Embryology and Anatomy of Ear - 1 - Free MCQ Practice Test with solutions,

The failure of fusion of 1st and 2nd arch leads to the formation of the “PRE-AURICULAR SINUS”.
This is seen in between the tragus (from 1st arch) and the ascending limb of helix (from 2nd arch) most commonly either above the tragus or at the root of helix 

The 1st cleft grows and meet 1st pouch medially to form the tympanic membrane.

• So the tympanic membrane is made from all the 3 layers, outer epithelial layer from ectoderm and inner endothelial layer from endoderm and in between these is the fibrous layer from mesoderm.
• Retina develops from neuro-ectoderm.
• Dura mater is derived from mesoderm. Pia and arachnoid are derived from neural crest.

• The mastoid is incompletely developed at birth and continues to develop till 18 years of age.
• The largest air cell of mastoid called as mastoid antrum is present at birth and is of adult configuration.
• The middle and inner ear structures attain adult size well before birth.
• TM is also of adult size but, in the absence of bony part of EAC, is horizontally placed.

• The tympanic membrane, middle ear and the inner ear structures are fully developed and are of adult size at birth.
• Orbital structures continue to grow after birth.

Maxilla and Parietal bone continue to grow after birth.

• Cartilaginous bones ossify from a cartilage model (endochondral bone formation), e.g. Bony labyrinth also called as Otic capsule 
• In contrast to cartilaginous bone, membranous bone also called as dermal bone does not form from cartilage that then calcifies. Dermal bone is formed within the dermis and grows by accretion only–the outer portion of the bone is deposited by osteoblasts. Examples of membranous or dermal bones are clavicle and patella.
• Inner ear bony labyrinth is present in the petrous part of temporal bone. It is known as petrous (which means rock like) as it is one of the densest bones of the body, though not the strongest. The strongest bone of the body is the femur.

• Pneumatic bones are those bones which contain an air filled cavity within them which make them light in weight. In humans, they are seen in relation to the nasal cavity, and enclose the paranasal sinuses. Besides making the skull light in weight, they also help in resonance of sound and act as air conditioning chambers for the inspired air.
• Pneumatic bones are–maxilla, frontal bone, sphenoid and ethmoid.
• The mastoid is also most commonly a pneumatic (air filled) bone which encloses numerous air cell spaces giving it a honeycomb appearance.

However in some people the mastoid may be diploic (marrow filled) or sclerotic (solid bone without air).

The posterior and superior semicircular canals fuse together to form a common crus called as “crus commune” which opens into the utricle.
Because of this the three semicircular canals open into utricle by five openings instead of six.

• Semicircular canals contain cristae which sense angular or rotational acceleration.
• Gravitational movements, head tilt and linear acceleration are sensed by maculae in the utricle and saccule. 
• Sound is sensed by organ of Corti present in the scala media, situated on the basilar membrane.