Embryology outline



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EMBRYOLOGY OUTLINE
Formation of the Human Eye - Oyster

I. OVERVIEW: key principles

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




  1. Specialized tissues are formed by collections of cells that have become specialized themselves.

    1. determination – the cells make a commitment to a particular developmental path

    2. differentiation – the cell begins to manufacture the proteins and intracellular organelles necessary for the lifestyle to which it has been committed




  1. Proliferation, movement, and differentiation in a cell group may require communication with other cells.

    1. Induction – the communication process between cells




  1. The blastocyst forms during the first week of embryogenesis

    1. cleavage – the earliest form of cell division in the embryo in which cells become smaller at each division

    2. morula – small cell cluster produced by the initial cleavage of the zygote, it then becomes a blastocyst

    3. blastocyst – ball of cells with a hollow interior (called a blastocoel)

    4. inner cell mass – dense cell cluster on one side of the blastocoel

    5. trophoblast – cells that surround the outside of the blastocyst




  1. The inner cell mass becomes the gastrula, which is divided into different germinal tissues

    1. gastrula

    2. epiblast – embryo forms from this layer cells

    3. hypoblast – becomes the lining for the yolk sac

    4. 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

    5. germ tissues – the three layers of cells in the gastrula are the primary germ tissues

    6. 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

    7. mesoderm – middle layer of cells that will become blood vessels, muscle, part of the urogenital system, bone , connective tissue, orbital bones, and extraocular muscles

    8. 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




  1. Neurulation begins the development of the nervous system

    1. gastrulation is followed by neurulation

    2. neural folds – thickening of ectoderm

    3. neural groove – separates neural folds

    4. notochord – elongated column of cells formed by mesoderm

    5. neural tube – tube of neural ectoderm that is the forerunner of the brain and spinal cord

    6. 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

    7. neural ectoderm – no longer connected to the surface ectoderm




  1. Ocular development begins in the primitive forebrain

    1. optic pits – beginnings of the eye, two small bumps on either side of the neural tube

    2. 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. The optic vesicle induces formation of the lens




  1. The optic cup and the lens form from different germinal tissues

    1. lens placode – thickened region of the surface ectoderm from which the lens develops

    2. lens vesicle – the lens before its interior is filled with cells

    3. optic cup – invagination of the lens placode




  1. The optic cup is initially asymmetric, with a deep groove on its inferior surface

    1. choroidal fissure – cleft along the underside of the optic cup that is the last part of the cup to be completed

    2. a primitive blood vessel the enters the fissure is also formed, and will eventually become the hyaloid artery and then the central retinal artery



  1. Closure of the choroidal fissure completes the optic cup




  1. The lens vesicle forms in synchrony with the optic cup




  1. The primitive lens is the first ocular structure to exhibit cell differentiation




  1. All future growth of the lens comes from the early lens cells, some of which are immortal stem cells

    1. stem cells - cells from which new cells of a particular cell type can be produced




  1. The precursors of the future retina, optic nerve, lens, and cornea are present by the sixth week of gestation




  1. In general, the eye develops from inside to outside




  1. One or both eyes may fail to develop completely

    1. degenerative anophthalmos – complete absence of the eye

    2. microphthalmos – one or both eyes are significantly smaller than normal




  1. Congenital absence of the lens may be an early developmental failure




  1. Incomplete closure of the choroidal fissure can produce segmental defects in the adult eye

a. colobomas – incomplete, improperly formed, or depigmented segments in structures such as the iris, lens, ciliary body, choroids, and retina, either singly or in combination
Ocular Embryology

OVERVIEW: key principles

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

2. Specialized tissues are formed by collections of cells that have become specialized themselves.


    1. determination – the cells make a commitment to a particular developmental path

    2. differentiation – the cell begins to manufacture the proteins and intracellular organelles necessary for the lifestyle to which it has been committed

3.Proliferation, movement, and differentiation in a cell group may require communication with other cells.

a. Induction – the communication process between cells, sometimes at a distance (not in direct contact).

Keep in mind – the structures are described separately, however the events are happening simultaneously.


  1. Germ Layer Development

    1. Embryonic plate – during embryonic development (~3rd week) the primary germ layers have formed in the embryonic plate.

    2. Embryonic primary germ layers

1. Ectoderm – forms neural plate/numerous eye structures

2. Mesoderm – Globe/ orbit connective tissue

3. Endoderm – no ocular structures


    1. Neural Plate

1. Thickening of ectoderm gives rise to CNS.

2. ~18 day – groove in neural plate forms ridges which grow into neural



folds.

3. ~22day – folds expand and grow toward one another forming neural



tube.

D. Neural Tube

1. Forms along the dorsal aspect of embryo.

2. Since the neural tube arises from ectoderm, the outer and inner linings of

the neural tube are the surface and neural ectoderms, respectively.

These lining have different locations anatomically and different

potentials for further development.

E. Neural Crest Cells and Mesoderm



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).

  1. Ocular structures and Lens Development

    1. Optic Pits – Indentations on both sides of neural tube in forebrain region. Before neural tube closes.

    2. 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.

    3. 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.

    4. Optic, Embryonic, or Fetal Fissure – 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.

    5. 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).

    1. 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.

    2. 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.

    3. Lens Capsule - ~5w. Evolves from the basement membrane of surface ectoderm and lens epithelium secretions.

    4. 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.

    5. 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.

    1. Fetal Nucleus – the area of the lens containing the Y sutures (Both ant. And Post.)

    2. Mitosis, cell elongation, and lens fiber formation continue throughout life.

    3. 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.

  1. Hyaloid Artery System (see figure 7.7 in handout)

    1. Hyaloid artery – formed by a branch of the internal artery entering the optic cup through the fetal fissure

    2. It produces a network that fills the vitreous cavity and forms the posterior tunica vasculosa lentis, which covers the posterior lens

    3. 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

      1. Tunica vasculosa lentis – carries nutrients to the developing lens until production of the aqueous begins

    4. 9 weeks - hyaloid vasculature reaches its peak development, begins to atrophy, and is reabsorbed during the 3rd and 4th months

    5. 7th month – no blood flow is present in the hyaloid vasculature




  1. Retinal Development (see figures 7.8, 9, 10 and 11)

    1. Retinal Pigment Epithelium

      1. The first retinal layer to differentiate

      2. Week 3 or 4 – cellular structures appear in the outer layer of the optic cup, and pigmentation of the retinal epithelium occurs

      3. Week 6 – RPE is 1 cell thick – basal surface faces developing choroids and apical surface faces inner layer of optic cup

    2. Neural Retinal (figure 7.8 and 9 are very helpful in understanding this)

      1. Week 4 or 5 – cells of the inner layer of the optic cup proliferate forming two zones

      2. Cells accumulate in the outer region, the proliferative zone

      3. The inner marginal zone is acellular

      4. Week 7 – cell migration occurs – forms the inner and outer neuroblastic layers

      5. Week 8 - Ganglion cells and amacrine cells differentiate in the vitreal portion of the inner neuroblastic layer – Muller cells also develop at this time

        1. ganglion cells migrate, forming a layer close to the basement membrane, and almost immediately send out their axonal processes

        2. the bodies of the Muller and Amacrine cells remain in the inner neuroblastic layer but move slightly towards the sclera

      6. 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)

      7. By week 12:

        1. Photoreceptor cells align along the outer side of the inner layer of the optic cup

        2. Horizontal, bipolar, amacrine and Muller cells are located in the inner nuclear layer

        3. Ganglion cell layer is evident

        4. Inner and outer plexiform layers are filling with cytoplasmic processes

      8. Synaptic complexes begin to appear

      9. cone pedicles develop before the rod spherules - synapse with bipolar cells before outer segments are completed

      10. 6 months – dense accumulation of nuclei in the macular region

      11. 7 months – ganglion cells and cells of inner nuclear layer move to the periphery of the macula

      12. 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

      13. The fovea is the last to reach maturity

    3. Retinal vessels

      1. Portions of the hyaloid artery within the optic stalk give rise to the central retinal artery

      2. 4 monthsvessels emerge from hyaloid artery near the optic disc and enter the developing nerve fiber layer

      3. continue to develop but are not completed until 3 months AFTER birth
  2. Cornea


    1. Corneal epithelium

      1. Arises form surface ectoderm

      2. Occurs at ~33 days

      3. All layers are present by 5th or 6th month

    2. Corneal endothelium

      1. Formed from first wave of neural crest that migrates into the space between the corneal epithelium and the lens

      2. It is 1 to 2 cells thick by week 6

      3. 4 months – it is a single row and Descemet’s membrane begins to form – tight junctions also begin to form

    3. Corneal Stroma

      1. 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

      2. Rapid growth of the stroma causes an increase in curvature relative to the rest of the globe

    4. At 3 months – all layers of the cornea are present except for Bowman’s layer, which develops during the 5th month

  3. Sclera Development

    1. Develops from neural crest mesenchyme

    2. Development begins near the limbus and continues posterior until it reaches the optic nerve

    3. 3rd month sclera has surrounded developing choriod

    4. 4th month lamina cribrosa begins to form

    5. 5th month sclera is well differentiated




  1. Uvea Development

    1. Choroid (from back to front and inside to outside)

  1. Develops from mesenchyme, starting near optic nerve

  2. Mesenchyme and developing pigmented epithelium must be in contact for choriocapillaris to form

  3. At 2 months vessels appear

  4. 5th month large and medium vessels are evident

  5. At midterm of fetal development Bruch’s membrane is present

  6. Basement membrane is the last component to appear

    1. Ciliary body

      1. Outer layer of optic cup forms nonpigmented epithelium

      2. Inner layer of optic cup forms pigmented epithelium

      3. 3rd month Epithelium begins to form ridges which become ciliary processes.

      4. 4th month major arteriole circle is formed

      5. 5th month Ciliary muscles begin developing

      6. 4-6 months Aqueous productions begins

    2. Iris

      1. End of 3rd month Optic cup begins to elongate and grows between the lens and cornea.

      2. 5th month, Iris sphincter pulls away from pupillary zone of epithelium and differentiates into smooth muscle

      3. 6th month, Iris dilator muscle fibers develop within epithelium

      4. Sphincter and Dilator muscles are completed by birth

      5. Sphincter and Dilator muscles originate from neural ectoderm

      6. Anterior Border layer is formed from large gaps between Mesenchymal cells

      7. Pigmentation begins to appear at 10 weeks and is complete by 7 months

      8. Iris stroma forms from accumulation of collagen fibers




  1. Pupillary Membrane – embryonic iris lacking a pupil

    1. Develops from 2nd neural crest wave during 3rd month forms anterior border & stroma.

    2. vascular network, anterior tunica vasculosa lentis joins to become iris vasculature

    3. Joins with long posterior ciliary arteries.

    4. Peripheral vessels contribute to minor arteriole circle of iris

    5. Central vessels fragment and are reabsorbed into anterior border layer.

  2. Anterior Chamber

    1. Trabecular Meshwork - ~4m – triangular neural crest cell mass becomes more and more organized at 9m. well developed beams and pores are present.

    2. Schlemm’s canal- derived from deep scleral plexus, tight junctions are present at 4m, endothelium derives from neural crest that invades at 3 m.

  1. Vitreous (See Fig. 7-7 for vitreous development)

  • Developing lens is crucial for vitreous accumulation

    1. Primary vitreous – fills vitreal space in early development

  • Made up of fibrils derived from the lens, retina, and hyaloid artery system

  • Reaches its greatest extent by 2 months

  • Vascular – will become Cloquets canal

  • Cloquet’s canal- apex at optic disc and base at the posterior lens

    • Well formed by the 4th month

    • persists to adulthood

    1. Secondary vitreous – forms around Cloquet’s canal (primary vitreous surrounding atrophying hyaloid system)

  • Avascular

  • Contains fibril network and hyalocytes which are produced by the primary vitreous

    1. Tertiary vitreous –an area between lens equator and ciliary body, not filled

  • Some texts do not mention this at all.

  • Once thought to give rise to zonule fibers that actually come from- thickening of internal limiting membrane of the ciliary body which is formed by the ciliary epithelium.

  1. Optic Nerve – (See Fig. 7-15)

  • Originates from the optic stalk

    • Optic stalk connects the optic vesicle to forebrain

    • The optic stalk becomes two-layered

    • 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.

  1. Ocular Adnexa

    1. 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

      1. Surface ectoderm – skin and conjuctiva epithelium, hair follicles and cilia, meibomian gland, Zeis glands, Glands of Moll

      2. Mesenchyme – orbicularis, levator, and tarsal muscles.

    1. 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.

    1. 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



    1. 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.

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VS112 Ocular Anatomy Page- - Last Updated 11/05/07


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