ESTIMACIÓN DE LOS MODELO LOGIT Y PROBIT SOBRE LA PROBABILIDAD DE DEMANDA DE SERVICIO DE TRANSPORTE, PUNO-

F i g u r e 16. The dynam ics of P D G F -aR an d PDGF-A expression in th e developing re tin a compared to some other key developm ental events. Dotted lines rep resen t uncertain ty about the precise tim e of onset of gene expression. Black boxes on lines indicate th e peaks of cell death in the corresponding cell layers. The lower p a rt of the Figure is adapted from Beazley et al. (1987).

E. PD G F-A a n d P D G F -a R i n th e o p tic n e r v e

In th e optic nerve, PDGF-A is expressed by cells in th e m eninges and throughout th e interior of th e nerve. Previous circum stantial evidence showed th a t type-1 astrocytes in th e r a t optic nerve express PDGF-A (R ichardson e t al., 1988; P rin g le e t al., 1989). T he re s u lts of double-labelling w ith antibodies ag ain st PDGF-A an d GFAP now confirm th a t view. 1 found a very close correspondence betw een the PDGF-A and GFAP signals, suggesting th a t th e m ajority of PDGF-A in th e optic nerve m ay be astrocyte-derived. 1 m ight have expected to see evidence of PDGF-A in th e RGC axons, since axons in the optic fibre lay er in th e re tin a ap p ear to contain PDGF-A (Figs 4B, 8A). It is possible th a t RGC-derived PDGF is only tran sp o rted a short distance into the axons an d does not reach th e optic nerve. A lternatively, its concentration in optic nerve axons m ay be below th e lim it of detection w ith our antibody.

P D G F - a R is e x p re ssed in th e optic n e rv e a n d e lsew h e re by oligodendrocyte precursors an d newly-formed oligodendrocytes (H a rt e t al.,

1989; M cKinnon et al., 1990; Pringle e t al., 1992). Oligodendrocyte lineage cells respond to PD GF by proliferation (R ichardson e t al., 1988; R aff e t al., 1988; Noble e t al., 1988; Pringle e t al., 1989) an d survival (B arres e t al., 1992b). O ligodendrocyte p rec u rso rs a re th o u g h t to m ig ra te in to th e developing optic nerve and other w hite m a tte r tra c ts from germ inal zones in the brain (Small e t al., 1987; Reynolds and Wilkin, 1988; Le Vine an d Goldman,

1988); th e w ay th a t PDGF-oR+ cells accum ulate in th e nerve, sta rtin g a t the optic chiasm and progressing tow ards th e eye, is consistent w ith th is idea (see Fig. 9C, E, H).

Since th e p rim ary function of oligodendrocytes is to in te ra c t w ith and m yelinate axons, it would m ake sense if th e axons them selves were a source of mitogens and/or survival factors for ofigodendrocytes and th eir progenitors. Indeed, there is evidence th a t axons are required for survival an d proliferation of th e oligodendrocyte lineage. T ransecting th e mouse or r a t optic nerve after b irth , which resu lts in W allerian degeneration of th e axons, causes a m arked reduction in th e to tal n u m ber of oligodendrocyte lineage cells th a t develop in th e nerve (P riv ât e t al., 1981; David e t al., 1984; B arres an d Raff, 1993a)

larg ely due to a red u c tio n in p ro liferatio n of oligodendrocyte p ro g en ito rs (B arres an d Raff, 1993a). Blocking electrical activity in optic nerve axons by in je ctin g te tro d o to x in (TTX) in to th e eye also in h ib its p ro life ra tio n of oligodendrocyte progenitors (B arres a n d Raff, 1993a). The effect of TTX can be reversed by deHvering PD GF from a n exogeneous source (B arres an d Raff, 1993a), suggesting t h a t blocking electrical activity somehow in te rru p ts th e endogenous su p p ly of PD GF. T his could be d irect effect of p rev e n tin g tra n sfe r of PD G F from th e RGC cell body in to th e nerve, or a n in d irect effect on th e release of PD GF from astrocytes in th e nerve (B arres an d Raff, 1993a). A t face valu e, m y failu re to d etect PD G F im m u n o reactiv ity in optic nerve axons tends to favour th e la tte r possibility.

F. PDG F-B a n d PDGF-BR in d e v e lo p in g a n d m a tu re b lo o d v e s s e ls

In th e eye, th e d istrib u tio n of PDGF-B mRNA is very sim ilar to th a t of PDGF-BR mRNA. B oth are found in sm all groups of cells a t th e back of th e lens, a t th e in n erm o st face of th e re tin a an d a t th e b o u n d ary b etw een th e in n e r n u c le a r a n d o u te r plexiform lay ers. G roups of cells t h a t ex p ress PD GF-B or PDGF-BR a re also s c a tte re d th ro u g h o u t th e optic n erv e, som etim es occurring in parallel "tram lines" suggestive of blood vessels. Blood vessels a re kno w n to be p re s e n t a t th e sites of PDGF-B a n d PDGF-BR expression in th e re tin a (E ngerm an an d M eyer, 1954). C apillary endothelial cells from b ra in have been show n to express PDGF-BR in culture (Sm its e t al., 1989), an d our in situ hybridization studies of th e r a t b ra in an d sp in al cord (H. M udhar an d W. R ichardson, unpublished) strongly suggest th a t th e sam e is tru e in vivo, so it seem s reaso n ab le to suppose th a t th e PDGF-BR+ cells t h a t we see in th e r e tin a a n d optic n erv e a re en d o th elial cells. T he co- localizing PDGF-B tran scrip ts could be co-expressed in endothelial cells, or in a n o th er cell type associated v d th blood vessels, such as pericytes. T h ere is evidence th a t cap illary en d o th elial cells in th e h u m a n p lac en ta co-express PDGF-B an d PDGF-BR (H olm gren e t al., 1991), suggesting t h a t th ese cells m ight undergo autocrine grow th stim ulation during development.

Blood vessels a re closely associated v d th astrocytes a t th e in n e r surface of th e r a t re tin a ; since r e tin a l a stro c y te s ex p re ss P D G F -a R th e y could

conceivably resp o n d to PDGF-BB from v ascu lar cells as well as PDGF-AA from RGCs. N etw orks of cell-cell interactions such as th is could be im p o rtan t for e n su rin g p ro p o rtio n al g ro w th a n d su rv iv al of in te rd e p e n d e n t tissu e elem ents.