Musculoskeletal System - Limb Development

Introduction

Adult appendicular skeleton

The mesoderm forms nearly all the connective tissues of the musculoskeletal system. Each tissue (cartilage, bone, and muscle) goes through many different mechanisms of differentiation.

The musculoskeletal system consists of skeletal muscle, bone, and cartilage and is mainly mesoderm in origin with some neural crest contribution.

The intraembryonic mesoderm can be broken into paraxial, intermediate and lateral mesoderm relative to its midline position. During the 3rd week the paraxial mesoderm forms into "balls" of mesoderm paired either side of the neural groove, called somites.

Somites appear bilaterally as pairs at the same time and form earliest at the cranial (rostral,brain) end of the neural groove and add sequentially at the caudal end. This addition occurs so regularly that embryos are staged according to the number of somites that are present. Different regions of the somite differentiate into dermomyotome (dermal and muscle component) and sclerotome (forms vertebral column). An example of a specialized musculoskeletal structure can be seen in the development of the limbs.

Skeletal muscle forms by fusion of mononucleated myoblasts to form mutinucleated myotubes. Bone is formed through a lengthy process involving ossification of a cartilage formed from mesenchyme. Two main forms of ossification occur in different bones, intramembranous (eg skull) and endochondrial (eg limb long bones) ossification. Ossification continues postnatally, through puberty until mid 20s. Early ossification occurs at the ends of long bones.

Musculoskeletal and limb abnormalities are one of the largest groups of congenital abnormalities.

Historic Embryology - Musculoskeletal
1853 Bone \| 1885 Sphenoid \| 1902 - Pubo-femoral Region \| Spinal Column and Back \| Body Segmentation \| Cranium \| Body Wall, Ribs, and Sternum \| Limbs \| 1901 - Limbs \| 1902 - Arm Development \| 1906 Human Embryo Ossification \| 1906 Lower limb Nerves and Muscle \| 1907 - Muscular System \| Skeleton and Limbs \| 1908 Vertebra \| 1908 Cervical Vertebra \| 1909 Mandible \| 1910 - Skeleton and Connective Tissues \| Muscular System \| Coelom and Diaphragm \| 1913 Clavicle \| 1920 Clavicle \| 1921 - External body form \| Connective tissues and skeletal \| Muscular \| Diaphragm \| 1929 Rat Somite \| 1932 Pelvis \| 1940 Synovial Joints \| 1943 Human Embryonic, Fetal and Circumnatal Skeleton \| 1947 Joints \| 1949 Cartilage and Bone \| 1957 Chondrification Hands and Feet \| 1968 Knee

--Mark Hill 09:25, 14 April 2010 (EST) Page Template only - content from original UNSW Embryology site currently being edited and updated.

Some Recent Findings

Spatially Controlled Cell Proliferation in Limb Bud Morphogenesis^[1]"Our data run contrary to the proliferation gradient hypothesis, indicating instead that oriented cell behaviours are important for driving elongation."

The apical ectodermal ridge (AER) can be re-induced by wounding.Cite error: Closing </ref> missing for <ref> tag]]

Four Concepts - much of the work has been carried out using the chicken and more recently the mouse model of development.

Limb Initiation
Proximodistal Axis
Dorsoventral Axis
Anteroposterior Axis

Limb Initiation

Fibroblast growth factor (FGF) coated beads can induce additional limb
FGF10 , FGF8 (lateral plate intermediate mesoderm) prior to bud formation
FGF8 (limb ectoderm) FGFR2
FGF can respecify Hox gene expression (Hox9- limb position)
Hox could then activate FGF expression

Note that during the embryonic period there is a rostrocaudal (anterior posterior) timing difference between the upper and lower limb development

this means that developmental changes in the upper limb can precede similar changes in the lower limb (2-5 day difference in timing)

Limb Identity

Forelimb and hindlimb (mouse) identity appears to be regulated by T-box (Tbx) genes, which are a family of transcription factors.

hindlimb Tbx4 is expressed.
forelimb Tbx5 is expressed.
Tbx2 and Tbx3 are expressed in both limbs.

Body Axes

Anteroposterior - (Rostrocaudal, Craniocaudal, Cephalocaudal) from the head end to opposite end of body or tail.
Dorsoventral - from the spinal column (back) to belly (front).
Proximodistal - from the tip of an appendage (distal) to where it joins the body (proximal).

Proximodistal Axis

Apical Ectodermal Ridge (AER) formed by Wnt7a
then AER secretes FGF2, 4, 8
stimulates proliferation and outgrowth

apical ectodermal ridge | AER and vascular channel

Dorsoventral Axis

Somites - provides dorsal signal to mesenchyme which dorsalizes ectoderm
Ectoderm - then in turn signals back (Wnt7a) to mesenchyme to pattern limb

Wnt7a

name was derived from 'wingless' and 'int’
Wnt gene first defined as a protooncogene, int1
Humans have at least 4 Wnt genes
Wnt7a gene is at 3p25 encoding a 349aa secreted glycoprotein
patterning switch with different roles in different tissues
mechanism of Wnt and receptor distribution still being determined (free diffusion, restricted diffusion and active transport)

One WNT receptor is Frizzled (FZD)

Frizzled gene family encodes a 7 transmembrane receptor

Fibroblast growth factors (FGF)

Family of at least 17 secreted proteins
bind membrane tyrosine kinase receptors
Patterning switch with many different roles in different tissues
FGF8 = androgen-induced growth factor, AIGF

FGF receptors

comprise a family of at least 4 related but individually distinct tyrosine kinase receptors (FGFR1- 4) similar protein structure
- 3 immunoglobulin-like domains in extracellular region
- single membrane spanning segment
- cytoplasmic tyrosine kinase domain

Anteroposterior Axis

Zone of polarizing activity (ZPA)
a mesenchymal posterior region of limb
secretes sonic hedgehog (SHH)
apical ectodermal ridge (AER), which has a role in patterning the structures that form within the limb
majority of cell division (mitosis) occurs just deep to AER in a region known as the progress zone
A second region at the base of the limbbud beside the body, the zone of polarizing activity (ZPA) has a similar patterning role to the AER, but in determining another axis of the limb

Wing as Limb Model

chicken wing easy to manipulate
- removal, addition and rotation of limb regions
- grafting additional AER, ZPA
- implanting growth factor secreting structures

UNSW Embryology - Axes Formation - Limb | Signal Factors - Wnt

References

↑ <pubmed>20644711</pubmed>| PMC2903592 | PLoS

Reviews

How to make a zone of polarizing activity: insights into limb development via the abnormality preaxial polydactyly. Hill RE. Dev Growth Differ. 2007 Aug;49(6):439-48. Review. PMID: 17661738

Articles

The apical ectodermal ridge (AER) can be re-induced by wounding, wnt-2b, and fgf-10 in the chicken limb bud. Satoh A, Makanae A, Wada N. Dev Biol. 2010 Mar 27. PMID: 20347761
Distinct roles of Hand2 in initiating polarity and posterior Shh expression during the onset of mouse limb bud development. Galli A, Robay D, Osterwalder M, Bao X, Bénazet JD, Tariq M, Paro R, Mackem S, Zeller R. PLoS Genet. 2010 Apr 8;6(4):e1000901. PMID: 20386744
Modification of the zone of polarizing activity signal by trypsin. Stefanov EK, Ferrage JM, Parchim NF, Lee CE, Reginelli AD, Taché M, Anderson RA. Dev Growth Differ. 2009 Feb;51(2):123-33. PMID: 19207183