Formation of the embryonic-abembryonic axis of the mouse blastocyst: relationships between orientation of early cleavage divisions and pattern of symmetric/asymmetric divisions

Setting aside pluripotent cells that give rise to the future body is a central cell fate decision in mammalian development. It requires that some blastomeres divide asymmetrically to direct cells to the inside of the embryo. Despite its importance, it is unknown whether the decision to divide symmetrically versus asymmetrically shows any spatial or temporal pattern, whether it is lineage-dependent or occurs at random, or whether it influences the orientation of the embryonic-abembryonic axis. To address these questions, we developed time-lapse microscopy to enable a complete 3D analysis of the origins, fates and divisions of all cells from the 2- to 32-cell blastocyst stage. This showed how in the majority of embryos, individual blastomeres give rise to distinct blastocyst regions. Tracking the division orientation of all cells revealed a spatial and temporal relationship between symmetric and asymmetric divisions and how this contributes to the generation of inside and outside cells and thus embryo patterning. We found that the blastocyst cavity, defining the abembryonic pole, forms where symmetric divisions predominate. Tracking cell ancestry indicated that the pattern of symmetric/asymmetric divisions of a blastomere can be influenced by its origin in relation to the animal-vegetal axis of the zygote. Thus, it appears that the orientation of the embryonic-abembryonic axis is anticipated by earlier cell division patterns. Together, our results suggest that two steps influence the allocation of cells to the blastocyst. The first step, involving orientation of 2- to 4-cell divisions along the animal-vegetal axis, can affect the second step, the establishment of inside and outside cell populations by asymmetric 8- to 32-cell divisions.     

C. elegans HAM-1 positions the cleavage plane and regulates apoptosis in asymmetric neuroblast divisions

Asymmetric cell division occurs when a mother cell divides to generate two distinct daughter cells, a process that promotes the generation of cellular diversity in metazoans. During Caenorhabditis elegans development, the asymmetric divisions of neural progenitors generate neurons, neural support cells and apoptotic cells. C. elegans HAM-1 is an asymmetrically distributed cortical protein that regulates several of these asymmetric neuroblast divisions. Here, we show that HAM-1 is a novel protein and define residues important for HAM-1 function and distribution to the cell cortex. Our phenotypic analysis of ham-1 mutant embryos suggests that HAM-1 controls only neuroblast divisions that produce apoptotic cells. Moreover, ham-1 mutant embryos contain many unusually large cell-death corpses. An investigation of this corpse phenotype revealed that it results from a reversal of neuroblast polarity. A misplacement of the neuroblast cleavage plane generates daughter cells of abnormal size, with the apoptotic daughters larger than normal. Thus, HAM-1 regulates the position of the cleavage plane, apoptosis and mitotic potential in C. elegans asymmetric cell divisions.     

Slow calcium waves accompany cytokinesis in medaka fish eggs

Animal cells are cleaved by the formation and contraction of an extremely thin actomyosin band. In most cases this contractile band seems to form synchronously around the whole equator of the cleaving cell; however in giant cells it first forms near the mitotic apparatus and then slowly grows outwards over the cell. We studied the relationship of calcium to such contractile band growth using aequorin injected medaka fish eggs: we see two successive waves of faint luminescence moving along each of the first three cleavage furrows at approximately 0.5 micron/s. The first, narrower waves accompany furrow extension, while the second, broader ones, accompany the subsequent apposition or slow zipping together of the separating cells. If the first waves travel within the assembling contractile band, they would indicate local increases of free calcium to concentrations of about five to eight micromolar. This is the first report to visualize high free calcium within cleavage furrows. Moreover, this is also the first report to visualize slow (0.3-1.0 micron/s) as opposed to fast (10-100 microns/s) calcium waves. We suggest that these first waves are needed for furrow growth; that in part they further furrow growth by speeding actomyosin filament shortening, while such shortening in turn acts to mechanically release calcium and thus propagates these waves as well as furrow growth. We also suggest that the second waves act to induce the exocytosis which provides new furrow membrane.     

Calcium signaling during the early meroblastic cleavages of zebrafish and medaka embryos

The regulation of cytokinesis ingianf' embryonic cells(i.e.,> 500 μm in diameter)presents exacting challenges that include long-range signaling with respect to time and space; the transport and assembly,followed by disassembly,of an extensive contractile apparatus; and the remodeling and addition of new surface membrane to the resulting daughter cells.     

Cortical development and asymmetric cell divisions

The development of the mammalian neocortex involves rounds of symmetric and asymmetric cell division of neural progenitors to fulfill needs of both self-renewal of progenitors and production of differe...     

Infertile couples with Robertsonian translocations: preimplantation genetic analysis of embryos reveals chaotic cleavage divisions

Preimplantation genetic diagnosis (PGD) may provide a feasible option for some Robertsonian translocation carriers who experience severe difficulty in achieving a normal pregnancy. We report on five PGD cycles for two such couples, 45,XY,der(13;14)(q10:q10) and 45,XX,der(13;21)(q10;q10), carried out by biopsy of two cells from day 3 post-insemination embryos generated by in vitro fertilisation. Locus-specific YAC probes for chromosomes 13, 14 and 21 were used to detect the chromosomes involved in the translocation using multicolour FISH. Three embryos transfers were carried out (two single embryo transfers and one double transfer) but no clinical pregnancies were established. In two cycles no embryos were transferred as all those biopsied were chromosomally abnormal. Combined results from both couples show 13% (6/45) of embryos analysed were normal for the translocation chromosomes and 87% (39/45) were chromosomally abnormal; these were categorised as 36% aneuploid or aneuploid mosaic and 51% chaotic where the chromosome constitution varied randomly from cell to cell. This suggests two factors may be acting to reduce fertility in these couples; the aneuploid segregation of the parental Robertsonian translocation and also a post-zygotic factor leading to uncontrolled chromosome distribution in early cleavage stages in an exceptionally high proportion of embryos. Received: 24 September 1997 / Accepted: 22 October 1997     

A model for cleavage plane determination in early amphibian and fish embryos

Current models for cleavage plane determination propose that metaphase spindles are positioned and oriented by interactions of their astral microtubules with the cellular cortex, followed by cleavage in the plane of the metaphase plate [1, 2]. We show that in early frog and fish embryos, where cells are unusually large, astral microtubules in metaphase are too short to position and orient the spindle. Rather, the preceding interphase aster centers and orients a pair of centrosomes prior to nuclear envelope breakdown, and the spindle assembles between these prepositioned centrosomes. Interphase asters center and orient centrosomes with dynein-mediated pulling forces. These forces act before astral microtubules contact the cortex; thus, dynein must pull from sites in the cytoplasm, not the cell cortex as is usually proposed for smaller cells. Aster shape is determined by interactions of the expanding periphery with the cell cortex or with an interaction zone that forms between sister-asters in telophase. We propose a model to explain cleavage plane geometry in which the length of astral microtubules is limited by interaction with these boundaries, causing length asymmetries. Dynein anchored in the cytoplasm then generates length-dependent pulling forces, which move and orient centrosomes.     

Comparison of the size of paternal and maternal homologous chromosomes during the first 2 cleavage divisions in mouse embryos

The mouse metaphase chromosomes of the 1st and 2nd cleavage divisions were prepared without colchicine and stained with trypsin-Giemsa. Both the homologues had the same pattern of differential staining (position and number of bands and interbands) in all pairs of chromosomes. The measurements of homologues of the 1st, 2nd, 3rd, 4th and 5th pairs of autosomes have shown that at the first cleavage division metaphase the paternal chromosomes are 1.2 times, on the average longer than the maternal ones, whereas at the second division metaphase no reliable differences in the length of homologues were found. In mice, thus, the heterocyclic pattern of the paternal and maternal sets of chromosomes manifested itself during the 1st cleavage division only and disappeared fully beginning from the 2nd division. This appears to be due to the early functional activity of chromosomes, i.e. to the fact that already in the 2-cell embryos both the maternal and paternal genes are expressed.     

中枢神经系统发育的不对称细胞分裂机制

无论在无脊椎动物还是脊椎动物中,组成中枢神经系统（CNS）的大多数细胞都是由极性神经祖细胞不对称分裂而来。通过简要综述果蝇（Drosophila melanogaste）成神经母细胞（NB）不对称分裂机制,并与近年来在脊椎动物不对称细胞分裂上取得的研究成果相比较,尝试找出两个系统的相似性和相异性。     

Evidence for asynchronous mitotic cell divisions in secondary spermatogonia of Drosophila

Examination of germ cell numbers within premeiotic as well as postmeiotic cysts of various Drosophila species gave evidence against any strict synchrony of mitotic cell division in secondary spermatogonia. The evidence was based on numbers of germ cells in primary spermatocyte cysts and spermatid bundles. Each species examined had its own distribution of primary spermatocyte cyst types in pupal testes, and the most common cyst type did not necessarily contain 2, 4, 8, 16 or (2) ⁿ germ cells which implies asynchrony of the previous spermatogonial divisions. Similar but not exactly the same distributions of germ cells were found in adult spermatid bundles, if allowances were made for a 4-fold increase in germ cell number during meiosis. This observation gives support to the operation of an age-dependent factor which controls germ cell numbers within cysts [1], The data thus suggest that the commonly accepted concept of a (2) ⁿ increase of spermatogonia via synchronous mitotic divisions is not true for the species of Drosophila studied.