Embryo MusculoSkeletal Development

Question 1

what are the 2 divisions of the skeleton

what are the 2 supporting tissues for the body?

what 2 paths lead to ossification?

what is the embryonic progenitor tissue for the skeleton?

mesencyme is derived from?

Answer 1

skeleton divided into axial (skull, spine, ribcage) and appendicular (limbs, shoulder girdle and pelvis)

1) cartilage: flexible and avascular support
2) bone: rigid support, mineralized, and vascularized

ossification:

a) intramembranous ossification
b) endochondral ossification

pregenitor: mesenchyme

• mesenchyme derived from the :
  1) paraxial and somatic mesoderm (remember somatic is from the lateral mesoderm)

2) ectoderm neural crest cells for parts of the cranium

Question 2

STFM has what important features

what does it stand for

Answer 2

skeletal tissue forming mesenchyme

1. skeletal progenitor mesenchyme migrate or are displaced from their site of origin
2. STFM condenses into pre skeletal condensations
3. differentiation is influenced by epithelial-mesencyhmal interactions
4. transcription factors:
   - CBFA1 gene: core binding factor is an osteoblast specific transcription factor.

the osteoblasts lay down new bone but the CBFA1 gene is not involved in patterning

why: bc when knock out this gene in mouse, no bones were formed
- Sox-9 is a transcription factor for cartilage

signaling molecules:
BMP

Question 3

initial step of cartilage development

Answer 3

condensation of the pre skeletal/precartialge mesenchyme cells. this is usually a epithelial-mesenchymal interaction

Question 4

cartilage formation occurs when

Answer 4

precartilage mesenchyme cells condense and the central cells are signaled via transcription facto Sox-9 to differentiate into chondrocytes

-forms chondrocyte center which secrete cartilage matrix of

surrounding cells will then form a perichondrium around the expanding cartilage matrix

Question 5

places that remain cartilage for life

Answer 5

nose,
larynx
coastal and auricular surfaces
IV discs

Question 6

how does cartilage become bone?

Answer 6

through ossification centers.

Question 7

primary ossification centers vs secondary ossification centers

what influencing factors for development?

Answer 7

primary centers initially appear in the shaft of a long bone or the center of a flat bone

-appear at 7 wks of development. osteoblasts produce bone matrix

secondary ossification centers appear later during perinatal, postnatal, or post puberal periods. These centers are located at the ENDS of long bonds and are called epiphysis
close by 20-30 years of age

*ossification centers under control of hormones.

Question 8

what is the bone age

Answer 8

amount of epiphyseal cartilage retained. compare the bone age and chronological age to determine skeletal growth and maturation

Question 9

vertebrae are formed from

Answer 9

sclerotome

-ventromedial part of somite

Question 10

what are the steps of vertebral development

actually pretty cool

Answer 10

1. sclerotome are placed medially
2. some slcerotme move even more medially to form the PERINOTOCHORDAL SHEATH
3. lateral sclereotome mesencyme arranges into a loose cranial half and a dense caudal half (on the bottom)
4. cells of the perionotochordal sheath also arrange into loose and dense areas

Question 11

primitive vertebrae are composed of what?

Answer 11

centrum and neural arches

centrum0 loose part of the perionotchordal sheath from 2 adj somite

neural arch- compact part of sclereotome

IV- compact part of perinotochordal sheath

Question 12

how is the development of the vertebrae patterned

Answer 12

axial patterning (cranial-caudal) which is used to develop regions of vertebrae being cervical, thoracic, and lumbar or sacral

this is controlled by hox genes and reinoic acid (think limb development fingers anter/postieror)

Question 13

where do the myotomes come in

Answer 13

differential arrangement of the sclerotome and periontochordal sheath results in the myotmes spanning of adjacent vertebrae process

Question 14

what are abnormalities of vertebral development:

abnormal regionalization

Answer 14

1. due to fusion would be klippel-fiel anomaly: shortened neck, fused cervical vertebral and low nuchal hair line.
   also get gintoruniary and cardiopulmonary defects and hearing loss (think organogenesis and lateral mesoderm)
2. change in number: change in number of the TYPE of vertebrae, but not the total number

Question 15

what are abnormalities of vertebral development:

defects of asymmetry

Answer 15

scoliosis- abnormal lateral curvature of spine

kyphosis- exaggeration of the outward curvature of the thoracic spine

lordosis- exaggeration of the inward curvature of the lumbar spine

Question 16

what are abnormalities of vertebral development:

defects of formation

Answer 16

hemivertebrae:
-failure of primary ossification centers to form

wedge shaped vetebrae
unsegmented bars

Question 17

what are abnormalities of vertebral development:

dysraphism

Answer 17

dysraphism is open vertebral column due to failure of the spinous process to fuse

1. rachischisis: spine division
   - extensive open in with the involvement of the CNS

SPina bifida: variation in length of opening and has several subtypes

1. SB occulta: spinous process have not fused but the spinal cord and meninges are in tact, you see tuft of hair along the back of neck
2. SB with meningocele: out of pocketing of the meninges and absence of spinous process. spinal cord is fine
3. SB with meningomyelocele:
   out of pocketing of meninges and spinal cord
4. myeloschisis: open spinal cord to amniotic fluid bc no bone and no meninges

myelo= spinal cord
meningo=meninges

Question 18

how are ribs formed.

abnormalities?

Answer 18

ribs formed by lateral extensions of the costal process in thoracic regions.

1. accessory: extra rib
2. abnormal: fused or forked rib

Question 19

how is the sternum formed

abnormalities?

Answer 19

fusion of sternal bands to first form cartilaginous sternum. then undergoes multiple primary ossification centers to ultimately become ossified sternum

abnormalities: cleft (hollow)

Question 20

what are the 3 parts of the skull that are developed?

• pregenitor cells
• type of ossification
• location

Answer 20

1) neurocranial: undergo endochondral ossification on the flat bones of the head. this is the cranium area. doesn’t fuse until after birth so have fontanelles in place until then.
pregenitor: neural crest and mesoderm

2) chondrocranium: has cartilage that allows for longitudinal growth of the head. these are at the base of the head. They come from neural crest and mesoderm
3) viscerocranial: like the jaw and mouth bones. come primarily from cranial and do intramembrous ossification

Question 21

what are abnormalities of skull formation?

Answer 21

1. marfans: fibrillin defect so get long limbs and hyper extensible joints along with aortic dissection and long limbs
2. congential dislocation of the hip. common in females
3. achondroplastia: shorter stature bc of early closure of epiphyseal plate. problem with FGFR3 gene. ** germ line mosaic bc parents don’t have it in their somatic cells
   * mosaic- achondroplastia
4. craniosynostosis: cranium closes too soon due to FGF gene again
5. dysraphic condition: skull doesn’t close and its open (cranioschisis)