Development of the optic nerve of the rat - Investigative ...

More documents

Recommendations

Info

Volume 14 Number 10 Rat optic nerve 743 Fig. 16. Development of the blood vessels in the optic nerve. A, 14-day-old fetus. Blood vessels are seen in the fissure. B, lG-day-old fetus. The fissure is closed but blood vessels are trapped within and are developing into large vessels. Both epoxy section, toluidine blue. x260. C, A blood capillary found in a newborn rat optic nerve. The capillary endothelium is thick-. The basement membrane is also thick, x 13,000. Invasion of blood vessels and connective tissue into the optic nerve begins postnatally. Thick basement membranes are always found along the invading vascular tissue. Although the septa is not conspicuous in the rat optic nerve, fibroblasts and collagen fibers are present regularly along the blood vessels of the adult tissue. At the time of optic stalk formation, numerous blood vessels are found in the loose mesenchymal tissue in and around the optic cup. The fissure is also filled with vascularized connective tissue. Some of these blood vessels seem to remain and develop into the large vessels of the retina and the optic nerve (Fig. 16). However, development of the optic nerve appears to take place without the aid of specific blood vessels. Discussion The developmental process of the rat optic nerve is summarized in the drawing (Fig. 17). The first step of the differentiation of the optic nerve apears to be elimination of excess cells by bionecrosis. Most of the neuroepithelial cells of the invaginated portion of the stalk fissure which are continuous to the developing retina degenerate and disappear immediately following the optic cup formation. Bionecrosis has been observed in the developing nervous tissue and its significance has been discussed. IS - l!1 Among several theories, production of inductive substances-" or energy sources for the development of the surrounding tissue'-' 1 has been believed to be the main purpose of the morphogenetic bionecrosis. The extensive degeneration of the developing optic nerve in the earliest stage, however, seems to be for elimination of unnecessary cells. The abundant neuroepithelial cells at the border zone between the optic stalk and the retina are apparently unnecessary in formation of the optic nerve. Only certain cells survive in
744 Kuwabara Investigative Ophthalmology October 1975 Fig. 17. Diagrammatic representation of the development of the optic nerve. Numbers indicate fetal days. See text for explanation. this area to become the optic stalk. After the elimination of many cells, the optic stalk becomes a tube consisting of a single cell layer wall. The stalk cells proliferate at their apical zone and fill up the tubular lumen rapidly. These cells appear to be designated to differentiate into glia cells. Astrocytes differentiate first from the proliferating wall cells. Development of the processes of the astrocyte appears to occur in two stages; the large processes divide the axons into large groups in the early stage, and finer processes extend to fill up the minute spaces between axons following the myelination. Oligodendroglia cells differentiate at a slightly later stage. Cells with darker cytoplasm become distinguishable in the cluster of the stalk cells. Both glia cells disperse rapidly throughout the optic nerve, but the oligodendroglia cells appear to proliferate in the later stage, mainly postnatally. Delayed maturation of the oligodendroglia cell has been demonstrated by Hirose and Bass. 22 Some small glia cells which have a cytoplasmic appearance similar to that of the third glial cell type 23 are found in the early developing optic nerve. However, its function was not demonstrated in this study. Although the cluster of the embryonal glia cells at the retrobulbar zone disappears during the differentiation of glia cells, it appears to be the origin of the highly cellular zone behind the lamina cribrosa of the adult optic nerve. This zone of the optic nerve may be serving as the germinal center of the glia cells throughout life. This study has demonstrated the exact location of the axonal invasion into the optic nerve at its early developmental stage. Axon bundles are seen first in the lateral intercellular spaces of the basal portion of the elongating stalk cells. Nerve fibers are not seen in the stalk lumen, fissure, or outside of the optic nerve. Bundles of the nerve fibers are divided into smaller groups by processes of the astrocytes which develop at an early stage. The presence of the relatively axon-free area halfway between the globe and the chiasm in 15-day-old fetuses suggests that the axons extend into the optic nerve from two origins. These findings indicate that a large number of centrifugal fibers invade into the retina in the early stage of eye formation. However, it is not possible to trace a further extension of these fibers into the retina in the present observation. Myelination of the axons occurs at a rather late stage. Several axons are surrounded by fine processes of the glia cell as a group. The first myelin is apparently formed by the collapse of these glia cell processes. Although axons are occasionally found within a large cytoplasm of the glia cell, it is rare to find spiral myelination around the individual axons in the early stage. The thinned myelin membranes appear to extend between axons sinuously. This finding is somewhat different from that of the myelination of the peripheral 21 and central nervous system. 25 This has not been described in the developing optic nerve by earlier investigators. However, the origin of these myelin-forming processes are the oligodendroglia cells, as earlier investigators have pointed out. 25 - 2G The active myelination process appears to occur for two to three weeks postnatally. Completion of the development of the
Page 1 and 2: Development of the optic nerve of t
Page 3 and 4: 734 Kuioabara Investigative Ophthal
Page 5 and 6: 736 Kuwabara Investigative Ophthalm
Page 7 and 8: 738 Kuwabara Investigntive Ophthalm
Page 9 and 10: 740 Kuwabara Investigative Ophthalm
Page 11: 742 Kuioabara Investigative Ophthal

Development of the optic nerve of the rat - Investigative ...

Create successful ePaper yourself

Delete template?

Save as template?