Postnatal development of the inferior olivary complex in the rat. III. A morphometric analysis of volumetric growth and neuronal cell number.

The overall volume of the three main inferior olivary (ION) subnuclei increases four-fold between the day of birth (PO) and P21. The rate of this increase is uneven; between P10 and P15 there is an abrupt acceleration which parallels the period of intense synaptogenesis and maturation of the neuropil. The growth of the various subnuclei occurs in an almost synchronous manner. A study discloses that the postnatal volumetric increase is the result of the development of the neuropil and the glial cells. The numerical estimation of neuronal perikarya was made for each ION subnucleus from P0 to P33. However, due to methodological problems, the quantitation becomes reliable only from P5 onwards. From P5 to P8, there is a small (10%) but consistent decrease in the number of olivary neurons, a decrease which is homogeneously shared by the 3 main subnuclei. This phase of reduction in neuronal numbers, indicative of cellular death, is followed by an increase of similar magnitude, between P10 and P15. Since the phase of apparent olivary cell death coincides with the peak of the regression of the multiple innervation of Purkinje cells (PCs) by climbing fibers (CFs), both processes might be interrelated. However, the slight amplitude of the cell death is inadequate to fully explain the whole process of synaptic regression. These results indicate a dual nature of the mechanism underlying the establishment of the one-to-one relationship between CFs and PCs: a small proportion of the regression results from cell death, while the largest proportion must be the result of a loss of collaterals from the olivary axons at the origin of the CFs. The problem of the increase in the number of ION neurons between P10 and P15 is discussed in relation to recent morphometric data indicating a late increase in the number of PCs of the cerebellum.