We understand why bird embryos depend on a yolk sac for nourishment, as they are in an eggshell. But human embryos also develop a similar sac, beginning about a week after conception. A few weeks later, it atrophies and disappears, without ever containing yolk substance. Could this be an evolutionary leftover from creatures who were hatched from eggs? That doesn't make much sense, because that was a long way back down the evolutionary path. We left eggshells behind, so why not yolk sacs?
A new study reveals that scientists have learned a lot about the function of a human yolk sac. This is where the very beginnings of blood cells are developed, both red and white cells. It's also the place where enzymes and other parts of what will become the human immune system are manufactured. Those important cells are made and sent to the embryo's developing liver, where they will divide and multiply (as biology bends mathematics) to populate the body. Once those cells are "seeded," the sac is no longer needed. Read more about what we've learned about the human yolk sac at ScienceAlert. -via Damn Interesting
(Image credit: Goh et al., Science, 2023)