Body Cavities and Lung Development

Question 1

The formation of the body cavities is accomplished by the […] folding of the embryo.

Answer 1

The formation of the body cavities is accomplished by the ventro-lateral folding of the embryo.

Question 2

What kind of cells line the body cavity?

Answer 2

serous cells

Question 3

Serous Cells

Answer 3

These cells secrete fluid for lubrication for the organs that will grow to occupy the space of the cavity.

Question 4

Mesenteries

Answer 4

As the folding closes the space, there are mesodermal connections that remain between the developing gut tube that project both ventrally and dorsally. These are called mesenteries. The ventral mesentery disappears in most locations except in the vicinity of the septum transversum (around the location that will eventually become the liver and diaphragm). This results in a single open abdominal (peritoneal) cavity.

Question 5

4 Coelomic Body Cavities

Answer 5

These will be:

1. a pericardial cavity
2. and 3. two pericardio-peritoneal cavities (the adult pleural cavities)
3. a peritoneal (abdominal) cavity.

Question 6

The Tissues That Make Up the Diaphragm

Answer 6

• The tissues that eventually make up the diaphragm are derived from several locations.
• These include the septum transversum, the pleuroperitoneal membranes, the dorsal mesentery of the esophagus and the body wall.

“Several Parts Build Diaphragm”

Question 7

Septum Transversum

Answer 7

•The septum transversum grows from the ventral body wall separating the developing heart and liver. The septum begins partitioning the pericardial and peritoneal coelom. It connects dorsally with the esophagus/foregut suspended by its associated dorsal mesentery closing off most of the space between the thoracic and abdominal cavities. In the adult, the septum transversum will produce the central tendon of the diaphragm.

Question 8

Pleuro-Peritoneal Folds and Dorsal Mesentary

Answer 8

• Two dorsolateral openings still connect the thorax and abdomen. These lie on either side of the foregut and are called pericardio-peritoneal canals (makes sense). These connections between the pleural and peritoneal cavites are closed by two membranes that grow medially to join with the septum transversum. These are the pleuro-peritoneal folds.
• When they eventually fuse with the septum transversum and the components of the dorsal mesentery of the esophagus they complete the formation of the diaphragm and the separation of the thoracic and abdominal cavities.

Question 9

Body Wall

Answer 9

•Finally, the most peripheral portions of the diaphragm become occupied by skeletal muscle that is derived from the body wall.

Question 10

Which nerves innervates the diaphragm?

Answer 10

phrenic nerve

Cervical spinal nerves C3, 4, and 5 contribute to the phrenic (C345 keeps the diaphragm alive!). This is a reminder that although the diaphragm will eventually come to rest in the thoracic vertebral levels, it originally arose in the cervical regions. As growth occurred, the diap

Question 11

What happens if the Pleuro-PEritoneal Folds do not fuse with the septum transversum and dorsal mesentary?

Answer 11

Fusion failure can lead to congenital diaphragmatic hernia (CDH), which if severe can cause problems with lung development because intestines can push into the thorax (typically unilaterally).

Question 12

5 Stages of Lung Development

Answer 12

1. Embryonic Stage
2. Pseudoglandualr Stage
3. Canalicular Stage
4. Terminal Sac Stage
5. Postnatal Stage

Question 13

Embryonic Stage

Answer 13

• (4 to 7 weeks = embryonic period overall)
• During this time the primitive airways are developing and the lungs are beginning to fill the pleural cavities.

Question 14

Pseudoglandualar Stage

Answer 14

• (8 weeks to 16 weeks)
• This is a continuation of the development of the airway conducting system of the lungs, prior to formation of the gas exchange regions. Histologically the organ looks like a gland at this point.
• Cells lining the airway are columnar.
• Lung arteries are beginning to form and follow the airways.

Question 15

Canalicular Stage

Answer 15

• (17 weeks to 26 weeks)
• This stage is characterized by the formation of respiratory bronchioles; thus, oxygen transport becomes possible during this period.
• Cells in the airway become ciliated cuboidal.
• There is intense ingrowth of blood vessels and formation of capillaries in the parenchyma/visceral mesoderm.

Question 16

Terminal Sac Stage

Answer 16

• (26 weeks to birth)
• During this stage, the alveoli form as buds from the respiratory bronchioles.
• The epithelial lining of the alveoli differentiates into type I and type II pneumocytes.
• Pulmonary surfactant is produced in the type II cells. This substance coats the inside of the alveoli and is necessary for appropriate opening and gas exchange. The production of surfactant is the major determining factor in the survival of preterm infants. Lack of surfactant causes respiratory distress syndrome.

Question 17

Postnatal Stage

Answer 17

• (birth to 5 years of age)
• Characterized by significant increase in the number of alveoli. At birth, only about 30 million alveoli exist. However, this number increases to 300 million in the mature lung. This occurs by a process called septation, in which, as alveoli get bigger they are subdivided by the outgrowth of a septum to divide each into two alveoli.

Question 18

Malformation of the Lung and Foregut

Answer 18

• Produces an aberrant connection known as an Esophageo-tracheal (or tracheoesophageal) fistula.
• Typically, this is a failure of the splanchnic mesoderm to push apart the endodermal tubes and is thus associated with other mesodermal malformations.
• This fistula (abnormal connection with a tube) is discovered when a newborn cannot take milk into the stomach, has respiratory complications (e.g., milk in the lungs) and a failure to thrive.

Question 19

Normal Surfactant Development

Answer 19

• Surfactant is a phospholipoprotein complex produced by Type II alveolar cells beginning around 20 weeks and increasing as the fetus approaches birth.
• Like soap, its function is to reduce surface tension in water and prevent the surface tension from collapsing alveoli (like bubbles) at the end of expiration.

Question 20

Surfactant is made of…

Answer 20

It is approximately:

• 40% dipalmitoylphosphatidylcholine
• 40% other phospholipids
• 5% surfactant proteins A, B, C & D
• the rest is a mixture of cholesterol and plasma proteins.
• It is made inside the Type II cells as lamellar bodies prior to secretion.

Question 21

Respiratory Distress Syndrome

Answer 21

•Babies born prior to 28 weeks usually have insufficient levels of surfactant. The lack of surfactant causes atelectasis or collapse of the alveoli in the lung and difficulty breathing.

Question 22

Prenatal RDS Treatment

Answer 22

• Prior to 34 weeks, mothers at risk of premature delivery may be given antenatal glucocorticoids to speed up surfactant production. Surfactant amounts are monitored by sampling amniotic fluid for the L/S (lecithin/sphingomyelin) ratio.

Question 23

Postnatal RDS Treatment

Answer 23

•Postnatally, premature infants are given artificial surfactants made up of lipid mixtures, bovine calf surfactant or a mixture of both. Infants may also be provided with moist oxygen. Optimization of these protocols is still under development but incidence of RDS is lower.

Question 24

Bronchopulmonary Dysplasia

Answer 24

• Babies born more than 10 weeks prematurely with RDS may have bronchopulmonary dysplasia (BPD), a version of RDS that persists and is more severe.
• Usually found in premature infants, BPD reflects long term lung damage due to early issues with surfactants and oxygen exposure.
• For example, the high oxygen pressure used to keep the immature alveoli open can cause lung, brain and eye damage.
• The incidence has decreased in recent times as protocols with artificial surfactant and oxygen have improved.