Рис. 2 Перестройки и слияния эпидермальных клеток у эмбриона. Цилиндрическая проекция эпителиальных клеток, создающая область в центральную части гиподермиса (эпидермиса) эмбриона, разрезанного вдоль вентральаной срединной линии и вид сбоку. (A) Эпителиальные клетки расположены в 6 параллельных рядов. (B) Центральные два ряда клеток мигрируют и взаимно проникают в друг друга(interdigitate) образуя один ряд гиподермальных дорсальных клеток.(C) После небольших миграций гиподермальных клеток, дорсальные клетки (1-17) сливаются с вентральными клетками 18-21 (вокруг экскреторной поры) и с клетками 22-23 (вокруг ануса)для формирования hyp7. Синцитий Hyp6 образуется за счет слияния a-f клеток. Латеральные h5 клетки мигрируют и сливаются чтобы образовать hyp5. Вентральные h4 клетки сливаются, чтобы образовать hyp4. Слияние клеток происходит до и во время элонгации (morphogenesis). (D) Левая и правая клетки мечены по Sulston et al., [1983].

Рис. 3 Генерация, миграция и слияние постэмбрионральных эпителиальных клеток. Цилиндрические проекции гиподермиса первого личиночного возраста (L1) разрез вдоль дорсальной срединной линии открывает вид сбоку.(A) Вновь вылупившиеся L1 клетки мечены по Sulston and Horvitz ([1977]) и Sulston et al. ([1983]). Имеется два продольных ряда клеток, называемых seam cells (H0-H2, V1-V6, T), на обеих сторонах L1, и два ряда вентральных Р клеток (P1/2-P11/12) все погруженные в цилиндрический синцитий, возникший во время эмбриогенеза (hyp5, hyp6, hyp7). (B) Клетки seam округляются и (C) делятся спустя 5 ч после вылупления за исключением H0 слева и справа, которые не делятся. (D) Передние дочерние V2-V6 (слева и справа) посылают цитоплазматические отростки в направлении вентральной срединной линии вдоль соединений между P клетками, пока не встретятся со своими симметричными парами. (E) Примерно 3 ч спустя они генерируются, черные дочерние клетки сливаются с окружающим hyp7 синцитием, изолируя 6 пар Р клеток. Ремоделирование слипчивых соединений с помощью этих отростков сопровождается ротацией изолированных 6 пар Pn клеток и в результате происходит реорганизации 12 Р клеток в один передне-задний ряд вместо двух параллельных рядов (Podbilewicz and White, [1994]).Позднее в результате делений Р клеток их задние дочерние клетки также сливаются с hyp7 за исключением 6 клеток, которые образуют вульву (Sulston and Horvitz, [1977]). За исключением наиболее передних seam клетки подвергают делению стволовые клетки в начале каждой личиночной ствдии. Примерно 3 ч. спустя происходит рождение non-stem тетраплоидных дочерних клеток, которые сужены в своих апикальных поверхностях и сливаются с hyp7 (Hedgecock and White, [1985]; Podbilewicz and White, 1994). Одновременно каждая стволовая клетка удлинняется пока не достигнет соседней стволовой клетки чтобы сформировать непрерывный ряд seam клеток на каждой стороне червя. Этот паттерн делений, слияний и совместной элонгации повторяется на каждой личиночной стадии вплоть до mid-L4, когда 15 удлинненых seam клеток в каждом ряду сливаются продольно, формриуя два непрерывных латеральных синцития, погруженных в синцитиальный hyp7

Рис. 4 Schematic representation of mature uterine syncytial structures. (A) Ventral view showing ut toroids, utse, uv cells, and spermathecal-uterine valve (i.e., junction: sujn). Following the nuclear divisions of the DU and the VU descendants, the primordial uterus is comprised of specialized cells that lie proximal to its midline (the du cell and the cell descendants) and two distal epithelial rows, dorsal and ventral. These rows comprise the ut cells and can be divided into eight groups of cells: four anterior and four posteriors. Each group contains four cells (two pairs of dorsal and ventral cells in each side) except for the most distal groups that contain six cells each. The groups are not the result of cell migration but rather are formed as a consequence of the orientation of the cleavage planes. EM and confocal reconstructions have revealed that the cells in each group fuse during mid-L4 to form the ut1-ut4 toroid syncytia, four anterior and four posterior (Newman et al., [1996]). This includes fusions between the dorsal and ventral cells (FDV) and transverse fusions between the left and the right side of each toroid (FT). The cytoplasm of the toroids becomes thinner when the lumen forms and enlarges resulting in a bi-lobe tube-shaped uterus in the A-P axis with eight toroidal syncytia engulfing the uterine lumen. At the distal ends of the uterus (anterior and posterior) lie the toroidal spermathecal valves (sujn), which like the ut toroids, are formed from fusion between DU and VU descendants. (B) Transverse view of an hermaphrodite showing the uterus where it sits over the vulva. The uv1 cells are directly dorsal to the vulva, and the utse and uv2 are dorsal to it. Dorsal to the vulva lies the du syncytium, which serves as the most dorsal cell in the uterus midline. This syncytium is the result of homotypic longitudinal and transverse fusions (FLT) between four DU descendants. The lateral attachments of the H shaped utse and the vulE toroid to the seam are also shown. Vulval toroids are color coded as in Figure 5. Modified from Newman et al., [1996];

Рис. 5 Formation of the vulva. (A) Lateral view of an L3 larva. (B) A somatic gonadal cell, the anchor cell (AC), through inductive signaling induces three (P5.p, P6.p, and P7.p) of the six vulval precursor cells (VPCs), P3.p-P8.p, to adopt vulval fates. The non-vulval cells- P3.p, P4.p and P8.p undergo a single proliferation step and fuse with hyp7 (h cell fate). (C) The other cell fates (a-f) are related to the terminal structural differentiated states of the vulval rings and not necessarily to the pattern of cell division of the VPCs (1°, 2°, and 3° sublineages). (D-H) Short-range migrations at the onset of the L4 stage, starting from vulE precursors and ending with vulA precursors, in each half the cells migrate toward the center until meeting with their symmetrical pairs. (F) The precursors of vulA fuse in each half in a longitudinal orientation prior to their migration. (H) The precursors of vulC fuse in each half in a transverse orientation at the end of their migration. As a result of these migrations and the successive remodeling of adherens junctions, the inner cells are pushed dorsally forming a stack of seven rings (vulA-vulF). (I) Intratoroidal fusions. With the exception of vulB1 and vulB2, the cells within the rings fuse to from syncytial rings (toroids). The AC is a black circle in panels D-H that penetrates the apex of the vulva and fuses with the dorsal utse uterine cell, creating a hole that connects the uterus and the vulva throughout its length (see Fig. 4B). (J) Lateral view of an L4 larva showing the invagination of the vulva and the gonad arms. In the last molt, a mature vulva is obtained after eversion of the dorsal rings and connection of specific rings to other tissues (see Fig. 4B). (C-H) are dorsal views of the vulva primordium. (I) This is a three-dimensional view of the late-L4 vulva. Vulval toroids are color coded as in Figure 4. Modified from Sharma-Kishore et al., [1999]

Рис. 6 Hox genes pattern Pn.p cell fates in C. elegans. (A) A drawing of a C. elegans first stage larva showing the 12 ectodermal Pn.p cells (numbered 1-12) located in the ventral cord. (B) The pattern of Pn.p cell fusion and Hox gene expression in hermaphrodites. (C) Pattern of Pn.p cell fusion and Hox gene expression in males. See text for details. Hatched backgrounds indicate lin-39 and mab-5 expression domains; note that overlaps are crosshatched. White circles represent the syncytial fate and black circles represent the unfused fate. Modified from Ch'ng and Kenyon, [1999];

Рис. 7 (495x453) Figure 7. A working model for the polarized membrane fusion events that occur during morphogenesis of the male tail tip in C. elegans (ventral view). (A) A hypothetical anterior signal triggers morphogenesis (green arrow). Cell fusions initiate at the anterior adherens junctions and progress to the posterior part of the tail. (B) Fluid is transferred through tail tip cells by intracellular vesicular transport (yellow spheres). These vesicles fuse in the posterior region of the plasma membrane. Through vesicle docking and fusion events, fluid is secreted and accumulates in the extracellular space of the tail spike (C). The syncytium (light green cytoplasm) retracts towards the anterior (small arrows), the syncytium moves towards the anterior of the worm (large orange arrow) and the nuclei (blue ovals) are transported towards the anterior part past the phasmid openings (Ph).

Рис. 8 The fusomorphogenetic model. (A) Three cells fuse in an anterior to posterior sequence. A pore is formed between the membranes of the anterior pair of cells, the pore(s) dilate and then the membranes vesiculate. (B) The posterior cells fuse and follow the same ultrastructural steps shown for the anterior pair. Vesicles derived from the transverse membranes that fused together are specifically transported to the lateral membranes (arrows). (C) The vesicles dock and fuse with the lateral plasma membranes. These polarized fusion events are coupled to a change in cell shape that results in longitudinal elongation. (D) Cells may undergo changes in shape without cell fusion by de novo synthesis of membranes through the secretory pathway or by the use of existing reserves of vesicular organelles that will fuse when required. (E) Vesicles derived from the Golgi apparatus (blue crescent shaped organelles) are targeted directly to the lateral membranes (arrows) where they fuse (F). The specific fusion of Golgi-derived vesicles to the lateral domains of the plasma membrane (D-F) can bypass the necessity for cell fusion and membrane recycling (A-C). A combination of both strategies (A-C and D-F) and a balance between the two (G and H) may drive morphogenesis. Caenorhabditis elegans and other nematodes with syncytial hypodermis may use cell fusion followed by polarized membrane recycling (A-C). Marine nematodes like Enoplus brevis may use de novo synthesis of membranes coupled to polarized secretion. Blue circles, nuclei; red lines, fusing plasma membranes or recycled membranes through red vesicular carriers. Small blue circles, newly synthesized vesicles in D-F. Adapted from Podbilewicz ([2000]).

Рис. 9 Schematic representation (left panel) and Nomarski photomicrographs (right panel) of ventral epidermal cell fate specification in Caenorhabditis elegans and Pristionchus pacificus. (A) Caenorhabditis wild-type: the vulval equivalence group is formed by P(3-8).p; P(3,4,8).p have the 3° sublineage (yellow); P(5-7).p form the vulva with P(5,7).p having the 2° sublineage (red) and P6.p having the 1° sublineage (dark blue). P12.pa forms part of the rectum (light blue). (B) Pristionchus wild-type: P(1-4, 9-11).p undergo programmed cell death; P(5,7).p form the vulva; P7.p (pink) is limited in its developmental competence, P8.p (green) is incompetent to adopt vulval fates. (C) Caenorhabditis lin-39(n1872) mutant: all vulval precursor cells (VPCs), P(3-8).p, adopt the fate of their anterior-posterior lineage neighbors and fuse with the hypodermis. (D) Pristionchus lin-39(sy319) mutant: VPCs undergo programmed cell death. (E) Caenorhabditis mab-5(e1239) mutant: most animals have a normal vulva. However, P(7,8).p differ in their developmental competence from wild-type animals. (F) Pristionchus mab-5(tu74) mutant; P8.p remains competent to form vulval tissue and eventually differentiates (D; arrow). V, vulva; E, epidermal; F, fusion. The Nomarski photomicrographs show only the central body region. Reprinted from Trends in Genetics, volume 15, Eizinger et al., Evolutionary change in the functional specificity of genes, pp. 197-202.

Рис. 10The fusomorphogenetic hypothesis applied to organ formation in different organisms. Cell fusion coupled to polarized membrane recycling may be a morphogenetic force in vertebrates and invertebrates. (A) Anterior-posterior elongation of the C. elegans embryo may be similar to the formation of myotubes in muscles (dorsal view). (B) Several mononucleated osteoclast precursors may fuse to form giant multinucleated osteoclasts that recycle the membranes to the specialized domain where bone resorption occur. Syncytial giant cells are also generated in certain tumors. (C) Intratoroidal fusions in the vulva result in a change in ring shape. The resulting ring has a larger lumen because the lateral membranes may be inserted into the central apical domain by intracellular vesicular traffic followed by docking and fusion (dorsal view). (D) Transverse section of the elongating C. elegans embryo showing two dorso-ventral cell fusions and one ventral left-right cell fusion that result in a hypodermal ring cell. The lateral membranes are transferred to the basal (internal ring) domain and the cytoplasm becomes thinner as the embryo elongates (dorsal is toward the top of the page). (E) Fusomorphogenesis applied to the formation of the syncytial trophoblast of the placenta to accomplish dynamic changes in cell and organ shapes (transverse section). Blue circles, nuclei; red lines, fusing plasma membranes or recycled membranes through intracellular red vesicular carriers.