Fertilization is an initiating step in development. Zygote, with the new genetic potential and its new arrangement of cytoplasm, now starts production of the multicellular organism. Between the events of fertilization and events of organ formation are two vital stages: cleavage and gastrulation. During cleavage, rapid cell divisions separate cytoplasm of fertilized egg in numerous cells. These cells then undergo dramatic displacements during gastrulation, a procedure whereby they move to different parts of embryo and obtain new neighbors. During cleavage and gastrulation, major axes of embryo are determined, and cells start to obtain the respective fates. While cleavage always precedes gastrulation, axis formation can start as early as oocyte formation. It can be completed during cleavage or extend all way through gastrulation (as it does in Xenopus). There are 3 axes which require to be specified: the anterior-posterior (head-anus) axis, dorsal-ventral (back-belly) axis, and left-right axis. Different species specify these axes at different times, employing different mechanisms meridional and are perpendicular to each other.
Blastula formationin sea urchin:
Blastula stage of sea urchin development begins at 128-cell stage. Here cells form hollow sphere surrounding a central cavity, or blastocoel. By this time, all cells are the same size, micromeres having slowed down their cell division. Each cell is in contact with proteinaceous fluid of blastocoel on inside and with hyaline layer on outside. At this time, tight junctions join the once loosely connected blastomeres in the seamless epithelial sheet which totally encircles blastocoel As cells continue to divide, blastula remains one cell layer thick, thinning out. This is achieved by adhesion of blastomeres to hyaline layer and by the influx of water which expands blastocoel.
These rapid and invariant cell cleavages last through ninth or tenth cell division, depending on species. After that time, there is midblastula transition, when synchrony of cell division ends, new genes become expressed, and several of the non-dividing cells develop cilia on their outer surfaces. Ciliated blastula starts to rotate inside fertilization envelope. Soon afterward, differences are seen in cells. Cells at vegetal pole of blastula start to thicken, forming the vegetal plate. Cells of animal half synthesize and secrete the hatching enzyme which digests fertilization envelope. Embryo is now a free-swimming hatched blastula.
Fate maps and the determination of sea urchin blastomeres:
Cell Fate Determination:
The fate map of sea urchin embryo was initially developed by observing each of the cell layers and what its descendants turned into. Recent investigations have refined these maps by given fates of individual cells injected with fluorescent dyes like dil. These studies have shown that by 60-cell stage, most of the embryonic cell fates are specified, but that cells are not irreversibly committed. Animal half of the embryo consistently gives rise to ectoderm larval skin and its neurons. Veg1 layer produces cells which can enter in either ectodermal or endodermal organs. Veg2 layer gives rise to cells which can populate three different structures endoderm, coelom (body wall), and secondary mesenchyme (pigment cells, immunocytes, and muscle cells). First tier of micromeres generates primary mesenchyme cells which form larval skeleton, whereas second tier of micromeres contributes cells to coelom.
Though early blastomeres have constant fates in larva, most of these fates are attained by conditional specification. Only cells whose fates are determined autonomously are skeletogenic micromeres. If these micromeres are isolated from embryo and placed in test tubes, they will still form skeletal spicules. Furthermore, if these micromeres are transplanted in animal region of blastula, not only will their descendants form skeletal spicules, but transplanted micromeres will modify fates of nearby cells by inducing the secondary site for gastrulation.
In the normal embryo, veg2 cells become specified by micromeres, and they, in order, help state veg1 layer. Without veg2 layer, the veg1 cells are able to generate endoderm, but endoderm is not specified as foregut, midgut, or hindgut. Therefore, there seems to be cascade in which vegetal pole micromeres induce cells above them to become veg2 cells, and the veg2 cells induce cells above them to suppose veg1 fates. Therefore, micromeres undergo autonomous specification to turn into skeletogenic mesenchyme, and these micromeres generate initial signals which specify other tiers of cells. Identities of signaling molecules involved in the procedure are just now becoming known. Molecule responsible for specifying micromeres (and their skill to induce neighboring cells) seems to be β- catenin. First, during normal sea urchin development, β-catenin accumulates in nuclei of those cells fated to become endoderm and mesoderm accumulation is autonomous and can take place even if micromere precursors are separated from rest of embryo. Second, this accumulation seems to be liable for specifying vegetal half of embryo.
In sea urchin blastula, cell fates line up along animal-vegetal axis established in egg cytoplasm before fertilization. Animal-vegetal axis also seems to structure future anterior-posterior axis, with vegetal region sequestering those maternal components essential for posterior development. As first cleavage plane can be parallel, perpendicular, or oblique with respect to eventual dorsal-ventral axis, it is probable that dorsal-ventral axis is not specified until 8-cell stage, when there are cell boundaries which correspond to the positions. Interestingly, in those sea urchins which bypass larval stage to develop directly in juveniles, dorsal- ventral axis is specified maternally in egg cytoplasm.
Cleavage in Snail eggs:
Spiral holoblastic cleavage is characteristic of numerous animal groups, comprising annelid worms, some flatworms, and most molluscs. It varies from radial cleavage in many ways. First, cleavage planes are not parallel or perpendicular to animal vegetal axis of the egg; rather, cleavage is at oblique angles, forming a spiral arrangement of daughter blastomeres. Second, cells touch one another at more places than do those of radially cleaving embryos. In fact, they suppose the most thermodynamically stable packing orientation, much like that of adjacent soap bubbles. Third, spirally cleaving embryos generally experience fewer divisions before they start gastrulation, making it likely to follow fate of each cell of blastula. When fates of individual blastomeres from annelid, flatworm, and mollusc embryos were compared, several of the same cells were seen in same places, and their common fates were identical. Blastulae generated by radial cleavage have no blastocoel and are known as stereoblastulae.
Direction of snail shell coiling is handled by the single pair of genes. In snail Limnaea peregra, most individuals are dextrally coiled. Rare mutants showing sinistral coiling were found and mated with wild-type snails. These matings illustrated that there is right-coiling allele D, which is dominant to the left-coiling allele d. However, the direction of cleavage is determined not by the genotype of the developing snail, but by the genotype of the snail's mother. Genetic factors involved in snail coiling are brought to embryo by oocyte cytoplasm. It is the genotype of ovary in which oocyte develops which finds which orientation cleavage will take. When injected the small amount of cytoplasm from dextrally coiling snails in eggs of dd mothers, resulting embryos coiled to right. Cytoplasm from sinistrally coiling snails didn't influence right-coiling embryos.
Cleavage in Early Amphibian Development:
Cleavage in Amphibians:
Cleavage in most frog and salamander embryos is radially symmetrical and holoblastic, just like echinoderm cleavage. Amphibian egg, though, has much more yolk. This yolk, that is concentrated in vegetal hemisphere, is the impediment to cleavage. Therefore, first division starts at animal pole and slowly extends down in vegetal region. Third cleavage, as expected, is equatorial. Though, due to vegetally placed yolk, this cleavage furrow in amphibian eggs is not actually at equator, but is displaced toward animal pole. It splits frog embryo in four small animal blastomeres (micromeres) and four large blastomeres (macromeres) in vegetal region. This unequal holoblastic cleavage establishes two main embryonic regions: rapidly dividing region of micromeres near animal pole and more slowly dividing vegetal macromere area. As cleavage progresses, animal region becomes packed with several small cells, while vegetal region has only relatively small number of large, yolk-laden macromeres. The amphibian embryo having 16 to 64 cells is usually known as a morula. At 128-cell stage, the blastocoel becomes evident, and embryo is considered the blastula. In fact, formation of blastocoel has been traced back to very first cleavage furrow. The first cleavage furrow widens in animal hemisphere to generate small intercellular cavity which is sealed off from outside by tight intercellular junctions. This cavity expands in subsequent cleavages to become blastocoel. Blastocoel probably provides two main functions in frog embryos:
(1) It allows cell migration during gastrulation, and (2) it prevents cells beneath it from interacting prematurely with cells above it. When took embryonic newt cells from roof of blastocoel, in animal hemisphere, and placed them next to yolky vegetal cells from base of blastocoel, these animal cells distinguished in mesodermal tissue instead of ectoderm. As mesodermal tissue is usually formed from those animal cells which are adjacent to vegetal endoderm precursors, it appears plausible that vegetal cells influence adjacent cells to distinguish in mesodermal tissues. Therefore, blastocoel seems to prevent contact of vegetal cells destined to become endoderm with those cells fated to give rise to the skin and nerves. While these cells are dividing, numerous cell adhesion molecules keep the blastomeres together. One of the most important of these molecules is EP-cadherin. The mRNA for this protein is supplied in the oocyte cytoplasm.
Cleavage in Fish Eggs:
In fish eggs, cleavage happens only in blastodisc, a thin region of yolk-free cytoplasm at animal cap of the egg. Most of the egg cell is full of yolk. Cell divisions don't completely divide egg, so this kind of cleavage is known as meroblastic. As only cytoplasm of blastodisc becomes embryo, this kind of meroblastic cleavage is known as discoidal. The calcium waves started at fertilization stimulate contraction of actin cytoskeleton to squeeze non-yolky cytoplasm in animal pole of egg. This converts spherical egg in more pear-shaped structure, with the apical blastodisc. Early cleavage divisions follow the very reproducible pattern of meridional and equatorial cleavages.
These divisions are rapid, taking approx 15 minutes each. First 12 divisions happen synchronously, forming the mound of cells which sits at animal pole of the large yolk cell. These cells comprise the blastoderm. Originally, all cells maintain some open connection with one another and with underlying yolk cell so that reasonably sized (17-kDa) molecules can pass liberally from one blastomere to next. First of these is yolk syncytial layer (YSL). YSL will be significant for directing some of cell movements of gastrulation. Second cell population differentiated at midblastula transition is enveloping layer. It is composed of most superficial cells of blastoderm, which form the epithelial sheet the single cell layer thick. EVL eventually becomes periderm, an extraembryonic protective covering which is sloughed off during later development.
Cleavage in Bird:
Ever since Aristotle first followed 3-week development, domestic chicken has been a favorite organism for embryological studies. It is available all year and is easily raised. Furthermore, at any particular temperature, developmental stage can be correctly predicted. Therefore, large numbers of embryos can be attained at same stage. Chick embryo can be surgically manipulated and, as it forms most of its organs in ways extremely similarly to mammals, it has frequently served as the surrogate for human embryos. Fertilization of chick egg takes place in oviduct, before albumen and shell are secreted upon it. Egg is telolecithal (like that of fish), with the small disc of cytoplasm sitting atop large yolk. Like fish eggs, yolky eggs of birds suffer discoidal meroblastic cleavage. Cleavage takes place only in blastodisc, a small disc of cytoplasm 23 mm in diameter at animal pole of egg cell. Fast cleavage furrow seems centrally in blastodisc, and other cleavages follow to create the single-layered blastodenn. As in fish embryo, these cleavages don't extend in yolky cytoplasm, so early-cleavage cells are continuous with each other and with yolk at their bases. After that, equatorial and vertical cleavages divide blastoderm in the tissue five to six cell layers thick. These cells become linked together by tight junctions. Between blastodenn and yolk is the space known as subgerminal cavity. This space is developed when blastoderm cells absorb fluid from albumin and secrete it between themselves and yolk. At this phase, deep cells in center of blastodenn are shed and die, leaving behind the one- cell-thick area pellucida. This part of blastodenn forms most of actual embryo. Peripheral ring of blastoderm cells which have not shed the deep cells constitutes area opaca.
Cleavage in Mammal:
Mammalian eggs are among the smallest in animal kingdom, making them difficult to influence experimentally. Human zygote, for example, is only 100 μm in diameter hardly visible to eye and less than one-thousandth volume of the Xenopus egg. Also, mammalian zygotes aren't produced in numbers similar to sea urchin or frog zygotes, so it is hard to get enough material for biochemical studies. Generally, fewer than ten eggs are ovulated by the female at the given time. As the final hurdle, development of mammalian embryos is achieved within another organism, rather than in external environment. Only lately has it been possible to duplicate some of the internal conditions and observe growth in vitro.
Mammalian oocyte is released from ovary and swept by fimbriae in oviduct. Fertilization takes place in ampulla of oviduct, region close to ovary. Meiosis is completed at this time, and first cleavage starts about a day later. Cleavages in mammalian eggs are among slowest in animal kingdom approx 12/24 hours apart. In the meantime, the cilia in oviduct push the embryo toward uterus; first cleavages take place along this journey.
Additionally to slowness of cell division, there are numerous other characteristics of mammalian cleavage which differentiate it from other cleavage types. Second of these differences is unique orientation of mammalian blastomeres with relation to one another. First cleavage is the normal meridional division; though, in second cleavage, one of the two blastomeres splits meridionally and other divides equatorially. This kind of cleavage is known as rotational cleavage.
Mammalian blastomeres don't all divide at same time. Therefore mammalian embryos don't increase exponentially from 2- to 4- to 8-cell stages, but often have odd numbers of cells. Fourth, unlike almost all other animal genomes, mammalian genome is activated in early cleavage, and generates proteins essential for cleavage to take place. Most research on mammalian development has focused on mouse embryo, as mice are comparatively easy to breed throughout year, have large litters, and can be housed easily.
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