Basic Eye Anatomy And Histology

Basic eye anatomy and histology Focusing  The cornea provides ~70% of focussing power, with the other 30% coming from the lens.  Lens undergo accommodation to change focus depending on distance. o Distant focus: light beams are closer together, hence lens don’t need to be wide. The ciliary muscles are relaxed. o Close focus: light beams are further away, hence lens need to be wide. The ciliary muscles are constricted. Snellen eye chart  Used to measure visual acuity.    Use 6m distant: o Smallest line read with normal acuity is the 6/6 or 20/20 line. o Symbols on the 6/6 line are 8.87mm tall and subtend an angle of 5 minutes of arc (top letters are 88.7mm tall). o The thickness of the lines and the spaces between the lines subtends one minute of arc. 3 lines above, where letters have twice the dimensions. o Normal acuity could read these letters at 12m. o This line is designated by the ratio 6/12. o If this is the smallest line a person can read, then acuity is “6/12”. o Roughly means that the letters need to be read at 6m, but can normally be read at 12m. Interpreting results: o Low vision: visual acuity<6/12. o Blindness: visual acuity<6/60. Epidemiology of visual impairment  246 million with low vision (160 million ≥50 years old).  39 million are blind (32 million ≥50 years old).  The number of Australians with low vision/blindness expected to double this decade. o Cataract more associated with low vision, whereas macular degeneration more associated with blindness. o There is no treatment for macular degeneration. o Both generate an expensive cost for surgery, care, etc. Review of stem cells  Stem cells divide many times, and can make: o Identical copies of itself. o Other, specialised cell types (i.e. differentiate).  Pluripotent stem cells: o Unlimited in vitro proliferative capacity. o Can make all the cells in the body. o Embryonic stem cells (from donated surplus IVF embryos) or induced pluripotent stem cells (‘reprogrammed’ somatic cells).  Tissue-restricted (‘adult’) stem cells: o Restricted in vitro proliferative capacity. o Can make only one or a few specialised cell types. o Involved in tissue homeostasis, tissue repair/regeneration.  Ocular stem cells are typically harder to obtain, purify and culture. Applications for stem cell research  Better understand ocular tissue development. o What genes or proteins are involved in normal tissue development? o What genes or proteins are involved in congenital defects, disease or injury? o Provide leads for finding new cures.  New drug development. o Make specialised cell types for drug screening.  Regenerative medicine. o Stimulate cells and tissues in the patient to re-grow or repair. o Grow replacement cells or tissues in the lab for transplantation (‘tissue engineering’). Corneal disease and injury  Penetrating keratoplasty allografts used for over 50 years. o Short-term survival of full-thickness allografts is ~90% at 1 year post-transplantation. o Long-term graft survival is 46% at 15 years posttransplantation.       Poor graft survival is due to rejection of donor endothelial cells. When corneal damage is limited to the surface epithelium, limbal cell use is possible. o Biopsy to harvest limbal stem cells. o Laboratory culture to expand stem cells and produce a graft containing these autologous cells. o Patient’s damaged conjunctival epithelium is removed and the graft transplanted. Initial long-term clinical assessment shows: o Grafts remain transparent. o Grafts can repair corneal damage. o Visual acuity is increased in most patients. Better cell substrates needed to: o Improve graft integrity. o Aid transplant manipulation and attachment. Australian proof-of-principle clinical trial showed: o Limbal biopsies can be cultured on a commercial contact lens. o These grafts provide successful treatment, with 3/3 patients showing improved corneal transparency and improved visual acuity. o Larger, long-term trials are needed. If limbal cells are unavailable (e.g. bilateral burn), oral mucosa cells can be used. o Many patients had improved visual acuity (follow-up ≤34 months). o Larger patient cohorts needed to establish long-term clinical benefits.