TRAINEES The embryology of the eye obody claims to like embryology. the keys to understanding pathology soft enough you can imagine he might At least nobody I know. It and disease. If you know the language sink into it deeply; imagine one of those has been a neglected part of you know the meaning. Let us look into nightmare scenarios where the mattress Nthe curriculum since time the language of eye development. The then proceeds to close around him and immemorial and a vicious cycle occurs embryology of the eye. trap him inside the mattress, so soft it is. in which those with an incomplete The man extends his arms out and what understanding fail to appreciate the Beginnings once formed a groove in the mattress by inherent beauty and relevance of A full and remembered understanding his arms now form outpouchings when embryology and thus a dry subject of eye embryology comes from an viewed from the outside, when, say, his becomes not only difficult to understand appreciation of how structures initially wife walks into the bedroom and sees two but also without relevance. This is a great form. The first stages of eye development bulging bits of mattress that is the only shame because of all subjects embryology are evident as early as the beginning of sign of her husband’s arms flailing around is the most fascinating and understanding the fourth week; indents called the optic inside the bed. its intricacies make a full appreciation of grooves or sulci appear at the cranial pole Initially these optic vesicles are best the human body possible. Our flaws and of the embryo and within the cranial fold described as crude bud-like forms diseases go from sudden and unexpected (see Figure 1a, b and c). As the fold closes to whose surface is continuous with the surprises to catch out the unwary doctor give rise to the space that is the beginning diencephalon. They grow laterally into to all too predictable design flaws that of the brain, the neural tube, these indents the mesenchyme and toward the surface can be expected and prepared for. One transform from being superficial features ectoderm and adopt a shape similar to a of the major advantages to speaking of negative space to being diverticuli of mushroom with an everted head, or goblet, Welsh is that 40% of its vocabulary is the forebrain itself (Figure 1d). What once known as the optic cup. Reciprocating derived from Latin and many difficult poked inward now pokes outward, albeit with this cup is a feature of the surface anatomical terms my English brethren from the inside. Say a very heavy man gets ectoderm called the lens pit which struggle with are readily understandable on a very very soft mattress with his arms deepens and invaginates into the space to us. Braich means arm, as in brachial. extended, and lies down on his stomach. created by the cup, until the touching sides Ffos means ditch, as in fossa. I could go He would initially be denting the mattress of the ectoderm fuse behind it leaving on. Understanding embryology gives one in and making a groove. If the bed is an independent spherical structure enveloped by the cup, but divided by a layer of mesenchyme (Figure 1e, f and g). This would be the equivalent of the man g in the bed, perhaps in his desperation and a fear of suffocation, extending his arms Lens outward as far as he can, his clenched fists Optic stalk being the optic vesicles and his arms the optic stalks. He then grabs onto the sides b Optic nerve of the mattress, opens his fists to form the optic cup, then grabs the external mattress f Optic grooves Optic cup surface and in a fit of desperation tears a piece off with both hands which then Retina c Optic fall lifelessly back into the soft mattress. nerve The pieces of mattress edge found in his fibres clenched fists are the lens vesicles. There Lens placode our analogy must end as the bed is not a living tissue and the torn pieces of external d mattress cannot heal closed. The mattress cover would be ectoderm, the man and his arms neuroectoderm and the mattress stuffing mesenchyme in this analogy though. e It is through this mesenchyme that blood vessels grow. The vessels run Figure 1: The stages of eye development. parallel to the optic stalk and soon eye news | DECEMBER/JANUARY 2016 | VOL 22 NO 4 | www.eyenews.uk.com TRAINEES organise themselves into a linear tract, pathology of retinal detachment. the choroidal fissure, within the surface The inner layer differentiates into a thick of the stalk and supply a pathway into the neuroeopithelium due to its proximity mesenchyme dividing the optic cup and to the lens vesicle and related growth immature lens. These vessels are called factors. It is this inner layer that divides the hyaloid artery and vein. and stratifies, initially into the inner and This summary describes approximately outer neuroblastic layers. These two days 21 to 35 of development and we will proliferative beds of cells are, in turn, now see how each individual part develops. surrounded by membrane; the external An understanding of this stage will be a limiting membrane and the internal foundation to the details that follow, and limiting membrane. The external faces ultimately make sense of it all. the retinal pigment epithelium and the internal faces the lens. The neuroblastic The optic nerve layers give rise to all of the six major Figure 3: At the half-way point of the lens (called the equatorial zone) these primary fibres are met by secondary The optic stalk that joins the forebrain cell classes that make up the mature lens fibres that have arisen from the anterior surface. and the interior of the optic vesicle is the retina. The order of this differentiation is future optic nerve. The choroidal fissure preserved by evolution and creates the tough and fibrous. that develops at the underside of the optic order we are familiar with (Figure 2): The sclera surrounds the whole vesicle extends into the stalk also. The 1. Inner limiting membrane optic apparatus and is continuous with axons of the retina and blood vessels pass 2. Nerve fibre layer the stroma of the cornea. The choroid through this opening. By the third month 3. Ganglion cell layer feathers into the ciliary processes and is the mesoderm that forms the connective 4. Inner plexiform layer responsible for the vascular centres of tissue of optic nerve along with minute 5. Inner nuclear layer these structures. capillaries enter the optic nerve. The outer 6. Outer plexiform layer 7. Outer nuclear layer covering of the nerve, i.e. dura, arachnoid The lens and pia, develop between three and seven 8. External limiting membrane As we have already discussed, the lens months, the lamina cribrosa develops 9. Photoreceptor layer – rods / cones begins development from an out-pouching later. Myelination of the optic nerve starts 10. Retinal pigment epithelium. of the surface ectoderm at approximately at about seven months gestation from day 35. It is surrounded by mesenchyme the cephalic end, extending towards the The macula and sits in the middle of the optic cup. The lamina and stopping short at this point, Development of the macula differs from deep, or posterior side, goes on to form the ideally. the rest of the retina. Initially there is fast lens epithelium made up of transparent development in area of macula up to the epithelial cells. This characteristic is due third month of life, then there is a slowing The retina to the form of the cells being very long The retina is formed from the optic cup. As of growth while rest of retina grows at and thin. They grow from the posterior a consequence of how it arises it is double the usual pace. This state of retardation surface forwards subsuming the space in layered; the outer layer becoming the persists up to the eighth month then it the middle of the sphere and are called retinal pigment epithelium and the inner starts growing in the same manner as the primary lens fibres. At the half-way point of layer, nearest the lens vesicle, becoming rest of the retina. Thus its development is the lens (called the equatorial zone) these the neural retina. Both of these layers are not complete by ninth month. To attain primary fibres are met by secondary lens continuous with the wall of the forebrain. full development macula has to wait up fibres that have arisen from the anterior The space between them fuses during the to four months of post natal life. At six surface (Figure 3). foetal development, however, this join months of foetal life it is thicker than the Initial lens development is nourished by displays weakness expressed through the rest of the retina. By the seventh to eighth the hyaloid artery and the tunica vasculosa months it starts thinning. The thinning is lentis, which is the vascular bed around due to spreading out of ganglion cells from the lens – the anterior part of which is the central part, i.e. the fovea, which at called the pupillary membrane. However, birth has only one layer of ganglion cells this ceases during the foetal period and left. The outer nuclear layer is also single the lens relies solely on diffusion from the layered. aqueous humour – its anterior relation. The hyloid artery retreats throughout The meninges foetal development leaving the hyloid An important and global concept in this canal through the middle of the vitreous discourse is an understanding of where the body.