Reproductive suppression in eusocial Cryptomys darnavensis colonies: socially-induced infertility in females

Inhibition of reproduction occurs in colonies of the Damaraland mole-rat ( Cryptomys damarensis ), where one female and usually one male are reproductively active. They remain the sole reproductive animals thoughout their stay in the colony, which in the field may exceed five years.
An 18-month study on a captive colony of C. damarensis shows that non-reproductive females remain anovulatory and always have concentrations of urinary progesterone lower than that of the reproductive female 10.7 8.8 nmols/mmol creatinine (n = 85), although their progesterone concentrations are slightly elevated when the reproductive female is early in pregnancy. In contrast the reproductively active female has elevated concentrations of progesterone 63.3 70.1 nmols/mmol creatinine (n= 14).
Standard histological together with immunohistochemical examination of the ovarian structure shows that follicular development in the non-reproductive females is halted at varying stages prior to ovulation and that the unruptured follicles luteinize. These unruptured luteinized follicles stain positive for 3β hydroxysteroid dehydrogenase—the enzyme catalysing the synthesis of progesterone from pregnenolone. It is possible that the low levels of circulating progesterone produced by the luteinized follicles in the non-reproductive females in the colony are sufficient to feedback on the hypothalamo-pituitary axis and prevent the surge of LH necessary for ovulation.
Behavioural studies provide corroborative evidence for sexual suppression. Thus the non-reproductive females in the colony are never involved in courtship or copulatory behaviour. nor do they actively solicit males in the colony. The reproductive female, however, plays an active role in mate selection and is the initiator in courtship behaviour.
The suppression of ovulation in non-reproductivc females is maintained for as long as there is a reproductive female in the colony.     

Luteinizing hormone-independent luteinization and ovulation in the hypophysectomized rat: a possible role for the oocyte?

Immature hypophysectomized, estrogen-treated rats were used to study the regulation of luteinization. Particular attention was focused on the potential role of the oocyte in this process. Rats were injected for 2 days with follicle-stimulating hormone (FSH) to stimulate follicular development. Within 48 h following FSH treatment, many follicles became luteinized, as determined by morphometric analysis. This luteinization occurred in the absence of detectable levels of luteinizing hormone (LH). The number of follicles undergoing luteinization was dependent on the FSH dose. In addition, ovulation occurred in some of the animals receiving the highest doses of FSH (3-micrograms or 5-micrograms injections). The majority of follicles undergoing luteinization or ovulation were greater than 400 microns in diameter. Luteinized follicles exhibited positive reactivity for cholesterol side-chain cleavage enzyme, 3 beta-hydroxysteroid dehydrogenase, lipid, and alkaline phosphatase, which was similar to that found in corpora lutea of the cycle. Serum progesterone (P0) and 20 alpha-hydroxypregn-4-en-one levels were elevated in animals with luteinized follicles, especially in those animals that also underwent ovulation. Morphological evaluation of oocytes showed that the majority of luteinized follicles contained a degenerating oocyte. Oocyte degeneration was highly correlated (r = 0.94) to luteinization. These results demonstrate that luteinization and ovulation can occur in the FSH/estrogen-primed hypophysectomized rats in the absence of detectable serum LH. Furthermore, LH-independent luteinization was strongly correlated to degenerative changes in the oocyte. These results provide new evidence to support the concept that the oocyte may be an intraovarian regulator of luteinization.     

Functional anatomy of the female genital organs of the wild black agouti (Dasyprocta fuliginosa) female in the Peruvian Amazon

This study examined anatomical and histological characteristics of genital organs of 38 black agouti females in the wild in different reproductive stages, collected by rural hunters in the North-eastern Peruvian Amazon. Females in the follicular phase of the estrous cycle had greater antral follicle sizes than other females, the largest antral follicle measuring 2.34mm. Antral follicles in pregnant females and females in luteal phase of the estrous cycle had an average maximum diameter smaller than 1mm. In black agouti females in follicular phase, some antral follicles are selected to continue to growth, reaching a pre-ovulatory diameter of 2mm. Mean ovulation rate was 2.5 follicles and litter size was 2.1 embryos or fetuses per pregnant female, resulting in a rate of ovum mortality of 20.8%. Many follicles from which ovulation did not occur of 1-mm maximum diameter luteinize forming accessory CL. The constituent active luteal tissues of the ovary are functional and accessory CL. Although all females had accessory CL, transformation of follicles into accessory CL occurred especially in pregnant females, resulting in a contribution from 9% to 23% of the total luteal volume as pregnancy advances. The persistence of functional CL throughout pregnancy might reflect the importance for the maintenance of gestation and may be essential for the continuous hormonal production. The duplex uterus of the agouti female is composed by two completely independent uterine horns with correspondent separate cervices opening into the vagina. In pregnant females, most remarkable observed uterine adaptations were induced by the progressive enlargement caused by the normal pregnancy evolution. The wild black agouti showed different vaginal epithelium features in accordance with the reproductive state of the female.     

Differential transport of embryos and degenerating ova by the oviducts of the long-tongued bat, Glossophaga soricina.

Female long-tongued bats which had been maintained in sexually segregated groups in captivity for more than 8 months were bred and killed at various intervals between Days 1 and 25 post coitum. Their reproductive tracts were then examined histologically. In 20 of the 28 bats carrying tubal embryos the remnants of 1-4 other ova were also observed in the oviductal ampullae. These remnants consisted of intact zonae pellucidae containing cellular debris, empty zonae, and/or zona fragments. Since long-tongued bats are monovular, the remnants had apparently been retained in the oviducts from previous, non-fertile reproductive cycles. In 27 of the 31 bats carrying implanting blastocysts, zonae pellucidae probably shed by the embryos had been retained in the oviducts. The remnants of 1-3 ova were also observed in the oviductal ampullae of 22 of the 31 bats carrying uterine blastocysts. In at least 14 of these bats the embryos had by-passed ovum remnants in the oviducts on their way to the uterus. No evidence of such remnants was found in the oviducts of 17 animals with tubal or uterine embryos, although old, as well as new, CL were present in the ovaries.     

Ovarian growth and folliculogenesis in breeding and nonbreeding females of a social rodent,the Zambian common mole-rat,Cryptomys sp.

Zambian common mole-rats are subterranean rodents that live in families with only one female breeding. Her offspring remain in the parental nest and do not reproduce. Behavioral experiments (Burda, '95) demonstrated that their apparent “sterility” is based on incest avoidance and individual recognition of family members. To elucidate whether some kind of morphologically apparent ovarian suppression still takes place in daughters, ovaries of females of known age, weight, and reproductive histories were examined histologically and morphometrically. The body mass of old females (more than 3 years of age) begins to decrease, and the ovaries seem to begin to atrophy at the age of about 3–6 years. Ovaries in neonates exhibited primordial and primary follicles, sometimes clustered in nests. Ovaries of adult nonbreeding females expressed all stages of the follicular development up to tertiary follicles. Many unruptured luteinized follicles were present, but true corpora lutea as a morphological sign of ovulation were missing. Unruptured luteinized follicles also could be found (additionally to true corpora lutea) in ovaries of breeding females. The number of primordial follicles dropped rapidly during the first 2 years of age; the number of primary, secondary, and tertiary follicles was subject to individual variation; and there was no clear correlation with age or reproductive status. While a tendency to form accessory unruptured luteinized follicles may just reflect taxonomic affinities of bathyergids to hystricomorphs, the otherwise complete folliculogenesis in “sterile” daughters and the presence of unruptured luteinized follicles even in breeding females are further evidence that there is no hormonal suppression of the ovarial cycle. We suggest that ovulation in nonbreeding females is not actively suppressed by the breeding female, but instead that it is not released because the triggering mechanisms, most probably repeated copulation, are missing. J. Morphol. 237:33–41, 1998. © 1998 Wiley-Liss, Inc.     

Postnatal regulation of germ cells by activin: the establishment of the initial follicle pool

Mammalian females enter puberty with follicular reserves that exceed the number needed for ovulation during a single lifetime. Follicular depletion occurs throughout reproductive life and ends in menopause, or reproductive senescence, when the follicle pool is exhausted. The mechanisms regulating the production of a species-specific initial follicle pool are not well understood. However, the establishment of a follicular reserve is critical to defining the length of reproductive cyclicity. Here we show that activin A (rh-ActA), a known regulator of follicle formation and growth in vitro, increased the number of postnatal mouse primordial follicles by 30% when administered to neonatal animals during the time of germline cyst breakdown and follicle assembly. This expansion in the initial follicle pool was characterized by a significant increase in both germ cell and granulosa cell proliferation. However, the excess follicles formed shortly after birth did not persist into puberty and both adult rh-ActA- and vehicle-treated animals demonstrated normal fertility. A follicle atresia kinetic constant (k(A)) was modeled for the two groups of animals, and consistent with the empirical data, the k(A) for rh-ActA-treated was twice that of vehicle-treated animals. Kinetic constants for follicle formation, follicle loss and follicle expansion from birth to postnatal day 19 were also derived for vehicle and rh-ActA treatment conditions. Importantly, introduction of exogenous rh-ActA revealed an intrinsic ovarian quorum sensing mechanism that controls the number of follicles available at puberty. We propose that there is an optimal number of oocytes present at puberty, and when the follicle number is exceeded, it occurs at the expense of oocyte quality. The proposed mechanism provides a means by which the ovary eliminates excess follicles containing oocytes of poor quality prior to puberty, thus maintaining fertility in the face of abnormal hormonal stimuli in the prepubertal period.     

Expression of vitellogenin receptor in the ovarian follicles during the reproductive cycle of the spotted ray Torpedo marmorata Risso 1880

The aim of this investigation was to identify the encoding sequence of vitellogenin receptor gene (vtgr), and its expression during the oogenesis in the spotted ray, Torpedo marmorata, in different phases of reproductive cycle. From an ovarian cDNA of vitellogenic female, we obtained a fragment of 581?bp, which corresponds to a partial sequence encoding the vitellogenin receptor (VTGR) in Torpedo (accession number: gi/193244760). This sequence shows a high identity with the VTGR of other vertebrates, particularly Leucoraja erinacea (89% identity) and Squalus acanthias (84% identity). We also showed that vtgr mRNA expression in the ovary modifies during the oogenesis and throughout the reproductive cycle. Indeed, in immature females, whose ovary contains only previtellogenic follicles, vtgr mRNA occurred in the oocyte cortex as well as within intermediate and pyriform cells. In mature females, whose ovary contains pre- and vitellogenic follicles, vtgr mRNA was detectable not only in the oocyte cortex and in intermediate and pyriform cells but also in small follicle cells present in the follicular epithelium of vitellogenic follicles. In ovulating females, that, as pregnant ones, show pre-and vitellogenic follicles, vtgr mRNA was evident in the oocyte cortex only, whereas in pregnant females, no vtgr mRNA was evident. The role of VTGR in the control of Torpedo vitellogenesis is discussed.     

Populations of granulosa cells in small follicles of the sheep ovary.

The aim of this study in sheep ovaries was to determine the total number of granulosa cells in primordial follicles and at subsequent stages of growth to early antrum formation. The second aim was to examine the interrelationships among the total number of granulosa cells in the follicles, the number of granulosa cells in the section through the oocyte nucleolus, and the diameter of the oocyte. A third aim was to examine whether proliferating cell nuclear antigen labelling occurred in flattened granulosa cells in primordial follicles or was confined to follicles containing cuboidal granulosa cells. The follicles were classified using the section through the oocyte nucleolus by the configuration of granulosa cells around the oocyte as type 1 (primordial), type 1a (transitory), type 2 (primary), type 3 (small preantral), type 4 (large preantral), and type 5 (small antral). In type 1 follicles, the number of granulosa cells and oocyte diameter were highly variable in both fetal and adult ovaries. Each type of follicle was significantly different from the others (all P < 0.05) with respect to oocyte diameter, number of granulosa cells in the section through the oocyte nucleolus and total number of granulosa cells. Follicles classified as type 2, 3, 4 or 5 each corresponded to two doublings of the total granulosa cell population. The relationships between oocyte diameter and the number of granulosa cells (that is, in the section through the oocyte nucleous or total population per follicle) could all be described by the regression equation loge chi = a + b loge gamma with the correlation coefficients R always > 0.93. For each pair of variables the slopes (b) for each type of follicle were not different from the overall slope for all types of follicle pooled. Immunostaining for proliferating cell nuclear antigen was observed in granulosa cells in type 1 follicles, as well as in the other types of follicle. These findings indicate that 'flattened' granulosa cells in type 1 follicles express an essential nuclear protein involved in cell proliferation before assuming the cuboidal shape. Thus, when considering factors that regulate specific phases of early follicular growth, it is important to consider: (i) the follicle classification system used; (ii) the animal model studied; (iii) whether type 1 follicles are all quiescent; and (iv) the likelihood that each follicle type represents more than one doubling of the population of granulosa cells.     

Activation of follicle development: the primordial follicle

Investigations of primordial follicle formation and growth are fundamental to our understanding of female gamete production. In all mammalian females the full complement of oocytes is established during fetal development. This store of primordial follicles is not renewable and serves the entire reproductive life span of the adult. The correct programming of fetal ovarian development and the number of primordial follicles formed will therefore limit the fecundity of the ovary. Primordial follicles are characterized by the presence of a single oocyte surrounded by a varying number of pregranulosa cells. The relatively small size, undifferentiated status and large numbers of primordial follicles make them prime candidates for use in basic and applied research in animal production, gene transfer and cloning. Furthermore, the development of cell culture systems that use primordial follicles as a source of oocytes for in vitro growth and maturation will enable us to maximize the potential of high genetic merit females and to shorten generation intervals. Despite these possibilities, primordial follicles are the least understood of all stages of follicle development. The factor(s) responsible for maintaining the primordial pool or, conversely, for activating primordial follicle growth remain elusive.     

Characterization of Luteinizing Hormone and Luteinizing Hormone Receptor and Their Indispensable Role in the Ovulatory Process of the Medaka

The molecular properties and roles of luteinizing hormone (Lh) and its receptor (Lhcgrbb) have not been studied for the medaka (Oryzias latipes), which is an excellent animal model for ovulation studies. Here, we characterized the medaka Lh/Lhcgrbb system, with attention to its involvement in the ovulatory process of this teleost fish. In the medaka ovary, follicle-stimulating hormone receptor mRNA was expressed in small and medium-sized follicles, while lhcgrbb mRNA was expressed in the follicle layers of all growing follicles. Experiments using HEK 293T cells expressing medaka Lhcgrbb in vitro revealed that gonadotropin from pregnant mare’s serum and medaka recombinant Lh (rLh) bound to the fish Lhcgrbb. The fish gonadotropin subunits Gtha, Fshb, and Lhb were essentially expressed at fairly constant levels in the pituitary of the fish during a 24-h spawning cycle. Using medaka rLh, we developed a follicle culture system that allowed us to follow the whole process of oocyte maturation and ovulation in vitro. This follicle culture method enabled us to determine that the Lh surge for the preovulatory follicle occurred in vivo between 19 and 15 h before ovulation. The present study also showed that oocyte maturation and ovulation were delayed several hours in vitro compared with in vivo. Treatment of large follicles with medaka rLh in vitro significantly increased the expression of Mmp15, which was previously demonstrated to be crucial for ovulation in the fish. These findings demonstrate that Lh/Lhcgrbb is critically involved in the induction of oocyte maturation and ovulation.     

Ultrastructural observations of previtellogenic ovarian follicles of the caecilians Ichthyophis tricolor and Gegeneophis ramaswamii

The ultrastructural organization of the previtellogenic follicles of the caecilians Ichthyophis tricolor and Gegeneophis ramaswamii, of the Western Ghats of India, were observed. Both species follow a similar seasonal reproductive pattern. The ovaries contain primordial follicles throughout the year with previtellogenic, vitellogenic, or postvitellogenic follicles, depending upon the reproductive status. The just-recruited primordial follicle includes an oocyte surrounded by a single layer of follicle and thecal cells. The differentiation of the theca into externa and interna layers, the follicle cells into dark and light variants, and the appearance of primordial yolk platelets and mitochondrial clouds in the ooplasm mark the transition of the primordial follicle into a previtellogenic follicle. During further development of the previtellogenic follicle the following changes occur: i) the theca loses distinction as externa and interna; ii) all the follicle cells become the dark variant and increase in the complexity of ultrastructural organization; iii) the nucleus of the oocyte transforms into the germinal vesicle and there is amplification of the nucleoli; iv) the primordial yolk platelets of the cortical cytoplasm of the oocyte increase in abundance; v) the mitochondrial clouds fragment and the mitochondria move away from the clouds, leaving behind the cementing matrix, which contains membrane-bound vesicles of various sizes, either empty or filled with a lipid material; vi) the perivitelline space appears first as troughs at the junctional points between the follicle cells and oocyte, which subsequently spread all around to become continuous; vii) macrovilli and microvilli develop from the follicle cells and oocyte, respectively; and viii) the precursor material of the vitelline envelop arrives at the perivitelline space. The sequential changes in the previtellogenic follicles of two species of caecilians are described.     

Increased beta-oxidation and improved oocyte developmental competence in response to l-carnitine during ovarian in vitro follicle development in mice

Oocyte developmental competence is acquired throughout folliculogenesis and is associated with appropriate differentiation and responsiveness to the luteinizing hormone (LH) surge. The recent development of a novel system for culturing ovarian follicles in a three-dimensional alginate matrix shows promise in phenocopying in vivo folliculogenesis. However, oocytes from follicles grown in vitro have a reduced capacity to complete nuclear maturation and be fertilized compared to oocytes matured in vivo. Oocyte metabolism is closely linked with oocyte quality, and we have recently shown that beta-oxidation of lipids is essential for oocyte developmental competence. Thus we investigated whether upregulation of beta-oxidation by treatment with the fatty acid transport cofactor l-carnitine could improve folliculogenesis and developmental competence of mouse follicles following three-dimensional culture. Ovarian hormones (androstenedione, estradiol, and progesterone) and the induction of cumulus matrix proteins (hyaluronan and ADAMTS1) were similar to in vivo follicles, indicating that appropriate differentiation of follicular cells occurs in cultured follicles after an LH/human chorionic gonadotropin (hCG) stimulus. l-carnitine did not alter survival, growth, or differentiation of follicles. However, l-carnitine supplementation significantly increased beta-oxidation, and markedly improved both fertilization rate and blastocyst development. Together, these results show that appropriate responsiveness of the follicle to the LH/hCG surge occurs following three-dimensional follicle culture but limitations on key metabolic requirements remain. l-carnitine supplementation during in vitro follicle culture increased lipid metabolism and improved oocyte developmental competence.     

Molecular characterization of the follicle defects in the growth differentiation factor 9-deficient ovary.

Growth differentiation factor-9 (GDF-9), a secreted member of the transforming growth factor-beta superfamily, is expressed at high levels in the mammalian oocyte beginning at the type 3a primary follicle stage. We have previously demonstrated that GDF-9-deficient female mice are infertile because of an early block in folliculogenesis at the type 3b primary follicle stage. To address the molecular defects that result from the absence of GDF-9, we have analyzed the expression of several important ovarian marker genes. The major findings of our studies are as follows: 1) There are no detectable signals around GDF-9-deficient follicles for several theca cell layer markers [i.e. 17alpha-hydroxylase, LH receptor (LHR), and c-kit, the receptor for kit ligand]. This demonstrates that in the absence of GDF-9, the follicles are incompetent to emit a signal that recruits theca cell precursors to surround the follicle; 2) The primary follicles of GDF-9-deficient mice demonstrate an up-regulation of kit ligand and inhibin-alpha. This suggests that these two important secreted growth factors, expressed in the granulosa cells, may be directly regulated in a paracrine fashion by GDF-9. Up-regulation of kit ligand, via signaling through c-kit on the oocyte, may be directly involved in the increased size of GDF-9-deficient oocytes and the eventual demise of the oocyte; 3) After loss of the oocyte, the cells of the GDF-9-deficient follicles remain in a steroidogenic cluster that histologically resembles small corpora lutea. However, at the molecular level, these cells are positive for both luteal markers (e.g. LHR and P-450 side chain cleavage) and nonluteal markers (e.g. inhibin alpha and P-450 aromatase). This demonstrates that initially the presence of the oocyte prevents the expression of luteinized markers, but that the absence of GDF-9 at an early timepoint alters the differentiation program of the granulosa cells; and 4) As demonstrated by staining with either proliferating cell nuclear antigen (PCNA) or Ki-67 and TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) labeling, the granulosa cells of GDF-9-deficient type 3b primary follicles fail to proliferate but also fail to undergo cell death. This suggests that granulosa cells of type 3b follicles require GDF-9 for continued growth and also to become competent to undergo apoptosis, possibly through a differentiation event Thus, these studies have enlightened us as to the paracrine roles of GDF-9 as well as the normal steps of granulosa cell and theca cell growth and differentiation within ovarian follicles.     

Profiles of oestradiol-17 beta and progesterone and follicular development during the reproductive season in mink (Mustela vison).

Plasma concentrations of oestradiol-17 beta and progesterone were studied in yearling mink females. The blood samples were collected from 2 March until 13 April in females not subjected to mating and in females mated on two consecutive days, early or late in the breeding season, or with 8-9 days between matings. Peaks in oestradiol-17 beta were recorded on the day of first mating, in relation to the second wave of growing follicles, and in early April, around the time when implantation should have occurred. Significant rises in progesterone were recorded from 17 to 21 March and were slightly later in females mated late in the season. Histological studies of ovaries from unmated females revealed that the number of 'active' follicles exceeded the number of degenerated or luteinized follicles until 7 April, after which the number of degenerated follicles increased rapidly. Degeneration was followed by luteinization. On 15 April, ovaries were collected from two females having 15 luteinized follicles each. These females had increased plasma concentrations of progesterone. These studies indicate that, in female mink, peaks in oestradiol-17 beta coincide with the first mating as a result of the copulatory act and that unmated females appear to experience a luteal phase in the absence of ovulation.     

Role of the double luteinizing hormone peak, luteinizing follicles, and the secretion of inhibin for dominant follicle selection in Asian elephants (Elephas maximus)

Elephants express two luteinizing hormone (LH) peaks timed 3 wk apart during the follicular phase. This is in marked contrast with the classic mammalian estrous cycle model with its single, ovulation-inducing LH peak. It is not clear why ovulation and a rise in progesterone only occur after the second LH peak in elephants. However, by combining ovarian ultrasound and hormone measurements in five Asian elephants (Elephas maximus), we have found a novel strategy for dominant follicle selection and luteal tissue accumulation. Two distinct waves of follicles develop during the follicular phase, each of which is terminated by an LH peak. At the first (anovulatory) LH surge, the largest follicles measure between 10 and 19.0 mm. At 7 ± 2.4 days before the second (ovulatory) LH surge, luteinization of these large follicles occurs. Simultaneously with luteinized follicle (LUF) formation, immunoreactive (ir) inhibin concentrations rise and stay elevated for 41.8 ± 5.8 days after ovulation and the subsequent rise in progesterone. We have found a significant relationship between LUF diameter and serum ir-inhibin level (r(2) = 0.82, P < 0.001). The results indicate that circulating ir-inhibin concentrations are derived from the luteinized granulosa cells of LUFs. Therefore, it appears that the development of LUFs is a precondition for inhibin secretion, which in turn impacts the selection of the ovulatory follicle. Only now, a single dominant follicle may deviate from the second follicular wave and ovulate after the second LH peak. Thus, elephants have evolved a different strategy for corpus luteum formation and selection of the ovulatory follicle as compared with other mammals.     

The effects of season and moderate nutritional restriction on ovarian function and oocyte nuclear maturation in cycling gilts

The fertility of female pigs is impaired during summer and in response to restriction of feed intake, resulting in reduced productivity of the breeding herd. This study determined the effect of season and moderate nutritional restriction on ovarian function and oocyte developmental competence of cycling gilts. Eighty prepubescent gilts were used across two seasons—summer (S: January to March) and winter (W: June to August)—and received either a high (2.5× maintenance) or a moderately restricted (1.5× maintenance) feeding level for the first 19 days of their second estrous cycle. On Day 19, ovaries were collected post-slaughter. Diameters of all surface follicles over 1 mm were measured. All follicles ≥4 mm were aspirated and cumulus–oocyte complexes underwent in vitro maturation for ∼44 hours to assess oocyte developmental competence on the basis of metaphase II (MII) attainment. Moderate dietary nutrition reduced daily liveweight gain but did not affect the ovarian follicle population or oocyte developmental competence. The number of large follicles (≥6 mm) was lower during summer (S: 10.7 ± 1.74 vs. W: 15.5 ± 1.15, P < 0.05), as was the proportion of oocytes at the germinal vesicle stage of meiosis (S: 0.06 ± 0.02 vs. W: 0.08 ± 0.02, P < 0.05). However, the proportion of oocytes attaining MII was similar in summer and winter (S: 0.72 ± 0.04 and W: 0.69 ± 0.06, P > 0.05). Intrafollicular concentrations of luteinizing hormone were higher in summer (S: 43.05 ± 6.44 vs. W: 12.05 ± 5.12 ng/mL, P < 0.001), whereas estradiol was lower (S: 1.27 ± 0.36 vs. W: 27.52 ± 5.59 ng/mL, P < 0.001). In conclusion, our data demonstrated that in summer, follicle growth beyond 6 mm is impaired during the periovulatory period, without affecting oocyte meiotic competence. Importantly, these data also demonstrated that ovarian follicle growth and the capacity of oocytes to reach MII in vitro appear unaffected by moderate nutritional restriction during the preceding estrous cycle.     

Pathways coordinating oocyte attrition and abundance during mammalian ovarian reserve establishment

The mammalian ovarian reserve is comprised of a finite pool of primordial follicles, representing the lifetime reproductive capacity of females. In most mammals, the reserve is produced during embryonic and early postnatal development with oocyte numbers peaking during mid‐to‐late gestation, and then experiencing a dramatic decline continuing until shortly after birth. Oocytes remaining after the bulk of this attrition are subsequently surrounded by a layer of somatic pre‐granulosa cells with these units then referred to as “primordial follicles.” The complex and varied cell death mechanisms intrinsic to this process are not only characteristic of, but also essential for, the proper formation of this pool of follicles, and as a result must be immaculately balanced to ensure long‐term fertility and reproductive health. Too few follicles can lead to Primary Ovarian Insufficiency, resulting in fertility loss and other features of aging, such as an overall shorter lifespan. On the other hand, whereas an excess of follicles might extend reproductive lifespan, this might also be the underlying etiology of other ovarian pathologies. The last decade, in particular, has vastly expanded our understanding of oocyte attrition and determinants of ovarian reserve abundance. By continuing to decipher the intricacies underlying the cell death processes and development of the initial primordial follicle pool, we may be in a much better position to understand idiopathic cases of premature follicle depletion and improve ovarian health in reproductive‐age women.     

New data on the factors of ovarian competence in the immature and mature guinea pigs

The effects of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) on ovulation and luteinization in premature and in mature female guinea pigs in different states of the estrous cycle were compared histologically. FSH and LH were administered in a horse pituitary extract (gonadormone, Byla) injected sc, and results were assessed from hematoxylin and eosin-stained serial sections of the ovaries, removed 24 hours later. In premature guinea pigs (mean weight 233 gm) the threshold dose of gonadormone was .1-1 U for luteinization, and results from different seasons did not differ, so experiments were pooled. At .5 U, 17 of 32 (53%) animals had luteinized follicles, compared to 44 of 56 (79%) given 1 U (p.02). Of these luteinized follicles 2 of 17 (12%) animals had ovulated, or .25 (coefficient of ovulation) of luteinized follicles at .1 U, while 10 of 44 (23%) animals ovulated, or .61 of luteinized follicles ovulated at 1 U. 35 or 70 mg atropine S04 per 100 gm body weight did not affect luteinization induced by 1 U gonadormone. In mature guinea pigs (mean weight 415 gm), 2 of 5 U gonadormone at the beginning of vaginal closure caused luteinization, usually with eggs enclosed (pseudopregnancy), or atresia, in more than 1/2 of the animals. On Day 8 after vaginal closure, 7 of 9 (78%) animals had corpora lutea with enclosed eggs, after receiving 1 U gonadormone. On Day 12, 18 of 51 (35%) animals had corpora lutea with enclosed eggs, 12 of 51 (24%) had postovulatory corpora lutea, and 9 of 51 (18%) had both. Atropine S04 again had no effect on luteinization. If the young guinea pigs given .1 U and the mature guinea pigs given 1 U were compared, the frequency of luteinization was 53% and 76%, respectively (p.05); the frequency of ovulation among animals with luteinization was 12% and 23%, respectively (p.01); and the coefficient of ovulation among luteinized follicles was .25 and .78, respectively (p.05). Therefore, degrees of competence can be assigned since mature follicles at the end of the cycle were more responsive than follicles from premature guinea pigs, whose follicles in turn were more responsive than early follicles of mature guinea pigs.