Ultrastructural study of nucleolar organizers in the quail oocyte during meiotic prophase I

Abstract

The localization of nucleolar organizers in the quail oocyte was studied during meiotic prophase I. During midpachytene, the oocyte nucleus contains chromocenters formed by association of the heterochromatic centromeric region of the microchromosomes. During late pachytene, the nucleolus always develops in strict relation to a chromosomal structure. The nucleolus may either be directly applied against the surface of a chromocenter or in contact with a specific region of a microchromosome emerging from a chromocenter. This microchromosome presents a juxtanucleolar knob of heterochromatic appearance. Two chromocenters, with their corresponding microchromosomes, may participate in nucleolar genesis, in which case the nucleolar constituents are arranged in a symmetrical manner with respect to the chromosomal structures. Penetration of DNP fibers of nucleolar organizer always takes place in a pale fibrillar zone (fibrillar center) contained within the nucleolus and surrounded by electron dense fibrillar strands. Ultrastructural data suggest that the nucleolar organizer may be situated in the pale fibrillar zone of the nucleolus. Nevertheless, no decisive argument exists excluding either the juxtanucleolar knob, which appears to be formed of facultative heterochromatin, or the DNP fibers of the nucleolonema.