This session discusses: 1. Development of body cavities 2. Development of the respiratory system

1. - Body Cavities
- Separation of Body Cavities
- The Diaphragm
- Respiratory System

2. Intraembryonic Coelom (Body Cavity)
 The primordium of the body cavity begins as isolated coelomic spaces in the
lateral plate mesoderm and cardiogenic area which coalesce to form a single
horseshoe-shaped cavity that communicates laterally with the extraembryonic
cavity

3. Intraembryonic Coelom (Body Cavity)
 The intraembryonic coelom can be understood by imagining a horseshoe of cavity placed
within the mesoderm of the embryo

4. Folding of the Embryo – Lateral Folding
 The spinal cord and somites stimulate lateral folding which causes incorporation of
another portion of the yolk sac forming the midgut, and ventrolateral body walls of the
embryo are formed; connection with yolk sac is reduced to yolk stalk (or omphaloenteric
duct) which is the site of the future umbilicus

5. Parietal and Visceral Membranes and Mesenteries

6. Parts of the Intraembryonic Cavity
 The intraembryonic cavity can be divided into the pericardial cavity, the
pericardioperitoneal canals, and the peritoneal cavity; the pericardioperitoneal
canals are lateral to the foregut, dorsal to the septum transversum
 The future heart occupies the pericardial coelom; the lungs occupy the
pericardioperitoneal canals; the contents of the gut occupy the peritoneal cavity

7. Shape of the Intraembryonic Cavity
The septum
transversum
lies between
the pericardial
cavity and the
vitelline duct

8. Organization of the Intraembryonic Cavity

9. Separation of Cavities
 When the lung primordia (bronchial buds) grow from the foregut into the
pericardioperitoneal canals, a pair of folds (later becoming membranes) form in each
canal: the pleuropericardial folds above, and the pleuroperitoneal folds below
 The pleuropericardial folds lie above and ventral to the bronchial buds, dorsal to the
primordial heart tube

10. The Pleuropericardial Membranes
 As the bronchial buds grow into the pericardioperitoneal canals, the canals enlarge ventrally around
the heart making the pleuropericardial folds larger (then known as membranes) and separating the
body wall at that area into the pleuropericardial membranes and the definitive thoracic wall
 Later the medial ends of the membranes fuse with the ventral mesentery of the esophagus thus
completely separating the pleural cavities from each other and from the pericardial cavity

11. The Pleuropericardial Membranes
 The contents of the pleuropericardial membranes are the phrenic nerves and the common cardinal
veins initially
 Since later the pleuropericardial membranes become the fibrous pericardium, the phrenic nerves are
found on them in the adult

12. The Phrenic Nerves
 Since later the pleuropericardial membranes become the fibrous pericardium, the phrenic nerves are
found on them in the adult

13. The Pleuroperitoneal Membranes
 The pleuroperitoneal membranes are attached dorsolaterally to the posterior abdominal
wall; they extend forward ventromedially and eventually fuse with the septum
transversum and the dorsal mesentery of the esophagus thus separating the plueral
cavities from the peritoneal cavity
 The right pleuroperitoneal membrane closes slightly earlier than the left one; the reason
could be due to the large size of the liver located on the right side

14. The Diaphragm
 The diaphgram has four origins: the septum transversum, the pleuroperitoneal
membranes, the mesentery of the esophagus, and muscular ingrowth from the body wall
 The septum transversum is first identified at the end of the 3rd week cranial to the
pericardial cavity; with the head fold, it comes to lie between the pericardial cavity above
and the peritoneal cavity below, although it does not completely separate them, leaving
the pericardioperitoneal canals behind

15. The Diaphragm
 The septum transversum is first identified at the end of the 3rd week cranial to the
pericardial cavity; with the head fold, it comes to lie between the pericardial cavity above
and the peritoneal cavity below, although it does not completely separate them, leaving
the pericardioperitoneal canals behind
 Later the septum transversum expands and fuses with the pleuroperitoneal membranes
and the mesentery of the esophagus

16. The Diaphragm
 The pleuroperitoneal membranes expand ventromedially from the posterior abdominal
wall to fuse with the mesentery of the esophagus and the septum transversum
 The pleuroperitoneal membranes later contribute a small amount to the diaphgram; the
esophageal mesentery forms the crura of the diaphgram; the septum transversum forms
the central tendon; and the muscular ingrowth forms the peripheral muscular part

17. The Diaphgram
 During the 3rd month, the expanding lungs and pleural cavities burrow into the lower part of the
lateral body walls, dividing it into two parts: an outer part which is continuous with the definitive
abdominal wall, and an inner part which contributes to the peripheral part of the diaphragm; this
results in the formation of the costodiaphragmatic recesses between the two layers in the adult

18. The Diaphragm

19. Position and Innervation of the Diaphragm
 Initially, after the head fold, the septum transversum lies opposite the 3rd to 5th somites and hence
receives myoblasts from them; these myoblasts bring their nerve supply (the phrenic nerves) with
them and the nerves pass through the pleuropericardial membranes (future fibrous pericardium)
 The diaphragm descends with further growth and takes its nerve supply with it; later some of the
dorsal bands of the diaphragm lie opposite the first lumbar vertebra; the myoblasts in the septum
transversum are distributed in the other three parts of the diaphragm as well and form the skeletal
muscle of the diaphragm
 Since some of the peripheral muscular fibers are derived from the body wall, they receive sensory
innervation from the lower intercostal nerves

20. The Respiratory System
 The respiratory diverticulum (or lung bud) appears from the ventral surface of the foregut
around the 4th week of development; initially there is open communication between the
diverticulum and the rest of the foregut; later, two tracheoesophageal ridges fuse and
form a septum that divides the foregut into a ventral portion (the trachea) and a dorsal
portion (the esophagus); the trachea remains in communication with the pharynx through
the laryngeal orifice
 Thus, the internal epithelial lining of the entire respiratory system is derived from
endoderm while all the cartilage, muscle, and connective tissue is derived from the
surrounding splanchnic mesoderm

21. The Respiratory Diverticulum
 The respiratory diverticulum (or lung bud) appears from the ventral surface of the foregut around
the 4th week of development; initially there is open communication between the diverticulum and the
rest of the foregut; later, two tracheoesophageal ridges fuse and form a septum that divides the
foregut into a ventral portion (the trachea) and a dorsal portion (the esophagus); the trachea remains
in communication with the pharynx through the laryngeal orifice

22. The Larynx
 The larynx forms from the cranial end of the diverticulum; its lining is derived from endoderm;
proliferation of the epithelial lining results in a period of occlusion and then vacuolization which
results in the formation of the laryngeal ventricles and vocal cords
 The splanchnic mesoderm surrounding the larynx belongs to the 4th and 6th pharyngeal arches, and all
the cartilages and muscles are derived from it; hence, the nerves of the 4th and 6th arches, the
superior laryngeal and recurrent laryngeal nerves, respectively, supply these structures in the adult
 With rapid proliferation and transformation of the mesenchyme of the arches, the laryngeal inlet
becomes T-shaped, then finally assumes the adult form

23. The Trachea
 The rest of the diverticulum forms the trachea and two lateral outpocketings (the bronchial buds)
which subsequently form the brochial tree down to the alveoli on each side
 The epithelium of the trachea and its glands are derived from endoderm; its cartilage, muscle, and
connective tissue are derived from its surrounding splanchnic mesenchyme

24. The Bronchi and Lungs
 The bronchial buds grow into the right and left main bronchi; then the right one forms three
secondary bronchi and the left one forms two; the division continues in this manner forming about 17
generations of subdivisions by the end of the sixth month and an additional six in postnatal life
 These bronchial divisions grow into the pericardioperitoneal canals which become the pleural
cavities; the splanchnic mesoderm surrounding the bronchial divisions forms the visceral pluera, while
the somatic mesoderm forms the parietal pleura, and the space between them is the pleural cavity

25. The Pleural Cavities
 These bronchial divisions grow into the pericardioperitoneal canals which become the pleural
cavities; the splanchnic mesoderm surrounding the bronchial divisions forms the visceral pluera, while
the somatic mesoderm forms the parietal pleura, and the space between them is the pleural cavity

26. The Alveoli
 Lung maturation passes through four phases: pseudoglandular, canalicular, terminal sac,
and alveolar stages
 Respiration becomes possible from the terminal sac stage (near the middle of the 6th
month) when the primitive alveoli (or terminal sacs) form and establish contact with the
surrounding capillaries
 In the last two months of pregnancy (beginning of alveolar stage), cells lining the alveoli
(called type I alveolar cells) become thinner and the surrounding capillaries protrude into
the sacs; another cell type, the type II alveolar cells, appears near the end of the 6th
month and begins secreting surfactant

27. Fluid and Movements of Lungs
 Before birth, the lungs are filled with a fluid with high chloride concentration, little protein, mucus
from bronchial glands, and surfactant from the type II alveolar cells
 At delivery, the fluid is removed by the following pathways:
 Mouth and nose of the fetus due to pressure exerted on fetal thorax in vaginal delivery
 Pulmonary vessels
 Lymphatics (which are larger and more numerous in near-term fetuses than in adults)
 Fetal breathing movements prepare and condition the lungs for respiration even before birth