Correlative cytological and histochemical studies on the avian oogenesis

Abstract

A correlative cytological and histochemical study has been made of oogenesis in birds (pigeon, Columbia livia; brown dove, Streptopelia senegalensis; ring dove, S. dacaocto; domestic fowl. Gallus domesticus; and quail, Coturnix coturnix). BALBIANI'S vitelline body consisting of large yolk nucleus, mitochondria, GOLGI bodies and lipid bodies of variable composition is gradually developed in the juxtanuclear cytoplasm of their young oocytes. Histochemical tests indicate that the yolk nucleus consists mainly of RNA, protein and lipoprotein. Mitochondria occur in the form of granules, rods and filaments having a phospholipid-protein composition. The lipid bodies are in the form of granules (composed of phospholipid and triglycerides) and spheres (made up of triglycerides and cholesterol and/or its esters). The organelles consisting of lipoprotein are preserved after classical techniques of cytology whereas the lipid bodies are mostly dissolved. With the growth of oocyte during previtello-genesis the various cell components of BALBIANI'S vitelline body are dispersed in the outer ooplasm where they further multiply and finally most of them are placed in the cortical layer. At the same time, many chromophilic and argentophilic elements of lipoprotein nature are formed in the follicular epithelium, adjacent to plasma membrane of oocyte and in the peripheral ooplasm during previtellogenesis, which gradually accumulate in the central ooplasm where the yolk vesicles first appear. Most of the lipid droplets consisting of triglycerides and cholesterol and/or its esters are depleted from the large previtellogenic oocytes. The possible significance of various ooplasmic structures, which accumulate during previtellogenesis, has been discussed. Corresponding to the development, multiplication and accumulation of various ooplasmic components during previtellogenesis the chromosomes develop the configuration of lampbrush and are the active sites for the synthesis of RNA and protein, which simultaneously accumulate in the ooplasm. The cytological and histochemical characteristics of follicle cells indicate that they are active in the secretion of lipids, glycogen, proteins and RNA which are apparently transported into the growing oocyte. The association of several lipid bodies with some phagocytotic vacuoles adjacent to the plasma membrane of growing previtellogenic oocyte has supported this suggestion. Several follicle cells as such degenerate during previtellogenesis and their products in the form of DNA, RNA, protein and lipoprotein also seem to be absorbed directly or indirectly by the growing oocyte. During early vitellogenesis, the yolk vesicles are formed in the central ooplasm by the activity of ooplasmic structures especially the chromophilic and argentophilic elements. They are ultimately transformed into yolk bodies of variable morphology and chemical composition, which are arrayed in alternating concentric spheres (yellow and white) and consist of proteinlipoprotein, triglycerides, carbohydrates, RNA etc...