HUMAN REPRODUCTION
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By now, you already know many things about human reproduction. You know, for instance, that it involves sexual organs, and that it produces sex cells, which are haploid (half of the DNA of a complete human!)
As soon as the two haploid cells get together, the egg is fertilized, and the action immediately starts to happen. We now call this fertilized egg a zygote. Within about 30 hours of the fertilization, the zygote divides by mitosis for the first time. From then on, cell division continues at such a fast rate, that there isn't even time for the cells to grow during interphase.
They just keep getting smaller and occupying the same spot!
As the cells divide, they reach a stage called morula, of about 8 to 32 cells. It's an important stage because at this stage, the cells can be anything -- a leg, a brain, a lip. And it's during this stage that the cells start getting really tight to each other, forming small little populations with distinct areas, which will eventually become just that! These little populations get really tight within their groups, so tight that the cells are no longer round, but more like a polygon.
Check out the picture above; the bigger area in the blastula will become the placenta, and the smaller mass will be the human.
After several rounds of division, and after travelling for about a week, the morula is having a blast. It starts dividing into two clear different masses. One is going to grow into a human being, and the other one will grow into the placenta! --This new structure is called a blastula, or blastocyst (see what I did there?)
Pause for Stem Cell Explanation!
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During the blastula stage, the cells can become anything. These are what we call stem cells. These cells are known as pluripotent cells. Under the right conditions, they could grow a lung, a heart, skin, and all different parts of an organism.
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For decades, scientists have been studying stem cells to produce specific cells, such as cells that produce insulin. Current uses include bone repair and cartilage damage. Stem cell transplant is a crucial treatment for some kinds of blood cancer, as it successfully replaces the cells that produce blood cells with new cells, which quickly learn what they are supposed to be doing.
During the blastula stage, and after a week of travel, this is about the time it arrives at the uterus and finally implant in the lining of the uterus, which is by now swollen like a pillow and ready to support life. This is where the development will continue.
Inside the blastula, specific areas start to form. These areas will be responsible for certain organs and systems.
Check out this half-cut blastula, and the areas already dividing!
Inside the blastula, specific areas start to form. These areas will be responsible for certain organs and systems.
Check out this half-cut blastula, and the areas already dividing!
If the implantation to the lining of the uterus goes well, the body sends chemical signals to stop your period. During this time, different hormones will also be released to support the pregnancy as well as to eventually produce milk. Over a period of about 38 weeks, a zygote undergoes a complex sequence of changes as the cells multiply and the mass grows to form the many billions of cells that make up an infant's body.
The embryonic stage lasts from about the third week of pregnancy until the end of the eight week of pregnancy. The mass of cells is then called a fetus until birth.
Each month of development corresponds to specific organs and systems being formed. Below, a table of all the major things that happen in human life development:
Having a baby may seem pretty simple, but many things can go wrong. Sometimes, a baby is born too early; this is called a preterm baby, or a preemie. The earliest baby that survived only spent 5 months in uterus. The main issue for preemies is the lungs; as organs develop in different stages, and lungs' function is to take oxygen from the air, other organs develop first because it only makes sense... the lungs should not be needed until the fetus is 9 months old. As a lot of growth happens on the last few months of pregnancy, preemies can be quite small.
Some people also have miscarriages, which is when a fetus gets released from the uterus too soon for survival. This can happen for many reasons; one of them, as we mentioned before, is when a mutation would have made it impossible for the fetus to survive, so the body aborts the fetus.
Twin Science
We already mentioned twins before, but let's review them again! Twins can happen in two different ways: They can be identical twins (monozygotic) or fraternal twins (dizygotic).
Fraternal twins have completely different genetic material from each other. This happens when two (or more!!! Eyooo) separate eggs are fertilized by two (or more!!!) different sperm, resulting in two (or a litter) genetically unique babies who can be different sexes. Identical twins result from a single fertilized egg that splits into two embryos, meaning the babies share the same genetic material and are always the same sex.
Fraternal twins have completely different genetic material from each other. This happens when two (or more!!! Eyooo) separate eggs are fertilized by two (or more!!!) different sperm, resulting in two (or a litter) genetically unique babies who can be different sexes. Identical twins result from a single fertilized egg that splits into two embryos, meaning the babies share the same genetic material and are always the same sex.
Vanishing Twin Syndrome
During the first trimester of a pregnancy, one of the twins can reabsorb the other twin. This can happen naturally when early enough, and the mother would not notice. When this happens, it can cause a condition called chimerism. This means one individual will be made of cells from two different people. In reality, this is more common than originally thought; we just rarely notice this in our daily lives.
Taylor Muhl is a talented singer. She also happens to be a chimera, someone who carries two distinct sets of DNA, each with the genetic code to make a completely separate person.
Sometimes, the absorption is not complete, which gives us cases of conjoined twins. Sometimes, the twins can be separated, but depending on how many organs are involved and how the vascular system is divided, they may not be able to be separated. There is speculation that conjoined twins can also happen if something goes wrong in the beginning of the cell division process, and somehow two beings are formed still attached to each other.
I put a documentary below which I have not watched, but the case of Lydia Fairchild is such an amazing case, I had to share. If we have time, let's watch it together!
I put a documentary below which I have not watched, but the case of Lydia Fairchild is such an amazing case, I had to share. If we have time, let's watch it together!
Where is the Human?
The embryos below belong to different animals: chicken, fish, human, salamander, tortoise and rabbit. Can you decide which one is which?
How about now? Did you change your answers?
Final one!
Why do you think there was such a similarity at early stages of development? How were your predictions?
Mutations in Humans
Mutations happen in any species. In humans, many mutations happen often enough that they can be tracked, compared and given names. That's the case with Down Syndrome, for instance. When a human is born with an extra chromosome 21, this can lead to changes in many areas of the body, such as appearance, development delays and poor muscle tone.
Other mutations can cause physical differences, but no development delays; this is the case of Waardenburg syndrome, which causes wide-set eyes that are often of a different colour, sometimes hearing problems and a characteristic white lock of hair in the front. This is an interesting syndrome as it is caused by mutations in several different individual genes, which is what causes the variations from person to person.
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Neurodivergences such as autism, ADHD and dyslexia, are caused by a complex interplay of genetic, biological, and environmental factors that lead to natural variations in brain development. These factors influence how a person's brain processes information and can be present from birth or influenced by events during a person's life. The ability to focus on things for an extended period of time makes some neurodivergent individuals extremely beneficial for humanity as a whole, and it is one of the reasons why many scientists and inventors throughout history are speculated to have been neurodivergent, based on their history, social challenges and intense areas of focus. Many people also suggest leonardo da Vinci may have had ADHD, due to his wide-ranging interests, difficulty finishing projects, and other behavioral patterns.
Ethics discussion: Reproductive Technologies?
Sometimes it is just hard for some people to carry a fetus to term. In those cases, options such as in vitro fertilization and surrogates can be used, for a price. In vitro fertilization (IVF) is a fertility treatment where eggs are retrieved from a woman's ovaries and fertilized by sperm in a laboratory. The resulting embryos are then transferred to the uterus to establish a pregnancy.
There is so much more about human reproduction we could talk about! But it's time for us to move into chemistry. I hope you guys have fun!
Vocabulary
fertilization
zygote
morula
blastocyst
embryo
fetus
monozygotic
dizygotic
chimerism
preterm
vanishing twin
conjoined twins
identical vs fraternal twin
miscarriage
mutation
in vitro
zygote
morula
blastocyst
embryo
fetus
monozygotic
dizygotic
chimerism
preterm
vanishing twin
conjoined twins
identical vs fraternal twin
miscarriage
mutation
in vitro
