Mark D. Olson

Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)




An electron microscopic investigation of the morphogenesis of photoreceptor cells in the domestic fowl (Gallus gallus) retina is presented in this study. The fine structural observations illustrate photoreceptor development ranging from five to twenty-one days of incubation and one to two days post-hatching. Retinal tissues were fixed in buffered aldehyde fixative, post-fixed in buffered 2% osmium tetroxide and otherwise conventionally prepared for electron microscopy.

The neural (sensory) retina of the chick is largely occupied by undifferentiated cells within the outer neuroblastic layer from the 5th to 9th days of incubation. This layer is later divided into the inner and outer nuclear layers as a result of the establishment of the outer plexi- form layer. The ellipsoid cells of the outer nuclear layer will develop as the photoreceptors of the chick retina. Numerous centrioles are conspicuous within these cells at the level of the outer limiting membrane.

The first indication of differentiation in the photoreceptor proper occurs during the 9th to 11th days, as the apical cytoplasm of the future receptor cells extends beyond the outer limiting membrane. These bulbous cytoplasmic projections occupy the optic ventricle and represent the forming inner segments with their characteristic ellipsoid. By the 16th day, the inner segments are extensively elongated. They are divided into the following two definitive portions: (1) the ellipsoid which occupies the distal one-third and (2) the myoid which represents the proximal two-thirds.

During the 17th day, irregularly arranged membranous discs are observed within the forming outer segments. This portion of the photoreceptor proper develops as a modified cilium which originates from a basal body within the ellipsoid region of the inner segment. The membranous discs are closely associated with the microtubules of the "connecting cilium" and many are continuous with cylindrical tubules measuring 240 to 400 A in diameter. The discs (135 A thick) become oriented at right angles to the plasmalemrna by the 19th day. Some of the membranous discs are continuous with the plasmalemrna. Groups of discs are found within the pigment epithelium during the second day post-hatching. This observation indicates that the pigment epithelium actively participates in the removal of membranous discs.

During the early stages of photoreceptor development, the basal portions of the receptor cells are not very conspicuous, with the exception of a large nucleus. By the 16th day basal arborizations begin to develop. Extensive synaptic terminals are plentiful and extend into the outer plexiform layer by the 19th day. They contain numerous synaptic vesicles and synaptic ribbons. Both conventional and invaginated (ribbon) synapses occur between the photoreceptors and secondary neurons (bipolar and horizontal cells).

This study indicates that the morphogenesis of photoreceptors in the chick retina corresponds well with that in other vertebrate species. Moreover, the basic fine structure of the mature photoreceptor correlates favorably with that in a number of vertebrates, including amphibians, other avian species and numerous mammals.