Development of the Muscular & Skeletal Systems

Question 1

When is the critical period in development?

Answer 1

weeks 3 - 8

Question 2

When do the upper limbs and lower limbs begin development

Answer 2

week 4 (lower limbs start after)

Question 3

Where does myotome come from and what comes from it

Answer 3

1) Somites (paraxial)

2) ALL MUSCLE

Question 4

What are the three types of somites and what do they give rise to

Answer 4

myotome - muscle
sclerotome - axial skeleton
dermatome - dermis (some)

Question 5

Describe the process of vertebral body formation starting with Somite division

Answer 5

1) The somite divides diagonally into the dorsal later part (myotome and dermatome) and the ventral medial part (sclerotome)
2) The sclerotome from the ventral medial part migrates down to surround the notochord to form the vertebrae
3) The Caudal Dense part of the sclerotome from the superior sclerotome A migrates down to merge with the inferior Cephalic Loose part of sclerotome B.
4) Merging of these parts leads to the creation of the vertebral body

Question 6

What forms along with the vertebral body and by what week

Answer 6

WEEK 5

1) Muscle movement (joints become crossed by muscles – myotome connect to vertebrae/neural tube)
2) Motor Nerves (sensory n. form later using motor as a template)

Question 7

When does the vertebral column form by

Answer 7

week 5

Question 8

Where do the following derive from:
1 - PNS sensory neurons
2 - PNS motor neurons
3 - PNS preganglionic neurons

Answer 8

1 - Neural Crest

2 & 3 - Neural Ectoderm

Question 9

Where do nerves come out in the cervical spine

Answer 9

Above the vertebrae whose number they share (i.e. second number rule) — different in the rest of the body

Question 10

What do motor nerves do by week 5

Answer 10

migrate between the vertebrae to innervate myotomes

Question 11

What parts of the adult vertebrae derive from A) Caudal Dense part and B) Cephalic Loose part

What about the spinous process?

Answer 11

Superior parts = Caudal Dense (superior articular process, pedicle, transverse process)

Inferior parts = Cephalic Loose (inferior articular process)

Spinous process = BOTH!!

Question 12

Where does the nucleus pulposus come from

Answer 12

notochord remnant – mesoderm

mucoid substance/jelly

Question 13

where does the annulus fibrosis come from

Answer 13

sclerotome

fibrous tissue/fibrocartilage

Question 14

Rib origin

Answer 14

from sclerotome cells that migrate out from the costal processes of thoracic vertebrae

Question 15

sternum origin

Answer 15

somatic lateral plater mesoderm

Mesenchymal condensation cells migrate to midline from the ribs to ossify as the sternum

Question 16

Hox genes (what parts of spine to they impact)

Answer 16

group of genes that determine bone shape and control the body plan on a cranial-caudal axis

Hox 4 + 5 = cervical 
Hox 6 = Ribs (thoracic)
Hox 9 = Floating Ribs (thoracic)
Hox 10 = lumbar
Hox 11 = sacral

Question 17

Caudalization (and Hox genes affected)

Answer 17

aka gain of function
leads to vertebrae transformation where vertebrae take on characteristics of vertebrae INFERIOR to it (i.e cervical become thoracic w/ ribs)

Hox 6 - cervical to thoracic (w/ ribs)
Hox 10 - thoracic to lumbar (no ribs)

Question 18

Cranialization (and Hox genes affected)

Answer 18

aka loss of function
leads to transformation of vertebrae to take on characteristics of vertebrae SUPERIOR to it

Hox 4 = C2-C3 will look like C1

Hox 5 = C4-T2 will look like C2

Hox 10 = look like thoracic vertebrae (with ribs)

Question 19

Cervical Rib abnormality

Answer 19

Rib development in cervical vertebrae (VERY RARE - lumbar more common)

Caused by gain of function (caudalization)

Contributes to THORACIC OUTLET SYNDROME —- subclavian A is compressed, causes issues in limbs

Question 20

Pectus carinatum

Answer 20

aka Pigeon chest

unusual growth of rib that makes chest jut out (sternum protrusion)

males more than females
noticed at age 11
no known causes

Question 21

Pectus excavatum

Answer 21

sternum depression (excavated chest)
aka funnel chest

Heart displaces almost entirely to the left side
Patients often have exercise intolerance
noticed by age 1

Question 22

What are the subclassifications of myotome (explain them)

Answer 22

1) epimere
- epaxial muscles — deep back muscles (LAYERS 4,5,6)
- dorsal/posterior rami innervated

2) hypomere
- hypaxial muscles —- flexor muscles of the trunk and limbs/all other skeletal muscles
- ventral/anterior rami innervated

Question 23

What part of the somite does epaxial, hypoaxial and dermatome derive from?

Answer 23

epaxial — dorsal part of the Dermomyotome
hypoaxial —- ventral part of the dermomyotome

dermatome – central part of the dermamyotome

Question 24

How does the migration of hypomere cells from the dermomyotome determine what skeletal muscles they become?

Answer 24

Some migrate to the Doral side of the limb bud, creating the anterior compartment muscles (triceps, extensors —- radial and axillary n.)

The others go to the ventral side of the limb bud to make the anterior compartment (flexors, pronators — lateral and medial cords)

Question 25

Poland syndrome

Answer 25

SEE TANK 26 in lab for an example!!!

• No pectoral major or minor present
• Ipsilateral breast hypoplasia (different musculature on each breast)

-Absence of 2-4 ribs (severe cases)

Question 26

Prune-Belly Syndrome

Answer 26

Partial or complete absence of abdominal muscles

• primarily affects males
• associated with:
  - cryptorchidism (failure of testicular drop)
  - poor formation of bladder/urinary tract