Cell Death in the Embryonic Chick Spinal Cord

Abstract

The events which occur in the death of visceromotor neurons of the cervical region of the chick embryo's spinal cord have been analyzed by electron microscopy . These normal degenerative events are compared with those in the lumbosacral cord where nerve cell death was induced by removal of peripheral organs . The initial set of degenerative changes include a decrease in nuclear size, the clumping of chromatin beneath the nuclear envelope, an increase in electron opacity of the cells, the disappearance of Golgi bodies, and the disaggregation of polysomes . These events are followed by the loss of the nuclear envelope and most of the endoplasmic reticulum, the appearance of bundles of filaments, and the formation of many ribosome crystals . Ribosome crystals are seen only in the dying cells .Their abundance may indicate a drastic reduction in RNA synthesis as one of the initial events which lead to the death of these neurons . The neurons are finally subdivided and engulfed by cells of the normal glial population, and further breakdown of the cell fragments occurs in large phagocytic vesicles of the gliocytes .Death of cells plays a very prominent part in the embryonic development of vertebrate organisms (1-3) . It occurs during the differentiation and sculpturing of various organs, and in the elimination of structures that are useful only during a limited phase of the embryonic period . The mechanism of cell death in embryonic systems has not for the most part been identified . Many studies have indicated that it is under genetic control (4), but how this genetic control is mediated on a cellular level is completely unknown .During normal development, massive cell death occurs in the cervical motor column of the chick spinal cord between the 4th and 5th days of incubation (5) . As a result of this degenerative process, at least two-thirds of the cell population in this region is reduced to cellular debris and pycnotic cells within a 24 h period . Careful analysis of this area using the Cajal silver staining method revealed that the dying cells are preganglionic neurons and part of an abortive visceral system (5), a conclusion supported by experiments involving transplantation of the cervical cord to the thoracic region (6) . Previous work has indicated that the dying cells are removed by macrophages which invade the motor column and ingest the degenerating cells (5, 7) . After the 6th day of incubation, however, macrophages could not be identified ; therefore, it was presumed that they had migrated from the cord .In the chick embryo, massive cell death does not normally occur in any other regions of the spinal cord . However, most of the motor neurons in the lateral motor column of the lumbosacral cord will die after extirpation of the ipsilateral leg bud of a 2% day old embryo (8) . Nerve processes are not severed in the course of the extirpation, since leg nerves do not reach the base of the leg bud until the 4th day ; therefore, this cell death does not result from nerve cell damage . Light

The role of temperature in the crystallization of ribosomes in chick embryos

“…Since ribosome crystallization increases in degenerating tissues when the temperature of the treatment rises from 4°C to 20"C, one may be tempted to conclude that the same treatments at 38°C should produce an even more pronounced increase in crystallization, but the evidence shows that this is not the case. The results of this study confirm instead the previous suggestion made by Mottet [7] that at 38°C ribosome crystals are not normally formed in dying cells and occur only in a restricted group of specialized tissues that are undergoing programmed processes of cell necrosis. Therefore, temperature does not have a linear or monotonic influence on ribosome crystallization even within the class of cells that are degenerating, and its effects appear, once again, to have not a kinematic but a physiologic basis.…”

Morphological Characteristics of Dying Cells in Axial Structures of Developing Human Embryos

“…At earlier development stages (5-6 weeks) apoptotic cells were found mostly in the ventricular zone, while at later stages they were seen in the intermediate zone of the neural tube. Other authors have also reported an apoptotic type of cell death in the neural tissue of the chick [O'Connor and Wyttenbach, 1974;Chu-Wang and Oppenheim, 1978] and of lizard embryos [Monzon et al, 1987]. Apoptosis has also been shown to occur not only during normal embryonic/fetal development, but in physiological tissue turnover in adults, in tumours, in tissues exposed to toxic agents and in neurodegenerative diseases [Kerr et al, 1974;Wyllie, 1992;Nijhawan et al, 2000].…”

Somatostatin (SRIF) Prevents Natural Motoneuron Cell Death in Embryonic Chick Spinal Cord

“…Neuronal cell death, occurring both during develop ment [2][3][4][5][6] and in the course of the more prominent adult neurodegenerative diseases, amyotrophic lateral sclero sis, Parkinson's disease and Alzheimer's disease [39], displays the morphological and temporal characteristics of the physiological cell death process, apoptosis. At the molecular level, apoptosis has been shown to be initiated by an increase in intranuclear Ca++ and to require the induction of a Ca+ + , Mg+ +-dependent endonuclease [40,41].…”

“…The morphologi cal characteristics of this process are those of apoptosis, or physiological cell death, which is initiated by extracel lular events, mediated by internally regulated intracellu lar events, and dependent upon Ca++ (2)(3)(4)(5)(6). In the rat spinal cord, motoneuron cell death begins prior to em bryonic day 15 (E15), with the greatest loss of cells occur ring between E15 and E16; by EI8, motoneuron loss is 95% complete [7].…”