SITES OF FORMATION OF THE EXTRA NUCLEOLI DURING EARLY OOCYTE GROWTH IN THE FRESHWATER TELEOST SALVELINUS FONTINALIS MITCHILL

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Abstract
The behavior of nuclear structures during early oocyte growth in Salvelinus fontinalis is described with special attention focussed on the sites at which the extra nucleoli first appear. In the species investigated, the process of extra nucleoli formation begins at about the mid-pachytene stage of meiotic prophase and proceeds in an uninterrupted fashion until the late strepsitene stage is reached; during that period of time, close to a 150 extra nucleoli form within the nuclear cavity. From the mid-pachytene up to about the mid-strepsitene stage, our observations suggest that, while some of the extra nucleoli arise by successive growth and detachment from a single large heterochromatic body adjacent to the oocyte's original nucleolus, the others develop in contact with minute chromo-centers located in the peripheral portion of the nuclear cavity. The relevant observational evidence would seem to favor the view that the heterochromatic body adjacent to the oocyte's original nucleolus contains one of the four alleged pairs of nucleolar organizing regions of the usual nucleolar chromosomes. During the mid-strepsitene stage, on the other hand, our observations reveal that, while the heterochromatic body and the chromocenters disappear from view, additional extra nucleoli develop at various points along the length of a number of chromosomes. These observations are taken to indicate that non-heterochromatic chromosomal regions distinct from the nucleolar organizing regions of the usual nucleolar chromosomes are, during that period of meiotic prophase, instrumental in the formation of extra nucleoli. The significance of the above findings is discussed in relation to the observations of other workers on the sites of formation of nucleoli in general.