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Biomedical subjects

R V Short

Publications and source records attributed to R V Short.

At least 37 records · Page 2Linked to original sources
PubMed1998

Dizygotic twinning as a measure of human fertility.

There is widespread concern about a possible decline in human fertility in recent decades. The spontaneous dizygotic twinning rate provides a way of measuring a combination of male plus female fertility as it reflects the frequency of double ovulation, the probability of fertilization, and the survival of the zygote. There was a decline in dizygotic twinning rates in developed countries which began around 1960 and continued until the late 1970s. The exact cause of the fall remains unknown. We suggest that it could have been due to a depression in the twin ovulation rate in women who stopped taking the oral contraceptive pill. The rise in the dizygotic twinning rates which occurred from the 1980s onwards in developed countries is almost certainly due to increasing use of ovulation-inducing agents, but this rise may have masked a continuing decline in dizygotic twinning. Monozygotic twinning rates have remained remarkably constant or increased only very slightly in recent decades. This makes it possible to use the dizygotic:monozygotic twinning ratio to monitor dizygotic twinning in populations where true incidence rates cannot be calculated, e.g. in hospitals where there may be selective referral of twins.

Adolescent↗
PubMed1998

Different fate of primordial germ cells and gonocytes following transplantation.

We have previously demonstrated that both donor primordial germ cells (PGCs) and gonocytes are capable of establishing spermatogenesis in the lumen of the seminiferous tubules of an adult host following transplantation in rats. Here we show that the PGCs, either in crude suspensions or after purification, undergo spermatogenesis only in the intraluminal compartment of the host's seminiferous tubules, while 4-5 days postpartum gonocytes also interdigitate with the host's seminiferous epithelium. The donor seminiferous epithelium was always in synchrony with the cycles of the host's spermatogenesis. It seems that the pattern of spermatogenesis of donor germ cells following transplantation in terms of its spacial location and the connection with the host's seminiferous epithelium depends on their developmental stages at transfer.

Animals↗
PubMed1998

Effects of cabergoline on reproduction in three families of Australian marsupials.

The effects of the prolactin inhibiting drug, cabergoline, on pregnant and lactating marsupials were investigated in four species from three diverse families: the tammar wallaby, Macropus eugenii, the quokka, Setonix brachyurus, the brushtail possum, Trichosurus vulpecula, and the fat-tailed dunnart, Sminthopsis crassicaudata. In tammar wallabies, 20 micrograms cabergoline kg-1 injected intramuscularly 1 day before expected birth did not alter the timing of parturition but neonates died within a day of birth, suggesting that the onset of lactation was compromised. During early lactation in tammars (56-69 days post partum), an intramuscular injection transiently retarded growth of the young, although they subsequently survived. This treatment induced reactivation of the quiescent corpus luteum and the blastocyst from diapause, so a new birth occurred 26-27 days later, despite the continued sucking of the young in the pouch. Intramuscular injection during late lactation (166-199 days post partum) apparently suppressed milk secretion since pouch young lost up to 20% of their bodyweight or died within 7 days of treatment. Oral administration of cabergoline had no effect on the growth of the young or on the quiescent corpus luteum and diapausing blastocyst. Quokkas showed similar responses to tammars after treatment in late lactation. Possums and dunnarts were less sensitive to injected cabergoline than the two macropodid species, and possums showed no response to oral administration. The lack of response of these marsupial species to oral cabergoline treatment suggests that accidental ingestion of baits, containing 20 micrograms cabergoline kg-1, used to control introduced eutherian pests such as the red fox, Vulpes vulpes, or the feral cat, Felis cattus, should not affect the reproduction of native marsupials.

Administration, Oral↗
PubMed1998

Male germ cell transplantation: present achievements and future prospects.

Germ cells are unique, since their surviving descendants can undergo meiosis and differentiate into gametes, which transmit genetic material from one generation to another. We now know that male germ cells, whether they be primordial germ cells in gonadal ridges, gonocytes, or stem spermatogonia, are transplantable. The donor cells can be transferred by direct microinjection into the seminiferous tubules, rete testis or efferent ducts, depending on the recipient species. Following transplantation, the donor cells undergo spermatogenesis in the host's seminiferous tubules in rats and mice, and have even sired offspring in mice. Interspecific germ cell transfer is possible if the recipient's immune system is defective; nude or SCID mice can even produce rat spermatozoa. However, the major obstacle restricting widespread use of this new technology is its extremely low success rate. This article discusses some ideas for improving the success rate of the transfer technique, and considers several potential applications.

Animals↗
PubMed1997

Use of dizygotic to monozygotic twinning ratio as a measure of fertility.

BACKGROUND: Monozygotic (Mz, identical) twinning occurs at a rate of around three per 1000 maternities in all populations, whereas dizygotic (Dz, fraternal) twinning is highly heritable, and varies with age and race. The Dz/Mz twinning ratio reflects the frequency of twin ovulations, and can provide a useful measure of human fertility. METHODS: 1625 pairs of twins of known sex were born in Isan Yuk Obstetric Hospital in Hong Kong during the period 1960-95. The yearly Dz/Mz ratio was calculated, and trends were analysed by chi 2 test with the Bonferroni correction. Ages of mothers of all opposite-sexed twins were recorded and trends analysed by ANOVA and linear regression. FINDINGS: The Dz/Mz ratio declined significantly from 1.12 in 1960 to 0.05 in 1978 (p < 0.001), and then rose significantly to an average of 0.86 in 1994-95 (p < 0.003). There was a significant declining trend in age of mothers of opposite-sexed twins from 1960 to 1978 (p < 0.001), but there were no significant changes in maternal age after 1978 (p = 0.38). INTERPRETATION: If we are correct in assuming that the frequency of Mz twinning remained constant during the study period, the declining Dz/Mz ratio from 1960 to 1978, which also occurred in many developed countries, could reflect some adverse environmental effect on human fertility. The increasing proportion of Dz twins in the past two decades is probably due to increasing use of ovulation-inducing drugs such as clomiphene citrate, which could mask a serious and continuing decline in human fertility. It is therefore important to continue to monitor the Dz/Mz ratio in the future in this and other subpopulations, after exclusion of any women who have taken drugs that stimulate fertility.

Adult↗
PubMed1997

Premature introduction of progestin-only contraceptive methods during lactation.

Experts on contraceptive technology concur that progestin-only methods can be used safely during lactation. However, very few studies exist of the effects on lactation of the introduction of progestin-only methods prior to the sixth postpartum week. Since progesterone withdrawal is the likely stimulus that initiates lactogenesis, it appears necessary for natural progesterone levels to decline to baseline before a progestin-only contraceptive is initiated. Therefore, the use of such contraceptive methods should be delayed for at least 3 days after the birth. Non-hormonal methods remain the first choice category of contraceptive methods for breastfeeding women, since there is no possibility that they will interfere with lactation. Progestin-only methods comprise a viable and often desirable next choice category, although the timing of their commencement must be determined with care in order to support lactation.

Animals↗
PubMed1997

The testis: the witness of the mating system, the site of mutation and the engine of desire.

There is now abundant evidence in a wide range of mammalian and non-mammalian species to show that the relative size of the testis and the morphology of the spermatozoa are infallible predictors of the mating system. Species with the largest testis/body weight ratios and the best spermatozoa have a multi-male or promiscuous mating system in which sperm competition operates. Judged by these criteria, men were not designed to be promiscuous. There is increasing evidence in humans to show that most spontaneous mutations of the germ line occur in the testis. Because these provide the variability on which natural selection can operate, the testis holds the key to evolution. Genes on the Y chromosome that control male fertility are particularly prone to mutations, perhaps because of the mutagenic metabolites produced by the metabolically active testis. Testicular descent into a scrotum, and cooling by countercurrent heat exchange between the spermatic artery and vein may have evolved as a way of holding the mutation rate in check. The hormones secreted by the testis, which control libido and aggression, ensure that these male mutations are disseminated as widely as possible throughout the population.

Animals↗
PubMed1996

Testosterone control of male-type sexual behavior in the tammar wallaby (Macropus eugenii).

In the tammar wallaby, Macropus eugenii, the expression of male-type sexual behavior is apparently determined by the activating effects of testicular hormones in adulthood. The incidence of male-type copulatory behavior and sexual checking behavior was compared in intact (control) males, control females, testosterone-treated females, and three groups of males castrated either postnatally (24-26 days of age), prepubertally (14.5 months of age), or in adulthood. All three groups of castrated male wallabies showed a very low incidence of male sexual behavior in adult life, comparable to that shown by the untreated females. Adult female wallabies with 100-mg testosterone implants showed a high incidence of male sexual behavior which was indistinguishable from that shown by intact males. The results suggest that sex differences in male-type behavior in the tammar wallaby are due to short-term inductive effects of testosterone acting on a sexually indifferent brain. There is no evidence of any long-term organizational effects of testosterone acting in fetal or neonatal life on the neural pathways controlling male-type sex behavior in this marsupial mammal.

Animals↗
PubMed1996

Sexual differentiation of the urogenital system of the fetal and neonatal tammar wallaby, Macropus eugenii.

In male tammar wallabies, the scrotum is the first organ to become sexually differentiated, 4-5 days before birth (day 22 of gestation). This is followed by enlargement of the gubernaculum and processus vaginalis one day before birth. However the indifferent gonad does not show any signs of testicular cord formation or androgen production until later, at around the time of birth; this is more pronounced at 2 days post-partum (p.p.), when the testis takes on a characteristic rounded appearance. Primordial germ cells proliferate throughout the testis at this time, although the testis does not become significantly heavier than the ovary until around 80 days p.p.. In females, the appearance of the mammary glands is the first sign of sexual differentiation 4-5 days before birth. The indifferent gonad first shows signs of developing an ovarian cortex and medulla 7 days after birth. The migrating germ cells are confined to the cortex, and first start to enter meiosis about 25 days after birth. The Wolffian (mesonephric) ducts are patent to the urogenital sinus in fetuses at day 21 of gestation. In the female they have started to regress by 10 days p.p. and only rudiments remain by day 25 p.p.. The Müllerian (paramesonephric) ducts develop adjacent to the cranial pole of the mesonephros at about day 25 of gestation and grow caudally to meet the urogenital sinus between days 2 and 7 p.p.. The Müllerian duct of the female develops a prominent ostium abdominale by day 9 p.p., but this structure has completely regressed in males by day 13 p.p.. The testis and ovary both migrate caudally, together with the adjacent mesonephros, at about day 10 p.p.. The ovaries remain around the level of lumbar vertebra 4 after about day 7 p.p., while the testes continue to descend. The testes enter the internal inguinal ring at about day 25 p.p., about the time that prostatic buds first appear in the urogenital sinus, and are in the inguinal canal from days 25 to 36 p.p.. They enter the scrotum at around day 36 p.p., and testicular descent is complete by days 65-72 p.p.. Melanin develops in the tunica vaginalis 72 days after birth. The overall development of the urogenital system in this marsupial is similar to that of eutherians but the sequence of events differs, with some aspects of genital differentiation preceding gonadal differentiation, apparently because they are directly controlled by X-linked genes, rather than indirectly controlled by gonadal steroids.

Animals↗
PubMed1995

Male germ cell transplantation in rats: apparent synchronization of spermatogenesis between host and donor seminiferous epithelia.

Primordial germ cells (PGC) and gonocytes from male Sprague-Dawley rat fetuses and neonates were transplanted via the rete testis into the lumen of the seminiferous tubules of recipient adult Long Evans rats. The donor germ cells apparently differentiated into mini-tubules or irregular segments of seminiferous epithelium within the lumen of the host seminiferous tubules, and exhibited qualitatively normal spermatogenesis in 10 out of 16 recipients. The stage of spermatogenesis of the intraluminal epithelium was synchronized closely with that of the adjacent seminiferous tubule epithelium, suggesting that the spermatogenic cycle is regulated locally by the intraluminal microenvironment. Male germ cell transplantation provides an interesting new tool for investigating the control of spermatogenesis.

Animals↗
PubMed1994

Women's reproductive cancers in evolutionary context.

Reproductive experiences for women in today's affluent Western nations differ from those of women in hunting and gathering societies, who continue the ancestral human pattern. These differences parallel commonly accepted reproductive risk factors for cancers of the breast, endometrium and ovary. Nutritional practices, exercise requirements, and body composition are nonreproductive influences that have been proposed as additional factors affecting the incidence of women's cancers. In each case, these would further increase risk for women in industrialized countries relative to forager women. Lifestyles and reproductive patterns new from an evolutionary perspective may promote women's cancers. Calculations based on a theoretical model suggest that, to age 60, modern Western women have a breast cancer risk as much as 100 times that of preagricultural women.

Biological Evolution↗
PubMed1994

What the breast does for the baby, and what the baby does for the breast.

Breastfeeding is nature's effective method of child-spacing, provided that feeding is exclusive and prolonged. Yet, its popularity has been limited in modern societies for a range of cultural, social and medical reasons. It is a matter of surprise and regret that no medical specialty has claimed overall 'ownership' of the physiology, function, pathology and management of this conspicuous organ, the female breast. The breast has major benefits for the baby in protecting against debilitating diarrhoea, necrotising enterocolitis and certain allergies, as well as providing highly specific nutritional requirements for the human neonate. It is less widely appreciated that, in addition to its major child-spacing benefits for the mother, breastfeeding also provides major protection against the development of breast cancer, up to 50% reduction in incidence in one study. Few other measures can approach this.

Breast Feeding↗