24 - Development of Skeletal Tissue, Spine, Ribs, and Sternum Flashcards
What are the two forms of skeletal tissue?
Bone and cartilage
What is the mesenchyme of the trunk and head derived from?
Trunk: paraxial mesoderm and somatic mesoderm.
Head: neural crest ectomesenchyme and head mesoderm.
What are the master genes for bone and cartilage formation?
Bone: RunX2 [CBFA1) osteoblast specific transcription factor
Cartilage: Sox9 is a chondroblast-specific transcription factor
These activate the expression of other genes that are activated in only bone or cartilage forming cells. These are not involved in patterning of skeletal elements.
What occurs if an ambryo is homozygous for runX2/CBFA?
Null mutant with no bones.
Skull is fibrous CT.
Skeleton is calcified cartilagenous skeletol with short limbs.
Fatal.
What is the initial step of developing supporting tissue? What else is involved in this process?
Initial step: condensation of preskeletal mesenchyme.
Epithelial mesenchyme interactions are involved:
- AER/mesenchyme forms the limb
- Neural tube/mesenchyme formsthe skull/vertebrae
- Notochord/mesenchyme forms the skull vertebrae
What occurs when the skeletal tissue forming mesenchyme preskeletal condenstation expresses sox9?
It becomes cartilage
How does skeletal tissue forming mesenchyme preskeletal condenstation become tissue that performs endochondrial ossification?
The preskeletal condensation expresses Sox9 and becomes cartilage model of bone.
That expresses to Ihh and VegF, which causes bone to replace the cartilage.
That expresses RunX2 causing endochondrial ossification.
How does skeletal tissue forming mesenchyme preskeletal condenstation become tissue that performs intramembranous ossification?
Mesenchyme cells express RunX2 and begin to form bone.
This type of ossification is seen in the flat bones of the skull and face.
What happens in your body when you have low serum Ca2+?
Calcium sensing receptors cause the parathyroid to release PTH.
PTH causes the kidney to make 1,25 (OH)2 D3 causing an increase in intestinal Ca absorption, and increased Ca2+ resorption in the kidneys, and increased bone Ca2+ mobilization.
What are ossification centers?
Areas in the sksletal primordium where ossification begins.
Can be used as a measures of skeletal maturation and development.
Where are the primary and secondary ossification centers? When do they occur during development?
Primary: initiatial center to appear; located at the shaft of long bone or center of flat bone. Appear around 7 weeks.
Secondary: Appear in perinatal, postnatal, or postpubertal period; ends of long bonds (epiphysis), heads of ribs. Close in 20-30s. Hormonal control of maturation (estrogenand thyroid hormone).
What is bone age?
The amount of epiphyseal cartilage retained - comparison of bone and chronological age is a measure of skeletal growth and maturation.
What are chracteristics of generalized skeletal tissue displasias?
- May affect all or part of skeleton
- Often affect growth andmay result in short or tall stature
- Often a component of the ECM is defeftive
- Often recognized genetic component
What is mucopolysaccharidoses?
Defect in synthesis, storage, or transport of a particular lysosomal enzyme; results in accumulation of substrate.
What are vertebrae derived from?
Scleratome cells
How do scleratomes form vertebrae?
- They organize into cranial and caudal subdividions with loose and dense areas of sclerotome divided by non-ebner’s fissures.
- Re-segmentation occurs so loose area of one sclerotome unites with a dense area of another to form vertebral precursor.
- Muscles attach to multiple tissues so the muscle can act on adjacent vertebra
What do the cells of the cranial (anterior) half of the scleratome contribute to?
Vertebral body, small parts of the neural arch and distal rib (in thoracic region).
What do the cells of the caudal (posterior) half of the scleratome contribute to?
Vertebral body, transverse processes, most of the neural arch and proximal part and main part of distal rib (in thoracic region).
How do forming vertebrae ossify?
There are 3-4 primary centers (by week 8): 2 at the centrum and one at each arach.
Closure of the neurocentral junction is at 3-6 years.
Secondary centers for at puberty.
What is dysraphism? What two diseases can be seen?
Open vertebral column due to failure of fusion of the spinous process.
Rachischisis: spine division - extensive opening with CNS involvement
Spinfa bifida: variation in length of opening
What are the two types of abnormal vertebrae formations? What is the worest case scenario when someone has scoliosis?
Hemi-vertebrae and block vertebrae (fused)
Worse case: hemivertabrea on one side and a fusion on the contralateral side. Very unstable.
What are two congenital defects that result in vertebral asymmatry?
Congenital kyphosis and congenital lordosis.
How does the sternum form?
Sternal bands fuse to become cartilage sternum, which ossifies to become an ossified sternum.
