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

9 Dec

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

Inf Mediastinum

Inf Mediastinum

Sup 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

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

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

aorta-azygos-system

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

posterior-mediastinum

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
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