Single Transverse Section Research Articles

Growth cones, dying axons, and developmental fluctuations in the fiber population of the cat's optic nerve.

We have studied the rise and fall in the number of axons in the optic nerve of fetal and neonatal cats in relation to changes in the ultrastructure of fibers, and in particular, to the characteristics and spatiotemporal distribution of growth cones and necrotic axons. Axons of retinal ganglion cells start to grow through the optic nerve on the 19th day of embryonic development (E-19). As early as E-23 there are 8,000 fibers in the nerve close to the eye. Fibers are added to the nerve at a rate of approximately 50,000 per day from E-28 until E-39--the age at which the peak population of 600,000-700,000 axons is reached. Thereafter, the number decreases rapidly: About 400,000 axons are lost between E-39 and E-53. In contrast, from E-56 until the second week after birth the number of axons decreases at a slow rate. Even as late as postnatal day 12 (P-12) the nerve contains an excess of up to 100,000 fibers. The final number of fibers--140,000-165,000--is reached by the sixth week after birth. Growth cones of retinal ganglion cells are present in the optic nerve from E-19 until E-39. At E-19 and E-23 they have comparatively simple shapes but in older fetuses they are larger and their shapes are more elaborate. As early as E-28 many growth cones have lamellipodia that extend outward from the core region as far as 10 microns. These sheetlike processes are insinuated between bundles of axons and commonly contact 10 to 20 neighboring fibers in single transverse sections. At E-28 growth cones make up 2.0% of the fiber population; at E-33 they make up about 1.0%; from E-36 to E-39 they make up only 0.3% of the population. Virtually none are present in the midorbital part of the nerve on or after E-44. At all ages growth cones are more common at the periphery of the nerve than at its center. This central-to-peripheral gradient increases with age: at E-28 the density of growth cones is two times greater at the edge than at the center but by E-39 the density is four to five times greater. Necrotic fibers are observed as early as E-28 in all parts of the nerve. Their axoplasm is dark and mottled and often contains dense vesiculated structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Dispersion of growing axons within the optic nerve of the embryonic monkey.

To determine whether individual optic fibers grow along constant sets of neighboring fibers, a group of 160 axons and 25 axonal growth cones were traced through a set of 500 serial electron micrographs of an optic nerve taken from a 39-day-old monkey embryo (Macaca mulatta). In single transverse sections, growth cones contact an average of 7.9 fibers, whereas axons contact 5.3 other fibers. The particular set of fibers in contact with one another changed rapidly, and, on average, growth cones and axons lost half of their original neighbors over a distance of only 8-10 micron. Between the first and last sections of the series, 92% of all initial contacts were lost. Individual axons moved freely between fiber fascicles, and the distance separating initial neighbors increased progressively. Most remarkably, the sets of fibers touched by the tips and the shanks of growth cones had no common neighbors in 17 out of 25 cases. These results demonstrate that, in primates, fibers in the optic nerve do not retain a particular set of immediate neighbors during their outgrowth.

Indirect Estimation of Crown‐rump Lengths of Incomplete Conceptuses from Analysis of Spinal‐cord Dimensions

The spinal cords of serially sectioned human embryos and early fetuses, ranging from 4mm to 112mm in crown-rump length, were examined in order to determine whether a mathematical relationship exists between certain spinal-cord dimensions and the crown-rump length of the conceptus. This information may be useful in estimating the crown-rump length, and hence the gestational age, of advanced human embryos and early fetuses when direct measurement is impossible. Five widely-spaced levels along the spinal cord were studied, but only those data relevant to the levels of the first thoracic (T1) and twelfth thoracic (T12) vertebral bodies are described in detail. Similarly, although eight distinct spinal-cord dimensions were measured at each level, only those measurements that bore a high enough correlation with crown-rump length to be of any predictive value are presented here, the most useful being the maximum spinal-cord width and the maximum grey-matter width, both of which increased with gestational age in a highly predictable way. The descriptive account of the morphological appearance of the cord at these levels also enables, though less accurately, an estimate of the crown-rump length of a conceptus to be made by direct inspection of a single transverse section through the thoracic region of the cord. The predictive value of these observations is discussed.