Topology of cleavage in the light of Pierre Curie principle

The stages of radial, spiral, and bilateral cleavage, including blastula, are considered as polyhedrons and projections of polyhedrons onto a plane: Wenn, Schlegel, and Euler projections. The blastula spatial organization is characterized by face numerals and Euler characteristics, as well by symmetry groups. The classes of equivalence of polyhedrons have been considered: duality and equal composition. The correspondence between different types of cleavage has been established by shift transformation on Schlegel projections and turn on a spherical noneuclidean surface. Determination of the prospective significance of blastomeres during cleavage was compared with the dichotomous division of the general notion in logic. This in view, the Wenn diagram of four figures has been reflected onto the sphere surface. Blastula faceting is interpreted as a reflection of hereditary information about the prospective significance of blastomeres onto a spherical surface. Topologically, the reflection represents a transformation of the linear orderliness of information contained in the genome into a two-dimensional orderliness of prospective properties on the blastula noneuclidean surface. Therefore, the elements of blastula symmetry can be considered as self in the sense of Pierre Curie principle. Cleavage stages are living colloid crystals.     

Essays on the application of nonclassical symmetries to natural biomorphs

The following symmetries are conventionally termed "nonclassical": conformal symmetry, inversion symmetry, Mikheev homology, curvilinear symmetry, colour symmetry, and antisymmetry. These symmetries are applicable to biomorphs differing in linear dimensions and proportions (such as the shells of bivalves, crustaceans, diatoms, etc.) and to such three-dimensional biomorphs as gastropod shells. These three-dimensional objects used to be substituted so far with two-dimensional images. The shift transformation causing inhomogeneous deformations is discussed in the framework of Mikheev homologies. Antisymmetry and colour symmetry are discussed by the example of flowers, duckweed plants, and the crustacean chela. Early stages of cleavage, like colloid crystals, may be represented as polyhedrons. They have Euler characteristics and face symbols, and, because of this, stages of cleavage may have symmetries of crystals. Mastering nonclassical symmetries may promote the progress of biosymmetrics. The potential of discrete (arithmetical) biomorphology in taxonomy and the potential of continuous (geomertical) biomorphology in biosymmetrics are discussed.     

An electron microscopic study of the development of amphioxus,Branchiostoma belcheri tsingtauense: Cleavage

Development of the Asian amphioxus, Branchiostoma belcheri tsingtauense, was investigated by scanning and transmission electron microscopy (SEM and TEM) from the fertilized egg through the blastula stage. The fertilized egg is spherical (mean diameter 115 μm after SEM preparation) and is covered with microvilli. Throughout cleavage, the second polar body remains attached to the animal pole. The cleavage type in this species is essentially radial, as revealed by SEM observations. At the third cleavage or 8-cell stage, and at later stages, a size difference between blastomeres in the animal and the vegetal halves is clearly discernible, but less marked than that reported for the European amphioxus, B. lanceolatum. During the period spanning the third to the fifth cleavage (8–32-cell) stages, blastomeres are arranged in tiers along the animal-vegetal axis. After the sixth cleavage, or 64-cell stage, the tiered arrangement of the blastomeres is no longer seen. At the 4-cell stage, the blastocoel or cleavage cavity is seen as an intercellular space, opening to the outside. The blastocoel remains open at the animal and the vegetal poles in later stages. Throughout early development, the cytoplasm of the blastomeres includes yolk granules, mitochondria, Golgi complexes, and rough and smooth endoplasmic reticulum. Chromatin in the interphase nucleus is not clearly demonstrated, and chromosomes in the mitotic phase are also extremely difficult to detect. As yet, regional differences have not been found in distribution and organization of cytoplasmic components with respect to prospective ectodermal, mesodermal, and endodermal areas in the fertilized egg and later cleaved embryos, although there are possibly fewer yolk granules in the region of the animal pole than in the vegetal polar zone.     

A fine structural study of cytodifferentiation during cleavage, blastula, and gastrula stages of Fundulus heteroclitus

The fine structure of cleavage, blastula, and gastrula stages of Fundulus heteroclitus was investigated. Cleavage blastomeres are relatively unspecialized, containing few or poorly developed organelles. Beginning in blastula stages, signs of differentiation were noted, including development of the endoplasmic reticulum and Golgi apparatus and appearance of a primary nucleolus and polyribosomes. More extensive structural specializations occur in gastrula stages, including further development of the endoplasmic reticulum and appearance of a granular component in the nucleolus. These changes are associated with cell differentiation and an increased capacity for protein synthesis, and may be preparatory to subsequent histogenesis. The periblast is a continuous syncytial cytoplasmic layer located between the blastodisc and yolk and is formed during late cleavage by incomplete division of the cytoplasm of the blastodisc. Cytoplasmic projections extend from the periblast (and from the basal region of cleavage blastomeres prior to formation of the periblast) into the yolk and function in uptake of yolk material in the absence of pinocytosis. Yolk material appears to be digested by the periblast and transferred into the segmentation cavity where it is available to the blastomeres. Protein granules, lipid droplets, glycogen, crystalline arrays, and multivesicular bodies are related to food storage and utilization by blastomeres. The yolk gel layer enclosing the yolk sphere was found to be a thin layer of cytoplasm continuous with the margin of the periblast and is renamed the yolk cytoplasmic layer.     

Indeterminate cell lineage of the zebrafish embryo

We have examined the clonal progeny descended from individual blastomeres injected with lineage-tracer dye in the zebrafish embryo. Blastomeres arising by the same cleavages in different embryos generated clones in which the types and positions of cells were highly variable. Several features of early development were correlated with this diversity in cell fate. There was no fixed relationship between the plane of the first cleavage and the eventual plane of bilateral symmetry of the embryo. By blastula stages the cleavages of identified blastomeres were variable in pattern. Moreover, cell fate was not easily related to the longitudinal and dorsoventral position of the clone in the gastrula. These results establish that single blastomeres can potentially generate a highly diverse array of cell types and that the cell lineage is indeterminate.     

Mechanism of an Alternate Type of Echinoderm Blastula Formation: The Wrinkled Blastula of the Sea Urchin Heliocidaris erythrogramma

While most indirect-developing echinoderms (possessing a feeding larval stage) form a hollow, smooth-walled blastula, most direct-developing species form a wrinkled blastula. The process of wrinkled blastula formation was examined in the direct-developing sea urchin, Heliocidaris erythrogramma . Approximately 5 hrs after fertilization the blastula epithelium contains folds along one, two or three orthogonal planes, which superficially appear like 2-, 4- or 8-cell stages, respectively. Microinjection of fluorescent dye into individual blastomeres of 2-, 4- and 8-cell embryos revealed that the wrinkles correspond with the first, second and third cleavage planes. Two factors appear to generate the wrinkled blastula epithelium. First, blastomeres undergo a partial separation along the first, second and third cleavage planes during early cleavage. Subsequent cell divisions are oriented such that the blastula epithelium is constructed with deep creases along these planes of cell separation. Second, there is no room for the expansion of the developing blastoderm within the tightly fitting fertilization envelope. Prior to hatching from the fertilization envelope, wrinkles in the blastula epithelium disappear, due to an increased packing and elongation of the cells. In addition, a substantial volume of cellular material is removed as lipids are secreted into the blastocoel in an apocrine fashion.     

Analysis of the embryonic development of Pogonophora in connection with the problems of phylogenetics

The development of the mesoderm in the Pogonophora being a point of argument, some stages of their ontogenesis are analyzed. The cleavage of Siboglinum caulleryi is considered as a modified spiral cleavage with the demonstration of the prospective significance of blastomeres. All mesoderm in Pogonophora is formed in enterocoelic mode from the anterior quadrant B. The spiral cleavage of pogonophores is compared to that of the Polychaeta and other animals. Some aspects of the formation and structure of the telosoma in the larva and adults are analyzed with a discussion of the nature of its segmentation. Some general problems of the evolution of the spiral cleavage are considered. The division of the Coelomata into 5 superphyla is confirmed, the Pogonophora being one of them.     

Formation of nucleus-like structure in the cytoplasm of lambda-DNA-injected fertilized eggs and its partition into blastomeres during early embryogenesis in Xenopus laevis

The fate of bacteriophage lambda-DNA was examined after injection into the fertilized eggs of Xenopus laevis. Injection of a large amount of lambda-DNA (ca. 24 ng) into a fertilized Xenopus egg induced the formation around the injected DNA of a giant nucleus-like structure which was surrounded by an apparently normal bilayered nuclear membrane with nuclear pore complexes. Southern blot analysis revealed the persistence of injected lambda-DNA until the blastula stage. The nucleus-like structure was partitioned into blastomeres during cleavage through a process of nuclear fission, and was maintained in a group of extraordinarily large blastomeres until the blastula stage.     

Periodic appearance and disappearance of microvilli associated with cleavage cycles in the egg of the ascidian, Halocynthia roretzi

The surface of eggs of the ascidian Halocynthia roretzi, observed with SEM, is essentially smooth until immediately before cell division when numerous microvilli appear and remain during cytokinesis. As the dividing blastomeres become closely adherent, however, the microvilli disappear and the eggs recover their smooth surface. This periodic appearance-disappearance of microvilli is repeated at each cleavage cycle up to at least the 32-cell stage. During blastomere adhesion, microvilli that have appeared near the plane of the first cleavage or of the bilateral symmetry seem to fuse together across the plane to form a zipper-like complex of cytoplasmic processes, which might be responsible for attachment of the two halves of these bilaterally symmetrical embryos via the blastomeres bordering the plane of symmetry.     

Structural and ultrastructural analysis of embryonic development of Prochilodus lineatus (Valenciennes, 1836) (Characiforme; Prochilodontidae)

This survey was performed to characterize the embryogenesis of Prochilodus lineatus. Seven stages of embryo development were identified--zygote, cleavage, blastula, gastrula, segmentation, larval and hatching--after a period of incubation of 22 h (24 degrees C) or 14 h (28 degrees C). The following cleavage pattern was identified: the first plane was vertical (2 blastomeres); the second was vertical and perpendicular to the first (4 blastomeres); the third was vertical and parallel to the first (4 x 2); the fourth cleavage was vertical and parallel to the second (4 x 4); the fifth was vertical and parallel to the first (4 x 8); and the sixth cleavage was horizontal (64 blastomeres). At the blastula stage (3.0-4.0 h (24 degrees C); 1.66-2.0 h (28 degrees C)) irregular spaces were detected and periblast structuring was initiated. At the gastrula stage (4.0-8.0 h (24 degrees C); 3.0-6.0 h (28 degrees C)) the epiboly, convergence and cell movements, as well as the formation of embryonic layers, had begun. The segmentation stage (10.0-15.0 h (24 degrees C); 7.0-10.0 h (28 degrees C)) was characterized by a rudimentary formation of organs and systems (somites, optic vesicle and intestinal delimitation). The embryo at the larval stage (16.0-21.0 h (24 degrees C); 11.0-13.0 h (28 degrees C)) showed a free tail, more than 25 somites, an optic vesicle and a ready-to-hatch larval shape. The blastomeres at cleavage stage had disorganized nuclei indicating high mitotic activity. At gastrula, the blastomeres and the periblast had euchromatic nuclei and a large number of mitochondria and vesicles. The yolk was organized into globose sacs, which were dispersed into small pieces prior to absorption.     

Orientation of the cleavage spindles in pulmonate mollusks. II. The role of the architecture of the intercellular contacts in III and IV cleavage spindle orientation

In the normal development of pulmonate molluscs, the variety of orientations of the III and IV cleavage spindles markedly reduces in the process of transition from pro- to meta- and anaphase. Even prior to the completion of spindle growth in these cleavage divisions the external faces of blastomeres become asymmetrical due to intercellular interactions but the whole system of external intercellular contacts (edges) is characterized by a certain symmetry. This symmetry coincides partially or fully with that of the system of definitive spindles. In the artificially obtained four-celled common pond snail embryos with the chain-like position of blastomeres, the III cleavage spindles were oriented at right angles to the polar axis, rather than in parallel with it (as in the normal development). The eight-celled embryos with symmetrical external faces of macromeres were also obtained. The variety of orientation of the IV cleavage definitive spindles in such embryos was markedly widened and in the macromeres with inverse asymmetry the inversion of the sign of declination of the spindles was observed. The spindle orientation depends, thus, on the form of adjacent region of the external face. This form as a whole is determined by the mutual position of blastomeres, curvature of surface and relative length of the face edges.     

Electron microscopic studies of giant nucleus-like structure formed by lambda DNA introduced into the cytoplasm of Xenopus laevis fertilized eggs and embryos

When bacteriophage lambda DNA was injected into the cytoplasm of the fertilized egg of Xenopus laevis, giant nucleus-like structures were assembled around the injected DNA. These nucleus-like structures survived during cleavage and were partitioned into blastomeres at the blastula stage. The nucleus-like structures formed in the uncleaved fertilized eggs and the blastula cells were both surrounded by a bilayer nuclear membrane with nuclear pore complexes. The ultrastructural features of the lambda DNA-induced nucleus-like structure were considerably different from those of the normal blastula nucleus: although the nuclear pore complexes appeared to be normal, the 'nucleoplasm' was much too homogeneous as compared with that of the normal nucleus.     

三疣梭子蟹胚胎发育早期的组织学研究

对三疣梭子蟹（Portunus trituberculatus)胚胎发育早期（卵裂至原肠胚期）进行了组织学观察。结果发现：卵排至体外约５２h后开始卵裂,卵裂方式为表面卵裂,卵裂至２５６细胞时,胚胎发育进行了囊胚期。囊胚为实囊胚,囊胚后期,１６个排成例嗽叭形的预定内胚层细胞与聚在其附近的其他细胞一起内陷形成原肠。预定内胚层细胞脱离原肠后,进行１次切向分裂,形成卵黄细胞和内胚层细胞,与此同时,胚工细胞不断分裂,产生视叶原基和胸腹原基,不久,２个胞腹原基逐渐愈合形成胸腹突。随胚胎发育,在似桥细胞带上出现大颚原基、大触角原基,随后大大触角原基与视叶原基之间的腹中线上发生口凹,在小触角原基产生后,胚肥发育进入卵内无节幼体期。     

Structural and functional polarity of starfish blastomeres

The cortex of the blastomeres of Asterina pectinifera are structurally polarized so that some kinds of granules in the cortex, which can be stained vitally with Nile blue (Nile blue-positive granules, NBGs), and microvilli were distributed mainly in the apical region. The blastomeres always faced the adjoining blastomeres and blastocoel with the NBG-free, smooth region during embryogenesis. To confirm whether such blastomeres are functionally polarized, we rotated one of the blastomeres in the 2-cell-stage embryo so that it faced the other with the NBG-containing region. As a result, all embryos developed into twin or partitioned blastulae. This shows that the blastomeres are functionally polarized and have to orient the basal cortex toward the inner side of the embryo in order to be integrated into a blastula together with the others. The cortical polarity was formed and maintained even in blastomeres of dissociated embryos. In such blastomeres the cleavage furrows were formed along the axis of polarity. When the blastomeres began to adhere closely to each other at the 256-cell stage, only the NBG-free (basal) region acquired adhesiveness. These facts make it possible to infer why the correct apicobasal orientation of blastomeres is necessary for embryonic integration, without considering intercellular communication during the cleavage stage.     

Pluripotency of cryopreserved blastomeres of the goldfish

To examine the pluripotency of cryopreserved blastomeres, we transplanted them into blastula. Donor blastomeres were prepared from blastula of goldfish (Carassius auratus) and cryopreserved in liquid nitrogen for two months. Fifty-five percent and 44% of blastomeres survived after thawing. Cryopreserved blastomeres were transplanted to the blastula of triploid crucian carp (C. a. longsdorfii), which reproduces gynogenetically in nature. At four days after the operation, resultant chimeric embryos transplanted with cryopreserved blastomeres showed a survival rate (41.6%) lower than that of embryos transplanted with unfrozen blastomeres (57.1%). Transplanted blastomeres were histologically identified in various organs derived from all three germ layers. A primordial germ cell differentiated from a cryopreserved blastomere was detected in one of the 32 chimeric fish examined. These results suggest blastomeres that survive after cryopreservation retain their pluripotency and are able to differentiate into both somatic and germ cell lines.     

Surface activity and locomotion of Fundulus deep cells during blastula and gastrula stages

The surface activity and locomotion of deep cells of the Fundulus blastoderm were studied in vivo with time-lapse cinemicrography. During late cleavage, the surfaces of the blastomeres begin to undulate gently. By early blastula, these undulations increase gradually in amplitude and hemispherical surface protrusions called blebs appear. These blebs form and retract rapidly, and at mid blastula some may be seen adhering to the surfaces of other cells. At the same time, they often expand into elongate lobopodia. Cell locomotion is first evident in mid blastula and continues throughout gastrulation. During locomotion, the leading edge of a deep cell behaves in various ways. When blebs and lobopodia adhere to a substratum (other deep cells, the undersurface of the enveloping layer, or the periblast) and retract, the cell may move in the direction of the shortening cell process. Alternatively, blebs and lobopodia may adhere, but not shorten. Locomotion is accomplished rather by protoplasmic flow into the protrusion. Blebs and lobopodia also may flatten and spread on the substratum as lamellipodia. Variations in the contact and locomotory behavior of deep cells and in the rate of their movement during blastula and gastrula stages are described in detail.     

Mesoderm formation by isolated and cultivated 8-cell stage blastomeres of the teleost, Leucopsarion ptersii (shiro-uo).

Isolation of cleavage-stage blastomeres and the study of their developmental potential has been used extensively for analyzing the mechanisms of embryogenesis in vertebrates, including amphibians and echinoderms. We devised a method to isolate 8-cell stage blastomeres in the teleost, shiro-uo, by utilizing its unique cleavage pattern of the horizontal 3rd cleavage plane. Removal of all the upper blastomeres at the 8-cell stage allowed almost normal embryogenesis from the remaining lower blastomeres and yolk cell mass. Isolated upper or lower blastomeres formed vesicles and spherical bodies, which later showed morphological changes during cultivation. Mesoderm formation was detected not only in the cultivated lower blastomeres or whole blastomeres but also in the upper blastomeres isolated from the yolk cell mass at the 8-cell stage, although at a lower frequency than the lower blastomeres. These results indicated the presence of very early signaling for mesoderm induction, which is independent from the currently postulated signals from the yolk syncytial layer at later stages. This also indicated non-equivalence or differentiation of the blastomeres from the very early cleavage stage in teleost embryos.     

CHANGES IN PROTOPLASMIC CONSISTENCY AND THEIR RELATION TO CELL DIVISION

1. The development of the amphiaster is associated with the formation of two semisolid masses within the more fluid egg substance. 2. The elongation of the egg during cleavage is possibly produced as a consequence of the mutual pressure of these two growing semisolid masses. 3. The division of the egg into two blastomeres consists essentially in a growth, within the egg, of two masses of material at the expense of the surrounding cytoplasm. When all the cytoplasm of the egg is incorporated in these two masses cleavage occurs. 4. After a certain period of time the semisolid masses revert to a more fluid state. In the eggs studied this normally occurs after the cleavage furrow has completed the separation of the two blastomeres. The formation of the furrow, however, may be prevented in various ways, upon which the egg reverts to a single spherical semifluid mass containing two nuclei. 5. An egg mutilated during its semisolid state (amphiaster stage) may or may not revert to a more fluid state. If the more solid state is maintained, the cleavage furrow persists and proceeds till cleavage is completed. If the mutilation causes the egg to revert to the more fluid state the furrow becomes obliterated and a new cleavage plane is subsequently adopted. 6. The nuclei of eggs in the semifluid state are able to alter their positions. In semifluid mutilated eggs the nuclei tend to move to positions which may assure symmetry in aster formation and cleavage.     

Cell polarity emerges at first cleavage in sea urchin embryos

In protostomes, cell polarity is present after fertilization whereas most deuterostome embryos show minimal polarity during the early cleavages. We now show establishment of cell polarity as early as the first cleavage division in sea urchin embryos. We find, using the apical markers G_M1, integrins, and the aPKC-PAR6 complex, that cells are polarized upon insertion of distinct basolateral membrane at the first division. This early apical-basolateral polarity, similar to that found in much larger cleaving amphibian zygotes, reflects precocious functional epithelial cell polarity. Isolated cleavage blastomeres exhibit polarized actin-dependent fluid phase endocytosis only on the G_M1, integrin, microvillus-containing apical surface. A role for a functional PAR complex in cleavage plane determination was shown with experiments interfering with aPKC activity, which results in several spindle defects and compromised blastula development. These studies suggest that cell and embryonic polarity is established at the first cleavage, mediated in part by the Par complex of proteins, and is achieved by directed insertion of basolateral membrane in the cleavage furrow.