Dr. Indran Development of Skeletal System

Question 1

What are the sources of origin of the skeltal system

Answer 1

§Paraxial mesoderm

§Somatic layer/parietal layer of lateral plate mesoderm

§Neural crest cells

Question 2

______ cells become ______, the embryonic connective tissue

Answer 2

Sclerotome

mesenchyme

8.5.6

Question 3

Mesenchyme in the _______ transforms to form the bony elements of the vertebral column and ribs

Mesenchyme in the _______ will transform to form the pectoral and pelvic girdles, and the bones of the upper and lower limbs

Answer 3

sclerotome

somatic mesoderm

Question 4

What are the two types of ossification?

Answer 4

Intramembranous:

The mesenchymal tissue will directly transform into bone forming cells to lay down bone

Ex: Skull bones, Bones of the face

**Endochondral:

The mesenchymal tissue will first give rise to a cartilaginous model of the bone:

The cartilage model will then ossify to from bone

Ex: long bones, irregular bones like vertebrae etc

Question 5

Development of the axial skeleton comes from the paraxial mesoderm except for?

Answer 5

Sternum (lateral)

8.5.6

Question 6

During 4th week, ______ cells migrate around the spinal cord and notochord to merge with cells from the opposite somite on the other side of neural tube

Answer 6

(blue dots)

Question 7

How does sclerotome resegment and what regulates it?

Answer 7

The caudal half of each sclerotome grows into and fuses with the cephalic half of each subjacent sclerotome

Thus, each vertebra is formed from combination of caudal half of one sclerotome and cranial half of its neighbor

Hox genes

Question 8

How do spinal nerves become intrasegmental initially?

Answer 8

With splitting and recombination of sclerotomes the originally segmental (intrasegmental) spinal nerves that sprout from each segmental neural tube, become intersegmental

Mesenchymal cells between cephalic and caudal parts of the original sclerotome segment do not proliferate but fill the space between two vertebral bodies: Fibrous part of the intervertebral disc

As the vertebrae form, two primary curves of the spine are established (thoracic and sacral curvatutures)

Question 9

The ______ forms the nuleus pulposus, how does that work?

8.5.6

Answer 9

Notochord

Notochord present inside vertebral body degenerates, whereas notochord present between vertebrae persists and forms nucleus pulposus

Question 10

_____ is when mesenchymal cells from one sclerotome fail to migrate, resulting in lateral curving of the spine (scoliosis)

Answer 10

Hemivertebra

Question 11

*** _____ occurs due to imperfect fusion or non-union of vertebral arches around a part of the spinal cord

____ is mild and most common

______ is when the posterior neuropore does not close, meninges come out through the gap, but spinal cord lies at its normal position

____ is when meninges and spinal cord both come out through the gap

Answer 11

Spina bifida

Spina Bifida Occulta:

Involve only the bony vertebral arches, leaving the spinal cord intact, Bony defect is covered by skin, A dimple, birthmark or hairy patch covering the area, No neurological deficits occur

Spina bifida cystica (meningocele)

MENINGOMYELOCELE

Question 12

______ results from fusion and shortening of cervical vertebrae

Answer 12

Klippel-Feil Anomaly

Results in:

Shortening of the neck

Limited movement of the neck

Low hair-line

Question 13

_____ is classified as dysplasic (congenital), isthmic (stress fracture), degenerative, or traumatic changes of the spine

This defecr may allow anterior (forward) displacement or slippage of the vertebra which is called _______

Answer 13

spondylolysis/spondylolisthesis

spondylolisthesis

A spondylolysis in a child or adolescent most commonly results from a defective formation of the pedicle

Question 14

Explain the image

Answer 14

Congenital Spondylolisthesis:

Pedicles of vertebral arches fail to fuse with the vertebral body

Vertebral body slips forward with respect to vertebrae below it

Causes lordosis

Most common in lower lumbar region (L5-S1)

Question 15

_____ is when either a benign or malignant tumor that arises from remnants of notochord

Answer 15

Chordorma

Most of them lie in the base of the skull, extending to nasopharynx

Can occur in lumbosacral region

Question 16

****______ happens when the nucleus pulposus prolapses into the vertebral canal through the defective annulus fibrosus

Answer 16

Intervertebral Disc Herniation

Question 17

How doe the ribs develop?

Answer 17

¨Ribs develop from the sclerotome in the thoracic region

¨Bony part is derived from central and lateral sclerotome (costal process) cells

¨Sclerotome cells that migrate into the lateral plate mesoderm form costal cartilages

Question 18

¨Sternum develops from the ______

Answer 18

somatic mesoderm

¨Two sternal bars form on either side of the midline

¨These later fuse to form the cartilaginous model of the manubrium, sternebrae (body) and the xiphoid process

Question 19

_____ is a depressed sternum, sunken posteriorly ***

_____ is when the sternum protrudes like a keel

Answer 19

Pectus exacvatum

Pectus Carinatum

8.5.6

Question 20

Development of the muscle overview

Answer 20

Question 21

True or False: ¨Each myotome receives its innervation from spinal nerves derived from the same segment as the muscle cells

8.5.6

Answer 21

8.5.6

Question 22

How does the development of trunk muscles work?

Answer 22

Epaxial Cells:

Form extensor muscles of vertebral column (erector spinae , tranverse spinalis)

Supplied by dorsal rami

Hypaxial Cells:

Form antero-lateral muscles of the body wall and muscles of the limbs

Supplied by ventral rami

Question 23

_____ is progressive muscle weakness and wasting, X-linked recessive disorder caused by a mutation in the gene for dystrophin on the short arm of chromosome X, Dystrophin attaches actin to laminin in the external lamina, common in males

____ is less severe form compared to DMD, late onset (8 to 25 years of age)

Answer 23

Duchenne muscular dystrophy (DMD)

Becker muscular dystrophy (BMD)

Question 24

____ is underdevelopment or absence of chest muscles on one side of the body

Answer 24

Poland syndrome

¨Absence of the pectoralis minor and partial loss of the pectoralis major

¨Nipple and areola are absent or displaced

¨Associated with webbing of fingers of the hand on the same side:

¨syndactyly (fused digits)

brachydactyly (short digits

Question 25

_____ is partial or complete absence of abdominal muscles

Answer 25

Prune belly syndrome

¨Abdominal wall is so thin, organs are visible and easily palpable

¨Associated with malformations of the urinary tract and bladder

¨Caused by non- migration/underdevelop-ment of myoblasts in the hypaxial myotomes

Question 26

How does limb development occur? ***

Answer 26

8.5.6

Question 28

How does AER help with limb outgrowth?

Answer 28

Limb outgrowth is initiated by the apical ectodermal ridge (AER) at the tip of the limb buds

Proceeds from proximal (i.e. shoulder or hip) to distal (i.e. hand or foot)

This process is absolutely dependent on FGF signaling from the AER

Any disruption to FGF signaling and/or formation and maintenance of the AER will result in arrested limb development