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oxygen z3 and descent" Just as with all acute moun-tain sickness, previous H.A.P.O. sufferers tend to get it

again.7 30 31 43 Signs of infection are usually absent43and pneumonia is a very uncommon finding at nec-

ropsy.z9 The cedema is fluffy, patchy, and irregular onchest X-ray29 and Kerley B lines are not seen. 32Could H.A.P.O. be a form of neurogenic pulmon-

arv oedema56? A massive central sympathetic dis-charge might shift blood from the high-resistancesystemic to the low-resistance pulmonary circula-tion, with resultant pulmonary hypertension, lunghxmorrhage and increased capillary permeability,malperfusion, and maldistribution of ventila-tion. 31 37 56 57 This seems a likely explanation, es-pecially since WEIL et al. 58 have detected a decreasein systemic venous compliance, mediated throughthe sympathetic system, triggered by hypocapniaand not potentiated by the hypoxia of ascent. CRUZet a1.59 confirmed the venoconstriction but con-cluded that hypoxia was responsible for the de-creased venous compliance and hypocapnia for theincreased resistance and decreased flow. Overperfu-sion of one lobe of a dog’s lung produces pulmon-ary oedema,5’ which supports the HULTGREN modelfor H.A.P.o.-namely, non-uniform vasoconstric-tion (or blockage) in some areas of the lung, withoverperfusion of others. 29 57

In most cases, acute mountain sickness is self-

limiting. Nevertheless, being vile at best and fatalat worst, it should be avoided. People who go uphigh mountains should climb slowly, spendingnights low down to acclimatise:8 60 61 acclimatisa-tion camps should be arranged and the schedule ofascent limited to 300 m a day above 2130 m. Pre-vention is vital. As HULTGREN says, fatalities aredue to failure in early diagnosis or failure to insiston early descent and failure in the provision ofoxygen.32 If anyone looks uncharacteristicallyweak or fatigued, has a dry cough, or is ataxic

(often hard to diagnose in a tent or on snow) orconfused, he should be brought down immediately:coma has been completely and rapidly reversed bycarrying a man from 4200 m to 3260 m. Oxygen,100’f at 6 to 8 1/min should be given if possible.Delaying descent (especially in the presence of cere-bral oedema) to give oxygen (which may run out)or to send for help through often inaudible radiosto possibly non-existent planes or helicopters, hasoften proved futile or dangerous to all concerned;carrying an unconscious man in deep, crevassedsnow is a climber’s nightmare. In established high-altitude pulmonary oedema, frusemide seems a

56. Theodore, J., Robin, E. D. Lancet, 1975, ii, 749.57. Hultgren, H. N., Robinson, M. C., Wuerflein, R. D. Circulation, 1966, 34,

suppl. 3, p 132.58. Weil, J. W., Byrne-Quinn, E., Battock, D. J., Grover, R F., Chidsey, C. A.

Clinical Sci. 1971, 40, 235.59. Cruz, J C., Grover, R. F., Reeves, J. T., Maher, J. T., Cymerman, A., Dennis-

ton, J. C. J. appl. Physiol. 1974, 40, 96.60. Rennie, D. in Mountain Medicine and Physiology (edited by M. Ward, and

E. Williams), p. 58. Alpine Club, London, 1975.61 Evans, W O., Robinson, S. M., Horstman, D. H., Jackson, R. E., Weiskopf,

R. B. Aviat Space Envir. Med. 1976, 47, 512.

sound treatment, despite the slight risk of vascularcollapse,62 with further reduction in tissue oxy-genation,63 and despite the fact that it has neverbeen shown, by controlled studies, to be effective.In addition, morphine may help by diverting bloodback into the systemic circulation.43 49 6a Preven-tion is far better than treatment. Acetazolamide,possibly by temporarily reducing cerebrospinal-fluid production in some people65 (as well as bycausing a slight diuresis and relative metabolic aci-dosis) undoubtedly reduces the incidence and sever-ity of acute mountain sickness66 67 (see p. 1149) aswell as the incidence of H.A.P.O.

Recently, Messner and Habeler made a pheno-menal alpine-style ascent of Gasherbrum I (8068m) in three days from 5100 m (though they hadbeen between 5100 m and 5900 m for two weeks

beforehand) .6’ These formidable climbers had thecourage, determination, luck, and physiology to doit. Many who try to emulate them will not be soblessed. Though HULTGREN and GROVER wrote, in1968, "Mountaineering parties are now alert to thedanger of high altitude pulmonary oedema and theavailability of oxygen at base camps in high alti-tude climbing will prevent future needless deaths,"they were prematurely optimistic, for the lesson hasnot been learnt: people will still climb too high, toofast, and without oxygen and some will die.

Mechanisms in Aplastic AnaemiaIN typical aplastic anaemia, pancytopenia is as-

sociated with a hypocellular trephine marrow. Un-treated, the severe form (reticulocytes less than10 000/[1-1,l neutrophils less than 100/1, and plate-lets less than 20 000/[1-12) is rapidly fatal. Patientswith milder disease may survive for many years,may have spontaneous remissions, and may seem torespond to androgen therapy. About half the casesare of unknown aetiology and the rest are mostlydrug-related or secondary to hepatitis.3 Under nor-mal circumstances, granulocyte, platelet, and red-cell precursors do not proliferate outside the mar-row. For growth they require a particular marrowmicroenvironment or "soil". Aplasia could theore-tically be caused by failure of this microenviron-

62. Hultgren, H. N. J. Am. med. Ass. 1975, 234, 589.63. Iff, H. W., Flenley, D. C. Lancet, 1971, i, 616.64. Nair, C. S., Gopinath, P. M. Aerospace Med. 1971, 42, 268.65. Rubin, R. C., Henderson, E. S., Ommaya, A. K., Walker, M. D., Ra

D. P. Neurosurgery, 1966, 25, 340.66. Forwand, S. A , Landowne, M., Follansbee, J. N., Hansen, J. E New Engl.

J Med. 1968, 279, 839.67. Gray, G. W., Bryan, A. C., Frayser, R., Houston, C. S., Rennie, I. D. B.

Aerospace Med. 1971, 42, 81.68. American Alpine Journal, 1976, 20, 541.1. Lohrmann, H.-P., Kern, P., Niethammer, D., Heimpel, H. Lancet, 1976, ii,

647.2. Lewis, S. M. Br. med. J. 1965, i, 1027.3. Williams, D. M., Lynch, R. E., Cartwright, G. E. Semin. Hœmat. 1973, 10,

195.

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ment to sustain growth or by failure of haemopoi-etic stem cells to proliferate.The microenvironment may be destroyed by

high-dose local irradiation. KNOSPE and CROSBy4gave 4000r to a localised area of rat femurs, andwithin four days there was depletion of haemopoi-etic cells. After two weeks the marrow had regener-ated from cells migrating from adjacent areas, butafter four weeks the irradiated area of the femurbecame totally aplastic and neither adjacent stemcells nor transplanted isogenic cells would grow inthis area, suggesting that the microenvironmenthad been destroyed. The genetically anaemic Steelmouse, SI/SId, is a model for a microenvironmentaldefect, in that SI/SId stem cells will repopulate nor-mal irradiated recipients’ marrow but donor marrowdoes not proliferate normally in SI/SId mice. It hasbeen postulated, by analogy with these animal

models, that aplastic anaemia may be caused by adefect of the microenvironment. Clinical investiga-tions, however, do not support this hypothesis. Ina prospective randomised multicentre trial of therapyfor severe aplastic anaemia, bone-marrow transplan-tation was compared with conventional treatmentsof transfusion support with and without androgens. 5There was full marrow reconstitution in 24 of 36

transplanted patients but in only 1 of 31 conven-

tionally treated patients. After intravenous infusionof donor hsemopoietic stem cells, repopulation is

usually evident within two weeks; and a "take" indi-cates that the recipient microenvironment is in-tact. (Encouraging results for bone-marrow

transplantation have come from a Los Angeles teamas well.6) An alternative explanation is thatmicroenvironment cells are injected along with thehaemopoietic cells, but the data of KNOSPE andCROSBY suggest that microenvironment cells turnover too slowly to be responsible for the rapid im-provement which is usual. Perhaps the few graftfailures may be due to defective microenvironment.Only further work will resolve this question.

If the stem cell is defective in most aplastic pa-tients, what is the nature of the defect? Cytotoxicdrugs and radiotherapy regularly cause reversiblemarrow hypoplasia, and when treatment is stoppedhxmopoietic stem cells proliferate and differentiateto repopulate the depleted marrow. In irradiatedmice stem cells are known to proliferate at the

expense of differentiation until replenishment of stemcells is complete.7 BoGGS and BOGGS8 have suggestedthat in aplasia the stem cell is abnormal in that itsrate of differentiation exceeds the required rate ofself-replenishment, so that a chronic state of stem-celldepletion arises. When human aplastic bone-marrowis grown in the agar culture system, it produces fewer

4. Knospe, W. H., Crosby, W. H. Lancet, 1971, i, 20.5. Camitta, B. M., Thomas, E. D., Nathan, D. G., Santos, G., Gordon-Smith,

E. C., Gale, R. P., Rappeport, J. M., Storb, R. Blood, 1976, 48, 63.6. U. C.L.A. Bone Marrow Transplant Team Lancet, 1976, ii, 921.7. Chervenick, P. A., Boggs, D. R. Blood, 1976, 48, 71.

8. Boggs, D. R., Boggs, S. S. ibid. p. 71.

colonies than normal marrow. 9-11 This assay proba-bly measures committed granulocyte/monocyte stemcells (the colony-forming cells or C.F.C.), so we haveindirect evidence of a depleted totipotent-stem-cell(colony-forming unit or c.F.u.) compartment.

It is possible that the stem cell may be directiydamaged by drugs or viruses, or that it may be thetarget of an immune response. There is experimentalevidence that both types of mechanism may cause

aplasia. MoRLEY and his colleagues’2 have studiedthe chronic aplasia induced in mice by a single largedose of busulphan. The animals had a minor reduc-tion in all peripheral-blood elements but a majorreduction of c.F.u. per femur as judged by the spleencolony assay, and also of C.F.c. as judged by the agarassay. This defect was correctable by transplantationof normal-mouse haemopoietic stem cells, suggestingthat the stem cell had been damaged by busulphan.MORLEY et al.13 went on to administer chlorampheni-col to busulphan-aplastic and normal control mice:the aplastic mice had a striking further reductionin c.F.u. and c.F.c. per femur, whereas control micehad no reduction. They postulated that patients whoacquire idiosyncratic drug-induced aplasia may havehad pre-existing unrecognised residual marrow

damage. Evidence to this effect came from the reportof HOWELL et al. : in one of their patients who hadrecovered from chloramphenicol-induced aplasia, areduced c.F.c. count in agar suggested residual

damage undetected by conventional blood and mar-row examination. Unfortunately there are no readymethods for measuring total marrow cellularity orturnover in man. In animals marrow depression hasbeen produced by administration of virus, and

aplasia may follow hepatitis in man. The stem cellmay thus be directly and irreversibly damaged byeither drugs or virus, but the mechanism is not known.

GOOD’S group at the Sloan-Kettering Institute14have shown that c.F.c. from a patient with aplasiapossibly induced by tetracyclines would proliferatein agar only if the associated lymphocytes in the mar-row sample were inactivated by antithymocyte glo-bulin and complement. This is preliminary evidencethat aplasia may be caused by cell-mediated immunemechanisms in man. Experimental aplasia in animalsmay be produced by injection of anti-marrow anti-bodies,15 and anti-red-cell-precursor antibodieshave been detected in pure red-cell aplasia.16 Inthis condition the anaemia responds to immuno-

suppressive therapy. Preparation for marrow trans-plantation includes immunosuppression, and someof the successes may be due to proliferation of the

9. Kurnick, J. E., Robinson, W. A., Dickey, C. A. Proc Soc. exp Biol Med

1971, 137, 917.10 Greenberg, P. L., Schner, S. L. Blood, 1973, 41, 753.11. Howell, A., Andrews, T. M., Watts, R. W. E. Lancet, 1975, i, 65.12. Morley, A., Trainor, K., Blake, J. Blood, 1975, 43, 681.13. Morley, A., Trainor, K., Remes, J. Br. J. Hœmat. 1976, 32, 535.14. Ascensão, J., Pahwa, R., Kagan, W , Hansen, J., Moore, M., Good, R Lancet.

1976, i, 669.15. Benestad, H. B. Acta med. scand. 1974, 196, 255.16. Krantz, S. B. Br. J. Hœmat. 1973, 25, 1.

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patient’s own marrow. But genetic-marker studiesleave no doubt that in many cases donor marrowdoes become engrafted.17 In our present state ofignorance it is probably wise to assume that mar-row becomes aplastic by several mechanisms. Workon human marrow has been bedevilled by lack ofmethods to quantitate marrow mass and thehuman stem cell. The report of a human stem-cell

assay’8 now opens exciting possibilities for furtherinvestigation.

CELL TRACERS IN DISEASE

VALUABLE and sometimes unexpected new insightsinto disease can be provided by tracing the origin of thecells involved. Abnormal chromosomes, when present,provide obvious markers for the progeny of any givencell, and on cytogenetic tracing many neoplasms in manand animals appear to be of clonal (single-cell) origin.’9Not all tumours show a distinctive marker chromosome,however, and even in the classic example of the Philadel-phia (Phi) chromosome in human chronic myeloidleukxmia (C.M.L.) it can be argued that the cytogeneticabnormality may have been induced independently inmany cells, in which case deductions about the clonalnature of the leukaemic process would be unjustified.This theoretical objection does not apply to immuno-globulin markers. In myeloma and Waldenstrom’s mac-rogtobulinaemia it has become clear that the M-band

protein represents the unique product of a greatlyexpanded single-cell clone .20 The same approach hasbeen applied to cell-surface immunoglobulins in otherproliferative disorders involving lymphoid cells of the Bseries.21-23A much more widely applicable principle is based on

the observation that in females, whose cells bear two Xchromosomes, at a given point early in embryonic devel-lopment either the maternal or the paternal X is inacti-vated and remains so throughout all subsequent cell

generations. The initial choice is evidently made at ran-dom for each cell; hence every tissue of the adult femalecontains a mixture of cells in which genes on either thematernal or the paternal X are expressed. If the two Xchromosomes differ at a given locus, both alleles will beexpressed in any normal tissue sample containing morethan a handful of cells but in a clone derived from a sin-gle cell only the allele present on the X which was activein the parent cell will be evident. The enzyme glucose-6-phosphate dehydrogenase (G.-6-P.D.) is coded for by agene on the X chromosome. In Black populations twoallelic forms (A and B) are commonly found,24 some 40%of females being heterozygous so that extracts of blood,skin, and other tissues contain enzyme types A and B.The presence of enzyme type A or type B only in tumourextracts from an individual whose normal tissues con-tain both types is then taken as evidence for clonal orig-in of the tumour. On the basis of G.-6-P.D. typing a

17. Storb, R., Thomas, E. D., Buckner, C. D., Clift, R. A., et al. Blood, 1974,43, 157.

18. Barr, R. D., Whang-Peng, J., Perry, S. Science, 1975, 190, 284.19. Nowell, P. C. in Chromosomes and Cancer (edited by J. German), p. 267.

New York, 1974.20. Natvig, J. B., Kunkel, H. G. Adv. Immun. 1973, 16, 1.21. Klein, E., Klein, G., Nadkarm, J. S., et al. Cancer Res. 1968, 28, 1300.22. Preud’homme, J. L., Seligmann, M. Blood, 1972, 40, 777.23. Aisenberg, A. C., Bloch, K. J., Long, J. C. Am. J. Med. 1973, 55, 184.24. Kirkman, H. N. Adv. hum. Genet. 1971, 2, 1.

clonal origin has been demonstrated for fibroids, com-mon warts, and thyroid adenomas among other benignneoplasms while, among malignant or potentially malig-nant conditions, those with a single enzyme phenotypehave included melanoma, reticulum-cell sarcoma, andcarcinomas of cervix and thyroid.25-27 In cases ofchronic myeloid leukaemia, myeloma, Burkitt’s lym-phoma,26-27 and, most recently, polycythaemia vera28G.-6-P.D. typing has tended to support evidence forclonal derivation previously obtained from cytogeneticand immunoglobulin analysis.

It is important to avoid the unjustified conclusion thatmany human tumours develop from a transformingevent affecting only a single cell. The emergence of suc-cessive waves of clones has repeatedly been demon-strated in the course of relapse and spread of C.M.L.,Burkitt’s lymphoma, and other neoplasms.19 29 30 If asimilar phenomenon of clonal selection occurs early inthe evolution of malignant disease, it is possible formany cells to undergo the initial "transformation" butfor the progeny of only one of these cells to form theoverwhelming mass of tumour by the time the conditionis clinically evident. This phenomenon is frequentlyobserved among populations of transformed cells invitro 31 33 and there are isolated examples from the useof cell tracers in viv034 35 which suggest strongly that agiven tumour can comprise clones from at least two cellswhich have independently undergone malignant trans-formation.The evidence does, however, indicate that most

tumours grow by the proliferation of one or a few com-mitted cells rather than by continuous "recruitment" ofnormal cells through the horizontal transmission ofsome oncogenic factor. Furthermore, the extent of thedisease, as revealed by cell tracers, can be surprising. Ithas long been known that in C.M.L. red-cell, monocyte,and platelet precursors, as well as the granulocyte series,carry the Ph’ chromosome marker.

36 37 G.-6-P.D. typingconfirms the common origin of all these cell types andestablishes the involvement of a pluripotent marrowstem cell in the initial lesson. 2627 Similarly, in polycythae-mia vera, cells of the erythrocyte, granulocyte, andplatelet series are all involved. Even during remission,when the patient’s haematological status is apparentlynormal, all circulating red cells, platelets, and granulo-cytes are derived from the same stem-cell clone, imply-ing an almost total suppression of maturation of normalstem cells in the marrow. 28The search continues for a usable X-linked polymor-

phism in White populations. Unfortunately the one pro-mising candidate so far, the Xg blood-group system,seems to be the exception which proves the rule of

25. Linder, D., Gartler, S. M. Science, 1965, 150, 67.26. Fialkow, P. J. Adv. Cancer Res. 1972, 15, 191.27. Fialkow, P. J. New Engl. J. Med. 1974, 291, 26.28. Adamson, J. W., Fialkow, P. J., Murphy, S., et al. ibid. 1976, 295, 913.29. de Grouchy, J., Turleau, C. in Chromosomes and Cancer (edited by J. Ger-

man); p. 287. New York, 1974.30. Gripenberg, V., Levan, A., Clifford, P. Int. J. Cancer, 1969, 4, 334.31. Makmo, S. Int. Rev. Cytol. 1957, 6, 25.32. Steel, C. M., McBeath, S., O’Riordan, M. L. J. natn. Cancer Inst. 1971, 47,

1203.33. Bloom, A. D., McNeill, J. A., Nakamura, F T. in Chromosomes and Cancer

(edited by J. German); p. 565. New York, 1974.34. Bentler, E., Collins, Z., Irwin, L. E. New Engl. J. Med. 1967, 276, 389.35. Fialkow, P. J., Klein, E., Klein, G J. exp. Med. 1973, 138, 89.36. Whang, J., Frei, E., Tjio, J. H., et al. Blood, 1963, 22, 664.37. Chervenik, P. A., Ellis, L., Pan, S., Lawson, A. L. Science, 1971, 174, 1134.