Development of the respiratory system

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After separation of the laryngo-tracheal tube from the esophagus.The tube retains its opening just behind the hypobrancheal eminence.Then the upper part of the tube becomes the larynx which opens in to the pharynx,while its caudal part divides in to 2 principal bronchi ,each one is surrounded by a mesodermal mass known as lung bud.

The cartilages & muscles of the larynx develop mainly from the mesoderm of the fourth branchial arch with some contribution from the fifth & sixth arches,while the cartilages & muscles of the trachea & bronchi develop from the splanchnic mesoderm which surrounds the laryngo-tracheal tube.

The lung bud comes to surround the terminal end of each bronchus( future lung substance).The bud invaginates itself in to the pleural sac to form the 2 layers of pleura along with the pleural cavity & at the mean time the lung bud is infiltrated by the bronchus & its segmentation(lobar,lobular,terminal & respiratory bronchioles up to 17 th generation of its branching)..Lung development passes through four stages as follows:

1-Pseudoglandular stage which is a solid mass like a gland between 5th-16th weeks.

2-Canalicular stage between 16th – 26 th weeks of gestation.

3-Terminal sac period( Type I alveolar epithelial stage) between the end of 6th month

To the mid of 7th month of intrauterine development.

4-Type 2 alveolar epithelial stage ( lung tissue of new born),where the cells produced

The enzyme Surfactant to prevent the collapse of the alveoli.

Congenital Anomalies

1-Tracheo-esophageal fistula ,due to incomplete fusion of the lips of the laryngotracheal


2-Abnormal branching of laryngo-tracheal tube may lead to absence of one lob or to formation of an accessory lobe or accessory bronchopulmonary segment.

3-Respiratory distress syndrome (RDS) due to absence or deficiency in surfactant E.

Development of The Tongue

The anterior two thirds develops from the followings:

1-Tuberculum impar which is an elevation situated between the ventral end of the first branchial arch (lies anterior to hypobranchial eminence )

2-Two lingual swellings arising from the ventral end of the first arch on each side of the tuberculum impar.These 2 swellings fuse with tuberculum impar to form anterior 2/3.

The posterior third develops from Hypobranchial eminence which is related to the third & fourth arches.This Hypobranchial eminence fuses with the anterior 2/3 of the tongue at a V- shaped line called sulcus terminalis.

The muscular component of the tongue forms by the occipital somites which migrates to enter the epithelial covering of the tongue and forms its muscles & they carry with them the Hypoglossal nerve.

Congenital Anomalies of The Tongue

1-Aglossia either complete or incomplete absence of the tongue.

2-Bifid tongue from its pointed anterior end and back ward.

3-Short tongue due to short frenulum.

4-Macroglossia which is abnormal large tongue.


Development of the limbs start as Limb Buds on the ventrolateral part of the body near the end of the 4th weeks from Mesenchymal cells in the lateral plate mesoderm,where the buds of the upper limb appears at day 27th and those of lower limbs appear at day 29th.Each bud at first is in fact a core of Mesenchyme covered by Ectoderm,then the buds elongate by the proliferation of mesenchymal cells & becomes flattened so that the pre-axial & post-axial surfaces appear.The buds of upper limbs are seen opposite the caudal cervical segments,while those of lower limbs opposite the lumbar & sacral segments and gain innervation from the Brachial & Lumbosacral Plexuses respectively..

The buds of upper limbs rotates laterally so that the Thumb is directed laterally,while of lower limbs rotates medially so that the Hallux is directed medially.Later on Digital rays appear in the hand plate to form the fingers by the end of 6th weeks ,and for toes in the foot plate at 7th weeks and the notches between digital rays disappear due to tissue break down.

Formation of Bones of the Limbs starts as chondrification centers late in 5th weeks,thus by the end of 6th weeks limbs skeleton is cartilaginous.At 7th weeks (upper limbs) and 8th weeks (lower limbs) ossification centers in the middle of the cartilage appears to form the bones of the limbs.Then primary ossification centers in the long bones are seen by the end of the 12th weeks of intrauterine life,while in the carpal bones starts at one year of the age.

The muscles development start from the Myotomes where each myotome gives rise to myoblasts that fused together forming muscle fibers with its striations.Each myotome gives rise to the Hypomere(on ventral aspect of body & innervates by ventral ramus) & Epimere (on dorsal aspect of body & innervates by dorsal ramus).

Limbs Rotations: The upper limbs rotates laterally 90 degrees so that the Elbow becomes directed posteriorly & the extensor muscles lie on the lateral & posterior aspects of limb.

The lower limbs rotates mediaaly 90 degree too so that the Knee is directed anteriorly & the extensor muscles on anterior aspect of limb..


1-Amelia(complete absence of limbs) & Meromelia(rudimentary stump) due to Thalidomide.

2-Cleft hand & foot as lobster claw deformity.

3-Brachydactyly which means very short digits .

4-Polydactyly which means supernumerly digits.

5-Syndactyly( cutaneous or osseous ) its incidence is 1 / 2200.

6-Congenital club foot( talipus equino varus) its incidence 1 / 1000.

7-Congenital dislocated hip which is more in female than in males its incidence is 1 / 1500 due to abnormal development of acetabulum & joint laxity.

The upper limb buds paddle finger like at 32 days ( 5th week)

Hand plate is formed at 35 days ( end of 5th weeks).

Digital rays in hand plate appears at 42-44 days .

Limbs bending at Elbow,fingers are short & webbed at days 46-48.

Fingers are distinct & are separated at days 54-56.

Those for the lower limbs about 2-3 days after the above date for each event.


1-Buds appear by the end of 4th wks Appears at mid or late in 5th wks

2-Contrictions for future joints Appears at middle

appears during late 6th wks of 7th weeks

3-Ossifications starts at 7th wks At beginning of 8th weeks

4- Muscles appear during 8th weeks Appears at beginning of 9th wks

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