The ultrastructural features of the synapses in the posterior horn of the spinal cord in the rat

Ultrastructural studies on postnatal differentiation of neurons in the substantia gelatinosa of rat cervical spinal cord

Neuroblasts of the substantia gelatinosa at birth were small with large oval nuclei and scanty cytoplasm. The cytoplasm possessed ribosomes and mitochondria. Granular endoplasmic reticulum and Golgi complexes were generally absent or rudimentary. Electron dense bodies were seldom observed. By the end of the first week, the nuclei of several cells demonstrated early nuclear invaginations; cytoplasm exhibited growth cones, a well developed granular endoplasmic reticulum and Golgi complexes. At several points the channels of endoplasmic reticulum became continuous with the perinuclear space. By the end of the second week, differentiation of the neuroblasts was more advanced. More nuclei showed invagination of their contour. The cytoplasm revealed well dev-loped granular endoplasmic reticulum and multiple Golgi complexes. Numerous vesicles and dense bodies were found adjacent to the Golgi complexes. Arrays of agranular endoplasmic reticulum also appeared late in the second week. By the third week, features of neuronal differentiation, such as nuclear invagination, granular endoplasmic reticulum agranular membrane configurations, multiple Golgi complexes and dense bodies in the cytoplasm became well established.

An investigation of the foetal rat spinal cord. I. Ultrastructural observations on the onset of synaptogenesis

Ultrastructural studies on the foetal rat spinal cord show that during synaptogeneis there is difficulty in recognizing true synaptic precursors. Symmetric densities are found at unusual sites forming, for example, somato-dendritic and somato-somatic junctions. They are also found between neurons and possible glial processes. Symmetric densities occur between nerve cells but may be confused with desmosomes. Profiles exhibiting membrane density, cleft material and 50 nm vesicles, which are the most reliable indicators of presumptive synapses, are found between neurones, but also at junctions between neurons and what may be glial processes. The picture is further confused by the presence of degeneration axodendritic synapses at early foetal stages. Caution must be exercised in defining an apparent synapse or precursor in foetal cord as that of a presumptive functional synapse because of the observed degenerating profiles and because of our knowledge, somato-somatic, somato-dendritic, and neurono-glial synapses have not been observed in adult cord. It is not known whether these structures are an unwanted consequence of development or play a role in guiding development.

Ultrastructure and function of growth cones and axons of cultured nerve cells

Dorsal root ganglion nerve cells undergoing axon elongation in vitro have been analyzed ultrastructurally. The growth cone at the axonal tip contains smooth endoplasmic reticulum, vesicles, neurofilaments, occasional microtubules, and a network of 50-A in diameter microfilaments. The filamentous network fills the periphery of the growth cone and is the only structure found in microspikes. Elements of the network are oriented parallel to the axis of microspikes, but exhibit little orientation in the growth cone. Cytochalasin B causes rounding up of growth cones, retraction of microspikes, and cessation of axon elongation. The latter biological effect correlates with an ultrastructural alteration in the filamentous network of growth cones and microspikes. No other organelle appears to be affected by the drug. Removal of cytochalasin allows reinitiation of growth cone-microspike activity, and elongation begins anew. Such recovery will occur in the presence of the protein synthesis inhibitor cycloheximide, and in the absence of exogenous nerve growth factor. The neurofilaments and microtubules of axons are regularly spaced. Fine filaments indistinguishable from those in the growth cone interconnect neurofilaments, vesicles, microtubules, and plasma membrane. This filamentous network could provide the structural basis for the initiation of lateral microspikes and perhaps of collateral axons, besides playing a role in axonal transport.

Cytoplasmic inclusions resembling nucleoli in sympathetic neurons of adult rats

A distinctive cytoplasmic inclusion consisting of a convoluted network of electron-opaque strands embedded in a less dense matrix was identified in the neurons, but not in the supporting cells, of rat sympathetic ganglia. This ball-like structure, designated "nematosome," measures approximately 0.9 micro and lacks a limiting membrane. Its strands (diameter = 400-600 A) appear to be made of an entanglement of tightly packed filaments and particles approximately 25-50 A thick. Cytochemical studies carried out with the light microscope suggest the presence of nonhistone proteins and some RNA. Usually only one such structure is present in a cell, and it appears to occur in most ganglion cells. Although they can be seen anywhere in the cell body, nematosomes are typically located in the perinuclear cytoplasm, where they are often associated with smooth-surfaced and coated vesicles. In fine structure and stainability, they bear a resemblance to the fibrous component of the nucleolus. Subsynaptic formations, which are a special feature of some of the synapses in sympathetic ganglia, appear similar to the threadlike elements in the nematosomes. The possibility that these three structures-nucleolus, nematosome, and subsynaptic formation-may be interrelated in origin and function is discussed.