Control of shape and pattern during the assembly of a large microtubule bundle. Evidence for a microtubule-nucleating-template

Electron microscopical observations on the ciliate Furgasonia blochmanni Fauré-Fremiet, 1967: Part II: Morphogenesis and phylogenetic conclusions

The ultrastructural events during cortical morphogenesis of Furgasonia blochmanni are studied by TEM. The kinetosomal proliferation in the somatic cortex at the beginning of morphogenesis produces kinetosomal triads. All kinetosomes of these triads have the same fibrillar systems as somatic monokinetids. Such somatic triads are also involved in the formation of the adoral membranelies of the opisthe. In the mature adoral membranelies the postciliary microtubules of the anterior and the middle kinetosomes of these triads as well as all kinetodesmal fibers are replaced by desmoses. Only the opisthe gets new adoral membranelles, since the parental adoral membranelles persist in the proter; however, the new paroral membrane of both the proter and the opisthe are newly formed as derivatives of the old paroral membrane. At the beginning of stomatogenesis the old paroral membrane divides into 2 parts of unequal length. The anterior part, which stays in the proter, splits longitudinally forming a new kinety 1' and the anlage of the new paroral membrane. In the adult cell the anterior kinetosomes of the paroral dyads (the right kinetosomes of the paroral membrane) are already orientated like somatic kinetosomes. Therefore, no rotation is necessary when these kinetosomes become part of the somatic monokinetids of kinety 1'. The posterior kinetosomes of the dyads of the anterior part of the paroral membrane (the anlage of a new paroral membrane of the proter) remain orientated perpendicularly to the longitudinal axis of the cell, an orientation which is necessary when these kinetosomes send postciliary microtubules towards the forming cytopharyngeal basket. All kinetosomes of the posterior part of the former paroral membrane also become orientated such that triplet 9 points to the left towards the presumptive oral opening of the opisthe. During stomatogenesis both kinetosomes of the new paroral membranes are rotated by 90° compared to the longitudinal axis of the cell and send postciliary microtubules towards the forming cytopharynx. In contrast to the adult cell, during stomatogenesis only the posterior kinetosomes of the paroral dyads are ciliated while the newly formed anterior kinetosomes are barren. At the end of stomatogenesis the cilia of the posterior kinetosomes are resorbed and new cilia grow at the anterior kinetosomes. During stomatogenesis all kinetosomes of the anlagen of the new paroral membranes possess postciliary microtubules, kinetodesmal fibers and in some cases transverse microtubules as well. Following stomatogenesis, the kinetodesmal fibers and transverse microtubules are resorbed, and the orientation of the anterior kinetosomes reverts from perpendicular to parallel to the paroral membrane axis. These data from F. blochmanni are compared with the ultrastructural data on morphogenesis from Paraurostyla, Tetrahymena and Coleps. Finally the phylogenetically significant characters obtained from studies on morphology and morphogenesis in F. blochmanni and other nassulid ciliates are discussed, and a "scheme of argumentation of phylogenetic systematics" is presented for the nassulids. It is concluded that F. blochmanni is correctly classified within the nassulid suborder Nassulina and that the Nassulida including F. blochmanni certainly are a monophyletic group within the subphylum Cyrtophora Small, 1976.

The patterning of ciliates

This past-presidential address considers the relationship between small-scale assembly and large-scale patterning in ciliates. Certain model examples of assembly-based patterning are reviewed, followed by the introduction of Williams and Honts' concept of "meta-assembly" as applied to the development of the oral apparatus. The major part of this review then explores two topics: the nature and origin of large-scale circumferential order, and the manner by which the large-scale order of cell directions influences the organization of membranelles of the oral apparatus. In this review, I summarize an existing formal description of large-scale positional order, allude to a more precise abstract theoretical model, and end with a brief discussion of the problem of searching for molecular mechanisms.

Origin and organization of pigment epithelial apical projections to cones in cat retina

The apical surface of the retinal pigment epithelial cells (RPE) in the cat extend long sheetlike membranes that wrap concentrically above and around cone outer segments forming the cone sheath. The origin and organization of these sheetlike projections were studied in serial sections by electron microscopy. The apical surface of the RPE cells was found to consist of a thin zone of anastomosing ridges, or microplicae, from which longer projections extend. The lamellar projections forming the cone sheath originate from the microplicae as small cytoplasmic tabs that rapidly expand into broader sheets. Growth of individual sheets to their final size and shape continues by lateral and longitudinal expansion, fusion, and subdivision of the membrane. The small area of connection to the cell body allows the lamellae to overlap and interdigitate in forming the complex organization of the sheath. Microfilaments but not microtubules extend into the apical processes. RPE cilia (9 + 0 microtubules) with associated basal bodies, striated rootlets, and microtubules mark the location of retinal cones. These structures may be part of a microtubule organizing center that participates in morphogenesis of the cone sheath. They also may be involved in anchoring the apical projections forming the sheath, or in the movement of apical projections during the phagocytosis of outer segment discs shed from cone tips.

Formation and positioning of surface-related structures in protozoa

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