Tuatara: Volume 11, Issue 2, June 1963
Review — General and Comparative Endocrinology
General and Comparative Endocrinology
E. J. W. Barrington
Clarendon Press, Oxford
xiii + 387 pp., N.Z. price 43/6
Endocrinology is a discipline which does not fall neatly into any of the major divisions of biology, or which may be studied using the techniques peculiar to any of these divisions. This does not mean that anyone can be an endocrinologist. but rather that the concerted efforts of anatomists, morphologists, histologist. physiologists, ecologists, biochemists, pathologists and geneticists are required for the advancement of this branch of biology, and that its material must be derived from all the divisions of the animal kingdom.
The book sets out to develop this proposition, in an orderly and logical framework, in fifteen chapters. Chapter I is introductory, explaining the principles applied to the exposition of the subject. In Chapter II one of the most primitive of the hormone systems — that concerned in the functioning of the alimentary tract — is dealt with at some length, for historical reasons, and because of the lessons that emerge from its description, regarding the pitfalls that abound for those who build elaborate generalisations on an insufficient coverage, both in width and depth, of the available animal material. Hence this chapter serves as an introduction to the methodology of the subject as a whole. As in subsequent chapters, the anatomy and histology of the relevant tissues are described, with adequate illustrations, as well as the physiological context, and due attention is paid to the evolutionary and phylogenetic aspects. The account is enlivened by intimate details of the actual discoveries, such as Bayliss and Starling's experiment on a denervated jejunal loop of January 16, 1902, which established the existence of secretin: I remember Staring saying: “Then it must be a chemical reflex” (when application of acid to the jejunal mucosa was followed by pancreatic response I. Rapidly cutting off a further piece of jejunum, he rubbed its mucous membrane with sand in weak HCI, filtered, and injected it into the jugular vein of the animal. After a few moments the pancreas responded by a much greater secretion than had occurred before. It was a great afternoon.’ It was from this occasion, too, that the term ‘hormone’ originated: this was suggested to them by Hardy, who derived it from the Greek hormaein, meaning ‘to impel or arouse to activity’; Bayliss later said that ‘although the property of messenger was not suggested by it, it has been generally understood as carrying this meaning.’page 111
The subsequent vicissitudes of this, and of other supposed hormones of the digestive tract, are followed in an equally fively way, which involves the consideration of its conditioned and unconditioned reflexes, and is epitomised in a series of criteria of the existence of hormonal mechanisms. This chapter requires very little specialised knowledge, and could be read with enjoyment and profit by a second-year student of zoology, physiology, or biochemistry.
The parts played by hormones in control of metabolism are lucidly set out in Chapter II, which deals largely with the functions of insulin and other hormones regulating carbohydrates metabolism in a wide variety of vertebrate phyla, and which attempts to link the data with evolutionary considerations.
Chapter IV introduces the pituitary gland, called by Langdon-Brown, in 1931, ‘The leader of the endocrine orchestra’; the role of conductor of this orchestra now tends to be assigned to the hypothalamus. Vesalius regarded the hypophysis as being no more than a funnel for draining condensed humours from the brain into the nose. After decades of hollow laughter at this naive concept, we have reverted to something similar; the neurohypophysis is now thought to be little more than a temporary receptacle for neuro-humours draining from the hypothalamus. Barrington extends the scope of neurosecretion to all nerve cells, and this theme is developed in this and in subsequent chapters. The functions and chemistry of various polypeptide hormones are described for a range of vertebrate phyla. Here the terminology used is confusing to the non-biochemist; the oxytocins and vasopressins are described as being octapeptides, taking cystine as being a single amino acid, yet the numbering of the amino acids (p. 84) is based on a nonapeptide convention, involving two cysteine residues, with the penultimate member of the chain (leucine, lysine or arginine) designated as being the eighth of a nine-residue sequence.
The nature and physiological functions of the hormones concerned in reproduction of vertebrates are dealt with in the next two chapters. The structure of the mammalian ovary is first outlined, and this is followed by a description of the cyclical changes taking place in it, and in the vagina, during the oestrous cycle. The female sex hormones are then enumerated, their structural formulae are shown, and rules of nomenclature are explained, not altogether correctly with regard to the alpha— & beta— series of oestrogens. Incidentally, the structural formula for cholesterol (p. 101) is wrong — there should be no H at C-5. The role of the liver in inactivating oestrogens by coupling with glucoronic and sulphuric acids is not mentioned, although it is of extreme importance, both to the foetal and to the maternal organisms. The structure and function of progesterone, of both luteal and placental origin, are next discussed, for a range of mammalian species.page 112
Much the same treatment is then given to the male gonad, with the structural formulae of the principal naturally occurring androgens. The author then proceeds to give a oclear description of the pituitary gonadotropins (follicle stimulating hormone, interstitial cell stimulating hormone, and prolactin), and of their complex interrelations with the gonadal and progestational hormones; thus he specifies ten or more hormones as being concerned in mammary gland functioning in the rat. The diverse effects induced by administration of prolactin to vertebrates other than mammals are described, and interesting speculations regarding its evolutionary history are advanced.
Having established that ovulation is regulated by release of follicle-stimulating hormone from the adenohypophysis. the author proceeds to an exposition of the evidence that this release is, in turn, under neural regulation, again from the hypothalamus, and probably mediated by the release into the infundibular portal system of a neurohormone. Finally, the feed-back mechanism of regulation of hypophyseal secretions, whereby the hormonal products of stimulation of the gonad suppress production and/or release of hypophyseal tropic hormones, is explained. Chapter V ends with a description of the phenomena of sexual periodicity in female mammals, and of the diverse factors, both intrinsic and environmental, associated with maintenance of the rhythm.
Chapter VI deals largely with the same problems as in the preceding chapter, as applied to vertebrates other than mammals, in particular, birds, amphibia, and fishes, and a fundamental uniformity of principle is shown to hold, side by side with a diversity of factors. A section of this chapter describes the effects of the various endocrine and environmental factors on behaviour, throughout the animal kingdom, and the possible implication of the thyroid gland is suggested. The problems of viviparity are expounded, and the contribution of the endocrine systems to their solution are explained. The role of the placenta as an endocrine organ in some mammals is described graphically as amounting to an early vote of no-confidence in the parent's capabilities by the embryo. Sexual differentiation in embryonic amphibians, birds, and mammals is followed in detail, and the phenomena of inter-sexuality and sex reversal are discussed in the light of the factors operative in determining foetal sex. Strangely enough, the author here makes no reference to the effects of excess or deficiency of X and Y chromosomes, as in the Klinefelter and Turner syndromes, although it was in 1959 that Jacobs et al. reported the association of the XXY karyotype with the former syndrome.
Chapters VII and VIII are devoted to the phylogeny, embryology and functions of the endocrine glands originating from the pharynx — the thyroid, parathroids, and thymus. In these chapters the author ranges far and wide in the field of thyroid functions. biochemistry, evolution, and pathology, giving a fascinating account page 113 of the history of the development of our understanding of its significance; it was first described by Wharton in 1656, who ascribed to it the purely ornamental function of smoothing out the contours of the neck of women, who have larger thyroids than men. In the discussion of the metabolic effects of thyroid hormones the difference between their effects on intact animals and on excised tissues is stressed (p. 191). However, although on p. 202 the effect on mitochondrial permeability is mentioned, nothing is said specifically about uncoupling of oxidation from phosphorylation of ADP. Yet it is this effect which seems to be directly responsible for the raising of the basal metabolic rate in hyperthyroidism. Another debatable opinion appears in the same section (p. 196); after noting that thyroxine treatment causes silvering of salmonids, due to deposition of guanine in the scales, the author says that ‘while it is often assumed that guanine is a product of the metabolism of nucleic acid, the precise sources of this material remains obscure’. Biochemists would not share this opinion — the pathways leading to guanine biosynthesis, and to its production as a product of degradation of nucleic acids, are both sufficiently well known; its accumulation in tissues appears to follow from deficiency of the enzymes guanine deaminase and or guanosine deaminase.
The well-known effects of thyroxine on metamorphosis of amphibians and teleost fishes, and the less-known effects on behaviour patterns of some fishes, are discussed impartially and judiciously, and the chapter closes with an account of mutual thyro-pituitary control.
Chapters IX and X deal with the adrenal gland. As in the earlier chapters the history of the elucidation of its anatomy and functions in different phyla is given, and this is followed by an account of the catechol hormones of the medullary portion of the gland, of which adrenaline was discovered by a Harrogate physician, Dr. Oliver, in 1896, while noradrenaline was only recognised as being physiologically the more important hormone fifty years later. The latter hormone is shown to be a specific neurosecretory product of the sympathetic system, and the importance of the sympathico-chromaffin complex in mammals and other vertebrates is discussed. Chapter IX ends with a brief discussion of the differences between hormones and neurohumours, arriving at the reasonable conclusion that no useful purpose is served by attempts at their rigid differentiation.
Chapter X is devoted to a consideration of the development, organisation, control, histochemistry, and products of the adrenal cortex. In view of the very voluminous literature of these subjects, the author is to be congratulated on the discrimination exercised in selecting from it the items strictly relevant to his main theme. Much more might have been included, such as, for example, the biochemistry of production of the corticosteroids, but in a relatively small book, such as is this one. much has, perforce, to be omitted.page 114
Colour changes in vertebrates are dealt with in Chapter XI. The role and structures of the melanocyte-stimulating hormones of the adenohypophysis of different mammalian species are discussed, as well as their activity in other vertebrates. Very recent work on the part played by the pineal gland, with it characteristic secretion melatonin, is included here. In discussing the evolutionary aspects of colour control, the interesting proposition is advanced that this had survival value in bridging the transition between living in a fully aquatic environment and under terrestrial conditions, involving exposure to extremes of temperature.
Chapter XII covers, in twenty-five profusely illustrated pages, the organisation and evolution of the pituitary gland of a range of animals, from mammals to tunicates.
Chapters XIII and XIV deal, respectively, with hormones in crustacea and insecta. The subject is treated with the reserve and caution imposed by the small amount of work so far done on a very small minority of representatives of these classes, and these chapters should be of real use to biologists who have been trained in vertebrate physiology only. A very interesting, though necessarily brief, survey of the ectohormones (pheromones) of arthropods is included in Chapter XIV.
The book closes with a chapter on some evolutionary aspects of endocrine systems. It is supposed, reasonably, that such systems are phylogenetically more ancient than are nervous systems, and it is suggested that the thyroid, steroid, and neurohormones are direct legacies from the most remote ancestors of the present metozoa, although modifications in their functions may well have developed with increasing complexity of metazoan organisation. The key position of neurohormones is pointed out, and it is postulated that they are the initiators of sequences which spread throughout the organism, giving rise to end-effects on various target tissues, remote by two or three steps from the neurohormone. The last sentence is a quotation from an unnamed source: ‘a worried frown has replaced the lifted eyebrow as the proper expression for pundits’.
The book has an adequate index, and a very useful bibliography, giving recommended reading for those wishing to make a more profound study of the subject matter contained in the various chapters. It is well printed, on good paper, and is singularly free of typographical errors; the only ones noted by the reviewer were: simulation for stimulation; responsible for responsive: and prostrates for prostates, on pp. 112, 132. and 147 (Fig. 49). respectively. A pleasing feature of the book is the absence of distracting footnotes, and an even more pleasing one is its low price (43/6 N.Z.); it would be hard to find a reference book containing 139 figures and seventeen plates at a price approaching this. The book can unreservedly be recommended for undergraduate teaching in zoology, and for reading by all those concerned in the life sciences, at any level.