The male gamete of the Gregarine Lecudina tuzetae has been studied with transmission electron microscopy and microcinematography. It is characterized by a flagellar axoneme of 6 + 0 pattern, a reduction of the chondriome, and the abundance of storage polysaccharide or lipid bodies.The movements of the flagella are of the undulating type and they are performed in the three dimensions of space. They are very slow, with a cycle time of about 2 s.The structure of the axoneme components are similar to those of flagella with a 9 + 2 pattern. Each doublet has overall dimensions of 350 x 220 ~; the space between the adjacent doublets is about 160 ~. The A subfiber bears arms like dynein arms. The diameter of the axoneme is about 1,000 ~. The basal body consists of a cylinder of dense material 2,500 ~ long and 1,300-1,400 ~ in diameter; a microtubule 200 ~ in diameter is present in the axis.This study shows that a 6 + 0 pattern can generate a flagellar movement. The mechanism of the flagellar movement of the male gamete of L. tuzetae does not require the presence of central microtubules and it would include molecular interactions of the dynein-tubulin type between the adjacent peripheric doublets.The slowness of the movements is discussed in terms of the axoneme's structure and its energy supply. Finally, the phylogenetic significance of this flagella is examined on the basis of the morphopoietic potentialities of the centriolar structures.L'armature fibrillaire ou axon~me des cils et des flagelles pr6sente le caract~re structural universel d'etre construit sur la base enn6an~me (9 doublets l~riph~riques + 2 fibres axiales), aussi la diff6r-ence entre ces deux organites concerne les mouvements. La flagelle a un mouvement ondulant. Le battement du cil est de type pendulaire, avec un coup actif et un coup de retour ou de repli.La constance du module 9 + 2 a ~:t6 6tablie par de nombreux travaux (35, 17, 1, 23)et maintes fois rappel6e dans les articles de revues (16,31, 3,59,61,9). Toutefois des exceptions au sch6ma 9 + 2 ont 6t6 signal6es; elles portent, soit sur le nombre de doublets p6riph6riques (48,2,12,39,47,43,8,65), soit sur le nombre de fibres axiales (57,2,58,13,36,11,26,40,28), soit sur la presence de microtubules ~ l'ext6rieur des doublets p6riph6-riques (2,39,47,40,18,7).Quand une exception au sch6ma 9 + 2 est signal6e, la question la plus importante concerne la motilit6 ou la non motilit6 du flagelle ou du cil. Bien que la conservation de la Iongueur des fibres p~riph6riques d6montr6e par les travaux de Satir (49, 51), permette de retenir la th6orie du glissement des fibres les unes sur les autres et non celle du raccourcissement des fibres, l'origine de la force de glissement n'est pas encore clairement pr6cis6e. Cette force de glissement pourrait btre engendr6e 492

Lecudina tuzetae is a parasitic protozoan (Gregarine, Apicomplexa) living in the intestine of a marine polychaete annelid, Nereis diversicolor. Using electron and fluorescence microscopy, we have characterized the dynamic changes in microtubule organization during the sexual phase of the life cycle. The gametocyst excreted from the host worm into seawater consists of two (one male and one female) gamonts in which cortical microtubule arrays are discernible. Each gamont undergoes multiple nuclear divisions without cytokinesis, resulting in the formation of large multinucleate haploid cells. After cellularization, approximately 1000 individual gametes are produced from each gamont within 24 h. Female gametes are spherical and contain interphase cytoplasmic microtubule arrays emanating from a gamma-tubulin-containing site. In male gametes, both interphase microtubules and a flagellum with "6 + 0" axonemal microtubules extend from the same microtubule-organizing site. At the beginning of spore formation, each zygote secretes a wall to form a sporocyst. Following meiotic and mitotic divisions, each sporocyst gives rise to eight haploid cells that ultimately differentiate into sporozoites. The ovoid shaped sporocyst is asymmetric and forms at least two distinctive microtubule arrays: spindle microtubules and microtubule bundles originating from the protruding apical end corresponding to the dehiscence pole of the sporocyst. Because antibodies raised against mammalian centrosome components, such as gamma-tubulin, pericentrin, Cep135, and mitosis-specific phosphoproteins, react strongly with the microtubule-nucleating sites of Lecudina, this protozoan is likely to share common centrosomal antigens with higher eukaryotes.

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Low density lipoproteins (LDL) were conjugated to colloidal gold to visualize the route for internalization of LDL in the cultured cells of human term placenta. Cells were obtained from placental villi (caesarian section) by a standard trypsin-DNase dispersion method followed in some cases by a Percoll gradient centrifugation step. Employing electron microscopy it was observed that after 3 days of culture, cells obtained by trypsin-DNase dispersion were a mixture of macrophages, mononucleated cells and large multinucleated cells. When the cells were incubated for 3 days after the Percoll purification, essentially multinucleated cells identical to the syncytiotrophoblast were present. The number of LDL receptor was increased by preincubation in medium with lipoprotein depleted serum. In binding experiments cells incubated at 4 degrees C for 2 h with medium containing gold LDL conjugates showed gold LDL attached to the plasma membrane without characteristic localization. After incubation with gold LDL at 37 degrees C for various times, the three cellular types showed ligand internalization. Gold LDL endocytosis involved first coated pits but also uncoated plasmalemmal invaginations. Then gold LDL was further observed in coated and non coated vesicles, smooth walled endosomes, multivesicular bodies and tubular vesicles. Lastly free gold particles were observed in lysosome like dense bodies. These results prove the internalization of gold LDL conjugates by human cultured placental cells, particularly by syncytiotrophoblast like multinucleated cells. This accumulation of LDL (the major cholesterol carrying protein in humans) is recognised to be responsible for the exogenous cholesterol supply indispensable to the progesterone biosynthesis and cellular growth of the placenta.