Lasting effects of early environmental influences

… the childhood shews the man As morning shews the day … JOHN MILTON Paradise Regained As commonly used, the phrase ‘early influences’ denotes the conditioning of behavior by the experiences of very early life. Early experiences, however, do more than conditioning behavioural patterns; they also affect profoundly and lastingly many biological characteristics of the adult. I shall show that, in animals, events occurring during the very first days of life determine the initial growth rate, the maximum adult size, the efficiency in utilization of food, and the resistance to infection, malnutrition, and other stressful stimuli. Early influences are, of course, at least [as] important in human life as they are in animal life. In fact, the experiments to be reported here were designed to provide experimental models for the study of socio-medical problems first recognized in human populations. During the past century, for example, there has been a constant trend toward earlier maturation of children. This phenomenon was first detected in the United States, then in other Westernized countries; it is now particularly striking in Japan. Evidence for earlier maturation is provided by the greater heights and weights of children at each year of age; by the faster growth rates during adolescence for both boys and girls; and by the earlier age of the first menstrual period. In England for example, the menarchal age was 15½ for the well-off townspeople in 1820, whereas it had fallen to 13 in 1960.1 Needless to say, the trend towards earlier maturation cannot be extrapolated far back in time. In fact there is evidence that the menarchal age was 14 in Shakespeare's time, at least in the favored social classes.2 The beginning of the nineteenth century apparently corresponded to a low ebb in the rate of physical and sexual development, perhaps because of the poor health conditions that prevailed at the end of the Napoleonic wars and during the early phases of the Industrial Revolution. What is certain in any case is that changes in the environment and in the ways of life during the past century have been associated with a marked increase in the growth rate of children throughout the Westernized world. The fall in mortality caused by diarrhea, tuberculosis, and other respiratory diseases during the past century in the United States and in other prosperous countries provides a spectacular illustration of the direct relationship that exists between high living standards and increase in resistance.3,4 There is overwhelming evidence, furthermore, that the fall in mortality has been especially marked among the young age groups (Figure 1). Infant mortality by prominent causes. Rates per 1000 births; New York City. (New York City Department of Health.) I shall not discuss the mechanisms of the fall in infantile mortality but shall emphasize instead that the essential cause of the improvement was greater resistance to disease, rather than control of pathogens or more effective medical treatment. The increased resistance to disease was brought about by social advances. The influence of social factors on growth and health has been particularly well documented in the countries of Central America. Very high infant mortality, slow rate of growth during childhood and adolescence, and physical and mental lethargy continuing throughout life are among the pathological manifestations common to all the deprived social groups of Central America. These disorders are not racially determined; they are found alike among the deprived Indians and among the populations of European origin who share their ways of life. In contrast, these disorders are rare among Indians and Latin people born and raised in social and economic environments similar to those now prevailing in the United States and in Europe. While it is thus certain that physical and mental development, as well as physical and mental health, are profoundly influenced by social factors, the complexity of the interplay between man and his total environment has handicapped the epidemiological and clinical study of such socio-medical problems. My purpose in the present report is to show that it is possible to create laboratory models useful in the study of human population problems. I shall emphasize in particular some of the lasting biological effects of early environmental influences. The development in our laboratory of models illustrating the effects of early influences was facilitated by the results of our earlier investigations with three mouse colonies that have the same genetic origin yet differ widely in several important biological characteristics.5–8 The colony of so-called standard Swiss mice (SS) has been maintained at the Rockefeller Institute (now Rockefeller University) for more than 50 years. The CFW colony was originally derived from the SS colony but has been in commercial production outside New York. The NCS colony was developed at the Rockefeller Institute 8 years ago out of nine animals obtained by Cesarean section from SS mice. Attempts have been made to maintain the NCS colony in a pathogen-free state at the Rockefeller University; a subcolony of it (NCS-D) has been continuously bred under semi-protected conditions in our own laboratories for the past 8 years. As reported elsewhere, the mice of the NCS colony, and especially of the NCS-D colony, differ profoundly from SS and CFW animals. Figure 2 illustrates differences in weight at weaning time, in growth rates of the young, in the size of the adults, and in nutritional requirements. Comparative growth rates of SS mice abnd NCS mice. All animals were born of mice fed a mixed natural diet (commercial pellets D & G) during gestation and lactation. Newborns were fed either casein diet or wheat gluten diet after weaning In the experiment illustrated in Fig. 2 all animals had been fed the same natural diet before mating as well as during gestation and lactation. Yet, the young of the SS (or CFW) animals were smaller that those of the NCS animals at weaning time. The difference in size between these two groups persisted from then on, irrespective of the composition of the diet the animals received after weaning. The difference between the SS (or CFW) and NCS animals becomes even more striking when the diet is inadequate. This is apparent in Figure 2 for the animals fed the wheat gluten diet (low in lysine and threonine). The NCS mice continued to gain weight on this amino-acid deficient diet, but, in contrast, the SS animals failed to grow altogether, or at best remained abnormally small. Earlier studies have revealed that the difference in efficiency of food utilization between NCS and SS (or CFW) mice is not genetically determined; rather, it is an expression of the different microbiota acquired by the animals during early life. The influence of the indigenous microbiota will not be discussed further at this time because the effects of early influences have been more sharply defined in other types of experiments now to be described. The general principle of these experiments was to introduce a variable of short duration during early life and to observe the delayed consequences of this intervention in animals that were maintained thereafter under optimum conditions. Despite the long inbreeding of the mouse colonies just described, and despite all efforts to standardize the breeding conditions, the litters exhibit marked differences in weaning weight, in growth rate, and in adult size. It was thought at first that these differences were due to the genetic constitution of the individual mice. For this reason, an effort was made to render the distribution of physical characteristics more uniform by pooling all the newborn animals and reallocating them randomly to foster mothers. The results of this pooling and random allocation, however, were very different from what had been expected. They showed that the growth characteristics of the newborn animals were highly uniform for each foster mother, but differed markedly from one foster mother to the other. In other words, individual variability had its origin not in the genetic endowment of each animal, but in the nursing effectiveness of the mother. In one particular experiment, 240 mice born the same day were separated from their mothers 2 days after birth and pooled. They were then reallocated randomly to these mothers, each of the latter receiving eight young. All foster mothers accepted the eight young, which grew normally. Figure 3 presents a typical group of results for three mothers. Newborn NCS mice pooled, then randomly allocated to three foster mothers on second day of life (8 per mother). All animals fed mixed natural diet (commercial pellets D & G) It will be noticed that the weaning weights of the young animals fell within a narrow range for each particular group. Furthermore, the relative order of weights of individual animals remained the same after they had been separated from their foster mothers and fed thereafter the same diet in the same room. In another experiment, the newborns were randomly allocated to foster mothers approximately 18 hours after birth. Table 1 shows their weight at that time and their subsequent rates of growth. It will be seen that the nursing effectiveness of the foster mother became apparent within a very few days and that its effect on the relative weight rank of the newborns persisted thereafter. Effect of foster mothera on neonatal growth of NCD-D micea NCS-D mice, 7 weeks of age, gave birth the same day. All newborns were pooled and reallocated randomly to these NCS-D mothers 1 day after birth, eight per foster mother. The table presents results for two foster mothers having received young which were comparable in initial weight. Effect of foster mothera on neonatal growth of NCD-D micea NCS-D mice, 7 weeks of age, gave birth the same day. All newborns were pooled and reallocated randomly to these NCS-D mothers 1 day after birth, eight per foster mother. The table presents results for two foster mothers having received young which were comparable in initial weight. The design of the two preceding experiments rules out that the groups of animals differed because of their genetic constitution, since they had been pooled and randomised; moreover, the differences among the groups did not have a prenatal origin, since the animals had been randomized after birth. All differences shown in Figure 3 and Table 1 could be traced, therefore, to the influence of the foster mothers during lactation. Quantity of milk, quality of milk, or other more subtle behavioral attributes of the foster mother account totally, or in part, for the differences in growth rates of the young. What is certain in any case is that the early experiences derived from the nursing mother had conditioned the development of the animals not only during lactation but also probably for long periods thereafter, and probably for their whole life span. The general improvement in child nutrition certainly accounts in large part for the acceleration of physical growth and of sexual maturity that is occurring everywhere in affluent societies. There is much reason to believe that the very early nutritional influences are particularly important in this regard. The following experiments illustrate that, in animals, the nutritional state of the mother during lactation affects not only the initial rate of development of the young but also the final stature of the adult. Pregnant NCS female mice were placed on various experimental diets just before delivery and were maintained on these diets throughout the lactation period. Their young were weaned at 3 weeks of age and from then on all were fed the same mixed natural diet (commercial pellets D & G). In other words, the differences in nutritional regimen were limited to the mother and to the period of lactation. The experimental regimen of the pregnant animals in one particular experiment consisted of a semi-synthetic diet containing 20% wheat gluten (supplemented with cysteine) as sole source of protein. The diet was, therefore, low in lysine and threonine but was otherwise adequate with regard to all growth factors known to be essential for the adult mouse. The young of females fed this gluten diet were compared with those of females fed the complete natural diet (D & G pellets). As will be noted in Figure 4 the young produced and nursed by mothers fed the gluten diet at weaning time than did those of mothers fed the complete Furthermore, the difference between the two groups was maintained from then on, even all animals were fed the same optimum diet and under the same conditions from the time of weaning throughout the of their life span. NCS female mice fed either mixed natural diet (commercial pellets D & G) or wheat gluten diet from the time of delivery and throughout lactation period All newborns fed D & G pellets after weaning at days of age groups of mice Figure the results the of age; the weight differences between the two groups of animals are present at the time of more than after the beginning of the As mice much than 2 it is that the nutritional by the mother during lactation has the development of young for their whole life span. experiment as Figure of the were separated from the females and placed in individual there is between the two groups of even they have been fed the same diet (commercial pellets D & G) since weaning time. The differences between the two groups at the time of after weaning. The of the two the effects of environmental factors In the preceding experiment 4 and the difference between the two groups of NCS mice was certainly caused by the low of lysine and threonine in the diet of the mother during because the effect could be by the gluten diet with a of these two more is the fact that a effect on weaning and adult weight also be by more subtle nutritional effects on the mother during the lactation period. and 7 present the results obtained by the nursing mothers a diet containing and all known essential growth is to since it growth of adult or females are fed diet during however, the weights of their young are at weaning time. these animals abnormally thereafter, even to an optimum diet after weaning. NCS-D females fed mixed natural diet (commercial pellets D & G) during then the following diets from the time of delivery the lactation diet diet with All newborns were fed D & G pellets after weaning days NCS females fed mixed natural diet (commercial pellets D & G) during then one of the following diet diet or mixed natural diet (D & G) In group diet the females were placed on diet days after in the three other they were placed on the experimental diets from the time of delivery the lactation period. All newborns were fed D & G pellets after weaning days The weight caused in the young by diet to the mother be by to the diet during lactation. As seen in Figure the weaning and adult weights of the young are then as high as those of animals nursed by mothers fed an optimum diet made of mixed natural it has not yet been possible to determine the of the in the The phrase an of widely for This was obtained the of few have been made to determine there exists a period during which the nutritional of the mother is for the young. As seen in Figure there was some of growth when the diet was first to the mother on the day after birth of young, but there was effect when the beginning of the regimen was delayed that time. The first days of therefore, a period for the physical development of the mouse. studies of human children that The control of childhood has probably been one of the factors to the acceleration of growth rates among children in the affluent As in our laboratory with three mouse colonies derived from the same genetic has provided experimental for the effect of early on the initial rate and subsequent of physical Table 2 presents the results obtained by newborn NCS mice a time two days after birth with of adult CFW mice or with obtained by this a As seen in Table the animals thus than the at weaning time, and they remained smaller thereafter. In contrast, the growth rate was not when the young mice were with derived from NCS-D adult animals. This is of particular since NCS-D animals have been raised under highly conditions in our own laboratories and were of mouse Effect of with on growth of mice NCS mice, 2 days were by on the 1 only of in received 1 of NCS-D mice are maintained in our laboratory and are of mouse they do not mice were obtained from a commercial The of adult NCS-D or CFW animals were in 4 of per are for groups of to whole The CFW was obtained by of CFW a the was a before Effect of with on growth of mice NCS mice, 2 days were by on the 1 only of in received 1 of NCS-D mice are maintained in our laboratory and are of mouse they do not mice were obtained from a commercial The of adult NCS-D or CFW animals were in 4 of per are for groups of to whole The CFW was obtained by of CFW a the was a before In many of newborn NCS mice with CFW did not cause any of disease other than weight even was In other however, of the was and many of the animals The of the CFW which are for the weight of newborns have not yet been it is that two different of pathogens are of in has been in in of newborn NCS mice days were with a of this of them developed but all of growth. The effect could be recognized as early as 7 days after (Figure The were separated from the females after weaning 3 weeks of As seen in Figure all remained smaller than the control the same was for the of NCS mice and of NCS mice 2 days after birth with a of originally obtained from a of of adult CFW mice. were separated from females on All animals were fed D & G pellets high of commonly results from during early or even in In fact, the of were by early to a state of resistance by of the after birth. the phrase has been to the increase in resistance to that be in human by them after birth with a of this It has long been of course, that mice in with the to the they are to with this even their not for out in our laboratory have brought to another striking of from It was found that certain colonies of mice are to caused by other colonies are highly to this As in the case of the resistance is to the fact that all mice are in of The be by large of to the animals. these conditions, the becomes and the animals of 3 and of mouse to with of within present in in of mouse to with of within present in in of by of by a of lasting effects from acquired during early even when the is not highly The effects in the preceding of development during the stature during and changes in the The effects some further While it is that the resistance to a lasting its to the is limited by its In any the is as a of as is well many in the of such conditions. It is in fact, that of the that such an important cause of disease in adult life have their origin in during very early life. In the experimental models the environmental factors on the during the very early periods of its Needless to say, the to to the environment and to be throughout its life. For example, the and of food and utilization by the adult animal are and in the of resistance to as it there is very the relationship between nutritional state and In fact, the in such relationship has been more by than by the of it is that the the animal (or the human the more and are the effects of nutritional on different models will be each illustrating the effect of a particular of regimen on the resistance to of young adult mice. have shown elsewhere, for that mice in the from the weight caused by only with large of a complete animals do not from the weight caused by when their diet is low in lysine and apparently increase and is adequate only when the diet has the It has been found also that the of young mice to to is the in the diet of that in certain natural This effect has been by as a either or a of with These results that the nutritional state at the time of first to certain pathogens conditions the of both the and nutrition during the early of life not only the of the caused by but also the subsequent of the young to the delayed effects of Needless to say, many types of early influences other than nutrition and effects that throughout the life span. For example, and conditions, and the of are a few of the many environmental factors that lastingly the initial rate of the size of the resistance to various of as well as mental It be or course, to the mechanisms for the effects by such environmental could be by development, and other of those mental My however, was not to discuss mechanisms but rather to illustrate that laboratory models be for the study of the of social I have to illustrate that the in the of laboratory models of the social conditions that have brought about the at the beginning of this the development of children among Westernized the changes in the of disease that have during the past century, and the physical and mental of deprived people in certain of Central America. The study of such experimental models on and in the of social for their for one of the human in the of During the first few of their the at a rate with the of the least its the of however, many them and their growth rate In fact, a very large of them at that time or after from a of pathological conditions. are in the that and have in several different types of pathogens from the There is of course, that a large in infantile and our experimental models that the nutritional state of the mother during the lactation period be as important as the of because it conditions the resistance of to The laboratory models in the present report were to some of the health problems of deprived populations. types of models have to be developed for the problems of affluent populations. other models be to study various types of behavioral and 18 are typical of in this the of the physical and social many problems of and age will be found to be the manifestations of environmental factors that were during the years of life. This relationship is well of course, with regard to experiences and mental It be even more for early biological experiences and of physical In affluent as well as in deprived populations the problems have their origin in the lasting and effects of early environmental influences. all of the child is the of the and for this reason to an experimental that be biological and the of human to the conditions of the present is conditioned by the biological of experiences of early life affect the biological characteristics of the adult in a lasting This phenomenon has been illustrated by epidemiological in man and by several experimental models in mice. It has been for example, that when newborn animals are nursed by mothers fed diets that are their size throughout their life even the young are fed an optimum diet after weaning. similar of growth be produced by after birth. in resistance to various of be brought about in young animals by various types of nutritional and environmental that their effects are not recognized when the animals are maintained under laboratory conditions. These the of laboratory models for the of many problems.