This is the first installment of blastocyst blog; but it's a bit of a pre-requisite. To give you a feel of where we are going, I will start with pictures of eggs and embryos and then blastocysts.
This is an egg. I doesn't really look like an egg. Part of the reason for this is that in this picture there are hundreds of cells, but just one that is an egg. The dark circle in the center is the egg. You can see how big eggs are compared to the rest of our cells. The surrounding specs are granulosa cells. These are the ovarian cells that line the inside of the follicle. Prior to ovulation, the egg's position in the follicle is along the edge, so the granulosa cells that are growing along the inside of the follicle surround the egg. When the egg ovulates, it carries some of these cells along. When an egg is retrieved during IVF, it is also surrounded by granulosa cells.
The granulosa cells make the estrogen (estradiol). So as the follicle grows, more granulosa cells form, and estrogen rises. In an IVF cycle, the more eggs there are, usually the higher the estrogen levels.
This is a picture of an egg a few hours after retrieval, after the granulosa cells have been removed. In the case of IVF using ICSI, the embryologist needs to remove the granulosa cells a few hours after retrieval. This is necessary so she can see the egg and to properly inject the sperm. If ICSI is not necessary, we can mix the eggs and sperm together, and the sperm will swim through the granulosa cells to get to the egg.
The little round object on the top is the first polar body, and this is an indication that the egg is mature. The first polar body contains chromosomes, as does the larger egg cell. For the egg to accept the DNA of the sperm, it needs to dump some of its own DNA, otherwise there will be too much. So the egg unloads some of the DNA into the polar body, which just withers away. Sometimes testing the DNA of the polar body can tell us about genetic diseases in the egg. For the most part, we can not use an egg that is not mature. There is some encouraging research looking at maturing eggs in the lab, but so far the process of maturing eggs in culture has not been widely accepted.
This is what we call a 2 pn zygote (or 2 pn embryo). The picture was taken one day after the retrieval. You can see a few granulosa cells still hanging around. The halo around the embryo is the zona pellucida. It's the shell of the egg. It has the consistency of a thin vitamin E capsule. Inside is the egg (or oocyte). In the middle of the egg, you can see 2 little round objects, and these are the pronuclei (pn). One contains the genetic material from the egg, the other from the sperm. In some animals we can tell which came from where, but not in the human, although as our microscopes improve, I suspect we will very soon be able to tell. So if we expose eggs to sperm, and look the next day, and do not see 2 polar bodies, fertilization has not occurred. Sometimes we see one, and this means fertilization possibly occurred. In this case we may or may not see 2 later in the day. The 2 pn will combine to complete the fertilization process.