Unsolved problems in male physiology: studies in a marsupial

WITHDRAWN: Utilizing comparative models in biomedical research

The Publisher regrets that this article is an accidental duplication of an article that has already been published in Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, Volume 255, 2021, 110593, https://doi.org/10.1016/j.cbpb.2021.110593. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.

Utilizing comparative models in biomedical research

This review serves as an introduction to a Special Issue of Comparative Biochemistry and Physiology, focused on using non-human models to study biomedical physiology. The concept of a model differs across disciplines. For example, several models are used primarily to gain an understanding of specific human pathologies and disease states, whereas other models may be focused on gaining insight into developmental or evolutionary mechanisms. It is often the case that animals initially used to gain knowledge of some unique biochemical or physiological process finds foothold in the biomedical community and becomes an established model. The choice of a particular model for biomedical research is an ongoing process and model validation must keep pace with existing and emerging technologies. While the importance of non-mammalian models, such as Caenorhabditis elegans, Drosophila melanogaster, Danio rerio and Xenopus laevis, is well known, we also seek to bring attention to emerging alternative models of both invertebrates and vertebrates, which are less established but of interest to the comparative biochemistry and physiology community.

Androgen receptor DNA methylation regulates the timing and androgen sensitivity of mouse prostate ductal development

Androgen receptor (AR) signaling initiates mouse prostate development by stimulating prostate ductal bud formation and specifying bud patterns. Curiously, however, prostatic bud initiation lags behind the onset of gonadal testosterone synthesis by about three days. This study's objective was to test the hypothesis that DNA methylation controls the timing and scope of prostate ductal development by regulating Ar expression in the urogenital sinus (UGS) from which the prostate derives. We determined that Ar DNA methylation decreases in UGS mesenchyme during prostate bud formation in vivo and that this change correlates with decreased DNA methyltransferase expression in the same cell population during the same time period. To examine the role of DNA methylation in prostate development, fetal UGSs were grown in serum-free medium and 5 alpha dihydrotestosterone (DHT) and the DNA methylation inhibitor 5'-aza-2'-deoxycytidine (5AzadC) were introduced into the medium at specific times. As a measure of prostate development, in situ hybridization was used to visualize and count Nkx3-1 mRNA positive prostatic buds. We determined that inhibiting DNA methylation when prostatic buds are being specified, accelerates the onset of prostatic bud development, increases bud number, and sensitizes the budding response to androgens. Inhibition of DNA methylation also reduces Ar DNA methylation in UGS explants and increases Ar mRNA and protein in UGS mesenchyme and epithelium. Together, these results support a novel mechanism whereby Ar DNA methylation regulates UGS androgen sensitivity to control the rate and number of prostatic buds formed, thereby establishing a developmental checkpoint.

Wolffian duct development

The Wolffian ducts (WDs) are the progenitors of the epididymis, vas deferens and seminal vesicles. They form initially as nephric ducts that acquire connection to the developing testis as the mesonephros regresses. The development of the WDs is dependent on androgens. Conventionally, the active androgen is believed to be testosterone delivered locally rather than via the systemic circulation. However, recent studies in marsupials show that 5α-reduced steroids are essential and that these can induce virilisation even when they are delivered via the systemic circulation. The development of the WDs involves an interplay between the duct epithelium and underlying mesenchyme; androgen receptors in both the epithelium and mesenchyme are needed. The epidermal growth factor and epidermal growth factor receptor may play a role, possibly via activation of androgen receptor. The formation of the epididymis involves a complex morphogenetic program to achieve the normal pattern of coiling, formation of septae, and regional functional differentiation. In part, this process may be mediated by inhibin beta A as well as by genes from the HOX cluster. Whilst the development of the WD is androgen dependent, it is clear that there is a complex interplay between androgens, genes and growth factors in the tissues that leads to the formation of the complex anatomy of the male reproductive duct system in the adult.

A dual role for SHH during phallus development in a marsupial

The mammalian phallus arises from identical primordia in both sexes and is patterned in part by the key morphogen Sonic hedgehog (SHH). We have investigated SHH and other morphogens during phallus development in the tammar wallaby. In this marsupial, testis differentiation and androgen production occurs just after birth, but it takes a further 50-60 days before the phallus becomes sexually dimorphic. One day before birth, SHH was expressed in both sexes in the urethral epithelium. In males, there was a marked upregulation of SHH, GLI2, and AR at day 50 postpartum, a time when testicular androgen production falls. SHH, GLI2, and AR were downregulated in female pouch young treated with androstanediol from days 24-50, but not when treatments were begun at day 29, suggesting an early window of androgen sensitivity. SHH, GLI2, and AR expression in the phallus of males castrated at day 23 did not differ from controls, but there was an increase in SHH and GLI2 and a decrease in FGF8 and BMP4 expression when the animals were castrated at day 29. These results suggest that the early patterning by SHH is androgen-independent followed by an androgen-dependent window of sensitivity and a sharp rise in SHH expression after androgen withdrawal at day 50.

Maturation of the growth axis in marsupials occurs gradually during post-natal life and over an equivalent developmental stage relative to eutherian species

The separation of a nutrition-responsive insulin-like growth factor (IGF) system and a growth hormone (GH) responsive IGF system to control pre- and post-natal growth of developing mammals may originate from the constraints imposed by intra-uterine development. In eutherian species that deliver relatively precocial young, maturation of the GH regulatory system is coincident with the time of birth. We measured the hepatic expression of the four key growth axis genes GH-receptor, IGF-1 and -2, and IGFBBP-3, and plasma protein concentrations of IGF-1 from late fetal life through to adult stages of a marsupial, the tammar wallaby. The data clearly show that maturation of GH-regulated growth in marsupials occurs gradually over the course of post-natal life at an equivalent developmental stage to that of precocial eutherian mammals. This suggests that the timing of GH-regulated growth in marsupials is not related to parturition but instead to the relative developmental stage.

Development of the penile urethra in the tammar wallaby

Hypospadias is increasingly common, and requires surgery to repair, but its aetiology is poorly understood. The marsupial tammar wallaby provides a unique opportunity to study hypospadias because penile differentiation occurs postnatally. Androgens are responsible for penile development in the tammar, but the majority of differentiation, in particular formation and closure of the urethral groove forming the penile urethra in males, occurs when there is no measurable sex difference in the concentrations of testosterone or dihydrotestosterone in either the gonads or the circulation [corrected]. Phalluses were examined morphologically from the sexually indifferent period (when androgens are high) to well after the time that the phallus becomes sexually dimorphic. We show that penile development and critical changes in the positioning of the urethra occur in the male phallus begin during an early window of time when androgens are high. Remodelling of the urethra in the male occurs between days 20-60. The critical period of time for the establishment urethral closure occurs during the earliest phases of penile development. This study suggests that there is an early window of time before day 60 when androgen imprinting must occur for normal penile development and closure of the urethral groove.

Reproductive and developmental manipulation of the marsupial, the tammar wallaby Macropus eugenii

The developing marsupial is an ideal animal for use in biomedical research. Marsupials are mammals that have been separated from eutherian mammals for over 130-140 million years. They all deliver altricial young that complete their growth and development after birth usually in a pouch, but not all marsupials have a pouch. Their lactation changes dynamically throughout the period of pouch life, and the mother controls their growth via the production of milk that is tailor-made for each stage of development. The tammar wallaby, Macropus eugenii, has been the experimental marsupial of choice for over five decades, as it is highly amendable to handling and breeds well in captivity. The tammar is especially interesting because it has both a lactational and a seasonal control of its reproduction and embryonic diapause that normally lasts 11 months. Standard molecular techniques can be used for most manipulations in marsupials. However, there are several special techniques for treating the young for experimental surgery and for organ culture that we detail below.

The role of oxidative mechanisms in the etiopathogenesis of pterygium: a preliminary study

BACKGROUND

To evaluate the role of light-induced oxidative mechanisms in the pathogenesis of primary pterygium.

METHODS

Pterygium specimens were obtained from 18 eyes of 18 patients undergoing pterygial removal (group A), and normal conjunctival specimens were obtained from 18 eyes of 18 patients undergoing cataract surgery (group B). In all specimens, malondialdehyde (MDA) levels, superoxide dismutase (SOD) and catalase (CAT) activities were studied and compared.

RESULTS

In group A, the mean SOD activity was lower, and the mean MDA level was higher when compared to group B. However, these differences did not reach clinical significance (p > 0.05). On the other hand, the mean CAT activity was statistically significantly higher in group A when compared to controls (p = 0.017).

CONCLUSION

This preliminary study indicates that oxidative mechanisms seem to play a role in the pathogenesis of pterygium. These mechanisms are apparently activated by ultraviolet irradiation. Further studies are required to establish the role of oxidative mechanisms in the pathogenesis of primary or recurrent pterygia.

The functional development of Leydig cells in a marsupial

Leydig cells are the major source of androgen in the male mammal. We describe here for the first time the development of the Leydig cell in a macropodid marsupial, the tammar wallaby, Macropus eugenii. Leydig cells are first recognized morphologically 2 days after birth with the appearance of lipid droplets in the cytoplasm of certain interstitial cells. Lipid content closely matches the steroid content of the developing testis and marks the maturation of the steroid synthesis pathway in the tammar testis. Morphologically mature Leydig cells, marked by distinct mitochondria with tubular cristae and an extensive anastomosing network of smooth endoplasmic reticulum, are developed by day 10 after birth - the time of peak testosterone content in perinatal tammar testes. The volume percentage of each cell type in the testis does not change over time so the growth of each cellular component keeps pace with growth of the whole testis. There was no morphological or quantitative evidence of a change from one population of Leydig cells to another in the tammar testis as has been reported in several other species including the rat, mouse and human. Maturation of the testis is also marked by the development of tight junctions between the cell membranes of adjacent Sertoli cells. These appear around day 30 after birth and coincide with the onset of mitotic arrest in male germ cells. Overall, the development of the Leydig cell in the tammar wallaby follows a similar pattern to that seen in other mammals, although the start of Leydig cell differentiation is, like many other organ systems in marsupials, post natal, not fetal and there appears to be only a single population of Leydig cells.

Reproduction in male swamp wallabies (Wallabia bicolor): puberty and the effects of season

This study describes pubertal changes in testes and epididymides and seasonal changes in the adult male reproductive organs and plasma androgen concentrations of the swamp wallaby (Wallabia bicolor). Pre-pubescent males had testes with solid seminiferous cords and spermatogenesis only to the stage of gonocytes. Their epididymides had empty lumina along their entire length. The testes of three males undergoing puberty had some lumen formation and mitotic activity. Their epididymides were similar in appearance to those of adult males but were entirely devoid of any cells within the lumen of the duct. Three other pubescent males showed full lumen formation in the testes and spermatogenesis up to the elongating spermatid stage. Their epididymides were similar in appearance to those of adult males but with no spermatozoa in the duct. However, cells of testicular origin were found in the lumen of the duct in all regions suggesting that testicular fluids and immature germ cells shed into the rete testes flow through the seminiferous tubules into the epididymis before the release of mature testicular spermatozoa. The weights of testes and epididymides of adult males showed no change throughout the year but prostate weight and plasma androgen concentrations varied significantly with season, with maximums in spring and summer and minimums in winter. The volume fraction of Leydig cells and seminiferous tubules was significantly lower in winter than in summer; but, despite this, maturing spermatozoa were found in the testes throughout the year. Females in the area conceived year-round, suggesting that seasonal changes in the male reproductive tract did not prevent at least some males from breeding throughout the year.

Society for Reproductive Biology Founders' Lecture 2006 - life in the pouch: womb with a view

Marsupials give birth to an undeveloped altricial young after a relatively short gestation period, but have a long and sophisticated lactation with the young usually developing in a pouch. Their viviparous mode of reproduction trades placentation for lactation, exchanging the umbilical cord for the teat. The special adaptations that marsupials have developed provide us with unique insights into the evolution of all mammalian reproduction. Marsupials hold many mammalian reproductive 'records', for example they have the shortest known gestation but the longest embryonic diapause, the smallest neonate but the longest sperm. They have contributed to our knowledge of many mammalian reproductive events including embryonic diapause and development, birth behaviour, sex determination, sexual differentiation, lactation and seasonal breeding. Because marsupials have been genetically isolated from eutherian mammals for over 125 million years, sequencing of the genome of two marsupial species has made comparative genomic biology an exciting and important new area of investigation. This review will show how the study of marsupials has widened our understanding of mammalian reproduction and development, highlighting some mechanisms that are so fundamental that they are shared by all today's marsupial and eutherian mammals.

The human fetal adrenal cortex and the window of sexual differentiation

Understanding normal development is fundamental to appreciating postnatal morphology, physiology and, in some instances, pathophysiology. Developmental biology tends to interrogate models in nonprimate species, for instance the mouse, where genetic manipulation gives privileged insight into the function of particular genes. Some human developmental processes, as occur in the adrenal gland, are not faithfully reproduced in these rodent models, yet have an impact on the pathophysiology and treatment of endocrine disorders, such as congenital adrenal hyperplasia. In this setting, in vitro research of normal human development complements clinical investigation of patients born with congenital disorders.

In humans, early cortisol biosynthesis provides a mechanism to safeguard female sexual development

In humans, sexual differentiation of the external genitalia is established at 7-12 weeks post conception (wpc). During this period, maintaining the appropriate intrauterine hormone environment is critical. In contrast to other species, this regulation extends to the human fetal adrenal cortex, as evidenced by the virilization that is associated with various forms of congenital adrenal hyperplasia. The mechanism underlying these clinical findings has remained elusive. Here we show that the human fetal adrenal cortex synthesized cortisol much earlier than previously documented, an effect associated with transient expression of the orphan nuclear receptor nerve growth factor IB-like (NGFI-B) and its regulatory target, the steroidogenic enzyme type 2 3beta-hydroxysteroid dehydrogenase (HSD3B2). This cortisol biosynthesis was maximal at 8-9 wpc under the regulation of ACTH. Negative feedback was apparent at the anterior pituitary corticotrophs. ACTH also stimulated the adrenal gland to secrete androstenedione and testosterone. In concert, these data promote a distinctive mechanism for normal human development whereby cortisol production, determined by transient NGFI-B and HSD3B2 expression, provides feedback at the anterior pituitary to modulate androgen biosynthesis and safeguard normal female sexual differentiation.

Male hormonal contraception: concept proven, product in sight?

Current male hormonal contraceptive (MHC) regimens act at various levels within the hypothalamic pituitary testicular axis, principally to induce the withdrawal of the pituitary gonadotrophins and in turn intratesticular androgen production and spermatogenesis. Azoospermia or severe oligozoospermia result from the inhibition of spermatogonial maturation and sperm release (spermiation). All regimens include an androgen to maintain virilization, while in many the suppression of gonadotrophins/spermatogenesis is augmented by the addition of another anti-gonadotrophic agent (progestin, GnRH antagonist). The suppression of sperm concentration to 1 x 10(6)/ml appears to provide comparable contraceptive efficacy to female hormonal methods, but the confidence intervals around these estimates remain relatively large, reflecting the limited number of exposure years reported. Also, inconsistencies in the rapidity and depth of spermatogenic suppression, potential for secondary escape of sperm into the ejaculate and onset of fertility return not readily explainable by analysis of subject serum hormone levels, germ cell number or intratesticular steroidogenesis, are apparent. As such, a better understanding of the endocrine and genetic regulation of spermatogenesis is necessary and may allow for new treatment paradigms. The development of an effective, consumer-friendly male contraceptive remains challenging, as it requires strong translational cooperation not only between basic scientists and clinicians but also between public and private sectors. At present, a prototype MHC product using a long-acting injectable testosterone and depot progestin is well advanced.

Sexual development of a model marsupial male

In eutherian mammals sexual differentiation occurs during fetal development, making experimental manipulation difficult, unlike in marsupials. We are investigating the roles of several key genes and hormones whose exact role in gonadal differentiation is still unclear using the tammar wallaby (Macropus eugenii) as a model. As in humans, unlike in mice, the testis-determining gene SRY is expressed in male tammar fetuses in many tissues over an extended period. Not all sexual differentiation depends on testicular hormones. Scrotum and mammary glands are under the control of X-linked gene(s). Our demonstration of DMRT1 expression in tammar and mouse ovaries suggests it has a wider role than previously thought. The Y-borne copy of ATRX (ATRY) is coexpressed with DMRT1 in developing testis. Gonadal sex reversal can be induced in males by neonatal oestrogen treatment and in females by grafting developing ovaries to males or culturing them in minimal medium. Treatments of developing young with various androgens, and studies of steroid metabolism have shown that the steroid androstenediol may have a previously unrecognised role in virilisation. Our studies using a marsupial model have given some surprising insights into the evolution and control of sexual development in all mammals.