(Transcribed from Dr. Martin’s lecture, 20 Mar 2000 by Brian Buschman)
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Like just about everything the story begins at the fourth week of development when the laryngotracheal diverticulum appears off of the primitive foregut. It is of endodermal origin and it’s smooth muscle, cartilage and C.T. are from splanchnic mesoderm. Soon the tracheoesophageal folds grow from the sides to form the tracheoesophageal septum which divides the laryngotracheal tube into the trachea and the esophagus.
The laryngeal epithelia is of endodermal origin and the laryngeal cartilages are of neural crests via the 4th and 6th arches.
The esophagus proliferates so rapidly that the foregut becomes occluded but it will recanalize around the 10th week of development. In the process a pair of the lateral grooves, the laryngeal ventricles, are formed which are bounded by the true and false vocal folds.
Tracheoesophageal fistula, as discussed with GI development, is a fistula from a blind ended esophagus to the trachea. A fistula of the inferior part will allow reflux of gastric contents into the lungs leading to respiratory distress.
The lung bud that forms off of the laryngotracheal tube divides into two bronchial buds. The right one grows more vertically then the left since the left must be more angled to fit around the heart. Foreign substances, therefore, will be more likely to end up in the right bronchus when inhaled.
The bronchial buds will develop into secondary bronchi, two on the left and three on the right, and they will develop into tertiary bronchi. The tertiary bronchi are the basis of the bronchiopulmonary segments (BPS) which are currently the smallest unit of the lung that is surgically removable. There will be 10 BPS on the right and 8 on the left.
The tubes are of endothelial origin but the muscles, visceral pleura, connective tissue and so forth are of splanchnic mesoderm.
The lungs develop in four stages. There is an overlap in time because the superior BPS develop faster then the inferior ones so the inferior BPS can be in an earlier stage while the superior ones are in a later stage.
1) The pseudoglandular period (5-17 weeks) is characterized by existence by all of the major structures but it is not yet functional. At this point it is impossible for a baby to survive outside of the mother’s uterus.
2) The canalicular period (16-25 weeks) is the period where the first respiratory segments develop. By the 24th week there are a few respiratory bronchioles that develop at the end of the terminal bronchioles. At the ends are the terminal sacs which are the first respiratory passages to form. This is the first point at which a baby can live outside of the human body.
3) The terminal sac period (24 weeks to term) is the period where more and more terminal sacs develop and their walls become thin. Their epithelia develop into alveolar cells (pnemunocytes). They exist at type I which are thin for exchange between the lungs and capillaries and type II which secrete a substance called surfactant. Surfactant is a fluid that breaks the surface tension of the watery secretions that line the alveoli. Without surfactant to break this surface tension inspiration would be impossible.
4) The alveolar period (birth until about age 8) is when the lungs continue to develop and form more alveoli.
At birth the amniotic fluid fills the lungs and is cleared by three routes:
1) Through the nose and mouth due to thoracic pressure.
2) Absorbed into pulmonary capillaries.
3) Into the lymphatics, pulmonary arteries and veins.
Pulmonary agenesis is the complete absence of all pulmonary structure due to the failure of the lung bud to form. Obviously life is completely impossible.
About 5-10% of the children in the US currently have asthma and that number of constantly rising.
Asthma results from an allergic reaction where mast cells release histamine in excess which causes constriction of smooth muscle, specifically bronchioconstriction. It is treated by the administration of epinephrine (a.k.a. adrenaline if you are British) which is a sympathetic agonist so it relaxed bronchial smooth muscle.
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