Preliminary observations on the role of the mesonephros in the development of the adrenal cortex

Early development of the human embryonic testis

Human gonadal development culminating in testicular differentiation is described through analysis of histologic sections derived from 33-day to 20-week human embryos/fetuses, focusing on early development (4-8 weeks of gestation). Our study updates the comprehensive studies of Felix (1912), van Wagenen and Simpson (1965), and Juric-Lekic et al. (2013), which were published in books and thus are unsearchable via PubMed. Human gonads develop from the germinal ridge, a thickening of coelomic epithelium on the medial side of the urogenital ridge. The bilateral urogenital ridges contain elements of the mesonephric kidney, namely the mesonephric duct, mesonephric tubules, and mesonephric glomeruli. The germinal ridge, into which primordial germ cells migrate, is initially recognized as a thickening of coelomic epithelium on the urogenital ridge late in the 4th week of gestation. Subsequently, in the 5th week of gestation, a dense mesenchyme develops sub-adjacent to the epithelium of the germinal ridge, and together these elements bulge into the coelomic cavity forming bilateral longitudinal ridges attached to the urogenital ridges. During development, primordial cells migrate into the germinal ridge and subsequently into testicular cords that form within the featureless dense mesenchyme of the germinal ridge at 6-8 weeks of gestation. The initial low density of testicular cords seen at 8 weeks remodels into a dense array of testicular cords surrounded by α-actin-positive myoid cells during the second trimester. Human testicular development shares many features with that of mice being derived from 4 elements: coelomic epithelium, sub-adjacent mesenchyme, primordial germ cells, and the mesonephros.

Integration of mouse ovary morphogenesis with developmental dynamics of the oviduct, ovarian ligaments, and rete ovarii

Morphogenetic events during the development of the fetal ovary are crucial to the establishment of female fertility. However, the effects of structural rearrangements of the ovary and surrounding reproductive tissues on ovary morphogenesis remain largely uncharacterized. Using tissue clearing and lightsheet microscopy, we found that ovary folding correlated with regionalization into cortex and medulla. Relocation of the oviduct to the ventral aspect of the ovary led to ovary encapsulation, and mutual attachment of the ovary and oviduct to the cranial suspensory ligament likely triggered ovary folding. During this process, the rete ovarii (RO) elaborated into a convoluted tubular structure extending from the ovary into the ovarian capsule. Using genetic mouse models in which the oviduct and RO are perturbed, we found the oviduct is required for ovary encapsulation. This study reveals novel relationships among the ovary and surrounding tissues and paves the way for functional investigation of the relationship between architecture and differentiation of the mammalian ovary.

Concerted morphogenesis of genital ridges and nephric ducts in the mouse captured through whole-embryo imaging

During development of the mouse urogenital complex, the gonads undergo changes in three-dimensional structure, body position and spatial relationship with the mesonephric ducts, kidneys and adrenals. The complexity of genital ridge development obscures potential connections between morphogenesis and gonadal sex determination. To characterize the morphogenic processes implicated in regulating gonad shape and fate, we used whole-embryo tissue clearing and light sheet microscopy to assemble a time course of gonad development in native form and context. Analysis revealed that gonad morphology is determined through anterior-to-posterior patterns as well as increased rates of growth, rotation and separation in the central domain that may contribute to regionalization of the gonad. We report a close alignment of gonad and mesonephric duct movements as well as delayed duct development in a gonad dysgenesis mutant, which together support a mechanical dependency linking gonad and mesonephric duct morphogenesis.

Developmental mechanisms of adrenal cortex formation and their links with adult progenitor populations

The adrenal cortex is the main steroid producing organ of the human body. Studies on adrenal tissue renewal have been neglected for many years, but recent intensified research has seen tremendous progress in our understanding of the formation and homeostasis of this organ. However, cell turnover of the adrenal cortex appears to be complex and several cell populations have been identified that can differentiate into steroidogenic cells and contribute to adrenal cortex renewal. The purpose of this review is to provide an overview of how the adrenal cortex develops and how stem cell populations relate to its developmental progenitors. Finally, we will summarize present and future approaches to harvest the potential of progenitor/stem cells for future cell replacement therapies.

Germ cell differentiation from pluripotent cells

Infertility is a medical condition with an increasing impact in Western societies with causes linked to toxins, genetics, and aging (primarily delay of motherhood). Within the different pathologies that can lead to infertility, poor quality or reduced quantity of gametes plays an important role. Gamete donation and therefore demand on donated sperm and eggs in fertility clinics is increasing. It is hoped that a better understanding of the conditions related to poor gamete quality may allow scientists to design rational treatments. However, to date, relatively little is known about human germ cell development in large part due to the inaccessibility of human development to molecular genetic analysis. It is hoped that pluripotent human embryonic stem cells and induced pluripotent stem cells may provide an accessible in vitro model to study germline development; these cells are able to differentiate to cells of all three primary embryonic germ layers, as well as to germ cells in vitro. We review the state of the art in germline differentiation from pluripotent stem cells.

On the structure of the adrenal gland of the common seal (Phoca vitulina vitulina)

The adrenal gland is a vitally important endocrine gland that occupies a central role in the regulatory mechanisms of the body metabolism. Environmental stress factors lead to permanent strain and overload of the body resulting in structural alterations of the adrenals that in turn are followed by hormonal imbalances. This leads to an increased susceptibility to bacterial and viral diseases. The recurrence of numerous fatalities in the different seal populations of the North Sea (during the years 1988, 1989 and 2002), of the Baikal Lake and Caspian Sea (during the years 2000 and 2001) were the motive for a morphological investigation of the species-specific structure of the adrenal gland of the common seal in order to differentiate environmental stress-induced pathological alterations from the physiological structure of this organ. The study was based on adrenals of 112 common seals (Phoca vitulina vitulina) using light microscopic and transmission and scanning electron microscopic methods. The phocine adrenal gland displays several structural characteristics. Originating from the connective tissue organ capsule, narrow and broad septa intersperse the adrenal cortex. These septa contain blastemata as a reserve for the regeneration of hormone-producing cortical cells. Such blastemata are also occurring in the form of an intermediate zone in between the zona glomerulosa and zona fasciculata in the phocine adrenal cortex. Another species-specific characteristic is an inverse part of the adrenal cortex encircling the central vein of the organ. These structural features have to be considered in assessment and definition of pathological alterations of the adrenals as observed in the form of exhausted blastema cell pools in the adrenocortex of seals perished in the mentioned phocine mass mortalities.

The Booroola (FecB) mutation is associated with smaller adrenal glands in young adult ewes

The Booroola (FecB) phenotype is associated with a mutation in the bone morphogenetic protein (BMP) receptor 1B. The BMP action is important during development; surprisingly the only differences so far observed in adult animals are restricted to the ovaries where precocious development of the antral follicles and increased ovulation rate of mutant ewes is observed. The internal organs of 17 ewes homozygous for the mutation (BB) and 18 wild-type ewes (++) were macroscopically examined and weighed. No macroscopic differences were found, and the weight of the heart, liver, lungs, kidneys, and spleen were similar for both genotypes (P > 0.05). In contrast, the adrenals of BB ewes were lighter than those of ++ ewes (P < 0.05). The effect of the mutation on the adrenal function of cortisol secretion was measured at basal level and after an adrenocorticotrophic hormone challenge, before and after dexamethasone suppression. The Booroola mutation had no effect (P > 0.05) in any of these conditions. These findings indicate that the Booroola mutation also affects the size of the adrenal glands and suggest that the mutated gene could be important in the development of other organs in addition to the ovary. However, in the mutant ewes the function of the adrenal glands is not compromised or it is compensated.

Functional glucocorticoid receptors in the mesonephros of the ovine fetus

BACKGROUND

At 27 days of gestation in the ovine fetus (term = 145 to 150 days), the only kidney is the mesonephros, and allantoic fluid represents fetal urine. The hypothesis tested in this study was that functional glucocorticoid receptors (GRs) are present in this early mesonephric kidney.

METHODS

Pregnant ewes, between 26 and 30 days, were infused with saline, dexamethasone (0.48 mg/hour), cortisol (5 mg/hour), or aldosterone (10 microg/hour) for 48 hours and were then killed for collection of fetuses and fetal fluids. GR mRNA was measured by real-time polymerase chain reaction in whole fetuses, and the location of gene expression was determined by hybridization histochemistry.

RESULTS

Significant changes in allantoic fluid composition were produced by the exposure of the fetus to maternally infused synthetic (dexamethasone) and natural (cortisol) glucocorticoids, over a period of two days, compared with fetuses of ewes infused with vehicle (isotonic saline; N = 8) or aldosterone (N = 8). Volume of fluid was unchanged by any treatment, but both dexamethasone (N = 10) and cortisol (N = 8) caused significant (P < 0.05) decreases in sodium and chloride concentrations and increases in concentrations of potassium, urea, glucose, and fructose. GR mRNA was detected in equivalent concentrations in the whole fetuses of saline, dexamethasone, and cortisol treatments. The GR mRNA levels were significantly decreased in the aldosterone group. By hybridization histochemistry, GR mRNA was detected in most of the tubular cells of the mesonephros.

CONCLUSION

These results suggest that functional GRs are present in the early ovine mesonephros.

The renin-angiotensin system and the development of the kidney and adrenal in sheep

1. The earliest form of the kidney, the pronephros, does not really occur in the ovine embryo; instead, a giant glomerulus forms at the anterior end of the mesonephros. 2. In the sheep, the mesonephros is present from 11-38% of total gestation (150 days) and produces a dilute urine, as well as expressing the genes for erythropoietin, renin, angiotensinogen, angiotensin-converting enzyme and the angiotensin II (AngII) receptors AT1 and AT2. 3. The ovine metanephros begins to develop at 18% of gestation and nephrogenesis is complete several weeks before birth. All components of the renin-angiotensin system (RAS) are expressed from at least 27% of gestation. 4. Both AT1 and AT2 receptors are expressed by the adrenocortical cells early in gestation but, at mid-gestation, exogenous AngII does not stimulate aldosterone secretion in vivo. 5. Preliminary results suggest that AngII has important roles in renal development in the ovine foetus but the role(s), if any, in adrenal development, remains to be investigated.

Three-dimensional structure of the developing mouse genital ridge

We are interested in understanding how the field of cells which forms the gonad arises, and how the testis-determining gene, Sry, controls morphogenesis of a testis within this field of cells. To appreciate changes in the three-dimensional structure of the mouse genital ridge at this time in development, whole-mount genital ridges taken from male and female embryos over the developmental period when the initiation of testis cord morphogenesis takes place, were stained with an antibody against laminin. Samples were visualized using confocal microscopy. Anti-laminin illuminates the elaborate array of mesonephric duct and tubules which occupy the cranial two-thirds of the mesonephros at the earliest timepoint. This complex structure gradually regresses as testis cords form in male gonads. No structural organisation is recognized by this antibody in the female gonadal region during this period. Confocal sections in the Z-plane reveal continuous cellular connections between 3-6 mesonephric tubules and the gonadal primordium. These cellular bridges are present in male and female gonads, so they do not depend on the expression of Sry. We consider the possibility that these bridges constitute the pathways of the founder cells of the gonadal primordium.

Germinal cell ectopism in the strepsirhine prosimian Galago crassicaudatus crassicaudatus

The presence of germinal cells outside of the embryonal and fetal gonads of the strepsirhine prosimian Galago crassicaudatus crassicaudatus is described. Forty-three embryos and fetuses from day 26 or 27 of gestational age to near term were studied: more than 90% possessed germinal cells in ectopic sites situated either far from (extragonadal ectopism) or close to the gonads (perigonadal ectopism). The first sites were the walls of the aorta and mesenteric artery, the stroma between the aorta and the cardinal vein, and the retroperitoneal neuroganglia. The second were the mesenchyme dorsal to the gonads and around the vestigia of the mesonephric glomeruli and tubules, and the rete ovarii and testis. The ectopic cells were generally present in conscpicuous numbers, in some animals being more numerous than in the gonads. Those situated far from the gonads underwent degeneration and decreased significantly in numbers during post-embryonal stages of development, while the others remained numerous and functionally active up to near term. While the differentiation of the extragonadal germinal cells after day 60 of gestational age could not be studied due to technical difficulties, the XX and XY cells in perigonadal sites appeared to follow patterns of differentiation identical to those of their entopic counterparts.

Germ cell differentiation in mouse adrenal glands

The differentiation of germ cells in the adrenal glands of 26 male and female Swiss albino mice was studied in sequential stages of development, from day 12 1/2 of intrauterine life to postnatal day 21; the study was performed by means of high-resolution light microscopy and electron microscopy. In 12 1/2- and 13-day-old embryos, the ectopic cells had morphologic characteristics typical of primordial germ cells, whereas in 14- and 15-day-old fetuses they were identifiable as oogonia. In male and female fetuses from day 17 to term, all ectopic germinal elements entered meiotic prophase, reached diplotene, and differentiated into oocytes in perfect adherence to mouse ovarian timetables. In the postnatal animals, females as well as males, all oocytes progressed through the postmeiotic phase of growth just as they normally do in ovarian follicles, and, in the 2- and 3-week-old animals, they displayed features identical to those exhibited by oocytes in large antral follicles, including a zona pellucida. Germinal elements were no longer seen in the adrenals of animals older than 3 weeks. Our study shows that mammalian germ cells are capable of developing even outside the gonads, and that in ectopic sites they all differentiate as oocytes irrespective of their genetic sex.

Mesonephric origin of the gonadal primitive medulla in chick embryos

The development of the gonadal primitive medulla in embryonic chick gonads was studied with the light microscope, using serial longitudinal sections from 72 h to 108 h of incubation. The sex of embryos was established from karyotypes. At 72 h, the germinal epithelium in the genital ridges was thickened. The nephrogenic cord was not differentiated into nephrons underneath, although the surrounding mesonephros displayed renal corpuscles and tubules. Clusters and trabeculae of mobilized mesonephric cells piled up under the germinal epithelium, forming the rudiment of the primitive medulla. From 78 h onwards, nephrotome-like structures existed in the mesenchyme underlying the germinal epithelium. Mesonephric cells became detached from their ventral walls and incorporated into the rudiment of the medulla. Finally, at 90 h, when the gonads were constituted, the primitive medulla was definitively formed without any contribution of the germinal epithelium. Adrenal cortical cells, also originating from the mesonephric blastema, showed tight relationships with the gonadal medullarian structures. Our observations support the concept of the mesonephric origin of the gonadal components having male potentialities in birds.

Ectopic germ cells: natural model for the study of germ cell sexual differentiation

In the course of a study on the morphogenesis of the adrenal gland in random-bred Swiss albino mice, we noted the presence of ectopic germ cells in the adrenal cortexes and medullas in animals of both sexes, from day 12 1/2 of fetal development to postnatal day 12. Up to day 15 of fetal development, the cells exhibited characteristics of primordial germ cells. At day 17, and irrespective of the sex of the fetus, they all entered meiosis in synchrony with those in the ovary. Postnatally, in females as well as males, all ectopic germ cells displayed morphologic characteristics identical to those of young oocytes in unilaminar ovarian follicles. No germinal elements were seen in the adrenal glands past day 12 of life. Our study shows that mammalian germ cells are capable of undergoing sustained differentiation outside the gonads and that, in ectopic sites, they all differentiate into oocytes as they normally would in the ovary, even in males.

The contribution of the mesonephros to the development of the sheep fetal testis

In this study, which was performed on 52 sheep fetuses aged 24 to 85 days, we examined the relationship between testicular development and the "giant" mesonephric nephron, a peculiar structure consisting of a large glomerulus and multiple tubules. The development of the testis occurred in two phases. The preparatory phase began at day 24 of gestation, evolved simultaneously with the involution of the glomerulus of the giant nephron, and was characterized by mobilization of the glomerular cells and by their colonization of the genital ridge. By day 31, a prominent mass of migrating mesonephric cells had developed; it extended uninterrupted from the giant glomerulus into the gonad where the mesonephric cells associated with the germinal cells forming the cellular template from which the testicular cords later became assembled. At complete involution of the glomerulus (day 52), the migratory mass implanted on the tubules of the giant nephron, which thus became continuous with the gonad. During the organizational phase, which began at day 29 with the formation of the tunica albuginea, the various components of this continuum became progressively organized along a testis-to-mesonephros direction into seminiferous cords, tubuli recti, the cords of the rete testis, and the ductules efferentes. These observations show that, in the sheep, the precursors of the Sertoli cells are mesonephric in origin, and that the genital tract proximal to the epididymis differentiates from a single mesonephric nephron.