Teorías del ciclo basadas en los desequilibrios sectoriales Spiethoff

The molecular mechanisms involved in endochondral ossification proceed in a time dependent and critically systematic order in the growth plate. Chondrocytes in the growth plate undergo changes that allow a population of progenitor cells in the

resting phase of the growth plate to develop sequentially into proliferative, prehypertrophic, hypertrophic, and then terminally differentiated hypertrophic chrondrocytes. Vascularization of the hypertrophied growth plate allows for colonization of osteoblasts and osteoclasts. These osteogenic cells trigger apoptosis of the terminally hypertrophied chondrocytes. Osteoblasts create the foundation for bone to be created on top of the scaffold left by the dying chondrocytes. It is this ordered gradual development of proliferative to hypertrophic chondrocytes or mature cartilage cells, which characterizes the growth plate. As the chondroblasts differentiate, they proceed in an orderly columnar array of cells that radiate from the epiphysis to the diaphysis. These chondrocytes secrete both type II collagen and aggrecan to create a suitable extracellular matrix for osteoblast development [94]. The end product is a post-mitotic, hypertrophic chondrocyte destined to undergo apoptosis. The extracellular matrix of the chondrocyte is now ready to act as a scaffold for the deposition of bone with the assistance of chondroclasts. Chondroclasts are specialized cells involved in calcification, and extracellular matrix remodeling of hypertrophic chondrocytes. The dead chondrocyte now can act as a site for the development of osteoblasts into osteocytes.

The resting zone, a source of growth plate chondrogenic progenitors

In reality, the synergy between growth plate markers is so critical to the development, that each marker could be thought to play a role in each growth phase due to the importance of timing. The factors with the most dramatic effect within a growth region will be focused on. The resting zone is an unorganized collection of

pre-proliferative chondrocytes. They act as a reservoir for chondrocytes that will proceed through the growth plate during the endochondral ossification process. The resting zone of chrondrocytes is sustained by bone morphogenetic protein (BMP) 2, parathyroid hormone like peptide (PTHlh, formerly named PTHrP) and Indian hedgehog (IHH). It is the combination of these markers that help to drive chondrocytes from this “resting zone” to an actively proliferative state. These cells become more chondrogenic in organization and morphology as they migrate towards the growth plate [82, 95, 96].

Proliferative growth zone

The proliferative zone of the growth plate is closely tied to the resting and hypertrophic zones[J12] in timing and ordered progression of development. Indeed,

the same growth factors, BMP-2, PTHlh and IHH, are crucial to how and when proliferative chondrocytes will proceed through the growth plate. Increased expression of IHH and BMP delays maturation and prolongs the proliferative state. However, these genes work in independent pathways [96]. Likewise, the levels of inhibitors of these pathways (e.g. BMPs inhibitor noggin) have the opposite affect, leading to decreased proliferation. The role of BMPs is often related to differentiation [97]. Excellent overviews of BMP function are presented by Hoffman and Gross (2001) and Wan and Cao (2005). The proliferative chondrocyte has a round morphology and produces type II collagen for the ECM as they begin to stack into columns of chondrocytes. In the most common form of achondroplasia, which is caused by mutations in the FGFR3 gene, the proliferative zone of growth is greatly reduced in size resulting in shortened limbs, decreased number of proliferative cells

and achondroplasia [98]. Additional proliferative growth markers include: BMP7 [95], collagen 2 (COL2) [99], and p57Kip2 [100].

Hypertrophic growth zone

As the cells progress through the proliferative state, their profile of gene expression changes, which leads to a switch from the proliferative to hypertrophic state. Many molecular factors are beginning to be explored; however, only a few critical pathways have been dissected in great detail. Several studies have utilized transcript profiling to identify possible mechanisms that control chondrocyte

hypertrophy [101-103]. Additional research of these pathways may provide insight into critical pathways that control bone elongation. Among the most characterized intracellular signaling pathways are FGFs and TGFβs. The role of type 10

collagen[J13] (COL10), an ECM component, has also been investigated thoroughly.

The FGFs have been explored extensively, because of the FGFR3 achondroplasia models for human dwarfism. Without proper FGF2 regulation, as occurs in the FGFR3 mutants, chondrocytes show decreased proliferation and limbs are

shortened due to decreased growth. As chondrocytes mature, they proceed from a prehypertrophic to a terminally differentiated state. The expression of Spp1 and MMP13 is critical to reaching this fully differentiated state [95]. FGF2 regulates IHH and COL10 mRNA expression levels. Expression of these markers regulates both proliferative and hypertrophic growth in chondrocytes [104]. The role of IHH has been discussed previously [105]. COL10 is a particularly important marker as it is an important structural transition into the prehypertrophic state. Without type 10

mineralization (including calcification) and ossification by chondrocytes [106, 107]. In fact, mutations in COL10 have already been shown to be responsible for

chondrodysplasias in the mouse, and pig [108-110]. The role of IHH is constant through out the entirety of growth phases and critical in regulating a host of growth factors in the growth plate [95, 96]. Considerable research with TGFβs has been explored. The role of TGFβs appears to be inhibition of hypertrophy in general, but this may be in part due to up regulation of PThlP [111, 112]. Additional hypertrophic growth regulators include: Alkaline phosphatase (ALP) [113, 114], BMP2 [96], Distal- less homeobox 5 (Dlx5) [115], Dlx6 [116], MMPs [117, 118], and osteopontin (OPN).

C. Overview of additional growth regulators in endochondral