AAOS Comprehensive Orthopaedic Review

Section 1 - Basic Science

Chapter 4. Skeletal Development

I. Cartilage and Bone Development

A. Formation of the bony skeleton

 

1. Intramembranous bone formation is achieved through osteoblast activity and is characterized by the formation of a calcified osteoid matrix in a cartilage framework. This type of bone formation can be found at the periosteal surfaces of bone as well as parts of the pelvis, scapula, clavicles, and the skull.

 

2. Endochondral ossification occurs at the growth plates and within fracture callus and is characterized by osteoblast production of osteoid on, not within, a cartilaginous framework.

 

B. Vertebral and limb bud development (

Table 1)

 

1. 4 weeks of gestation

 

a. The vertebrate limb begins as an outpouching of the lateral body wall.

 

b. Formation of the limb is controlled along three cardinal axes of the limb bud: proximal-distal, anterior-posterior, and dorsal-ventral.

 

c. Interactions between the ectoderm and mesoderm characterize development along each axis and are governed by the interaction of fibroblast growth factors, bone morphogenetic proteins, and several homeobox genes.

 

2. 6 weeks of gestation

 

a. The mesenchymal condensations that represent the limbs and digits chondrify.

 

b. The mesenchymal cells differentiate into chondrocytes.

 

3. 7 weeks of gestation

 

a. The chondrocytes hypertrophy and the local matrix begins to calcify.

 

b. A periosteal sleeve of bone forms in a circumferential fashion around the midshaft of each anlage, and intramembranous bone formation begins to occur via direct ossification.

 

4. 8 weeks of gestation

 

a. Vascular invasion into the cartilaginous anlage occurs as capillary buds expand through the periosteal sleeve.

 

b. The capillaries deliver the blood-borne precursors of osteoblasts and osteoclasts and thus create a primary center of ossification. This process occurs first at the humerus, and signals the transition from the embryonic to the fetal period.

 

C. Formation of endochondral bone and ossification centers

 

1. As development continues, the osteoblasts produce an osteoid matrix on the surface of the calcified cartilaginous bars and form the primary trabeculae of endochondral bone.

 

2. The osteoblasts create the medullary canal by the removal of the primary trabecular bone. This process of formation and absorption enlarges the primary center of ossification so that it may become the growth region.

 

[Table 1. Limb Bud Development]

3. These growth regions differentiate further and become well-defined growth plates.

 

4. Division within the growth plate is coupled with the deposition of bone at the metaphyseal side of the bud, and long-bone growth begins.

 

5. At a specific time in the development of each long bone, a secondary center of ossification develops within the chondroepiphysis.

 

6. The secondary center of ossification typically grows in a spherical fashion and accounts for the centripetal growth of the long bone.

 

7. The rates of division within the centers of ossification ultimately determine the overall contour of each joint.



II. Normal Growth Plate

A. Structure, organization, and function

 

1. The function of the growth plate is related to its structure. In its simplest form, the growth plate comprises three histologically distinct zones surrounded by a fibrous component and bounded by a bony metaphyseal component.

 

2. The three cellular zones of the growth plate are the reserve, proliferative, and hypertrophic zones (

Figure 1).

 

[Figure 1. Photomicrograph showing the structure and zones of the growth plate, ×220.]

a. The reserve zone is adjacent to the secondary center of ossification and is characterized by a sparse distribution of cells in a vast matrix.

 

i. Cellular proliferation in this zone is sporadic, and the chondrocytes in this region do not contribute to longitudinal growth.

 

ii. Type II collagen content is highest here.

 

iii. Blood supply to this zone is via the terminal branches of the epiphyseal artery, which enter the secondary center of ossification.

 

b. The proliferative zone is characterized by longitudinal columns of flattened cells. The uppermost cell in each column is the progenitor cell, which is responsible for longitudinal growth.

 

i. The total longitudinal growth of the growth plate depends on the number of cell divisions of the progenitor cell.

 

ii. The rate at which the cells divide is influenced by mechanical and hormonal factors.

 

iii. The matrix of the proliferative zone comprises a nonuniform array of collagen fibrils and matrix vesicles.

 

iv. Proliferative zone chondrocytes are also supplied by the terminal branches of the epiphyseal artery; however, these vessels do not penetrate the proliferative zone but rather terminate at the uppermost cell. These vessels deliver the oxygen and nutrients that facilitate the cellular division and matrix production that occur within this zone.

 

c. The cells in the hypertrophic zone are 5 to 10 times the size of those in the proliferative zone.

 

i. The role of the hypertrophic zone chondrocytes is the synthesis of novel matrix proteins.

 

ii. The hypertrophic zone has the highest content of glycolytic enzymes, and the chondrocytes participate in matrix mineralization through the synthesis of alkaline phosphatase, neutral proteases, and type X collagen.

 

iii. The hypertrophic zone is avascular.

 

3. Metaphysis

 

a. The metaphysis begins distal to the hypertrophic zone and removes the mineralized cartilaginous matrix of the hypertrophic zone.

 

b. The metaphysis is also involved in bone formation and the histologic remodeling of cancellous trabeculae.

 

c. The main nutrient artery of the long bone enters at the mid-diaphysis, then bifurcates and sends a branch within the medullary canal to each metaphysis.

 

d. The capillary loops of these arteries terminate at the bone-cartilage interface of the growth plate (

Figure 2).

 

4. Surrounding the periphery of the growth plate is the groove of Ranvier and the perichondral ring of LaCroix.

 

a. There are three cell types in the groove of Ranvier: an osteoblast-type cell, a chondrocyte-type cell, and a fibroblast-type cell.

 

b. These cells are active in cell division and contribute to bone formation, latitudinal growth, and anchorage to the perichondrium.

 

c. The ring of LaCroix is a fibrous collagenous network that is continuous with both the groove of Ranvier and the metaphysis and functions as mechanical support at the bone-cartilage junction.

 

B. Biochemistry

 

   1. Reserve zone

 

a. Has the lowest intracellular and ionized calcium content.

 

b. Oxygen tension is low.

 

   2. Proliferative zone

 

a. Oxygen tension is highest in this zone secondary to its rich vascular supply.

 

b. The presence of abundant glycogen stores and a high oxygen tension supports aerobic metabolism in the proliferative chondrocyte.

 

   3. Hypertrophic zone

 

a. Oxygen tension in the hypertrophic zone is low, secondary to the avascular nature of the region. Because of this low oxygen tension, energy production in the hypertrophic zone occurs via anaerobic glycolysis of the glycogen stored in the proliferative zone.

 

b. In the upper hypertrophic zone, a switch from adenosine triphosphate (ATP) production to calcium production occurs. Once the glycogen stores have been depleted, calcium is released. This is the mechanism by which the matrix is calcified.

 

c. The region of the hypertrophic zone where mineralization occurs is known as the zone of provisional calcification.

 

d. Slipped capital femoral epiphysis (SCFE) involves the hypertrophic zone.

 

   4. Cartilage matrix turnover

 

a. Several enzymes are involved in this process, including metalloproteinases, which depend on the presence of calcium and zinc for activity. Collagenese, gelatinase, and stromelysin are produced by the growth plate chondrocytes in an inactive form and are then activated by interleukin-1, plasmin, or tissue inhibitor of metalloproteinases.

 

[Figure 2. Structure and blood supply of a typical growth plate.]

b. The metaphysis removes the mineralized cartilage matrix as well as the unmineralized last transverse septum of the hypertrophic zone.

 

c. The unmineralized portion is removed via lysosomal enzymes, and the cartilaginous lacunae are invaded by endothelial and perivascular cells.

 

   3. Metaphysis

 

a. Characterized by anaerobic metabolism, vascular stasis, and low oxygen tension. This is secondary to the blood supply to the region.

 

b. After the removal process is complete, osteoblasts begin the remodeling process, in which the osteoblasts progressively lay down bone on the cartilage template, creating an area of woven bone on a central core that is known as primary trabecular bone. The primary trabecular bone is resorbed via osteoclastic activity and replaced by lamellar bone, which represents the secondary bony trabeculae.

 

c. This remodeling process occurs around the periphery and subperiosteal regions of the metaphysis and results in funnelization, a narrowing of the diameter of the metaphysis to meet the diaphysis.

 

[

Table 2. Skeletal Dysplasias Associated With Genetic Defects]

C. Pathophysiology

 

1. Overview

 

a. Most growth plate abnormalities can be attributed to a defect within a specific zone or to a particular malfunction in the system.

 

b. Most growth plate abnormalities affect the reserve zone; however, there is currently no evidence to suggest that any disease state originates from cytopathology unique to the reserve zone.

 

c. However, any disease state that affects the matrix will have an impact on the proliferative zone.

 

2. Achondroplasia (Table 2 and

Figure 3)

 

a. Originates in the chondrocytes of the proliferative zone.

 

b. The disorder usually results from a single amino acid substitution, which causes a defect in fibroblast growth factor receptor 3 (FGFR-3).

 

[Figure 3. Histologic image showing the disorganized arrangement seen with achondroplasia.]

3. Jansen dysplasia

 

a. A mutation in the parathyroid hormone-related protein (PTHrP) receptor affects the negative feedback loop in which PTHrP slows down the conversion of proliferating chondrocytes to hypertrophic chondrocytes.

 

b. The mutation in the receptor results in a constitutively active state that is the molecular basis for Jansen chondrometaphyseal dysplasia.

 

c. Because this receptor is the shared receptor for PTH, hypercalcemia and hypophosphatemia can occur in Jansen dysplasia.

 

D. Growth plate mineralization

 

1. Growth plate mineralization is a unique process because of the specialized blood supply to the growth plate, its unique energy metabolism, and its handling of intracellular calcium stores.

 

2. The major factors that affect growth plate mineralization are intracellular calcium homeostasis and the extracellular matrix vesicles and extracellular macromolecules. Various microenvironmental factors and systemic hormones also modulate this process.

 

a. Intracellular calcium

 

i. The role of intracellular calcium in matrix mineralization is so significant that the mitochondria in the chondrocytes are specialized for calcium transport.

 

ii. Compared to nonmineralizing cells, the chondrocyte mitochondria have a greater capacity for calcium accumulation as well as the ability to store calcium in a labile form so that it can be used for release.

 

iii. Histologic studies have demonstrated that mitochondrial calcium accumulates in the upper two thirds of the hypertrophic zone and is depleted in the lower chondrocytes.

iv. When the mitochondrial calcium is released in the lower cells, matrix mineralization occurs (

Figure 4).

 

b. Extracellular matrix vesicles

 

i. The initial site for matrix calcification is unclear, though data exist to support the role of the matrix vesicle in this process.

 

ii. The matrix vesicles are rich in alkaline phosphatase and neutral proteases, which are critical to promote mineralization.

 

c. Extracellular macromolecules

 

i. The major collagen in the hypertrophic zone is type II; however, the terminal hypertrophic chondrocytes also produce and secrete type X collagen.

 

ii. The appearance of this collagen in the matrix initiates the onset of endochondral ossification.



III. Effects of Hormones and Growth Factors on the Growth Plate

A. Influence on growth plate mechanics

 

1. Hormones, growth factors, and vitamins have been shown to influence the growth plate through mechanisms such as chondrocyte proliferation and maturation, macromolecule synthesis, intracellular calcium homeostasis, or matrix mineralization.

 

2. Each growth plate zone may be targeted by one or more factors that help to mediate the cytologic characteristics unique to that zone. These factors may be exogenous or endogenous to the growth plate.

 

[Figure 4. The factors influencing growth plate chondrocyte function and matrix mineralization. ER = endoplasmic reticulum, N = nucleus, PM = plasma membrane, Mito = mitochondria.]

a. Paracrine factors are produced by the cell within the growth plate and act within the growth plate, but on another cell type.

 

b. Autocrine factors act on the cells that produced them.

 

B. Thyroid hormones and PTH

 

1. The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), act on the proliferative and upper hypertrophic zone chondrocytes through a systemic endocrine effect.

 

a. Thyroxine is essential for cartilage growth. It increases DNA synthesis in the cells of the proliferative zone and affects cell maturation by increasing glycosaminoglycan synthesis, collagen synthesis, and alkaline phosphatase activity.

 

b. Excess T4 results in protein catabolism; a deficiency of T4 results in growth retardation, cretinism, and abnormal degradation of mucopolysaccharides.

 

2. PTH also acts on the proliferative and upper hypertrophic zone chondrocytes.

 

a. PTH has a direct mitogenic effect on epiphyseal chondrocytes. Furthermore, PTH stimulates proteoglycan synthesis through an increase in intracellular ionized calcium and the stimulation of protein kinase C.

 

b. PTHrP is a cytokine with autocrine or paracrine action.

 

c. The common PTHrP-PTH receptor has a role in the conversion of the small cell chondrocyte to the hypertrophic phenotype.

 

3. Calcitonin is a peptide hormone that is produced by the parafollicular cells of the thyroid and which acts primarily in the lower hypertrophic zone to accelerate growth plate calcification and cell maturation.

 

C. Adrenal corticoids

 

1. Adrenal corticoids, or glucocorticoids, are steroid hormones primarily produced by the adrenal cortex. These hormones primarily affect the zones of cellular differentiation and proliferation.

 

a. The primary influence of the glucocorticoids is a decrease in proliferation of the chondroprogenitor cells in the zone of differentiation.

 

b. Supraphysiologic amounts of these hormones result in growth retardation through a depression of glycolysis and a reduction of energy stores.

 

2. Sex steroids, or androgens, function as anabolic factors.

 

a. The primary active androgen metabolite is postulated to be dihydrotestosterone, based on the presence of this receptor in both male and female growth plate tissue.

 

b. The role of the androgens is to regulate mineralization in the lower part of the growth plate, increase the deposition of glycogen and lipids in cells, and increase the number of proteoglycans in the cartilage matrix.

 

D. Growth hormone (GH) and vitamins

 

1. Growth hormone

 

a. GH is produced by the pituitary and is essential for growth plate function. The effects of GH are mediated by the somatomedins, a group of peptide factors.

 

b. When GH binds to epiphyseal chondrocytes, insulin-like growth factor 1 (IGF-1) is released locally. Therefore, GH regulates not only the number of cells containing the IGF receptor, but also the synthesis of IGF-1 in all zones of the growth plate.

 

2. Vitamin D

 

a. The active metabolites of vitamin D are the 1,25- and 24,25-dihydroxylated forms, both of which are produced by the liver and kidneys.

 

b. A direct mitogenic effect has been reported with 24,25-dihydroxy vitamin D.

 

c. The metabolite significantly increases DNA synthesis and inhibits proteoglycan synthesis.

 

d. The highest level of vitamin D metabolites are found in the proliferative zone; no metabolites are found in the hypertrophic zone.

 

3. Vitamin A

 

a. Vitamin A (the carotenes) are essential to the metabolism of epiphyseal cartilage.

 

b. A deficiency of vitamin A results in impairment of cell maturation, which ultimately causes abnormal bone shape.

 

c. Excessive vitamin A leads to bone weakness secondary to increases in lysosomal body membrane fragility.

 

4. Vitamin C

 

a. Vitamin C is a cofactor in the enzymatic synthesis of collagen.

 

b. It is therefore necessary for the development of the growth plate.



IV. Biomechanics of the Growth Plate

A. Growth plate injury

 

1. The weakest structure in the ends of the long bones is the growth plate, and the weakest region within the growth plate itself is the hypertrophic zone.

 

2. Growth plate injuries occur when the mechanical demands exceed the mechanical strength of the epiphysis-growth plate metaphysis complex.

 

3. The factors that determine the incidence of injury are the ability of the growth plate to resist failure and the nature of the stresses introduced to the bone.

 

4. The mechanical properties of the growth plate are described by the Hueter-Volkmann law, which states that increasing compression across a growth plate leads to decreasing growth (

Figure 5).

 

B. Growth plate properties

 

1. The morphology of the growth plate allows it to adapt its form to follow the contours of principal tensile stresses. The contours allow the growth plate to be subjected to compressive stress.

 

2. The tensile properties of the growth plate have been determined by controlled uniaxial tension tests in the bovine femur. The ultimate strain at

 

[Figure 5. The histologic zone of failure varies with the type of load applied to the specimens.]

   failure has been shown to be uniform throughout the growth plate.

 

3. The growth plate has been shown to be stronger and stiffer in the anterior and inferior regions.

 

4. Mechanical forces can influence the shape and length of the growing bone, and studies have demonstrated that mechanical forces are present and can influence bone development during the earliest stages of endochondral ossification.

 

5. The biologic interface between the metaphyseal ossification front and the adjacent proliferative cartilage is partially determined by mechanical forces, initially in the form of muscle contractions.

 

6. Both the function of the growth plate and its mechanical properties appear to be influenced by both internal structure and external mechanical factors.



V. Pathologic States Affecting the Growth Plate

A. Genetic disorders (

Tables 3 and

4)

 

1. Cartilage matrix defects—All produce some form of skeletal dysplasia, with varying degrees of impact on articular and growth plate cartilage.

 

a. Abnormalities of type II collagen are the cause of Kniest dysplasia and of some types of Stickler syndrome and spondyloepiphyseal dysplasia.

 

b. Abnormalities of type IX collagen are the cause of some forms of multiple epiphyseal dysplasia.

 

c. Defects in type X collagen cause the Schmid-type metaphyseal chondrodysplasia.

 

2. Diastrophic dysplasia—Classic example of a defect in proteoglycan metabolism.

 

a. The disorder is caused by a mutation in the sulfate transporter molecule, which results in undersulfation of the proteoglycan matrix.

 

b. The phenotype is short stature and characteristic severe equinovarus feet.

 

3. Mucopolysaccharidoses—A group of disorders that result from defects in the proteoglycan metabolism (Table 3).

 

a. These disorders are caused by a defect in the enzymes involved in proteoglycan metabolism with a resultant accumulation of undegraded glycosaminoglycans (Table 4).

 

b. The clinical presentation of each mucopolysaccharidosis depends on the specific enzyme defect and the resultant glycoprotein accumulation.

 

c. Common to all six disease states is a toxic effect on the central nervous system, skeleton, or ocular or visceral system.

 

4. Metabolic mineralization disorders

 

a. Hypophosphatasia is an autosomal recessive defect in alkaline phosphatase with resultant normal serum levels of calcium and phosphate but inability of the matrix to calcify. The hypertrophic zone widens, but there is no mineralization of the osteoid that is laid down. The zone of provisional calcification never forms. The histologic appearance and effect are similar to nutritional rickets, with a resulting inhibition of growth.

 

b. Hypophosphatemic familial rickets is a sex-linked dominant disorder characterized by low serum calcium and phosphorus. Alkaline phosphatase activity is high, with resultant abnormal conversion of vitamin D to its metabolites. The skeletal changes seen are those typical of nutritional rickets, which is discussed below.

 

B. Environmental factors

 

1. Infection

 

a. The metaphyseal portion of the growth plate is typically affected by bacterial infection. This is due to the slow circulation, low oxygen tension, and deficiency of the reticuloendothelial system in this area.

 

b. Bacteria become lodged in the vascular sinusoids, with the resultant production of small abscesses in the area.

 

c. If the infection extends into the Haversian canals, osteomyelitis of the cortical bone ensues, with associated subperiosteal abscess.

 

d. In the first year of life, cartilage canals may persist across growth plates and serve as an additional conduit for the spread of infection. Severe infection may cause local or total cessation of growth, and in most instances inhibited or angular growth results.

 

2. Irradiation—Depending on the dose, irradiation can result in shortened bones with increased width as a result of the preferential effect of irradiation on longitudinal chondroblastic proliferation, with sparing of latitudinal bone growth.

 

C. Nutritional disorders

 

1. Nutritional rickets

 

a.

Results from abnormal processing of calcium, phosphorus, and vitamin D

b.

The common disorder is failure to mineralize the matrix in the zone of provisional calcification.

 

[Table 3. Genetic Abnormalities With Musculoskeletal Manifestations]

c.

The hypertrophic zone is greatly expanded, with widening of the growth plate and flaring of the metaphysis noted on plain radiographs (

Figure 6).

 

2. Scurvy

 

a. Caused by vitamin C deficiency, with a resultant decrease in chondroitin sulfate and collagen synthesis.

 

b. The greatest deficiency in collagen synthesis is seen in the metaphysis, where the demand for type I collagen is the highest during new bone formation.

 

c. Characteristic radiographic findings of scurvy are the line of Frankel (a dense white line that represents the zone of provisional calcification) and osteopenia of the metaphysis.

 

d. Clinical findings include microfractures, hemorrhages, and collapse of the metaphysis.

 

[Table 4. Genetic Defects Associated With Metabolic Bone Diseases]

[Figure 6. A, Radiographic features of rickets in the distal radius and ulna. Note the widened growth plates and flaring of the metaphyses. B, Histologic features of rickets. The zone of proliferation is largely unaffected, but the hypertrophic zone is markedly widened.]

Top Testing Facts

1. Formation of the bony skeleton occurs via either intramembranous bone formation or endochondral bone formation. Intramembranous bone formation occurs through osteoblast activity; endochondral ossification occurs at the growth plates and within fracture callus.

 

2. In the primary center of ossification, blood-borne precursors of osteoblasts and osteoclasts are delivered by the capillaries. This process signals the transition from the embryonic to the fetal period and occurs first at the humerus.

 

3. The total length of the growth plate depends on the number of cell divisions of the progenitor cell.

 

4. The region of the hypertrophic zone where mineralization occurs is known as the zone of provisional calcification.

 

5. SCFE involves the hypertrophic zone.

 

6. The genetic mutation in achondroplasia is a defect in FGFR-3.

 

7. Growth plate injuries occur when the mechanical demands of bone exceed the strength of the epiphysis-growth plate metaphysis complex. The Hueter-Volkmann law states that increasing compression across the growth plate leads to decreased growth.

 

8. Diastrophic dysplasia is a defect in proteoglycan sulfation.

 

9. Bacterial infection affects the metaphyseal portion of the growth plate.

 

10. Scurvy is caused by a vitamin C deficiency with resultant decrease in chondroitin sulfate and collagen synthesis.



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