Transcript 1._Embryology,_Anatomy_&_Function_of_the_Eye

Embryology of the eye
The eye is formed from both ectoderm and
mesenchyme. The neuroectoderm that is derived
from the neural tube gives rise to (the retina, the
fibers of the optic nerve, and the smooth muscle of
the iris). The surface ectoderm on the side of the
head forms( the corneal and conjunctival
epithelium, the lens, and the lacrimal and tarsal
glands). The mesenchyme forms( the corneal
stroma, the sclera, the choroid, the iris, the ciliary
musculature, part of the vitreous body, and the cells
lining the anterior chamber).
The eyeball
The rudimentary eyeball develops as an
ectodermal diverticulum from the lateral aspect
of the forebrain. The diverticulum grows out
laterally toward the side of the head, and the end
becomes slightly dilated to form the optic
vesicle, while the proximal portion becomes
constricted to form the optic stalk. At the same
time, a small area of surface ectoderm overlying
the optic vesicle thickens to form the lens
placode. The lens placode invaginates and sinks
below the surface ectoderm to become the lens
vesicle, the optic vesicle becomes invaginated to
form the double-layered optic cup.
Orbit and Ocular Adnexa
During infancy and childhood, the orbital volume
increases, the shape of the orbital opening becomes
less circular and more like a horizontal oval, the
lacrimal fossa becomes more superficial, and the
angle formed by the axes of the 2 orbits assumes a less
divergent position.
The palpebral fissure measures approximately 18 mm
horizontall y and 8 mm vertically at birth and changes
very little during the first year of life, but a rapid
increase in palpebral fissure length occurs during the
first decade. causing the round infant eye to acquire an
elliptical adult shape.
Iris, Pupil, and Anterior Chamber
Most iris color changes occur over the first 6
to 12 months of life. as pigment accumulates
in the iris stroma and melanocytes, but
iris pigmentation may continue.
The optic nerve
The ganglion cells of the retina develop
axons that converge to a point where the
optic stalk leaves the posterior surface of
the optic cup. This site will later become
the optic disc. The axons now pass
among the cells that form the inner layer
of the stalk. Gradually, the inner layer
encroaches on the cavity of the stalk until
the inner and outer layers fuse.
Developmental Malformations and
Anomalies
Eyelids
Epicanthic folds
Signs- bilateral vertical folds of skin
that extend from the upper or lower
Lids towards the medial canthi;
Coloboma
1. Definition- partial or full-thickness
Eyelid defect occurring when eyelid
Development is incomplete.
2. Pathogenesis failure of migration
of lid ectoderm to fuse the lid folds,
or mechanical forces such as
Amniotic bands.
3. Upper lid - at junction of the middle
and inner thirds.
4. Lower lid - at junction of the middle
and outer thirds
Globe
Microphthalmos
1. Definition- total axial length at
least2 standard deviations below
age-similar controls usually
unilateral.
2. Pathogenesis non-specific growth
Failure in response to a variety of
Prenatal insults.
3. Classification
a. Simple- isolated.
b. Complex(colobomatous\
Anophthalmos
1. Pathogenesis either acomplete
Failure of budding of the optic vesicle
or early developmental arrest.
2. Simple- associated with absence of
Extraocular muscles,short conjunctival
sac. and microbleoharon.
3. Anophthalmos with cyst (congenital
cystic eyeball) - globe is replaced
by a cyst
Iris, Pupil, and Anterior Chamber
Most iris color changes occur over the first 6 to
12 months of li fe. as pigment accumulates
in the iris stroma and melanocytes, but iris
pigmentation may continue. Compared with
References
1. Lecture notes in
ophthalmology
2. Parson’s disease of the
eye