The eye develops rapidly during gestation and in the first years of life. During the 4th-8th week of gestation, the optic vesicle forms and invaginates to become the optic cup, which later develops into the neural retina and retinal pigment epithelium. The lens placode also forms during this time. After birth, the eye continues growing, with the axial length increasing most in the first year of life. The retina and visual cortex also continue developing postnatally. Proper development of the eye and visual system during these critical periods is required for normal visual function.

1. Eye in infancy
Anatomical and physiological considerations of eye

3. The neural tube is at first a simple tube. Later, its cranial end differentiates froms two
constrictions. Three vesicles i.e. forebrain, midbrain and hindbrain are formed.
Telencephalon gives rise to cerebrum.
The optic cup develops in the diencephalon.

4. Embryogenesis and eye development : establishment of primary organ
rudiments
Finishes around the end of the third gestational week
Soon after gastrulation (formation of the three layers -ectoderm, mesoderm, and
endoderm) begins, the eye field is specified in the anterior neural plate.
The first morphologic landmarks are bilateral indentations (optic sulci or pits),
at approximately 22 days, in the neural folds at the cranial end of the embryo.
Eye organogenesis – development of primary organ rudiments
(4th to 8th week)
Fourth week
In the fourth week, the optic sulci deepen and form the optic vesicles
which are evaginations of the lateral walls of the diencephalon.
The proximal part of the optic vesicle becomes elongated to form the optic stalk.
Optic stalk eventually forms the optic nerve.

5. Interaction between the OV and surface ectoderm (SE) induces the lens placode, and the
wall of the OV in contact with the SE thickens to form the retinal disk.
Towards the end of the fourth week invagination begins to transform the OV into the optic
cup (OC). Simultaneously, the primordia of the extraocular muscles appear as
condensations in the periocular mesenchyme.

6. The optic cup is not continuous, and forms a fold that is continuous with the optic stalk.
This fold k/a embryonic or optic fissure allows the passage of hyaloid artery into the optic
cup.
Disruptions in these early steps lead to severe congenital anomalies, including
anophthalmia, microphthalmia, and optic fissure closure defects (coloboma).

7. Fifth week
The process of invagination of the OV to form the OC predominates in the fifth week.
Invagination of the retinal disk of the OV leads to formation of the inner layer of the OC which becomes the
neural retina, while the external layer of the OC will become the retinal pigment epithelium (RPE).

8. The primary vitreous develops around the hyaloid vasculature.
The process of invagination also involves the lens placode (plate), which leads to the formation of the lens
pit. The lens pit deepens to become the lens vesicle. Further development leads to the lens vesicle
separating from the SE.
The lens vesicle is large and fills the OC.
The SE becomes the corneal epithelium.
Sixth week
In the sixth week the optic fissure closes after the edges of the OC that border the fissure become closely
apposed.
The embryonic fissure closure begins in the middle and then extends anteriorly and posteriorly.
The development of retina progresses with the RPE forming a single layer of cuboidal cells. A primitive Bruch’s
membrane arises.
The sensory retina thickens due to proliferation of cells in the germinative zone of the inner layer of the OC. At
this stage, retinal ganglion cell axons, which form the optic nerve fibers, first enter the optic stalk to exit the
primitive eye.
The secondary vitreous, a cellular structure with associated extracellular matrix (ECM), forms and remodels the
primary vitreous filling the remaining retrolenticular space.

9. Seventh week
The main events during the seventh week include the maturation of the RPE, the
development of the sensory retina with the formation of outer and inner neuroblastic layers
at the posterior pole.
Primary lens fibers form to obliterate the cavity in the lens vesicle.
The anterior periocular mesenchyme in the mammal has contributions from neural crest and
mesodermal cells.
The mesenchymal cells migrate forward so that cells of neural crest and mesodermal origin
contribute to the corneal stroma, endothelium, and trabecular meshwork: Schlemm’s canal (SC)
is of mesodermal origin.

11. Eighth Week
In the eighth week, there is marked development of the optic nerve as ganglion
cells differentiate; by the end of this week, 2.67 million axons have formed. Optic
nerve axons start to make contact with the brain and establish a rudimentary chiasm.
The RPE nears maturation with the appearance of melanosomes.
Müller cells appear now and extend radial fibers inwards to form the internal limiting
membrane and outward toward the future external limiting membrane.
Corneal differentiation includes endothelial cells starting to form Descemet’s membrane; the
corneal stroma consists of 5–8 rows of cells and the corneal epithelium is evolving to a
stratified squamous epithelium.

12. The lens develops rapidly during this period. The
primary
lens fibers fill the lens vesicle. The intracellular
organelles disappear.
The equatorial epithelial cells begin to divide and new
cells are pushed posteriorly, then elongate and
become the secondary lens fibers. With the formation
of the secondary lens fibers, there is development of
the lens bow which represents the nuclei of the
secondary lens fibers. They form an arc with a
forward convexity. The lens “sutures” develop where
the secondary lens fibers meet in a linear pattern at
the anterior and posterior poles of the lens. The
sutures initially are in a Y shape anteriorly and an
inverted Y posteriorly.
The four rectus muscles insert into the sphenoid bone
and
the trochlea develops. The lacrimal glands form from
the
superotemporal quadrant of the conjunctival sac.

15. POST NATAL DEVELOPMENT

16. Dimensions of the Eye
Most of the growth of the eye takes place in the first year of life. The change in the axial
length of the eye occurs in 3 phases. The first phase (birth to age 2 years) is a period of rapid growth.
The axial length increases by approximately 4 mm in the first 6 months of life and by an additional 2 mm
during the next 6 months. During the second (age 2 to 5 years) and third (age 5 to 13 years) phases, growth
slows, with axial length increasing by about 1 mm per phase.

17. Similarly, the cornea grows rapidly during the first year of life. Keratometry
values change markedly in the first year, starting at approximately 52.00 D at birth, flattening
to 46.00 D by age 6 months, and reaching adult measurements of 42.00- 44.00 D by age
12 months.
The average horizontal diameter of the cornea is 9.5-10.5 mm in newborns and increases to 12.0 mm in adults.
Mild corneal clouding may be seen in healthy newborns and is common in premature infants. It resolves as the
cornea gradually thins, decreasing from an average central thickness of 691 um at 30- 32 weeks' gestation to
564 um at birth.
Keratocyte density is around 60,000 cells per cubic millimetre in infancy with a decline of 0.3% per year
through life.
Endothelial cell counts exceed 10,000 cells per square millimetre at 12 weeks of gestation, 50% of this at birth
and 4,000 cells per square millimeter in childhood.

18. Mean values (dots) and standard deviations (bars) for calculated lens power as determined by modified SRK formula, plotted
with respect to age.
The power of the pediatric lens decreases dramatically over the first several years of life-an important consideration when
intraocular lens implantation is being planned for infants and young children after cataract extraction.

19. Orbit and ocular adnexa
During infancy and childhood, the orbital volume
increases and the shape of the orbital opening
becomes less circular, resembling a horizontal oval.
The lacrimal fossa becomes more superficial, and
the angle formed by the axes of the 2 orbits
becomes less divergent.

20. At first angle between the orbital axes is nearly 180 degree in IUL. With continuous
growth, the axes gradually become more oriented frontally. At birth, the angle is
reduced to approximately 71 degrees.
The adult condition of 68 degree is achieved at adolescence.

21. The palpebral fissure measures approximately 18 mm horizontally and 8 mm vertically at birth and changes
very little during the first year of life. However, from age 1 to 10 years, the palpebral fissure length increases
rapidly, causing the round infant eye to acquire its elliptical adult shape.
The palpebral fissure lengths are 15 ± 2 mm at 32 weeks of gestation, 17 ± 2 mm at birth,
24 ± 3 mm at 2 years of age, and 27 ± 3 mm at the age of 14.
Inner canthal distance and outer orbital distance are 16 and 59 mm, respectively,
in premature infants; 20 ± 4 and 69 ± 8 mm in newborn babies; 26 ± 6 and 88 ± 10 mm
at the age of 3; and 31 ± 5 and 111 ± 12 mm at the age of 14.

22. Volume of orbit is 10.3 ml at birth, doubling by 1 year to 22.3 ml, and reaching
adult volume of 30 ml by 6-8 years.
Birth- 10.3 mm3
1 year- 22.3 mm3
6-8 years- 39.1 mm3
Adult : Males: 59.2 mm3
Females: 52.4 mm3

23. Histologic studies show that the nasolacrimal duct is not fully canalized in many newborns, but
most are asymptomatic.

24. Cornea, Iris, Pupil and Anterior Chamber
Average central corneal thickness (CCT) decreases during the first 6-12 months of life. It then increases from 553
mm at age 1 year to 573 µm by age 12 years and stabilizes thereafter.
Most changes in iris color occur over the first 6-12 months of life, as pigment accumulates in the iris stroma and
melanocytes.
Compared with the adult pupil, the infant pupil is relatively small. A pupil less than 1.8 mm or greater than 5.4
mm in diameter is suggestive of an abnormality. The pupillary light reflex is normally present after 31 weeks'
gestational age.
At birth, the iris insertion is near the level of the scleral spur, but during the first year of life, the lens and ciliary
body migrate posteriorly, resulting in formation of the angle recess.
Trabecular meshwork is less pigmented.
Angle of anterior chamber attains adult-like morphology around 1 year of age.

27. Intraocular Pressure
Measurement of intraocular pressure (lOP) in infants can be difficult, and normal
pressures vary depending on the method used to obtain them. Nevertheless, normal lOP
is lower in infants than in adults, and a pressure of greater than 21 mm Hg should be
considered abnormal. CCT influences the measurement of lOP, but this effect is not well
understood in children.
Normal IOP
Birth-6m 9.5-11.5 mm Hg
1-2 y 10-12 mm Hg
2y <12 mm Hg
Shallow anterior chambers, miotic pupils and bluish irides are features of prematurity.

28. Extraocular Muscles
The rectus muscles of infants are smaller than those of adults; muscle insertions, on
average, are 2.3-3.0 mm narrower in infants than in adults; and the tendons are thinner in infants.
In newborns, the distance from the rectus muscle insertion to the limbus is roughly 2 mm less than that in
adults; by age 6 months, this distance is 1 mm less; and at 20 months, it is similar to that in adults.
Enlargement of the posterior segment occurs during the first 2 years of life, resulting in a separation of 4-5
mm between the superior and inferior oblique insertions and migration of the inferior oblique insertion
temporally.
Extraocular muscle function continues to develop after birth.
Vestibular-driven eye movements are present as early as 34 weeks' gestational age.
Conjugate horizontal gaze is present at birth, but vertical gaze may not be fully functional until 6 months of
age.
Intermittent strabismus is present in approximately two-thirds of young infants but resolves in
most by 2-3 months of age.

30. Retina
The macula is poorly developed at birth but changes rapidly during the first 4
years of life. Most notable are changes in macular pigmentation, the annular ring,
the foveal light reflex, and cone photoreceptor differentiation.
Improvement in visual acuity with age is attributed to 3 processes: differentiation
of cone photoreceptors, narrowing of the rod-free zone, and increase in foveal
cone density.
Retinal vascularization proceeds in a centrifugal manner, starting at the optic disc
at 16 weeks' gestational age and reaching the temporal ora serrata by 40 weeks'
gestational age.

32. Visual cortex development
Development of the cortical visual centers has been investigated using Macaque monkeys.
The lateral geniculate nucleus (LGN) can first be identified at an age that corresponds to 8 to 11 weeks
in a human gestational age with ganglion cells reaching the LGN at 10 weeks gestational age.
The lamination that characterizes the LGN develops between 22 and 25 weeks gestational age.
Concurrently, as the LGN is developing, cells that will form the striate cortex are developing between 10 to 25
weeks.
Formation of ocular dominance columns takes place between 26 weeks and term, and a significant amount of
cortical visual development continues postnatally.
Just as the foveal development is incomplete at birth, so is the lateral geniculate nucleus as well as striate cortex.
Synaptic connections in the striate cortex develop to reach a maximum degree of interconnection 8 months
postnatally with further refinement that occurs over several years. This refinement of organization is dependent
upon a clear retinal image being focused upon the eye transmitted through the optic nerve and received by the
developing striate cortex.

33. There is a critical period of cortical development during which any impediment of formed
vision leads to permanent abnormal cortical development.

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