Lecture 17: Development of the Musculoskeletal Systems

Question 1

When do the Muscle Systems start to develop?

Answer 1

Week 4

Question 2

Where does all of skeletal muscle in the body come from?

(Bonus: What is the one exception?)

Answer 2

The Myotome of the Paraxial Mesoderm

(Constrictor / Dialator Pupilae of the Eye)

Question 3

For organization of the Somite, what two parts is the Somite broken into, and what occupies those parts?

Answer 3

Ventromedial Part

Sclerotome

Dorsolateral Part

Dermatome and Myotome

It makes sense that the Sclerotome would be more medial, since it forms the vertebrae that are the very center line of the body.

Question 4

The Sclerotome (Mesoderm) becomes Mesenchyme before it makes cartilage and bone. What is Mesenchyme?

Answer 4

Embryonic connective tissue. It has mesoderm in it, but may have other cell types as well.

Question 5

What are the parts of the Sclerotome?

Answer 5

Cephalic Loose part, on top of a Caudal Dense part

Question 6

What do the Sclerotomes do to create the vertebral column?

Answer 6

They separate between the Cephalic Loose part and the Caudal Dense part, and then merge with the other half of the scleratome above / below them.

Question 7

When are muscles being innervated?

Answer 7

Late Week 3 / Early Week 4

Question 8

What structures of a typical vertebra are composed of the Cephalic Loose part of the Sclerotome?

What about the Caudal Dense part?

Answer 8

The Cephalic Loose part of the Sclerotome gives rise to the inferior half of the vertebral body.

Everything else comes from the Caudal Dense portion, including all of the processes and facets.

Question 9

What forms the Nucleus Pulposus as it degenerates?

Answer 9

The Notocord

Question 10

What forms the Annulus Fibrosus?

Answer 10

Fibrous tissue from the Sclerotome.

Question 11

Where do Ribs come from?

Answer 11

They grow out of the Costal Processes of Thoracic Vertebrae

(Therefore they are also made of Sclerotome)

Question 12

Where does the Sternum come from?

Answer 12

Somatic Layer of the Lateral Plate Mesoderm

(Starts as two separate sternal bars)

Question 13

What clinical problem can be caused by a Cervical Rib?

Answer 13

Thoracic Outlet Syndrome

Question 14

What is Pectus Carinatum?

Answer 14

Pidgeon Chest (Sternal Protrusion)

Question 15

What is Pectus Excavatum?

What are the complications / Symptoms?

Answer 15

Funnel Chest

(A sternal depression of the chest)

The heart can become compressed, leading to shortness of breath, exercise intolerance, etc.

Question 16

What does a gain of function of a Hox gene cause in the nearby Vertebrae?

Answer 16

Caudalization

Superior vertebrae begin to look more inferior

• Eg: Cervical vertebrae begin to look more like Thoracic vertebrae - gain of function of Hox 6*
• Hox pulls down.*

Question 17

What does a loss of function of a Hox gene cause?

Answer 17

Cranialization

Vertebrae begin to look more superior.

• Hox pulls down,*
• so less Hox goes up.*

Question 18

Where do Hox 4, 5, and 6 act?

Answer 18

Hox 4: Upper Cervical

Hox 5: Mid to Lower Cervical

Hox 6: Lower Cervical to Upper Thoracic

Question 19

Where do Hox 9, 10, and 11 act?

Answer 19

Hox 9: Lower Thoracic / Upper Lumbar

Hox 10: Upper Lumbar / Lower Sacral

Hox 11: Upper Sacral / End Coccygeal

Question 20

What specific structure requires Hox 9 to exist?

Answer 20

Floating Ribs

Question 21

What organizational part of the Somite gives rise to Muscles?

Answer 21

The Dorsolateral Part

(This part becomes both the Myotome and the Dermotome)

Question 22

What two parts does a Myotome split into?

Answer 22

Epimere (Epaxial / true muscles of the back / dorsal rami)

Hypomere (Hypaxial muscles / all of the other muscles / ventral rami)

Think about the embryo on its belly like a lizard. Up would be dorsal (epi) and down would be ventral (hypo)

Question 23

What are the four divisions of the Hypaxial part of Myotomes?

Answer 23

Cervical, Thoracic, Lumbar, Sacrococcygeal

Question 24

What does the Cervical division of the Hypaxial Myotome give rise to?

Answer 24

Scalene, Prevertebral, Geniohyoid, and Infrahyoid Muscles.

Muscles of the front / sides of the neck

Question 25

What does the Thoracic division of the Hypaxial Myotome give rise to?

Answer 25

Lateral and Ventral Flexor Muscles

(Rectus Abdominus and Obliques)

Question 26

What does the Lumbar division of the Hypaxial Myotome give rise to?

Answer 26

Just one muscle:

Quadratus Lumborum

Question 27

What does the Sacrococcygeal division of the Hypaxial Myotome give rise to?

Answer 27

Muscles of the pelvic diaphragm

Question 28

What is Poland Syndrome?

What is it associated with?

Answer 28

Absence or underdevelopment of Pectoralis Major or Minor Muscle.

(Hypomere doesn’t migrate into the chest)

Associated with abnormal breast development on the side of the deficiency and syndactyly.

Other muscles will compensate, and function is not reduced

Question 29

What is Prune Belly Syndrome?

Who is primarily affected?

What is associated?

Answer 29

Absence of Abdominal Musculature

Males

“Cryptorchidism” Failure of the testicles to decend.

Malformation of the Urinary Bladder