Development of Cranium and ossification Flashcards
Paraxial mesoderm
Separates into blocks called somites
Somites
Gives rise to vertebrae, ribs, skeletal muscle of the body walls and limbs
Lateral plate mesoderm
Forms all connective tissue (ligaments and cartilages), blood vessels and bones of the appendicular skeleton (limb bones and bones of the limb girdle) and sternum
Skeletal System 4th week
Develop form paraxial, lateral late mesoderm and neural crest cells
Somites differentiate into:
Sclerotom and Dermomyotome
Sclerotome
Ventromedial part
Cells form the vertebrae and ribs
Dermomyotome
Dorsolateral part
Cells from its myotome region form myoblasts
Cells from its dermatome region form the dermis
Myoblasts
primordial muscle cells
At the end of the 4th week Sclerotome becomes…
Mesenchyme
Mesenchymal cells migrate and differentiate into
Fibroblast
Chondroblasts
Osteoblasts
Mesenchyme also derives from…
neural crest cells
Mesenchyme in the head region
migrate into the pharyngeal arches and form the bones and connective tissue of craniofacial structures
Types of bone ossification:
Intramembranous and Endochondral
Intramembranous Ossification
Mesenchymal condensation in the dermis differentiates into bone
Ex. Flat bones of the skull
Endochondral Ossification
Mesenchymal cells first give rise to hyaline cartilage models
ex. long Bones and skull base bones
Cartilage and bone extracellular matrix consist of…
Collagen type I
Intramembranous Ossification
Mesenchymal cells condensate
* Osteoblastssecretetheorganicmatrix (osteoid)
* Calcium phosphate is then deposited and the matrix reorganized into compact bone
* Osteoblasts in the osteoid become osteocytes.
* Continuous remodeling of bone occurs at fetal and postnatal life due to the activity of osteoblasts and osteoclasts.
Endochondral Ossification
Ossification of the limbs
* Mesenchyme cells begin to condense and differentiate into chondrocytes
* Chondrocytes form a cartilaginous model of the prospective bone
* Primary Ossification Center
* Blood vessels Invade the center of the cartilaginous model, bringing osteoblasts to the
diaphysis
* Secondary Ossification Center
* Blood vessels invade the epiphyses
Primary Ossification Center
Blood vessels Invade the center of the cartilaginous model, bringing osteoblasts to the
diaphysis
Secondary Ossification Center
Blood vessels invade the epiphyses
Development of the cranium
Cranium consist of
Neurocranium and Viscerocranium
Neurocranium
Membranous part:
* Forms a protective case around the brain via intramembranous ossification
* Cartilaginous part:
* AKA chondrocranium
* Endochondral ossification forms the bones of the base of the skull
Viscerocranium
Forms the skeleton of the face
* Mainly form by the first two pharyngeal arches
Cartilaginous neurocranium
Occipital bone
Ethmoid
Lesser and greater wings
Body of sphenoid
Petrous part of temporal bone
Membranous neurocranium
Frontal Bones
Parietal Bones
Cartilaginous viscerocranium
Incus
Stapes
Malleus
Hyoid
Thyroid cartilage
Membranous viscerocranium
Nasal Bone
Maxilla
Mandible
Squama temporalis
Development of the cranium
Calvaria
Sutures
Fontanelles
Calvaria
Cranial vault
Formed by intramembranous ossification in the mesenchyme at the sides and top of the brain
Sutures
fibrous joints composed of dense connective tissue
that joins the flat bones
Sutures (types seen on photo)
Frontal or metopic Suture
Lamboid Suture
Coronal Suture
Sagittal Suture
Fontanelles
6 large fibrous areas where several sutures meet
* The softness of the bones and their loose connections at the sutures enable the calvaria to undergo changes in shape (“molding”) during birth.
Fontanelles (6)
Fontanela anterior
Fontanela posterior
Fontanela Mastoidea (Posterolateral) (2)
Fontanela Esfenoidea (Anterolateral) (2)
Clinical Correlations
Cranium
Acrania
Craniosynostosis
Acrania
Absent calvaria without defects of the vertebral column.
Associated with anencephaly (partial absence of the brain)
occurs in 1/1000 births and is incompatible with life.
Etiology: results from failure of the cranial end of the neural tube to close during the 4th week of development.
Craniosynostosis
Most cranial deformities result from premature closure of the cranial sutures; more common in males than females; prenatal closure results in the most severe abnormalities.
* Mutations of the transcription factors MSX2, TWIST1 and fibroblast growth factor receptors (FGFR1, FGFR2 and FGFR3) have been implicated in cases of craniosynostosis and other cranial deformities
Three types of craniosynostosis
Scaphocephaly
Brachycephaly
Plagiocephaly
Scaphocephaly
Premature closure of the sagittal suture (skull becomes long and narrow); constitutes 57% of all cases.
Brachycephaly
Premature closure of the coronal suture results in a short skull.
Plagiocephaly
The coronal or lambdoid suture closes prematurely on one side only (cranium is twisted and asymmetric).
* Regulation of suture closure involves secretion of various isoforms of transforming growth factor-β.
