Table Of ContentDevelopmental Toxicology
Developmental Toxicology
Edited by Keith Snell
'If
Croom Helm London
©1982 Keith Snell
Softcover reprint of the hardcover 1s t edition 1982
Croom Helm Ltd, 2-10 St John's Road, London SWll
British Library Cataloguing in Publication Data
Snell, Keith
Developmental toxicology.
1. Abnormalities, Human
I. Title
616'.043 RG626
ISBN-13: 978-1-4615-9792-6 e-ISBN-13: 978-1-4615-9790-2
DOl: 10.1007/978-1-4615-9790-2
Typesetting by Elephant Productions, London SE1S
Biddies Ltd, Guildford and King's Lynn
Contents
Preface
1. Model Systems in Teratology Research Felix Beck 11
2. The In Vitro Approach to Teratogenicity Testing
Nigel A. Brown and Sergio E. Fabro 31
3. The Use of Cell Culture Methods for Exploring
Teratogenic Susceptibility
Ruth M. Clayton and Ahmet Zehir 59
4. Biochemical Mechanisms of Teratogenesis
Kenneth E. Williams 93
5. The Distribution of Drugs and Other Agents in the Fetus
Sven Ullberg, Lennart Dencker and Bengt Danielsson 123
6. The Differentiation of Drug Metabolism in Relation
to Developmental Toxicology Olavi Pelkonen 165
7. The Role of the Placenta in Developmental Toxicology
Mont R. luchau 187
8. Developmental Carcinogenicity Paul Kleihues 211
9. Behavioural Teratogenicity
Charles V. Vorhees and Richard E. Butcher 247
10. Developmental Enzyme Pathology Keith Snell 299
Notes on Contributors 340
Index 341
Preface
Since the 1930s and the work of Hale and Warkany on birth defects
produced by variations in dietary vitamin A, it has been recognised that
the developing fetus is particularly vulnerable to adverse influences in the
environment. . Studies of malformations at birth remained largely in the
hands of paediatricians who, for instance, quickly established the con
nection between rubella infection in early pregnancy and the birth of
severely affected infants. However, it was through the tragic events of
1962, when dramatic increases in the incidence of phocomelia in newborn
infants in Germany, the United Kingdom, Japan, and other countries were
traced to the use of the apparently non-toxic sedative thalidomide by
pregnant women, that toxicologists were brought face to face with the
devastating possibility of chemically-induced developmental abnormalities.
It had been shown earlier that chemotherapeutic agents could cause
damage to the developing organism, but the absence of any known examples
of drug-induced birth defects had produced an air of complacency which
was reinforced by the lack of any specific regulatory requirements for
safety evaluation in this area. The magnitude of the thalidomide tragedy,
affecting some 10000 malformed children, was sufficient to cause an
immediate, and some would say hasty, reaction by government drug
regulatory agencies throughout the world to construct a test protocol
which would detect potential teratogenic substances that might give rise
to anatomical malformations. Such empirically based protocols paid scant
attention to the spectrum of possible developmental abnormalities that
may befall the developing organism and more recent revised regulatory
guidelines require investigations that include reproduction studies, tera
tology studies and postnatal studies.
The latter category is noteworthy in that it raises two fundamental
issues in developmental toxicology. One is the recognition that develop
ment, and the susceptibility of the developing organism, is not confined to
the prenatal phase of life but continues during postnatal life. This is a
particularly significant concept for toxicology in the light of the known
Preface
transmission of therapeutic agents and environmental pollutants to the
suckling infant in the breast milk of the nursing mother, the so-called
translactational route of exposure (cf. transplacental exposure). The other
is the question of the latency of developmental toxic responses which
may only become overtly manifest as maturation proceeds. Thus, although
the critical toxic event itself may have occurred during gestation, the
defect or disturbance may be functional rather than structural and may
not be revealed until these functions and other associated processes are
brought into action at a later maturational stage, often in the postnatal
period. Moreover, for toxic interactions which are initiated postnatally,
the eventual derangement may not occur until much later in life. In these
cases, where cause and effect are temporally separated, it is difficult to
explore the mechanisms just by examining the end-results. It is equally
difficult to devise testing procedures which would be able to detect the
full range of possible developmental toxic phenomena.
The aim of this book is to provide a forum for a discussion of the inter
actions between toxic agents and developmental processes. It is not a
recipe book for test procedures for detecting teratogenicity or develop
mental toxicity. Rather it seeks to show the current status of various
aspects of research in developmental toxicology which should comprise
the background knowledge required to approach the problem of the
detection of such hazards in a rational fashion governed by scientific
rather than empirical considerations. It is axiomatic in toxicology, and
developmental toxicology is no exception, that the discipline as a whole is
made up of many varied specialist areas of expertise. It is important for the
scientific advancement of the subject that these individual specialisms
should be intelligible to each other so that their contributions can be
properly evaluated. In this spirit the various contributors to the book have
been charged with the task of presenting a review of their specialist area
which should not only describe the current research efforts in that area
and the concepts which are emerging, but also place these in perspective
and discuss them in a way which will allow them to be appreciated by the
non-specialist in that area.
The book, then, is aimed at a broad readership which will include
practising toxicologists, especially those concerned with evaluating develop
mental hazards whether their role is in the laboratory or in the legislative
departments of government and industry. It will also include those research
scientists who are interested in the effect of toxic agents on the processes
of mammalian development and the mechanisms of such interactions.
Hopefully, this book will also be read by those members of the medical
Preface
profession, particularly paediatricians, who are faced with the consequences
of the untoward effects of therapeutic and environmental agents on the
developing human organism. Finally, it is intended that the book should
prove useful to those students in universities and young scientists in
industrial training who are embarking on a career in toxicology and who
would benefit from an overview of the current preoccupations of develop
mental toxicology.
The book begins with a consideration of various aspects of teratology,
particularly the use of model systems using techniques in vitro. These are
proving useful in dissecting the mechanistic events of teratogenesis free
from the secondary events which can lead to ambiguity with studies in vivo
(Beck). The applications of some of these in vitro systems for use in testing
chemicals for teratogenic potential are described by Brown and Fabro.
The use of in vitro cell culture systems for exploring the nature and timing
of teratogenic susceptibility is illustrated with reference to examples of
known teratogens by Clayton and Zehir. Williams discusses the biological
and biochemical bases for teratogenic actions. One of the factors deter
mining the susceptibility of the developing organism to developmental
toxicity in general is the manner and extent to which chemical substances
are distributed (Ullberg et al.) or undergo bioactivation or detoxification
(Pelkonen; Juchau) in the materno-placental-fetal unit. Bioactivation is a
key factor in determining the carcinogenicity of many agents and Kleihues
discusses the effect of carcinogens on the developing organism, with
particular emphasis on tumours of the nervous system and the mechanisms
which determine carcinogenic susceptibility. Functional developmental
aberrations which only become manifest at later periods after the original
toxic insult are illustrated by examples of behavioural teratogenicity
(Vorhees and Butcher) and of biochemical or metabolic teratogenesis and
developmental toxicity (Snell). This final chapter also describes the
possible use of developmental biochemical parameters (particularly
enzymes) in the early detection of developmental aberrations induced by
developmental toxicants and carcinogens (Snell).
As editor of the book I am indebted to my colleagues and co-authors
for the enthusiastic and diligent manner in which they approached their
various contributions. Even if, in certain cases, the gestation period proved
somewhat longer than expected, the finished articles more than justified
their conception. I am grateful to Tim Hardwick of Croom Helm for
prompting the initial implantation and for guiding the whole conceptus on
its journey into life. I should also like to acknowledge with thanks the
ministering efforts and secretarial co-operation ofPrue Levers and Janet Cole
Preface
in assisting with the delivery. In the end of course, as with any newly
created being, this book must justify its existence on its own merits. It is
my hope that they approach closely the original aspirations.
Keith Snell
Toxicology Division,
Biochemistry Department,
University of Surrey,
Guild ford
CHAPTER ONE
MODEL SYSTEMS IN TERATOLOGY
RESEARCH
Felix Beck
CONTENTS
I Introduction 13
II. In Vivo Systems 14
A. Screening for Potentially Embryopathic Agents 14
B. The Study of Teratogenic Mechanisms 16
III. Some Model Systems to Study Teratogenic Mechanisms
Using Surgical Techniques 20
IV. Some Model Systems Using In Vitro Techniques 24
A. Embryo Culture 24
B. Organ Culture 25
C. Cell Culture 27
V. Conclusion 27
References 28
I. Introduction
Broadly speaking, teratology research falls into two main categories. The
first is essentially concerned with identifying extrinsic and intrinsic factors
responsible for embryopathies. In pursuing such objectives account is taken
not only of the biological effects of potentially embryotoxic agents in
isolation but also of the so-called dramatype of the experimental animal
used (i.e. the combination of the maternal phenotype with the multiple
environmental f,!ctors which play upon it) as well as a consideration of the
embryonic genotype and its contribution in facilitating or hindering the
action of agents which tend to disturb development. Clearly the widest
application of this type of work is in the testing of drugs and other con
sumer products for possible embryopathic effects. A quartet of effects are
relevant in this context; namely, fetal death (with consequent resorption
or abortion), fetal malformation (true teratologies in the morphological
sense) abnormalities of function (perhaps associated with faulty histo
genesis) and intra-uterine growth retardation.
The second principal aim of teratology research is the precise elucidation
of the natural history of a congenital disorder. Here Wilson's (1977)
subdivision of the process into an initial mechanism (such as a gene
mutation) which leads to a pathogenic event (for example impeded morpho
genetic movement) with a subsequent final defect being produced by a
restricted number of common pathways (such as too few cells to effect
localised morphogenesis) is a useful concept, though it is probably in
complete and oversimplified.
Teratology research, therefore, is a wide field with an important unifying
13
