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