1. Embryogenesis begins with cell proliferation, cell movement, and changes in cell shape.
a. proliferation is an increase in cell number by mitosis and division
b. motility is the ability of cells to move around and change shape
Specialized tissues are formed by collections of cells that have become specialized themselves.
determination – the cells make a commitment to a particular developmental path
differentiation – the cell begins to manufacture the proteins and intracellular organelles necessary for the lifestyle to which it has been committed
Proliferation, movement, and differentiation in a cell group may require communication with other cells.
Induction – the communication process between cells
The blastocyst forms during the first week of embryogenesis
cleavage – the earliest form of cell division in the embryo in which cells become smaller at each division
morula – small cell cluster produced by the initial cleavage of the zygote, it then becomes a blastocyst
blastocyst – ball of cells with a hollow interior (called a blastocoel)
inner cell mass – dense cell cluster on one side of the blastocoel
trophoblast – cells that surround the outside of the blastocyst
The inner cell mass becomes the gastrula, which is divided into different germinal tissues
epiblast – embryo forms from this layer cells
hypoblast – becomes the lining for the yolk sac
primitive streak – develops in the epiblast, defines it’s long axis at which cells migrate down and laterally to form a three layered disc of cells
germ tissues – the three layers of cells in the gastrula are the primary germ tissues
ectoderm – upper layer of cells closest to the amnion, will give rise to the nervous system, epidermis, and other epithelial layers, and the major parts of the eyes
mesoderm – middle layer of cells that will become blood vessels, muscle, part of the urogenital system, bone , connective tissue, orbital bones, and extraocular muscles
endoderm – inner layer of cells next to blastocoel, forms lining of gut, respiratory system, and related organs; no direct role in formation of eye and orbit
Neurulation begins the development of the nervous system
gastrulation is followed by neurulation
neural folds – thickening of ectoderm
neural groove – separates neural folds
notochord – elongated column of cells formed by mesoderm
neural tube – tube of neural ectoderm that is the forerunner of the brain and spinal cord
neural crest – cluster of cells from the roof of the neural tube that migrate out to form peripheral nerve ganglia, corneal stroma and endothelium, trabecular meshwork, iris stroma, and ciliary muscle
neural ectoderm – no longer connected to the surface ectoderm
Ocular development begins in the primitive forebrain
optic pits – beginnings of the eye, two small bumps on either side of the neural tube
optic vesicle – cell proliferation of the optic pit produces large spherical bulge, connected to the neural tube by a short, cylindrical optic stalk (both vesicle and stalk made of epithelial cells, meaning they are hollow)
1. Neural crest cells – an area of cells that separates from the ectoderm
on the crests of each of the neural folds. They then settle to lie
between neural tube and surface ectoderm.
2. Mesoderm – the middle embryonic layer is located between the neural
tube and surface ectoderm.
3. Mesenchyme – Neural crest cells and mesoderm form connective tissue of globe and orbit
See figure 7.1 of Remington (page 104). Ocular structures and Lens Development
Optic_Pits'>Optic Pits – Indentations on both sides of neural tube in forebrain region. Before neural tube closes.
Optic Vesicles - ~25days lateral, sac-shaped extensions are formed from optic pits. After the neural tube closes, the vesicle wall is initially in contact with surface ectoderm but is separated from it by cells of the neural crest origin and mesoderm. While it is in contact with the surface ectoderm, it thickens and flattens to form the retinal disc.
Optic Stalk – invagination of optic vesicle and constriction of the tissue joining the vesicle to the neural tube form the optic stalk. Inner surface cells are ciliated, and outer surface cells are covered by basal lamina.
Optic, Embryonic, or FetalFissure – the lower wall of the optic vesicle and optic stalk begin to buckle and move inward toward the upper and posterior walls which form a cleft called optic, embryonic, or fetal fissure. Edges of the fissures grow toward each other and begin to fuse after 5 weeks, fusion starts in the center an proceeds anteriorly toward the rim of the optic cup and posterior along the optic stalk. Closure is complete after 7 weeks forming the 2 layers optic cup.
Optic cup ~ 7 weeks. Composed of two layers separated by intraretinal space. Outer layer –RPE, outer NPE of ciliary body, anterior iris epithelium. Inner layer – neural retina, inner PE of ciliary body, posterior iris epithelium.
Flashback: Remember the pictures of the coloboma (incomplete iris) – this is where that initially occurs – incomplete closure of optic fissure. See Figure 7.2 -7.3 on pp. 105-106 of handout (Remington chapter). Lens- development begins ~27 days (5 weeks). Invagination of the optic cup occurs around the same time the surface ectoderm thickens around the optic vesicle and forms the lens plate or lens placode. Thickening occurs by elongation of ectodermal cells by regional increase in cell division.
Lens pit – invagination of the center of the lens plate forms the lens pit. Further invagination forms the lens vesicle which pinches off from the surface ectoderm (~33 days) and becomes a hollow sphere of a single layer of cells surrounded by a basil lamina. Upon further development, the basil lamina becomes the lens capsule.
Lens Capsule - ~5w. Evolves from the basement membrane of surface ectoderm and lens epithelium secretions.
Embryonic nucleus and primary lens fiber cells – cells on the posterior side of the hollow sphere, adjacent to the future vitreous cavity, elongate into the center of the sphere to fill the empty space (lumen) within the lens vesicle. These epithelium cells become primary lens fiber cells and form embryonic nucleus – no sutures. This makes explains why there are only epithelial cells on anterior surface of the lens.
Sutures and Secondary Lens Fiber cells - ~7w.
Equatorial cells begin undergoing mitosis, elongation anteriorly and posteriorly around the embryonic nucleus forming secondary lens fibers. 1st layer of secondary fibers is complete after 7 weeks. Anteriorly, the secondary lens fiber cells meet and form upright Y suture. Posteriorly, the secondary lens fibers meet and from inverted Y sutures. These sutures are visible after the 3rd month. The region containing Y sutures continues until birth forming the fetal nucleus.
Fetal Nucleus – the area of the lens containing the Y sutures (Both ant. And Post.)
Mitosis, cell elongation, and lens fiber formation continue throughout life.
FYI: At birth, lens has complete capsule with zonular fibers inserted, an anterior epithelium, ~1.1 million lens fibers, and ~1300 lens shells, 6.5mm dia. An aged lens has ~ 3.6 million lens fibers and ~ 2500 shells, 9.5mm diam.
See figure 7.5 and 7.6. Hyaloid Artery System(see figure 7.7 in handout)
Hyaloid artery – formed by a branch of the internal artery entering the optic cup through the fetal fissure
It produces a network that fills the vitreous cavity and forms the posterior tunica vasculosa lentis, which covers the posterior lens
Branches near the equator anastomose with the annular vessel, which sends loops forward onto the anterior surface of the lens to form the anterior tunica vasculosa lentis Tunica vasculosa lentis – carries nutrients to the developing lens until production of the aqueous begins
9 weeks - hyaloid vasculature reaches its peak development, begins to atrophy, and is reabsorbed during the 3rd and 4th months
7th month – no blood flow is present in the hyaloid vasculature
Retinal Development(see figures 7.8, 9, 10 and 11)
Retinal Pigment Epithelium The first retinal layer to differentiate
Week 3 or 4 – cellular structures appear in the outer layer of the optic cup, and pigmentation of the retinal epithelium occurs
Week 6 – RPE is 1 cell thick – basal surface faces developing choroids and apical surface faces inner layer of optic cup
Neural Retinal (figure 7.8 and 9 are very helpful in understanding this)
Week 4 or 5 – cells of the inner layer of the optic cup proliferate forming two zones
Cells accumulate in the outer region, the proliferative zone
The inner marginal zone is acellular
Week 7 – cell migration occurs – forms the inner and outer neuroblastic layers
Week 8 - Ganglion cells and amacrine cells differentiate in the vitreal portion of the inner neuroblastic layer – Muller cells also develop at this time
ganglion cells migrate, forming a layer close to the basement membrane, and almost immediately send out their axonal processes
the bodies of the Muller and Amacrine cells remain in the inner neuroblastic layer but move slightly towards the sclera
Bipolar and Horizontal cells migrate from the outer neuroblastic layer and settle near the Muller and Amacrine cells, eliminating the Transient fiber layer (of Chevitz)
By week 12:
Photoreceptor cells align along the outer side of the inner layer of the optic cup
Horizontal, bipolar, amacrine and Muller cells are located in the inner nuclear layer
Ganglion cell layer is evident
Inner and outer plexiform layers are filling with cytoplasmic processes
Synaptic complexes begin to appear
cone pedicles develop before the rod spherules - synapse with bipolar cells before outer segments are completed
6 months – dense accumulation of nuclei in the macular region
7 months – ganglion cells and cells of inner nuclear layer move to the periphery of the macula
Birth – a single layer of ganglion cells and a thin inner nuclear layer still covers the foveal area – it is not until 4 months post partum, that the layers are displaced to the sloping walls of the fovea, leaving the cones as the only neural cell bodies in the center of the depression
The fovea is the last to reach maturity
Retinal vessels Portions of the hyaloid artery within the optic stalk give rise to the central retinal artery
4 months – vessels emerge from hyaloid artery near the optic disc and enter the developing nerve fiber layer
Corneal endothelium Formed from first wave of neural crest that migrates into the space between the corneal epithelium and the lens
It is 1 to 2 cells thick by week 6
4 months – it is a single row and Descemet’s membrane begins to form – tight junctions also begin to form
Corneal Stroma Week 8 – second wave of neural crest migrates between developing epithelium and endothelium – it splits and the posterior portion gives rise to the pupillary membrane and the anterior portion gives rise to the stroma
Rapid growth of the stroma causes an increase in curvature relative to the rest of the globe
At 3 months – all layers of the cornea are present except for Bowman’s layer, which develops during the 5th month
Sclera Development Develops from neural crest mesenchyme
Development begins near the limbus and continues posterior until it reaches the optic nerve
3rd month sclera has surrounded developing choriod
4th month lamina cribrosa begins to form
5th month sclera is well differentiated
Uvea Development Choroid (from back to front and inside to outside)
Develops from mesenchyme, starting near optic nerve
Mesenchyme and developing pigmented epithelium must be in contact for choriocapillaris to form
At 2 months vessels appear
5th month large and medium vessels are evident
At midterm of fetal development Bruch’s membrane is present
Basement membrane is the last component to appear
Ciliarybody Outer layer of optic cup forms nonpigmented epithelium
Inner layer of optic cup forms pigmented epithelium
3rd month Epithelium begins to form ridges which become ciliary processes.
4th month major arteriole circle is formed
5th month Ciliary muscles begin developing
4-6 months Aqueous productions begins
Iris End of 3rd month Optic cup begins to elongate and grows between the lens and cornea.
5th month, Iris sphincter pulls away from pupillary zone of epithelium and differentiates into smooth muscle
Outer layer becomes neuroglial sheath (surrounding the optic nerve) and glial components of lamina cribosa
Inner layer- vacuoles formed for retinal ganglion axons to pass through, and other cells form optic nerve glial cells
Ganglion cell axons fill the lumen of the optic canal
Grow toward the central nervous system
Nerve myelination begins at the lateral geniculate body during the 5th month, reaches the chiasm during the 6th month and stops at the lamina cribosa 1 month after birth.
Ocular Adnexa Eyelids - (See Fig. 7-16) At 2months surface ectoderms folds begin to grow anterior to developing cornea. They meet and fuse ~ 10 weeks and remain fused until the lid structures have developed. (See Fig. 7-17)
Fusion of the eyelids protects the developing eye from amniotic fluid
May also prevent keratinizing of the the cornea and conjunctiva
At 5-6 months eyelids separate due to breakdown of desmosomes formed for lid fusion (breakdown is possibly caused by Meibomian gland secretions
Development of Eyelid Components
Surface ectoderm – skin and conjuctiva epithelium, hair follicles and cilia, meibomian gland, Zeis glands, Glands of Moll
Mesenchyme – orbicularis, levator, and tarsal muscles.
Orbit- (See Fig. 7-18)
Bones fuse and ossify at 6-7 months.
The angle between orbits decreases over time from initially 180° during early development to 70° at birth and 68° at adulthood
Globe reaches adult size by age 3
Orbit reaches adult size by age 16
Orbial fat and connective tissue originate from neural crest cells.
EOMS (Extraocular Muscles)- mesenchymal origin.
Mesoderm forms muscle while connective tissue has neural crest origin.
Recent studies seem to indicate the muscle origin, belly, and insertion grow simultaneously
Nasolacrimal system – not well known
Main lacrimal gland has a possible epithelium & mesenchyme origin
Continues to develop after birth and is not fully developed until 3-4 years of age
Drainage system – develops from surface ectodermal cord buried below maxillary mesenchyme.
VS112 Ocular Anatomy Page- - Last Updated 11/05/07