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Summary of the Main Patterns of Cleavage

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Summary of the Main Patterns of Cleavage Model organisms in development Cell and embryology A few have been studied extensively; each has advantages and disadvantages. Model Systems Xenopus laevis: development is independent (in vitro), easy catch and observation but poor genetics. Model organisms: vertebrates (frog, mouse, zebrafish) Chick: available, surgical manipulation and in vitro culture but poor Model organisms: invertebrates (sea urchin, Drosophila, nematode) genetics. Mouse: surgical manipulation, good genetics, transgenic model, Identifying development genes mammalian but development is in utero . Drosophila: great genetics, great development (recent Nobel Prize to Textbook: Wolpert L, Beddington R, Jessell T, Lawrence P, Meyerowitz E, Smith J. (2007) Principles of Development. 3th ed. London: Oxford university press. Lewis, Nusslein-Volhard & Wiechaus). C. elegans: has less than 1000 cells and is transparent. Gilbert SF. (2003) Development Biology. 7th ed. Sunderland: Sinaure Sea Urchin : in vitro Associates Inc. 1 Arabidopsis thaliana: flowering plant. 2 Summary of the main patterns of cleavage Lecithal 3 4 1 Model organisms: vertebrates All vertebrate embryos undergo a similar pattern of development. 1) fertilization 2) Cleavage (cell number ↑, but total mass X) Fig. 2. 1 3) blastulation (blastcoel formation and three germ layers) 4) gastrulation (where ectoderm covers embryo, endoderm and The skeleton of a mouse embryo illustrates the mesoderm are inside), A-P axis (body plan), notochord vertebrate body plan formation, embryo affected by yolk in egg. In mammalian, yolk to small but have extra-embryonic structure of placenta for nutrition. 5) Phylotypic stage, at which they all more or less resemble each other an show the specific features of notochord, somites and neural tube. Fig. 2.2 5 6 The phylotypic stage Xenopus laevis: egg At the end of gastrulation all embryos appear to be similar (the (Amphibians) phylotypic stage). Structures that are common to the phylotypic stage of the Advantage: easy observation, fertilized, catch (sperm, egg), low infection vertebrates are: 1) the notochord (an early mesoderm structure along A/P axis), The egg is composed of an animal and a vegetal Animal 2) the somites (blocks of mesoderm on either side of notochord region, bo th covere d by v ite lline mem brane (ge l which form the muscles of the trunk & limbs), coat). Fig.2.4 3) the neural tube - ectoderm above notochord forms a tube (brain Meiosis is stopped at 1st division with apparent 1 polar and spinal cord). body (the 2nd polar body comes after fertilization). Box 2A Extra- After fertilization, the cortex (the layer below plasma embryo membrane) rotates to determine future dorsal region nic at a position opposite to the site of sperm entry . vegetal Vertebrate embryo tissue to through a phylotypic state, but differences in form before Fig. 2.3 gastrulation 7 8 2 Box 2A Cleavage of a frog egg. 9 10 Early developmental stages of Xenopus laevis Xenopus laevis : fertilization and early growth 1. one sperm enters animal region (grow to embryo, plant pore to yolk) morula Blastula 2. completes meiosis 3. egg and sperm nuclei fuse 4. v ite lline mem brane lifts 5. yolk rotates down (15 minutes) 6. cortical rotation occurs (60 minutes). 囊胚 7. 1st cleavage occurs (90 mins) Animal / Vegetal (A/V) 8. Every 20 mins, one cleavage 2.5 hpfp 3.5 hpfp 5 hpfp 10 hpfp 9. 2nd cleavage (110 mins) A/V 90 degrees to 1st 10. 3rd cleavage (130 mins) equatorial (4 small animal and 4 large blastocoel - vegetal= 8 , it is blastomeres). 11. Continued cleavage → blastomeres ↓, cells at vegetal region hpf: hours post-fertilization large than those at the animal region. 11 12 3 Xenopus laevis: blastulation The blastula (after 12 divisions) has radial symmetry. The marginal zone will become mesoderm and endoderm. Marginal zone, the belt of tissue around the equator , plays a crucial part in future development. Internalization of the mesoderm and endoderm starts at the blastopore. Fig 2.3 Life cycle of the frog Xenopus laevis. In blastula stage, it is in the form of a hollow sphere with radial symmetry 13 14 Types of cell movement during gastrulation Xenopus laevis: gastrulation Gastrulation step: 1. Mesoderm and endoderm converge and begin to move inwards at dorsal lip of the blastopore. 2. Mesoderm and endoderm extend in along A/P axis. 3. Ectoderm spreads to cover embryo (epiboly). 4. Dorsal endoderm separates mesoderm from the space between the yolk cells, the archenteron (future gut). Do not forget, mesoderm come from ectoderm 5. Lateral mesoderm spread to cover inside of archenteron. 6. dorsal mesoderm is beneath dorsal ectoderm 7. mesoderm spread to cover gut 8. epiboly - ectoderm covers embryo 9. yolk cells are internalized (food source), dorsal mesoderm develops into a) notochord (rod along dorsal midline) and b) somites (segmented blocks of mesoderm along notochord). Invagination Blastopore Involution ↓ Ingression Archenteron ↓ Delamination Large Eiboly: ectoderm covers embryo ↓ Blastocoel ↓ Close ↓ 15 gut 16 4 Xenopus laevis: Neurulation • Neuralation or neural tube formation: 1) The neural plate is the ectoderm located above notochord and somites. 2) The edge of the neural plate forms neural folds which rise towards midline. 3) The folds fuse to form neural tube. 4) The neural tube sinks below epidermis. • The anterior neural tube becomes brain. Mid and posterior neural tube becomes spinal cord. Gastrulation → neurulation → neural plate → fold → tube notochord Neural crest cell Anterior posterior Autonomic nerves ↓ ↓ 17 Brain spinal cord18 Fig. 2.7 Neurulation in amphibian Xenopus laevis: Somites The somites formation, after neurulation The dorsal part of somites have ready begun to differentiate into dermatome (future dermis). The rest of each somite becomes vertebrae and trunk muscles (and limbs). Lateral plate mesoderm becomes heart, kidney, gonads and gut muscles. VlVentral meso dbderm becomes bldblood-fiiforming tissues. Also at this stage, the endoderm gives rise to the lining of the gut, liver & lungs. Brain andspinal Brain Fig. 2.8 A cross-section through a stage 22 Xenopus embryo just after gastrulation and neurlation are completed Notochord begins to form in the midline Neural plate develops neural folds 19 20 5 The major lineages of the mesoderm Xenopus laevis: tail bud stage • After gastrulation comes the early tail bud stage In the anterior embryo: a) the brain is divided, b) eyes and ears form, c) 3 branchial arches form (anterior arch later becomes the jaw. In the posterior embryo, the tail is formed last from dorsal lip of blastopore by extension of notochord, somites and neural tube. Circulatory body cavity Fig. 2. 9 The ear ly ta ilbu d system stage of Xenopus embryo Scler Myo tome Cartilage skeletal dermis 21 22 Schematic representation of neural crest formation Xenopus laevis : neural crest cells (in chick embryo) Neural folds meet and adhere Neural crest cells come from the edges of the neural folds after neural tube fusion. Neural crest cells can form from the dorsal side of the Cells at this junction form neural crest closed neural tube Neural crest cells detach and migrate as single cells between the mesodermal tissues to become: 1) sensory and autonomic nervous systems 2) skull 3) pigment cells Closure not simultaneous 4) Cartilage → bone Only vertebrate Cell adhesion molecular expressed dependent Closed tube detaches – change Epidermal and neural plate/tube interactions may generate crest cells in adhesion molecule 23 expression 24 6 Zebrafish ( Danio rerio) -- A Vertebrate Model •It is 3 cm long •Short generation time •Large clutch size •External fertilization •Transparent embryos •Rapid development http://zfin.org/ and http://www.nih.gov/science/models/zebrafish/ 25 26 Sphere 29h 48h 27 28 7 Fish (Zebrafish) embryo: •Human disease model •Transgenics •Reverse genetics tool Fig. 2.26 29 30 The development of Zebrafish Characterization of Fish embryo Telolecithal: most of the egg cell is occupied by yolk Zebrafish Meroblastic: the cell divisions not completely divide the egg development occurs Discoidal: since only the blastodisc becomes the embryo, this type of very rapidly. In 24 hr meroblastic cleavage is call discoidal. hours of embryogenesis, Cleavage can take place only in the blastodisc, a thin region of yolk free shown here, the 1 cytoplasm at the animal pole of the egg. cell zygote becomes into a vertebrate embryo with a tadpole-like form. Fig. 2.27 Cleavage of the zebrafish embryo 31 32 8 Fish embryo: blastula stage Three cell populations: At about the 10th cell division -- the onset of the About 10 cell division, the onset of mid-blastula transition: gene transcription MBT begins, divisions slow and cell move. And formed three distinct cell mid-blastula transition populations: 1. Yolk syncytial layer (YSL) (1)YSL (yolk syncytial layer): location of vegetal edge of the blastoderm and 2. Deep cells -- forming the embryos proper fusion produces a ring of nuclei within the part of the yolk cell cytoplasm 3. Enve lope layers (EVL) -- fithidlforming the epidermal that just beneath the blastoderm. It is important for directing some of the cell movement of gastrulation. ANIMAL BODY Internal YSL: the yolk syncytial nuclei move under the blastoderm External YSL: some cell move vegetally, stay ahead of the blastoderm margin (2)Enveloping layer (EVL): Made up of the most superficial cell from the blastoderm, which form an epithelial sheet a single cell layer thick. (3) Deep cells Blastoderm Both YSL and EVL are the deep cells, that give rise to the embryo 4 hpf: hours post-fertilization 33 proper. 34 Fish embryo: gastrulation The fate map of the deep cells after mixing has stopped The blastoderm at 30% completion of Internal epiboly (4.8 hr) YSL This stage, no mesoderm, ectoderm The fate of the early blastoderm cells are not determined. After much cell mixing during cleavage 35 36 9 Types of cell movement during gastrulation Close-up of the marginal region Formation of the hypoblast, either by involution of cells at the Invagination marggpgin of the epibolizing Involution balstoderm or by Ingression delamination and Delamination ingression of cells from Eiboly: ectoderm covers embryo the epiblast (6hr) The formation of germ layers is started.
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