Embryology Exam 3 TQs

What is the intraembryonic coelom?

The embryonic body cavity, which subdivides into:

• Pericardial cavity (1)

• Pericardioperitoneal canals (2)

• Peritoneal cavity (1)

What are the adult derivatives of the intraembryonic coelom?

Pericardial cavity (heart), pleural cavity (lungs), peritoneal cavity (abdominal viscera)

What is the septum transversum?

The crescent-shaped, shelf-like divider of the embryonic body cavity which will develop into the central tendon of the diaphragm; formed from mesoderm, between heart and liver

What is lateral to the foregut and dorsal to the septum transversum?

Pericardioperitoneal canals

What are the 2 walls of the embryonic cavities?

Parietal wall (somatic mesoderm, form parietal membranes)

Visceral wall (splanchnic mesoderm, form visceral membranes)

What is a mesentery?

A double layer of peritoneum which attaches a segment of gut to the body wall and sandwiches vessels/nerves

What are the 2 embryonic mesenteries?

Ventral (attach gut to ventral wall of intraembryonic coelom), dorsal (to dorsal wall)

Which mesentery disappears (mostly)?

Ventral mesentery

What are the 2 sets of folds in the lateral walls of the intraembryonic coelom?

Pleropericardial folds (divide heart from lungs), pleuroperitoneal folds (divide pleural cavities from pericardial cavity)

What is the pleuropericardial membrane?

The completed partition between pleural and pericardial cavities

What are the pleuroperitoneal membranes?

Structures formed from pleuroperitoneal folds’ growth into the pericardioperitoneal canals

What is the diaphragm?

The most important muscle for inspiration; a dome-shaped fibromuscular structure, separating the thoracic and abdominal cavities

What are the 4 structures that form the diaphragm?

1. Septum transversum

2. Pleuroperitoneal membranes

3. Dorsal mesentery of the esophagus

4. Muscle of the lateral walls

Why is the septum transversum an incomplete partition?

The pericardioperitoneal canals form openings on the dorsal side, on either side of the esophagus

What does the dorsal mesentery of the esophagus form?

Left and right crus (of diaphragm)

What is the aortic hiatus?

The opening in the diaphragm through which the aorta passes

What is the esophageal hiatus?

The opening in the diaphragm through which the esophagus passes

What level does the septum transversum lie at in the 4th week of development?

3rd-5th cervical somites

Which nerve fibers form the phrenic nerve? What does it supply?

Contributions from C3-5, supplying the diaphragm

If a patient has referred pain from their thorax/abdomen to their shoulder, which nerve is most likely involved?

The phrenic nerve

What is congenital diaphragmatic hernia?

Herniation of abdominal viscera into thorax through a posterolateral defect of the diaphragm

Usually unilateral

Arises from defective formation of the pleuroperitoneal membrane

Most common cause of pulmonary hypoplasia, results in severe breathing difficulties

When does the respiratory system begin to develop?

Week 4

What is the first structure of the respiratory system to form?

The laryngotracheal groove, in the caudal anterior wall of the pharynx

What does the respiratory endoderm form?

Respiratory epithelium, glands of larynx/trachea/bronchi

What does the respiratory splanchnic mesoderm (external to foregut) form?

Cartilages and smooth muscle of trachea/bronchi

How does the laryngotracheal groove develop?

Evaginates to form the laryngotracheal diverticulum (ventral to foregut)

Distal end enlarges → respiratory bud

Opening to pharynx → primordial laryngeal inlet (the “wrong pipe” when you inhale your drink)

What develops as the laryngotracheal diverticulum elongates?

2 longitudinal tracheoesophageal folds, which grow together and fuse into the tracheoesophageal septum

What does endoderm typically form in visceral systems?

Epithelium and glands of visceral organs

What does splanchnic mesoderm typically form in visceral systems?

Stroma and smooth muscle walls of visceral organs

How does the tracheoesophageal septum divide the laryngotracheal diverticulum?

Ventral part (laryngotracheal tube) → larynx, trachea, bronchi, lungs

Dorsal part → esophagus (cont. of pharynx)

What do the 4th and 6th pharyngeal arches form in the respiratory system?

All but one laryngeal cartilage (thyroid, cricoid, arytenoid, corniculate, cuneiform)

Formed from neural crest cells

What do the epiglottis and epiglottic cartilage develop from?

Hypopharyngeal eminence (caudal part)

What does the hypopharyngeal eminence form?

Caudal part: epiglottis, epiglottic cartilage

Rostral part: oropharyngeal part of tongue

What do 8 of the 9 pairs of intrinsic laryngeal muscles develop from?

6th pharyngeal arch

Served by the recurrent laryngeal branch of CN X

What does the cricothyroid muscle do? What does it develop from?

Raises voice, from 4th pharyngeal arch (unique!)

Served by the superior laryngeal branch of CN X

What is unique about humans’ larynx?

Low position: increases size, facilitates speech, prohibits swallowing and breathing at the same time

Descends in first 1.5-2 years, when speech develops

What does the trachea develop from? What forms its respiratory epithelium and glands?

Laryngotracheal tube (dist. to larynx), endoderm

What does splanchnic mesoderm form in the trachea?

C-ring cartilages, smooth muscle

What is a tracheoesophageal fistula?

An abnormal opening between the trachea and esophagus, arising from incomplete division of laryngotracheal tube from esophagus (by tracheoesophageal folds)

Goes with esophageal atresia (esophagus ends in blind pouch)

Infant cannot swallow, may reflux gastric acid into lungs

When does the respiratory bud develop?

Week 4

What does the respiratory bud divide into?

Two primary bronchial buds (left and right)

When do the primary bronchial buds enlarge into main bronchi?

Week 5

How many divisions come from each main bronchus?

Right has two, splits to three: upper (superior lobar bronchus), lower (middle and inferior lobar bronchi)

Left has only two: superior and inferior lobar bronchi

When have 10 segmental bronchi formed?

Week 7

What surrounds the segmental bronchi? What are they together (bronchi and surrounding tissue)?

Splanchnic mesoderm; 10 bronchopulmonary segments

How many orders of bronchioles are present by 6 months? How many develop postnatally? How many in total?

17 orders by 6 months; 7 more layers postnatally; 24 tiers total

What forms the epithelium and glands of the bronchi/bronchioles?

Endoderm which lines the bronchial buds

What forms the cartilaginous rings/plates of the bronchi and their smooth muscle?

Splanchnic mesoderm

What forms the visceral pleura of the lungs?

Splanchnic mesoderm

What forms the parietal pleura?

Somatic mesoderm

What forms the pleural cavity?

The space between parietal and visceral pleura

What are the 4 stages of lung development?

Pseudoglandular (6-16 weeks)

Canalicular (16-26 weeks)

Terminal sac (26 weeks - birth)

Alveolar (32 weeks - 8 years)

Which lung maturation stages are survivable?

Canalicular (if 24-26 weeks), terminal sac (unassisted IF surfactant production is sufficient (26-28 weeks)