The eye and ear are highly
complex special senses, and both make their initial appearances
early in development - before the end of the 1st month of
development. The eye is derived from ectoderm and mesoderm, while
the ear is derived from all three germ layers - ectoderm, mesoderm,
and endoderm.
Both senses become partially
functional before birth: the fetus is startled by a loud noise or a
bright light from about the 6th months onwards. But maturation
continues after birth, especially in the neural mechanisms involved
in interpreting visual and auditory stimuli.
developing eye
Development of the eye has
been studied in detail in several species, and much of our knowledge
about embryonic mechanisms has come from these studies. For example,
much of the early work on induction (the influence of one region of
the embryo on the development of a nearby region) was carried out on
the embryonic eye, particularly the relationship between the optic
vesicle and the lens placode. Induction of lens development by the
optic vesicle appears to be a means of co-ordinating development -
it ensures that the lens is formed in the right place at the right
time. If this process fails, the eye fails completely to develop, a
condition known as anophthalmia.
The importance of parallel
arrays of microtubules in the folding of embryonic cell layers was
first noticed in an electronmicroscopic study of the invaginating
lens placode.
Note that the neural retina,
which is derived from the indented region of the optic vesicle, is
really a modified part of the brain wall. Thus, the optic nerve
connects two different parts of the brain, and perhaps therefore
should be considered a tract rather than a nerve.
Once the optic cup and lens
vesicle have been formed, the adjacent mesenchyme condenses to form
the outer coats of the eyeball - the sclera and choroid - and the
transparent cornea at the front of the eye.
Development of the eye
developing ear
The ear develops alongside
the embryonic hindbrain - note also its position in relation to the
1st and 2nd pharyngeal arches.
The outer ear develops from
the 1st pharyngeal cleft - which forms the external auditory meatus
- and six small swellings around the cleft which combine to form the
auricle.
The middle ear cavity
develops from the tip of the 1st pouch, and retains its connection
with the pharynx (the pharyngotympanic tube). Before birth, the
middle ear cavity is filled with fluid, but after birth the fluid is
replaced by air. The ossicles of the ear develop in 1st and 2nd arch
mesenchyme alongside the 1st pouch. Later, the middle ear cavity
enlarges and 'engulfs' the ossicles, so that they form a bridge
across the cavity.
The inner ear first appears
as a spherical otic vesicle, formed by the invagination of the
ectodermal otic placode. The different components of the inner ear -
ie: the semicircular canals, ultricle, saccule, and cochlea - are
then fashioned by modelling of the otic vesicle.
Note the formation of the tympanic membrane from all
three germ layers.
Development of the ear
Questions on development
of the eye and ear:
1 How is the optic cup
formed?
2 Give an
embryological origin of the iris. From which germ layer are the
muscle fibres in the iris delivered?
3 What is anophthalmia?
4 What is believed to
be the primary fault in cyclopia?
5 What is the
embryological origin of the ossicle of the ear?
6 Describe development
of the tympanic mambrane.
7 If a mother becomes
infected with Rubella virus during the first trimester, there is an
increased risk of her child being born deaf. Which part of the
developing ear is most commonly affected by this virus?
8 Is the ear sometimes
involved in the first arch syndrome?
