Embryology of the musculoskeletal system

Question 1

Which three sources does the musculokeletal system develop from? and what do they form

Answer 1

The paraxial mesoderm

• Axial skeleton but not sternum or facial bones

The parietal layer of the lateral plate mesoderm

• Bones of the shoulder and pelvic girdle, limbs and sternum

The neural crest cells

• neutral crest cells in the head partly form the skull bones and face

Question 2

What are the three divisions of mesoderm

Answer 2

Paraxial

Intermediate

Lateral plate

Question 3

Which division of the mesoderm does skeletal muscles form from?

Answer 3

Paraxial mesoderm

Question 4

Describe the process by which mesoderm forms skeletal muscle

Answer 4

1. In the 4th gestational week paraxial mesoderm segments into somites on either side of the neural tube and notochord
2. The ventral and medial (dorsal) walls divide into 2 masses of cells: sclerotome and dermomyotome
3. The ventral part of the somite, the sclerotome, will form cartilage and bone of the vertebral column and ribs
4. Dorsal part of the somite, the dermomyotome will differentiate into dermatome and myotome. Dermatome will form the dermis of the back and neck and myotome cells form the skeletal mucles

Question 5

Describe the divison of the lateral plate mesoderms and what it forms

Answer 5

Somatic (parietal)

• Bone, skeleton muscle and connective tissue of limbs, pelvis and sternum
• Splanchnic (visceral) smooth muscle and cardiac

Splanchnic (visceral)

• Smooth muscle and cardiac

Question 6

When does the development of bone and muscle begins?

Answer 6

Fourth gestational week

Question 7

Describe the process by which mesoderm form connective tissues

Answer 7

1. Paraxial Mesodermal cells gve rise to loosely organised tissue - mesenchyme
2. Mesenchyme cells are pluripotent
3. Mesenchymal cells migrate and differentiate into different tissue types
4. They may become fibroblasts, chondroblasts or osteoblasts under the influence of vascular changes

Question 8

a) Name the two types of bone formation?
b) What do they form?
b) What does the occurence of both processes first require?

Answer 8

a)

• Intramembranous ossification
• Endochondral ossification

b) Intramembranous ossification - cranial vault, many facial bones and the clavicle

Endochondral ossification - base of the skull, some faial bones, bones of the limb and girdles, the vertebral column, ribs and the sternum

c) The condenstation of mesenchymal cells - loosely organized connective tissue

Question 9

When does primary and secondary ossification start?

Answer 9

aPrimimary ossification starts at 56 days and secondary ossification is after birth (knees ossify just before)

Question 10

Describe the formation of epiphyseal plates due to primary and secondary ossification

Answer 10

• Primary ossification starts at the centre of the shaft and grows out
• Secondary ossification is at the extremities of the develping bone
• As the limb grows, the primary and secondary ossification centres do not fuse together until after - forming epiphyseal plates

Question 11

a) What is intramembranous ossification?
b) What does it form?
c) Describe the process of intrammeranous ossification

Answer 11

a) Intramembranous ossification is the process of bone development from fibrous membranes.
b) Forms the flat bones of the skull, the mandible, and the clavicles.

c)

1. Ossification begins as mesenchymal cells form a template of the future bone.
2. They then differentiate into osteoblasts at the ossification center.
3. Osteoblasts secrete the extracellular matrix and deposit calcium, which hardens the matrix.
4. The non-mineralized portion of the bone or osteoid continues to form around blood vessels, forming spongy bone.
5. Connective tissue in the matrix differentiates into red bone marrow in the fetus.
6. The spongy bone is remodeled into a thin layer of compact bone on the surface of the spongy bone.

Question 12

a) What is endochondral ossification?
b) What does it form?
c) Describe the process of endocondral ossification

Answer 12

a) Endochondral ossification is the process of bone development from hyaline cartilag
b) All of the bones of the body, except for the flat bones of the skull, mandible, and clavicles, are formed through endochondral ossification.

c)

1. In long bones, chondrocytes form a template of the hyaline cartilage diaphysis.
2. The matrix begins to calcify.
3. This calcification prevents diffusion of nutrients into the matrix, resulting in chondrocytes dying and the opening up of cavities in the diaphysis cartilage.
4. Blood vessels invade the cavities, and osteoblasts and osteoclasts modify the calcified cartilage matrix into spongy bone.
5. Osteoclasts then break down some of the spongy bone to create a marrow, or medullary, cavity in the center of the diaphysis.
6. Dense, irregular connective tissue forms a sheath (periosteum) around the bones.
7. The periosteum assists in attaching the bone to surrounding tissues, tendons, and ligaments.
8. The bone continues to grow and elongate as the cartilage cells at the epiphyses divide.
9. In the last stage of prenatal bone development, the centers of the epiphyses begin to calcify.
10. Secondary ossification centers form in the epiphyses as blood vessels and osteoblasts enter these areas and convert hyaline cartilage into spongy bone.
11. Until adolescence, hyaline cartilage persists at the epiphyseal plate (growth plate)

Question 13

What are the two ways that the bones of the skull are formed?

Answer 13

• Endochondral ossification
• Intramembranous ossification

Question 14

During the maturation of the skull, it is categorically divided into two main parts. What are these two main parts?

Answer 14

The viscerocranium and the neurocranium.

Question 15

a) What is the role of the neucranium
b) Describe the divisons of the neurocranium and what they form

Answer 15

a) Protecting the brain
b) Membranous neucranium - flat bones

Cartilnaginous neucranium - baseof the skull

Question 16

a) Describe what viscerocranium forms
b) What is it mainly formed by?

Answer 16

a) Forms facial skeleton
b) Formed mainly by the first two pharyngeal arches and some elements of the frontal bone

Question 17

a) What are skull frontanelles?
b) What are the different skull frontanelles and when do they close?

Answer 17

a) These are the growth areas of the skull that are present since before birth up until certain ages in young children.

b)

• Anterior - closure: 1-3 years post birth
• Posterior - closure: 2 months post birth
• Sphenoidal - closure: 2 months post birth
• Mastoid - closure: 6-18 months post birth

Question 18

Describe the process of limb and appendicular system development

Answer 18

1. The appendicular skeleton includes the bones of the limbs and girdles
2. The formation of these structures begin by the end of the fourth gestational week, where limb buds become visible as outpocketings from the ventrolateral body wall.
3. They consist of a core of mesenchymal cells - derived from the somatic layer of the lateral plate mesoderm - covered by a layer of ectoderm.
4. Ectoderm at the distal boarder of the limb thickens to form Apical Ectodermal Ridge (AER)
5. AER – Exerts an inductive influence of the adjacent mesenchyme causing it to remain undifferentiated, rapidly proliferating cells - this region is known as the progression Zone
6. Cells further from the AER differentiate into cartilage and muscles.
7. Development of the limbs thus proceed proximodistally. Zone of Polarizing Activity – anterior-posterior axis first to be fixed
8. Cell death in the AER creates separate ridges for each digit forming webbed fingers and toes.
9. Further cell death in the interdigital spaces are what creates the separation of the digits.

Question 19

The structural development of the upper limbs and lower limbs are similar but with two exceptions: What are these two exceptions?

Answer 19

1. The development of the lower limb is approximately 1 to 2 days behind that of the lower limb
2. Upper limb rotates 90° laterally, placing the flexor muscles anterior/extensors posteriour and the lower limb rotates 90° medially, placing the flexors posterior /extensors anterior