Development of the Eye in Vertebrates and Cephalopods and its Implications for Retinal Structure
The biggest difference between the cephalopod and vertebrate eye is the structure of their retinas. To understand the implications of this difference, and how it affects their visual systems, an understanding of each eyes' organogenesis is necessary.
During embryological development in the cephalopod, eye formation begins at two oval areas of cells along each side of the ectoderm, which cluster and differentiate to form the preliminary retina (Williams). Next, the nuclei of the retinal anlage move to either the end or the middle of the cell, which causes the epithelial layer to appear as two layers instead of one. The ends of the retinal anlage then begin to curve and eventually form the outer wall of the eyeball. From these optical walls of ectodermal tissue, come the retina, lens, and ciliary body (Harris 1997). Between these two layers of ectodermal tissue, is a thin layer of mesodermal tissue that produces the sclera and lens ligaments (Callaerts et al. 1997). Around this time in development, two additional layers of ectodermal and mesodermal tissue begin to grow around the original optic vesicle (Tomarev 1997). The inner layer will become the iris and the outer layer will become the cornea and the covering of the eye. The eyeball will be separated from this covering (Williams).
The vertebrate eye on the other hand develops through a series of invaginations and begins as an out growth of the forebrain. In the forebrain region an optic pit begins to grow and overtime it expands and deepens to form the optic vesicle (Tomarev 1997). The optic vesicle continues to grow outward, and will remain connected to the forebrain by the optic stalk (Tomarev 1997). (The next step in organogenesis in the vertebrate eye is very important in understanding the difference between it and the cephalopod eye and why the vertebrate retina is considered to be reversed). The optic vesicle invaginates to produce the optic cup and this leads to the inner layer of cells becoming the neural retina and the outer layer the pigmented epithelium (Doughty et al. 1995). During this time, a lens is forming on the ectodermal surface and eventually the developing lens vesicle will separate from the surface of the epithelium and cause the ectoderm to differentiate into the cornea (Callaerts et al. 1997).
The differences in organogenesis mentioned above cause the photoreceptors in each eye to be located in different places on the retina. In cephalopods, the photoreceptors of the retina are located at the front, facing all incoming light while those in the vertebrate eye are on the inside on the retina, facing away from incoming light. Because of this, all incoming light must pass through multiple layers of cells (ganglion and bipolar) before it strikes any photoreceptors. Further, since the vertebrate eye develops as an outgrowth of the forebrain, it has a blind spot not found in the cephalopod eye. This blind spot lacks any photoreceptors (rods and cones) and is the area of the eye, where neural fibers gather to form the optic nerve that transmits signals to the brain in order to form images (Rodieck 1998, Davson 1972, Ondracek 2002). The reason cephalopods do not have a blind spot in their eye is because during organogenesis their optic nerve is on the exterior side of the eye and does not enter into the actual eyeball.
Differences in retinal structure between the two eye types is discussed in more detail at The Cephalopod Eye 1
Image 3 Embryological Development of the Vertebrate and Cephalopod Eye (Image used by permission of William A. Harris. Pax-6: Where to be conserved is not conservative. Proc. Natl. Acad. Sci. USA Vol. 94, pp. 2098-2100, March 1997; taken from http://www.pnas.org/cgi/content/full/94/6/2098/F1. October 5, 2003)
The Cephalopod Eye 1 The Cephalopod Eye 2 Cephalopod Statocysts Further Comparisons Vocabulary Links References