ANA301 Term Test 2

What does the intraembryonic coelom give rise to?

Pleural, pericardial, and peritoneal cavities

Parts of the respiratory system that are endoderm-derived

Respiratory epithelium

Parts of the respiratory system that are mesoderm derived

Intraembryonic splanchnic mesoderm

Cartilage/CT/SMCs of trachea + bronchi

Visceral pleura

Blood vessels of the lungs

Beginning of respiratory development (1st step)

Primordial foregut (cranial aspect) => pharynx

Paired mesenchymal swellings = pharyngeal arches => face/pharnynx/largyngeal structures

Respiratory outgrowth forms

What gives rise to pharyngeal arches

Mesoderm + neural crest cells

How many pharyngeal arches

6 pairs (5th pair regresses)

Respiratory outgrowth (what/where/when)

Ventral outgrowth that gives rise to the respiratory system

At the level of the 4th-6th pharyngeal arches

Week 4

How does the trachea separate from the esophagus

1: laryngotracheal groove forms

2: evagination ventrally to form laryngotracheal diverticulum (week 4)

3: distal end of diverticulum = respiratory bud

4: elongation + formation of tracheoesophageal folds

5: folds fuse = tracheoesophageal septum (end of week 5)

Tracheoesophageal folds

Growth that separates the trachea from the esophagus

Laryngeal inlet

Cranial opening between the primordial pharynx and developing trachea

Formation of the bronchioles

Respiratory bud bifurcates (= L + R primary bronchial buds) = week 4

Branching = secondary and tertiary bronchial buds = week 6-8

Tracheoesophageal fistula

Tracheoesophageal folds fail to fuse

Abnormal connection between the airway and the esophagus

Adult respiratory system structures

Nasal cavity

Oral cavity

Pharynx

Larynx

Trachea (15-20 cartilage rings + trachealis muscle)

Primary bronchi

Secondary bronchi

Segmental bronchi

Bronchioles

Alveoli

R + L lungs

Diaphragm

Carina

Wedge-shaped cartilage

Located at the bifurcation point of trachea

Superior = cough reflex

Inferior = no cough reflect

Right vs Left primary bronchus in the adult

Right = wider, shorter, vertical

Left = narrow, longer, horizontal

Right vs Left secondary (lobar) bronchi in adults

Right = 3

Left = 2

Right lung structures

3 lobes = superior, middle, inferior

Horizontal fissure (superior/middle)

Oblique fissure (middle/inferior)

Left lung structures

2 lobes = superior + inferior

Cardiac notch + lingula in superior lobe

Oblique fissure (superior/inferior)

Hilum of the lung

Depression on the mediastinal surface of the lungs

Contains the root of the lung (pulmonary arteries, pulmonary veins/primary bronchus/nerves/lymphatics)

State of the body cavities after folding

Embryonic folding = intra-embryonic coelom (U-shaped cavity)

- cranial = primitive pericardial coelom

- more caudal = paired pericardioperitoneal canals

- most caudal = peritoneal cavities (temporary communication with extraembryonic coelom

Development of the pleural cavities

Bronchial buds grow into the pericardioperitoneal canals

Expansion of the lungs reduces the cavity size => visceral and parietal pleura come into close apposition = pleural cavities

Pleural cavity in the adult

Lined by visceral pleural lining (adjacent to lung tissue) and parietal pleura (adjacent to ribs)

Space between linings filled with small amount of pleural fluid

Lung maturation stages

Pseudoglandular Stage (5-17 weeks)

Canalicular Stage (16-25 weeks)

Terminal Sac Stage (24 weeks - birth)

Alveolar Stage (late fetal period - 8 years)

Pseudoglandular stage

Rapid growth of conducting airway

Terminal bronchioles = exocrine glands

Gas exchange not possible

- epithelium = cuboidal

- capillaries = not closely associated with bronchioles

Canalicular stage

Development of respiratory elements

Gas exchange possible but limited

- epithelium = largely cuboidal

- capillaries = more & some apposed

Terminal sac stage

Many more terminal sacs bud off

Gas exchange is possible

- epithelium = thin + squamous (type 1 pneumocytes) & type II pneumocytes make surfactant

- capillaries = close apposition

Surfactant (time + purpose)

Begins in weeks 20-22

Functional levels around 26-28 weeks

Reduces surface tension + increases lung compliance

Low levels = neonatal respiratory distress syndrome (NRDS)

Alveolar stage

Terminal sacs mature into alveoli

Gas exchange gets more efficient

Diaphragm anatomy in adult

Attaches to margins of inferior thoracic aperture

Inserts in the midline at the central tendon

Hiatus

1. Caval opening for inferior vena cava

2. Esophageal hiatus

3. Aortic hiatus

Location of each hiatus in the diaphragm

I8 10 Eggs At 12

Inferior vena cava (T8)

Esophagus (T10)

Aorta (T12)

Embryonic components that give rise to the diaphragm

Septum transversum = central tendon

Pleuroperitoneal membranes = close pericardioperitoneal canals

Dorsal mesentery of esophagus = crura

Muscular ingrowth from body wall = peripheral skeletal muscle

Septum transversum for diaphragm development

Mesoderm derived structure

Week 3 = appears cranial to pericardial cavity

Week 4 = cranial folding moves it caudal

Extends dorsally from the ventrolateral body wall

- fuses with the mesenchyme and pleuroperitoneal membranes

Pleuroperitoneal membranes for diaphragm development

Grows ventromedially from dorsolateral body wall

Fuses with septum transversum + dorsal mesentery of esophagus (week 6)

Important in the fetus but minor contribution in the infant (replaced by muscle)

Dorsal mesentery of the esophagus for diaphragm development

Suspends esophagus to dorsal body wall

Located in the midline

Fuses with septum transversum + pleuroperitoneal membranes

Myoblasts from cervical somites (paraxial mesoderm) infiltrate + form crura

Muscular ingrowth from body wall for diaphragm development

Myoblasts from cervical somites (paraxial mesoderm) migrate into the lateral body wall

- form the muscular portion of the diaphragm

Congenital diaphragmatic hernia (CDH)

Defect in the diaphragm = abdominal viscera enter thoracic cavity

- can impair lung development (lack of space)

Most commonly due to incomplete fusion of pleuroperitoneal membranes with other components

When can heart beat be detected in the fetus

4th week (gestational week 6)

Before this point, blood flow = ebb + flow

Germ cell origins of the cardiovascular system

Heart = intraembryonic splanchnic mesoderm

Pericardial cavity = intraembryonic coelom + visceral + parietal pericardium

Aortic arches = neural crest cells

Formation of the heart tube

1. Primary heart field develops in the splanchnic mesoderm (week 3)

- U-shaped, bilateral

2. Primary heart field (mesenchymal cells) = angioblasts = aggregate to form angioblastic chords = canalize to form bilateral endocardial heart tubes

3. Secondary heart field = progenitor cells to elongate the arterial pole

What does lateral folding do for heart development (+ when)

Paired endocardial heart tubes moved to the midline

- Fuses = single primitive heart tube

Paired pericardial coeloms into midline

- fuse = pericardial cavity

Day 20-22

Suspension of the heart tube

Heart tube lies within the pericardial cavity (attached dorsally by a mesentery = dorsal mesocardium)

Mid-portion often the dorsal mesocardium degenerates = transverse pericardial sinus (day 28)

Anchoring points of the primitive heart tube (day 22-28)

Cranial = truncus arteriosus (arterial outflow)

Caudal = sinus venosus (venous inflow)

Layers of the embryonic heart wall

Endocardium = endothelial lining (derived from endocardial tube)

Myocardium = middle muscular layer (derived from splanchnic mesoderm), separated by cardiac jelly from the endocardium

Epicardium = outer layer (derived from the secondary heart field), coronary vessels

Constrictions + dilations of the heart tube

Day 23 - 28

Truncus arteriosus (TA) = arterial outflow, continuous with pharyngeal arch arteries

Pharyngeal arch arteries (PAA)

Bulbous Cordis (BC) = future R ventricle

Primitive ventricle (PV) = largest dilation = future L ventricle

Primitive Atrium (PA) = future L+R atria

Sinus Venosus (SV) = venous inflow segment

Septa in the adult heart

Atrioventricular septum

Interatrial septum

Interventricular septum

Heart looping

Day 23 (linear) - 28 (S-shaped)

Caused by differential growth in a confined space

- Bulbus cordis and primitive ventricle grow faster than the rest

Cranial end buckles ventrally, caudally, slightly to the right

Caudal end buckles dorsally, cranially, and slightly to the left

Trabeculation

Late week 4 - early week 5

Smooth myocardial surface => muscular ridges (Trabeculae)

Dextrocardia

Looping occurs in the opposite direction

Chambers on the wrong side of the

General mechanisms of septum formation

Tissue growth

- opposing ridges proliferate + extend across the lumen (from opposite sides or from a single wall)

Differential remodelling

- septal ridge present but fixed

- adjacent chambers expand/dilate

Formation of septa in the heart

1. Dorsal + ventral endocardial cushions form in the AV canal (end of week 4) + left + right cushions narrow the AV canals

2. Active growth of cushions + fusion = separation into R and L atroventricular canals (week 5)

3. Septum primum (thin membrane) extends from the roof of the atrium towards the cushions

- gap is left called ostium primum

4. Septum primum fuses with the cushions but cell death makes a new opening (ostium secundum)

5. Septum secundum forms (crescent-shaped with a gap (foramen ovale)

Function of the ostium secundum

R to L shunting during fetal life (no need for pulmonary circulation

Fossa ovale

Depression in the R atrial wall

Location where the septum primum and septum secundum fused after birth

Patent foramen ovale

Septa (primum + secundum) fail to fuse

- stay closed under normal pressure conditions

How do the atria develop

Primitive right atrium enlarges - pectinate

Encorporates right horn of the sinus venosus (into posterior wall) = sinus venarum - smooth-walled

Primordial pulmonary vein sprouts from the dorsal wall of the primitive atrium (week 4) + connects to vascular plexus

- bifurcates into 4 pulmonary veins

Left atrium enlarges and incorporates the pulmonary veins (until week 8)

Interventricular septum anatomy in the adult

Membranous part = small, superior, posterior

- results from the fusion of the right conotruncal (bulbar) ridge, left conotruncal (bulbar ridge), fused dorsal + ventral endocardial cushions

Muscular part = large, inferior, anterior

- leaves a gap called interventricular foramen

Timeline for heart development

Mostly between day 23 and day 28

Role of the membranous part of the interventricular septum

closes up the interventricular foramen

Formation of the aorticopulmonary septum

Swellings develop in the walls of the truncus arteriosus and bulbus cordis

- L + R truncal ridges in TA

- L + R bulbar ridges in BC

Neural crest cells migrate into the ridges

Ridges align, spiral, and merge = aorticopulmonary septum

Outflow tract partitioned into ascending aorta and pulmonary trunk

What happens to the interventricular foramen

Fusion of L + R (conotruncal) bulbar ridges

Fusion of dorsal + ventral endocardial cushions

Fusion to the superior margin of the muscular interventricular septum

Lateral expansions of the sinus venosus

L + R sinus horns (bilateral by day 26)

Concomitant with heart looping

Cardinal vein (anterior/posterior),

Umbilical vein (placenta)

Vitelline vein (UV)

Flow of blood in the embryo (day 26)

L+R head => L+R anterior cardinal vein =>

+

L+R body => L+R posterior cardinal vein =>

common cardinal vein => sinus venosus

Anatomy of the adult right atrium

Inflow vessels = superior + inferior vena cava

Pectinate muscles (Trabeculated part of the wall)

Sinus venarium (smooth part of atrial wall)

Interatrial septum (wall to L atrium)

Fossa Ovalis (depression with rim from septum secundum and floor from septum primum

Right Auricle (ear-like flap)

Right AV valve (tricuspid valve)

Adult anatomy of the right ventricle

AV valve (tricuspid)

Trabecular carnae (muscular ridges on wall of ventricle)

Interventricular septum

Pulmonary valve (semilunar)

Outflow vessels (pulmonary trunk + R and L pulmonary artery)

Adult anatomy of the left atrium

Inflow vessels (pulmonary veins (superior + inferior right + left))

Pectinate muscles (rough part of anterior wall)

Smooth posterior wall

Interatrial septum

Valve of foramen ovale (thinning in septum)

Left auricle (ear-like flap)

AV valve (bicuspid/mitral)

Adult anatomy of the left ventricle

AV valve (bicuspid/mitral)

Chordate tendinae (close valve)

Papillary muscles (close valve)

Trabecular carnae (muscular ridges)

Interventricular septum

Aortic semilunar valve

Outflow vessels (ascending aorta, aortic arch, brachiocephalic, left common carotid, left subclavian arteries)

Where is the heart located

Middle mediastinum

Between 2 pleural cavities

Enclosed in the pericardium (outer fibrous + inner serous layer)

Superior to diaphragm

Inner serous layer of pericardium

Parietal layer (somatic mesoderm)

Visceral layer (splanchnic mesoderm) = epicardium

Pericardial cavity contains pericardial fluid

Fate of the sinus venosus in the embryo

R atrium enlarges + incorporates right horn into posterior wall (sinus venarium in adult) = week 6

Venous return shifts to the R, so left horn of sinus venosus shrinks (persists as coronary sinus in adult) = week 8

Coronary sinus

Blood vessel on the posterior side that drains blood from coronary circulation into the right atrium

Coronary arteries that supply the heart tissue

Right Coronary A

- branch of ascending aorta

- in atrioventricular sulcus

- R marginal branch (R ventricle)

- Posterior interventricular artery

- supplies RA, RV, some LV

Left Coronary A

- branch of ascending aorta

- posterior to pulmonary trunk + bifurcates

- Anterior interventricular artery (anastomosis with posterior interventricular artery at apex)

- circumflex a (AV sulcus) (anastomosis with R coronary artery)

- left marginal a (branch of circumflex a)

- supplies LA and LV)

Fate of Anterior Cardinal veins

Day 26 = bilateral venous drainage from head

Week 7-8 = anastomosis between L+R anterior cardinal vein = shift blood flow from Left to Right

Caudal L anterior cardinal vein + L common cardinal vein regress

Anastomosis = L brachiocephalic vein

Caudal R anterior cardinal vein + R common cardinal vein = superior vena cava

Cranial R anterior cardinal vein = R brachiocephalic vein

Fate of Posterior Cardinal Veins

Day 26 = bilateral venous drainage

Posterior cardinal veins regress

- except contribute to the root of the Azygos vein

Flow of blood in week 8

L upper body => L brachiocephalic vein => SVC => right atrium

R upper body => R brachiocephalic vein => SVC => right atrium

Fate of the umbilical veins

R umbilical vein disappears (7 weeks)

L umbilical vein persists

- cranial aspect (heart - liver) degenerates

- caudal aspect (placenta - liver) persists as umbilical vein

- ductus venosus between L umbilical vein and IVC (bypass the liver)

Blood flow from placenta

Placenta => umbilical vein => ductus venosus => IVC => right atrium

Fate of the vitelline veins

Bilateral at Day 26

Week 7 = left vitelline vein regresses

Right vitelline vein = IVC + hepatic portal system (return nutrient-rich blood from abdominal region to heart)

Blood flow from the vitelline veins

Abdominal venous blood =>

portal vein =>

hepatic veins =>

IVC =>

right atrium

Outflow of blood pathway in embryo (day 26)

Primordial atrium

Aortic Sac (distal Truncus Arteriosus)

Pharyngeal Arch Arteries

Dorsal Aorta

- dorsal segmental arteries => intercostal + lumbar arteries in adult

- vitelline artery (to UV via omphaloenteric duct)

- umbilical artery

Fate of the vitelline arteries

Multiple arise from the dorsal aorta (day 26)

Most regress during weeks 5-6

3 unpaired ventral branches present at the end of week 6

- celiac artery (foregut)

- superior mesenteric artery (midgut)

- inferior mesentery artery (hindgut)

Adult anatomy of outflow vessels

Thoracic aorta (between esophagus + vertebral bodies)

Paired posterior intercostal arteries

Passes through the diaphragm at the aortic hiatus (T12) = Abdominal aorta

Branches

- paired lateral branches (ex. Renal)

- unpaired: celiac, superior mesenteric, inferior mesenteric arteries

Terminates at L4, bifurcates:

- L+R common iliac arteries, branch:

- internal illiac artery (bladder/rectum/reproductive organs)

- superior vesicle artery

- medial umbilical ligaments

- external iliac artery (lower limb)

Fate of umbilical arteries

Initially = travel through connecting stalk

Remodelled in week 5-6 as placental circulation is established

In adult:

- Distal portions regress (=medial umbilical ligaments

- Proximal portions persist (internal iliac arteries + superior vesical arteries)

Fate of dorsal aortae

Cranial portions remain bilateral

- left = primordial aorta

- right = regress + remnants = proximal part of right subclavian artery + distal internal carotid arteries

Caudal portions fuse to single thoracic/abdominal aorta (by day 37)

- 30 intersegmental branches = intercostal arteries/lumber arteries/common iliac artery

Pharyngeal arch arteries

6 pairs, (1 per pharyngeal arch)

Form during heart looping (4th week)

Blood flow from distal aspect of the truncus arteriosus (aortic sac) into pharyngeal arch arteries (6th => 1st)

5th pharyngeal arch artery regresses early (in 50%)

Fate of the pharyngeal arch arteries (week 7)

1st + 2nd also mostly regress by 6 weeks

1st = maxillary arteries (ears, teeth, muscles of face)

2nd = dorsal remnants form stems of the small stapedial arteries (middle ear)

dorsal aorta degenerates between 3rd & 4th = physical separation between 3rd and 4th outflows

3rd = internal carotid artery (both sides)

4th = bilateral (parallel to 3rd PAA)

6th = ductus arteriosus

Fate of the pharyngeal arch arteries (week 8 onwards)

3rd = proximal aspect of internal carotid artery + common carotid artery (both sides)

4th =

- R = proximal segment of right subclavian artery

- L = arch of aorta between L common carotid and L subclavian

6th =

- R = degenerates & proximal = R pulmonary artery

- L = proximal = L pulmonary artery + distal = ductus arteriosus

ductus arteriosus

Shunt between pulmonary artery and aorta

Allows blood to flow from pulmonary trunk to L dorsal aorta (future descending aorta)

Fate of the aortic sac

Terminates at the 3rd pharyngeal arch

- 6th pharyngeal arch branches from the caudal aspect

Elongates => sac (week 6) => tube (week 7) => ascending aorta (week 8)

Contributes to the ascending aorta, part of the arch of the aorta, brachiocephalic artery

Right subclavian artery derivatives

Proximal segment = 4th pharyngeal arch artery

Distal = R dorsal aorta, 7th intersegmental artery

Adult vasculature coming off the arch of aorta

Brachiocephalic artery

- R subclavian

- R Common carotid (R external carotid + R internal carotid)

L Common Carotid (L internal carotid + L external carotid)

L subclavian artery

Changes in blood flow between the fetus and neonate

Umbilical vein = ligamentum teres that attaches inferior liver to umbilicus

Umbilical arteries = distal parts medial umbilical ligament on either side of the bladder. Proximal part = superior vesical arteries of the bladder

Oval foramen = closed by valve of septum primum (fuses at 3m of age)

Ductus venosus (bypasses liver) = ligamentum venosum

Ductus arteriosus (bypasses lungs) = ligamentum arteriosum

Adult derivatives of the foregut

Pharynx

Esophagus

Stomach

Proximal duodenum

Liver

Gallbladder

Cystic duct

Pancreas

Respiratory system

Adult derivatives of the midgut

Distal duodenum (starting after major duodenal papilla)

Jejenum

Ileum

Cecum

Appendix

Ascending colon

Proximal 2/3 of transverse colon

Adult derivatives of the hindgut

Distal 1/3 of transverse colon

Descending colon

Sigmoid colon

Rectum

Proximal anal canal

Epithelium of urinary bladder + urethra

Allantois

Omphaloenteric duct/vitelline duct/yolk stalk

Connects midgut to umbilical vesicle

Classification of mesoderm surrounding gut tube

Developing tube is invested in intraembryonic splanchnic mesoderm

Enclosed by the intraembryonic coelom (peritoneal cavity)

Intraembryonic somatic mesoderm surrounds overlying ectoderm

Tissues derived from the Endoderm in the digestive system

Epithelial lining of gastrointestinal tract

Epithelial cells of the accessory digestive organs

Blood supply of the digestive system in the adult

Foregut = celiac trunk

Midgut = superior mesenteric artery

Hindgut = inferior mesenteric artery

Name of the somatic and splanchnic mesoderm in the adult

Somatic mesoderm = parietal peritoneum

Splanchnic mesoderm = visceral peritoneum

Serous cavity

Cavity between the parietal and visceral membranes

Mesentery

Site of attachment

Tether the gut tube to abdominal wall

Contain neurovascular components that supply the gut tube