Development Of The Muscular System

Question 1

Paraxial mesoderm

Answer 1

Somites become…

• myotome (muscle)
• Scleratome (axial skeleton)
• dermatome (dermis)

Question 2

Intermediate mesoderm

Answer 2

Urogenital, becomes…

• Kidneys
• gonads

Question 3

Lateral plate mesoderm

Answer 3

Connective tissue, like…

• blood
• lymph
• mesenteries
• cardiovascular

Question 4

Somites line up along the neural fold, can be used to what?

Answer 4

Counting Somites can be used to determine the age of the embryo

Question 5

Two parts of the Somite

Answer 5

1. Dorsolateral»dermatome + myotome

2. Ventromedial»scleratome ( will migrate around notochord and neural tube.

Question 6

Describe the two paths for bone development to the axial skeleton.

Answer 6

1. Scleratome»mesenchyme»bone
   - called intramembranous ossification
   - flat bones of skull, many face bones, and clavicle
2. Scleratome»mesenchyme»hyaline cartilage»bone
   - called endochondral ossification
   - long bones, pelvis, pectoral girdle

Question 7

Formation of the vertebral column

Answer 7

• Caudal dense portion fuses with cephalic loose portion of the Scleratome inferior to it
• neural tube sends nerves out between the newly formed vertebra and attach to myotome
• newly innervated muscle attach to vertebra above and below

Question 8

What parts of the vertebral column are formed by which parts of the scleratome?

Answer 8

Caudal dense—forms superior body and spinous process

Cephalic loose—forms inferior vertebral body

Question 9

Intervertebral discs have two parts.

What are they made from?

Answer 9

1. Nucleus pulposus
   - made of notochord
   - notochord expands, degenerates, and globs up
   - creates a mucoid substance
2. Annulus fibrosis
   - fibrous tissue from the scleratome
   - encircles the globs of degenerating nototchord

Question 10

Formation of ribs

Answer 10

• scleratome cells that grow out from the costal processes of the thoracic vertebra
• costal cartilage also comes fro the scleratome

Question 11

Formation of the sternum

Answer 11

• develops in the somatic lateral plate mesoderm
• sternal bars start as mesenchymal condensations
• sternal bars fuse together to become sternum

Question 12

Cervical ribs

Answer 12

• contribute to thoracic outlet syndrome

- brachial plexus can become compressed leading to motor and sensory issues

Question 13

Pectus cariatum

Answer 13

• sternum protrusion
• “pigeon chest”
• no deficits, only cosmetic
• boys more affected
• becomes more pronounced as puberty progresses

Question 14

Pectus excavatum

Answer 14

• sternum depression
• “funnel chest”
• can compress and shift hear to the left
• complications include SOB, pain, fatigue, exercise intolerance
• on a spectrum, can be mild or severe.

Question 15

Hox genes

Answer 15

Genes that control the body plan along the cranio-caudal axis

Question 16

Gain of function of hox genes leads to what?

Answer 16

Caudalization

• changes in the combinatorial code, results in transformation of the vertebrae
• ex. If hox 6, which usually is for C6-T6, extends up pinto C4+ C5, those cervical vertebra will appear more thoracic

Question 17

Loss of function of hox genes leads to what?

Answer 17

• cranialization
• changes in the combinatorial code, results in transformation of the vertebrae
• ex. If hox 5, which usually plans C3-T2, is deleted, all of those vertebrae will appear more cervical in nature.

Question 18

The dorsolateral part of the paraxial mesodermal somite, becomes the dermatome and the myotome.

The myotome gives rise to two different structures, what are they and what do they become?

Answer 18

1. Epimere
   - give rise to epaxial muscles (true muscles of the back)
   - innervated by dorsal rami
2. Hypomere
   - hypaxial muscles (all other muscles)
   - innervated by ventral rami

Question 19

Epaxial muscles

Answer 19

• form extensor muscles of neck and vertebral column

Question 20

Hypaxial muscles

Answer 20

• cervical myotome: scalene, prevertebral, genioohyoid, infrahyoid muscles
• thoracic myotome: lateral and ventral flexor muscles of the vertebral column (obliques, rectus abdominus)
• lumbar myotome: quadratus lumborum
  Sacrococcygeal myotome: muscles of pelvic diaphragm

Question 21

Poland syndrome

Answer 21

• complete or partial absence of pec major/ pec minor
• due to absence of migration of cells of hypomere into chest
• can also lead to absence of ribs 2-4
• often associated with syndactyly (fusion of digits)

Question 22

Prune belly syndrome

Answer 22

• partial or complete absence of abdominal musculature
• primarily affect males
• associated with cryptochidism(failure of one or both testicles to descend) and malformation of urinary tract/bladder»> urethral obstruction
• caused by abnormal migration of hypomere cells.