Devo Week 3

Front 1

Longitudinal folding of embryo

Back 1

--Early in Fourth week
--Head fold: rapid grwoth of developing forebrain results in its growth beyond oropharyngeal membrane, and it overhangs the developing heart. portion of the endoderm of the yolk sac becomes incorporated into the embryo as the future foregut
Tail fold: rapid growth of caudal neural tube causes caudal eminence to project over the cloacal membrane. portion of endoderm of yolk sac incorporates as future hindgut.

Front 2

Transverse/Horizontal plane folding

Back 2

--left and right lateral folds arise from rapid growth of spinal cord and somites-->cylindrical embryo
--intrercellular clefts form in lateral plate mesoderm that deepend to form intraembryonic cavity/coelom. initially continuous with extraembryonic coelom, but eventually sides will meet (and incorporate part of yolk sac to form midgut)

Front 3

lateral plate mesoderm

Back 3

--as transverse folding continues, forms two layers:
--somatic/parietal mesoderm that lines walls of the future peritoneal, pleural, and pericardial cavities
--splanchnic/visceral mesoderm: cover abdominal organs, lungs, and heart
--the two layers meet as the dorsal mesentery

Front 4

Mesentary

Back 4

--a double layer of peritoneum that originates as part of the visceral peritoneum covering body organs
--connect organs to the body wall and transmit vessels/nerves to the organs
--initially, dorsal and vetnral mesentaries divide peritoneal cavity into right and left, but ventral mesentary disappears except in caudal part of foregut at level of liver-->continuous peritoneal cavity

Front 5

Primitive Pericardial cavity

Back 5

--curved portion of the horseshoe shaped embryonic body cavity will become pericardial cavity. Here the primitive heart will invaginate.
--iniitially located in front of neural tube and oropharyngeal membrane
--Cranial-caudal folding shifts it vetnrally and caudally

Front 6

Pericardioperitoneal canals

Back 6

--arms of the horseshoe shaped embryonic body cavity are the pericardioperitoneal canals and the peritoneal cavities
--lateral folding brings the two peritoneal cavities together to form a right and left peritoneal cavity (separated by dorsal and ventral mesenteries)
--ventral mesentery degenerates

Front 7

partitioning of the pericardial and pleural cavities

Back 7

--pericardial cavity + pericardioperitoneal canals = future thoracic cavity
--pleuropericardial folds are formed (in part by bronchial bud grwoth), separating the primitive pericardial cavity from the pericardio-peritoneal canals, aka the future pleural cavities
--developing lungs will fill this space and drive development
--pleuopericardial folds-->fibrous pericardium, creating definitive pericardial and pleural spaces (problems sometimes arise with formation on left side)

--region of thorax between the newly formed pleural cavities is called the mediastinum

Front 8

Development of the diaphragm

Back 8

--drives partitioning of peritoneal cavity from pericardial/pleural.
--Four separate events:
*mass of mesoderm called septum transversum grows dorsally from ventrolateral body wall-->will become central tendon of diaphragm
*early in development, it doesn't completely separate thoracic and abdominal cavities due to passage of pericardioperitoneal canals
*pleuroperitoneal folds aris dorsolaterally and grow into caudal ends of pericardioperitoneal canals, uniting with dorsal mesentery of esophagus-->form the posteriolateral musculature of the diaphragm (opening on right closes earlier than left)
*septum transversum and pleuroperitoneal membranes fuse with dorsal mesentery
*dorsal mesentery of esophagus retained to from crura of diaphragm and esophageal hiatus

Front 9

Positional changes of diaphragm

Back 9

--Septum transversum lies at cervical level C3-C5 prior to completion of folding process.
--myoblasts from cervical somites at these levels migrate into margins of septum transversum and bring their innervations with them-->phrenic nerve
--by 41 days, diaphragm at mid thoracic levels
--at beginning of week 8, dorsal portion of diaphgram at L1
--in adults, phrenic nerve 30 cm long due to this shift
--phrenic nerve both motor and sensory; margins of diaphragm get some innervation from intercostal nerves

Front 10

Congenital diaphragmatic hernia (CDH)

Back 10

most common anomaly of diaphragm
--posterolateral defect due to faulty closure of pleuroperitoneal membrane
--defect often referred to as foramen of Bochdalek
--allows abdominal cavity contents to enter pleural cavity-->respiratory distress, cardiac shift, underdevelopment of lungs
--most common on left due to later fusion and presence of liver on right side
--other hernias can occur but are less common (at esophageal hiatus and retrosternal

Front 11

Eventration

Back 11

from faulty muscle development producing a thin diaphragm which balloons uperiorly into the thorax; abdominal organs displaced superiorly
-->hypoplastic, compressed lung

Front 12

Cleft sternum

Back 12

--body wall defect resulting from failure of fusion of lateral mesoderm responsible for formation of sternum (only a problem if heart pokes through...aka ectopia cordis)

Front 13

Ectopia cordis

Back 13

Heart protruding through cleft sternum or due to absence of lower third of sternum

Front 14

Gastroschisis

Back 14

herniation of abdominal content through a body wall defect lateral to the umbilical cord (not covered in membrane, must be kept hydrated to prevent dessication until can be put back inside)

Front 15

Bladder exstrophy

Back 15

pelvic wall defect that results in bladder forming outside of the body cavity

Front 16

Cloacal exstrophy

Back 16

--a more severe pelvic wall defect resulting in an exposed bladder

Front 17

Development of trachea

Back 17

--laryngotracheal groove is primordium of the respiratory system
--appears as endodermal out pouching from the floor of hte pharynx caudal to the 4th pharyngeal pouch
--by end of fourth week, envaginates to form laryngotracheal diverticulum
--thickens to form tracheoesophogeal folds
--as diverticulum elongates, respiratory buds appear at its distal end
--tracheoesophogeal folds fuse to form the tracheoesophogeal septum, formed from mesoderm and endoderm
--neural crest cells from brachial arches 4 and 6 will give rise to laryngeal cartilages and muscles
--airway lining and poulmonary alveoli are endodermal in origin; laryngeal lumen is obliterated by epithelial proliferation and then recanalized-->failure to recanalize leads to laryngeal stenosis or atresia

Front 18

Tracheoesophageal fistula

Back 18

results from faulty formation and separation of the trachea from the esophagus by the tracheoesophageal septum

--1 in 3000-5000 births, more common in males

--85% of cases associated with esophageal atresia

--several forms, but all result in direct or indirect aspiration of food into respiratory airway

--most common type is esophageal atresia with distal fistula

Front 19

Polyhydramnios

Back 19

often associated with esophageal atresia because fluid cannot passed into GI system for absorption and transferthrough placenta to mother's blood stream

Front 20

Lung Development overview

Back 20

--laryngotracheal tube grows caudally. concurrently:
--respiratory bud at end of tracheal diverticulum divides into two primary bronchial buds
--these buds expand within splanchnic mesoderm as they grow laterally into pericardio-peritoneal canals
--near end of 4th week, secondary and tertiary bronchial buds
--in fifth week, left and right main bronchi begin to form, which subdivid into secondary bronchi that form lobar, segmental, and intrasegmental branches
--by 24 weeks, 17 order of branches have formed
--7 orders of branching occur after birth
--mesothelial tissue lining inner body becomes parietal pleura
--further growth and branching after birth

Front 21

4 Stages of lung development

Back 21

Pseudoglandular (6-16 weeks)
Canalicular (16-24 or 26)
Terminal sac (week 24 or 26 to birtth)
Alveolar (32 weeks to 8 years)


**these stages overlap b/c cranial end lung is developing faster than caudal end of lung

Front 22

pseudoglandular stage of lung development

Back 22

weeks 6-16
--lung resembes exocrine gland with numerous endodermal tubules lined with columnar epithelium which are surrounded by mesoderm with only rudimentary capillary network
--no gas exchange structures present. no survival if born

Front 23

Canalicular stage of lung development

Back 23

16-24 or 26 weeks
--Airway lumens enlarge
--respiratory bronchioles, alveolar ducts, and primitive alveoli present
--alveolar ducts are lined with simple cuboidal epithelium
--surrounding mesoderm contains enriched capillary network, but no contact with terminal sacs have been developed
--Alveolar type I and II pneumocytes begin to differentiate and type II start making surfactant around 20 weeks
--partial overlap with pseudoglandular stage
--survival possible if born at this stage

Front 24

Terminal sac stage of lung development

Back 24

week 24 or 26 until birth
--more terminal sacs (primitive alveoli) are formed and become separated from each other by primary septa
--cuboidal epithelium differentiate into type I pneumocytes which make intimate contact with capillaries and establish blood-air barrier and type II pneumoytes which secrete surfactant

--survival if born between weeks 24 and 26, but with respiratory distress due to surfactant deficiency
--between weeks 26 and 28 suually have neough terminal sacs and surfactant to survive

Front 25

Aleolar stage of lung development

Back 25

--32 weeks to 8 years
--terminal sacs begin partitioning by secondary septa to form adult alveoli
--respiratory membrane matures (thins) as capillaries bulge into the alveoli and surfactant production increases prior to birth
--95% of alveoli produced after birth
--after birht, increase in lung size due primarily to increase in the number of respiratory bronchioles and number of primordila alveoli and not an increase in the size of alveoli

Front 26

Surfactant

Back 26

lipoprotein that reduces surface tension forces within alveoli and facilitates their expansion with air and maintains their patency

--50% cholesterol, 40% dipalmitoylphosphatidylcholine,10% surfactant proteins A,B,C

--produced at greatest amount in 2 weeks before birth

Front 27

Respiratory Distress syndrome

Back 27

aka hyaline membrane disease
--deficiency or absence of surfactant
--prevalent in premature infants, causes 50=70% of premature infant deaths

--treatments include administration of corticosteroirds to mother prior to impending birth to increase surfactant production, postnatal administration of artificial surfactant, and high frequency ventilation

Front 28

Fetal breathing movements

Back 28

--aspiration of amniotic fluid, essential for normal lung development

--pattern of fetal breathing can be used to predict labor and fetal outcome in a preterm infant

--at birth, much of liquid is absorbed by capillaries and rest is expelled into trachea

--surfactant, however, remains.