Embryology of the Orbit

• Development – 4^th week, hox genes, pax 6
• Derived from 4 sources :
  • Neuroectoderm of forebrain
  • Surface ectoderm of the head
  • Mesoderm b/t the above layers
  • Neural crest / mesechyme cells
  • Optic groves appear in the neural folds at the cranial end of the embryo. Then these optic grooves→ optic vesicles. The distal part of your optic vesicles expand and their connections with the forebrain constricts, this forms a hollow optic stalk. Then surface ectoderm adjacent to the optic vesicles thickens to form the lens placode. (lens placode = primordia of the lens) Lens placode invaginate and sink deep top the surface ectoderm forming lens pits → lens vesicles . 1˚ lens fibers closes the lumen of the lens vesicles and must last a lie time. 2˚ lens fibers elaborate through out life.
  •  As lens is developing the optic vesicles form optic cups around the lens. At this point lens vesicles loose their connection with surface ectoderm, and enter the cavities of the optic cups.
  • Outer, thinner layer of the optic cup =  retina pigmented epithelium, which is anchored to the choroid. The inner, thicker layer, forms the neural retina. In between these layers there is an inter-retinal space, which will eventually obliterate.
  • Optic fissures develop on the ventral surface of the optic cups, and this is the location where the hyaloid blood vessels develop. The hyaloid artery supplies the inner layer of the optic cup, the lens vesicle, and mesechyme of the optic cup. The hyaloid artery and vein will become the central artery and vein of the retina.
  • The cavity of the optic stalk is gradually obliterated as the axons of the many ganglion cells of the retina to form the optic nerve. The optic nerve will become myelinated to the optic disc, and the is completed 10 weeks AFTER BIRTH.
  • The developing lens is supplied by the distal part of the hyaloid artery, as it degenerates the lens becomes avascular and it is dependent on diffusion from the aqueous humor from the anterior part of the eye.
  • The tunica vasculosa lentis is the vascular mesechyme layer that invest the developing lens , it eventually degenerates, however the lens capsule produced by the anterior lens epithelium and lens fibers will persist.

 

Ectoderm and Neural Crest Derivatives:

 

Neuroectoderm:                                                                      Surface Ectoderm:

            Retina                                                                                      Lens

            Posterior layers of the iris                                                       Corneal Epi.

            Optic Nerve                                                                             Lacrimlal gland

            Dilator and Sphincter Pupillae

 

 

Mesoderm:                                                                              Neural Crests:

            Fibrous and Vascular coats of eye                                           Choroid

                                                                                                            Sclera

                                                                                                            Corneal endo.

 

Anomalies: most common are defects in closure of the optic fissure

 

• Congenital detachment of the retina - This occurs when the inner and outer layers of the optic cup fail to fuse during the fetal period to form the retina and obliterate the inter-retinal space.

• Coloboma of the Retina - Localized gap in the retina, usu. inferior to the optic disc. A typical coloboma results from defective closure in the optic fissure and is localized inferiorly during the 6th week.

• Cycolpia - Eyes are partially or completely fused, forming a single median eye, enclosed in single orbit, usu. assoc’d w/ tubular nose (proboscis) superior to the median eye

• Microphthalmos - Small eye, assoc’d with other ocular defects or it may be a normal appearing mini eye.

• Anaophthalmia - Denotes congenital absence of all tissues of the eye. Eyelids are present!

 

• Coloboma of the Iris - Defect in the inferior sector of the iris, or a notch in the papillary margin (keyhole appearance of the pupil).

• Congenital Anairdia - Almost complete absence of the Iris. Assoc’d with congenital glaucoma. Results from an arrest in the development @ the optic rim of the optic cup during 8th week.

• Congenital glaucoma - Abnormal elevation of intraocular pressure in infants that is usu. a result of abnormal drainage of the aqueous humor.

• Persistence of the Hyaloid Artery - Small part  of the artery persists distally, nonfunctional vessel, appears as a cyst or duct.

• Congenital Aphakia - Absence of the lens, which results from failure of the lens placode to form, in 4th week.

• Congenital Cataracts - Opacity of the lens, usu. bilateral. Can be caused by maternal rubella b/c of inflammatory adhesions

• Coloboma of the Eyelid - Characterized by a small notch in the upper eyelid. Not assoc’d with a defect in the optic fissure closure.

• Coloboma of the lens - Not assoc’d with a defect in the optic fissure closure.

• Anterior Cleavage syndrome - Assoc’d with congenital glaucoma

Tags: coloboma of the iris, eye embryology, mesoderm derivative, neural crest, neuroectoderm, optic groves, surface ectoderm, tunica vasculosa

This entry was posted on Friday, January 16th, 2009 at 5:45 am and is filed under Embryology. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.