Respiratory and Body Cavities Development

Question 1

What are the three divisions of the gut tube at 4 weeks? What will they give rise to?

Answer 1

1. Foregut - pharynx + its derivatives, larynx, trachea, lungs, esophagus, stomach, and proximal duodenum
2. Midgut - small intestine, and a portion of large intestine (cecum, ascending colon, ~2/3 of transverse colon)
3. Hindgut - distal 1/3 of transverse colon, descending and sigmoid colon, rectum up to pectin line, urinary bladder + urethra

Question 2

Where is the respiratory system derived from? How is this regulated?

Answer 2

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

Question 3

What is the respiratory diverticulum?

Answer 3

TBX-4 induced ingrowth of endoderm + mesoderm also called the lung bud or laryngotracheal diverticulum which develops in the foregut region of the embryo

Question 4

What are tracheoesophageal ridges? What do they form?

Answer 4

Parallel longitudinal ridges that initially separate the respiratory diverticulum from the gut tube. They will fuse at midline to form the tracheoesophageal septum

Question 5

What is the laryngotracheal tube? What does it open into?

Answer 5

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

Question 6

What will the respiratory diverticulum give rise to as it elongates caudally?

Answer 6

It will give rise to a pair of lung buds which form the right and left primary bronchi and lungs

Question 7

What will the endoderm of the respiratory diverticulum form?

Answer 7

Epithelium and glands of trachea, larynx, bronchi, and lungs

Question 8

What will the splanchnic mesoderm give rise to?

Answer 8

Smooth muscle, connective tissue, and cartilages associated with trachea, bronchi, and bronchioles

Question 9

How is the larynx formed?

Answer 9

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

Question 10

What produces the 2 lateral laryngeal swellings? What do they form?

Answer 10

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)

Question 11

Where does the epiglottis form?

Answer 11

Between 3rd and 4th pharyngeal arch.

Question 12

What is the innervational significance of each of the pharyngeal arches for forming the larynx?

Answer 12

1. Arch 4 - Superior laryngeal nerves of CN X - innervates mucosa and muscles superior to true vocal folds
2. Arch 6 - Recurrent laryngeal nerves of CN X- Innervates muscles and mucosa inferior to true vocal fold

Question 13

What is recanalization?

Answer 13

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.

Question 14

How are the right and left primary bronchi formed? How does this relate to secondary and tertiary bronchi?

Answer 14

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

Question 15

What is the pseudoglandular period?

Answer 15

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.

Question 16

What is the canalicular period?

Answer 16

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

Question 17

What characterizes as respiratory bronchiole?

Answer 17

Characterized by smooth muscle, absence of cartilage, and giving rise to 3-6 alveolar ducts

Question 18

What is the terminal sac period?

Answer 18

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

Question 19

What are primitive terminal sacs?

Answer 19

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

Question 20

Why does the baby breath before birth?

Answer 20

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

Question 21

When does the number of terminal sacs increase in number?

Answer 21

During the last 2 months of development, for several years following birth. There are 6 more divisions of the tree post-natally

Question 22

What is the significance of Type II alveolar cells at ~34 weeks? What does this cause

Answer 22

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

Question 23

Where does the initial growth of the lung proceed to?

Answer 23

Fills the pericardioperitoneal canals

Question 24

What forms the visceral pleura?

Answer 24

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

Question 25

What forms the parietal pleura?

Answer 25

Serous mesothelium derived from somatic mesoderm. This forms the walls of the pleural cavity, including costal, diaphragmatic, and mediastinal

Question 26

What are the pleuropericardial folds?

Answer 26

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.

Question 27

How does the pleural cavity remain open to the peritoneal cavity?

Answer 27

Despite the fusing of the pleuropericardial folds, it remains open via the pericardioperitoneal canals.

Question 28

What is the septum transversum?

Answer 28

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

Question 29

What happens when the pleuroperitoneal folds fuse with the mesentery of esophagus + septum transversum?

Answer 29

All three cavities - pericardial, pleural, and peritoneal, are individually compartmentalized from eachother

Question 30

What are the pleuroperitoneal folds?

Answer 30

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

Question 31

What gives rise to the central tendon of the diaphragm?

Answer 31

Septum transversum

Question 32

What gives rise to the right and left diaphragmatic crura?

Answer 32

Mesentery associated with the esophagus

Question 33

What forms the muscular part of the diaphragm?

Answer 33

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

Question 34

What innervates the different portions of the diaphragm? Why?

Answer 34

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

Question 35

What is atresia?

Answer 35

A blunt ended closure of a normal opening or passage

Question 36

What is a fistula?

Answer 36

Pathological sinus or abnormal passage leading from one organ structure to another

Question 37

What is esophageal atresia? What is the most common type?

Answer 37

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.

Question 38

What is full esophageal atresia? What does it cause in utero?

Answer 38

No tracheoesophageal fistula, just two blind ends of the esophagus. Will lead to polyhydramnios since the fetus cannot swallow

Question 39

What is tracheoesophageal fistula and what characterizes it at birth?

Answer 39

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

Question 40

What is congenital diaphragmatic hernia?

Answer 40

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

Question 41

What is parasternal hernia?

Answer 41

Small intestines enter mediastinum when muscle fibers of diaphragm don’t form near sternal attachment

Question 42

What is esophageal hernia?

Answer 42

Underdevelopment of esophagus leads to upper portions of stomach to be retained in thorax