Week 7: Ocular Embryology 3

Question 1

What is the embryonic development timeline?

Answer 1

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

Question 2

What is the retinal development timeframe?

Answer 2

• 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

Question 3

What is mesenchyme?

Answer 3

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

Question 4

How is eyecolour determined?

Answer 4

By melanin found in iris stroma

Question 5

What do the optic stalks do ?

Answer 5

Form a critical connection to the forebrain during iris growth (36 days)

Question 6

What does the Mesenchyme’s fiberous layer (anterior optic cup) become ?

Answer 6

The cornea

Question 7

What does the mesenchyme’s corresponding vascular layer become?

Answer 7

Iridopupillary membrane- at periphery will form iris

Question 8

What does the Fiberous mesenchyme in posterior optic cup become?

Answer 8

The sclera & EOM

Question 9

What does the vascular mesenchyme become?

Answer 9

Choroid & ciliary body

Question 10

What type of muscle develops from surrounding mesenchyme on the ciliary muscle?

Answer 10

Smooth muscle

Question 11

What are the 2 layers that the mesenchyme surrounding the optic cups external surface turn to at 6-7 weeks?

Answer 11

• inner, pigmented, vascular layer = choroid

- outer fiberous layer = sclera

Question 12

How does the cornea develop and become transparent?

Answer 12

—-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

Question 13

What layer of the mesencyme forms the cornea?

Answer 13

Fiberous layer of mesenchyme surrounding anterior part of optic cup forms cornea

Question 14

When do the eyelid begin to develop and form what cells?

Answer 14

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)

Question 15

What is between the eyelids and cornea before week 27 when eyelids separate?

Answer 15

Conjunctival sac which contains tear ducts

Question 16

What position do the eyes begin?

Answer 16

—- 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

Question 17

What is the vitreous formed of?

Answer 17

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)

Question 18

What do the EOM develop from and when?

Answer 18

-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

Question 19

When does the eye develop and how many somites?

Answer 19

Develops from day 22 & 8 somites

Question 20

What is the embryonic origins of the lens + cornea & retina + iris + ciliary body epithelial?

Answer 20

• 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