14. The pleura and pericardium. Bone formation. Formation of the blastocyst and the bilaminar germ disc.

14. The pleura and pericardium. Bone formation. Formation of the blastocyst and the bilaminar germ disc.

Flash cards:

Pleura 1

Pleura 2 - reflections

Pericardium 1

Pericardial reflections

Anatomy:  The pleura and pericardium.

Pleura:

• serous membrane covering the Lung
• Double layer:
  • Inner visceral – covers lung itself
  • Outer parietal -covers inner surface of thoracic wall
• B/w 2 layers = Pleural cavity – 4 ml of serous fluid
• Function:
  • lubricates the 2 pleural surfaces
  • allows layers of pleura to slide smoothly over each over during respiration
  • surface tension allows lung surface to stay touching thoracic wall
  • Creates a seal b/w 2 pleural surfaces
• The two layers combine around the root of of the lung – so the root of lung has no pleural coverage, the layers combine to form the pulmonary ligament, which runs inf and attaches the root of the lung to the diaphragm
  
• CLINICAL NOTE: When parietal pleura is punctured, lungs collapse due to air rushing (air pressure higher outside than in) = pneumothorax

Pleural surfaces and recesses

Parietal Pleura – senses PAIN, lines inner surface of thoracic wall
4 parts:

1. Cupula pleura (aka cervical pleura) – part of pleura which project above clavicle
   • located w/in scalenus tent
   • strengthened by a layer of endothoracic fascia call the suprapleural membrane = Sibson’s fascia
2. Diaphragmatic pleura – faces domes of diaphragm inf
   • , also separated from diaphragm via endothoracic fascia,
   • only attached to lateral sides of diaphragm
   • as central tendon of diaphragm attaches to pericardium
3. Sternocostal pleura – attaches to rib and sternum
   • separated from ribs via a part of endothoracic facsia called phrenicopleural fascia
4. Mediastinal pleura – faces vert column and midline space b/w two areas of pleura = mediastinum

3 Pleural reflections: Lines where the surfaces of parietal pleura meet each other

• Sternal line = costal –> mediastinal (ant)
• Costal line = costal –> diaphragmatic (inf)
• Vertebral line = costal –> mediastinal (post)

* first 2 have a sharp,defined border, the last is a gradual rounded border

Nerve supply:

• IC n (sternocostal pleura, and peripheral part of diaphragmatic pleura)
• Phrenic n (central diaphragmatic pleura, and mediastinal pleura)

Blood supply = Int thoracic a, post IC, sup phrenic a, sup IC a

Visceral Pleura – sensitive to STRETCH, lines lung ext and dips into all fissures
Nerve supply = contains vasomotor fibers and sensory ending of CN X for respiratory reflexes

Blood Supply = bronchiol a, pulmonary v

Surface Projection:

1. Lat –  lat wall thorax = lat wall of pleura
2. Sup = Cupula Pleura= dome of pleura,
   • projects into neck
   • above neck of 1st rib
   • 2 cm above clavicle @ med end
   • middle 1/3 clavicle b/w midclavicular line and midline
3. Med border:
   • R – R sternoclavicular joint –> desc to lower border of R 6th rib
   • L – goes toward midline from apex, but never reaches it, follows the lung and desc down to 4th rib, runs a bit laterally and then comes back towards midline @ 6th rib, to form the pleural cardiac notch
4. Inf border
   • @ parasternal line – inf border crosses border of 6th/7th rib
   • @ midclavicular line – inf border crosses 8th rib
   • @ mid axillary line – 10th rib
   • @ paravertebral line – T12, 11th rib

Surface Projection of Pleura

Pleural recesses:
If you notice, the surface projection of pleura is larger than the surface projection of the lung itself.

In between parietal pleura, are a series of recesses:

1. Costo-mediastinal recesses – vertical in direction, costal & mediastinal pleura meet
2. Phrenico-mediastinal recess – ant-post direction, b/w mediastinal & diaphragmatic pleura
3. Costo-diaphragmatic recess – b/w costal & diaphragmatic pleura
   • largest of the 3, horseshoe shaped,
   • accumulates fluid when standing
   • potential space that allow lung to expand into them when inhaling
   • can collect sample of pleural fluid from here
   • deepest point of sinus is @ mid-ax line where space b/w lung and pleura = 12 cm, or 4 fingers

Pleural Recesses

Pericardium.
strong fibrous layer, double walled sac w/ heart w/in
originates from intraembryonic somato and splanchno pleura

Layers of Heart itself: (sup –> deep)

1. Fibrous pericardium –
   • bound to central tendon of diaphragm for deep inspiration,heart follows diaphragm, so heart is more vertical,
   • in deep expiration, heart rises due to upward movement of diaphram – heart is more horizontal
2. Serous pericardium
   • Has a parietal layer – stong dense reg CT, prevent heart from over dilating
   • Visceral layer – simple squamous epithelium (mesothelium), that lines the heart itself aka Epicardium
   • B/w 2 layers = Pericardial cavity
3. Subepicardial CT
4. Myocardium – muscular wall – striated cardiac m
5. Subendocardial CT
6. Endocardium – simple squamous epithelium (endothelium), same epith as blood vessels

Layers of Pericardium

Pericardium moves along w. movements of structures around it b/c:

• fused w/ tunica adventia of great vessesl entering/leaving heart
• attached to post surface of sternum by sternopericardial ligaments
• fused w. central tendon of diaphragm

Blood supply = mostly from pericardiophrenic a (int thoracic a) musculophrenic a (int thoracic a), bronchiol, esophageal, sup phrenic a (thoracic aorta), coronary arteries

Innervation = phrenic n (C3-C5), vagus n (CN X), SNS trunk

Reflections of pericardium: where 2 layers of pericardium meet each other

1. @ arterial end – Transverse sinus
   • where pul a and aorta leave heart,
     
   • when doing surgery on aorta or pulmonary a, can stop circulation to this area by making a stitch through this sinus,
   • reach by reaching under pulmonary trunk and ant to SVC
2. @ venous end – Oblique sinus =
   
   • where SVC,IVC pulmonary v enter heart,
     
   • reach via inserting finger under apex of heart and pushing up and right towards root of R lung

Development of Sinuses:

• Form during embryonic life due to folding of embryonic heart tube
• As heart tube folds, venous end moves posterior and up, so venous end then is up by arterial end, separated by transverse sinus
• As veins of heart grow and expand, oblique sinus is formed, a recess, a blind sac behind post side of the heart,

Development of pericardium

Histology: Bone formation.

Embryology: Formation of the blastocyst and the bilaminar germ disc.

Blastocyst

Image via Wikipedia

• fluid is secreted within the morula (16 cell) = blastocyst cavity
• now called blastocyst, and has 2 distinct cell layers
  
• inner cell mass = embryoblast –> is the future embryo
• outer cell mass = trophoblast –> will form the placenta

Bilaminar Germ Disk

Bilaminar Disc Formation:

• In the 8th day, the blastocyst has the inner cell mass & outer cell mass
• Each cell mass  differentiated into 2 different cell layers
• Trophoblast = outer cell mass – splits into:
  • Cytotrophoblast – inner layer of mononucleated cells – distinct cell barriers
  • Syncytiotrophoblast – outer layer of mutlinucleated cells – no  obvious cell boundaries
  • cells are probably created in cytotrophoblast, and then migrate into syncytiotrophoblast
• Embryoblast = inner cell mass – splits into:
  • Epiblast – layer of high columnar cells, next to amniotic cavity
  • Hypoblast – layer of small cuboidal cells, next to blastocyst cavity
  • small cavity forms in epiblast – is amniotic cavity , epiblast cells next to the cytotrophoblast = amnioblasts
  • Cells at periphery of hypoblast migrate over inner surface of cytotrophoblast – forming thin layer extraembryonic endoderm = Heuser’s membrane
  • Blastocyst cavity = is then called primitive yolk sac
• Uterus endometrium has interstitial edema (increased interstitial fluid b/w cells), and increased # of a/v