The multifunctional choroid
Introduction
The choroid does not appear to be a mysterious tissue. It consists mostly of blood vessels, it supplies the outer retina, and choroidal defects cause degenerative changes and neovascularization. However, it is becoming increasingly evident that the choroid has at least three other functions: thermoregulation, adjustment of the position of the retina by changes in choroidal thickness, and secretion of growth factors. The last of these is likely to play an important role in emmetropization–the adjustment of eye shape during growth to correct myopia or hyperopia. What remains mysterious, at present, are the mechanisms behind the changes in choroidal thickness, the nature of its secretory functions and the relationship between these two processes.
In this review, we will summarize the anatomy, histology, innervation and functions of the choroid, discuss the control of choroidal thickness by visual signals, show evidence for a secretory role for the choroid and speculate on the relationship between changes in choroidal thickness and ocular elongation, and therefore, emmetropization.
Section snippets
Structure and “classical” functions of the choroid
The choroid is the posterior part of the uvea, the middle tunic of the eye (Fig. 1). The uvea develops from the mesenchyme surrounding the two vesicles that bud off the embryonic forebrain at the end of the first month in humans, eventually becoming the eyes. At about that time, melanocyte precursors migrate into the uvea from the neural crest; these do not differentiate into pigmented melanocytes until 7–8 months of gestation. The mesenchyme that forms the choriocapillaris at about 2 months
Modulation of choroidal thickness
Long ago, it was suggested that the choroid might participate in refractive adjustment as a slow accommodative mechanism (Kajikawa, 1923, Walls, 1942). Over 50 years later, renewed interest in the choroid was sparked by the confirmation of this hypothesis by showing that, in chickens, the choroid can increase its thickness in response to myopic defocus (image focused in front of the retina) by as much as 1 mm (>17 D) pushing the retina towards the image plane and compensating for much of the
The choroid and emmetropization
As the eye develops from birth to maturity it undergoes adjustments of its optical components so that most eyes eventually become emmetropic (focused for objects at distance). It is generally accepted that this “emmetropization” is determined by a combination of environmental (i.e., visual) and genetic influences. When this process goes awry, the eyes develop refractive errors (hyperopia or myopia). In the United States approximately 25% of the population is myopic (Sperduto et al., 1983) while
The diurnal rhythm in choroidal thickness and ocular growth
As mentioned above, the thickness of the chicken choroid shows diurnal oscillations, thickening during the night and thinning during the day (Nickla et al., 1998, Papastergiou et al., 1998), in approximate anti-phase to the diurnal rhythm in axial elongation. Both rhythms free-run in darkness and so are driven by an endogenous circadian oscillator, rather than being a response to darkness and light (Nickla et al., 2001). Furthermore, isolated scleral tissue exhibits a circadian rhythm of
Significance to human refractive disorders
The reasons behind the high incidence of myopia, particularly among educated people, remain obscure. The finding that animals can be made myopic by presenting them with hyperopic defocus by spectacle lenses argues that refractive error is under homeostatic control. Because the eye can alter its growth even if the optic nerve is severed, it follows that the retina is able to detect the sign of the defocus and control the eye's growth. Because chemical signals from the retina must pass through
Future directions
In this review, we have tried to bring together the literature bearing on the unusual aspects of the anatomy and function of the choroid. The knowledge concerning most of these aspects remains incomplete and hypotheses about choroidal functions remain tentative. In these final paragraphs, we will point to some research directions that might prove useful.
The most puzzling component of the choroid is the lamina fusca, about which nearly nothing is known. It would be interesting to assess what it
Acknowledgements
Supported by: NIH-EY-013636 (DLN) and EY-02727 and RR03060 (JW).
The authors are grateful to the following people for their important contributions: Jeffrey Kiel for his insightful discussions on the choroid and thermoregulation, and Falk Schrödl, Maria Egle De Stefano and Gerald Lutty for their excellent critical readings of the manuscript.
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