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BIORHEOLOGY

OBITUARY

Benjamin W. Zweifach1910-1997

Biorheology. Vol. 54, No.6, pp. ¥-ox, 1997Copyright C 1998 Ebcvier Science LtdPrinted in the USA. All rights reserved

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Benjamin W. Zweifach, eminent scientist, researcher and teacher of humanphysiology and bioengineering, died on Thursday, October 23rd, in San Diegoat the age of 86. His pioneering application of engineering principles andmethods to studies of the microcirculation ushered in a new age of medicalresearch and contributed to the development of biorheology as an establishedscientific discipline.

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Ben Zweifach was born in New York City, the son of Eastern Europeanimmigrants. He received a B.S. degree in biology from the College of the Cityof New York in 1931, a M.S. degree and in 1936 a Ph.D. degree, both in cellularphysiology from New York University. His first paper, published in 1933, wasthe first direct micromanipulative study of the capillary blood vessels in a livingtissue. In it he showed that these smallest blood vessels in the body were notcontractile, as proposed previously by August Krogh. This seemingly innocentconclusion has dramatically influenced microcirculatory theory for the past halfcentury. This study set the tone for Zweifach's lifelong devotion to the study ofthe smallest blood vessels in the circulation.

He was Assistant Professor and then Associate Professor of Physiology inCornell Medical School from 1947 to 1952. He returned to New York Universityand served as Associate Professor of Biology and Pathology until 1958, and thenas Professor of Pathology until 1966. He received an Established InvestigatorAward from the American Heart Association from 1950 to 1955, and a CareerInvestigator Award from the Health Research Council of the City of New Yorkfrom 1955 to 1966. During this time, he established a brilliant record in thestudy of the physiology and pathology of small blood vessels.

In 1964, Dr. Zweifach won the Claude Bernard Medal in Physiology fromCanada. He then made a major decision to learn and use engineering methodsto further his research in microcirculation. He went to the California Institutefor Technology in Pasadena as a visiting Professor, and made friends withengineers such as Harold Wayland, Yuan-Cheng Fung, Marcos Intaglietta andGiles Cokelet. In 1966 he accepted an appointment as Professor ofBioengineering at the newly established San Diego campus of the University ofCalifornia.

Dr. Zweifach served as a rallying point for many engineers and physiologistsin the development and application of new techniques to study themicrocirculation. He dismantled the barriers that, at the time, existed betweenengineering and medicine, taking full advantage of both disciplines to solveproblems in the microcirculation. He realized the importance of biorheologyto studies of the microcirculation and supported many efforts world wide . Inhis own work, he elected to stay close to jhe microcirculation in living tissue,and to develop new techniques of intravital microscopy to study the flow ofblood in living blood vessels even as small as capillaries.

Dr. Zweifach recognized that the application of engineering to studies ofthe microcirculation entailed more than just placing numerous experimentalobservations on a firm quantitative foundation. He viewed engineering as aphilosophical framework that helped to formulate practical questions andextract useful answers that might serve to "unravel the mysteries ofmicrovascular function." Toward this goal, he undertook many collaborativestudies with his newfound colleagues in La Jolla. He was instrumental instimulating Y.C. Fung to undertake studies of the elastic properties of the truecapillaries, and also in steering Marcos Intaglietta toward seeking anunderstanding of the pulsatility of microvascular blood flow. With MarcosIntaglietta he refined the Landis micro-occlusion technique for themeasurement of capillary permeability into a useful tool with which one couldsystematically evaluate the transcapillary exchange of fluid across the walls ofsmall blood vessels. Through his numerous collaborations with Intaglietta, Dr.Zweifach served as a catalyst for the development of new instruments for studiesof the microcirculation by intravital microscopy, which Dr. Intagliettapopularized with refinements of the Wiederhielm servo-null micropressuresystem and the Wayland:Johnson method for the measurement of red cell

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velocity. Under Zweifach's guidance, La Jolla quickly became the microcir­culation capital of the world.

Blood rheology held a special interest for him when he recognized in theearly 1970's that the mechanics of fluid flow and shear stresses in the circulationwere important determinants of microvascular function. In general, heentertained and nurtured the application of blood rheology studies of themicrocirculation, often encouraging rheologists such as Giles Cokelet, AlCopley, Shu Chien and Shunichi Usami to perform studies relevant tomicrovascular blood flow. It was at that time that Ben also realized thatrheological studies were a natural complement to his exploration of the basisfor pressure and flow relations in the microvasculature. In a bold move (for thetime), he took on several bioengineering graduate students (including HerbLipowsky and Geert Schmid-Schonbein) to work directly in his laboratory. WithLipowsky, he pioneered studies of the in vivo measurement of the resistance toblood flow in small blood vessels and established the first measurements of thein vivo apparent viscosity of blood in microvessels, as well as the application ofnetwork analysis techniques to elucidate the distribution of pressures, flows andwall shear stresses in the microvascular network. With Schmid-Schonbein, hebegan a period of collaboration that would continue for more than twodecades and initiated the first attempts to examine details of red blood cellvelocity profiles in microvessels.

Dr. Zweifach had a long-standing interest in leukocytes as part of theclassical inflammatory response. He personally knew the Clarks, who did workwith the rabbit ear chamber in the 1930's, and had a first hand appreciation oftheir careful description of leukocyte behavior in wound repair. He had hisown working experience with various approaches to studies of these cells in themicrocirculation. His interest in leukocytes was spawned by a collaborationwith Lester Grant at New York University College of Medicine in the sixties whostudied chemotaxis in vivo using a small Laser burn. The focus of these studieswas on the site of adhesion in a microvascular network, in particular, onhumoral control of migration in the tissue and the mechanisms of leukocyteadhesion and phagocytosis at sites of inflammation. In San Diego, he becameinterested in studying leukocyte adhesion in postcapillary venules. Why wouldleukocyte attachment be mostly limited to postcapillary venules and be lessfrequent in arterioles or capillaries? Was this a phenomenon related to the fluidshear stress or was it due to a factor related to the endothelium in arterioles orvenules? It was characteristic of Ben to ask many questions but rarely was heready to draw a firm conclusion in favor of one mechanism versus another.

His interest in the role of leukocytes started to expand in the late seventiesas evidence became available that capillary obstruction in some conditions maybe brought about by circulating leukocytes. Thus the earlier notion thatleukocytes may be exclusively beneficial cells, became challenged. The evidencepointed to the fact that impairment of microcirculatory homeostasis inconditions as varied as diabetes, hypertension, hemorrhagic shock, ischemiaand reperfusion, sepsis, and others may be accompanied by a deleterious formof leukocyte-endothelial interaction that may lead to loss of cell integrity andcell death on a multiorgan scale. His formal retirement period at UCSD thatstarted in 1981 was a fruitful time during which he continued to work on analmost full-time basis with many of the bioengineering graduate students, and itushered in the era of molecular techniques to examine leukocyte adhesion. Hefollowed this development with great interest. The molecular approach offerednew tools, such as monoclonal antibodies directed at active membraneadhesion sites, which permitted intervention against the adhesion of leukocytesto endothelium. Ben felt that the in vivo approach offered a unique tool to

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investigate leukocyte rheology since the microvascular network, the specificand, in many cases, heterogeneous function of the endothelial cell and thepattern of parenchymal cell signaling provides an environment for leukocytebehavior that cannot be reproduced in any idealized system. Illustrations insupport of this particular point of view became available on many occasions.

Ben played an especially active role in the discussion about the influence ofleukocyte activation on the microcirculation. He frequently asked about criteriathat constitute an activated state and the nature of the experiments that defineactivation. The evidence, obtained by David Tung in 1995, which showed thatneutrophils may be activated and migrate into the tissue, but 'in spite of thismay produce only a limited degree of parenchymal cell damage, exemplifies histhinking that in vitro evidence by itself may be quite limited. The discovery byFariborz Moazzam that fluid shear stress serves to control, in addition to manyendothelial and platelet functions, most forms of leukocyte activation, isanother example that illustrates the uniqueness of the in vivo approach that heso relished. But for biorheologists perhaps the most surprising examples werethe observations by Don Sutton and later by Brian Helmke, one of the laststudents he advised in the microcirculation laboratory. Brian demonstratedthat, against most expectations, even a very small number of leukocytes relativeto red cells may have a pronounced influence on whole organ hemodynamicresistance. Such low numbers of leukocytes are required to observe an elevationof apparent viscosity in vivo, which in whole blood viscosity measurements areentirely unaffected, within the accuracy of current viscometers. Only the mostrecent analysis in skeletal muscle microcirculation has started to shed light onthis phenomenon, suggesting that the motion of red cells in their preferredcenter line position in capillaries may be disturbed in the presence of slowermoving leukocytes, leading to an elevation of the apparent viscosity by red cells.Thus, while even a few white cells serve to raise the apparent viscosity of redcells, their own apparent viscosity plays a minor role in raising microvascularnetwork resistance. Activation of leukocytes has a profound affect on organresistance by an amplification of the same affect even at low cell counts. All ofthese observations required in vivo experimentation and are undetectable inviscometers or glass tubes unless they actually have the small dimensions of truecapillaries.

Ben had a long-standing interest in hypertension research. He felt thathypertension is a vascular disease that involves the microcirculation. Early onhe supported efforts at UCSD and at other institutions to initiate researchprograms designed to examine the role of the microcirculation in thehypertension syndrome. In fact, to a good degree this problem dominated hiswork for the last two decades of his life. He felt that the pressure distribution inthe microcirculation needs to be adjusted to preserve normal physiologicalfunctions in spite of the challenge produced by an elevated central bloodpressure. He demonstrated, in vivo, the unique role of the terminal arteriolesin the adjustment of capillary pressure, vessels that are not only uniquelyendowed with specialized adrenergic innervations as well as endothelium butmay constitute the actual manifestation of the precapillary sphincters due totheir ability to completely close the lumen by folding and compression of theendothelial cells.

One of the problems that Ben frequently discussed was the question ofwhat constitutes the actual manifestation of the disease in hypertension. It wasdifficult to obtain direct evidence that the elevation of blood pressure, even insome experimental models that develop extreme levels of pressure elevations,constitutes the single factor that leads to cell and organ dysfunction with lesionformation and eventual organ failure. Ben was quick to point out that most

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experimental models of hypertension suffer from a plethora of functionaldisorders, some of which were by no means directly coupled with the elevatedarterial blood pressure. He supported the notion that cofactors are present inhypertension, some of which may be even more fundamental to the progressionof the disease than hemodynamic parameters per se. He was led to thisquestion after a highly detailed analysis of pressure and flow distribution in themicrocirculation using state of the art technology. But as usual, Ben Zweifachwas not committed to one point of view, instead he patiently listened and triedto support all arguments.

While Ben Zweifach's illustrations of microvascular network architectureswere published in the medical literature in the fifties, microanatomy interestedhim for the rest of his life. He saw the need for a detailed picture of thevascular display at every step of microvascular analysis long before biomedicalengineers pointed out the necessity for a realistic set of network geometries inorder to define boundary conditions for their analysis. Ben developed and usednumerous techniques to visualize the network anatomy, and he was fascinatedby some of the most exotic vascular networks such as those found inamphibians or in specialized organs. As anatomist he identified the importanceof precapillary sphincters, shunts, and metarterioles and he introduced manyterms that are standard vocabulary today, such as arcade arterioles, transversearterioles, metarterioles, collecting venules and others. He saw the power ofstereology as one of the tools to quantify geometric parameters in themicrocirculation. Thus it is not surprising that he supported the effort of EricEngelson and Thomas Skalak who, in 1980, set out to reconstruct a completenetwork topology in the rat spinotrapezius muscle. This was the first time athen incomplete and fragmented picture of the actual vascular connectionsbetween the different hierarchies of microvessels was completed for a singleorgan. It became possible to evaluate older and often simplified vesselclassifications against the microvascular network anatomy that is actuallypresent in the tissue. Ben Zweifach was skeptical of so-called simplemicrovessel classifications, especially if they were supposed to serve as the basisfor a comparison between vessels from disease models and controls. Therewere many instances of enlightenment in his study of microvascular anatomyand perhaps one of his greatest was the discovery, with Thomas Skalak in 1984,of a lymph pump mechanism in skeletal muscle which was based on theidentification of arteriolar-lymphatic pairs in the arcade arterioles of the musclemicrovasculature. As usual Ben was modest in his expression of enthusiasm, hesimply concluded that the study was "ready for publication."

Ben felt that the study of the microvascular architecture should serve otherpurposes as well; he was looking for the application of microcirculatorytechniques in man. With Bruce Fenton, Steve Kovalcheck, and Peter Chen heexplored the possibility of examining the microcirculation in the conjunctiva ofpatients with diabetes, hypertension, and other chronic conditions. His ideawas that the early manifestation of diabetes may be detectable in themicrocirculation, possibly long before clinical signs of the disease aredetectable, a situation that calls for early interventions and continuousmonitoring. Under Ben's guidance, the team was able to document remarkablechanges in the microvasculature of patients for periods of several years, most ofwhich are still not understood at any level and have remained essentiallyunderutilized as a clinical tool.

Benjamin Zweifach's passion was to serve microvascular research.Biorheology was an integral part of it. He served biorheology as a pioneer andas an educator. He has left us with more than a legacy and an astonishingrecord of contributions to the understanding of blood flow. Any historian who

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looks at the life of Benjamin Zweifach will note the persistence and longperspective with which he applied himself to science. He shaped micro­circulatory research in this century. and he shaped the individuals that he drewinto the field. His record of original publications spans the period from 1933 to1998.

Rather than reference many of Dr. Zweifach's publications, we invite thereader to consult the complete list of his publications which is available on theweb site at:

http://www-bioeng.ucsd.edu/research/research_groups/microcirc/zweifach_pub.html,

A hard copy of his publications and books are stored in the UCSD LibrarySpecial Collections.

Geert w: Schmid-SchonbeinHerbert H. Lipowsky