The fine structure of the growth stage oocytes of a lancelet (= amphioxus),Branchiostoma lanceolatum

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Abstract
Oocytes of the European lancelet (Branchiostoma lanceolatum) were fixed for transmission electron microscopy at Banyuls (French Mediterranean) in mid spring, which is just before the spawning season. Special attention is given to the growth stage (= diplotene) oocytes and to their relations with non-germinal cells of the ovary. At the time of year studied, the ovaries contain both relatively small (7–57 μm) and relatively large (97–127 μm) oocytes, but none of medium-size. In addition to the usual cell organelles (like free ribosomes, mitochondria, and endoplasmic reticulum), the oocytes include a number of more specialized structures. Nuage is present throughout the growth stage, both adjacent to the nucleus and scattered elsewhere in the cytoplasm. Other specialized structures do not begin to appear until the oocytes attain a certain diameter. The following structures are first detected at the following oocyte diameters: cortical granules, always in close association with Golgi complexes (20 μm); vacuoles containing presumed precursors of the vitelline layer (30 μm); yolk granules (35 μm); and a central vacuole within the nucleolus (50 μm). The cytoplasm of the largest oocytes includes a few annulate lamellae and some very unusual striated fibers resembling ciliary rootlets. These fibers, which are banded with a 70-nm periodicity (but are evidently not associated with centrioles), occur only at the animal pole of the oocyte, and their functional significance is unknown. The smaller oocytes have relatively smooth plasma membranes, except where endocytotic pits are abundant, whereas the larger oocytes bear abundant microvilli and are covered by a vitelline layer of dense, granular material. During oocyte growth, areas of contact between the oocytes and neighboring non-germinal cells shrink progressively. In the large oocytes, such contacts are limited to the animal pole, where a few processes from non-germinal cells pass through the vitelline layer and terminate on the oocyte plasma membrane at adhaerens type junctions (perhaps mixed with gap junctions). The possible functions and phylogenetic significance of such junctions are discussed.