SISTEMA DE TRATAMIENTO DE AGUA POTABLE TIPO PAQUETE

Although expression studies have localized FGFR transcripts in a variety of tissues, the expression of each receptor within a given tissue is generally specific for different cell types (Patstone at a/., 1993). FGFR4 is expressed only at low levels in developing chondrocytes, and does not appear to play an important role in skeletogenesis.

1.3.3.3.1 FGFRs and endochondral ossification

In the chick, FG FRI expression is prominent in osteoblasts scattered in the space between the periosteum and the degenerating cartilage (Patstone ata/., 1993). High levels of FGFR2 mRNA can be detected in the periosteum

surrounding long bones, contrasting with relatively lower levels in the bone themselves. These findings are consistent with a role for FGFR1 in osteogenic differentiation and the involvement of FGFR2 in regulating stern cell proliferation (Iseki et al., 1999). FGFR3 is highly concentrated in the chondrocytes, but greatly reduced in hypertrophic chondrocytes compared with less mature cells (Patstone et a/., 1993). In humans, at the onset of chondrification in mesenchymal condensations, around 40 days of gestation, cells stop expressing first FGFR2 and then FGFR1. Both receptors are then restricted in their expression to the perichondrium, whereas FGFR3 transcripts are distributed uniformely in the cartilage (Delezoide et a/., 1998). At week 11, in contrast to the mouse or chick, FGFR3 is expressed in hypertrophic chondrocytes. Interestingly, FGFR2 is also detected both in epiphyseal chondrocytes lining the joints and in articular cartilage.

In the mouse, FGFRS is expressed in resting, proliferating and prehypertrophic chondrocytes, whereas FGFRI is expressed in hypertrophic chondrocytes (Peters eta!., 1993; Deng et a!., 1996). FGFRS null mice exhibit overgrowth of the long bones with growth plate expansion restricted to the hypertrophic zone, as a result of enhanced proliferation and maturation of proliferating chondrocytes (Colvin et a/., 1996; Deng et a/., 1996). This indicates that FGFRS is a negative regulator of bone grov/th. This idea is supported by recent data showing that FGFRS induces cell cycle inhibitors (Sahni et a/., 1999; Su et al., 1997). Other evidence also suggests that FGFRS may regulate chondrocyte differentiation. Indeed, mice overexpressing FGFRS bearing the mutation that causes achondroplasia in humans, exhibit stunted growth and slowed chondrocyte differentiation (Naski et al., 1998). However, the small hypertrophic zone encountered in these mice could also be explained by enhanced apoptosis of hypertrophic chondrocytes. Although no differences of TUNEL labelled cells were observed in these mice, chondrocytes isolated from cartilage of TD I human fetuses were apoptotic (Legeai-Mallet et al., 1998). Additional effects of the FGFRS mutation include abnormal growth plate vascularisation and ossification (Segev et al., 2000).

1.3.3.3.2 FGFRs and intramembranous bone formation

In the chick, expression of FGFR genes in the early frontonasal mass could relate closely to the differentiation of cartilage in the centre of the pnmordium and eventual outgrowth. FGFR2 transcripts are seen throughout the head mesenchyme overlapping with FGFRS in the chondrogenic regions of the infero-lateral region of the mandible and the hyoid arch (Wilke et a/., 1997). It has been shown that epithelium is required to maintain FGFR2 expression in facial mesenchyme (Matovinovic and Richman, 1997). At later stages, FGFR1

heavily labels the osteoblasts lining the trabeculae whereas FGFR2 mRNA is confined to the outer layer of undifferentiated mesenchyme. FGFRS is barely detectable at this stage (Patstone et al., 1993).

In the mouse, although the Fgfr2 lllb isoform is detected in long and cranial bones, the level of expression of the lllc isoform in these tissues is much higher (Orr-Urtreger et al., 1993). At later stages ( E l6.5), Fgfr2 is highly expressed in the osteogenic fronts as they approach each other in the sutures (Kim et al., 1998). Wholemount in situ hybridisation of mouse heads clearly shows the developing cranial bones outlined by Fgfr2 expression, demarcating the expression domain of osteopontin, an early bone marker whilst remaining mutually exclusive from it (Iseki et al., 1997). Its expression coincides with areas of rapid cell proliferation of cells considered to be osteogenic stem cells and is associated with low levels of FGF2. The onset of differentiation is accompanied by down-regulation of Fgfr2, up-regulation of

F g fri and subsequent expression of osteopontin, suggesting that signalling through FGFR2 regulates stem cell proliferation, whereas signalling through FGFRI regulates osteogenic differentiation. However, F g fri, osteonectin and alkaline phosphatase are subsequently down regulated indicating that they are not essential for maintaining the differentiated state (Iseki et al., 1999).

Very recently, Fgfri has been shown to regulate intramembranous bone formation by modulating Cbfal expression (Zhou et al., 2000). FgfrS is expressed weakly in osteogenic cells at the periphery of the osteoid on the outer side of the suture, as well as more intensely in the thin layer of cartilage underlying the lower part of the coronal suture. Interestingly, Fgfri, 2 and 3

are also expressed in the cartilages of the cranial base, where a growth defect could have the secondary effect of bringing the calvarial bones closer (Rice et al., 2000) Detailed analysis of ^OFR splicing variants in skull development shows that F g fri lllb is generally weaker compared to F g fri lllc, and that

Fgfr2 lllc is stronger than Fgfr2 lllb except in the skin. Finally, Fgfr3 lllb is weakly expressed compared with Fgfr3 lllc in the cartilage (Rice at al., 2000). In humans, FG FRI expression is marked in the entire head mesenchyme at 6 weeks of development. Later, high expression of FG FR I and 2 is observed along the sides of the sutures, in the preosseous mesenchymal condensations and osteoid, contrasting with the low levels of FGFRS (Delezoide at al., 1998). Work from our laboratory has further investigated the spatial distribution of FGFRI and 2 expression in the human embryo from 6 weeks of development in which both genes are expressed in sheets of condensed mesenchyme before overt chondrogenic differentiation. Distinct patterns of expression are established by 8 weeks, FGFR2 is expressed evenly throughout developing cartilage and bone, whereas FGFRI transcripts predominate in perichondria and periostea (Chan and Thorogood, 1999).