How does middle ear cleft develop embroyologically?

The middle ear cleft consist of

• The tympanic cavity,
• The Eustachian tube and
• The mastoid air cell system.

During early embryonic life in the mesenchyme surrounding the pharynx, five or six parallel thickenings develop as buds that surround the pharynx. There are the branchial arches which are numbered 1 to 5 from head to tail.

On the external surface there arches are separated by deep grooves, known as branchial clefts and this is matched by branched pouches on the inner pharyngeal surface.

 

Formation of the Eustachian tube middle ear and mastoid antrium:

The first pharyngeal pouch on the inside expands due to the rapid growth of the surrounding mesenchyme dragging some of  the 2^nd pouch endoderm.

The Eustachian tube lumen develops in the embryo by the lateral extension of the endoderm of the first pharyngeal pouch as it touches the inner surface of the ectoderm of the first branchial cleft. During this process, the distal portion of the pouch expands, forming the tubotympanic recess, which will later evolve into the middle ear cavity. The proximal portion narrows to form the Eustachian tube. This process takes place during the first 10 weeks of gestation.

The cartilage and muscles of the Eustachian tube develop from the surrounding mesoderm during the ensuing weeks. The levator veli palatini and the tensor veli palatini muscles seem to develop earlier than the cartilage and glandular tissue.

The cartilaginous portion of the tube elongates during the middle and third trimester until it reaches approximately 13 mm in length at term. Other morphological changes also occur during that time with further development of the glandular structures and folding of the epithelium.

As the skull base grows down, the angle of the Eustachian tube changes gradually from horizontal to oblique. This process continues after birth and well into adulthood.

Formation of ossicles:

The ossicle develop from the outer end of the first arch (Meckel’s) and second arch(Reichert’s) cartilages that lie above and below the first pharyngeal pouch.

Meckel’s cartilage – Malleus , Incus

Reichert’s cartilage – stapes

Muscles of middle ear:

The muscles in the middle ear cavity are derived from:

1^st branch arch – Tensor tympani

2^nd branch arch – stapedius.

The endoderm of the slit like sac that is the precursor of the middle ear lies against the ectoderm of the first pharyngeal groove by 4^th week. Mesenchyme grows in between these 2 layers to form the middle layer of the future tympanic membrane. The underlying sac expands and as it reaches the developing ossicle and labyrinth the epithelium is draped over the structure and associated muscles, tendons and ligament so that a complex series of the mucosal fold is formed.

A few mastoid “air cells” may be present at birth, albeit filled with amniotic fluid. About 90% of air cell formation being completed by the age of 6 and the remaining 10% takes place up to age of 18 years.

 

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