Two of the dogmas that most biology students learn in school are that cardiac muscle is not renewed, and that women (and almost all female mammals) are born with their full complement of eggs - they can't make new ones. Two recent studies have shown that these dogmas may not be as clear cut as they originally seem to be.
The first of these studies (New York Times article) was carried out by a team at the Karolinska Instituet in Sweden. They wanted to see if any of the cardiac muscle cells in humans were formed after the person's birth. In biology, the classic method to do such a tracing experiment in animals is to feed the animals some sort of radioactive tracer, which would then become incorporated into the DNA of cells that are being formed during the period that the the radioactive tracer was being fed to the individual. Cells formed after that period would not have radioactive tracer in their DNA. Obviously there are ethical implications of feeding humans radioactive tracers, but the experiment has already been done on unwitting subjects at a global scale.
During the 1950s, Cold War powers tested nuclear bombs above ground, releasing significant and measurable quantities of carbon 14 into the atmosphere. Individuals born during that time would have incorporated this carbon 14 signal into their tissue. After the test ban treaty in 1963, carbon 14 levels gradually diminished, and this changed isotope ratio can be detected in DNA synthesized after that date. The team applied their method to heart muscle cells, and found that for individuals born before the test ban, some of their heart muscle cells had a lower carbon 14 isotope ratio, meaning that those cells were produced after that date - evidence for regeneration.
The second study was carried out at the Shanghai Jiaotong University, and involved mice and not humans (NY Times article, Science feature). They also used a really cool technique, but one which operates on a less grandiose scale. Before this study, the debate in the field was whether supposed female germline stem cells (FGSCs) in mouse ovaries thought to be responsible for generating new oöcytes were in fact capable of doing so. The method used to isolate the FGSCs is called immunomagnetic isolation. Antibodies to a protein found only on FGSC surfaces were raised, and coated onto magnetic particles. Therefore, a magnetic filter would be able to isolate the supposed FGSCs. After isolation, they were transformed with green fluorescent protein, and introduced into the ovaries of sterile mice. The sterile mice were then mated with normal males, and offspring, expressing green fluorescent protein, were produced. This demonstrates that ovaries possess stem cells that are capable of regenerating oöcytes. Given the similarity of mouse and human reproduction, it is possible that humans have similar capabilities.
In both cases, therapeutic applications are a long way off, but it is still intellectually exciting to be aware that what has long been "known" as fact is still open to reassessment by pure empirical work, showing that biology is still a young and growing field.