messenger RNA and transfer RNA in order to complete growth as quickly as possible in order to become independent early on. Other creatures, such as fishes, reptiles, and birds, produce a huge yolk, which will provide nutrients for the organism as it grows much more slowly. Mammals, which have the advantage of developing while inside of the mother, take a longer time to go through the stages of development. These eggs are called isolecithal eggs. Since this project focuses on the gene therapy at early stages of growth for mammals (blastomere germline gene therapy), we will focus on the mammalian isolecthical eggs.
Growth of Mammals-Isolectithal Eggs

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Cleavage of Human Embryo

Cleavage


Mammalian cleavage is holoblastic, where the entire mass divides. Unlike other organisms, however, the divisions are asynchronous, so that one cell divides into two cells, which divide into three cells, and then four cells. After four days of cleavage, cells begin to form around a small cavity, in a process called cavitation. The small space is called the blastocoel and the whole embryo is called a blastocyst. At this stage, the outer layer of cells is called the trophoblast while the small grouping of cells inside of that is the inner cell mass. (43)
 

Gastrulation

The inner cell mass has two layers. The top layer is called the epiblast, which will eventually form the mass of the body and the bottom layeris called the hypoblast, which will migrate around the trophoblast to become the yolk sac. Cells from the epiblast migrate between the layers through a process called ingression. The first cells to ingress form the later endoderm, the next form the mesoderm, and the cells that remain in the epiblast form the ectoderm. The embryo now has three layers. (43)

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5-day old Human Blastocyst

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The Process of Ingression

Regulation and Manipulation of Growth


Through many experiments it has been found that during the very early stages of development, cells have regulative abilities that allow formation of the embryo to proceed even if major changes are made to the embryo. Cells can react to their surrounding environments, and therefore at initial formation, their fates are not predetermined. One cell from a two or four cell embryo can develop into an entire organism if taken on its own.


It also means that even when cells are added or deleted very early on, development can usually continue normally. Cells added from other sources produce chimeras or mosaics, offspring with mixed genetic components. For mammalian development, experiments using marked cells have determined which cells in the early cleavage stage give rise to the trophoblast versus the inner cell mass. (44)
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