Embryogenesis

Question 1

What are somites?

Answer 1

Paraxial mesoderm → somites → sclerotomes + dermomyotomes

Question 2

What do sclerotomes form?

Answer 2

1) Vertebrae
2) Intervertebral disc
3) Ribs
4) Connective tissue
5) Tendons

Question 3

What do dermamyotomes form?

Answer 3

Dermatome → dermis + SQ tissue of neck/trunk

Myotome → skeletal muscles

Question 4

Describe the process of intramembranous ossification.

Answer 4

1) Mesenchymal cells → initial ossification centers

At ossification centers:
2) Mesenchymal cells condense → osteoblasts + capillaries

3) Osteoblasts secrete bone matrix → calcify → trabeculae

4) Trabeculae merge & vascularised → spongy bones

5) Spaces between trabeculae occupied by bone marrow

Question 5

Describe the process of endochondral ossification.

Answer 5

Intiation
1) Mesenchymal cells differentiate into chondrocytes → hyaline cartilage model

Diaphysis:
2) 1° OC forms in diaphysis
3) Chondrocytes hypertrophy → ECM calcifies
3) Chondrocytes apoptose → spaces within cartilage

Periosteum:
4) Perichondrium differentiate into osteoblasts
5) Osteoblasts initiate formation of compact bone on outer edge of diaphysis

Vascularisation:
6) Blood vessels penetrate diaphysis → recruit osteoblasts
7) Osteoblasts continue to deposit bone matrix towards ends of long bone

Bone remodelling:
8) Osteoclasts appear and replace cartilage w trabecular bone → spongy bone

Medullary cavity:
9) Osteoclasts break down center of trabecular bone → medullary cavity

Epiphyseal growth plates:
10) Epiphyseal growth plates from @ ends
11) Blood vessels & osteoblasts invade → form 2° OC
12) Cartilage replaced by bone

Question 6

Describe the process of limb development.

Answer 6

Both composed of ectodermal epithelium and inner mesenchymal cells

1a) Upper limb buds form from lateral mesoderm @ C5-T1 (~25 days into development)

1b) Lower limb buds form from lateral mesoderm @ L1-L5 (after upper limbs)

2) Distal growth
- Apical ectodermal ridge induce cell proliferation → precursors to cartilage → undergoing endochondral ossification

3) Organisation
- Zone of polarising activity organise cells of limb bud into cranial to caudal arrangement

4) Digits:
- limb buds flatten → plates → hands and feet
- mesenchymal cells → early digits
- mesenchymal cells separated into digit rays by cells that will apoptose → webbing