Abstract
Ovaries from a series of fetal and neonatal mouse (from 12.5th day after birth) were examined by electron microscopy.
The oogonium in a resting phase was characterized by a large, round nucleus having one or more well developed nucleoli and sometimes intranuclear iuclusion bodies simnlar to the nuclear envelope. Two types of the inclusion bodies were observed. One of them was the type of the ‘annulate lamellae’ and the other was of the ‘double walled vesicles’, even in the wall of the latter, in sometimes, several annuli could been found (this pored structure was termed ‘annulate vesicle’). (Cf. Figs. 1 and 2.)
Many mitotic figures of oogonia mingled with those of the resting phase were also encountered in the ovary at 13.5th day post coitum. In some of the mitotic cells several tiny vesicles are noticed in a rather wide space between the nuclear envelope still imcompletely restored and telophase chromosomes. (Cf. Fig. 3.)
A causal relation between such the vesicles and the intranuclear inclusion bodies was assumed and discussed. Double nucleated oogonia were also found in this stage, but never in stages later than the late pachytene.
The features of oocytes in the onset of the prophase (the pre-leptotene) were revealed preferentially in the nucleoli. The nucleolonema frame-work in the nucleolus was thicker, denser, and simpler than that of the oogonia. This finding was followed by a dense, dotlike appearance of the nucleoli in the leptotene, and then, again by more complicated network formations in later stages. (Cf. Fig. 4.)
The nuclei of oocytes at the leptotene stage contained many of single, dense, thread-like structures of 300-400Å in diameter. A pole of the one of these thread-like structures was in contact with a higher electron dense dot embedded in a moderately dense patch lying at the periphery of the nucleus. But any attachment of the endings of another thread to the nuclear membrane without accompanying with the moderately dense patch (synizesis) has not been observed. (Cf. Fig. 6.)
In the following zygotene stage the single threads became tripartite strands consisting of bilateral thicker fibrils (300-400Å) and a central finer filament (about 150Å). This structure is considered to be identical with the ‘synaptinemal complex’ which has been detected by MOSES in the bivalent chromosomes of crayfish spermatocytes. (Cf. Fig. 7.)
In the early pachytene, chromosomal configulations became thicker and discerned easily from one another. The long set of the complex was often observed in the center of individual chromosomes and recognized more or less to be twisted around tis long axis. (Cf. Fig. 8.)
In the late pachytene such an extended form of the complex has no longer been seen, presumably due to its coiling or bending. (Cf. Fig. 9.)
The diplotene chromosomes possessed only single strands similar to those seen at the leptotene, which strands were thought to be not differed essentially from those of the leptotene. (Cf. Figs. 10 and 11.)
Shortly after the oocytes entered into the dictyate or the resting stage, these filamentous structures disappeared from the nucleus. (Cf. Fig. 12.) From these observations it is conceivable that the single strands represent the ‘cores’ of individual ‘asynaptic’ chromosomes, and that the tripartite sets of strands imply the presence of synapsis between homologous chromosomes.
On the other hand, in the oocyte cytoplasm at the onset of meiotic prophase, the evidence of a small vacuolization in some mitochondria was revealed. The evidence may be regarded as a sign of transformation from oogonium to oocyte. This phenomenon, furthermore, became more prominent in the later stages of development except only in the case of the diplotene. Mitochondria in the diplotene oocytes had no vacuole and showed an approximately normal arrangement to cristae in the most case observed
