Development of coordinated movements was quantitatively assessed in adult opossums (Monodelphis domestica) with thoracic spinal cords transected by (1) crushing 7-8 d after birth [postnatal days 7-8 (P7-P8)]; at 2-3 years of age, systematic behavioral tests (e.g., climbing, footprint analysis, and swimming) showed only minor differences between control (n = 5) and operated (n = 10) animals; and (2) cutting on P4-P6; at 1 month these opossums exhibited coordinated walking movements but were unable to right themselves from a supine position, unlike controls (n = 6). When tested at 2 or 6 months, they could right themselves and showed remarkable coordination, albeit with more differences from controls than after a crush. No animals with spinal cords that were crushed at P14-18 survived because of cannibalism by the mother. Morphological studies (n = 10) 3 months-3 years after crush at 1 week showed restoration of structural continuity and normal appearance at the lesion site. Animals with cut rather than crushed cords showed continuity but greater morphological deficits. That lesions were complete was demonstrated by examining morphology and nerve impulse conduction immediately after crushing or cutting the spinal cord in controls. After lumbar spinal cord injection of 10 kDa dextran amine, retrogradely labeled cells were found rostral to the lesion in hindbrain and midbrain nuclei. Conduction was restored across the site of the lesion. Thus complete spinal cord transection in neonatal Monodelphis was followed by development of coordinated movements and repair of the spinal cord, a process that included development of functional connections by axons that crossed the lesion.

Optical methods were used to examine the spread of electrical potentials and the distribution and time course of calcium transients in individual identified nerve cells isolated from the leech. A photodiode array detected voltage transients by measuring absorbance changes of cells stained with the voltage-sensitive dye RH155 added to the bath. Calcium transients were recorded by measuring absorbance changes of the dye arsenazo III, which had been injected into the cells. In addition, Lucifer yellow was injected to outline the some and processes. Calcium changes resulting from individual action potentials were recorded from N, P, and Retzius cells without averaging. Signals from T cells and anterior pagoda (AP) cells were weaker but could be detected with averaging. These results are in accord with previous studies on calcium contributions to action potentials in these cells. For all cells, larger or wider action potentials gave bigger signals. Calcium changes from each of a train of action potentials were of equal amplitude, showing no sign of facilitation. Calcium transients from Retzius cells that had formed chemical synapses with P cells had properties similar to those of isolated cells. We were also able to detect responses from prolonged subthreshold depolarizations to -40 mV from a hyperpolarized membrane potential (-65 mV). These signals rose throughout the duration of the pulse (1–2 sec). With the photodiode array we mapped the distribution of the calcium signals. The amplitudes from each pixel are proportional to the amount of calcium entering that element in response to the stimulating pulse, if the simplifying assumption is made that the calcium buffering of the cytoplasm is uniform throughout the cell. The largest signals were detected over the axon stump left from the cell isolation procedure. Large signals were also detected from the soma. Weak signals were detected from the processes of some cells. From many Retzius cells, no signals at all were detected from the newly formed processes. Using the photodiode array, we also recorded voltage transients from the cells. Signals were recorded from all over the arborization of the neuron, with no obvious variation in time course, showing that the entire cell, including fine slender processes and broad growth cones, was essentially isopotential. Combining these observations with the measured distribution of calcium transients in the same cell suggests that the density of calcium channels in most cells is less in the outgrowing processes than in the soma or axon stump.

The ability of neurons in the central nervous system to grow through a lesion and restore conduction has been analyzed in a developing spinal cord. The preparation consists of the entire central nervous system of the newly born opossum (Monodeiphis domestica), isolated and maintained in culture. Cell division, cell migration, and reflexes are maintained in such preparations for up to 8 days in culture. In the present experiments, massive lesions were produced by crushing the spinal cord, which abolished all conduction for a day. By 2-3 days after injury, electrical conduction across the crush could be observed. After 4-5 days, clear recovery had occurred: the amplitude of the conducted volley was comparable to that in acute preparations. In such preparations, the spinal cord had largely regained its normal appearance at the crush site. Axons stained by carbocyanine dyes or horseradish peroxidase had, by 4 days, grown in profusion through the lesion and several millimeters beyond it. These experiments demonstrate that neurons in the central nervous system of newly born mammals, unlike those in adults, can respond to injury by rapid and extensive outgrowth in the absence of peripheral nerve bridges or antibodies that neutralize inhibitory factors of myelin. With rapid and reliable regeneration occurring in vitro, it becomes practicable to assay the effects of molecules that promote or inhibit the restoration of functional connections.Damage to the central nervous system (CNS) of adult mammals is usually followed by minimal regrowth or repair (1). Axons can regenerate and make connections with their targets only after grafts of peripheral nerves have been supplied (2-4) or after inhibitory growth molecules have been neutralized (5-7). Oligodendrocytes in particular have been shown to produce molecules that actively prevent neurons from growing. By contrast, neurons within the CNS of a developing mammalian embryo are unmyelinated, grow profusely, and form synapses in the virtual absence of glial cells. The aim of the present experiments was to determine whether damaged immature mammalian CNS might be better able to repair itself after an injury. Such tests have not been made on embryos in the past, for technical reasons. Although axonal outgrowth can occur in neonatal hamsters following injury to pyramidal-tract fibers, the regenerating axons are not able to grow across the lesion (8).A suitable experimental preparation for studying regeneration after an injury is provided by the central nervous system of the neonatal opossum (Monodelphis domestica). This animal, being a marsupial, is extremely immature at birth. The newly born pup has only rudimentary eyes, ears, and hindlimbs and is unable to walk or to right itself. Its CNS is incomplete and corresponds to that of a 14-day rat embryo, having no cerebellum and only a rudimentary forebrain (9). Yet this neonatal animal is able to suck, to breathe, and to perform selected vital functions. Earlier experiments (10-12) have shown that the CNS can be removed in ...

An analysis has been made of intrinsic mechanisms influencing growth patterns of 2 identified leech interneurons in tissue culture. These small cells (known as DL and VL) display unusual arborizations in the ganglion. The distinctive branching patterns resemble the letter "T" for VL and the letter "Y" for DL. DL and VL cells contain serotonin and can be identified in situ with Neutral red. 1. DL and VL cells were isolated and cultured on 2 homogeneous substrates, concanavalin A (Con A) and leech extracellular matrix extract (ECM). Individual DL and VL cells in culture retained their ability to fire in spontaneous, rhythmical bursts. The synaptic connections formed in culture were specific and differed from those made by other serotonergic neurons. 2. On both Con A and ECM the neurons sprouted to reproduce their characteristic patterns. Every DL or VL cell that grew processes on ECM developed the appropriate Y- or T-shaped pattern. On Con A, most but not all of the cells showed specific Y or T patterns. The probability of sprouting was higher on Con A than on ECM. 3. Disruption of microtubular assemblies in freshly isolated DL and VL cells by treatment with nocodazole did not affect the later outgrowth of specific patterns in culture. 4. These results provide evidence that adult DL and VL leech neurons retain intrinsic information for determining a variety of their properties in culture: In particular, they reproduce type-specific neuritic patterns in the absence of extrinsic guidance cues. Moreover, the intrinsic pattern determining mechanism is not lost after the destruction of microtubular arrays within the cytoskeleton.

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