Optic Cup and Lens Vesicle

The developing eye appears in the 22-day embryo as a pair of shallow grooves on the sides of the forebrain (Fig. 17.1). With closure of the neural tube, these grooves form outpocketings of the forebrain, the optic vesicles. These vesicles subsequently come in contact with the surface ectoderm and induce changes in the ectoderm necessary for lens formation (Fig. 17.1). Shortly thereafter the optic vesicle begins to invaginate and forms the double-walled optic cup (Figs. 17.1 and 17.2 A). The inner and outer layers of this cup are initially separated by a lumen, the intraretinal space (Fig. 17.2B; see also Fig. 17.4A), but soon this lumen disappears, and the two layers appose each other (see Fig. 17.4). Invagination is not restricted to the central portion of the cup but also involves a part of the inferior surface (Fig. 17.2 A) that forms the choroid fissure. Formation of this fissure allows the hyaloid artery to reach the inner chamber of the eye (Figs. 17.3 and 17.4; see also Fig. 17.8). During the seventh week, the lips of the choroid fissure fuse, and the mouth of the optic cup becomes a round opening, the future pupil.

During these events, cells of the surface ectoderm, initially in contact with the optic vesicle, begin to elongate and form the lens placode (Fig. 17.1). This placode subsequently invaginates and develops into the lens vesicle. During the fifth week, the lens vesicle loses contact with the surface ectoderm and lies in the mouth of the optic cup (Figs. 17.2C, 17.3, and 17.4).

Mouse EmbryoDay Embryo Optic Cup

Figure 17.1 A. Transverse section through the forebrain of a 22-day embryo (approximately 14 somites), showing the optic grooves. B. Transverse section through the fore-brain of a 4-week embryo showing the optic vesicles in contact with the surface ectoderm. Note the slight thickening of the ectoderm (lens placode). C. Transverse section through the forebrain of a 5-mm embryo showing invagination of the optic vesicle and the lens placode. D. Scanning electron micrograph showing a frontal view of a mouse embryo at a stage similar to that shown in B. E. Scanning electron micrograph of a mouse embryo during formation of the optic vesicle. The embryo has been cut sagit-tally to reveal the inside of the brain vesicles and outpocketing of the optic vesicle (arrow) from the forebrain. H, hindbrain; and M, midbrain.

Figure 17.1 A. Transverse section through the forebrain of a 22-day embryo (approximately 14 somites), showing the optic grooves. B. Transverse section through the fore-brain of a 4-week embryo showing the optic vesicles in contact with the surface ectoderm. Note the slight thickening of the ectoderm (lens placode). C. Transverse section through the forebrain of a 5-mm embryo showing invagination of the optic vesicle and the lens placode. D. Scanning electron micrograph showing a frontal view of a mouse embryo at a stage similar to that shown in B. E. Scanning electron micrograph of a mouse embryo during formation of the optic vesicle. The embryo has been cut sagit-tally to reveal the inside of the brain vesicles and outpocketing of the optic vesicle (arrow) from the forebrain. H, hindbrain; and M, midbrain.

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