The choroid is a vascular structure in the eye that supplies the outer retina. It has several unique features, including large membrane-lined lacunae that function in lymphatic drainage and can change volume dramatically, affecting choroidal thickness. The choroid also contains non-vascular smooth muscle cells and intrinsic choroidal neurons, which may control these muscles and modulate blood flow. These neurons are innervated by sympathetic, parasympathetic, and nitrergic pathways. The choroid has multiple functions, including supplying oxygen and nutrients to the outer retina, regulating retinal temperature, and secreting growth factors. It also plays a role in the drainage of aqueous humor and may contribute to thermoregulation. The choroid's thickness can change in response to retinal defocus, influencing the position of the retina and photoreceptors. These changes are linked to eye growth and may play a role in emmetropization, the adjustment of eye shape during growth to correct myopia or hyperopia. The choroid's structure includes layers such as Bruch's membrane, the choriocapillaris, and vascular layers. It contains various cell types, including melanocytes, fibroblasts, and smooth muscle cells. The choroid's thickness can vary significantly between species and is influenced by factors such as visual signals and growth. Changes in choroidal thickness are associated with changes in the sclera and may be linked to the homeostatic control of eye growth, affecting the development of myopia and hyperopia. The choroid's blood flow is crucial for retinal nourishment and may also play a role in thermoregulation. It has been shown to exhibit some degree of autoregulation in response to changes in perfusion pressure. The choroid is also involved in age-related macular degeneration, where changes in choroidal blood flow and structure can lead to retinal damage. The choroid's thickness can change in response to refractive errors, with thickening in response to myopic defocus and thinning in response to hyperopic defocus. These changes are thought to be mediated by the expansion and contraction of lacunae, which are fluid-filled spaces in the choroid. The mechanisms underlying these changes include alterations in the synthesis of osmotically active molecules, changes in vascular permeability, and fluid movement across the retinal pigment epithelium. These processes are complex and involve multiple factors, including the autonomic nervous system and the extracellular matrix. The choroid's ability to modulate its thickness in response to visual stimuli is an important aspect of eye development and function.The choroid is a vascular structure in the eye that supplies the outer retina. It has several unique features, including large membrane-lined lacunae that function in lymphatic drainage and can change volume dramatically, affecting choroidal thickness. The choroid also contains non-vascular smooth muscle cells and intrinsic choroidal neurons, which may control these muscles and modulate blood flow. These neurons are innervated by sympathetic, parasympathetic, and nitrergic pathways. The choroid has multiple functions, including supplying oxygen and nutrients to the outer retina, regulating retinal temperature, and secreting growth factors. It also plays a role in the drainage of aqueous humor and may contribute to thermoregulation. The choroid's thickness can change in response to retinal defocus, influencing the position of the retina and photoreceptors. These changes are linked to eye growth and may play a role in emmetropization, the adjustment of eye shape during growth to correct myopia or hyperopia. The choroid's structure includes layers such as Bruch's membrane, the choriocapillaris, and vascular layers. It contains various cell types, including melanocytes, fibroblasts, and smooth muscle cells. The choroid's thickness can vary significantly between species and is influenced by factors such as visual signals and growth. Changes in choroidal thickness are associated with changes in the sclera and may be linked to the homeostatic control of eye growth, affecting the development of myopia and hyperopia. The choroid's blood flow is crucial for retinal nourishment and may also play a role in thermoregulation. It has been shown to exhibit some degree of autoregulation in response to changes in perfusion pressure. The choroid is also involved in age-related macular degeneration, where changes in choroidal blood flow and structure can lead to retinal damage. The choroid's thickness can change in response to refractive errors, with thickening in response to myopic defocus and thinning in response to hyperopic defocus. These changes are thought to be mediated by the expansion and contraction of lacunae, which are fluid-filled spaces in the choroid. The mechanisms underlying these changes include alterations in the synthesis of osmotically active molecules, changes in vascular permeability, and fluid movement across the retinal pigment epithelium. These processes are complex and involve multiple factors, including the autonomic nervous system and the extracellular matrix. The choroid's ability to modulate its thickness in response to visual stimuli is an important aspect of eye development and function.