The mediastinum. The histology of the esophagus. Fertilization and cleavage.

The mediastinum. The histology of the esophagus. Fertilization and cleavage.

Inf Mediastinum

Sup Mediastinum

Anatomy: The mediastinum.

Superior Mediastinum:

Borders:

• ant = manubrium of sternum
• post = T1-T4
• lat = mediastinal pleura, 1st rib
• sup = thoracic inlet
• inf = line b/w sternal angle –> Intervertebral disk b/w T4,T5

Superficial Dissection of Superior Mediastinum

Layers/Contents:
1. Thymus – in youth only, mostly adipose in adults

• lymphoid organ
• gradually replaced w/ fat as an adult
• produces T cells throughout life, but does T cell education during youth
• Blood Supply =   ant IC & ant mediastinal br of int thoracic a, internal thoracic and inf thoracic v

2. Veins and phrenic n

• L & R brachiocephalic v – formed by union of subclavian and internal jugular v = angulus venosus, combine to form SVC
  • L is 2x as long as left b/c has to cross many structure in midline to get to SVC – receives thoracic duct
  • R rec R lymphatic duct
• SVC – will desc to enter R atrium @ 3rd cc, rec blood from all structures above diaphragm, except lungs and heart, is lat to trachea and asc aorta
• Inf thyroid v
• Int thoracic v
• Phrenic n – supply diaphragm, SS, runs b/w subclavian a and origin of brachioceph v, will desc and run in front of root of lung on both sides

3. Arteries and vagus n (remember that vagus will run behind root of lung, and phrenic in front)

• arch of aorta – originates @ 2nd R sternocostal joint @ level of sternal angle, and starts to desc @ 2nd L sternocostal joint
• Brachiocephalic trunk – 1st branch of arch, ant to trachea and behind v – @ R SC joint, becomes R common carotid and subclavian a
• L common carotid a – asc in carotid triangle
• L subclavian a – runs behind L SC joint, w/ L common carotid
• L & R vagus n

4. Trachea and Esophagus

• Trachea – see topic # 8, divides @ sternal angle into R & L main bronchus
• Esoph – fibromuscular tube that leads from pharynx –> stomach, runs b/w trachea and vertebral column, thoracic duct is left to it, runs down to pass thru esophageal hiatus in diaphragm

5. Prevertebral structures

• Prevertebral M (see topic #4, ex/ longus capitis, longus colli, rectus anterior)
• Post IC a/v/n
• SNS trunk, white and gray branches
• Azygos system
• Thoracic duct (more on that in topic #16)

Deep Dissection of Superior Mediastinum

Inferior Mediastinum:

Inf Mediastinum – divided into ant, middle, posterior

Borders:

• ant = sternum, ribs
• post = vert column
• lat = mediastinal pleura
• sup = line b/w sternal angle and intervertebral disk b/w T4,T5
• inf = diaphragm

Anterior Mediastinum – b/w sternum and ant side of pericardium, larger in infants b/c of size of thymus

• int thoracic a/v
• ant IC a/v
• Transverse thoracic m
• Parasternal lymph nodes
• sternocardial ligaments
• Areolar CT

Middle Mediastinum – w/in pericardium, see pericardium topics/heart

• heart
• last part of SVC
• last part of IVC
• Asc aorta
• pulmonary trunk
• terminal part of 4 pulmonary v
• phrenic v
• pericardiophrenic a/v
• bronchiol a/v
• bifurcation of trachea
• R & L main bronchi

Posterior Mediastinum – post side of pericardium –> vertebral column
Step by Step showing of layers of Post mediastinum : http://home.comcast.net/~WNOR/thoraxlesson5.htm

1. thoracic part of desc aorta – continuation of desc aorta, begins from inf border of T4,  and desc thru post mediastium, till it goes thru aortic hiatus of diaphragm @ about midline

• • surrounded by thoracic aortic plexus
  • behind root of lung, pericardium and esophagus

Branches of thoracic aorta:
has parietal and visceral branches
Parietal:

• post IC – 3rd – 11th IC spaces
• subcostal
• sup phrenic – anastomose w/ musculophrenic, pericardiophrenic from int thoracic a

Visceral:

• bronchial – 1 R, 2 L
• pericardial – post wall of pericardium
• esophageal – 4-5 br
• mediastinal – supply lymph nodes and minor structures

2. Azygos system –
On R side:

Azygos v –  runs to R of T4-T12, arches of root of R lung –> empty into SVC

• receives R post IC v, mediastinal v, esophageal v, R bronchial v
• anatomosis w/ vertebral venous plexus
• originates below diaphragm as asc lumbar v –> asc on R side of vertebrae –> diaphragm –> then called azygos v

On L side:
Hemiazygos v – collects L lower post IC v (7-12 IC v)

• Hemiazygos crosses midline @ T6-T7 , joins azygos v –> SVC

Accessory hemiazygos v – collects L upper post IC v (1-6 IC v), rec bronchiol v on L side

• joins hemiazygos v or runs sup to join brachiocephalic v

3. Esophagus:

• pass post and R to arch of aorta
  
• just behind L atrium
• goes thru esoph hiatus of diaphragm @ T10
• has impressions on it made by aortic arch, L bronchus, diaphragm

4. Vagus n – desc lat to esoph on both sides

• L & R recurrent laryngeal n –
  
  • R recurrent laryngeal hooks around R subclavian a,
    
  • L recurrent laryngeal desc and hooks around arch of aorta
• Each contribute to esophageal and pulmonary plexus on each side, R vagus n also gives br to cardiac plexus
• forms ant/post vagal trunk after diaphragm b/c will run ant/post to esophagus in this area, instead of lateral

Br of vagus n:

• recurrent laryngeal n
• pulmonary plexus
• esophageal plexus
• cardiac n
• Vagus runs w/ esophagus in inf mediastinum

5. Thoracic duct – details in topic # 16

6. Posterior mediastinal & paraaortic l.n

7/8. Nerves of Post mediastinum:

• Thoracic part of SNS trunk – cont w/ cervical & lumbar SNS trunk
• Greater/Lesser/Least splanchnic n – supply viscera below diaphragm, have presynaptic fibers from 5th -12th SNS ggl, synapse w/ ggl in abdomen

9. Other structures:

• 4-11th post IC a/v
• 4-11th IC n
• Subcostal a/v/n

Histology of the esophagus.

Embryology: Fertilization and cleavage.

Fertilization:

• occurs in ampulla of uterine tube, close to ovary
• @ ovulation, sperm again are motile (they sit in the ovarian tube)
• Before fertilization = sperm has to go through a few reactions:
  • Capacitation – epithelial interaction b/w sperm and mucosal surface of tube –> glycoprotein coat removed, plasma protein removed from plasma membrane over head of sperm
  • Acrosome reaction – after binding to zona pellucida, zona proteins induce the release of enzymes to help the penetration

Phases of Fertilization:

Penetration of Corona Radiata:

• aided by action of sperm & uterine tube mucosal enzymes

Penetration of Zona pellucida:

• Zona = glycoprotein shell around egg
• The Zona induces acrosome reaction –> release of enzymes = acrosin
• Permeability of zona pellucida changes when head of sperm come in contact w/ oocyte
• sperm contact w. cell membrane of secondary oocyte = cortical reaction –> release of cortical granules from oocyte cytoplasm  –> this inhibits sperm penetration
• zona pellucida & oocyte membrane = impermeable to other sperm

Fusion of sperm & oocyte:

• both membranes bind and then breaks down near the fusion area
• the entire sperm enters the cytoplasm of secondary oocyte
  • mitochondria and tail degenerate
  • sperm nucleus = male pronucleus
• secondary oocyte completes meiosis II –> mature ovum
• ovum nucleus = female pronucleus

Cleavage

• series of mitotic divisions = blastula = 2 cell –>4 –> 8 of blastomeres = totipotent
• formation of morula by undergoing compaction
• tight junctions form between cells in  outer cell mass
• inner cell mass is connected via gap junctions
• this separates inner and outer cell mass from each other

9. The valves of the heart. The histology and development of the heart.

9. The valves of the heart. The histology and development of the heart.

Anatomy: The valves of the heart.

Valves of Heart
all of valves of heart originate from anulus fibrosus
2 valves on arterial end and 2 valves at AV Jxn

View of valves @ level of anulus fibrosus

Semilunar valves

• made of 3 semilunar cusps
• NOTE – Pumonary valve has an R, L, and ant cusp & Aortic valve has a R,L, and post cusp —> So it is always P & A – Pulmonary has Ant, and Aortic has Post
  
• Each cusps has a pars flaccida, and a pars tecta
• The side of each cusp, thin CT area = lunule
• peak of every cusp, fibrous = nodule
• When valve closes, the lunules and nodules meet in the center.

Pulmonary Valve:

• @ border b/w conus arteriosus of R ventricle & pul trunk
• has an ant, R, L cusps
• opened in ventricular systole & shuts after aortic valve closes
• SP = behind med end of L 3rd costal cartilage & sternum connects there
• Auscultation Point = heard best @ L 2nd IC space, just lat to sternum
• each cusp is concave from superior view, project into pul trunk, but flatten against wall when blood is flowing through
• cusps open up when they close, and catch backflow and prevent blood from going back into R ventricle
• just above each cusp of valve, pul trunk bulges out, forming the pulmonary sinuses – these prevent cusps from sticking to the walls of the pulmonary trunk.
  

Aortic Valve:

• b/w L ventricle and asc aorta
• has R,L, post cusps
• each cusp is concave from superior view, project into ascending aorta, but flatten against wall when blood is flowing through
• cusps open up when they close, and catch backflow and prevent blood from going back into L ventricle
• just above each cusp of valve, asc aorta bulges out, forming the aortic sinuses – these prevent cusps from sticking to the walls of the pulmonary trunk.
• R&L coronary a originate from the aortic sinuses above R&L cusps
• Post cusp’s sinus does not have an a. originate from it
• blood pumped into coronary a in diastole
• SP = behind L 1/2 sternum @  3rd IC space
• Auscultation Point = heard best @ R 2nd IC space, just lat to sternum
• closed during ventricular systole
• 2nd Heart sound

Parts of Semilunar/Cuspid Heart valves, showing lunules, nodules, etc

Cuspid Valves
3 main structures:

1. Cusps w/ core of CT
2. papillary m (emerging from myocardium) –  cone shaped m that attach to cordae tendinae, valves open when these contract
   
3. chordae tendinae – threads of dense reg CT connecting papillary m w/ cusps of valve

Location and Parts of cuspid valves

Tricuspid Valve = R AV valve

• B/w R atria & R ventricle
• SP = opposite 4th IC space, covered by endocardium
• Has Ant, Post, Septal cusps = Ant works the most!
• closed during ventricular systole ( contraction)
• Auscultation Point = heard best @ R lower sternum body

Bicuspid valve = mitral = L AV valve:

• b/w L atria and ventricle
• SP = behind L 1/ sternum @ 4th costal cart
• has Ant cusp & smaller post cusp
• closed @ onset of ventricular systole
• Ausculation point = heard best @ L 5th IC space @ midclavicular line

Mechanism of contration:

1. Papillary m contract during ventricular systole, wall of ventricle, contracts and shorten, cusps stay closed
2. If no contraction occurs, chords will relax, cusps flip open to atria
3. If pap m. shorten too much, chordae pull down cusps, and they will open during diastole
4. NOTE – b/c action of valve opening is active, when musc not contracted, valve closes passively, and shape of each cusp stops backflow back into atria

Histology: Histology of the heart.

Embryology: development of the heart.

Anim 1 = Early Heart and Primitive Heart Tube Folding

Anim 2 = Overview of Heart Tube Folding and Early Heartbeat

Anim 3 = Interatrial Septum Development

Anim 4 = Division of the Atrioventricular Canal

The primordium of the heart forms in the cardiogenic plate located at the cranial end of the embryo. Angiogenic cell clusters which lie in a horse-shoe shape configuration in the plate coalesce to form two endocardial tubes. These tubes are then forced into the thoracic region due to cephalic and lateral foldings where they fuse together forming a single endocardial tube.

The tube can be subdivided into primordial heart chambers starting caudally at the inflow end: the sinus venosus, primitive atria, ventricle, and bulbus cordis.

The heart tube begins to grow rapidly forcing it to bend upon itself. The result is the bulboventricular loop. Septa begin to grow in the atria, ventricle and bulbus cordis to form right and left atria, right and left ventricles and two great vessels- the pulmonary artery and the aorta. By the end of the eighth week partitioning is completed and the fetal heart has formed.