19: Musculoskeletal System Development

Question 1

Where is paraxial mesoderm + what is it continuous with?

Answer 1

Longitudinal columns along the developing neural tube, continuous with intermediate mesoderm

Question 2

What is paraxial mesoderm derived from?

Answer 2

Primitive node

Question 3

What is lateral plate mesoderm continuous with?

Answer 3

Extraembryonic mesoderm covering umbilical vesicle and amnion

Question 4

When do somites form?

Answer 4

End of 3rd week

Question 5

Sequence of somite formation

Answer 5

Craniocaudally

Question 6

How many somite pairs are there initially -> by the end of the 5th week

Answer 6

28-40 -> 42-44 pairs

Question 7

What do somites give rise to?

Answer 7

Most of axial skeleton and associated musculature and dermis

Question 8

When does the sclerotome appear?

Answer 8

End of 4th week

Question 9

What will the sclerotome develop into?

Answer 9

Vertebrae and ribs

Question 10

What causes sclerotome to have cranial and caudal portions?

Answer 10

Gene expression and cell density

Question 11

What forms the centrum?

Answer 11

Adjacent cranial + caudal segments of the sclerotome

Question 12

Why do we have 8 cervical nerves and only 7 vertebrae?

Answer 12

The occipital bone is also formed by somites

Question 13

What do ribs develop from?

Answer 13

Costal processes on T vertebrae

Question 14

When do ribs ossify?

Answer 14

During fetal period (are cartilagenous in embryonic period)

Question 15

Sternum development

Answer 15

Paired sternal bars move medially -> fuse at 10 weeks

Question 16

What do the sternal bars arise from?

Answer 16

Lateral plate mesoderm (somatic division)

Question 17

What do Hox boundaries correlate with in vertebrae?

Answer 17

Changes in vertebral shape/type of vertebrae

Question 18

Thoracic outlet syndrome

Answer 18

A supernumerary rib on C7 can impinge the brachial plexus and/or subclavian A.

Question 19

Pectus excavatum

Answer 19

Funnel chest; depressed sternum

Question 20

Pectus carinatum

Answer 20

Pigeon chest; bilateral flattening of chest with anterior sternal protrusion

Question 21

Steps in myogenesis

Answer 21

1. Mesenchyme nuclei and cell bodies elongate -> myoblasts
2. Myoblasts fuse -> myotubes
3. Myofilaments develop in myotubes
4. Myotubes + CT -> fascicles

Question 22

When do skeletal muscles develop?

Answer 22

Most before birth; remaining ones are formed by one year

Question 23

Poland syndrome

Answer 23

Absence of pec minor + partial loss of pec major -> ipsilateral breast hypoplasia

Question 24

Two conditions Poland syndrome can be associated with

Answer 24

Syndactyly or brachydactyly

Question 25

Population most at risk for Poland syndrome

Answer 25

Males

Question 26

Prune belly syndrome

Answer 26

Partial or complete abs ente of abdominal musculature + cryptorchidism (failure of testes to descend) + malformation of urinary tract