Development of Cranium and ossification

Question 1

Paraxial mesoderm

Answer 1

Separates into blocks called somites

Question 2

Somites

Answer 2

Gives rise to vertebrae, ribs, skeletal muscle of the body walls and limbs

Question 3

Lateral plate mesoderm

Answer 3

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

Question 4

Skeletal System 4th week

Answer 4

Develop form paraxial, lateral late mesoderm and neural crest cells

Question 5

Somites differentiate into:

Answer 5

Sclerotom and Dermomyotome

Question 6

Sclerotome

Answer 6

Ventromedial part
Cells form the vertebrae and ribs

Question 7

Dermomyotome

Answer 7

Dorsolateral part
Cells from its myotome region form myoblasts
Cells from its dermatome region form the dermis

Question 8

Myoblasts

Answer 8

primordial muscle cells

Question 9

At the end of the 4th week Sclerotome becomes…

Answer 9

Mesenchyme

Question 10

Mesenchymal cells migrate and differentiate into

Answer 10

Fibroblast
Chondroblasts
Osteoblasts

Question 11

Mesenchyme also derives from…

Answer 11

neural crest cells

Question 12

Mesenchyme in the head region

Answer 12

migrate into the pharyngeal arches and form the bones and connective tissue of craniofacial structures

Question 13

Types of bone ossification:

Answer 13

Intramembranous and Endochondral

Question 14

Intramembranous Ossification

Answer 14

Mesenchymal condensation in the dermis differentiates into bone
Ex. Flat bones of the skull

Question 15

Endochondral Ossification

Answer 15

Mesenchymal cells first give rise to hyaline cartilage models
ex. long Bones and skull base bones

Question 16

Cartilage and bone extracellular matrix consist of…

Answer 16

Collagen type I

Question 17

Intramembranous Ossification

Answer 17

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.

Question 18

Endochondral Ossification

Answer 18

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

Question 19

Primary Ossification Center

Answer 19

Blood vessels Invade the center of the cartilaginous model, bringing osteoblasts to the
diaphysis

Question 20

Secondary Ossification Center

Answer 20

Blood vessels invade the epiphyses

Question 21

Development of the cranium
Cranium consist of

Answer 21

Neurocranium and Viscerocranium

Question 22

Neurocranium

Answer 22

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

Question 23

Viscerocranium

Answer 23

Forms the skeleton of the face
* Mainly form by the first two pharyngeal arches

Question 24

Cartilaginous neurocranium

Answer 24

Occipital bone
Ethmoid
Lesser and greater wings
Body of sphenoid
Petrous part of temporal bone

Question 25

Membranous neurocranium

Answer 25

Frontal Bones
Parietal Bones

Question 26

Cartilaginous viscerocranium

Answer 26

Incus
Stapes
Malleus
Hyoid
Thyroid cartilage

Question 27

Membranous viscerocranium

Answer 27

Nasal Bone
Maxilla
Mandible
Squama temporalis

Question 28

Development of the cranium

Answer 28

Calvaria
Sutures
Fontanelles

Question 29

Calvaria

Answer 29

Cranial vault
Formed by intramembranous ossification in the mesenchyme at the sides and top of the brain

Question 30

Sutures

Answer 30

fibrous joints composed of dense connective tissue
that joins the flat bones

Question 31

Sutures (types seen on photo)

Answer 31

Frontal or metopic Suture
Lamboid Suture
Coronal Suture
Sagittal Suture

Question 32

Fontanelles

Answer 32

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.

Question 33

Fontanelles (6)

Answer 33

Fontanela anterior
Fontanela posterior
Fontanela Mastoidea (Posterolateral) (2)
Fontanela Esfenoidea (Anterolateral) (2)

Question 34

Clinical Correlations
Cranium

Answer 34

Acrania
Craniosynostosis

Question 35

Acrania

Answer 35

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.

Question 36

Craniosynostosis

Answer 36

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

Question 37

Three types of craniosynostosis

Answer 37

Scaphocephaly
Brachycephaly
Plagiocephaly

Question 38

Scaphocephaly

Answer 38

Premature closure of the sagittal suture (skull becomes long and narrow); constitutes 57% of all cases.

Question 39

Brachycephaly

Answer 39

Premature closure of the coronal suture results in a short skull.

Question 40

Plagiocephaly

Answer 40

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.