Abstract
We examined six types of cells that form the ovary of the earthworm Dendrobena veneta ogonia, prooocytes, vitellogenic oocytes, trophocytes, fully grown postvitellogenic oocytes and somatic cells of the gonad. The quantitative stereological method revealed a much higher “volume density” of mitochondria in all of the types of germ-line cells except for the somatic cells. Fluorescent vital stain JC-1, however, showed a much higher oxidative activity of mitochondria in the somatic cells than in the germ-line cells. The distribution of active and inactive mitochondria within the studied cells was assessed using the computer program ImageJ. The analysis showed a higher luminosity of inactive mitochondria in all of the types of germ-line cells and a higher luminosity of active mitochondria in somatic cells. The OXPHOS activity was found in somatic cells mitochondria and in the peripheral mitochondria of the vitellogenic oocytes. The detection of reactive oxygen species (ROS) revealed a differentiated distribution of ROS in the different cell types. The amount of ROS substances was lower in somatic cells than in younger germ-line cells. The ROS level was also low in the cytoplasm of fully grown postwitellogenic oocytes. The distribution of the MnSOD enzyme that protects mitochondria against destructive role of ROS substances was high in the oogonia and in prooocytes and it was very high in vitellogenic and postvitellogenic oocytes. However, a much lower level of this protective enzyme was observed in the trophocytes and the lowest level was found in the cytoplasm of somatic cells. The lower mitochondrial activity and higher level of MnSOD activity in germ-line cells when compared to somatic cells testifies to the necessity of the organisms to protect the mitochondria of oocytes against the destructive role of the ROS that are produced during oxidative phosphorylation. The protection of the mitochondria in oocytes is essential for the transfer of healthy organelles to the next generation.
Highlights
Numerous studies on somatic tissues that have been carried out since the second half of the 20th c. have shown a direct connection of mitochondrial activity with their quantity in the cytoplasm of a cell [1, 2, 3, 4, 5, 6, 7]
Some studies on germ-line cells have reported a high level of activity of mitochondria in Xenopus and zebrafish oocytes [17, 18] or in the germinal plasm in Drosophila embryos [10] while other studies that were recently carried out on human, mouse, bovine and, Xenopus oocytes and eggs using confocal microscopy and substances that indicate the level of mitochondrial activity have shown that the mitochondria in germ-line cells behave in a different way than the mitochondria in somatic cells and that during oogenesis or at least at certain stages of this process, a large number of these organelles remains inactive [19, 20, 21, 22, 23]
At the free end of the ovary there are huge postvitellogenic oocytes wrapped with several layers of somatic cells in zone IV, (Fig. 1A, G), and after losing the cytoplasmic connections with the oocytes the trophocytes eventually degenerate (Fig. 1A, G)
Summary
Numerous studies on somatic tissues that have been carried out since the second half of the 20th c. have shown a direct connection of mitochondrial activity with their quantity (volume density, i.e. the percentage of the cell volume that is occupied by mitochondria) in the cytoplasm of a cell [1, 2, 3, 4, 5, 6, 7]. Nerve cells or liver cells have a higher relative volume density of mitochondria than bone or cartilage cells Based on these assumptions some researchers have studied germ-line cells in an attempt to interpret the energy consumption of those cells by studying mitochondria distribution within a cell [10, 11] or by determining the relative volume of mitochondria in the cytoplasm [12, 13, 14, 15]. The data from invertebrates are few and scarce [10, 43]
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