Development of the respiratory system
Definitions & Key takeaways
The respiratory system develops during week 4 of gestation, where it is initially with a primitive gut tube, which later becomes the gastrointestinal system. At around week 4, the respiratory diverticulum detaches as an outpocketing on the proximal part of the primitive gut tube.
A longitudinal ridge called the tracheoesophageal septum forms between the gut tube and the respiratory diverticulum, becoming two separate structures. The respiratory diverticulum later gives lung buds, from which the lungs and trachea develop. The larynx arises from the fourth and sixth pharyngeal arches around week 12.
The respiratory system starts developing during week 4 of intrauterine life, and it begins when a lung bud sprouts out of the primitive digestive tract.
The lung bud is an outgrowth of the foregut portion of the digestive tract, and it turns into lower respiratory tract structures such as the larynx, trachea, and lungs.
Around week 4, the embryo has developed all three embryonic germ layers, and of the three, respiratory tract structures arise from the endoderm and mesoderm.
The larynx starts developing at the beginning of week 4, as nothing more than a slit between the 4th and the 6th pharyngeal arches.
The pharyngeal arches are paired, symmetrical embryonic structures that sprout from the foregut and arch towards the embryo’s front midline.
They consist of a mesoderm core and that mesoderm is made up mostly of mesenchyme - a soupy, fetal tissue that eventually turns into circulatory tissue, lymphatic tissue, and musculoskeletal tissue.
The arches are covered on the outside with by the pharyngeal cleft, made of ectoderm and lined on the inside with the pharyngeal pouch on the inside, made of endoderm.
In fish, these arches develop into gills, but in our case they serve as a foundation for many important structures around the head and neck.
The endoderm of the 4th and 6th pharyngeal arches forms the laryngeal epithelium and glands and the mesoderm forms the laryngeal cartilages, while the arches themselves carry the laryngeal branches of the vagus nerve to these structures - all in all, these two arches give us our ability to talk.
The epithelium inside the larynx turns into laryngeal ventricles, which give rise to the vocal chords. In week 6, the epiglottis forms, and in week 12, the laryngeal opening has its adult shape, as well as thyroid, cricoid and arytenoid cartilages.
.in the core The lung bud has endoderm which develops into the epithelial and glandular structures of the trachea and lungs, and around the core it also has visceral mesoderm around that core which develops into muscles, cartilages, and connective tissue.
Now, the lung bud basically just hangs off the foregut, and at its loose end it bifurcates into two bronchial buds. These bronchial buds give rise to the lungs, which grow and develop inside the pleural cavities.
The bronchial buds develop into lungs in four stages. The first stage is the pseudoglandular stage and it lasts from week 5 to week 16.
During this time, the bronchial buds divide into a left and right main bronchus, and these divide again into three secondary bronchi for the three right lung lobes and two secondary bronchi for the two left lung lobes.
Secondary bronchi turn into 10 tertiary bronchi on the right side, and 8 on the left - corresponding to the number of lung segments on either side.
Tertiary bronchioles divide repeatedly until all the terminal bronchioles are formed. So, now the lungs enter the second stage of development, called the canalicular stage.
It lasts from week 16 to week 26, and during this time the terminal bronchioles continue to divide and form the respiratory bronchioles.
Each respiratory bronchiole then divides into 3 to 6 alveolar ducts. A great number of pulmonary capillaries also form during this stage, making the lungs highly vascular organs.
Starting with week 24, the first primitive alveoli appear in the lung regions closer to the trachea. However, these alveoli are immature, in that they have a lining of cuboidal epithelial cells that is not perfectly adapted to gas exchange just yet - and there’s still very few of them!
Next is the third stage, called the terminal sac stage. It lasts from week 26 to birth, and during this time more primitive alveoli, sometimes called terminal sacs, form around each alveolar duct, In addition, the epithelial lining differentiates into either type I or type II pneumocytes, and more capillaries form around the primitive alveoli.
Type I pneumocytes are flat cells that are in direct contact with the capillaries. At the point where the endothelium of the capillaries meets that of the type I pneumocytes, the blood-air barrier forms which also includes the basement membrane in between.
Type II pneumocytes also form, and they’re large, cuboidal cells that produce pulmonary surfactant. Surfactant is a fluid whose name is actually short for “surface active agent” - because it decreases surface tension at the blood-air barrier, by lowering the attractive forces between water molecules in the alveoli.
This allows the lungs to fill up with air more easily, and it also prevents the collapse of the tiny alveoli during expiration.
The fourth and final stage is the alveolar stage, and it lasts from week 36 to about 8 years after birth. So the lungs continue to develop for quite some time!
The alveolar stage overlaps with the terminal sac stage for four weeks. During the alveolar stage, thin walls called septa partition the primitive alveoli, giving rise to many more adult alveoli, and the number of respiratory bronchioles also increases.
To put this into perspective - we are born with 20 to 70 million primitive alveoli, and by the age of 8, that number increases to a whopping 300-400 million.
So feel free to take a deep breath now, because you’ve got the numbers for it! Oh, and speaking of breathing - fetal breathing movements actually begin before birth, so that the lungs and the respiratory muscles can learn how to work together.
Since there is no air in the uterus, the fetus breathes in amniotic fluid. At birth, when the baby takes its first breath, that fluid gets resorbed into the pulmonary capillaries, leaving behind a layer of surfactant on the alveolar cell membranes.
In premature babies, lung function depends heavily on how much surfactant has been produced already. All right, as a quick recap: the respiratory system starts developing during the week 4 of pregnancy, and it’s mature enough so that a premature baby can breathe on its own by 28 weeks.
The larynx is completely formed by the week 12, and it’s formed by the 4th and 6th pharyngeal arches. The lungs and trachea develop from the lung bud starting with the week 4, and the lungs mature in 4 stages: the pseudoglandular period, the canalicular period, the terminal sac period and the alveolar period.
During the last two stages, type II pneumocytes secrete alveolar surfactant, which prevents the alveoli from collapsing during expiration.
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