Musculoskeletal embryology 1

Trilaminar disc

Ectoderm, mesoderm, endoderm
Mesoderm - forms nearly all connective tissue of musculoskeletal system
Some ectodermal - neural crest cells
Mesodermal cells give rise to loosely organised tissue - mesenchyme
Mesenchyme - multipotent
Neural crest cells differentiate into mesenchymal cells - forms bones of the face

First pairs of somites develop in the cranial region
New pairs develop sequentially - cranial to caudal, head to toe (38 pairs)
Form paired bead-like elevation along the dorsolateral surface of the embryo
Shortly after formation their ventral and medial walls divide into 2 masses of cells
- Sclerotome - ventromedial
- Dermomyotome - dorsilateral

Line the intraembryonic cavity and surround organs, also the peritoneal, pleural and pericardial cavities
Somatic (parietal)
- Bone and connective tissue of limbs, pelvis, and sternum
- Dermis of skin in body wall and limbs
Splanchnic (visceral) - smooth muscle and cardiac

Paraxial mesoderm - axial skeleton, but not sternum or facial bones
Form vertebral column, ribs and most of the base and back of the skull
Sclerotome cells form loosely organised mesenchyme tissue or embryonic connective tissue - end of fourth week
Mesenchymal cells migrate and differentiate into different tissue types
May become fibroblast, chondroblast or osteoblast - under influence of vascular changes
Lateral plate mesoderm forms bones of the shoulder, pelvic girdle, limbs and sternum
Neural crest cells in the head form the craniofacial bones of the skull
Other skull bones formed by occipital somites and somitomeres of the paraxial mesoderm

Endochondral ossification
Mesenchymal condensations fifth week - embryonic connective tissue
Chondrification centres
Mesochyme cells differentiate into chondroblasts - secrete collagen and ground matrix
Collagenous/elastic fibres deposited in matrix and a cartilaginous (hyline) bone model forms - sixth week

Primary ossification centres form in the diaphysis
Chondrocytes increase in size and the matrix calcifies and cells die
Thin layer of bone forms in the area surrounding the diaphysis to form the periosteum
Invasion of vascular connective tissue from surrounding periosteum blood vessels break up cartilage
Osteoblast reach developing bone via vascular connective tissue
Bone forms from the centre out towards to epiphysis - sites of secondary ossification centres

End of fourth week - sclerotome cells form mesenchymal tissue with bone forming capacity
Sclerotomic mass migrates between the neural tube, notochord and body wall
Other mesodermal cell migrate around notochord and surrounded by sclerotome cells

Develops from lateral plate (parietal) mesoderm and costal cartilage from paraxial mesoderm

Human skull made up of 45 separate bony elements
Neurocranium - protecting the brain
- Membranous part - flat bone
- Cartilaginous part - base of the skull
Viscerocranium - facial skeleton
- Formed mainly by the first two pharyngeal arches
Fetal skull vault is large compared to face skeleton
Neural crest cells - frontal
Paraxial mesoderm - parietal (back)
Intramembranous ossification
- Mesenchyme condense and osteoblasts form matrix - calcium phosphate deposits
- Bone spicules radiate from the primary ossification centres within membranous sheaths
- Enlarge by apposition of new layers on the outer surface and osteoclatic resorption inside
Flat bones separated by sutures