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Showing posts with label embryology. Show all posts
Showing posts with label embryology. Show all posts

Derivatives of the pharyngeal pouches

The pharyngeal pouches are balloonlike diverticula that formed on the endodermal side between the pharyngeal arches.The pairs of pouches develop in a craniocaudal sequence between the arches

During the embryonic development pharyngeal pouches develop between the branchial arches .The first pharyngeal pouches pouch is situated between the first and second branchial arches. There are 4 pairs, the 5th pouch is absent or very small.The pharyngeal pouches form on the endodermal side between the pharyngeal arches and the pharyngeal grooves or clefts form the lateral ectodermal surface of the neck region to separate the arches.The endoderm of the pharyngeal pouches and the ectoderm of the branchial grooves contact each other to form the branchial membranes and this will seperate  the pharyngeal pouches and the branchial grooves.

The primordial pharynx , derived from the foregut it widens cranially where it joins the primordial mouth or stomodeum.It narrows caudally where it joins the esophagus
Derivatives of the 1st pharyngeal pouch 
1st pharyngeal pouch expands into a tubotympanic recess
The expanded distal portion of the recess contacts the 1st branchial groove .It is the only branchial membrane to persist in the adult.This will be contributing to the formation of the tympanic membrane or eardrum.
Only the 1st branchial groove persists in the adult as the external acoustic meatus .
The tubotympanic recess gives rise to the tympanic cavity and the mastoid antrum.
Connection between the tubotympanic recess and the pharynx elongates to form the auditory tube.
Mandibular nerve supply derivatives of first pharyngeal pouch
Derivatives of first pharyngeal pouch
  • Middle ear
  • Mastoid antrum
  • Inner layer of the tympanic membrane
  • The endoderm lines the future auditory tube
Derivatives of the 2nd pharyngeal pouch
2nd pharyngeal pouch contributes to the formation of the following Palatine tonsil  and epithelial lining of the fauces.The endoderm of the pouch proliferates and grows into the underlying mesenchyme,the central parts of these buds form crypts .The endoderm of the pouch forms the surface epithelium and the lining of the tonsillar crypts. At about 20 weeks the mesenchyme around the crypts differentiates into lymphoid tissue.These tissues soon organize into the lymphatic nodules of the palatine tonsil
2nd pouch derivatives are supplied by the facial nerve.
Derivatives of second pharyngeal pouch
  • Palatine tonsil
  • Epithelial lining of the fauces
  • Middle ear
Derivatives of the 3rd pharyngeal pouch
3rd pharyngeal pouch has dorsal and ventral wings and contributes to the formation of the inferior parathyroid glands (week 5- bulbar portion and the thymus (elongate portion). which migrate inferiorly past the superior parathyroid glands of the 4th pouch.
Derivatives of this pouch is supplied by glossopharyngeal nerve .
Derivatives of 3rd pharyngeal pouch
  • Derivatives of the dorsal wings include the inferior parathyroid glands,
  • Ventral wings fuse to form the cytoreticular cells of the thymus
Derivatives of the 4th pharyngeal pouch
4th pharyngeal pouch contributes to the formation of the superior parathyroid gland (bulbar portion) and the parafollicular cells or calcitonin cells of the thyroid gland (elongate portion form ultimobranchial body).
Superior laryngeal nerve supply the derivatives of this pouch
Derivatives of 4th pharyngeal pouch
  • Superior parathyroid glands 
  • Ultimobranchial body which forms the parafollicular C-Cells of the thyroid gland
  • Musculature and cartilage of larynx (along with the sixth pharyngeal arch).
Derivatives of the 5th pharyngeal pouch
It is a rudimentary structure and becomes part of the fourth pouch contributing to formation of thyroid C-cells.
Derivatives of the 6th pharyngeal pouch 
The sixth pharyngeal pouch does not exist. The fourth and sixth arches contribute to the formation of the musculature and cartilage of the larynx. These structurs are supplied by Recurrent laryngeal nerve.

What are the derivatives of the branchial arch muscles ?

1st branchial arch give rise to the following structures
  • Muscles of mastication
  • Mylohyoid and anterior belly of the digastric
  • Tensor tympani
  • Tensor veli palatini
2nd branchial arch give rise to the following structures
  • Muscles of facial expression
  • Stapedius
  • Stylohoid
  • Posterior belly of the digastric
3rd branchial arch give rise to the following structures
  • Stylopharyngeus
4th and 6th branchial arches give rise to the following structures
  • Cricothyroid
  • Levator veli palatini
  • Constrictors of the pharynx
  • Intrinsic muscles of the larynx
  • Striated muscles of the esophagus

Development of interatrial septum and its defect (ASD)

Atrial septal defect is the most common congenital heart diseases in adults. Intratrial septum is developed from septum primum and septum secundum.
Septum primum forms the lower part of atrial septum
Septum secundum forms the upper part of atrial septum.

There are 4 different types of atrial septal defect which include :
  • Ostium secundum type of ASD seen in 70% cases.
  • Ostium primium type of ASD seen in 20% case
  • Sinus venosus type
  • Coronary sinus type
Ostium secundum type of ASD is developed from septum secondum which is also called as fossa ovalis.
Ostium primum type of ASD develops from septum primum. It is also called as partial AV canal defect.
Sinus venosus type is situated superior to fossa ovalis in relation to the superior venocaval opening into the right atrium. 
Coronary sinus type the defect is seen near the opening of coronary sinus.

Following associations are seen in atrial septal defect
Ostium secundum – May be associated with mitral valve prolapse and partial anomalous pulmonary venous connection on right side.
Ostium primum ASD – is associated with left mitral or tricuspid valve and first degree heart block.
Sinus venosus ASD –may be associated with partial anomalous pulmonary venous connection.
Coronary sinus – May be associated with superior venacaval opening into left atrium.
Syndromes in relation to atrial septal defect
Down syndrome patients have higher rates of ASDs, 
Ebstein's anomaly about50% of individuals with Ebstein anomaly have an associated shunt between the right and left atria, either an atrial septal defect or a patent foramen ovale.
Fetal alcohol syndrome 25% of patients with fetal alcohol syndrome has either an ASD or a ventricular septal defect.
Holt-Oram syndrome – Both the osteium secundum and osteum primum types of ASD are associated with Holt–Oram syndrome
Lutembacher's syndrome – the presence of a congenital ASD along with acquired mitral stenosis.

SCIMITAR syndrome
What is SCIMITAR syndrome: Here there is fatal or patial anomalous pulmonary venous drainage into the inferior venacava

Lutembacher syndrome 
It is congenital ASD with acquired mitral stenosis.

Development of human Arterial System

Arterial system develop from Pharyngeal arch arteries and Aortic sac.
Aortic sac - remodels to form 2 horns, Right horn forms brachiocephalic artery, left horn forms common carotid artery

Pharyngeal arch arteries
Pharyngeal arch artery 1 and 2 undergoes regression
Pharyngeal arch artery 3 and the associated dorsal aorta form the paired internal carotid arteries, these in turn generate the external carotids
Left pharyngeal arch artery 4 becomes the the aortic arch
The paired dorsal aorta forms from the  paraxial mesoderm 
The head mesenchyme forms aortic arches

Connecting stalk contains umbilical (placental) arteries
The dorsal aortas give rise to two arteries 
  • Vitelline arteries which connect to capillaries on yolk sac
  • Intersegmental arteries located between somites
Venous System

  • Cardinal veins contribute nearly all systemic venous system.
  • Common cardinal veins is  ducts of Cuvier
  • Hepatic veins  drain de-oxygenated blood from the liver into the inferior vena cava.
  • Internal iliac vein drains to hypogastric vein

Development of interatrial septum

Through all stages of development of fetus blood shunts from right atria to left atrium so it bypass lungs.
Inter atrial septum is developed from  Septum Primum and Septum Secundum
Septum Primum
Septum Primum is a crescent shaped membranous extension of dorso-cranial wall. 
It will grows downward towards endocardial cushions
The opening between the Septum Primum and endocardial cushions is foramen primum (ostium primum), which  serves as a shunt.
A series of perforations will develop in cranial end that coalesce to form foramen secundum (ostium secundum)
In the later stages of development the septum primum fuses with endocardial cushions – this will obliterates foramen primum
Septum Secundum
Septum Secundum is a crescent shaped septum begins to form to the right side of of septum primum
Grows as septum primum downwards, it does not fuse with endocardial cushion, the opening is called foramen ovale
Because of the arrangement of the two septa it act as one-way valve.

Separation of the Atria from the Venticles in the embryo

Endocardial Cushions separate the atria from ventricle
They form initial division between atria and ventricles
This is formed  on the dorsal and ventral wall of atrioventricular canals
Endocardial cushion grow into canal  to meet and fuse to separate atrioventricular canal into right and left  heart channels

The anterior and posterior cushions fuse but the lateral cushions remain unfused