Class Notes (807,465)
Canada (492,646)
Biology (2,161)
BIOL 303 (133)

BIOL 303 Study Questions Set 9 & 10

12 Pages
Unlock Document

University of Waterloo
BIOL 303
Dragana Miskovic

BIOL 303 Study Questions Set 8 Part II and Set 9 1. What distinguishes the animal pole from the vegetal pole of the Xenopus egg?  Visually, animal pole is dark and vegetal pole is light.  Animal pole = upper part of egg, no yolk. During embryogenesis, cells divide rapidly.  Vegetal pole = lower part of egg, yolk. Serves as food for developing embryo and cells divide more slowly. 2. At what stage of meiosis is the Xenopus egg at the time of fertilization? Where on the egg does fertilization occur?  Fertilization can occur anywhere on the animal hemisphere. Point of sperm entry determines orientation of dorsal and ventral axis (point of sperm axis will be ventral). 3. What is the grey crescent and how does it arise? What is the destiny of the grey crescent?  Sperm centrioles organize the microtubules in the egg. Microtubules in vegetal pole are arranged parallel to one another. The microtubule tracks allow for the cortical cytoplasm to rotate (30 degrees) within the inner cytoplasm. The cortical rotation can reveal a gray band of inner cytoplasm called the gray crescent.  A rotation of the cortical cytoplasm 30 degrees with respect to the internal cytoplasm in the marginal region of the 1-cell amphibian embryo.  The cortical rotation breaks the radial symmetry of the amphibian egg, specifying the orientation of the embryonic body axes. The entire outer cortex of the fertilised egg rotates relative to the mass of inner cytoplasm by an angle of about 30° about an axis perpendicular to the primary animal-vegetal axis. As a result of this cortical rotation, 'dorsal determinants', factors able to trigger the formation of the 'organiser' region of the gastrula, are displaced from the vegetal pole region to a more equatorial position where they become activated.  Gastrulation starts here. 4. What pattern of cleavage is observed in the Xenopus embryo?  Radially symmetric and holoblastic  First two cleavage planes are meridonial (1 bisects the gray crescent), 3 is equatorial – but displaced towards animal pole – this makes four smaller animal blastomeres and four larger vegetal blastomeres  The second cleavage may actually start in animal pole before first is completed in the vegetal pole  Cleavage continues for 12 cycles.  Results in many smaller cells in animal pole than vegetal pole due to yolkiness in vegetal pole.  Becomes morula when 16  64  Becomes blastula at 128 cells 5. What purposes might we attribute to the Xenopus blastocoel?  Permits cell migration during gastrulation  Prevents early cell-cell interactions 6. When does the mid-blastula transition occur and what molecular events are associated with zygotic gene activation of Xenopus?th  Mid-blastula transition occurs late in the 12 cycle (last cycle)  Gene transcription and cell cycle adds G phases  Blastomeres become motile  DNA demethylation (promoters are demethylated so it promotes transcription) and histone methylation (associated with active transcription) 7. What are the first signs of gastrulation in Xenopus?  Purpose of gastrulation: bring some cells inside to form endoderm, leave some on the outside to form the ectoderm, and get some inbetween to form the mesoderm.  In Xenopus, mesoderm is derived from deep blastula layers and endoderm and ectoderm are derived from more superficial layers.  Gastrulation initiates on future dorsal side of embryo.  Cells invaginate to form a slit-like blastopore. The invaginating cells change shape to become bottle cells.  Vegetal rotation: 2 hours before bottle cells are observed, cell rearrangements on the dorsal floor of the blastocoel – cells pushed up towards animal cap  Involution of the marginal zone cells. The animal cells undergo epiboly (thinning and spreading of layers) and converge and blastopore (opening into archenteron – cavity forming primitive gut).  Migrating marginal cells turn inward at dorsal lip and travel along blastocoel roof. First cells to pass over the lip are prospective pharyngeal endoderm (forms the salivary glands, thyroid gland, pharyngeal pouches). Next is prechordal plate (forms head mesoderm) and then chordamesoderm (forms notochord – induces developmental patterns in nervous system). 8. Draw a diagram of the cross section of the early Xenopus gastrula. Label the blastocoel, archenteron, and blastopore. 9. What are some of the molecular events associated with convergent extension?  What drives the movement during gastrulation? Cells in the involuting marginal zone and the noninvoluting marginal zone that are initially several cell layers deep become one thin broad layer. This is called radial intercalation.  When the deep cells pass around the blastopore lip, they involute and undergo convergent extension (cell movement and tissue elongation) along mediolateral axis. This results in a long narrow band of mesoderm.  Polarized cell adhesion: – involuted mesoderm cells send out protrusions to contact one another – directed toward midline of embryo and depend on extracellular layer of fibronectin  Differential cell cohesion – cell type specific expression of different cadherins  Calcium flux – waves of intracellular calcium surge across dorsal tissues undergoing convergent extension (regulates actin filament contraction) 10. The Xenopus egg is already polarized at the time of fertilization. In which axis is the unfertilized egg polarized? An in situ hybridization of what transcript attests to the above statement?  The unfertilized egg is polarized in the animal-vegetal pole axis.  Vegetal cells determine the body formation plan. The mRNA of transcription factors VegT is anchored to cortex of vegetal hemisphere. They differentiate to form the endoderm and induce cells above them to become the mesoderm. VegT activates transcription of TGF-β related genes (paracrine factors required for mesoderm induction). By knocking out VegT transcripts, the embryo develops all epidermis.  Xenopus tropicalis VegT. whole-mount in situ hybridization analysis of tVegT mRNA expression in tropicalis oocyte. tVegT mRNA is localized to the vegetal half of a stage VI oocyte.  In Xenopus A/P is linked with the establishment of D/V – and this is set up by events occurring at fertilization  Dorsal is established opposite point of sperm entry  Subsequent movement of involuting mesoderm sets up the A/P axis  Inductive interactions: the first endomesoderm moves around the dorsal lip induces ectoderm above it to form anterior structures  Inductive interactions: later involuting mesoderm induces ectoderm above it to form posterior structures 11. Transplantation experiments involving early and late gastrulation give dramatically different results. What is the general explanation for these observations – or what concept was developed from these types of experiments?  Early gastrula: presumptive neural ectoderm transplanted to site of presumptive epidermis  resulted in epidermis  therefore conditional, regulative  Late gastrula: presumptive neural ectoderm transplanted to site of presumptive epidermis  resulted in two neural plates  therefore autonomous, mosaic 12. What is the Xenopus “organizer” (aka Spemann-Mangold organizer) and how was it discovered?  This process, whereby the central nervous system (ectoderm) forms through interactions with the underlying mesoderm, has been called primary embryonic induction and is one of the principal ways that the vertebrate body becomes organized. Its discoverers called the dorsal blastopore lip and its descendants “the Organizer,” and found that this region is different from all the other parts of the embryo.  Spemann’s Organizer: More correctly, the Spemann-Mangold Organizer. In amphibians, the dorsal lip cells of the blastopore and their derivatives (notochord and head endomesoderm). Functionally equivalent to Hensen’s node in chick, the node in mammals, and the shield in fish. Organizer action establishes the basic body plan of the early embryo.  Ligation experiments: demonstrated nuclear equivalence. The first ligature was set up in the same plane as the first division (plane of cytokinesis)  2 normal embryos developed.  Alternate ligation experiment: similar experiment but the ligature was set up perpendicular to the plan of cytokinesis  1 normal embryo and 1 bauchstuck developed (ball of unorganized ventral cells [ectoderm, mesoderm, endoderm], no dorsal structure [nervous system, notochord, somites])  Ligature experiments proved that something about the grey crescent is critical for proper development.  They also found one special region of the amphibian region had an autonomous fate: a self-determining tissue – the dorsal lip of the blastopore – derived from the gray crescent cytoplasm – named the organizer.. Transplantation of dorsal lip tissue into recipient belly skin region had the remarkable effect of initiating gastrulation and embryogenesis in surrounding tissue. 13. What is the Nieuwkoop centre?  In Xenopus embryos, the Spemann-Mangold organizer establishes the dorsal- ventral and anterior-posterior axes. According to the prevalent model of early development, the organizer is induced by the dorsalizing Nieuwkoop signal, which is secreted by the Nieuwkoop center. Formation of the center requires the maternal Wnt pathway, which is active on the dorsal side of embryos. Nieuwkoop center produce dorsalizing signals in vitro.  After the sperm entry (determines D/V axis
More Less

Related notes for BIOL 303

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.