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We are the leaves on the Tree of Life

We are the Leaves on the Tree of Life  (Wolf Krebs, retired Prof. of Anatomy)

The unit of life is the cell. All living things on this planet are composed of cells. There are cells, like bacteria, that lead a solitary life and others cooperate to form multicellular organisms, some of which are highly complex and sophisticated like plants and animals. Cells that form animals or plants are nucleated. That means they consist of two compartments, cytoplasm and nucleus. The cytoplasm is the domain of protein molecules. Proteins are the most important molecules of life. They do everything needed for the function of the cell. They can synthesize other molecules needed for specific functions, they can sense physical and chemical conditions outside and inside the cell and they can make the cell react to stimuli in an appropriate manner. Proteins move and stabilize cells. They can do all this and more. However there is one thing they cannot do: they cannot replicate themselves. In the cytoplasm, ribosomes, protein manufacturing units made up of proteins, can read the information of how to compose any protein from molecules called “ribonucleic-acid (RNA).”

The RNA molecules themselves are the product of still other molecules called “deoxyribonucleic acids (DNA).” The DNA molecules are the chemical equivalent of the genes and each DNA molecule represents the blueprint of a specific protein. DNA molecules cannot function in the aggressive environment of the cytoplasm. They have to hide in the nucleus of the cell which is separated from the cytoplasm by a double membrane. If there is a need for a certain new protein, the cytoplasm requests the particular RNA and the proper gene (DNA) in the nucleus produces it with the instruction of how to make the requested protein and sends it to the cytoplasmic ribosomes. This RNA is called “messenger RNA (mRNA).”

All cells on this planet are the product of an uninterrupted chain of cell divisions that may have started with a single living cell about 3.8 billion years ago. Thus, our cells are as old as life itself.

In the course of billions of years, living cells have changed the surface of our planet to suit their needs. They created the biosphere. Although they have been alive for billions of years, all cells in multicellular organisms alive today on Earth will routinely die within the next few years. Only very few survivors, fewer than one in many billions, will sprout into new individuals of their respective species.

Initially single cells ruled the world. A billion years later they started to cooperate to form multicellular organisms which evolved into countless species that populated the oceans. About 1.5 billion years ago the first macroscopic multicellular organisms appeared and only 450 million years ago animals began to invade the dry land. Cells began to grow into humans about 2 million years ago.

You, my fellow human are composed of about 80 trillion cells if you are of average size and weight. Your cells are diploid. That means each of them contains two sets of genes. One set of genes came from your father and the other one from your mother. Your cells are small, invisible to the naked eye. Like life itself, that began with a solitary cell, your life as an individual human started with a single cell too.

Let us go back about 280 days before you were born. At this time, you are two separate cells. One of them is a gigantic cell in your mother’s uterine tube, an ovum (egg cell) about 200 micrometers in diameter, and the other one is a tiny cell in your father’s seminal vesicle, a sperm. The cytoplasm of the ovum is filled with countless yoke vesicles. It is surrounded by a thick membrane and covered with your mother’s epithelial cells.

While you, the egg cell are waiting in your mother’s uterine tube, you, the sperm sperm cell are sleeping in your father’s seminal vesicle together with millions of other sperms. You, the sperm, are tiny. You are a very simple cell and you resemble a microscopic tadpole. You have a large head, filled with one set of your father’s genes. On top of the head is a cap filled with enzymes and at your other end is a cylindrical piece of cytoplasm containing mostly mitochondria, little particles that can produce energy by burning sugar with oxygen. This energy you will need later when you have to use your long swimming tail that is attached to your mitochondria containing middle piece.

Suddenly you are wide awake. You and your spermatic brothers and sisters find yourselves in your mother’s cervical canal now. You sense an odor. You know you have to swim toward it as fast as you can. You do not know that over a million of other sperms are doing the same. You are starting a race of life and death. You are swimming frantically. You give it all you have. You swim through your mother’s uterine cavity, you reach her uterine tube and you arrive at the place where you, the ovum are waiting and where the odor came from. Maybe a hundred of your fellow sperms are arriving with you. Now you are using the enzymes in your cap to drill a tunnel through the epithelial layer and the membrane that protects the ovum. The other sperms are doing the same. However, you are the first to arrive at the cytoplasmic membrane of the ovum, which is you also, and which had waited for just this moment. You, the ovum sucks you, the sperm into its cytoplasm.

At that very moment, these things happen simultaneously: The still diploid ovum (you) rejects any other sperm. The thick surrounding membrane is hardening und becomes kind of a shell. You ( the ovum) expel one set of your mother’s genes. Your spermatic cytoplasm and tail are discarded and metabolized in the egg cell’s cytoplasm. Your spermatic head becomes a second nucleus in the ovum.

Now you are a complete new human individual with two sets of genes, one set from your father and one from your mother.

You just have survived the most dangerous phase of your entire life. As an ovum you are one of very few survivors of over a million competing cells. At puberty your mother had over a million oogonia in her ovaries. Oogonia are the precursor cells of oocytes, the egg cells in the ovaries. You and your siblings are the only winners. The odds on your father’s side are even worse. During his life your father produces over six billion sperms and only one sperm per each of his children survives. You are one of them.

However, you are still in mortal danger. Now, that you have a combination of both your parents’ genes you will produce proteins with your new and unique genetic signature. You are a foreigner in your mother’s body. Should her defending forces, antibodies and white blood cells discover you, they will destroy you mercilessly. That’s why you need the protection of your shell envelope which still has all of your mother’s characteristics.

Before you can begin with the business of growing, your two nuclei have to be combined. A billion year old dance begins. As your genes cannot be in the cytoplasm unprotected and since both nuclear envelops must be opened, the genes in both of your nuclei hide in chromosomes. Chromosomes are long proteinaceous structures which have pockets for the genes. The human cell has 46 chromosomes, 23 per set. Each chromosome is unique and it has specific hiding places for specific genes. Right now 23 chromosomes are filled with your father’s genes and 23 contain your mother’s. Your paternal and maternal individual chromosomes match each other perfectly with one possible exception, the 23rd chromosome. This is the sex chromosome which is better known as the X or the Y chromosome. If you are female, you have gotten the genes that belong into the X chromosome from your father and from your mother. Thus, you have two X chromosomes. If you are male your mother gave you the genes for one X chromosome and your father provided those that fit in the Y. The Y chromosome contains only a fraction of the genes that belong in the X and it is much shorter. So, as a male you have one long X chromosome partnered by a short Y. Thus, genetically males are imperfect females.  As soon as all your genes are inside of their chromosomes, both of your nuclear membranes disappear. Your chromosomes perform a mating dance choreographed billions of years ago. Once the corresponding maternal and paternal chromosomes are paired, a new nucleus forms with a proper nuclear membrane. Inside of it, the chromosomes release their genes and dissolve. You are ready for growing.

You begin to divide. All cellular divisions that follow involve the formation of chromosomes. However your paternal and maternal genes are now mixed within each chromosome pair. During the first cellular divisions you are not growing in volume. With each division your cells get smaller until they have reached the proper size of human cells. As an ovum you have been about two thousand times larger than an average cell. Thus, you still fit into your protective shell for a while, drifting down the uterine tube toward the uterine lumen. Early during this time, your cells split into two different cell lines: body cells called soma or somatic cells, and germ cells. The somatic cells are the ones doing all the growing. The germ cells just wait at the periphery of the action. The somatic cells are changing while dividing. By activating some genes and blocking others, they differentiate into various cell types. They become the typical cells of the tissues of the human body and they will form all your organs and body parts. Once they have differentiated into tissue cells, they have to remain in this form until they die. The germ cells remain perfect copies or the original fertilized egg cell and each of them has the potential to become a perfect clone of you. This happens if you have identical siblings.

You have to hurry. When you reach the uterine cavity you have outgrown your capsule. You hatch by cracking the shell, and naked now, you are exposed to your mother’s defenses. You slip through the uterine epithelium into the uterine wall where you are safe for now.

You are sending out chemical signals and your mother comes to the rescue. Together with her, you are growing a protecting skin-like envelop, the chorion. The inner leaflet of the chorion is made up by your own cells and the outer one is covered by your mother’s cells thus camouflaging you from her own antibodies. The chorion becomes a fluid filled bubble in which you swim. Nutrients and oxygen are absorbed by the chorion and sent to you via a thick stalk that connects you to the wall of the chorion.

You will be safe in the bubble of the chorion and you will stay in it until your birth. In addition to the chorion, you surround yourself with a second bubble, the amnion. Both of these bubbles are filled with a fluid that resembles the chemical composition of the ocean in which you swam about 500 million years ago. Inside of your double walled cocoon, you safely can get back into the uterine cavity. The part of the chorion that connects to you via the above mentioned stalk will remain attached to the uterine wall. It will develop into the placenta, where your blood capillaries are suspended in your mother’s blood. The stalk becomes the umbilical cord. You get all your nutrients and oxygen through the placenta from now on. And you will send chemical signals to your mother to make her provide you with whatever you need.

Your somatic cells remember their past in a sketchy and distorted manner. At some point, they begin growing into gills of the fish, that they had been millions of years ago. However, you do not need gills in your mini ocean. You get everything you need from your mother via the placenta. Thus, your gills change into structures of your neck and face.

Your body cells do not know what sex they are. So, they begin to grow into testes and ovaries simultaneously. And that is the moment, the germ cells have been waiting for. By now they are a few thousand in number, and they know exactly what sex they are. As soon as they sense, that ovaries and testes are being developed, they start moving. Male germ cells will migrate toward the testes and you will become a man. The female germ cells will invade the ovaries and you will grow into a woman. Once established in either testes or ovaries, the germ cells become oogonia or spermatogonia. From now on, the germ cells direct your sexual development. They force the somatic cells to abandon the further development of the gonade of the opposite sex. They force them to finalize the proper sexual organs and to grow into gender specific humans.

As you are growing, you become more and more perfect. All your tissues and organs are in place and functioning. Approaching the 280th day of your life you are ready to face the world. You signal to your mother that it is time and she initiates the sequence of your birth.

In a little less than ten months you have grown from a single cell into an organism of about four trillion cells. During your childhood and adolescence you grew into a mass of about 80 trillion well organized somatic cells that are hosting several millions of germ cells. If you have children one germ cell per child could escape mortality by sprouting into a new human. All the other cells, somatic and germ, are condemned to die.

We are the leaves on the tree of life and the germ cells are its branches.

#Prenatal life #Becoming a human Human biology Popular Science

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