Embryology of Musculoskeletal System ✅ Flashcards
What does development of the MSK system require?
Differentiation into specific cell types, and co-ordination to produce an integrated and functional system
What does the MSK system develop from?
Mainly the mesodermal germ layer, with some neural crest contribution
What is mesoderm subdivided into?
- Paraxial
- Intermediate
- Lateral
What happens to the mesoderm in the 3rd week of gestation?
The paraxial mesoderm forms ‘little balls’ (somites), which are paired each side of the neural groove
What happens to the somites?
They differentiate differently in different regions
What can the somites differentiate into?
- Sclerotome
- Dermomyotome
What happens to the sclerotome in development?
It splits segmentally, giving rise to the vertebral column
What does the dermomyotome develop into?
Dermal and muscle components
What creates the limb buds?
The migration of the lateral mesoderm and the dermomyotome to the limb field
What gives rise to the mesenchyme?
Mesodermal cells
What is the mesenchyme?
Loosely organised connective tissue
What do mesenchymal cells differentiate into?
They are pluripotent, and differentiate into many different cell types, including bones and cartilage
What are the ways that bones can develop?
- Direct ossification of the mesenchyme
- Endochondrial ossification
What bones form from direct ossification of the mesenchyme?
- Cranial bones of the skull
- Clavicle
What bones form from endochondrial ossification?
The bones of the limb and girdle
What is endochondrial ossification?
The ossification of a cartilaginous precursor of the long bones of the skeleton
What is the first step in endochondrial ossification?
Condensation of lateral plate mesenchyme occurring in a rod-like structure along the axis of the limb bud
What happens after the condensation of lateral plate mesenchyme in the limb buds?
Chondrocytes (cartilage cells) respond to growth factors and differentiate in a process called chondrification.
What do the chondrocytes secrete?
Proteoglycans and collagen
What is the result of the proteoglycan and collagen secretion by chondrocytes?
Formation of cartilage
Where is there deposition of cartilage initially?
Around the entire limb condensations
Where does further chondrification occur after the initial deposition of cartilage?
Limited to future bone sites, sparing the inter-zone regions
What does the sparing of inter-zone region in further chondrification lead to?
Site of future joints
What is the initial step in the formation of the joints of the long bones?
Mesenchymal cells at the interzones of long bones differentiate into multiple fibroblastic connective tissue layers
What happens to the fibroblastic connective tissue layers at the interzones of long bones?
They differentiate further to provide the articular cartilage at either end of the joint, and connective tissue in the middle forms the internal structures of the joint
What internal structures of the joint are produced from connective tissue in the middle of the fibroblastic connective tissue layers?
- Synovial tissue
- Menisci
- Ligaments
How is the joint cavity formed?
Vacuoles form within the connective tissue, which then becomes the joint cavity
How is the joint capsule formed?
The mesenchymal sheath becomes the joint capsule
How are fibrous joints, or immobile joints which connect bones (e.g. skull, pelvis), formed?
Also developed from interzones, which differentiate into a single layer of fibrous connective tissue
What happens following chondrification?
Ossification
Where does ossification occur from?
The primary ossification centre
What happens to mesenchymal cells in the primary ossification centre?
They differentiate into bone cells (osteoblasts) in response to growth factors
What do the osteoblasts secrete?
The calcium matrix of mineralised bone
What appears alongside the osteoblasts?
Osteoclasts
What do osteoclasts do?
Reabsorb bone, enabling remodelling of growing bone
What does the limb musculature develop from?
Two condensations of somitic mesoderm
How does the somitic mesoderm form the limb musculature?
In the 5th week, the condensations of somitic mesoderm invades the limb bud, one ventrally and one dorsally
What does the ventral somatic mesoderm give rise to?
Mainly flexors, pro atoms, and adductor muscles
What does the dorsal somatic mesoderm give rise to?
Mainly extensors, supination, and abductor muscles
What do cells in the condensations of somitic mesoderm differentiate into?
Myoblasts (muscle cell precursors)
What happens to myoblasts?
They fuse together to form syncytial
How does the inner action of the limb muscles develop?
From branches that develop from spinal nerve axons in a multi step process
What do branches of the ventral spinal nerve inner age?
Ventral muscles
What do branches of the dorsal spinal nerve innervate?
Dorsal muscles
