Week 7: Ocular Embryology 3 Flashcards
What is the embryonic development timeline?
Weeks 3 - 4 = Eye Fields-Optic Vesicle
Weeks 5 - 6 = Optic Cup, Lens Vesicle, Choroid Fissure, Hyaloid Artery
Weeks 7 - 8 = Cornea, Anterior Chamber, Pupillary Membrane, Lens, Retina
Weeks 9 - 15 = Iris, Ciliary Body
Weeks 8 - 10 = Eyelids
What is the retinal development timeframe?
- The inner limiting membrane:5th week of gestation
- The nerve fibre layer: 6th week
- The ganglion cell layer : 10th week
- The inner plexiform layer : 10th week
- The inner nuclear layer : 10th week
- The outer plexiform layer: 10th-12th week
- The outer nuclear layer
- The outer limiting membrane
- The rod & cone layer : 10th week
- The pigment epithelium : 7th week
What is mesenchyme?
Connective tissue, mainly found during embryogenesis
-ciliary body muscles (accommodation) arise from overlying mesenchyme
-As optic cup develops, inner vascular layer of
mesenchyme is transported into cup through the choroidal fissure
-As fissures close, mesenchyme is trapped and enclosed in optic cup
-Mesenchyme’s fibrous layer (ant. optic cup) becomes the cornea
-Mesenchyme’s corresponding vascular layer becomes iridopupillary membrane. At periphery, will form iris
-Fibrous mesenchyme in posterior optic cup will become sclera & EOM
-Vascular mesenchyme = choroid & ciliary body
How is eyecolour determined?
By melanin found in iris stroma
What do the optic stalks do ?
Form a critical connection to the forebrain during iris growth (36 days)
What does the Mesenchyme’s fiberous layer (anterior optic cup) become ?
The cornea
What does the mesenchyme’s corresponding vascular layer become?
Iridopupillary membrane- at periphery will form iris
What does the Fiberous mesenchyme in posterior optic cup become?
The sclera & EOM
What does the vascular mesenchyme become?
Choroid & ciliary body
What type of muscle develops from surrounding mesenchyme on the ciliary muscle?
Smooth muscle
What are the 2 layers that the mesenchyme surrounding the optic cups external surface turn to at 6-7 weeks?
- inner, pigmented, vascular layer = choroid
- outer fiberous layer = sclera
How does the cornea develop and become transparent?
-As lens develops, cornea forms from the surface ectoderm cells
- By week 5, this 1st corneal epithelium is 2 cells thick, and the cells of Golgi apparatus form the primary stroma
- Ectoderm cells number doubles, creating the corneal epithelium
- Corneal endothelial cells (from mesodermal mesenchyme) migrate over the primary stroma’s inner surface
-These cells continue to form an inner corneal lining, and produce lots of hyaluronic acid, which is secreted into primary stroma, making it swell
-Swelling signals the settling of fibroblasts, and synthesise hyaluronidase
-Stroma (containing fibroblasts) is now called the secondary stroma and it dehydrates to make the cornea transparent
What layer of the mesencyme forms the cornea?
Fiberous layer of mesenchyme surrounding anterior part of optic cup forms cornea
When do the eyelid begin to develop and form what cells?
Week 6, from neural crest cells and surface ectoderm tissue (anterior to lens)
- eyelids begin as 2 folds of skin that meet over cornea and are attached until week 27 (when separate)
- The orbicularis oculi (eyelids) forms from the 2nd branchial arch, along with other facial expression muscles (humans born with open eyes unlike other animals)
What is between the eyelids and cornea before week 27 when eyelids separate?
Conjunctival sac which contains tear ducts
What position do the eyes begin?
- As facial structure develops, eyes that began to form on either side of embryo’s head are carried forward
-This allows for the convergence of the optical axes
What is the vitreous formed of?
Vitreous forms in centre of optic cup, posterior to lens
-Comprised of vitreous humour which is derived from mesenchymal cells of neural crest origin (primary vitreous)
- Later on, more vitreous humour added, from the neuroectoderm (secondary vitreous), which starts as a homogenous gel that expands and pushes primary vitreous to behind lens
- Tertiary vitreous - large number of collagen fibres develop with formation of zonular fibres (between ciliary processes and lens, involved in accommodation)
What do the EOM develop from and when?
-week 4
- The extraocular muscles develop from 3 preotic somites (anterior to developing ear)
- Each preotic somite is supplied by its own cranial nerve
- 3 different cranial nerves (III, IV and VI) supply the EOMs
- The somite supplied by the 3rd cranial nerve forms 5 of the 7 EOMs. The remaining 2, give rise to 1 muscle each
When does the eye develop and how many somites?
Develops from day 22 & 8 somites
What is the embryonic origins of the lens + cornea & retina + iris + ciliary body epithelial?
- Lens and cornea = surface ectoderm
- Retina, and iris & ciliary body epithelia = anterior neural plate - Mesenchyme cells migrate into cornea (become future corneal stroma)
-Likewise, iris and ciliary body form from optic cup:
- The outer layer becomes RPE. Inner layer forms neural retina (6 different cell types) - The retinal ganglion cells grow toward optic stalk, forming the optic nerve
