Final answer:
In a 7-year period, most cells in the human body are replaced at least once, with variation in replacement rates based on cell type and function. Specialized cells like neurons and cardiac muscle cells may never divide, whereas epithelial cells typically have a turnover rate of a few days.
Step-by-step explanation:
In a 7-year period, most of your cells are replaced at least once, making option (d) the correct answer. The length of the cell cycle and the frequency of cell turnover vary greatly depending on the cell type. For example, epithelial cells might turnover every few days, whereas some specialized cells, like cortical neurons or cardiac muscle cells, spend their entire lifetimes without dividing, remaining in the G0 phase of the cell cycle. This is due to a combination of internal mechanisms, such as the specific roles and functions of these cells, and external influences on the body.
The differences in longevity among tissue cells, such as those in muscle, nerve, adipose, liver, and others, stem from their unique functions and environmental interactions. Cells that are frequently damaged or wear out, like skin cells, consistently divide and replace themselves at a much faster rate than those in more protected environments or with less direct physical functions.