Embryology of the Muscular and Skeletal Systems

Question 1

Define somites

Answer 1

Somites are segments of paraxial mesoderm (form alongside the notochord)
Somites form the Dermatome, Myotome, and Sclerotome

Question 2

Define Dermatome, Myotome, Sclerotome

Answer 2

Dermatome: forms from somites (paraxial mesoderm); migrates to form the dermis layer (medial layer of skin)

Myotome: forms from somites (paraxial mesoderm); migrates everywhere to form skeletal muscles

Sclerotome: forms from somites (paraxial mesoderm); forms “hard” structures like bones (axial skeleton) and cartilage

Question 3

Define Intracartilaginous/Endochondral Ossification

Which bones use this method to form?

Answer 3

One of two ways in which bone can form!
Sclerotome-derived bones (most bones! e.g. vertebrae, ribs, sternum) all use this method!
1. mesenchyme condenses
2. CHONDROBLASTS in the mesenchyme form a CARTILAGE MODEL of the bone
3. the cartilage model becomes ossified by osteoblasts

Question 4

Define Intramembranous Ossification

Which bones use this method to form?

Answer 4

One of two ways in which bone can form!
Importantly, used to form SKULL

1. mesenchymal sheets (membranes) condense but do not form cartilage
2. osteoblasts differentiate and deposit BONE SPICULES

Question 5

Define epiphysial growth plate

Answer 5

this is a cartilage plate that keeps forming new cartilage model cells (on the epiphyseal side) that are ossified (on the diaphysial side) and cause long bones to continue growing until about age 20

Question 6

Define achondroplasia

Answer 6

“dwarfism”: premature or abnormal ossification of cartilages, particularly affecting long bones (short limbs, short stature)

Question 7

Define scoliosis

Answer 7

Scoliosis is a lateral curvature of the spine caused by (for the purposes of this class) an error in vertebral body formation or fusion, for example hemivertebra

Question 8

Define Epaxial and Hypaxial myotome

Answer 8

epiblast -> mesoderm -> paraxial (along notochord) mesoderm -> somites -> dermatome/myotome/sclerotome -> myotome -> epaxial and hypaxial divisions

Epaxial division: gives rise to EXTENSORS of the trunk region (e.g. erector spinae)

Hypaxial division: gives rise to the FLEXORS of the trunk region (including muscles of the ventral body wall)

Question 9

What gives rise to skeletal muscle?

What gives rise to smooth (visceral) muscle?

Answer 9

Most but not all skeletal muscle arises from the myotome (from somites from paraxial mesoderm).

Smooth muscle around the gut arises from splanchnic (visceral) mesoderm, which is the inner lining of the Intraembryonic Coelom (with midgut, bit of yolk sac, inside of it)

Question 10

What is prochordal mesoderm?

What does it form and how does it form this?

Answer 10

Prochordal mesoderm is a region of mesoderm that is at the rostral end, rostral to the oropharyngeal membrane.

1. Cavitation splits it into dorsal (somatic) and ventral (splanchnic) halves.
2. Splanchnic prochordal mesoderm forms angiogenic cells.
3. The angiogenic cells coalesce into a solid tube.
4. Canalization of the tube forms a pair of hollow endocardial tubes which will form the inner lining of the heart.

ALSO: prochordal mesoderm will become CARDIAC MUSCLE

Question 11

What are the branchial arches and what will they become?

Answer 11

NEURAL CREST cells migrate and form “gills” (branchial, or pharyngeal, arches)
These will become MOST MUSCLES IN THE HEAD AND NECK

Question 12

What is branchial arch mesenchyme derived from?

Answer 12

from neural crest

Question 13

What is the Apical Ectodermal Ridge?

Answer 13

An area of ectoderm at the tip of a forming limb that is an INDUCTIVE structure! It sends out Fibroblast Growth Factors 8 & 10 that induce the different structures of the limb.

Question 14

How are digits formed?

Answer 14

The AER splits and induces the formation of digits…

DIGITAL RAYS are sculpted by utilizing apoptosis between each “fingertip” of apical ectodermal ridge.

Question 15

What are AMELIA and MEROMELIA?

Answer 15

These are errors in limb induction or pattern formation.
Amelia: complete
Meromelia: partial

Question 16

What are POLYDACTYLY and SYNDACTYLY?

Answer 16

These are errors in digital ray formation and/or the programmed cell death between rays.
Polydactyly: too many digits
Syndactyly: too few digits (fused)

Question 17

What parts of the musculoskeletal system are derived from somites and what parts are not?

Answer 17

Derived from somites:
dermis (from dermatome)
skeletal muscle (from myotome)
most of the axial skeleton (from sclerotome)

Not derived from somites:

• -cranial vault (from head mesoderm)
• -visceral smooth muscle (from splanchnic mesoderm of lateral plate)
• -smooth muscle of blood vessels and lymphatics (from somatic mesoderm of lateral plate)
• -cardiac muscle (from prochordal mesoderm)
• -head and neck muscles (from branchial arch mesenchyme from neural crest)

Question 18

What are the embryonic origins of the axial and appendicular skeleton?

Answer 18

axial skeleton: sclerotome-derived cells form vertebrae, ribs, sternum via intracartilaginous ossification
head mesoderm forms cranial vault via intramembranous ossification

appendicular skeleton: made of ectoderm, somatic mesoderm, and paraxial mesoderm via intracartilaginous ossification

Question 19

What is the fate of the notochord?

Answer 19

the notochord is replaced by sclerotome cells from somites (-> vertebrae) and the remnant of the notochord is the NUCLEUS PULPOSUS, in the intervertebral discs

Question 20

Why are the vertebral bodies considered INTERSEGMENTAL structures?

Answer 20

The boundary between vertebrae shifts by half a segment.

1. Sclerotome cells migrate to notochord in a segmented fashion.
2. The caudal halves of each segment form cartilages.
3. The rostral halves stay less condensed.
4. The caudal half of one segment ossifies with a rostral portion of the next segment back.
5. The intervertebral disc is formed from the rostral portion that was not ossified with the caudal segment ahead of it.

Question 21

What is the contribution of the branchial arch mesenchyme to the musculoskeletal system?

Answer 21

Branchial arch mesenchyme (neural crest-derived) froms most muscles in the head and neck.

Question 22

What types of tissues participate in limb formation?

Answer 22

Muscles of limbs are formed from MYOTOME cells (from the somites from paraxial mesoderm).

Appendicular skeleton is made of:
ectoderm
somatic mesoderm
paraxial mesoderm

Question 23

What is the developmental origin of cardiac muscle?

Answer 23

prochordal mesoderm

Question 24

What is the developmental origin of smooth muscle?

Answer 24

splanchnic mesoderm from lateral plate -> visceral smooth muscle

somatic mesoderm from lateral plate -> smooth muscle of blood vessels and lymphatics

Question 25

What is a dermatome?

Answer 25

Dermatomes: the third division of the somites (others are myotome, sclerotome)

–form the connective tissue layer of the skin (dermis)

–dermatomes originally are segmentally arranged

–dermatomes grow out with limb buds in an orderly pattern (which later gets stretched and twisted as the limbs develop)

Question 26

What is the relationship between dermatomes and spinal nerves?

Answer 26

–dermatomes are innervated by spinal nerves, which are also segmental

–dermatome by definition is “the area of skin supplied by a bilateral pair of spinal nerves and their spinal ganglia”

–dermatomes overlap (really, the areas innervated by spinal nerves overlap)

Question 27

What does a numb dermatome segment mean, from a clinical point of view?

Answer 27

There is a problem at the spinal nerve level