9.14.11+-+Embryology+of+Respiratory+Tract

Embryology of Respiratory Tract **1) Define the following**

**Intraembryonic Coelom (IEC)-** Originally a cavity forming in the mesoderm. When dorsal mesoderm (somatic) splits from ventral mesoderm (splanchnic) the remaining gap is the Intraembryonic Coelom. It will give rise to one pericardial cavity, one peritoneal cavity and two pleural cavities. Briefly communicates with the extraembryonic coelom.

**Pericardioperitoneal Canals-** Not true canals. Arise from IEC**,** connect the pericardial cavity with the future peritoneal cavity (IEC open to the outside). Lung buds grow into pericardioparitoneal canals to form pleural cavities

**Pleuropericardial Membranes-** Membrane arising which seperates growing pleural cavity from pericardial cavity

**Pleuroperitoneal Membranes-** Membrane separating growing pleural cavity from peritoneal cavity

**Stomodeum (ectodermal)****-** an ectodermally lined pit, most rostral segment of the gut

Rathke's Pouch- a diverticulum off of the stomodeum, later helps form the pituitary

**Thyroid Diverticulum****-** a diverticulum off the foregut which forms the thyroid

**Thyroglossal Fistulas****-** a malformation in which the thyroid develops but an opening to the skin remains open

**Laryngotracheal Diverticulum (Lung Bud)****-** endodermal bud that comes ventrally off the foregut


 * Tracheoesophageal Fistula****-** malformation in which a secondary fistula connects the trachea with the esophagus.


 * Respiratory Bronchioles (containing Terminal Sacs)****-** bronchioles which come in contact with blood vessels.


 * Terminal Sacs (primitive Alveoli)****-** appear at the end of the respiratory bronchioles.


 * Alveoli****-** develop from the primitive terminal sacs.


 * Surfactant****-** a surface acting lipid which decreases surface tension and allows for independent breathing by the fetus

The IEC will give rise to the thoracic body cavities. These include one pericardial cavity which houses the heart, one (originally two) peritoneal cavity, and two pleural cavities which house the lungs.
 * 2) Describe the division of the intraembryonic coelom (in broad terms: what does it give rise to?)**

Prior to the formation of the primitive mouth, the OP membrane (the membrane remaining from the migration/formation of the IEM) serves as the mouth landmark. It is formed from both endoderm and ectoderm. At the OP membrane there is an overgrowing at all sides of the membrane by the branchial/pharyngeal arches (made of ectodermal spinal crest cells). This leads to the formation of an ectodermally lined pit (the base of the pit formed by the OP membrane) called the Stomodeum. When the OP membrane breaks down, these ectodermal arches are able to migrate up to the line of endoderm established by the foregut. This creates a mouth which is ectodermally based ventrally and endodermally based dorsally and caudally.
 * 3) How does the primitive mouth come to be lined by endoderm and ectoderm (in different regions)?**


 * 4) What are the consequences of persistent portions of the thyroid diverticulum**

Persistent thyroid diverticulums lead to thyroglossal cysts and fistulas.


 * 5) How does the pericardio-peritoneal canals give rise to the pleural cavities**

The lung buds (arising from the foregut) begin to produce two bronchial buds which expand laterally into the pericardioperitoneal canals of the IEC. As they continue to push into the canal, the canal envelops the developing lungs, producing the pleural cavities.


 * 6) Describe the separation of the esophagus from the trachea**

As the esophagus develops from the foregut, it begins to develop the laryngotracheal diverticulum which grows ventrally to the esophagus. The diverticulum is separated from the developing esophagus by a squeezing or longitudinal folding of splanchnic mesoderm. The gap that develops between the esophagus and the tracheal tube is called the tracheoesophageal septum.


 * 7) What are the consequences of incomplete septation of the esophagus from the tracheal tube**

A complete separation can result in a fistula forming between the esophagus and trachea, a malformation called a tracheoesophageal fistula.


 * 8) How does ectopic thyroid tissue arise**

The thyroid develops first as a thyroid diverticulum from the foregut. It descends through splanchnic mesoderm until it arrives in the neck. In normal development the thyroid gland is transiently connected to the foregut via the thyroglossal duct. Ectopic thyroid tissue is a thyroid that has not properly migrated from the base of the tongue (where its development begins) to the neck; there are two different ways this can happen. If the thyroid is also present where it's supposed to be, then the additional thyroid tissue is called an accessory thyroid. It is formed from an incomplete degeneration of the thyroglossal duct. An ectopic thyroid is an incomplete descent of the thyroid diverticulum. In this case, there is no thyroid tissue present in the proper position - the neck. An ectopic thyroid may be fully or partially functional, but it typically leads to hypothyroidism.

The lung buds (laryngotracheal diverticulum) branch from the endodermal foregut. As they grow, they branch into the two bronchial buds that expand into the splanchnic mesoderm lining the pericardioperitoneal canals of the IEC. The canals come to form the pleural cavities.
 * 9) How does the lung come to be made of layers of endoderm and mesoderm?**

(Ps: Cut The Air)
 * 10) What are the 4 official stages of lung development**

Pseudoglandular period: From weeks 5-17, fatty thick-walled bronchii, branching into splanchnic mesoderm. Cut through are thick circles like glands. Canalicular period: From weeks 16-25, slightly thinner walled bronchii, they have started to have hollow centers that when cut through look like canals. Also, angiogenesis and vasculogenesis happening. Terminal sac period: From weeks 24-birth, First occurrence of bronchioles, the location of respiration. Indicated by the first occurrence of terminal sacs, defined by having distal tips of squamus cells. Blood vessels closely oppose squamus cell tips. Alveolar period: From 25 weeks-8 years old. Terminal sacs continue to differentiate into alveoli, blood vessels sink into endothelium, one set of lung cells produce surfactant.


 * 11) When, in general terms, is lung development complete?**

Approximately age 8

Respiratory Distress Syndrome is caused by insufficient production of surfactant. Hyaline Membrane Disease is the effect of Respiratory Distress. Hyaline Membrane Disease is presented as accumulations of shed alveolar cells, because without sufficient surfactant the fragile endothelial lining of the alveoli have too much friction when they rub past each other.
 * 12) What is the major cause of respiratory distress syndrome and hyaline membrane disease in newborns?**