ANHB2212 - Respiratory System Development

When does the larynx, trachea and lungs arise and from what?

week 4 from an outgrowth of foregut

What does endoderm give rise to in the respiratory system?

• mucosal lining of larynx, trachea and bronchi

• epithelial cells of alveoli (simple squamous)

What does mesoderm give rise to in the respiratory system?

• smooth muscle and cartilage supporting larynx, trachea & bronchi (lateral plate mesoderm)

• visceral pleura covering the lung (splanchnic mesoderm)

• parietal pleura covering the thoracic cavity: costal, mediastinal & diaphragmatic regions (somatic mesoderm)

• Vasculature

When does development of the respiratory system begin?

day 22

Describe the development of the primary bronchi, trachea and esophagus.

1. respiratory diverticulum (RD) forms as a ventral outpouching of the endodermal foregut

2. RD grows ventro-caudally through the splanchnic mesenchyme surrounding the foregut

3. RD splits into right and left primary bronchi in a process called bifurcation at days 26-28

4. proximal end of RD forms trachea and larynx

5. larynx opens into pharynx via glottis

6. as primary bronchi form, the stem of the RD separates from the overlying portion of pharynx to become the esophagus

When does first bifurcation occur?

day 26-28

What is the original point of evagination of the diverticulum?

glottis

What is the pattern of branching of the lung endoderm regulated by?

signaling from the surrounding splanchnic mesenchyme

What are the muscles of the larynx innervated by?

vagus nerve

What happens when the foregut fails to separate into the trachea and esophagus 1) from birth 2) before birth?

1. fluid (milk) can enter lungs = infection and death

2. blind-ending esophagus prevents swallowing amniotic fluid which is usually returned to the mother via placenta circulation = excess amniotic fluid and distention of the uterus

Summarise the anatomy of the lung tubes (bronchi and bronchioles)

1. primary bronchi enter lungs at hilum

2. lobar bronchi enter at lobes

3. tertiary bronchi enter at bronchopulmonary segments

4. bronchioles

5. terminal bronchioles

6. respiratory bronchioles

7. alveolar ducts

8. alveoli

Describe branching of the bronchi.

Late week 4 - two primary bronchi

Early week 5 - secondary bronchi (3 right, 2 left)

Late week 5 - tertiary bronchi (10 right, 8 left) in bronchopulmonary segments, lungs assume a more caudal position

Week 5 to 24 - terminal bronchioles give rise to respiratory bronchioles that give rise to 3-6 alveolar ducts that end in primitive alveoli. Respiratory membrane starts to form as type 1 alveolar cells and capillaries become closer together and type 1 cells become thinner.

Week 24 - type 2 alveolar cells develop to produce surfactant

At the end of the 6th gestational month how many generations of subdivisions have occurred?

17 generations (postnatally 6 more occur)

Surfactant

A substance produced by type 2 alveolar cells that prevent alveolar collapsing during respiration.

When do fetal breathing movements occur?

weeks 10-12

Why do fetal breathing movements occur?

• stimulate lung development

• condition respiratory muscles

What happens at birth regarding amniotic fluid and surfactant?

amniotic fluid is absorbed whilst surfactant is not.

Pleura

A single layer of squamous cells (mesothelium) that lines the pleural cavity that is subdivided into visceral (covers lungs) and parietal pleura (covers thoracic cavity).

What is pleural (parietal and visceral) developed from?

parietal - somatic lateral plate mesoderm

visceral = splanchnic lateral plate mesoderm

Where is pleural fluid produced?

Mesothelium

What is the purpose of pleural fluid?

• allows for parietal and visceral pleura to slide over each other when the thoracic wall moves (reduces friction)

• combination of parietal pleura, visceral pleura & pleural fluid keep the lungs attached to the costal, diaphragmatic & mediastinal surfaces of the thoracic cavity

• when thorax moves lungs volume changes because visceral pleura is “stuck” to thoracic cavity surfaces via parietal pleura

What are the 3 types of parietal pleura?

• costal

• mediastinal

• diaphragmatic

Describe the development of the pleural and pericardial cavities

1. pericardio-peritoneal canals become too small as lung buds expand caudolaterally within them, causing lungs to expand dorsally, laterally and ventrally into body wall mesenchyme

2. in early week 5, 2 pleuro-pericardial folds develop from the lateral body walls and project into the undivided thoracic cavity

3. these folds grow medially towards each other between the heart and lungs, eventually fusing at the end of week 5 with the foregut mesenchyme

4. the primitive pericardial cavity is divided into 2 dorsolateral pleural cavities and 1 ventral definitive pericardial cavity

What are the pleural and pericardial cavities lined by?

mesothelium

What is the function of the fluid secreted by mesothelium?

• reduce friction during movement

• pleural fluid keeps lungs stuck to thoracic surfaces

Where do phrenic nerves arise from?

joined branches of cervical roots 3, 4, and 5

Describe the innervation of the diaphragm.

Phrenic nerves follow the diaphragm as it descends into the lower thoracic vertebrae through the fibrous pericardium.

Describe the motor and sensory innervation of phrenic nerves.

• motor = diaphragm muscles

• sensory = fibrous & parietal serous pericardium, mediastinal & diaphragmatic pleura, peritoneum covering inferior surface of diaphragm

What organisms have a diaphragm?

mammals only

What are the 2 functions of the diaphragm?

• drive inspiration

• barrier between thorax and abdomen

What 3 major tubes pass between thorax and abdomen and how?

• Inferior vena cava (IVC) - through the central tendon (T8)

• Esophagus – through diaphragm muscle (T10)

• Aorta – behind the crura (muscle) (T12)

What happens to the inferior vena cava, esophagus and aorta when the diaphragm contracts?

• inferior vena cava - Venous return to heart via IVC increases due to increased abdominal pressure & decreased thoracic pressure

• esophagus - Muscle surrounding the esophagus reinforces the cardiac sphincter of the stomach

• aorta - unaffected because it passes behind the crura

What 4 regions of the intraembryonic coelom does the septum transversum create?

• Large pericardial cavity

• Large peritoneal cavity

• 2 x Pericardio-peritoneal canals

Pericardio-peritoneal canals

• short channels located dorsal to the ST & lateral to the proximal part of future esophagus

• spaces into which lungs first start to grow

pleuro-peritoneal folds

• 2 x pyramidal shaped structures between thoracic & abdominal cavities

• Originate in the cervical region

Describe the development of the diaphragm.

• pleuro-peritoneal folds (PPFs) expand dorsally and ventrally across the septum transversum.

• PPFs give rise to diaphragm muscle connective tissue.

• first muscle progenitors emigrate from cervical somites (C3-5) to PPFs. Axons of the phrenic nerve follow the developing muscle and project into PPFs.

• Once the muscle progenitors are encased by PPF connective tissue, the PPFs, diaphragm muscle, developing heart, lungs & liver start to descend from cervical levels to final location in thorax.

• some muscle develops in the central tendon but this muscle fails at it matures due to lack of innervation

What regulates diaphragm muscle development?

Pleuro-peritoneal folds

True Ribs

• Ribs 1-7

• Directly connected to the sternum by costal cartilage

False Ribs

• Ribs 8-10

• do not articulate directly with the sternum but are connected to it via fusing with the costal cartilages of the rib above to rib 7

Floating Ribs

• ribs 11-12

• no connection to the sternum

What are the symphyses of the thoracic cavity?

• intervertebral discs

• manubriosternal joint

What are the synchondroses of the thoracic cavity?

• costochondral junction (where the ribs meet hyaline cartilage)

• sternocostal junction of rib 1

• xiphisternal joint

What are the synovial joints of the thoracic cavity?

• costovertebral

• costotransverse

• sternocostal junctions of ribs 2-7

• interchondral joints of ribs 7-10

Describe the vertebral body facets of the thoracic vertebrae

T2-9 = demi-facets

T1, T10-12 = full facets

Describe the transverse process facets of the thoracic vertebrae.

T1-7 = rounded facet

T8-10 = flat facet

T11-T12 = no facet

Describe the zygapophyseal joint facet orientation of the thoracic vertebrae.

T1-T11 = typical orientation

T12 = superior and inferior facets have different orientation

Describe the head of each rib.

R2-9 = 2 articular facets

R1, R10-R12 = 1 articular facet

Describe the tubercle, neck and angle of the ribs.

R1-10 = present

R11, R12 = absent

External intercostal muscles

• from tubercles of the ribs to costal cartilages

• most active in inspiration

• oriented distally towards the sternum (\)

• external intercostal membrane present (thin layer of connective tissue)

Internal Intercostal Muscles

• from parasternal regions to angles of ribs

• oriented opposite to the external intercostal muscles

• most active in expiration

Innermost Intercostal Muscles

• same orientation as internal intercostals (opposite external)

• most obvious in lateral thoracic wall

What are the 3 types of chest wall movements?

• bucket handle

• pump-handle

• calliper

Bucket Handle Movement

True ribs (1-7) elevate, swinging laterally.

Pump Handle Movement

- rib 1 lifts manubrium & carries sternum forwards

- manubriosternal joint ‘bends’ (symphysis) reducing the sternal angle

- rib 1 moves sternum, therefore, all true and false ribs move = movement of sternocostal joints 2-7 and costovertebral & costotransverse joints

Calliper Movement

- False ribs (8-10) act on the costal margin, not sternum

- The lower thorax widens laterally to become ‘broader

What are the functions of the floating ribs?

Support diaphragm & spread (‘lengthen’) thorax (supero-inferiorly)

Torque

force applied over a distance (lever arm = rib) causing rotation about a fulcrum (axis of rotation = spine)

= force x distance from axis

Describe what happens in this scenario regarding movement. What is this modelling?

External intercostals:

• muscle insertion point 1 is further away from the axis compared insertion point 2

• assuming equal amount of force, when the muscle contracts, torque is greater at insertion point 1 compared to point 2

• so movement will occur in an upwards and forward direction (rib elevation)

Describe what happens in this scenario regarding movement. What is this modelling?

Internal intercostals:

• muscle insertion point 1 is further away from the axis compared insertion point 2

• assuming equal amount of force, when the muscle contracts torque is greater at insertion point 1 compared to point 2

• so movement will occur in a downwards direction (rib depression)

Describe pleural pressure

Having lungs stuck to the pleural cavity means they are very stretched at rest The resulting tension force means the lungs are always pulling on the body wall so pleural pressure is always negative

What are recesses between visceral and parietal pleura? What are the 3 types?

spaces where lungs or pleural fluid can move to or accumulate that are important for accommodating changes in lung volume during breathing.

• costomediastinal

• costovertebral

• costodiaphragmatic

Costamediastinal Recess

Pleural fluid recess either side of the sternum, between body wall and pericardium

Costovertebral Recess

Pleural fluid recess that extends to rib 12 where the lungs do not

Costodiaphragmatic Process

Pleural fluid process between the inferior margin of the lungs and inferior margin of the pleural cavities. At the mid axillary line lungs extend to rib 8 and pleura extends to rib 10. Safe place to take pleural fluid sample.

What is the largest pleural recess?

costodiaphragmatic recesses

Describe the sloping of ribs in adults and how it impacts intrathoracic volume.

In adults at rest, ribs slope downward ventrally Elevation of ribs, therefore, increases intrathoracic volume

Describe the sloping of ribs in newborns and how it impacts intrathoracic volume.

In newborns, ribs are oriented more horizontally As the ribs are already elevated, further elevation has little impact on intrathoracic volume.

Compare the adult and newborn diaphragm.

the newborn is very reliant on the diaphragm during ventilation BUT the neonatal diaphragm is much flatter compared to the adult. This results in a proportionally smaller range of displacement of air when the diaphragm moves

Why do babies breathe rapidly?

the neonatal diaphragm is much flatter compared to the adult This results in a proportionally smaller range of displacement of air when the diaphragm moves

What 2 arteries do the lungs receive blood from?

• bronchial arteries

• pulmonary arteries

Bronchial arteries

bronchial arteries (2%): responsible for lung physiology: supply bronchi, bronchioles, CT, visceral pleura & the larger pulmonary BVs

Pulmonary arteries

pulmonary arteries (98%): whole body physiology & supply alveoli

Describe the breasts

• Breasts are modified sweat glands

• They consist of mammary glands, superficial fascia, and overlying skin

• The breast is related to the thoracic wall and associated upper limb structures - therefore, vascular supply and drainage can occur by multiple routes

How are the breasts (and nipple) innervated?

Anterior and lateral cutaneous branches of the 2nd to 6th intercostal nerves

Nipple is innervated by the fourth intercostal nerve

Sternal Foramen

An anatomical variant of the sternum that occurs due to incomplete fusion of the sternal ossification centre.

What ligaments hold the rib to the vertebrae?

• radiate ligament of head of rib (to body)

• superior costotransverse ligament (to transverse process)

Describe the attachment of the diaphragm

the back of the sternum, along the costal arch to the tip of the 12th rib then arise on each side of L2 from lumbar fascia.

Where is the breast located?

Above the level of the 2nd rib to the 6th rib. It extends from the edge of the sternum to midaxillary line.

Axillary Tail

The prolongation of the breast from the midaxillary line posteriorly.

What structures are associated with the costal groove

Intercostal vein, artery & nerve.

What bones form the boundary of the thoracic inlet?

T1 vertebrae, manubrium, first rib

What bones form the boundary of the thoracic outlet?

T12 vertebrae, tip of rib 12, tip of rib 11, costal margin (ribs 7-10ish),
xiphoid process of sternum

What structure closes off the thoracic outlet?

diaphragm

The costal cartilage of which rib articulates with the manubriosternal joint?

rib 2

Describe the contribution of upper ribs to the mechanism by which the volume of the thoracic cavity is increased during filling of the lungs

Bucket handle pivots ribs up, overall thoracic circumference increases

Describe the contribution of lower ribs to the mechanism by which the volume of the thoracic cavity is increased during filling of the lungs

Calliper action widens the transverse dimension of lower thorax

Describe the contribution of the manubrium to the mechanism by which the volume of the thoracic cavity is increased during filling of the lungs

No angle change with rib 1, but the angle with sternal body increases
on the anterior / ventral body surface due to movement of rib 1.

Describe the contribution of the sternal body to the mechanism by which the volume of the thoracic cavity is increased during filling of the lungs

Angle of the manubriosternal joint changes as described above, body
of sternum moves out and up to increase antero-posterior thoracic
diameter.

where do the crura attach to the vertebral column?

left = L1-2

right = L1-2

Why is it advantageous for the diaphragm to pierce the muscle of the diaphragm?

If the oesophagus was not surrounded by muscle, food would be forced up from
the stomach because each contraction of the diaphragm increases abdominal
pressure. The oesophagus being surrounded by muscle prevents this reflux

What embryological structure develops into the central tendon?

The region of the pleuro-peritoneal folds where muscle cells fail to develop

Explain the reason for the difference in sound above and below the right dome of the diaphragm.

Below the line, the solid liver in the abdomen deadens percussion, causing a muffled sound.
Above the line, the thorax sounds hollow because of the air spaces in the lung.

Which layers of the intercostal muscles do the neurovascular bundles run between?

Internal and Innermost intercostals

Function of the scales during breathing

active during breathing to attach to ribs 1 and 2 to elevate the ribcage.

Function of the sternocleidomastoid during breathing

active during deep\laboured breathing to elevate the sternum

Function of the serratus posterior superior during breathing

active during deep/laboured breathing to elevate ribs 2-5

Function of the serratus posterior inferior and quadratus lumborum during breathing

active during deep/laboured breathing to depress the lower ribs

obstructive respiration

Obstructive respiration occurs during lung disorders such as Chronic Obstructive
Pulmonary Disease (COPD), which is a group of lung diseases that impede normal airflow.

Function of the rectus abdominus and oblique abdominus during breathing.

active in forced exhalation to pushes the abdominal contents up against the diaphragm to decrease thoracic volume

Function of the pectoralis major during breathing

used during obstructive respiration: If the arms are fixed, e.g. by holding on
to a chair, pectoralis major expands the ribcage anteriorly