Embryology of Musculoskeletal System ✅

Question 1

What does development of the MSK system require?

Answer 1

Differentiation into specific cell types, and co-ordination to produce an integrated and functional system

Question 2

What does the MSK system develop from?

Answer 2

Mainly the mesodermal germ layer, with some neural crest contribution

Question 3

What is mesoderm subdivided into?

Answer 3

• Paraxial
• Intermediate
• Lateral

Question 4

What happens to the mesoderm in the 3rd week of gestation?

Answer 4

The paraxial mesoderm forms ‘little balls’ (somites), which are paired each side of the neural groove

Question 5

What happens to the somites?

Answer 5

They differentiate differently in different regions

Question 6

What can the somites differentiate into?

Answer 6

• Sclerotome

- Dermomyotome

Question 7

What happens to the sclerotome in development?

Answer 7

It splits segmentally, giving rise to the vertebral column

Question 8

What does the dermomyotome develop into?

Answer 8

Dermal and muscle components

Question 9

What creates the limb buds?

Answer 9

The migration of the lateral mesoderm and the dermomyotome to the limb field

Question 10

What gives rise to the mesenchyme?

Answer 10

Mesodermal cells

Question 11

What is the mesenchyme?

Answer 11

Loosely organised connective tissue

Question 12

What do mesenchymal cells differentiate into?

Answer 12

They are pluripotent, and differentiate into many different cell types, including bones and cartilage

Question 13

What are the ways that bones can develop?

Answer 13

• Direct ossification of the mesenchyme

- Endochondrial ossification

Question 14

What bones form from direct ossification of the mesenchyme?

Answer 14

• Cranial bones of the skull

- Clavicle

Question 15

What bones form from endochondrial ossification?

Answer 15

The bones of the limb and girdle

Question 16

What is endochondrial ossification?

Answer 16

The ossification of a cartilaginous precursor of the long bones of the skeleton

Question 17

What is the first step in endochondrial ossification?

Answer 17

Condensation of lateral plate mesenchyme occurring in a rod-like structure along the axis of the limb bud

Question 18

What happens after the condensation of lateral plate mesenchyme in the limb buds?

Answer 18

Chondrocytes (cartilage cells) respond to growth factors and differentiate in a process called chondrification.

Question 19

What do the chondrocytes secrete?

Answer 19

Proteoglycans and collagen

Question 20

What is the result of the proteoglycan and collagen secretion by chondrocytes?

Answer 20

Formation of cartilage

Question 21

Where is there deposition of cartilage initially?

Answer 21

Around the entire limb condensations

Question 22

Where does further chondrification occur after the initial deposition of cartilage?

Answer 22

Limited to future bone sites, sparing the inter-zone regions

Question 23

What does the sparing of inter-zone region in further chondrification lead to?

Answer 23

Site of future joints

Question 24

What is the initial step in the formation of the joints of the long bones?

Answer 24

Mesenchymal cells at the interzones of long bones differentiate into multiple fibroblastic connective tissue layers

Question 25

What happens to the fibroblastic connective tissue layers at the interzones of long bones?

Answer 25

They differentiate further to provide the articular cartilage at either end of the joint, and connective tissue in the middle forms the internal structures of the joint

Question 26

What internal structures of the joint are produced from connective tissue in the middle of the fibroblastic connective tissue layers?

Answer 26

• Synovial tissue
• Menisci
• Ligaments

Question 27

How is the joint cavity formed?

Answer 27

Vacuoles form within the connective tissue, which then becomes the joint cavity

Question 28

How is the joint capsule formed?

Answer 28

The mesenchymal sheath becomes the joint capsule

Question 29

How are fibrous joints, or immobile joints which connect bones (e.g. skull, pelvis), formed?

Answer 29

Also developed from interzones, which differentiate into a single layer of fibrous connective tissue

Question 30

What happens following chondrification?

Answer 30

Ossification

Question 31

Where does ossification occur from?

Answer 31

The primary ossification centre

Question 32

What happens to mesenchymal cells in the primary ossification centre?

Answer 32

They differentiate into bone cells (osteoblasts) in response to growth factors

Question 33

What do the osteoblasts secrete?

Answer 33

The calcium matrix of mineralised bone

Question 34

What appears alongside the osteoblasts?

Answer 34

Osteoclasts

Question 35

What do osteoclasts do?

Answer 35

Reabsorb bone, enabling remodelling of growing bone

Question 36

What does the limb musculature develop from?

Answer 36

Two condensations of somitic mesoderm

Question 37

How does the somitic mesoderm form the limb musculature?

Answer 37

In the 5th week, the condensations of somitic mesoderm invades the limb bud, one ventrally and one dorsally

Question 38

What does the ventral somatic mesoderm give rise to?

Answer 38

Mainly flexors, pro atoms, and adductor muscles

Question 39

What does the dorsal somatic mesoderm give rise to?

Answer 39

Mainly extensors, supination, and abductor muscles

Question 40

What do cells in the condensations of somitic mesoderm differentiate into?

Answer 40

Myoblasts (muscle cell precursors)

Question 41

What happens to myoblasts?

Answer 41

They fuse together to form syncytial

Question 42

How does the inner action of the limb muscles develop?

Answer 42

From branches that develop from spinal nerve axons in a multi step process

Question 43

What do branches of the ventral spinal nerve inner age?

Answer 43

Ventral muscles

Question 44

What do branches of the dorsal spinal nerve innervate?

Answer 44

Dorsal muscles