Mesoderm Flashcards
What induces the primitive streak in chick embryos?
the PMZ: Posterior Marginal Zone
expresses GDF3 gene = secreted signaling protein
What genes are involved in the induction of endoderm and mesoderm?
-GDF3 + Nodal (proteins of TGFbeta fam)
-form heterodimers = 1 Nodal protein + 1 GDF3 protein
(more biologically active than homodimers)
-evidence that these signaling proteins act as morphogens expressed in primitive streak
(inducing endoderm at highest concentration, then axial mesoderm, then more caudal mesoderm)
What do mesodermal cells ingressing through the node during gastrulation form?
ingression through node = axial mesoderm:
notochord + prechordal mesoderm (mesenchyme cells, will contribute to mesoderm of head, pattern forebrain/midbrain)
What is evidence that the node is an organiser?
grafting experiments:
- grafting node into host embryo = 2 perfect embryos with mesoderms completely sequences (v similar to amphibian organiser - conjoined twin)
- graft forms notochord + parts of neural tube and mesoderm
- most of neural tube + mesoderm formed by host cells (in birds, mammals)
What is the importance of Chordin and BMP? Where are they expressed?
-Chordin expressed by node + axial mesoderm
-BMP expressed by posterior Primitive Streak, lateral plate mesoderm and non-neural ectoderm
-BMP is a morphogen: mesodermal cells read BMP concentration to become different types of mesoderm
-function of Chordin: degenerate function of BMP (chordin is a non-signaling protein which binds to BMP, stopping it from activating receptors)
-Axial mesoderm: high Chordin, low BMP
vs Lateral plate mesoderm: low Chordin, high BMP
What is the contribution of the notochord to the spinal chord?
-in amniotes: notochord less needed for axial support
-mostly short-lived structure: destruction when axial skeleton formed
-retained only in spongy centre of invertebral discs
=nucleus pulposus
Explain the formation of somites
-somites come from paraxial mesoderm:
bud off from rostral to caudal end
-FGF concentration gradient: high concentration in unsegmented paraxial mesoderm - creates threshold
-hes1 expression moves up until meets FGF concentration threshold (after which no FGF)
=hes1 stops, expressed in caudal part of forming somite = new somite, cycle starts again
(cells themselves don’t move, only hes1 transcription!)
Describe the development of somites after budding off.
-somites continue to develop after budding off
-3 regions formed by different signals:
-SHH (produced by notochord/floor plate)
= Sclerotome
-WNT (+SHH) (signals from dorsal neural tube +
lower concentration of SHH)
= Epaxial Myotome (Epimere)
-WNT (signals from epidermis)
= Dermotome
-BMP (+SHH) (from lat plate mesoderm, low SHH)
= Hypaxial Myotome (Hypomere)
What do sclerotome cells form?
- migrate around notochord + neural tube
- form cartilage, then bone of vertebral column
What does the dermatome form?
- migrates around inner surface of epidermis
- forms connective tissue of skin = dermis
What does the myotome form?
Myotome:
- epimere (dorsal) forms epaxial skeletal muscle
- hypomere (ventral) forms hypaxial skeletal muscle
How are muscles developed?
-skeletal muscle made by mesoderm cells in somites
-signals (ex: WNT from neural tube) induce expression of myogenic factors (ex: MyoD1)
-at first, cell proliferation encouraged by FGF (myoblasts)
-then, inhibited by MyoD1 (transcription factor in nucleus of myoblasts)
= become myotubes (then differentiate into myofibre)
Composition of skeletal muscle?
-myofibres < fascicles < epimysium
=muscle (bundles within bundles)
-myofibres made up of contractile sarcomeres wiht characteristic cross-striations
=myosin (dark band) and actin (light band) slide past each other during muscle contraction
