CELL-DEATH OF MOTONEURONS IN CHICK-EMBRYO SPINAL-CORD .1. LIGHT AND ELECTRON-MICROSCOPIC STUDY OF NATURALLY OCCURRING AND INDUCED CELL LOSS DURING DEVELOPMENT

The onset and sequence of cell death in the lateral motoneurons of the lumbar spinal cord of the chick embryo was studied between 4 and 18 days of incubation by light and electron microscopy. The naturally occurring degeneration of a few motoneurons begins already on day 4 of incubation, but the cell loss becomes much more frequent from day 5.5 to day 9, during which 40% of the neuron population degenerates. In the light microscope, the degenerating neurons are seen to undergo shrinkage and condensation. Ultrastructurally, two types of degeneration can be recognized. In the first, termed Type I, the polyribosomes are dissociated and appear free in the cytoplasm. Ribosomes also become detached from rough endoplasmic reticulum, although small pieces of cisternae of rough endoplasmic reticulum are still recognizable. Most of the mitochondria are vacuolized. These changes are accompanied by the appearance of a pyknotic nucleus which contains condensed chromatin masses. The most characteristic feature in what we term Type II degeneration is the striking dilatation of the endoplasmic reticulum, the nuclear envelope and the Golgi apparatus. Ribosomes still form distinctly rosette‐like polyribosomes. A few mitochondria show signs of degeneration. The nuclear profile in most cells of this type is rounded whereas the chromatin is becoming condensed. In the late stages of Type II degeneration, the dilated membrane systems break down into numerous vesicles some of which still have ribosomes attached. Only late in the sequence of Type II degeneration do polyribosomes then dissociate to free monoribosomes. The degeneration process in both types leads finally to cell death and complete cellular breakdown: the entire process being due to an autolytic mechanism. The nuclear envelope breaks down and the nuclear content becomes mixed with lytic cytoplasm. The dying cell finally either condenses into one big globule or several smaller fragmented globules. All the globular debris are highly osmiophilic and compact. Only at this stage of breakdown is the cell debris phagocytozed by radial ependymal processes and mononuclear leukocytes. The entire degenerative process in these immature neurons is strikingly rapid. In the case of induced cell death by removal of the limb bud on day 2.5, we found that both types of degeneration described above also occur in the peripherally deprived lateral motor column (LMC). The only obvious difference is that limb bud removal increases the speed and the number of cells undergoing degeneration. By day 10, about 90% of the neuron population in the deprived LMC have completely disappeared. Copyright © 1978 The Wistar Institute Press