Basic Science

Adam S. Levin

MUSCULOSKELETAL GROWTH DEVELOPMENT

Embryology

Initial three layers of embryologic development:
- Endoderm
- Mesoderm
- Ectoderm
Musculoskeletal system and connective tissues derive from the mesoderm.
Limb bud development
- 4 to 8 weeks’ gestation
- Central core becomes skeleton.
- Three cardinal axes (Figure 1.1):
  - Proximal-distal
    - Largely governed by apical ectodermal ridge (overlying ectoderm)
    - Fibroblast growth factors, homeobox (HOX)
  - Anterior-posterior
    - Governed by zone of polarizing activity
      - Preaxial—radial, tibial
      - Postaxial—ulnar, fibular
    - Sonic hedgehog (SHH)
  - Dorsal-ventral
    - Bone morphogenetic proteins, Wnt signaling

Axial skeleton

Paired consolidations of mesoderm (somites) develop.

Somites consist of skeletal and dermatomal elements.

Figure 1.1 Limb bud. Apical ectodermal ridge extends from anterior to posterior along dorsal/ventral boundary of growing limb bud. Proximal to apical ectodermal ridge is the progress zone (an area of proliferating mesodermal cells). In posterior mesoderm is the zone of polarizing activity, an important signaling center. These centers are interconnected, so limb patterning and growth are partly dependent on coordinated function. From Donohue CM. Congenital digital deformities: ectrodactyly. In: Southerland JT, ed. McGlamry’s Comprehensive Textbook of Foot and Ankle Surgery. Vol 2. 4th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013:1109-1116. Redrawn after Jorde LB, Carey JC, Bamshad M, White RW. Medical Genetics. London, England: Mosby; 1999.

Cartilage growth and development
- Appendicular skeleton derives from endochondral ossification.
- Cartilage differentiation at approximately 6 weeks’ gestation
- Vascular invasion into the cartilage by 8 weeks—primary ossification center

Bone growth and development (Figure 1.2)

Bone is formed from remodeling of collagenous scaffold (intramembranous ossification), calcified cartilage (endochondral ossification), or other bone (lamellar bone formation).

Figure 1.2 Photomicrographs demonstrating the development of a long bone. A, Essential dumbbell shape of the long bone organizes from the primitive mesenchymal tissue. B, Initially, the structure comprises cartilage. C, A calcified coliform in the mid-diaphysis, which is associated with vascular channels. D, Later, secondary ossification centers develop in the epiphysis. E, As skeletal maturation approaches, the physis attenuates and disappears. From Vigorita VJ. Basic science of bone. In: Orthopaedic Pathology. 3rd ed. Philadelphia, PA: Wolters Kluwer; 2016:1-54.

Figure 1.2 (continued)

Endochondral development
- Periosteal sleeve allows capillary invasion into the cartilage anlage.
- Capillaries deliver osseous precursors to produce calcified cartilage.
  - Primary ossification center
  - As bone replaces cartilage, perichondrium is replaced by periosteum.
- Development of physis (Figure 1.3)
  - Secondary ossification center at epiphysis
    - Secondary ossification center has largely spherical growth pattern, rather than longitudinal pattern.
Intramembranous development
- Osteoblasts form bone directly onto a collagen scaffold.
- Typically flat bones (clavicle, pelvis)

Physeal growth (see Figure 1.3)

Growth plate structure (Figure 1.4)

Reserve zone
- Adjacent to epiphysis/secondary ossification center
  - Vascularity through epiphysis (epiphyseal artery branches)
  - Vessels pass through, but give little blood supply
  - Low partial pressure of oxygen (PO₂), anaerobic metabolism
- Minimal longitudinal growth
- Relatively fewer cells in abundant matrix
  - High type-II collagen
Proliferative zone
- Columns of flat, ovoid cells
- Responsible for longitudinal growth
  - Based on rate of cell division
- Nutrient supply from epiphysis, with diffusion through the matrix
- High PO₂, aerobic

Hypertrophic zone

Plump, rounded cells

Responsible for matrix production and calcification

Figure 1.3 Sites of bone growth. The metaphysis grows in length from the physis (bottom) but the epiphysis itself grows radially in three dimensions from the deep zone of the articular cartilage (top). Both articular cartilage and physeal cartilage maintain a consistent architecture, which is critical for function. From Salter RB. Disorders of epiphyses and epiphyseal growth. In: Textbook of Disorders and Injuries of the Musculoskeletal System. 3rd ed. Baltimore, MD: Lippincott Williams & Wilkins; 1999:340.

Essentially lacks vascularity
Increasingly anaerobic toward metaphysis
Three parts:
- Maturation zone
- Degeneration zone
- Zone of provisional calcification
  - Attaches to the primary spongiosa of the metaphysis

Vascularity from metaphyseal branches of the nutrient artery and metaphyseal arteries

Figure 1.4 At the growth plate, resting cartilage at the subchondral plate begins to proliferate in columns. Following this incipient growth, cartilage cells begin to hypertroph, and following vascular penetration, undergo calcification forming a primary spongiosa or calcified cartilage zone (inset). This primary tissue remodels into bone or secondary spongiosa. From Vigorita VJ. Basic science of bone. In: Orthopaedic Pathology. 3rd ed. Philadelphia, PA: Wolters Kluwer; 2016:1-54.

Perichondrial ring of La Croix
- Ring of woven bone around the physis for structural support
Ossification groove of Ranvier
- Responsible for appositional growth of the physis/epiphysis

Nerve growth and development
- Nervous system originates from ectoderm.
  - Neural crest—sensory neurons
  - Neural tube—brain, spinal cord, motor neurons
- Axonal growth guided by growth factors and matrix glycoproteins
Abnormalities of musculoskeletal growth and development
- Skeletal dysplasias
  - Many skeletal dysplasias and disorders of growth are related to the specific function within defined areas of the growth plate (Table 1.1).
- Connective tissue disorders (Table 1.2)

BONE STRUCTURE AND FUNCTION

Bone has multiple functions:
- Structural support
- Attachments for muscle activity
- Mineral homeostasis
- Hematopoiesis (bone marrow)

Long bones

Epiphysis

Trabecular bone surrounded by thin cortical rim
Bordered by subchondral plate and physis/physeal scar

Apophysis—secondary ossification centers that are nonarticular (eg, greater trochanter, humeral epicondyles)

Table 1.1 Skeletal Dysplasias and Growth Disorders Related to Areas of the Growth Plate

Condition by Growth Plate Location			Genetic Mutation	Inheritance
Epiphysis
	Multiple epiphyseal dysplasia		COMP or type-IX collagen	AD
Reserve zone
	Pseudoachondroplasia		COMP	AD
	Diastrophic dysplasia		Diastrophic dysplasia sulfate transporter	AR
	Kniest dysplasia		COL2A1	AD
Proliferative zone
	Achondroplasia		FGFR3	AD
	Hypochondroplasia		FGFR3	AD
	Spondyloepiphyseal dysplasia		Type-II collagen	AD (congenital)
			Type-II collagen	AR (tarda)
			Unknown	XLD
Hypertrophic zone
	Mucopolysaccharidosis
		Hurler syndrome	Alpha-L-iduronidase	AR
		Morquio syndrome	Beta-galactosidase	AR
		Hunter syndrome	Iduronate sulfatase	XLR
	Rickets (provisional calcification)		1,25(OH)₂D₃
Primary spongiosa
	Metaphyseal chondrodysplasia
		Schmid type		AD
		Jansen type	PTH receptor	AD
Secondary spongiosa
	Osteogenesis imperfecta
		Type I	COL1A1	AD
		Type II	COL1A1	AR
		Type III	COL1A1	AR
		Type IV	COL1A1	AD
	Osteopetrosis		M-CSF	AR
AD, autosomal dominant; AR, autosomal recessive; COL1A1, collagen type-I alpha-1 chain; COL2A1, collagen type-II alpha-1 chain; COMP, cartilage oligomeric matrix protein; FGFR3, fibroblast growth factor receptor-3; M-CSF, macrophage colony-stimulating factor; PTH, parathyroid hormone; XLD, X-linked dominant; XLR, X-linked recessive.

Metaphysis
- Trabecular bone surrounded by thin cortical rim
- Typically a flare of the bone to connect the diaphysis to the epiphysis
- Often a site of ligamentous and tendinous attachments, and site of anastomosing vessels

Diaphysis

Frequently cylindrical tube of thick cortical bone, surrounding marrow and trabecular bone

Table 1.2 Connective Tissue Disorders and Associated Genetic Abnormalities

Condition		Genetic Mutation	Inheritance
Apert syndrome		FGFR2	AD
Ehlers-Danlos syndrome		Multiple subtypes—most commonly COL5A1	Varies
Marfan syndrome		Fibrillin-1	AD
Charcot-Marie-Tooth disease
	Type 1A	PMP22	AD
	Type 1B	Myelin protein zero	AD
	Type 2	Various defects in peripheral axon	Typically AD
	Type 3 (Dejerine-Sottas)	PMP22 or myelin protein zero	AD
	Type 4	Various defects	AR
	X-linked	Connexin 32	XLR
Friedreich ataxia		Frataxin	AR
Spinal muscular atrophy		Survival motor neuron	AR
Duchenne muscular dystrophy		Dystrophin	XLR
Becker muscular dystrophy		Dystrophin	XLR
AD, autosomal dominant; AR, autosomal recessive; COL5A1, collagen type-V alpha-1 chain; FGFR2, fibroblast growth factor receptor-2; PMP22, peripheral myelin protein-22; XLR, X-linked recessive inheritance.