Zoo 470 Unit 3 learning objectives

Provide evidence that, counterintuitively, ectoderm is normally fated to become neural unless it receives epidermalizing signals

BMPs and Wnts normally make ectoderm epidermal

• on neural plate, sox2(neural marker) is only expressed there because of BMPs lead to epidermis cells everywhere else in ectoderm

• in BMP absence, all ectoderm expresses sox2

• so, without BMP ventral signals, every single ectoderm cell defaults to its neural identity

Explain how gastrulation positions mesoderm to send inductive signals to ectoderm that leads to neural induction and provide classical evidence that both planar and vertical inductive signals contribute to neural induction.

• organizer mesoderm lies at surface to send signals along the surface to the ectoderm (planar)

  • pieces of early gastrula that include what would normally become neural ectoderm are excised and then placed side by side with each other. If neural ectoderm forms, the only type of signals it can receive are planar signals since the mesoderm lies alongside ectoderm.

• later, mesoderm lies under ectoderm to send signals vertically. Posterior, planar signals could still continue

  • anterior mesoderm placed into early gastrula induces head with eyes and forebrain

  • posterior mesoderm placed into early gastrula induces a trunk and tail. this proves that vertical signals can pattern neural ectoderm based on the type of underlying axial mesoderm

Explain how anteriorizing and posteriorizing signals pattern the neural ectodermal along the anterior-posterior axis and be able to name molecules associated with each signal.

• Anteriorizing signals (like noggin and chordin) block both BMPs and Wnts to keep the neural default state and induce head and brain.

• posterior signals induce tail structures have high levels of BMP and Wn

• the middle blocks only BMPs but still have Wnt expression

• BMP inhibiotrs result in spinal cord

• Wnt inhibior lead to future brain

• Wnts, FGF,, and RA posterize the spinal cord

State major developmental events accomplished during gastrulation in amphibians.

• gastrulation initiates at the dorsal blastopore lip

• mesoderm involutes beneath the ectoderm

• archenteron begins to form

• blastocoel displaced towards future anterior

• head and other mesodermal mesenchyme forms

Identify the type of morphogenetic movements performed by each major region of the amphibian gastrula, including the animal cap, bottle cells, involuting and non-involuting marginal zones, leading edge mesoderm, and vegetal endoderm

animal cap: epibloy via radial intercalation

bottle cells: invagination and later respreading via apical constriction

IMZ: convergent extension and involution

NIMZ: convergent extension and radial intercalation

LEM: directed migration

Vegetal endoderm: rotation

Identify the germ layer and basic tissues to which deep and superficial cells of the involuting and non-involuting marginal zones contribute

Superficial cells: epithelial cells

deep cells: loosely organized

IMZ: superficial forms endoderm, deep cells form mesoderm; dorsal IMZ deep cells are mesoderm from organizer; superficial dorsal becomes part of primitive gut

NIMZ: dorsal becomes neural ectoderm, ventral becomes epidermis

Explain how Keller explants can be used as a tool to study detailed movements of cells during convergent extension, the basic protrusive behavior of rearranging cells, and evidence that non-muscle myosin is important for convergent extension

• both the neural and axial mesoderm elongate dramatically therefore dorsal NIMZ and IMZ both undergo convergent extension

• cells become polarized, extending protrusions along an axis perpendicular to the axis of extension

• non-muscle myosin forms complex networks within IMZ cells therefore is required for convergent extension

Provide experimental evidence for the importance of the planar cell polarity (PCP) pathway in promoting polarized protrusive activity during convergent extension.

• expressing Dsh dominant negative (dirsupts PCP) results in extension not occuring

Provide experimental evidence for the importance of fibronectin in promoting directional migration of leading-edge mesoderm.

antibodies injected into blastocoel disrupt binding of cells to fibronectin and don’t allow leading egde cells to migrate

Compare and contrast zebrafish and amphibian axis specification and gastrulation, identifying the Organizer equivalent in fish, and the molecular steps that lead to axis specification.

Zebrafish:

• most embryo forms on top of large yolk cell

• hypoblast forms a floor for the embryo at the edges of the cellular portion of the embryo

• epiblast is the surface layer of embryo

• shield is the Organizer equivalent in fish in the future dorsal midline of the embryo

• beta-catenin accumulates in dorsal side of embryo

  • nodal-related proteins are produced by the yolk syncytial layer

  • BMPs specify ventral tissues; BMP antagonists are produced by the shield

  • Wnts and FGFs promote posterior structures; Wnt antagonists are produced by the Organizer

Provide experimental evidence that the shield is the Organizer equivalent in fish, and that BMP signaling is important for ventral differentiation

• possible to transplant bits of the shield from one embryo to another. The extra set of axial structures that result from the shield transplant indicate the equivalency

• loss of Bmp7 leads to loss of ventral structures and dorsalized embryo

Explain the basic morphogenetic movements that drive fish gastrulation, and how meroblastic cleavage and a telolecithal egg appear to lead to modifications compared to amphibians.

• radial intercalation causes thinning of the epiblast

  • enveloping layer is somewhat like the superficial cells of the frog gastrula

  • deep cells undergo radial intercalation to thin epiblast

• mesodermal deep cells involute

  • hypoblast forms via delamination

  • epiblast cells appear to involute

• cells converge and extend

  • largely appear to migrate as individuals which differs from frogs

  • axial mesodermal cells converge and extend

Provide evidence that PCP signaling regulates fish gastrulation.

• trilobite is a mutation in strabismus, a PCP component

• convergent extension is reduced in trilobite mutants showing importance of PCP signaling

Describe differences between amniotes and other vertebrates, including why the amniotic egg allows for life on land.

• produce specialized amniotic membranes to prevent them drying out on land and remove the need for aquatic breeding

Describe how the hypoblast arises in chicks, and where the majority of the embryonic endoderm comes from in amniotes.

• from epiblast cells that ingress

• from posterior marginal zone (not the same as in frogs)

• primitive streak is equivalent to amphibian blastopore

• koller’s sickle acts like the nieukwoop center in frogs

• embryonic endoderm forms from some the first epiblast cells to ingress

Provide experimental evidence that Henson’s node is the Organizer equivalent in chicks, and molecular evidence that it functions in a similar way.

• cells ingress through primitive streak

• definitive endoderm comes from the epiblast

• hypoblast forms extra embryonic endoderm

• notochord mesoderm moves anteriorly around henson’s node

• other mesoderm takes various trajectories after engrossing though the streak

Describe the basic movements of presumptive mesoderm in chicks, and how mesoderm is internalized via ingression at the primitive streak

• through preamative streak

Describe the movements of material that enters the streak at Henson’s node, what type of mesoderm this material forms, and how regression of Henson’s node is tied to formation of the notochord.

• notochord mesoderm

• henson’s node is the bird equivalent of the organizer

• henson’s node sits at the anterior and then shifts to the posterior

  • as the node retreats, it leaves behind a trail of cells (notochord)

Describe the striking differences in developmental timing in anterior vs. posterior regions of amniotes during gastrulation

Provide experimental evidence that cadherin-mediated compaction is crucial for early mammalian development

Describe evidence from monozygotic twinning that the inner cell mass retains pluripotency until immediately prior to gastrulation

Explain key molecular regulators of pluripotency and trophoblast development in the early mammalian embryo, including the role of Cdx2 and the Oct4/Nanog pathway

Identify key transcription factors that are expressed in differentiated cell types in the developing embryo after the early blastocyst stage.

Describe the basic movements of gastrulation in placental mammals and be able to compare them to other amniotes.

Identify the mammalian equivalents of the Organizer and anterior mesoendoderm and describe experiments that demonstrate similar molecules are expressed in this tissue and required for normal axis specification and neural induction.

Compare and contrast – in very general terms – the similarities and differences between various vertebrates, in terms of gastrulation and Organizer differentiation.

Explain why biological “facts” may be an insufficient basis for ethical decision-making regarding embryos.

Explain why procedures involving the intentional destruction of embryos may be ethically controversial.

Explain the concept of a worldview, and how ontology, ethics, and religion shape worldviews as they relate to the status of the human embryo.

Provide examples of the role of intuition in ethical decision-making as it relates to stem cells.

Provide examples of consequentialist and deontological arguments for and against destruction of embryos.

Provide examples of how religious worldviews influence ethical decision-making about embryos.

Explain the basic procedures involved in PGD, why the totipotency of 8-cell uncompacted (or earlier) blastomeres was key to an early version of this technique, and how more recent procedures perform PGD

Identify how PGD could be used for sex selection and using other genotypic criteria, including single-gene genetic disorders.

Provide evidence that sex-selective abortion has led to sex-specific imbalances in live births in some countries worldwide, and that it has been used to eliminate Down syndrome individuals from some human populations.

Identify ethical argumentation regarding human embryos in everyday human discourse, using classic, culturally embedded television documentary footage as an example.

Explain why technologies related to synthetic embryos (review) and ex utero culturing of mammalian embryos have the potential to complicate ethical discussions related to human embryos.