Synaptic emergence and development in the duck olfactory bulb was quantitatively studied by electron microscopy from the 14th day of incubation (E 14) to the adult stage. Overall synaptic density in this bulb grew considerably during the last weeks of embryonic life and the first postnatal week. The pattern of synaptic density development was similar in the four main architectonic layers of the bulb. However, lower density values were observed in the mitral and inner granule cell layers. In the glomerular layer (GL), axodendritic synapse density was always higher than dendrodentritic synapse density. In the external plexiform layer, most synapses were dendrodendritic and were established between the gemmules of the granule cells (GC) and the dendrites of the mitral cells (MC) or tufted cells (TC). Synapses established by MC and TC on GC gemmules, or by GC on MC and TC dendrites had densities very similar to each other at all the stages studied. Reciprocal synapses already appeared at E 14; their density grew until a week after birth (P7) and thereafter remained stable. In the internal granular layer, the density of asymmetrical synapses was always higher than that of symmetrical synapses. Excitatory synapses formed earlier on MC and TC than inhibitory synapses. The ratio of inhibitory-to-excitatory synapses rose rapidly after birth, reaching 2.5 in the adult duck. The density of excitatory synapses received by granule cells was as high in the external plexiform layer as in the inner granule layer, at all stages of GC development. However, the ratio of received-to-formed synapses fell in these cells from 8.42 at E 14 to 2 after birth. These results are discussed as a function of the evolution of the different synaptic balances during olfactory bulb development. Synaptic development in the duck olfactory bulb at birth is relatively close to the adult state. It appears sufficiently advanced to enable the olfactory system to function in a way compatible with the relatively independent behavior displayed by the duckling.
The effects of neonatal hypothyroidism on synaptic organization, glial surrounding and cytoplasmic structures of the Purkinje cell perikaryon of the 21 days old rat were studied by electron microscopy. Hypothyroidism decreases the size of the perikaryon but does not change the nucleo-cytoplasmic ratio of the cell. At 21 days the axonal endings that make synapse on the Purkinje cell perikaryon, already show the adult morphological features and hypothyroidism does not change these. The general synaptic density on the Purkinje perikaryon is not significantly altered, but the cells of hypothyroid animals still recieve 1/3 of their axo-somatic synapses on somatic spines, whereas euthyroid animals have practically no synapses on somatic spines. Hypothyroidism leads the disappearance of the synchronism which normally exists between the translocation or resorption of the Purkinje cell somatic spines and the establishment of the basket cell synapses. The basket cell axons of the normal animals form synapses only on the smooth surfaces of the Purkinje cell perikaryon while they also establish it on the somatic spines of the hypothyroid animals. The density of the axon terminals of basket cells is decreased, while that of the endings of Purkinje cell axon collaterals and of climbing fibers are increased. Hypothyroidism produces an increase in the size of the glial sheath around the Purkinje perikaryon. It does not alter the proportion of the cytoplasmic area occupied by mitochondria, but in thyroid deficiency there is an increase in the numbers of mitochondria which are reduced in average size. The Golgi apparatus is only slightly affected. Thyroid defiency does not alter the density of the cytoplasmic agreggates of free ribosomes but there is a decrease in density of the ergastoplasm and a disorganization of the Nissl bodies.
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