The morphogenesis of the ciliary body of the avian eye: II. Differential enlargement causes an epithelium to form radial folds

Abstract

The morphogenetic mechanism responsible for the radial folding of the anterior retina of the chick eye as it forms the ciliary body has been investigated in two ways. First, eye growth and cell division have been assayed to find out the origins of the extra fold material, and second, possible mechanisms have been tested, and in some cases excluded, by artificially increasing the size of the embryonic eye in vitro under a range of restrictive conditions. Growth studies show that, while on average the eyeball increases linearly in area by a factor of about 14 over the period 4–8 days or stages 24–33, there is a slowing down in growth at stage 28 which is followed by a rapid catch up as the surface area increases by about two-thirds in the 12 hr between stages 29 and 30, just as the ciliary body forms. Thymidine incorporation studies show that cell division is roughly uniform over the eye at this stage. The sudden increase in the overall size of the eye is not, however, matched by growth in the region of the pupil at the retinal tip; this ring of tissue grows slowly and its diameter remains virtually constant over the period of ciliary body morphogenesis. These observations suggest a simple morphogenetic mechanism. Tissue near the retinal tip, unlike such tissue in the rest of the eye, is unable to swell uniformly under intraocular pressure as it is constrained by the rigid pupillary ring; the resulting complex tensions cause radial folding. This process is facilitated by lateral cell detachment in the neural retinal epithelium (NRE) and nucleated by existing radial capillaries superficial to the retina. It is a prediction of the mechanism that any differential growth of the eyeball should cause stage 29 eyes to fold. Such growth has been induced in vitro by immersing early stage 29 eyes in 50% ethanol and water, a solution which causes an 8% or so increase in eye diameter in a few minutes, equivalent to about several hours growth in vivo. Pupillary expansion, however, lags some 15 min behind that of the rest of the eye. In most cases, immersed eyes generate large numbers of radial folds over about half their circumference in about 10 min after immersion, the degree of folding expected for similar growth in vivo. This result argues against morphogenetic mechanisms based on localised growth or on other slow developmental events. Such radial fold formation in vitro after treatment of eyes in vivo with colchicine and cytochalasin B has also been observed, results which argue against any morphogenetic mechanism based on microtubules or microfilaments. Some folds also form with ethanol swelling after the cornea and peripheral mesenchyme have been removed from the eye, a result which excludes any major directive role for this part of the tissue. The observations therefore support the simple differential growth stimulus for ciliary body morphogenesis.