Quantitative electron microscopic analysis of the optic nerve of the turtle, Pseudemys

The postnatal development of the optic nerve of a reptile (Vipera aspis): A quantitative ultrastructural study

The number of axons in the optic nerve of the ovoviviparous reptile Vipera aspis was estimated from electron micrographs taken during the first 5 weeks of postnatal life. One to two days after birth, the optic nerve contains about 170,000 fibres, of which about 9% are myelinated. At the end of the fifth postnatal week, the number of optic fibres has fallen to about 100,000, of which about 42% are myelinated. This fibre loss continues after the fifth postnatal week, since in the adult viper the nerve contains about 60,000 fibres, of which 85% are myelinated; overall, about 65% of the optic nerve fibres present at birth disappear before the number of axons stabilises at the adult level. This study shows, for the first time, that the mode of development of the visual axons of reptiles is not that of anamniote vertebrates but similar to that of birds and mammals.

Developmental changes in the fibre population of the optic nerve follow an avian/mammalian-like pattern in the turtle Mauremys leprosa

The changes in the axon and growth cone numbers in the optic nerve of the freshwater turtle Mauremys leprosa were studied by electron microscopy from the embryonic day 14 (E14) to E80, when the animals normally hatch, and from the first postnatal day (P0) to adulthood (5 years on). At E16, the first axons appeared in the optic nerve and were added slowly until E21. From E21, the fibre number increased rapidly, peaking at E34 (570,000 fibres). Thereafter, the axon number decreased sharply, and from E47 declined steadily until reaching the mature number (about 330,000). These observations indicated that during development of the retina there was an overproduction and later elimination of retinal ganglion cells. Growth cones were first observed in the optic nerve at as early as E16. Their number increased rapidly until E21 and continued to be high through E23 and E26. After E26, the number declined steeply and by E40 the optic nerve was devoid of growth cones. These results indicated that differentiation of the retinal ganglion cells occurred during the first half of the embryonic life. To examine the correlation between the loss of the fibres from the optic nerve and loss of the parent retinal ganglion cells, retinal sections were processed with the TUNEL technique. Apoptotic nuclei were detected in the ganglion cell layer throughout the period of loss of the optic fibres. Our results showed that the time course of the numbers of the fibres in the developing turtle optic nerve was similar to those found in birds and mammals.

Comparative retinal ganglion cell and optic nerve morphology

The optic nerve is divided in four regions: intraocular, intraorbital, intracanalicular, and intracranial. The vertebrate retinal ganglion cells are classified by morphology, physiology and soma size. Species differences and similarities occur with retinal ganglion cells. Alpha retinal ganglion cells have large somata, large dendritic fields, large-diameter axons, and are most dense in the peripheral retina. Beta retinal ganglion cells have smaller diameter somata, smaller dendritic fields, small diameter axons, and predominate in the central retina. Gamma retinal ganglion cells are a heterogenous class of cells and have small diameter axons, and slow axon conduction velocities. The spatial distribution and organization of the retinal ganglion cells extends retinotopically through the nerve fiber layer, optic nerve, optic chiasm, optic tract, lateral geniculate nucleus, and visual cortex. The retinal nerve fiber layer thickness decreases from the optic disk toward the periphery of the retina. The retrobulbar optic nerve axon counts and axon density vary by species, with larger nerves having higher axon counts. Decussation of the optic nerve axons at the optic chiasm varies with 100% decussation in most birds and fish, 65% in cats, 75% in dogs, 80-90% in large animals, and 50% in primates. Centrifugal axons also occur in the optic nerve and may represent a method by which the brain can influence retinal activity.

Optic tectum of the eastern garter snake, Thamnophis sirtalis. IV. Morphology of afferents from the retina

The morphology of single retinal terminals in the optic tectum of the eastern garter snake was demonstrated by orthograde filling from extracellular injections of horseradish peroxidase (HRP) into the optic tract. HRP-filled terminals share a characteristic shape and structure. Their parent axons course caudally in the stratum opticum within fascicles of 200-300 fibers of varying diameters. Single axons exit a fascicle and course into either the stratum fibrosum et griseum superficiale, ventrally, or the stratum zonale, dorsally, where they bifurcate successively two or three times into preterminal branches. Each preterminal branch gives rise to many thin, terminal branchlets laden with boutons. The arbors are ellipsoidal with their long axes oriented mediolaterally and their short axes oriented rostrocaudally. Arbors vary in their overall size (from 45 to 150 micron), in the diameters of their parent axons (from less than 0.5 to 3.0 micron), and in the size of their terminal boutons (from 0.5 to 3.5 micron). Bouton size increased with increasing diameter of the parent axon. The great majority of arbors are confined to one of three retinorecipient sublayers in the superficial tectum. However, the full range of arbor sizes and axon diameters is present in each sublayer.

Conduction velocity, size and distribution of optic nerve axons in the turtle, Pseudemys scripta elegans

Electrophysiological and morphological techniques have been used to characterize optic nerve axons in the red-eared turtle. Three distinct groups of axons are identified on the basis of conduction velocity and axon diameter. The first group (T1) is a small population of axons with large diameters (2.8-4.5 microns) and mean conduction velocities of 13 m/sec. The second group (T2) is a large population of axons with medium diameters (0.4-2.8 microns) and mean conduction velocities of 3 m/sec. The third group (T3) is a medium sized population of small diameter (0.2-0.6 micron), mostly unmyelinated axons with mean conduction velocities of 1 m/sec. There is a significant regional variation in the size, density and myelination of axons in the optic nerve. Large axons are found dorsally and ventrally, while smaller axons and the majority of unmyelinated fibers are found along a dorsotemporal to ventronasal axis through the nerve. Fink-Heimer techniques were used to trace the trajectories of axons of different sizes from the retina to the brain. Large diameter axons can be traced along the dorsal and ventral portions of the optic tract, with a dorsal group leaving the tract in the pretectum and a ventral group entering the basal optic tract. These observations suggest that the distribution of axons within the optic nerve reflects in part the distribution of ganglion cell somata in the retina. However, there is also some segregation of axons of different sizes according to their various central targets.

A detailed morphometrical analysis of the pyramidal tract of the rat

The present study is aimed at a quantitative analysis and comparison of the fibers of the pyramidal tract of the rat at two levels: the pyramis medullae and the second cervical segment. For this purpose both levels of the pyramidal tracts of 5 rats have been studied morphometrically at the ultrastructural level. Fiber numbers as well as fiber diameter distributions are presented for both myelinated and unmyelinated axons. At the level of the pyramis medullae the number of unmyelinated fibers (140,000 +/- 7000) exceeds the number of myelinated axons (103,000 +/- 6000). In contrast at the level of the second cervical segment the numbers of fibers of the axon populations studied are not significantly different, viz. 43,000 +/- 2000 myelinated and 35,000 +/- 8000 unmyelinated fibers. However, these numbers mean a significant decrease of myelinated axons (48%) compared with the pyramis medullae level and an even much larger decrease (75%) in the numbers of unmyelinated fibers. Diameter distributions, obtained from the minimal fiber diameter, show a similar, monomodal shape for all axon and myelin profiles. For unmyelinated axons the minimal diameter ranges from 0.05 to 1.21 micron with a mean of 0.18 +/- 0.03 micron. For myelinated axons a mean minimal diameter of 0.72 +/- 0.12 micron has been found (range 0.13-4.92 micron), whereas their diameter measured with myelin sheath measure 1.08 +/- 0.13 micron (range 0.25-6.03 micron). The average thickness of the myelin sheath is 0.2 micron and is strongly correlated to axon diameter. Furthermore, g-ratio has been computed at a mean of 0.65 and is fairly constant throughout the diameter spectrum. Several possibilities concerning the functional and anatomical qualities of the numerically important unmyelinated axon population in the pyramidal tract of the rat are discussed.