Emmetropization - What are the contributions of the choroid & sclera?
A novel finding in the chick that has since been corroborated in other animals is that the choroid can rapidly modulate its thickness, in so doing, varying the position of the retina and thus the state of focus of the eye. Traditionally, the vascular choroid has assigned the far similar role of supplying surrounding tissues with nutrients. "Choroidal accommodation" can lead to increases in thickness of over 100 m? in a day in the chick. We understand relatively little about the mechanisms underlying these thickness changes and their regulation. Choroidal blood flow changes occur in the same direction as the thickness changes, yet precede them, raising the possibility that they trigger the latter. The thickened choroids have increased protein and water content, with the main expansion taking place mainly in the outer layers where lacunae, lymphatic-like vessels, reside.
The sclera constitutes the outer support structure of the eye, and also contributes to defocus compensation although it responds more slowly than the choroid. Choroidal thinning is coupled to increased scleral growth and vice versa, choroidal thickening with decreased scleral growth. Thus, at an optical level, their responses are complementary. Whether they represent two independent mechanisms that are independently regulated is yet to be resolved. Another piece of hot news is that retinoic acid (RA) may play a critical regulatory role; impose focusing errors produce opposite effects on the levels of RA in the retina and choroid (thick choroids have increased RA), and RA also inhibits scleral proteoglycan synthesis. There is interest in the sclera as a target for pharmacological intervention in myopia. The interrelationship between biochemical and biomechanical changes in the sclera, and intraocular pressure, and their significance for eye size changes is another area of on-going research.
A novel finding in the chick that has since been corroborated in other animals is that the choroid can rapidly modulate its thickness, in so doing, varying the position of the retina and thus the state of focus of the eye. Traditionally, the vascular choroid has assigned the far similar role of supplying surrounding tissues with nutrients. "Choroidal accommodation" can lead to increases in thickness of over 100 m? in a day in the chick. We understand relatively little about the mechanisms underlying these thickness changes and their regulation. Choroidal blood flow changes occur in the same direction as the thickness changes, yet precede them, raising the possibility that they trigger the latter. The thickened choroids have increased protein and water content, with the main expansion taking place mainly in the outer layers where lacunae, lymphatic-like vessels, reside.
The sclera constitutes the outer support structure of the eye, and also contributes to defocus compensation although it responds more slowly than the choroid. Choroidal thinning is coupled to increased scleral growth and vice versa, choroidal thickening with decreased scleral growth. Thus, at an optical level, their responses are complementary. Whether they represent two independent mechanisms that are independently regulated is yet to be resolved. Another piece of hot news is that retinoic acid (RA) may play a critical regulatory role; impose focusing errors produce opposite effects on the levels of RA in the retina and choroid (thick choroids have increased RA), and RA also inhibits scleral proteoglycan synthesis. There is interest in the sclera as a target for pharmacological intervention in myopia. The interrelationship between biochemical and biomechanical changes in the sclera, and intraocular pressure, and their significance for eye size changes is another area of on-going research.