Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
14.10
Somite maturation over time: the somite separates into four regions
Class 16: Differentiation of Somites: Muscle and Bone
G 14.10
dermatome Hypaxial myotome
sclerotomeMigrating myoblast
Epaxialmyotome
Cell types within the somite
Multiple signals determine derivatives of the somite Epaxial myotome: makes back muscles
Reguires Wnts from neural tubeShh from notochord
Hypaxialbody wall, limb muscles
Hypaxial myotome: makes limb and body wall musclesRequires BMP4, FGF from lateral plate mesoderm
Wnts from overlying ectoderm
14.11
Sclerotome: makes bone (vertebrae and ribs)Requires Shh from notochord
How is muscle made?The discovery of a “master” regulatory factor for muscle
MyoD expression in the E11.5 mouse embryo
somites
forelimb
hindlimb
eye
bHLH transc. FactorsShow binding domain, etc
A “master regulatory gene” for muscle differentiation:MyoD, bHLH family of transcription factors
14.12
Myogenesis: Stages in determination and differentiation of myoblasts
Myotome
(only proliferating)
Myoblast
(starting to differentiate)
FGF
no FGF
Three additional members of the MyoD family were found,each with specific roles and different expression patterns
Myf-5: expressed in epaxial (back) myotome
MRF-4: expresed as myoblasts differentiate intomyotubes and myofibers
Myogenin: expresed as myoblasts differentiate intomyotubes and myofibers
MyoD: expressed in hypaxial (limbs) myotome
Phenotypes of mouse knockout mutants for myogenic genes
MyoD -/- Normal muscle (developmental delays)
Myf5 -/- Normal muscle (developmental delays)
MyoD -/- ; Myf5 -/- Paralyzed, no myoblasts or muscleMyogenin -/- Myoblasts, but few myotubes
MRF4 -/- Muscle fibers (but delayed)
14.12
Myogenesis: Stages in determination and differentiation of myoblasts
Committed, express MyoD but still dividingMrf-4
Myogenin
MyoD family members are only active as heterodimerswith other bHLH proteins
Regulation of MyoD family members
Basic Helix 1 Loop Helix 2
MyoD
E12
DNACONTACT
DIMERIZATION
CANNTGGTNNAC
MyoD E12
Timing of differentiation is controlled by levels of the regulators
NegativeIdMist1MyoR
(don’t bind DNA)
PositiveE proteins (E12 and E47)(bind DNA)
There are both positive and negative regulators of MyoD family members
A4 22-43
Other regulators:Myostatin negatively regulates muscle size
Belgian Bluemyostatin mutants
An interesting tidbit:
Muscle can regenerate because of a unique population of cells that normally are “quiescent”
Satellite Cells
Inactive stem cells that are triggered by growth factors to undergo division and produce myoblaststhat then develop into myofibers
Studied here in MCDB by Dr. Brad Olwin
A regulatory pathway for myogenesis
Myotome
Myoblasts
Myotube Striated myofiber
Wnt
Shh
Pax3
MyoD
Myf5
Myogenin
MRF4
Myosin, actin, etc.
Somite
FGF
Myofiber
dermatome Hypaxial myotome
sclerotomeMigrating myoblast
Epaxialmyotome
Osteogenesis: Bone formation
Cartilage and bone are derived from several sources:
1) Sclerotome (trunk- ribs, vertebrae)
2) Lateral plate mesoderm (limbs)
3) Neural crest (head)
There are two forms of osteogenesis:
1) Endochondrial ossification (cartilage template)
2) Intramembraneous ossification (direct production of bone)
Axial bone derives from the sclerotome
14.11
Intramembranous ossification: skull
Endochondrial ossification (all other bones)
Cell types:
1) Chondrocytes (cartilage formation)
2) Osteoblasts (bone formation)
3) Osteoclasts (bone resorption)
14.13
Endochondrial ossification
Molecular control of cartilage formation
1. Mesenchyme cells à chondrocytes.
Sox-9 is required for mesenchyme condensation, the firstrequirement for bone formation. The eventual size of the bone depends on how much mesenchyme condenses
2. Chondrocyte proliferation
•Stimulated by growth factors (IGF) and a hedgehog signal
3. Chondrocytes à hypertrophic chondrocytes
•Favored by CbfaI, transcription factor
•Hypertrophy requires estrogens and testosterone
•This sets the stage for bone formation
14.13
Blood vessels bring in chondroclasts (which get rid ofcartilage) and osteoblasts, the bone precursors
Factors required for bone formation
Osteoblasts are bone producing cells: they are brought in via the blood vessels --Require Cbfa1, a transcription factor, for differentiation--Without Cbfa1, skeleton remains cartilagenous
Red: boneBlue: cartilage
Wild type Cbfa1 knockout
Osteoclasts are derived from haematopoietic stem cells: These cells break down the bone, leaving an area for bone marrow, which will be site of blood synthesis (and future synthesis of osteoclasts)
Osteoclast numbers are tightly regulated
•Inhibited by estrogen and testosterone
estrogentestosterone
14.13
Blood vessels bring in the osteoclasts, which “remodel” the bone