Transformation: development and maturation of the lungs. Why premature babies aren’t “little people” yet»
November 23, 2020
On July 19, 2012, the respected new England Journal of Medicine published an article titled ” Evidence of lung growth in an adult.” There was a sensation behind the dry name. What happened? A thirty-three-year-old woman suffering from cancer had her right lung removed. Numerous post-operative examinations over the course of fifteen years showed that the left lung was gradually growing. At first, this fact did not seem worth attention, since the remains of the lungs after surgery often increase in size — they simply have more space in the chest. But in this case, the increase in size was due not just to expansion, but to the growth of new tissue in the left lung. Nothing like this has ever been observed before.
This medical report was a double sensation. First of all, he refuted all previous ideas about the development and growth of the lungs. Previously, it was considered beyond doubt that the lungs develop in the womb and to some extent continue to do so until the child is 10 years old — by which time the number of alveoli reaches a maximum. Further, although the lungs increase in size (due to the volume of the chest), but the formation of new lung tissue is finally stopped. However, it turned out that the lungs can grow even in adults, that nature has managed to re-launch the already completed program for the development of this organ. Isn’t this the key to treating lung diseases, the consequences of which were previously considered irreversible? Maybe it’s possible to grow new lungs? And what explains this phenomenon-perhaps it’s all about intensive breathing training after surgery? Regeneration of damaged lungs through the formation of new tissue is the dream of any pulmonologist, and suddenly there is a new approach to its implementation-through the original code, through the program “lung Development”! After all, if you study the process of lung development in the womb, you can master the necessary tools. Knowing how the lungs are formed gives you a chance to “build” them again, re-run the program and establish medical control over it. How does the lung development program work?
On the one hand, this process, like pregnancy in General, is a giant construction project, which is based on a mysterious code. On the other hand, it is also a love story. It tells the story of how two partners search for and find each other and how they then jointly realize their life’s purpose: it is a long and mysterious process, full of obstacles, dangers, contradictions and mistakes. Like any love story, it has an ending, and it’s almost always a happy one. And sometimes tragic, unfortunately. It is also a story of wonder. The partners do not know each other and do not even guess (apparently) about each other’s existence, and yet some unknown force directs their actions, corrects the direction of movement if necessary, pushes until they find each other and merge, ensuring further life. This is the story of connecting two vital systems — the respiratory system and the circulatory system. If there is no border area where oxygen from the inhaled air passes into the blood, life outside the mother’s womb will be impossible. The way the two systems meet each other is a miracle, because they grow and develop separately. There are no ready-made “miniature lungs” in the embryo, which just need to grow during pregnancy. Doctors, researchers, anatomists, physiologists, biologists, philosophers look at this process and are amazed, and at the same time learn. With each new experiment and observation, they understand more and more how this happens, but less and less why. Specialists know the drawings and design plans, they have already seen most of the actors in this microscopic world. But who controls the process, who manages it? What is the driving force? Who is the conductor and who is the choreographer of this multimillion-dollar ballet? The best and most honest experts recognize that you do not have the slightest idea. The miracle of this love story is also that once you start following it, it is impossible to break away, the observer falls in love with the story and feels happy. See for yourself: when you really feel bad, do not listen to tearful songs, but rather look through the textbook on embryology. I’m not kidding: read something about the development of life in the womb, and if reading is difficult, just look at the pictures. You can even take some children’s book on this topic, if there are any. And there you will find the greatest miracle of our time. I guarantee that you will not be able to break away, even if you know how events will develop and how it will end. This reading is addictive, similar to the way we watch the same old movie on TV at Christmas, and each time it touches us to the core.
Do you like a happy ending? So this love story is just for you, it almost always has a happy ending. The respiratory and circulatory systems find each other and live together happily until they die.
“Is that all?”
— It doesn’t sound romantic.”
— But you promised something about love.”
“Okay, I’ll give you a few more details.
Let’s go back to the very beginning, about the third week of the embryo’s existence. At this stage, your heir is shaped like a ball. Imagine a candy: a hard layer of chocolate on the outside, and a soft sweet filling underneath. Then this candy has three separate formations: the head, chest and stomach — like a snowman. So far, everything is simple. But how are those tubes formed through which air and food will enter the body? But here’s how: in one place of the candy, a recess appears, deep enough. Something similar happens when you press your finger on a balloon. Thus, part of the chocolate shell is inside the filling, forming something like a tube. From it, then, the respiratory tract and gastrointestinal tract are formed. Over the course of several days, the tube grows in length, and then divides into anterior and posterior parts. From the anterior Airways develop, from the posterior — the esophagus, and a little later, the stomach and intestines. If the separation was made with errors, a “short circuit” may occur between the esophagus and the respiratory tract — the so-called fistula, or fistula. For a newborn, this is dangerous, since food can enter the respiratory tract through the fistula, which is why such a defect is eliminated immediately after birth. The first four weeks of pregnancy are extremely important for the development of the lungs, as well as for all other organs. The embryo, as a rule, cannot survive the organic defects that have arisen at this stage.
But if this phase passed without problems, then about 25 days after conception, small thickenings can already be found at the lower end of the tube — the right and left lungs. In the next four weeks, thin “tentacles” of the bronchi appear, which give the first branches. The right lung is divided, as an exception, into three parts (therefore, the right lung is divided into three lobes, and the left — only two), otherwise everything corresponds to a strictly defined program, the bronchi are divided into exactly two branches at each stage. This occurs whenever one of the air-bearing tubules reaches a certain length. Thin channels of the bronchi permeate the filling and gradually displace it. But the filling itself plays an important role in the maturation of the lungs. First, it is the starting material for the supporting connective tissue of the respiratory tract. From its cartilage and muscle cells, characteristic ring-shaped thickenings are formed, which allow the bronchi to maintain a stable shape. Secondly, it helps to give birth to the second side of our love story — the blood vessels of the lungs, which are formed not from a lung tube, but from a tiny heart, which is formed in the filling near the lungs around the 30th day of pregnancy. This small ball of blood vessels immediately goes in search of his betrothed. To the touch, like a blind man, he breaks through the darkness of toppings, wanting to reach the growing respiratory tract. It’s like a long, long game of blind man’s buff. Heat. Cold. Still cold. Warmer. Heat. Hot! Finally, at the seventh week of pregnancy, blind groping leads to the result, there is the first contact between the blood vessels and the respiratory tract. Everything happens by chance, as if by itself and unnoticed by both partners, because it takes some time before they form a real love couple. The moment of this first touch is reflected in one of the most beautiful poems of the English poet Christina Rossetti:But if this phase passed without problems, then about 25 days after conception, small thickenings can already be found at the lower end of the tube — the right and left lungs. In the next four weeks, thin “tentacles” of the bronchi appear, which give the first branches. The right lung is divided, as an exception, into three parts (therefore, the right lung is divided into three lobes, and the left — only two), otherwise everything corresponds to a strictly defined program, the bronchi are divided into exactly two branches at each stage. This occurs whenever one of the air-bearing tubules reaches a certain length. Thin channels of the bronchi permeate the filling and gradually displace it. But the filling itself plays an important role in the maturation of the lungs. First, it is the starting material for the supporting connective tissue of the respiratory tract. From its cartilage and muscle cells, characteristic ring-shaped thickenings are formed, which allow the bronchi to maintain a stable shape. Secondly, it helps to give birth to the second side of our love story — the blood vessels of the lungs, which are formed not from a lung tube, but from a tiny heart, which is formed in the filling near the lungs around the 30th day of pregnancy. This small ball of blood vessels immediately goes in search of his betrothed. To the touch, like a blind man, he breaks through the darkness of toppings, wanting to reach the growing respiratory tract. It’s like a long, long game of blind man’s buff. Heat. Cold. Still cold. Warmer. Heat. Hot! Finally, at the seventh week of pregnancy, blind groping leads to the result, there is the first contact between the blood vessels and the respiratory tract. Everything happens by chance, as if by itself and unnoticed by both partners, because it takes some time before they form a real love couple. The moment of this first touch is reflected in one of the most beautiful poems of the English poet Christina Rossetti:
I wish I could remember the first day, And the hour, and the first meeting with you.
Do you remember? How did you remember your first meeting with your future husband, partner, wife, or girlfriend? No, not the first date or the first kiss. That would be too easy. Where did you first see each other? On a Dating site? Yes, you are clearly not a fan of romance. But in the womb, everything happens the old-fashioned way, otherwise a human being will not be born.
After the first meeting, the blood vessels invariably follow the same route as the respiratory tract, twining around them like a vine. They are always a little behind, a couple of steps behind. Those who carelessly stray from the road are punished, as a result, not every bride is destined to meet her fiance. Fully formed lungs have about 20 percent more blood vessels than Airways.
Another miracle is happening nearby. The diaphragm is growing there! This organ of muscles and tendons is also formed on the basis of the internal filling of the embryonic ball. From the fourth week of pregnancy, the chest cavity is increasingly separated from the abdominal cavity. In the tenth week, the diaphragm starts moving for the first time! Due to these first uncoordinated and convulsive contractions, amniotic fluid is sucked into the fetal airway and pushed out. The diaphragm trains in performing breathing movements, but not with air, but with liquid, as in fish. At the same time, one very remarkable event occurs: the first breaths of an unborn child cause a powerful release of serotonin — the hormone of happiness-into the blood. Do you need more convincing evidence that breathing and happiness are linked? But diaphragm exercises not only bring your baby into a state of euphoria, they have a much more important goal. The fact is that by the time of birth, the diaphragm should be fully ready for action. From a biological point of view, the sudden transition of a small organism from life support through the uterus to Autonomous breathing is an unprecedented and very risky step. Anyone who has ever been present at a birth can remember how infinitely long the seconds seem between cutting the umbilical cord and the first breath, the first cry of the baby.
The flow of amniotic fluid into the growing bronchi as a result of movements of the diaphragm contributes to the further growth and division of both the bronchi themselves and the accompanying blood vessels. The pairs formed from them take up more and more space in the chest cavity over the following weeks. Soon we can record 18 points of branching of the respiratory tract. There is less and less free space. The Airways and blood vessels are on the eve of the “wedding” and, like many young couples, lose weight, because you need to look prettier in photos. Initially, rather rough and thick bronchi with each division become thinner and more tender and by the end of the 20th week they reach such a state that gases are able to pass directly from the respiratory tract to the blood vessels and Vice versa. The alveoli are not yet fully formed, but they are already able to provide gas exchange to some extent.
This moment of “engagement” is crucial for the unborn child. Even if it is born before its time, it will now have a good chance of survival. Until the 20th week, alveoli are absent and gas exchange is impossible. Even the most sophisticated device of artificial ventilation of lungs is not able to solve this problem. The earliest of the premature live babies were born after 21 weeks. But these are extreme cases, a kind of medical miracle that defies the theory of probability. Most premature babies born before the 24th week die. And this is the tragic side of our love story. Lovers do not find each other and die. However, after the 20th week, the chances of survival increase dramatically with each new day, and by the 30th week they already exceed 80 percent. Starting from the 24th week, fully formed alveoli are formed. By this time, the bronchi are divided for the 23rd — and last-time, and rounded thickenings appear on their tender endings, permeated with blood vessels. Every day there are more and more new pairs. In addition, after the 24th week, the alveoli begin to produce a stabilizing liquid-surfactant. The chemical composition of the surfactant is similar to the usual liquid for rinsing dishes. This surfactant prevents the alveoli from collapsing and sticking together during exhalation. The production of surfactant in the alveoli can be caused by hormones, in particular cortisone, so if there is a threat of premature birth, pregnant women are prescribed this hormone. In addition, preterm babies are given artificial surfactant after birth, improving their respiratory function and increasing their chances of survival. Ideally, a newborn has between 50 and 100 million fully formed alveoli at the time of birth. But the love story doesn’t end there. By the 15th year of life, the number of alveoli increases to about 400 million, and only then development stops. The total surface area of all the alveoli is 90 m2, which is about half of the tennis court. This is three times more than the gut, and 45 times more than the skin.
Is it possible to fully understand this wonderful story? Unlikely. You can watch and wonder. And take action. Indeed, despite progress in the care of premature infants, the immaturity of their lungs may be affected throughout life. Then only a few manage to reach the maximum possible lung volume, and many have frequent infections or diseases such as asthma or COPD. The history of the respiratory tract and blood vessels reminds us how vulnerable children are in the prenatal phase and what can happen to them in the womb. We do not tire of emphasizing that care for the lungs of the unborn child should begin long before birth, and that both pregnant women and their partners have a great responsibility. After all, pregnant women share their body with a fragile creature that can’t say anything and can’t stand up for itself. Do not forget that after childbirth, the lungs continue to develop, they remain vulnerable and need protection. The world is full of threats for them, and not all of us are able to control. However, with the main viruses that cause colds, the child copes independently. But when they are added to passive Smoking, air pollution with harmful substances, it becomes too difficult. The child has no choice, then asthma and other chronic respiratory diseases. There is no point in worrying about the fact that your child is small and will never Shine on the catwalks of Paris and new York. But the fact that he will never have one hundred percent lung capacity should make you sad! Equality of opportunity applies not only to education and social status, but also to physical development. Of course,” crazy ” parents who blow every speck of dust off their child can get on everyone’s nerves at parent-teacher meetings. But if we are talking about the prenatal phase, then I have nothing against this “madness”. On the contrary, at this stage of development, we would like to have more of it.