lecture 2

• Embryology


  Understanding the anatomy and function of the eye by studying how it develops in the embryonic stages
• Inverted retina


  • The eyes of all vertebrates develop in a pattern which produces an "inverted" retina, in which the initial detection of light rays takes place at the outermost portion
  • Light enters the cornea and travels though transparent nerve fibres and nuclei before striking photoreceptors (cons or rods)
  • The retinal pigment epithelium provides the energy requirements, removes excess heat and absorbs stray light and reduces scattering or visual disturbance factors like shadows

  
• Embryogenic eye development


  1. Starts at week four of the gestational period

  2. Gastrulation stage

  3. Blastula is the result of multiplication of punch of cells from which zygot then it continue to differentiate and travel from uterine tube and then implant to the womb wall

  4. Gastrulation is the process produces 3 germ lyres (starts at the 3rd week of gestational period)

  5. Ectoderm, mesoderm and endoderm

  6. Zygote

  7. Morula (16 cells)

  8. Blastocyst

  9. Two discs (bilaminar germ disk layers)

  10. Three discs (trilaminar germ disk or layers) ===Gastrulation (A) Day 16

  11. Neural tube=== Neurulation (B) Day 22

  
• Embryonic plate stage


  • The earliest embryonic stage at which ocular structures can be differentiated from the rest of the fetus
  • The site of the eye is indicated by flattened areas on both sides of the anterior end of the neural groove
  • The neural tube (neuroectoderm) produce the retina proper and its associated pigment cell layer
  • The mesoderm of the head region, produce the corneoscleral and uveal tunics
  • The surface ectoderm, produce the lens

  
• Germ layers that contribute to eye development


  • Surface ectoderm
  • Neural ectoderm
  • Neural crest cells (NCC)
  • NCC + Mesoderm=Mesenchyme

  
• Eye development


  • The eye begins to develop as pair of optic vesicles on each side of the forebrain
  • Only ectoderm (surface ectoderm and neuroectoderm) and mesoderm are involved in the development of the eye
  • The development of the eye is a complex and rapid process that involves a precise interaction of neuroectoderm, surface ectoderm, and mesoderm
  • The sensory components of the eye develop by twenty weeks
  • The accessory structures develop by thirty-two weeks

  
• Early embryogenic eye development


  1. Optic vesicles bilaterally protrude from either side of the forebrain approaching the thickened surface ectoderm (lens placodes)

  2. The interaction between the optic vesicle and the lens placode of the surface ectoderm results in optic vesicle invagination, optic cup formation and lens placode evagination (lens pit)

  3. Continued evagination of surface ectoderm leads to the formation of an independent lens vesicle

  
• Optic Vesicle Stage


  • The earliest stage of eye development is the formation of the paired optic vesicles on either side of the forebrain
  • These growing diverticula expand laterally into the mesoderm of the head and narrowed to develop a stalk-like connection to the main portion of the primary central nervous system (primary brain)
  • The forming stalks will eventually become the basics of the optic nerves

  
• Optic Cup Stage


  1. The surface ectoderm thickens to form a lens vesicle, a region visible on the surface of the embryo

  2. The expanding optic vesicle begins to invaginate to form a cup-shaped structure, and also to fold along its centerline, enclosing a small amount of angiogenic mesenchyme as it does so

  3. The inner layer of the optic cup will eventually form the retinal tunic, including its light-sensitive elements

  4. The outer layer of the optic cup will form the pigment epithelium layer

  5. The uveal and corneoscleral tunics eventually will differentiate from the surrounding mesoderm

  
• 1. week stage


  1. The surface ectoderm of the lens vesicle has thickened and is beginning to differentiate two distinct areas

  2. The lens vesicle separates completely from the surface ectoderm to lie free in the rim of the optic cup

  3. The surface ectoderm and the mesoderm beneath it differentiate into the cornea and the eyelids

  4. The retina has begun to develop; the intra-retinal space will eventually be obliterated

  5. The lens has detached from the surface and the cornea is forming from the thickened ectoderm

  
• 1. week stage


  The choroidal fissure narrows until it becomes completely closed, leaving one small permanent opening at the anterior end of the optic stalk through which pass the hyaloid artery until the 100-mm (4-month) stage and the central retinal artery and vein thereafter
• Coloboma


  • Failure of fusion of choroidal fissure by 6th/7th week results in coloboma formation
  • Anterior extreme➔Coloboma of iris
  • Posterior extreme➔Coloboma of posterior fundus and optic nerve
  • If anterior and posterior aspect of cup are open for longer time ➔ it is exposed to teratogenic insults
• 1. Month Stage


  1. The ultimate general structure of the eye has been determined

  2. Further development consists of differentiation into individual structures

  3. Differentiation occurs relatively more rapidly in the posterior than in the anterior segment early in gestation and more rapidly in the anterior segment later in gestation

  
• Embryonic Origins of Individual Eye Structure


  • From neural ectoderm: Retina includes retinal pigment epithelume, Optic nerve, Epithelium of iris, Smooth muscles of iris (sphincters and dilators), Epithelium of ciliary body
  • From surface ectoderm: Lens, Epithelum of conjunctive, Epithelium of cornea, Epithelium of skin of the eye lids, Glands of eyelids and lacrimal gland
  • From mesoderm (NCC+Mesoderm): Sclera, stroma of cornea, conjunctiva, iris, ciliary body, choroid, extraocular muscles, lids (except epithelium and conjunctiva), hyaloid system (gone by birth), sheaths of the optic nerve, connective tissue and blood supply of eye, bony orbit, and vitreous

  
• Retina


  • Develops from the two layers of the optic cup
  • The sensory retina is formed by the inner layer of the optic cup
  • The retinal pigment epithelium is formed by the outer layer of the optic cup

  
• Embryology of the Eye