Wk.1 L2 - Musculoskeletal Development - from conception to birth Flashcards
LO
- Describe mesoderm and somite development.
- Describe bone and muscle development.
- Explain the intramembranous and endochondral ossifications, and synovial joint development.
Mesoderm Development:
- Gastrulation
- Migration of Mesoderm
- Neural Plate Formation
Gastrulation
Week 2:
- Epiblast and Hyoblast is formed
- Primitive streak forms at caudal end of epiblast to define body axes.
Week 3:
- Migration of cells through the primitive streak forming to Definitive Endoderm
- Epiblast cells migrate between endoderm and epiblast to form the intraembryonic mesoderm
- The three germ layers are formed
- Ectoderm, Mesoderm, Endoderm
Migration of Mesoderm
- Mesoderm cells migrate billaterally from the primitive streak
- Mesoderm cells migrate cranially to form the notocord
- Cells migrating from lateral of the notocord become paraxial mesoderm
Neural plate formation
Day 18 Neural plate is formed
- Cranial to primitive node on the primitive streak
Week 4:
- The neural plate folds into the neural tube
- Neuralation process occurs
[heft]
Muscle development
- Paraxial Mesoderm
- Somites
- Somite differentiation
Paraxial Mesoderm
- In the head, paraxial medoserm is undifferentiated/ unsegmented
- Caudal to the occipital region, paraxial mesoderm is segmented and organised into Somites
Somites
Somites give rise to most of the axial skeleton, skeletal muscles of neck, trunk, limbs and dermis of the body.
- Appears in crainiocaudal sequence of ~3pairs per day
Week 5:
- 42-44 pairs from Occipital to Coccygeal regoins (Coccygeal will dissapear)
- The remaining somites become axial skeleton and voluntary muscle and dermis
Somite differentiation
- Pre-somatic mesoderm arrange themselves into a ball of epithelial cells with a lumen (epithelialisation)
- Shortly after, they form Sclerotome and Dermomyotome
Sclerotome = Bone & Cartilage
Dermomyotome = Muscle & Dermis
Each Somite forms its own Sclerotome, Myotome and dermatome
- Myotomes and Dermatomes have their own segmental nerve component.
Somite Differentiation: Dermomyotomes that migrate
Anteriorly:
- Give rise to trunk and limb muscles (Hypaxial muscle)
Posteriorly:
- Give rise to intrinsic back muscles (epaxial muscles)
Bone Development
2 Processes:
Intramembranous Ossification
- Skull, clavicle, mandible - flat bones
Endochondral Ossification
- Long bones, vertibrae, ribs
Intramembranous Ossification
Bones develop from mesenchyme (loose CT)
Week 8:
- Mesenchymal cells bunch to form an Ossification centre which differentiates into Osteoprogenitor cells and Osteoblasts
Osteoblasts sectrete components of the bone matrix
- Undergoes mineralisation, and trapped osteoclasts become osteocytes
- Osteocytes get seperated, but communicate
Osteoblast = produce matrix
Osteocyte = maintain matrix
Eventually the immature bone is replaced by compact bone in the periphery and spongey between
- Spaces between the trabecular are occupied by bone marrow cells, arriving in blood vessels
Endochondral Ossification
Mesenchyme cells differentiate into Chondrocytes which produce Cartilage matrix
Cartilage model can grow in length & width and calcify.
- Blood vessels errode and invade the cartilage wall and create a marrow cavity
As a primary ossification centre develops, the endochondral bone is formed on specules of calcified cartilage
A second centre is established in the epiphyses
- With growth of the long bone, the distal epiphyseal cartilage dissapears
- Then the Proximal epiphyseal cartilage disappears and the metaphysis becomes continuous with the epiphysis
[heft]
Once grown, the epiphyseal line remains where it last existed
Synovial Joint Development
Mesenchyme of the interzone between chonrifying bone primordia differentiates into fibroblastic tissue
[heft]
At both proximal and distal ends, chondrification differentiates the fibroblastic tissue into articular cartilage, covering 2 facing bone primordia
- Proximally and distally the CT in the central region condenses to form the synovial tissue.
- Vacuoles form within the CT to form the Joint cavity filled with synovial fluid
- The central zone of the Interzone can give rise to menisci and internal joint ligaments
- Joint capsule arises from the interzone
