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

Intraembryonic Coelom (Body Cavity)y 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

Intraembryonic Coelom (Body Cavity)y The intraembryonic coelom can be understood by imagining a horseshoe of cavity placed

within the mesoderm of the embryo

Folding of the Embryo Lateral Foldingy 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

Parietal and Visceral Membranes and Mesenteries

Parts of the Intraembryonic Cavityy 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 y The future heart occupies the pericardial coelom; the lungs occupy the pericardioperitoneal canals; the contents of the gut occupy the peritoneal cavity

Shape of the Intraembryonic Cavity

The septum transversum lies between the pericardial cavity and the vitelline duct

Organization of the Intraembryonic Cavity

Separation of Cavitiesy 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 y The pleuropericardial folds lie above and ventral to the bronchial buds, dorsal to the primordial heart tube

The Pleuropericardial Membranesy 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 y 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

The Pleuropericardial Membranesy The contents of the pleuropericardial membranes are the phrenic nerves and the common cardinal

veins initially y Since later the pleuropericardial membranes become the fibrous pericardium, the phrenic nerves are found on them in the adult

The Phrenic Nervesy Since later the pleuropericardial membranes become the fibrous pericardium, the phrenic nerves are

found on them in the adult

The Pleuroperitoneal Membranesy 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 y 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

The Diaphragmy The diaphgram has four origins: the septum transversum, the pleuroperitoneal

membranes, the mesentery of the esophagus, and muscular ingrowth from the body wall y 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

The Diaphragmy 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 y Later the septum transversum expands and fuses with the pleuroperitoneal membranes and the mesentery of the esophagus

The Diaphragmy The pleuroperitoneal membranes expand ventromedially from the posterior abdominal

wall to fuse with the mesentery of the esophagus and the septum transversum y 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

The Diaphgramy 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

The Diaphragm

Position and Innervation of the Diaphragmy 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) y 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 y Since some of the peripheral muscular fibers are derived from the body wall, they receive sensory innervation from the lower intercostal nerves

The Respiratory Systemy 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 y 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

The Respiratory Diverticulumy 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

The Larynxy 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 y 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 y With rapid proliferation and transformation of the mesenchyme of the arches, the laryngeal inlet becomes T-shaped, then finally assumes the adult form

The Tracheay 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 y 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

The Bronchi and Lungsy 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 y 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

The Pleural Cavitiesy 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

The Alveoliy Lung maturation passes through four phases: pseudoglandular, canalicular, terminal sac,

and alveolar stages y 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 y 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

Fluid and Movements of Lungsy 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 y 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 y Pulmonary vessels y Lymphatics (which are larger and more numerous in near-term fetuses than in adults)y

y Fetal breathing movements prepare and condition the lungs for respiration even before birth