At birth, a woman has approximately one million eggs stored in her ovaries. At puberty 400,000 will have survived. Of these, only about 500 will be released through ovulation . Each of them is surrounded by other cells in a structure formed by a thickening in the wall of the ovary. Once every four weeks one of these structures bursts and releases an egg. If at that moment there is a stream of sperm in the fallopian tube, it is likely that one of them will get past the wall of the egg and fertilize it. The resulting cell, the zygote , already contains all the information necessary to build a human being.
Over the next four days, as it makes its way from the fallopian tube to the uterus, the zygote begins to divide. By the time it reaches its destination , 8 to 16 cells have already developed . For the next two days, division continues until it becomes something like a thick-walled, hollow sphere with a bulge at one point: the blastocyst . From the protuberance the embryo will develop and from the wall -along with material from the mother- the placenta. Six days after fertilization, the blastocyst implants in the uterus: it is the embryo. Sixty days later, the term that describes the future human being is called the fetus.
Being human is not easy, even from the moment of fertilization. Of all the zygotes, only 75% implant correctly in the uterus and of those only 60% survive the second week. In other words, only 45% of the fertilized eggs will end up giving the unequivocal signal to the woman that she is in the process of becoming a mother. But from there the situation does not improve. Only 72% of what we can call pregnancies end in birth. Therefore, less than a third of all conceptions lead to a fetus.
We are not so different
Billions of years of evolution must leave their mark. The system for producing the proteins that direct the chemical reactions of the fertilized egg is the same in all living things. At this stage of development, the human being, the fish, the sequoia and the fungus resemble each other like two drops of water. In a way, we are like 2,000 million years ago, when the first eukaryotic cell appeared, with a nucleus. Now, at no point in the development of a fetus do humans share structures with prokaryotes, with bacteria – despite the fact that a large part of our genome is made up of genes from bacteria and viruses.
With the first cell divisions, our resemblance to unicellular beings, plants and fungi disappears. In flowering plants, the first cell division produces offspring that are programmed to carry out certain well-defined tasks. One of them divides to produce a structure analogous to an ovary, while the others produce structures intended to attach that “ovary” to the outer covering of the seed. It is as if the first division of a plant zygote produces a kind of miniature plant. This does not happen in animals, although in the seventeenth century some scientists believed that the sperm housed a tiny human being inside it and that the egg was the pantry from which it fed.
In the first divisions of the human zygote, when it goes from 1 to 2 to 4 to 8 cells -the blastula-, we have a system similar to that of the rest of the animals . It becomes so similar that at the beginning of the 20th century, scientists who wanted to study the development of the blastula used sea urchins.
On the nineteenth day the central nervous system begins to form and at the end of the fourth week a very clear line is perceived that will end up becoming the spine, while the formation of the intestine, liver and heart begins. Right now our resemblance to other vertebrates, from fish to mammals, is uncanny. It was this resemblance that gave rise to the famous phrase “ontogeny (the development of an individual) summarizes phylogeny (the development of a species)”.
At eight weeks the embryo is perfectly differentiated from reptiles and birds , it continues to retain the resemblance to other mammals and the distinctive characteristics of primates begin to be prepared, which will clearly acquire them at the end of the first trimester. It is curious to see that by the fourth week the nervous system of a human embryo is practically identical to that of a hen.
what makes us human
The development of the brain in the embryo and fetus reminds us of the evolution of vertebrates . Fifteen days after conception the embryo’s brain resembles that of the frog, with an olfactory bulb protruding in front of a tiny cerebrum, followed by the eye bud, and finally the cerebellum and spinal cord. In the following weeks, the heart of the diencephalon grows – the place where emotions settle and which regulates blood pressure; comes from the midbrain of reptiles and frogs – while the olfactory bulb shrinks almost to nothing. The two hemispheres of the cortex rapidly grow to surround the diencephalon. At the end of the third month, its appearance is very similar to the adult human brain, but it lacks connections in its cerebral cortex.
Despite what we may think, what distinguishes our brain from that of other animals is not that it has completely new structures , but that an existing one has grown to such an extent that it has acquired new and totally unexpected functions. If we go up the scale of vertebrates, the weight of the cortex is increasing until it reaches us, where it reaches 70% of the total weight of the brain. We are not different because we have bark, but because it is big.
But what makes us human is not having a brain, but rather that its cells perform the function for which they have been chosen. So, when can we say that the brain of the human fetus begins to function as such? Most of the brain cells are produced in the early stages of pregnancy . From the second to the fifth month they grow and migrate towards their final positions, in the sixth they differentiate and form the structure of the cortex. And when most of the nerve cells are in place and everything is ready, large-scale synaptic firing kicks in: the fetus is in its seventh month of life. In fact, different studies -such as the fundamental ones carried out by the dean of the Albert Einstein College of Medicine in New York, Dominic Purpura- show that the connections in the beginning of the visual cortex begin at 25 weeks.
But not everything ends here. After birth, the brain multiplies by three in size and the number of synapses continues to increase for several years. All this to end up having a brain with some ten or fifteen billion nerve cells and a hundred billion synaptic connections. Just for comparison, the intestinal parasite, the Ascaris roundworm, has 162 nerve cells, not one more, not one less. And with them he lives without complications: he learns, has a memory and acts according to the information that comes from outside. His mental capacity isn’t great, but he manages pretty well. And what about the busy bee, which with its 7,000 nerve cells (a factor of two million less than us) maintains a hive, detects colors, knows the time of day, finds and remembers paths, estimates distances, transmits information and makes complicated flights?
Own elaboration
