This resource is prepared for researchers and academic institutions studying reproductive biology using research-use-only (RUO) peptide compounds in pre-clinical models. All compounds discussed are for in vitro and pre-clinical investigation and are entirely distinct from clinical fertility treatment or contraceptive applications. This hub is distinct from the reproductive research hub (ID 77159), the PCOS hub (ID 77526), the male fertility hub (ID 77367), the endometriosis hub (ID 77369), and the Kisspeptin-10 fertility post (ID 77047), providing an integrated mechanistic framework covering HPG axis activation, gonadotrophin biology, oocyte maturation, spermatogenesis, and reproductive ageing.
The Hypothalamic-Pituitary-Gonadal Axis: Architecture and Regulation
The hypothalamic-pituitary-gonadal (HPG) axis operates as a hierarchical neuroendocrine system controlling reproductive function in both sexes. Hypothalamic gonadotrophin-releasing hormone (GnRH; 10 aa; pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH₂) is synthesised and released in pulses from approximately 1,500 specialised GnRH neurons of the hypothalamus, primarily concentrated at the arcuate nucleus (ARC) and the medial preoptic area (mPOA), projecting axons to the median eminence portal vasculature.
GnRH pulse frequency and amplitude are the critical determinants of pituitary gonadotrophin output. Low-frequency pulsatility (~60–90 min intervals) preferentially drives FSH secretion; high-frequency pulsatility (~30–60 min intervals) drives LH secretion. In females, the mid-cycle LH surge is triggered by a positive oestrogen feedback mechanism (switching from negative to positive feedback at oestradiol levels >200 pg/mL sustained >36h), activating GnRH surge via KNDy neurons and glutamatergic kisspeptin neurons in the anteroventral periventricular nucleus (AVPV/PeN). Continuous GnRH receptor agonism (as with GnRH agonist therapy) leads to receptor downregulation and pituitary desensitisation — exploited in gonadotrophin suppression protocols and distinguishing research models requiring pulsatile vs continuous GnRH delivery.
KNDy Neurons and Kisspeptin-10: The GnRH Pulse Generator
The KNDy neuron network — co-expressing Kisspeptin (KISS1 gene product), Neurokinin B (NKB; TAC3), and Dynorphin A (Dyn; PDYN) — constitutes the intrinsic GnRH pulse generator in the arcuate nucleus. NKB provides autosynaptic excitatory drive via NK3R (TACR3); Dynorphin provides the resetting inhibitory phase via KOR (OPRK1); Kisspeptin provides the ultimate output to GnRH neurons via KISS1R (GPR54), which signals via Gαq/11→PLCβ→IP3→Ca²+ and Gαs→cAMP→PKA activation of GnRH neuron depolarisation and pulsatile release.
Kisspeptin-10 (KP-10; the C-terminal decapeptide Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH₂; ~1302 Da) is the minimal active fragment retaining full KISS1R agonism with EC₅₀ ~1–2 nM. Kisspeptin peptides vary in length: KP-54, KP-14, KP-13, and KP-10 share the C-terminal RF-amide motif essential for receptor binding. In male rodent research models: KP-10 (50–100 nmol i.c.v. or 1–10 nmol i.v.) produces LH pulses of 2.4–3.2 ng/mL amplitude (baseline 0.8–1.2 ng/mL) within 30–45 min, accompanied by testosterone rises of 2–3-fold at 60–90 min. In anovulatory female models (oestrous-suppressed), KP-10 restores LH pulsatility (inter-pulse interval restored from >120 min to 60–80 min) and triggers ovulation in 72–86% of animals when given in the appropriate hormonal context.
LH and FSH Biology: Gonadotrophin Receptor Signalling
LH (luteinising hormone; heterodimeric glycoprotein, α-subunit shared with FSH/TSH/hCG, β-subunit unique, ~28 kDa) and FSH (follicle-stimulating hormone; ~32 kDa with distinct β-subunit) act through leucine-rich repeat-containing GPCRs: LHR (LHCGR) and FSHR (FSHR), both Gαs-coupled activating cAMP/PKA pathways with supplementary β-arrestin/ERK/PI3K signalling in specific cell types.
In males: LH stimulates Leydig cell LHR activation, driving StAR (steroidogenic acute regulatory protein) cholesterol import into mitochondria → CYP11A1 (P450scc) cleavage to pregnenolone → CYP17A1 (17α-hydroxylase/17,20-lyase) → androstenedione → 17β-HSD conversion to testosterone. FSH acts on Sertoli cells (FSHR), inducing androgen-binding protein (ABP), inhibin B, activin, GDNF, and creating the seminiferous tubule microenvironment essential for spermatogenesis. Seminiferous tubule testosterone concentrations (100–200× serum) maintained by Sertoli ABP are required for meiosis and spermiogenesis.
In females: FSH drives granulosa cell aromatase (CYP19A1) induction (via CREB/SF-1 transcription), converting theca-produced androgens (driven by LH/LHR) to oestradiol — the classic two-cell, two-gonadotrophin model. FSH also induces granulosa LHR expression in dominant follicles, enabling LH surge responsiveness. The mid-cycle LH surge (40–100-fold amplitude increase over basal) triggers follicle rupture (via PGE2/cAMP/ADAM10/17 metalloprotease cascade), resumption of meiosis I in the arrested oocyte (via CDC25B/CDK1 dephosphorylation of MPF), and luteinisation.
Folliculogenesis and Oocyte Maturation: Molecular Biology
Ovarian folliculogenesis proceeds from primordial follicles (~180,000–400,000 at birth; declining to ~1,000 at menopause) through primary, secondary, preantral, early antral, antral, and dominant (Graafian) stages — a 3–6 month gonadotrophin-independent pre-antral phase followed by a 14-day FSH-dependent terminal development phase. Primordial follicle activation requires PI3K/AKT/mTOR signalling in oocytes (via PTEN suppression from KIT ligand/Kit receptor and mTORC2/Rictor) — dysregulation drives premature ovarian insufficiency (POI).
Oocyte growth depends on bidirectional communication with surrounding granulosa cells via gap junctions (CX37/GJA4 connexin) and paracrine GDF-9/BMP-15 oocyte-derived signals that drive granulosa proliferation and prevent luteinisation. Within the fully grown oocyte (~115–120 µm diameter in humans), germinal vesicle breakdown (GVBD) — resumption of meiosis from prophase I arrest — is triggered by CDK1/cyclin B (MPF) activation following cAMP drop in response to LH surge (LHR-Gαs→EPAC-PDE3A→cAMP↓→PKA↓→CDC25B→MPF). Metaphase II arrest (MII) occurs at fertilisation readiness, released by Ca²+ oscillations at sperm entry. Mitochondrial content (>100,000 copies mtDNA/oocyte) is the primary energy substrate for oocyte maturation and early embryonic cleavage.
Spermatogenesis: Stem Cell to Mature Spermatozoon
Spermatogenesis involves self-renewal of spermatogonial stem cells (SSCs; expressing PLZF/GFRA1/RET) and transit-amplifying progenitors (type A → type B spermatogonia), meiotic division (primary and secondary spermatocytes), and spermiogenesis (round spermatid to elongated spermatozoon). The complete cycle takes 74–76 days in humans, organised in 6 stages in mice (12 stages in humans) within the seminiferous epithelium.
Key molecular events: (1) SSC self-renewal — GDNF (from Sertoli cells) activates GFRα1/Ret/Src/PI3K maintaining PLZF expression and suppressing RAR-α-dependent differentiation; (2) meiosis — SPO11-induced DSBs and homologous recombination via DMC1/RAD51; synaptonemal complex (SYCP1/SYCP3) assembly; sex body formation (H2AK119ub/H3K27me3); (3) spermiogenesis — acrosome biogenesis from Golgi (PICK1/GOPC pathway), flagellum assembly (CFAP43/44/69 axonemal components), histone-to-protamine replacement (TH2B→TP1/TP2→PRM1/2) achieving 10-fold DNA compaction; (4) spermiation — MMP-2/9/plasminogen/LIF Sertoli retraction; BTB (blood-testis barrier) dynamic restructuring via FAK/Src/claudin/occludin.
Reproductive Ageing: Mechanisms of Fertility Decline
Female reproductive ageing involves primordial follicle pool exhaustion (atresia-driven, ~11.7 years to zero from menopause in mice; ~10–13 years in humans), declining oocyte quality (aneuploidy rate increasing from ~5% at age 25 to ~40% at 40 and >70% at 45 — driven by cohesin degradation, spindle assembly checkpoint weakening, centromere cohesion loss via age-dependent cohesin reduction on chromosomes), and endometrial receptivity decline (reduced LIF/Hoxa10/progesterone receptor signalling).
Mitochondrial dysfunction in aged oocytes: reduced mtDNA copy number (from ~150,000 in young to ~80,000 in aged), elevated mtROS (MitoSOX fluorescence 2–3-fold higher), impaired spindle assembly (AURKA/AURKB; BubR1 checkpoint protein), and ATP production deficits (MII arrest ΔΨm 30–40% lower in aged vs young oocytes). Male reproductive ageing involves declining Leydig cell LHR expression and testosterone output, Sertoli cell number reduction (−20–30% by age 70), increased SSC senescence (p21+/PLZF+ cells), sperm DNA fragmentation index (DFI >15% threshold for fertility impact, rising to 25–35% in elderly males), and reduced sperm motility (total motile sperm count declining 1–2% annually after age 40).
Kisspeptin-10 in HPG Axis Research
Research applications of Kisspeptin-10 in reproductive biology focus on HPG axis interrogation, pulse generator characterisation, and anovulation models. In prepubertal male rats, KP-10 (10 nmol i.v.) triggers premature LH/testosterone pulses — confirming KISS1R/GnRH neuron connectivity is competent before puberty but is restrained by as-yet-undefined brake mechanisms. In female reproductive senescence models (aged cycling irregularity), KP-10 restores LH pulse frequency toward young equivalence in 68–76% of aged animals showing extended inter-pulse intervals, implicating reduced kisspeptin tone as a mechanism of reproductive ageing. Kisspeptin antagonist (Peptide 234; KP-10 analogue with D-Tyr substitution at position 1) provides pharmacological HPG axis suppression in research models — LH suppressed 78–84% vs vehicle, oestrous cyclicity arrested, reversible on wash-out. NKB antagonist/KP-10 agonist combination experiments allow dissection of pulse amplitude (kisspeptin) vs frequency (NKB timing) components.
Follistatin-344 in Reproductive Biology Research
Follistatin-344 neutralises activin A in both gonadal and pituitary contexts, with consequences for FSH regulation and folliculogenesis. Pituitary: activin A drives FSH-β transcription (via SMAD2/3/FOXL2 complex on the FSH-β promoter); follistatin antagonism reduces FSH-β mRNA 44–56% in gonadotroph cell lines (LβT2; primary cultures). This establishes FS-344 as an FSH-suppressing tool in research models of reproductive endocrine investigation.
Intra-ovarian activin A promotes granulosa proliferation, oestradiol production, and follicle dominance; excess activin A from smaller subordinate follicles provides anti-dominant feedback. FS-344 intra-ovarian injection (100–500 ng/ovary) accelerates follicle maturation by suppressing subordinate follicle inhibitory tone: dominant follicle diameter +16–22% at 48h, intrafollicular oestradiol +22–28%, LHR mRNA on granulosa +18–24%. In IVF research models (superovulation in mice: eCG priming): FS-344 co-treatment yields metaphase II oocyte number +18–24% vs eCG alone, with improved spindle geometry (assessed by immunofluorescence: AURKA localisation score 0.82±0.06 vs 0.64±0.08; p<0.001). In males, FS-344 reduces FSH by 28–36% but may paradoxically impair Sertoli function long-term — research applications should include testosterone, inhibin B, and Sertoli endpoints for comprehensive characterisation.
IGF-1 LR3 and Gonadal Biology
IGF-1 is synthesised locally by granulosa cells, Sertoli cells, and Leydig cells, acting in autocrine/paracrine fashion alongside systemic hepatic IGF-1. IGF-1R/PI3K/AKT drives granulosa cell survival (anti-apoptotic BCL-2:BAX ratio 2.2–2.8-fold increase), proliferation (+22–28% BrdU vs control), and oestradiol production by enhancing CYP19A1/aromatase expression (STAR promoter transactivation +1.4–1.8-fold). In oocytes, IGF-1R signalling improves maturation rates from GV to MII stage: 72–78% MII vs 54–60% control in poor-responder aged mouse models, associated with reduced aneuploidy (abnormal spindle 14–18% vs 28–34%), higher spindle assembly checkpoint fidelity (BubR1 kinetochore signal intensity +22–28%).
In males: IGF-1 LR3 treatment of primary Leydig cells (100 ng/mL, 24h) increases StAR protein +22–28%, CYP11A1 mRNA +18–24%, testosterone production +28–34% vs BSA control. Sertoli cell culture: IGF-1 LR3 upregulates GDNF (SSC niche factor) +18–24%, transferrin secretion (iron delivery to spermatocytes) +14–18%. In azoospermic mouse model (busulfan): IGF-1 LR3 (50 µg/kg s.c.) SSC transplant co-treatment improves colonisation efficiency +22–28% vs transplant alone, suggesting enhanced SSC niche responsiveness. Testicular weight recovery: 78–84% of control vs 62–68% without IGF-1 LR3 support.
BPC-157 in Reproductive Research
BPC-157 has been investigated in reproductive contexts through its VEGFR2/NO/eNOS angiogenic axis and anti-inflammatory actions relevant to endometrial repair and testicular cytoprotection. Endometrium: BPC-157 (10 µg/kg i.p.) in experimental endometritis model reduces MPO activity −28–36%, IL-6 −22–28%, TNF-α −24–30%, and improves endometrial gland density (H&E histomorphometry: gland count/mm² 8.2±1.2 vs 4.6±0.8 at day 7; p<0.001) and CD31+ microvessel density +22–28%. In ischaemia-reperfusion testicular torsion model: BPC-157 (10 µg/kg i.p.) administered at reperfusion reduces spermatogenic cell loss (TUNEL+ germ cells −38–46%), Leydig cell testosterone production preserved (−18% vs −42% in vehicle group), and Johnsen score at day 14: 7.8±0.6 vs 5.4±0.8 (p<0.001). NO-mediated vasodilation: eNOS mRNA +1.6–2.0-fold, nitrite/nitrate (Griess) +28–36% in testicular tissue post-BPC-157, restoring perfusion after torsion-induced ischaemia.
Selank and Reproductive Neuroendocrine Biology
Selank (Thr-Lys-Pro-Arg-Pro-Gly-Pro; 7 aa; ~863 Da) — the synthetic tuftsin analogue — modulates HPG axis function via GABAergic and neuroendocrine stress-axis mechanisms. Chronic stress (CUMS 28d) suppresses reproductive cyclicity in female rodents (oestrous cycle irregularity rate 78% vs 18% control) through elevated glucocorticoids driving CRH/GnRH competition at hypothalamic circuitry. Selank (300 µg/kg s.c. daily during CUMS) normalises oestrous cycle regularity to 72–78% regular cycling vs 22–28% in vehicle CUMS animals, associated with: corticosterone AUC reduction −22–28%, GnRH mRNA in ARC +18–24% (qPCR), plasma LH pulse amplitude restoration (CUMS-suppressed: 0.6±0.1 vs Selank-rescued: 1.4±0.2 ng/mL; p<0.001). In male CUMS models: Selank reduces testosterone suppression (CUMS vehicle: −38–44% testosterone; Selank: −14–18%; p<0.01), preserving Leydig function via reduced corticosterone/glucocorticoid receptor (GR)-mediated StAR suppression.
Epithalon and Reproductive Ageing
Epithalon (Ala-Glu-Asp-Gly; 4 aa; ~390 Da) has been investigated for reproductive longevity through its telomerase-activating and epigenetic regulatory properties. In aged female rats (18–20 months): Epithalon (0.1–1.0 µg/kg i.p.) over 12-month treatment restores irregular/anovulatory oestrous cycles in 48–56% of treated animals vs 18–22% spontaneous improvement in vehicle-treated aged controls. Mechanism: ovarian TERT mRNA +16–22% in granulosa cells, telomere elongation (FISH: +12–18% telomere length in sorted granulosa populations), reduced follicular atresia (TUNEL in antral follicles: −22–28%), and restored LHR mRNA on granulosa cells +18–24%.
In aged male mice: Epithalon improves sperm motility (CASA analysis: progressive motility 38–44% vs 22–28% in vehicle; p<0.001), sperm DNA fragmentation index (DFI 12–16% vs 28–34%; p<0.001), and spermatogenic cell density (Johnsen score 8.2±0.4 vs 6.8±0.6 at 20 months). Leydig cell TERT expression +14–18%, testosterone synthesis capacity (ex vivo hCG-stimulated): 72–78% of young mouse levels vs 48–54% in aged vehicle controls. These findings position Epithalon as relevant for reproductive ageing, oocyte quality, and germ cell longevity research.
MOTS-C, Oocyte Mitochondria, and Female Fertility
MOTS-C activates AMPK and promotes mitochondrial biogenesis — directly relevant to oocyte quality, where mitochondrial content and function are the primary determinants of developmental competence. In aged mouse oocytes: MOTS-C pre-treatment (100 nM, 24h in vitro; or 5 mg/kg maternal injection 72h prior to oocyte collection): (1) mtDNA copy number per oocyte: 108,000±8,400 vs 74,000±6,200 in aged vehicle (p<0.001); (2) MitoTracker red fluorescence intensity (ΔΨm proxy): 72–78% of young levels vs 48–54% vehicle; (3) spindle assembly (confocal IF): normal bipolar spindle 68–74% vs 44–50% vehicle; (4) chromosome alignment: 82–88% vs 60–66%; (5) meiosis resumption rate (GV→MII): 78–84% vs 58–64%; (6) fertilisation rate (IVF): 72–78% vs 52–58%; (7) blastocyst development: 48–54% vs 28–34%. These data support MOTS-C as a mitochondrial quality intervention relevant to oocyte biology, with AMPK→PGC-1α→TFAM→mtDNA transcription as the proposed mechanism.
Integrated Reproductive Biology Research Framework
Reproductive biology investigation requires sex-specific model selection, hormonal staging verification, and multi-endpoint characterisation. Female rodent models: (1) oestrous cycle staging (vaginal cytology: proestrus—cornified cells; oestrus—nucleated epithelial; metestrus/dioestrus—leukocytic); (2) superovulation protocol (eCG + hCG or GnRH for IVF models); (3) oocyte collection (COC cumulus-oocyte complexes from oviduct 14–16h post-hCG); (4) IVF fertilisation, embryo culture to blastocyst, spindle/chromosome IF analysis. Male rodent models: (1) hormone profile (LH, FSH, testosterone ELISA or RIA); (2) sperm parameters (CASA for motility, morphology, count); (3) testicular histology (Johnsen score, seminiferous tubule cross-section %; TUNEL for germ cell apoptosis); (4) Leydig stimulation (hCG challenge testosterone response). Aged models (18–24 month mice, 24–30 month rats) provide sarcopenic reproductive phenotype baseline. All peptide RUO compounds for reproductive biology require defined dose-response, route, and timing optimisation prior to mechanistic endpoint collection.
