Embryology Exam 3

intraembryonic coelom

embryonic body cavity, formed week 4 via embryonic folding, has 3 divisions

pericardial cavity

surrounds the heart

pericardioperitoneal canals

surrounds the lungs, lie lateral to foregut and pass dorsal to the septum transversum

septum transversum becomes the

central tendon of the diaphragm

parietal wall

covered with somatic mesoderm, forms the parietal layer of membranes (pleura, pericardium, peritoneum)

visceral wall

covered with splanchnic mesoderm, forms visceral layer of membranes (pleura, pericardium, peritoneum)

why might the pleural cavity not be present initially?

because the lungs develop over time

mesentery

a double layer of peritoneum which attaches a segment of gut to the body wall

2 initial mesenteries

ventral- attaches gut to ventral wall of intraembryonic coelom, dorsal- attaches gut to posterior wall of intraembryonic coelom; both divide the peritoneal cavity into left and right halves, only ventral disappears through much of its length

pleuropericardial folds

grow and eventually form partitions that separate the pericardial cavity from the pleural cavities that surround each lung

pleuroperitoneal folds

grow into pericardioperitoneal canals forming pleuroperitoneal membranes

diaphragm

most important muscle of inspiration, dome-shaped fibromuscular structure that separates thoracic cavity from abdominal cavity

diaphragm develops from

septum transversum, pleuroperitoneal membranes, dorsal mesentery of the esophagus, muscle of lateral walls

septum transversum

crescent shaped, shelf-like structure formed of mesoderm, grows dorsally from ventrolateral body wall and passes between heart and liver to form an incomplete partition between thoracic and abdominal cavities

pleuriperitoneal membranes

grow ventromedially to fuse with septum transversum and dorsal mesentery of esophagus

dorsal mesentery of esophagus forms

left and right crus of the diaphragm

crus of the diaphragm

pass in front of aorta forming aortic hiatus, right crus forms boundaries of esophageal hiatus

septum transversum movements

travel with nerve fibers from C3-5 in its decent and form phrenic nerve, supplying the diaphragm, 4th week is at the level of the 3rd-5th cervical somites, later through differentiated growth of body wall is lies at adult position of diaphragm between thoracic and abdominal cavity

congenital diaphragmatic hernia

herniation of abdominal viscera into thorax- due to presence of posterolateral defect of diaphragm, usually unilateral and results from defective formation of pleuroperitoneal membrane, results in large opening in diaphragm though which viscera protrude, most common cause of pulmonary hypoplasia, results in sever breathing difficulties

respiratory system develops

week 4 with appearance of laryngotracheal groove in caudal anterior wall of pharynx

endoderm of laryngotracheal groove

forms epithelium and glands of visceral organs

splanchnic mesoderm external to foregut- larynx, trachea, bronchi

forms stroma and smooth muscles of visceral organs- trachea and bronchi

the distal end of the laryngotracheal diverticulum enlarges to form ___ while the opening of the laryngotracheal diverticulum into the pharynx becomes the ___

respiratory bud; primordial laryngeal inlet

the laryngotracheal diverticulum is initially

in open communication with the pharynx

2 longitudinal tracheoesophageal folds

develop in the laryngotracheal diverticulum as it elongates, grow toward each other and fuse to form tracheoesophageal septum

tracheoesophageal septum divides laryngotracheal diverticulum into

ventral part- forma larynx, trachea, bronchi, lungs ; and dorsal part- forms esophagus as a direct continuation of pharynx

respiratory epithelium of larynx develops from

endoderm lining the laryngotracheal tube

epiglottis and its cartilage

develop from caudal part of hypopharyngeal eminence

8 of 9 intrinsic muscles of larynx develop from

myoblasts of 6th pharyngeal arch, 1 exception is cricothyroid muscle(controls innervation) that develops from 4th pharyngeal arch

where is the larynx of most mammals found and what’s the purpose?

in a high position in the neck dorsal to the soft palate, creates separate respiratory and digestive channels allowing mammals to swallow and breathe at the same time

larynx of adult humans

lies in a low position increasing the size of the pharynx- facilitates speech and prohibits swallowing and breathing at the same time

where is the larynx in an infant?

it is born high up similar to other mammals, but during the first 1.5-2 years after birth it descends to adult position corresponding to time of speech development

trachea develops from

laryngotracheal tube distal to larynx

endoderm lining of laryngotracheal tube forms the

respiratory epithelium and glands of the trachea

splanchnic mesoderm external to laryngotracheal tube forms

the c-ring cartilages and smooth muscle of the trachea

tracheoesphageal fistula

an abnormal opening between trachea and esophagus, results from incomplete division of laryngotracheal tube from esophagus due to incomplete fusion of tracheoesphageal folds, usually associated with esophageal atrasia, infant cannot swallow and reflux of gastric acid into lungs may occur- associated with polyhydramnios

esophageal atrasia

upper part of esophagus ends as a blind pouch

a single respiratory bud develops where?

the end of the laryngotracheal diverticulum during week 4, it then divides into 2 primary bronchial buds

beginning of week 5

each primary bronchial bud enlarges to form right and left main bronchi- carry air to and from right and left lung

right main bronchus splits into

upper division and lower division, lower division then splits again

upper bronchi division becomes

superior lobar bronchus, supplies superior lobe of right lung

lower bronchi division

splits again to become middle and inferior lobar bronchi, supply middle and inferior lobes of right lung

left main bronchus

splits only once into superior and inferior lobar bronchi, supply superior and inferior lobes of left lung

by week 7 the lobar bronchi form

10 segmental bronchi in each lung

10 bronchopulmonary segments of each lung

the segmental bronchi and the splanchnic mesoderm that surrounds it

splanchnic mesoderm surrounding bronchi and bronchioles form

cartilaginous rings and plates of bronchi, smooth muscle of bronchi and bronchioles

as the outer lungs develop

splanchnic mesoderm on its outer surface forms visceral pleura

somatic mesoderm covering inside of thoracic wall forms

parietal pleura

pleural cavity

space between parietal and visceral pleura

4 stages of lung development

pseudoglandular stage: 6-16 weeks, canalicular stage: 16-26 weeks, terminal sac stage: 26 weeks-birth, alveolar stage: 32 weeks-8 years

pseudoglandular stage

6-16 weeks, chracterized by branching of bronchioles and formation of bronchopulmonary segments, by end all major parts of lung have formed except those involved with gas exchange

T/F premature infants born in canalicular stage may survive

F- respiration cannot occur, premature infants at this stage cannot survive

canalicular stage

16-26 weeks, characterized by expansion in diameter of bronchi, vascularization of the lung, formation of respiratory bronchioles and alveolar ducts, by end some terminal sacs have developed and lungs have become highly vascularized

T/F a premature infant born toward the end of canalicular stage may survive

T- some gas exchange can occur, however it will require intensive care and mortality risk is high

terminal sac stage

26 weeks-birth, characterized by formation of numerous terminal sacs which represent primordia of alveoli of mature lung, walls of terminal sacs become thin as cuboidal cells of respiratory bronchioles transform to squamous flattened epithelial cells (type I pneumocytes)

type I pneumocytes

flattened epithelial cells, make up terminal sac epithelium that’s 1 cell thick, capillary beds surround the terminal sacs and bulge into their walls, facilitates gas exchange

type II pneumocytes

rounded cells that secrete surfactant

surfactant

a mixture of phospholipids and proteins that form a thin film over inner walls of terminal sacs and later alveoli, reduces surface tension between air and alveolar surfaces which is essential to allow alveolar sacs to expand and prevent their collapse during expiration

surfactant secretion begins

20-22 weeks and reaches adequate levels for unassisted survival at 26-28 weeks

alveolar stage

32 weeks-1 year, characterized by development of mature alveoli, 95% of mature alveoli develop postnatally- mostly complete by 3 years but some form up to 8 years

3 factors that determine if a fetus can make transition successfully from dependence on placenta for gas exchange to air-breathing respiration

adequate surfactant, formation of functional alveoli, establishment of pulmonary/systemic circulation

why is it important to advise parents not to smoke around infants/small children?

the alveoli are still developing and inhaling smoke could alter/disrupt the developing alveoli

at birth lungs are filled with what? what happens to that fluid?

at birth lungs are about half-filled with amniotic fluid, removal of the amniotic fluid is essential to allow inflation of the lungs

3 ways clearance of amniotic fluid occurs

expulsion through mouth and nose due to pressure on thorax during parturition (c-section babies have more fluid in lungs), via pulmonary blood vessels, via pulmonary lymphatics

fetal breathing movements occur intermittently during

3rd trimester of gestation and some amniotic fluid is aspirated during these movements

fetal breathing movements function to

exercise muscles of respiration, stimulate lung development

are pulmonary lymphatics larger and more numerous in adults or a fetus?

they’re larger and more numerous in a fetus to facilitate drainage of amniotic fluid

oligohydramnios and hypoplasia

when insufficient amniotic fluid is present, expansion of the lungs is restricted due to pressure from the uterine walls, results in pulmonary hypoplasia

respiratory distress syndrome

rapid, labored breathing that develops shortly after birth, affects 2% of neonates, major cause is surfactant deficiency

digestive system is initially closed by the

oropharyngeal membrane (cranial end) and cloacal membrane (caudal end)

when does the digestive system begin development?

week 4 when folding of the embryo incorporates the dorsal part of umbilical cord

what germ layer(s) is the digestive system derived from?

all 3- endoderm, mesoderm, ectoderm

endoderm

forms most of the digestive system, epithelium of gut, parenchyma of accessory organs (liver, pancreas) and glands

ectoderm

forms epithelium of primitive oral cavity (stomodeum) and distal end of anal canal (proctodeum or anal pit)

mesoderm

splanchnic mesoderm forms the smooth muscle and connective tissue of gut wall and stroma of accessory organs

the 3 embryological components the digestive system

foregut, midgut, hindgut

foregut

forms the distal esophagus through second part of duodenum where the bile duct enters

midgut

forms the duodenum below entrance of bile duct through right 2/3 of the transverse colon

hindgut

forms left 1/3 of transverse colon through upper part of anal canal

foregut derivatives are supplied by

branches of the celiac trunk

esophagus develops from

pharynx as the laryngotracheal diverticulum and is divided from laryngotracheal tube by tracheoesophageal septum

the esophagus consists of

upper 1/3, middle 1/3, lower 1/3

lower third of esophagus

consists of smooth muscle derived from splanchnic mesoderm surrounding the foregut

middle third of esophagus

consists of mixed striated and smooth muscle

upper third of esophagus

consists of striated muscle

esophageal atresia

upper part of esophagus ends as a blind pouch, usually associated with tracheoesophageal fistula, results from deviation of tracheoesophageal septum in posterior direction

what clinical condition results in a fetus being unable to swallow amniotic fluid and results in polyhydramnios?

esophageal atrasia

the ventral and dorsal mesenteries of the stomach are called

ventral and dorsal mesogastrium

ventral mesogastrium

attaches stomach to ventral body wall

dorsal mesogastrium

attaches stomach to dorsal body wall, contains developing spleen

which direction does the stomach rotate?

90 degrees clockwise

stomach

dilation of the foregut occurs week 4, broadens ventrolaterally, dorsal border grows faster than ventral

ventral and dorsal borders of the stomach becomes which curvatures?

ventral border becomes lesser curvature and dorsal border becomes greater curvature of stomach

after a 90 degree clockwise rotation the ventral border comes to lie

to the right and the dorsal border comes to lie to the left

after a 90 degree clockwise rotation the left surface and right surfaces become

the ventral and dorsal surfaces of the stomach

after a 90 degree clockwise rotation the left and right vagus nerves supply

ventral and dorsal surfaces of the stomach

as the stomach rotates the dorsal mesogastrium elongates and forms

a cavity that becomes the omental bursa- a subdivision of the peritoneal cavity

after a 90 degree rotation the left/right vagus nerves become the

anterior/posterior vagal trunks

the dorsal mesogatrium expands

caudally to overhang intestines and its 2 layers fuse to form greater omentum

the ventral mesogastrium becomes the

lesser omentum and falciform ligament of liver