ORIGIN OF THE EYEBALL
MACULAR AREA AND FOVEA CENTRALIS:
The macular area first develops as a localized increase of superimposed nuclei in
the ganglion cell layer, later to the optic disc, just after midterm. During the seventh month
there is a peripheral displacement of the ganglion cells, leaving a central shallow depression,
the fovea centralis. The inner segment of the foveal cones decrease in width, but the outer
segments are elongated. This permits an increase in foveal cones density. At birth, the
ganglion cells have been reduced to a single layer of fovea, and by 4 months of age the
cones nuclei in the center of the fovea have no ganglion cells covering them. The reason for
the newborn’s imperfect central fixation is that the cones do not fully develop till several
months after birth.
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 layer fuse. The
cells of the optic stalk form neuroglial supporting cells to the axons, and the cavity of the
stalk disappears. The stalk, together with the optic axons forms the optic nerve. The axons
of the optic nerve begin to develop their myelin sheaths just before birth, but the process of
myelination continues for some time after birth. The partial decussation of the axons of the
two optic nerves forms the optic chiasma. The hyaloid artery and vein become the central
artery and vein of the retina.
The optic nerve axons leave the optic chiasma and grow backward as the optic
tracts and the majority pass to the lateral geniculate body and to the tectum of the midbrain.
THE LENS:
The rudimentary lens is first seen as a thickening of the surface ectoderm, the lens
placode, at 22 days'gestation: it overlies the optic vesicle. The lens placode invaginates and
sinks below the surface ectoderm to form the lens vesicle, which consists of a single layer of
cells covered by a basal lamina.
The cells forming the posterior wall of the lens now rapidly elongate and become
filled with proteins called crystallins, which make them transparent. These densely packed
elongated cells are known as the primary lens fibers. The base of each elongating cell
remains attached to the basal lamina posteriorly while their apices grow forward the anterior
lens epithelium so that the cavity of the lens vesicle gradually becomes obliterated. The
lengthening of the cells first occurs at the center of the posterior wall, which projects forward
into the lens cavity. The nuclei of the lens fibers more anteriorly within the cells to form a line
convex forward called the nuclear bow. The primary lens fibers now become attached to the
apical surface of the anterior lens epithelium and their nuclei disappear.
All additional fibers are formed by the mitotic division of the anterior epithelial cells
at the equator. These are known as the secondary lens fibers. New secondary lens fibers
are formed throughout life and persist throughout life. The basal ends of the fibers remain
attached to the basal lamina, while their apical ends extend around the primary fibers. Thus,
each new set of lens fibers is added superficially to the previous layer at the equator and the
lens enlarges and becomes more ellipsoid. Since the lens fibers are laid down concentrically,
the lens on section has a laminated appearance.
MACULAR AREA AND FOVEA CENTRALIS:
The macular area first develops as a localized increase of superimposed nuclei in
the ganglion cell layer, later to the optic disc, just after midterm. During the seventh month
there is a peripheral displacement of the ganglion cells, leaving a central shallow depression,
the fovea centralis. The inner segment of the foveal cones decrease in width, but the outer
segments are elongated. This permits an increase in foveal cones density. At birth, the
ganglion cells have been reduced to a single layer of fovea, and by 4 months of age the
cones nuclei in the center of the fovea have no ganglion cells covering them. The reason for
the newborn’s imperfect central fixation is that the cones do not fully develop till several
months after birth.
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 layer fuse. The
cells of the optic stalk form neuroglial supporting cells to the axons, and the cavity of the
stalk disappears. The stalk, together with the optic axons forms the optic nerve. The axons
of the optic nerve begin to develop their myelin sheaths just before birth, but the process of
myelination continues for some time after birth. The partial decussation of the axons of the
two optic nerves forms the optic chiasma. The hyaloid artery and vein become the central
artery and vein of the retina.
The optic nerve axons leave the optic chiasma and grow backward as the optic
tracts and the majority pass to the lateral geniculate body and to the tectum of the midbrain.
THE LENS:
The rudimentary lens is first seen as a thickening of the surface ectoderm, the lens
placode, at 22 days'gestation: it overlies the optic vesicle. The lens placode invaginates and
sinks below the surface ectoderm to form the lens vesicle, which consists of a single layer of
cells covered by a basal lamina.
The cells forming the posterior wall of the lens now rapidly elongate and become
filled with proteins called crystallins, which make them transparent. These densely packed
elongated cells are known as the primary lens fibers. The base of each elongating cell
remains attached to the basal lamina posteriorly while their apices grow forward the anterior
lens epithelium so that the cavity of the lens vesicle gradually becomes obliterated. The
lengthening of the cells first occurs at the center of the posterior wall, which projects forward
into the lens cavity. The nuclei of the lens fibers more anteriorly within the cells to form a line
convex forward called the nuclear bow. The primary lens fibers now become attached to the
apical surface of the anterior lens epithelium and their nuclei disappear.
All additional fibers are formed by the mitotic division of the anterior epithelial cells
at the equator. These are known as the secondary lens fibers. New secondary lens fibers
are formed throughout life and persist throughout life. The basal ends of the fibers remain
attached to the basal lamina, while their apical ends extend around the primary fibers. Thus,
each new set of lens fibers is added superficially to the previous layer at the equator and the
lens enlarges and becomes more ellipsoid. Since the lens fibers are laid down concentrically,
the lens on section has a laminated appearance.
