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UNC-Chapel Hill BIOL 205 - Lecture 3 Early Cleavage Patterns

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BIOLOGY 205/SECTION 7 DEVELOPMENT-LILJEGRENLecture 3Early Cleavage PatternsCleavage and Gastrulation are two critical stages of embryogenesis. ie ~18% of fertilizedembryos fail to complete cleavage and become successfully implanted.1. After fertilization, early development in most animals starts with a series ofrapid cell divisions called CLEAVAGE that serves several purposes:a. rapidly increases cell number, dividing the cytoplasm of the egg into smallerand smaller cells called blastomeres i. Length of "cell cycle" is regulated in most cell types. ii. In the early embryo it's much shorter- many divisions very rapidly,ie. in Drosophila cleavage-stage embryos, mitosis every 10minutes for >2 hours iii. How so fast? No growth phases (just mitosis and DNA synthesis) &relies on stored material- no transcription. So faster, and no volumeincreases iv. Transition from fertilization to cleavage caused by activation of MPF(mitosis-promoting factor). Active MPF dependent on presence ofcyclin B, which accumulates during S and is then degraded after cellsreach M phase. v. Cyclin regulators stored in egg cytoplasm, so cell cycle doesn’t requirezygotic transcription for numerous cell divisions. We’ll talk about thismore when we discuss the Mid-blastula transition.b. distributes egg contents, often asymmetrically, to the cells2. What controls the process of cleavage?c. The cleavage plane forms perpendicular to the mitotic spindles(microtubules made of tubulin).d. The contractile ring (microfilaments made of actin) that forms creates acleavage furrow.e. Placement of the mitotic spindle is regulated by attachment sites that formon the inside of the cell membrane. The composition of the sites is unknown,but in some animals is controlled by cytoplasmic factors made bymom. This is an example of a MATERNAL EFFECT, in which the phenotype ofthe EMBRYOS is controlled by the genotype of the MOM.3. Different organisms have different patterns of cleavage.a. the amount and location of yolk in the eggb. factors in the egg cytoplasm influence the angle of the mitotic spindleand the timing of its formation.1. Placement of mitotic spindles determines cleavage planes:• MERIDIONAL cuts through embryo poles like a geographic meridian• EQUATORIAL cuts through the area of greatest diameter2. timing of mitotic spindle formation• cleavages can be SYNCHRONOUS, occuring at the same time. ie. seaurchin• or ASYNCHRONOUS, occuring at different times. ie. mammals3. symmetric (equal) vs. asymmetric (unequal) cell divisions:• 4 cells in animal tier divide to form 8 mesomeres of same size• 4 cells in vegetal tier divide to form 4 macromeres and 4 micromeres4. Mammalian cleavage is unique in several ways:a. First, initial cell divisions in mammalian are slow (12-24 hours) compared toembryos that develop outside the mother (ie. flies 10 minutes), presumablybecause mammalian embryos are protected.b. Second, mammals have a ROTATIONAL cleavage pattern. For radialcleavage (ie. sea urchins), the first two cleavages are MERIDIONAL (cleavageplane goes through the embryo poles). In mammals, the first cleavage isMERIDIONAL, but during the second cleavage, one blastomere dividesmeridionally and the other blastomere divides equatorially.c. Third, Mammalian cleavage is asynchronous from an early timepoint. Inother words, mammlian blastomeres do not all divide at the same time. So,embryos frequently contain odd numbers of cells, ie. 3 or 6 blastomeres,instead of exponential 2-to 4-to 8-cell stages.d. Fourth, unlike most other animals, zygotic transcription occurs duringearly cleavage, and produces proteins necessary for subsequent cleavagesto occur. In mice, this switch from maternal to zygotic control occurs at the2-cell stage!e. Fifth, at the 8 cell stage the mammalian embryo undergoes COMPACTION.The outer cells form tight junctions with each other, while the inner cells formgap junctions, enabling small molecules and ions to pass between them.Compaction requires the cell adhesion protein E-cadherin.f. The 16 cell stage embryo is called the morula, and it has a small group ofinternal cells surrounded by a larger group of outer cells. This environment ofouter vs. inner cells sets the stage for differentiation in development.g. Most of the descendents of the outer cells form the TROPHOBLAST(trophectoderm), while the inner cells become the INNER CELL MASS.These cells will give rise to all parts of the embryo proper and someextraembryonic structures, while the trophoblast cells will participate inplacenta formation and implantation. This distinction betweentrophoblast and inner cell mass cell fate is the first differentiationevent in mammalian development.h. The BLASTOCYST forms at 32-64 cells by the outer (trophoblast) cellssecreting fluid into the morula to form a cavity called the BLASTOCOEL.i. Hatching from the zona pellucida is prevented until the blastocyst reaches theuterus. Premature hatching/implantation in the oviduct known as ectopic ortubal pregnancy can cause a life-threatening hemorrhage.5. After fixed number of divisions, Cleavage ends in a controlled manner calledthe MID-BLASTULA TRANSITION (MBT)a. you can imagine that if it ended haphazardly there would be huge differencesin cell numbers among embryos of the same species.Cell divisions=9-10 (sea urchin), 12 (frog), 10 (fish), flies (13)b. it seems to be controlled by the nuclear/cytoplasmic ratio, which is almost0 in a fertilized egg with tons of cytoplasm, but approaches 1 in cells aftermany nuclei are made without extra cytoplasm.c. during the MBT the cell cycle lengthens and often G1 is used for the first time.d. the embryo starts to synthesize its own RNA (often maternal RNA is activelydestroyed at this time). Remember this is a difference between mammalsand other animalse. the cells become motile (prelude to gastrulation).Gastrulation1. GASTRULATION is a complex series of cell movements that:a. rearranges cells, giving them new neighbors. These rearrangements put cells in a newenvironment, with the potential to receive new signals.b. results in the formation of the 3 GERM LAYERS (do not confuse with germline) that willform most of the subsequent embryo: ECTODERM, ENDODERM and MESODERM.2. While all animals gastrulate, gastrulation can seem very different in differentorganisms (mostly due to geometric differences due to yolk content and distribution),


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UNC-Chapel Hill BIOL 205 - Lecture 3 Early Cleavage Patterns

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