epithelial layers contain no blood vessels, so they must receive nourishment via diffusion of substances from the underlying connective tissue , through the basement membrane.
Do you mean "why"?
They are the main source for all living organisms because they provide energy, relatively short term, for any creature that consumes them.
Chromosomes<span> first become </span>visible during<span> prophase in mitosis, and they are fully</span> visible during<span> metaphase and anaphase. They lose their visibility </span>during<span> telophase and are not </span>visible during<span> cytokinesis, which follows mitosis.</span>
