Embryonic Cells
The concept of each normal, mature cell having a specific morphology and function (or set of functions) is astounding when one considers that all humans started life as a single cell. Although human embryonic cells, those present from conception until postconception day 8 are clearly normal, for a short period of time their behavior and characteristics are very different from the behavior and characteristics of mature differentiated cells.
Growth Characteristics.
The main activity for an early embryonic cell is to undergo mitosis and expand the size and cell numbers of the embryo. Cell division for early embryonic cells is controlled but very rapid.
Cell Cycle.
Early embryonic cells move through each phase of the cell cycle in a specific sequence. Conditions surrounding the embryo are so favorable that as quickly as a cell completes mitosis, it is ready to re-enter the cell cycle. Although these cells are under restriction point control (checkpoints), they do not spend a significant amount of time in G 0 . It appears that early embryonic cells continue to enter the cell cycle even when contacted on all surfaces by other cells. Thus, they do not exhibit density-dependent contact inhibition.
Phenotypic Characteristics.
Early embryonic cells are functionally and morphologically undifferentiated. Anatomic features that distinguish the future neuron from the future myocyte are absent. Early embryonic cells appear small and round (anaplastic morphology) and are unable to perform differentiated functions. In addition, they do not synthesize any of the intracellular or cell-surface proteins found on normal differentiated cells. As a result, they loosely adhere to each other and continually reposition themselves within the growing embryonic ball. At this developmental stage, embryonic cells have the potential to mature into any kind of differentiated cell. This flexible state is referred to as pluripotent, multipotent, or totipotent. The characteristic of pluripotency coupled with rapid mitosis allows early embryonic cells to survive and progress even when conditions are unfavorable. If exposed to lethal conditions at this stage, resulting in the destruction of 90% of the embryo, the remaining 10% continue mitosis and simply replace the lost cells. Such a situation would delay but not disrupt cellular development.