Formation of polar cytoplasmic domains (teloplasms) in the leech egg is a three-step segregation process

Segregation and proliferation of mitochondria, leading to formation of the teloplasms (pole plasms), were studied in eggs of the leech T. rude by immunocytochemistry, fluorescent time lapse video imaging, confocal and electron microscopy. The translocation of mitochondria was analyzed after loading the egg with either Rhodamine 123 or a Mitotracker. Mitochondrial proliferation was assessed after pulse labeling with BrdU. The involvement of the cytoskeleton in the segregation process was determined by drug action. The teloplasms form during the first interphase as consequence of a 3-step sequential process of mitochondrial redistribution throughout the egg cytoplasm. The first step is a microtubule dependent process of ectoplasm thickening due to centrifugal mitochondrial transportation from the neighboring endoplasm. During the second step mitochondria move in the plane of the ectoplasm to become concentrated at the wall of rings (polar rings) and bands of contraction. This process appears to mostly depend on actin. The furrowing pattern of the egg during this step can be modified by cold treatment and seems to be determined during oogenesis. During the third step the ectoplasm flows to either of the poles in conjunction with bipolar displacement of the polar rings and shortening of the contraction bands. This step depends on both microtubules and microfilaments. Mitochondria of first interphase eggs have three special features: (1) they move in clusters, (2) their movement depends on both microtubules and microfilaments and (3) they proliferate continuously. During the first interphase the polarized meiotic egg becomes a bipolar cell.