Oocyte Differentiation and Vitellogenesis in the Roach

The ovary of the roach Periplaneta americana has been studied by techniques of light and electron microscopy. Each ovariole (panoistic type) contains a linear array of oocytes in varying stages of development. Newly formed oocytes become encased by a layer of follicle cells and begin pinocytosis. All subsequent growth stages of the oocytes are dependent, in part, on this phenomenon. All of the pinocytotic caveolae show an unique surface modification ; i.e., on their internal surface they have an amorphous or filamentous substance and their external surface is studded with many fine radially oriented spike-like projections. The pinosomes of early oocytes do not contain a demonstrable internal structure; they are thought to contain nutritive substances for the developing oocytes rather than yolk precursors. When the oocyte enters its last stage of growth, characterized by yolk deposition, the caveolae become filled with a dense material which is thought to be the precursors of yolk. Hence the conclusion is drawn that yolk formation is independent of any cytoplasmic organelle system of the oocyte and that the precursors of this deutoplasmic substance are manufactured outside the ovary and are internalized by the process of pinocytosis. Under the phase-contrast microscope the nucleoli of early oocytes are large irregular masses and show the phenomenon of nucleolar emission (fragmentation). These "emissions" become randomly dispersed in the nucleoplasm and some of them come to be intimately associated with the fenestrated nuclear envelope. After this process ceases, the main nucleolar mass becomes vacuolated. Electron micrographs suggest that the constituent particles of the nucleolar emissions migrate from the nucleus through patent pores of the nuclear envelope. Within the past several years investigators have presented pictorial evidence which makes it clear that oocytes of invertebrates and vertebrates possess the same basic substructural organizationwithstanding the agreement which has been reached concerning the basic microanatomy of mature oocytes, no such consensus is evident with respect to the mechanisms of vitellogenesis. Although unanimity may be reached concerning particular features of this phenomenon within the class Insecta, apparently no generalization can be drawn for the entire animal kingdom. In order to inquire into the complicated dynamic processes involved in the development of oocytes (including those in which the cytoplasm is devoid of yolk and those in which it becomes filled with copious amounts of yolk at certain stages), it is evident that a detailed knowledge of the ultrastructural changes which presumably take place during …