Respiratory and Body Cavities Development Flashcards
What are the three divisions of the gut tube at 4 weeks? What will they give rise to?
- Foregut - pharynx + its derivatives, larynx, trachea, lungs, esophagus, stomach, and proximal duodenum
- Midgut - small intestine, and a portion of large intestine (cecum, ascending colon, ~2/3 of transverse colon)
- Hindgut - distal 1/3 of transverse colon, descending and sigmoid colon, rectum up to pectin line, urinary bladder + urethra
Where is the respiratory system derived from? How is this regulated?
Ventral surface of the foregut in midline between 4th and 6th pharyngeal arches.
Retinoic acid is released by underlying splanchnic mesoderm, which stimulates TBX4 transcription factor in adjacent endoderm at the site of the respiratory diverticulum. This transcription factor induces its formation
What is the respiratory diverticulum?
TBX-4 induced ingrowth of endoderm + mesoderm also called the lung bud or laryngotracheal diverticulum which develops in the foregut region of the embryo
What are tracheoesophageal ridges? What do they form?
Parallel longitudinal ridges that initially separate the respiratory diverticulum from the gut tube. They will fuse at midline to form the tracheoesophageal septum
What is the laryngotracheal tube? What does it open into?
The tube which connects the esophagus to the developing larynx, trachea, and lungs.
Opens via the laryngeal orifice / inlet, which is the midline communication of the developing respiratory system with the pharynx
What will the respiratory diverticulum give rise to as it elongates caudally?
It will give rise to a pair of lung buds which form the right and left primary bronchi and lungs
What will the endoderm of the respiratory diverticulum form?
Epithelium and glands of trachea, larynx, bronchi, and lungs
What will the splanchnic mesoderm give rise to?
Smooth muscle, connective tissue, and cartilages associated with trachea, bronchi, and bronchioles
How is the larynx formed?
Between arches 4 and 6, below the developing tongue:
Endoderm - gives rise to internal lining of larynx
Mesenchyme (primarily neural crest) - gives rise to laryngeal cartilages and muscles of the larynx
What produces the 2 lateral laryngeal swellings? What do they form?
Rapid proliferation of mesenchyme of arches 4 and 6. They form the T-shaped laryngeal opening between the epiglottal swellings (arch 4) and arytenoid swellings (arch 6)
Where does the epiglottis form?
Between 3rd and 4th pharyngeal arch.
What is the innervational significance of each of the pharyngeal arches for forming the larynx?
- Arch 4 - Superior laryngeal nerves of CN X - innervates mucosa and muscles superior to true vocal folds
- Arch 6 - Recurrent laryngeal nerves of CN X- Innervates muscles and mucosa inferior to true vocal fold
What is recanalization?
The reopening of the larynx after it was solid / occluded via cavitation of the endothelium. This forms the paired ventricles + false and true vocal folds.
How are the right and left primary bronchi formed? How does this relate to secondary and tertiary bronchi?
Release of sonic hedgehog via endothelial cells stimulates surrounding splanchnic mesoderm to release FGF, causing bronchial buds to lengthen and enlarge.
The secondary and tertiary bronchi continue developing via the same stimulation to ultimately form 10 bronchopulmonary segments on the right and 8-9 on the left
What is the pseudoglandular period?
Development of lungs from 4-16 weeks where there are no respiratory bronchioles, and thus no alveoli with blood vessel contact. Survival of premature birth is not possible at this stage.
What is the canalicular period?
Weeks 16-26, where the primitive gas exchange development occurs. Terminal bronchioles will divide to give rise to several respiratory bronchioles. Since minimal branching has occurred, it would be “heroic” for a baby to survive a premature birth at this stage
What characterizes as respiratory bronchiole?
Characterized by smooth muscle, absence of cartilage, and giving rise to 3-6 alveolar ducts
What is the terminal sac period?
Period from 26 weeks to birth, baby can survive premature birth. The alveolar ducts end as primitive sacs, and by the 6th month, there are 17 generations of branching. 7 months is when you can definitely survive as you have enough alveolar branchings
What are primitive terminal sacs?
The end of alveolar ducts, which branched from respiratory bronchioles. They consist of a single layer of flattened endothelial cells in close proximity to capillaries within the underlying CT
Why does the baby breath before birth?
They are breathing amniotic fluid which fills the lungs. This is important for development and strengthening of respirator muscles (thoracic diaphragm and intercostal muscles) as well as for further lung development
When does the number of terminal sacs increase in number?
During the last 2 months of development, for several years following birth. There are 6 more divisions of the tree post-natally
What is the significance of Type II alveolar cells at ~34 weeks? What does this cause
Surfactant secretions are released into alveolar duct and mix with amniotic fluid that fills the lungs.
- > stimulates alveolar macrophages to produce immune system proteins, including IL-1beta
- > amniotic fluid containing macrophages and this protein will cross the placenta and increase production of prostaglandins by the uterus, stimulating uterine contractions at high concentrations
Where does the initial growth of the lung proceed to?
Fills the pericardioperitoneal canals
What forms the visceral pleura?
A serous mesothelium which is derived from splanchnic mesoderm, will completely invest the surface of the right and left lungs as they fill the pericardioperitoneal canals
What forms the parietal pleura?
Serous mesothelium derived from somatic mesoderm. This forms the walls of the pleural cavity, including costal, diaphragmatic, and mediastinal
What are the pleuropericardial folds?
Two lateral folds forming of the wall of the lateral + ventral pericardioperitoneal canals, which grow medially and hold the right or left phrenic nerve + cardinal veins. They will fuse with eachother and separate the pericardial cavity from the pleural cavity by week 7.
How does the pleural cavity remain open to the peritoneal cavity?
Despite the fusing of the pleuropericardial folds, it remains open via the pericardioperitoneal canals.
What is the septum transversum?
A ventral midline thickened plate of splanchnic mesoderm which is between primitive thoracic and abdominal cavities, only allows communication between these two via the pericardioperitoneal canals
What happens when the pleuroperitoneal folds fuse with the mesentery of esophagus + septum transversum?
All three cavities - pericardial, pleural, and peritoneal, are individually compartmentalized from eachother
What are the pleuroperitoneal folds?
A pair of crescent shaped folds which close off the pericardioperitoneal canals by extending medially and ventrally, fusing with septum transversum + mesentery of esophagus
Will form the pleuroperitoneal membrane between the abdomen and the thorax
What gives rise to the central tendon of the diaphragm?
Septum transversum
What gives rise to the right and left diaphragmatic crura?
Mesentery associated with the esophagus
What forms the muscular part of the diaphragm?
Myoblasts from cervical somites 3, 4, and 5 which infiltrate the pleuroperitoneal membranes + and septum transversum.
This explains the motor and sensory innervation provided by the right and left phrenic nerves
Some around the edge comes from the intercostal nerves
What innervates the different portions of the diaphragm? Why?
Since the lungs inform via elongation, they move from their cervical starting position to the lower thoracic / upper lumbar level.
Peripheral rim - innervated by lower intercostal nerves for sensory
Central portion - right and left phrenic nerves for motor and sensory
What is atresia?
A blunt ended closure of a normal opening or passage
What is a fistula?
Pathological sinus or abnormal passage leading from one organ structure to another
What is esophageal atresia? What is the most common type?
Proximal part of esophagus ends in a blind pouch due to abnormal partitioning by the tracheoesophageal septum
~90% occurs with a tracheo-esophageal fistula and can thus be surgically treated by attaching the fistula portion to the atresia end.
What is full esophageal atresia? What does it cause in utero?
No tracheoesophageal fistula, just two blind ends of the esophagus. Will lead to polyhydramnios since the fetus cannot swallow
What is tracheoesophageal fistula and what characterizes it at birth?
Abnormal connection between esophagus and trachea, associated with esophageal atresia.
Feeding will result in violent coughing and choking as food can only pass through trachea due to proximal end of esophagus being blocked. Can lead to pneumonia + difficulty breathing
What is congenital diaphragmatic hernia?
Common malformation in newborn caused by failure of one or both pleuroperitoneal membranes to close the pericardioperitoneal canals. The abdominal viscera can be found in the pleural cavity in 90% of cases
What is parasternal hernia?
Small intestines enter mediastinum when muscle fibers of diaphragm don’t form near sternal attachment
What is esophageal hernia?
Underdevelopment of esophagus leads to upper portions of stomach to be retained in thorax
