Final answer:
Human development from a single fertilized egg cell, or zygote, to a multicellular organism with trillions of cells involves rapid cell division, which slows down as the organism reaches adulthood. Cell division continues throughout life mainly for tissue maintenance and repair, influenced by both internal mechanisms and external factors like growth hormone levels. The cell cycle, however, is highly regulated and disruption can lead to diseases such as cancer.
Step-by-step explanation:
Cell Division Throughout the Human Lifespan
From the moment of conception, a single zygote commences a series of cell divisions to develop into a multicellular organism with trillions of cells. During the initial stages of human development, cell division is rapid, particularly in the embryonic phase, where cell cycle times can be as short as a few hours.
As development progresses and an organism grows, cell division continues but at a rate that balances with cell death, thus maintaining tissue function and organismal homeostasis.
As an individual reaches adulthood, certain cell populations, like blood and skin cells, continue to divide frequently, whereas other specialized cells such as cortical neurons or cardiac muscle cells enter into a phase called Go, where they remain without dividing for the individual's lifetime.
The cell cycle is influenced by both internal and external factors. For instance, growth hormones like human growth hormone (HGH) promote cell division, while the cell's size and nutrient availability can limit growth and trigger division when the cell becomes inefficient.
In pathological conditions, such as cancer, regulation of the cell cycle is disrupted leading to uncontrolled cell proliferation.
Thus, the balance between cell division and cell death is a finely tuned process that changes throughout the lifespan, from rapid division in embryonic development to a more regulated and specific pattern of division in adulthood for maintenance and repair.