Consider for a moment that at one point you were a single cell or a zygote. Over less than a year you went from a single cell to a helpless infant. This is my son when he was about a day old, and now he's learned to walk, talk, run and there's still so much that he's learning. Maybe someday he'll donate a cell with another adult and have a tiny human of his own. No matter what your life has been like, you've had an incredible journey from a cell to an adult human being. An interesting artifact of our current academic system is that if you want to take a course that covers conception to birth, you'd look in a biology department for a class in developmental biology. If you wanted to take a course from birth to adulthood, you'd look not in a biology department, but rather in a psychology department and take human growth and development. This module is going to combine both. We're going to start with conception and move through to adulthood and then close with a discussion of regenerative medicine. So the beginning, in course 2, we talked about the amazing things that you can do with the single cell. One example was to make a new human. At all animals have a special subset of cells called gametes. We talked about gametes briefly in course 2. Gametes are special because they are half cells. They have half as much genetic material as the rest of yourselves. That way when the female gamete or the egg meets the male gamete or the sperm, they fuse together to form a zygote and the resulting cell or zygote will have genetic material from each parent, be a wholly unique individual, and also have the right amount of DNA. Mom typically only has one egg available for consumption each month. So what happens if the egg isn't fertilized? She gets her period. Eggs are only viable or healthy for a short period of time. There's only a small window each month when conception can actually occur. Sperm can live longer in the reproductive track, and if sperm are present prior to ovulation, which is when an egg is produced, conception will occur. Conception is the process where the egg and the sperm fuse together to make a zygote. What happens next? The zygote first mitotically divides, so remember mitosis from course 2? Mitosis is the process of making identical cells. Remember PGD, one of the genetic technologies we discussed in course 2, and I'm always plucking off one of these identical cells and sequencing the DNA. After dividing to form more cells the zygote is instead called a blastocyst. The blastocyst finds its way to the uterus and implants. You can think of the uterus is like a large muscular bag that provides a home for a developing fetus that's able to expand out as a fetus grows. Once implantation occurs, mom and the future baby have now established a physical connection for the first time. You ever heard of an ectopic pregnancy before? This occurs when the zygote doesn't find its way to the uterus and implants somewhere else, causing a life threatening medical emergency. Implantation takes anywhere from 7-14 days to occur. Some women say they can feel it, others will observe a small amount of bleeding at the time of implantation. Once implantation occurs, the real fun and biological magic begins and positive pregnancy test typically follows a few days later. My personal experience was that it was very obvious that I was pregnant even before I took a pregnancy test. This picture was taken about a month before my due date. So how did I know I was pregnant? Well, my breasts were soar, and every time I sat down, I would fall asleep. Even more interesting, a few days before my period was due, I was at the doctor's office really really sick. I had strep throat and bronchitis at the same time. The doctor's response before anything else is said was, "Is there any chance you're pregnant?" So what does bronchitis and strep throat have to do with a pregnancy? Well, one of the biological changes that happens at the beginning of a pregnancy is changes with the immune system. Our immune system is a system in our body that helps protect us from foreign invaders, that keeps us from getting sick. To be able to sustain a pregnancy, a woman's immune system needs to relax because the child can be seen as a foreign invader just the way that germs can be. If our immune system is relaxed so we don't reject the pregnancy, well, well someone is more likely to get sick too. That's why I simultaneously having multiple infections at once without any underlying medical conditions was a tell-tale sign that I was pregnant. So now that implantation has occurred, what happens next? The blastocyst will begin going through a process called gastrulation. Gastrulation is when the specific cell and tissue types begin to form. Those identical cells and the blastocysts are now going to become programmed to become different cell types. They are going to beginning their job so to speak, this is also when the placenta begins to form. The placentas are really fascinating structure, it's an organ that's grown especially for its pregnancy and then lost at the end of the pregnancy. The placenta and umbilical cord connect mother and child and facilitate the exchange of nutrients and wastes. The placenta also helps to protect the developing fetus. For example, there are enzymes in that placenta that breakdown stress hormones before they can reach the fetus and mitigate any possible damage. We talked a bit about prenatal programming and epigenetics in course 2. Around four weeks after conception, the blastocyst graduates to embryo status and has primitive organ structures like a rudimentary heart and the brain. At 12 weeks post-conception, the embryo graduates and becomes a fetus. The fetus is the last stage until birth, is when the rudimentary organ systems continue to develop while the fetus grows exponentially in size. I really like the following quote by Veronica Van Heyningen, a medical geneticist to describe prenatal development. "The amazing thing about mammalians development is not that it sometimes goes wrong, but that it ever succeeds." Like we talked about in course 2, with the development of cancer in order to get a fully functional healthy baby, development relies in certain genes being turned on at the right place and at the right time. Some genes are needed at certain times to form correct patterns. For example, sonic hedgehog has a protein involved in pattern formation in our hands. If sonic hedgehog is not expressed properly, you get extra fingers just like those hemingway cats we talked about in course 3. It isn't just hiccups and gene expression, neither they can cause trouble, the environment is also important too. Consider for a moment, what are some things that are off-limits to pregnant women? Teratogens are a common example. A teratogen is something in the environment that can cause damage to the fetus. This includes cigarette smoke, alcohol, and certain prescription medicines. Unfortunately lacking good research on the safety of many prescriptions and over-the-counter drugs, they tend to be off-limit to pregnant women because we just don't know if they're safe. We don't have good data on many common medicines because the potential risk of testing women and their unborn children is considered to be too great. In certain situations, the risks to the mother outweighs the potential for harm for the baby, and so the mother will take the medicine but be very closely monitored. We see the same issues with breastfeeding as well as many medications will also pass into breast milk at some level. There are some foods that are off-limits too. I really missed eating unpasteurized soft cheeses like Gorgonzola when I was pregnant. Since soft cheeses are unpasteurized, which is when we treat the milk to kill off microorganisms, they have great flavor, but also come with an increased risk of listeria infection. Listeria infection when pregnant can lead to a miscarriage or a stillbirth. Also, as we talked about in course 2, many varieties of seafood are also off-limits due to concern with mercury. Since prenatal exposure to mercury can lead to significant physical and intellectual problems in the child. Some periods during prenatal and postnatal development are called sensitive periods. These are times during development when a child is particularly susceptible to environmental influence for the good or the worse. For example, perhaps a child may be more sensitive to environmental toxins during early organ development. Alternatively, there are periods of development when a child is more likely to pick up a new skill, such as being able to learn a new language. All right, so now that we've covered prenatal development, let's transition over to what happens next and into the realm of psychology.
