Introduction

In a relatively condensed manner, this chapter attempts to portray some of the highlights during formation of the eutherian gonads. These organs are uniquely derived from migration and movement of germinal and somatic cells, and give the first clear evidence of sexual dimorphism in specific tissues. The treatment that follows focuses on the question of their origins during early embryogenesis and then on the nature of the interactions as the cell lines of the gonad differentiate and develop. Although discussion at the level of cellular organisation is both stimulating and illuminating as a teaching approach, and in the present instance emphasises the proximity between embryonic renal, gonadal and adrenal tissues, the ultimate and incisive problem for the 1990s must concern the regulation of gonadal formation at the molecular level. Precisely how does gene action programme cellular differentiation to establish the respective lineages? At the time of writing, an appropriate body of information is not available on this topic, although it is assumed that specific proteins are induced to regulate transcription.

Numerous reviews exist on the processes involved in formation of the gonads in eutherian mammals. Extensive treatments include those of Gillman (1948), Witschi (1951), Burns (1961), Clermont & Huckins (1961), Clark & Eddy (1975), Zuckerman & Weir (1977), Jones (1978), McLaren (1981 a), Baker (1982), Byskov (1981, 1982, 1986), Byskov & Høyer (1988), Wartenberg (1989) and Jost & Magre (1993).

Introduction

These opening pages represent no more than the briefest survey of highlights upon a canvas extending from the speculations of classical antiquity to the findings of contemporary molecular genetics. The viewpoint offered is, of course, a personal one and there is little doubt that other authors would – in part at least – have focused on different events and different material. In this regard, inspection of some of the volumes cited as references should give access to alternative interpretations and indeed to modified historical perspectives. Readers may perhaps detect a certain partiality towards contributions from the University of Edinburgh in the last portion of this short chapter. Taking into consideration the author's earlier connections with this institution (see Preface), and its extensive influence in the field of mammalian reproductive physiology, then it is hoped that the significance of the studies mentioned will not be judged as completely inappropriate.

Ancient Greek philosophers

As discussed by Short (1969), the dual problems of sex determination and sexual differentiation have fascinated mankind since the dawn of history, and sexual abnormalities of one sort or another have become the centrepiece of many a legend and fable. Primitive man undoubtedly had a keen appreciation of the vital parts of the anatomy, this being well illustrated in cave paintings, carvings, and frequently in the nature of mutilations inflicted upon rivals. Anomalous sexual anatomy certainly attracted attention and sometimes even social esteem, a situation which has persisted in some cultures up to the present day.

Historical sweep

It was noted in the opening chapter that the reproductive musings of the ancient Greeks unwittingly contained some elements of truth, for example those concerning the left–right differences between gonadal size and type in hermaphrodites, but little real progress in understanding the formation and function of the sexual organs took place until growth of the Italian medical schools in the late Middle Ages; even then, progress was slow. The dissecting room paved the way to ever more accurate descriptions and illustrations of reproductive tissues but, in terms of explaining the mysteries of generation, powerful imagination was an invariable substitute for solid facts. Only with the advent of reasonable quality microscopes were the respective gametes and their gonadal origins discovered, these giving way to general descriptions of the process of fertilisation and eventually to a visualisation of mammalian chromosomes. Although the dual nature of embryonic duct systems had been more or less appreciated since the studies of Caspar Wolff and Johannes Müller and also, in due course, the dual potential of the embryonic gonads, a rational understanding of the decisive elements in mammalian sex determination had to await the technique of karyotyping 150 years later. In effect, it was the crucial clinical evidence presented during 1959 on the involvement of the Y chromosome in imposing the condition of maleness irrespective of the number of X chromosomes, coupled with developments in the tender young discipline of molecular biology, that enabled today's grasp at the level of individual genes.

Introduction

Anomalous sexual development in domestic farm animals has been recognised since ancient times, and overtly bizarre conditions appear to have enjoyed special significance in the fertility rites of various societies. Indeed, what might be regarded as selective breeding programmes were even imposed to generate an increased incidence of such animals on islands of the New Hebrides (see Baker, 1925). Because anatomical and behavioural abnormalities have attracted attention for many years, quite detailed reports exist for diverse conditions in cattle (Marcum, 1974), sheep (Bruere & Macnab, 1968), goats (Hamerton et al., 1969) and pigs (Crew, 1924; Baker, 1925; Brambell, 1929; Breeuwsma, 1970). In a majority of these reports, the emphasis was descriptive rather than analytical, well illustrated as long ago as 1779 when John Hunter was drawing attention to the problem of freemartinism in cattle. More recent studies have progressed from morphological and histological observations to karyotyping and chromosome banding studies in a search for possible genetic lesions underlying the aetiology of sexual abnormalities. However, a molecular approach involving, for example, the use of probes for Y-related DNA sequences is still in its relative infancy in farm animals, even though such probes are becoming available in domestic species for the purpose of monitoring the success of treatments to select or predetermine the sex of embryos prior to transplantation (Kirkpatrick & Monson, 1993; Machaty et al., 1993).

The anomalous development referred to in the first sentence of this introduction invariably concerns anatomical aberrations of a female reproductive tract and gonads that have been taken to reflect an intersex condition (reviews by Biggers & McFeely, 1966; Biggers, 1968; Short, 1969; Bishop, 1972).

Introduction

Because the laboratory mouse has been used so widely in biological research, it will come as little surprise to learn that abnormalities have been observed throughout the reproductive system, not least in inbred strains of mice. These abnormalities extend from an absence of functional gonads to the formation of hermaphrodites in which ovotestes are observed unilaterally or comprising both gonads. The origin of such conditions would appear to reside in spontaneously arising mutant genes. Experimentally generated mice such as chimaeras or transgenics also demonstrate a variety of reproductive abnormalities, and highlight the apparent vulnerability of gonadal and genital tissues to major derangement expressed as modification of structure. Even so, derangement that involves generation of both male and female tissue within the same gonad immediately raises questions concerning sex determination and differentiation. For such a situation to arise, it would seem possible to argue that the downstream gene programmes for testicular and ovarian development are both present in mammalian zygotes, and that sex determination involves active suppression of one of the programmes, a step prompted by a gene located on a sex chromosome. An inappropriate or untimely instigation or regulation of the suppression mechanism could permit generation of an ovotestis.

The objective of this chapter is to comment on some of these abnormal conditions, and to introduce concepts that will be relevant to the ensuing chapter dealing with man.

Introduction

Diverse mechanisms are involved in the determination of sex in lower animals, such as the ratio of the number of X chromosomes to autosomes in Drosophila (Hodgkin, 1990) or the ambient temperature during incubation of the embryo in turtles, alligators, crocodiles and some lizards (Charnier, 1966; Bull, 1980, 1983, 1987; Head, May & Pendleton, 1987; Deeming & Ferguson, 1988; Ewert & Nelson, 1991). However, the dogma for eutherians remains that it is the genetic nature of the spermatozoon penetrating and activating the oocyte that is the primary determinant of the sex in the resultant zygote. Such a genetic mechanism is taken to indicate evolutionary progress from the lower vertebrates in which environmental influences exercise such a large and often decisive rôle in directing the path of sexual differentiation. The genetic mechanism for sex determination in mammals is viewed as relatively stable, inferring that mammals have thereby gained an increased control over their environment (Mintz, 1968).

Because females are the homogametic sex in mammals, bearing two X chromosomes in diploid cells, it falls to the heterogametic population of spermatozoa to impose the decision and developmental programme according to the type of spermatozoon fusing with the oocyte and delivering its haploid complement of chromosomes into the vitellus. Activation of the egg by a Y-bearing spermatozoon is classically held to give rise to a male embryo, if the events of fertilisation and early development proceed successfully.

Introduction

Diverse disorders of sexual development exist in man, many of them having been recognised since ancient times and indeed some of them conferring on their possessors positions of dubious privilege in society (Fig. 7.1). Whilst scientific description of the disorders has tended to focus primarily on the condition of the gonads, and may find expression in the external genitalia and non-reproductive tissues, the disorders in fact stem from genetic anomalies involving the chromosome complement or perhaps even individual genes. Such anomalies were first critically analysed in the 1950s with the development of reliable methods for karyotyping and, with the ever increasing sophistication of techniques culminating in those at the molecular level, so the precision of diagnosis has increased. One outcome of the cytogenetic or molecular genetic approach has been the revelation that sexual disorders are frequently a consequence of errors at meiosis in the maternal or paternal germ cell line. Another is the realisation that errors in man usually have their counterparts in laboratory rodents and sometimes also in the large farm animals (see Chapters 5 and 6).

Introduction

In Greek mythology, a chimaera was a monster bearing a lion's head, goat's body and serpent's tail (Fig. 8.1). In the present chapter, however, chimaeras will be regarded as composite animals containing genetically different cell populations derived from more than one zygote, and primary chimaeras are those in which the genetically different cell populations have coexisted from a very early stage of embryogenesis or from fertilisation itself. In a mosaic, by contrast, the cells are derived from a single zygote lineage (Mintz, 1974). Artificially generated chimaeras have been used quite extensively in experimental embryology since the 1960s (e.g. Tarkowski, 1961, 1964; Mintz, 1962, 1964, 1965a, b), and especially during the 1970s and 1980s to examine somatic cell lineages (McLaren, 1976; Gardner, 1978, 1982; Gardner & Rossant, 1979; Beddington, 1982; Le Douarin & McLaren, 1984; Bradbury, 1987), not least with the focus on malignancy and on possible ways of controlling the growth of tumours (Brinster, 1974; Illmensee & Mintz, 1976; Hardy et al., 1990). The approach of bringing cell lineages together from two or more different individuals to produce a viable conceptus has permitted analysis of a number of steps in the process of differentiation such as the time of allocation of commitment, and has shed light on the origin and fate of individual cell lines (Gardner, 1985). The experimental generation of chimaeras has also enabled interactions between germ cells and the surrounding soma to be analysed under a variety of conditions, and has led to conclusions concerning differentiation and organogenesis, and survival of oogonia in testicular tissue or spermatogonia in ovarian tissue.

This book has been written from the perspective of a university teacher whose research has been in the field of mammalian reproductive physiology but who is also much interested by developments in animal genetics and molecular biology. Preparing the individual chapters offered an exciting opportunity for bringing these disciplines together in various ways. The result is seen primarily as a text for advanced (Honours) undergraduates in Schools of Biological Sciences, Medicine, Veterinary Medicine and Animal Science. It should also appeal to those on taught MSc courses and to PhD students interested in both developmental biology and reproductive physiology in the higher mammals.

The book was planned during my time in the Faculty of Veterinary Medicine, University of Montréal, but the administrative commitment there – together with lecturing and activities in the operating theatre – meant that a serious spell of writing had to await my return to Edinburgh. In fact, the chapters were prepared in draft between May 1991 and June 1993, and then brought up to date as far as December 1993 on the basis of the extensive journal coverage available in the University of Edinburgh libraries. A small number of 1994 references has also been included.

As to the origins of this work, they almost certainly date back to the author's post-doctoral days in Paris (1968–1970) listening to lectures on sexual differentiation by the late Professor A.

The use of narrative as a socializing tool is most apparent when myths or traditional tales, embodying the collective wisdom of a people, are told in a deliberate attempt to edify or enlighten. Among the Western Apache, for example, stories about the early history of the group are directed at transgressors on the belief that they will be moved to reflect upon and correct their misconduct (Basso, 1984). In such cases narrative serves overtly as a “culture-preserving instrument” (Sapir, 1933/1949). Group stories function not only to maintain community values among their members but also to instill those values in the young (Gates, 1989). In some cultures, telling stories about the mythic origins or history of the group is even institutionalized as part of the formal preparation for adult roles and responsibilities (Herdt, 1981). Thus, the socializing power of sacred and traditional narrative is widely recognized. More than 60 years ago Malinowski (1926/1984) wrote,

The present essay focuses on a type of narrative that has received less attention with respect to its socializing potential, namely the informal, mundane, and often pervasive narrative accounts that people give of their personal experiences.

The remembered self can refer to memory of past concepts of self or to the way memory of the past structures and changes present construal of self. These two senses of the remembered self no doubt interact to give shape and definition to the present self, but a full discussion of both aspects would be more than I could accomplish in a short essay. Consequently, I want to confine the present discussion to the way memory of the past structures and changes present self-construal. Students of the self only vaguely understand this process. Probably the best articulations of the relation between memory and self grow out of work in narrative psychology (see Bruner, 1990; Howard, 1991; Spence, 1982; Viederman, 1979). From the perspective of a narrative psychologist, selves are construed through autobiographical narrating. Starting with the observation that people engage continuously in the interpretation of present and past experiences, narrative psychologists contend that this interpretation takes the form of story telling. For them, the self plays the role of both protagonist and narrator in these stories, and through these roles, people come to terms with who and what they are.

Memory contributes to self-actualizing narrative telling because it serves as the raw material for the narrative. As raw material, memories do not constitute the self. Without the interpretive molding provided by narrative telling, memories would have little chance of being much more than unconnected bits of information. My recollections of past jobs, loves, and tragedies are each fairly meaningless memories unless they can be placed in a larger context.

Accuracy implies correspondence between what is remembered and an earlier state of affairs in the world. There are two influential points of view that assume that memory is not, or cannot, be accurate. The first, essentially a postmodern view of the world (see Gergen, chap. 5 of this volume), rejects the possibility of correspondence between memory and the event remembered on the grounds that there is no single valid interpretation of the original event against which to attempt a match. By this view, past realities are always being constructed anew and any match is illusory. Another view grants that a kind of accuracy is possible – events may leave a record – but still rejects any simple correspondence model. Memory is seen as a process of reconstruction, not reappearance (Bartlett, 1932; Neisser, 1967). By this view, it is highly unlikely that remembering will be entirely faithful to the original event. There may be occasional correspondence, or accurate remembering, but normal remembering is dynamic. Still, no matter how passive or dynamic one's theory of memory function, it would be very surprising from an evolutionary standpoint if our memories had little to do with the events in our past at all.

Several independent lines of thought come together in this book. The first of these is an ecological/cognitive analysis of the self that was initially proposed by one of us (Ulric Neisser) in 1988. Five different sources of self-relevant information were identified in that analysis and described in terms of the different “selves” that they establish. The “ecological” and “interpersonal” selves, based on perception, have been considered in a preceding volume called The Perceived Self. The “private” and “conceptual” selves will be the subject of a volume currently in preparation. Here we are concerned with what was initially called the “temporally extended” self-that is, with memory and the self-narrative.

The second group of ideas that animates this book comes from recent studies of memory development. The research of the last few years, including our own (Robyn Fivush), has made it obvious that remembering does not just happen. Instead it is a skill that must be learned, a socially motivated activity with a specific developmental history in early childhood. This means that the remembering self has a course of development too, one that is explored in several of these chapters.

Our third theme is one of the more prominent currents in late 20th-century intellectual life. The concept of narrative has recently become important across a surprisingly wide range of disciplines. The seven fields listed on the contributor information page of the Journal of Narrative and Life History – anthropology, education, folklore studies, linguistics, literary criticism, psychology, and sociology – are just the tip of the iceberg; history, philosophy, and theology are among many that could be added.