The ovary is an immunologically active organ in which tightly regulated interactions between immune cells, cytokines, and reproductive hormones are essential for follicle and oocyte development, ovulation, luteal function, and maintenance of the ovarian reserve. Immune checkpoints such as PD-1, PD-L1, and CTLA-4 are increasingly recognized as key regulators of immune tolerance beyond classical lymphoid tissues, yet their roles within the ovary remain poorly defined. Immune checkpoint inhibitors (ICIs), which target these pathways to enhance anti-tumor immunity, have transformed cancer treatment and are increasingly administered to women of reproductive age. While traditional cancer therapies are well known to compromise ovarian function and fertility, the reproductive consequences of ICIs are only beginning to emerge. Here we synthesize current knowledge of ovarian immune biology, with a focus on T cells and immune checkpoint signaling, and integrate clinical, preclinical, and mechanistic evidence linking immune checkpoint blockade to ovarian dysfunction. We discuss immune-related adverse events (irAEs), their management, and emerging evidence that the ovary may be uniquely sensitive to immune checkpoint perturbation. Collectively, this review highlights emerging knowledge of T cells and immune checkpoints in the ovary, highlights critical gaps in our understanding of immune - ovarian crosstalk, and discusses the urgent need to define the reproductive risks of ICIs to inform fertility preservation strategies and clinical decision-making for women with cancer.
Since the first description of "stem cells" in spermatogenesis in 1885, our understanding of spermatogonial stem cells (SSCs) has evolved over time. Normal function of SSCs is critical for the survival of a species, and thus, a balanced control of SSC fate (self-renewal or commitment) is required to transmit genetic information adequately to future generations. A microenvironment that surrounds SSCs, the SSC niche, is widely considered to generate a dominant force for this balancing mechanism and "dictate" SSC fate. The components of the SSC niche are thought to be somatic cells, particularly Sertoli cells, and acellular substrates, such as the extracellular matrix. The aim of this chapter is to provide different perspectives into our current understanding of the SSC-niche relationship. Notably, as SSCs cannot be visualized unequivocally, our understanding of the SSC-niche communication has been underdeveloped. As a result, discussions presented here involve assumptions and speculations. Under this premise, I will discuss three hypotheses. First, SSCs and germ cells are the driving force of spermatogenesis and the fundamental constituents of the SSC niche. Second, the somatic niche provides a permissive environment that supports, rather than dictates, the functions of SSCs and their descendant germ cells. Third, SSCs and their niches communicate reciprocally to fine-tune the fate of SSCs while orchestrating spermatogenesis. The overarching concept is that SSCs induce their niches by forming a community of their descendant germ cells while somatic cells passively respond to the community. I will further attempt to apply this germ cell-dominant concept of SSC niche to human spermatogenesis, since the uniqueness of human spermatogenesis casts significant questions on the soma-dominant concept of SSC niche that has been generated using the mouse model.
The testis is a unique organ that simultaneously supports spermatogenesis and steroidogenesis, and it must maintain host defense against infection while suppressing excessive immune responses in an environment that includes germ cell antigens that emerge after sexual maturation. Testicular macrophages reside in the testicular interstitium starting in fetal life and localize near Leydig cells and around seminiferous tubules. Through cytokine and lipid mediator production, phagocytosis of dying cells, and antigen presentation, testicular macrophages can influence both steroidogenesis and spermatogenesis. In this chapter, we summarize the key components underlying testicular immune privilege, including the blood-testis barrier (BTB), immunosuppressive factors, and immune cell constituents in the testis, and we provide an overview of the development and maintenance of testicular macrophages and their interactions with major testicular cell types such as Leydig cells. We also outline changes that occur during inflammation and summarize links between testicular macrophages and disease states, including infertility and aging. Finally, we discuss future research directions and potential intervention strategies focused on testicular macrophages.
Primordial follicle activation represents the pivotal and rate-limiting transition that governs the size and longevity of the ovarian reserve, thereby shaping female reproductive lifespan. Although the vast majority of primordial follicles remain quiescent, a small fraction is periodically activated through tightly regulated signaling events. Emerging evidence reveals that dormancy is not a passive state, but an actively maintained program orchestrated by intrinsic oocyte pathways, surrounding pre-granulosa cells, and the ovarian microenvironment. Key intracellular regulator PI3K-Akt-FOXO3 signaling establishes the oocyte's threshold for activation, and pre-granulosa cell cues, including KITL-KIT interactions, mechanical constraints, and extracellular matrix dynamics, modulate this core signaling. Additionally, paracrine signals from stromal cells, local hypoxia, and biomechanical forces contribute to the spatial patterning of activation within the ovary. Integration of these inputs ensures both the lifelong preservation of a dormant pool and the continuous supply of follicles entering the growth phase. Recent advances in single-cell technologies, imaging, and lineage tracing systems have begun to redefine the temporal and spatial complexity of primordial follicle activation. Dysregulation of this system underlies a spectrum of reproductive disorders, from premature ovarian insufficiency to diminished ovarian reserve and aberrant follicle recruitment. Understanding the molecular logic of activation not only illuminates the biology of ovarian aging but also informs emerging therapeutic strategies, including targeted modulation of signaling pathways, ovarian tissue engineering, and fertility preservation interventions. This chapter describes current knowledge of the mechanisms governing primordial follicle activation and highlights key gaps that will shape future research in ovarian biology.
To produce haploid gametes, mitotic germ cells in both sexes must transition to the specialized meiotic program. The molecular details underlying this transition - termed meiotic initiation - are poorly defined in higher eukaryotes, especially mammals. In mammals, the current mechanistic dogma is the signal, or 'meiosis inducing substance' (MIS), triggering germ cell meiotic initiation is provided by retinoic acid (RA). However, over the past twenty years a considerable number of reports using both genetic and pharmacologic approaches have been published that alternatively support or challenge RA's role as the mammalian MIS. Here, we attempt to fairly describe this conflicting evidence in the hopes that it will stimulate future conversations and design of studies to elucidate the molecular and cellular mechanisms driving meiotic initiation in the mammalian germline.
The follicle is the functional unit of the ovary and has been the historical focus of ovarian biology research. However, recently a greater emphasis has been placed on the role of the ovarian microenvironment, or stroma, in supporting the follicle and germ cell within. The ovarian stroma is composed of the ovarian surface epithelium, blood and lymphatic vasculature, nerves, immune cells, fibroblasts, mesenchymal cells, and the extracellular matrix (ECM). The stroma provides structural, biomechanical, biochemical, and metabolic support for follicle activation and development, in turn impacting gamete quality. The ovary is one of the first organs to age characterized by a loss of gamete quantity and quality. In addition, the stroma exhibits significant age-related changes - becoming fibrotic, inflammatory, and stiff. This review summarizes the physiological functions of ovarian stromal cell populations and components in supporting ovarian function, alterations in the ovarian stroma that occur with advanced age, and the resulting reproductive consequences.
Soma-oocyte interactions are a key component in female fertility. First the ovary must adopt a female fate and become populated with supporting somatic cells. We discuss the origins of each somatic cell type with a particular focus on pre-granulosa cell origins and differentiation. Upon germ cell arrival into the ovary, the oogonia develop into cysts and begin their interactions with neighboring pre-granulosa cells. Cell to cell contacts change over time as the germline cysts form and then break down to facilitate primordial follicle formation. These changes do not happen in isolation, rather, they are mediated by signaling cascades and changes in the physical structures. The processes and developmental changes explored in this chapter are crucial for the longevity and success of the primordial follicle and thus, an essential part of female fertility.
The cell cycle of the mammalian oocyte arrests in prophase around the time of birth and remains in meiotic arrest as it grows interdependently with increasing layers of surrounding somatic cells as a structure called an ovarian follicle. Within preovulatory follicles, gap junction communication between somatic cells and the oocyte allow cyclic nucleotides to maintain equilibrium concentrations that prevent premature meiotic resumption. Intercellular communication is also required for the mid-cycle surge in luteinizing hormone to alter cyclic nucleotide dynamics and cause the oocyte cell cycle to resume. In this review, the increasingly well-understood mechanisms by which meiotic arrest and meiotic resumption occur will be summarized, along with recent developments made possible by an improved cyclic nucleotide sensor and imaging techniques. These studies have uncovered new aspects of this process and helped clarify the required role of epidermal growth factor receptor signaling in meiotic resumption. Growing evidence that the cyclic nucleotide-associated participants in oocyte cell cycle regulation, first described in rodents, appear to be universal across mammals will also be summarized. This understanding has fostered new approaches to assisted reproductive technologies (ARTs) in domesticated animals and humans.
Communication between cells is essential for orchestrating tissue, organ, and whole-body cohesion. The communication can occur within the same cell (autocrine signaling), between neighboring cells (paracrine signaling), from distant cells (endocrine signaling), and between adjacent cells (juxtacrine signaling). Extracellular vesicle secretions, where messaging molecules like proteins, RNAs, and lipids are embedded in a membrane-enclosed particle released by the signaling cell, allow signaling to occur upon uptake and release of the cargo. Intercellular communications in synchrony between folliculogenesis and oogenesis involve both long-distance and short-distance messaging within the ovarian follicle, which is segmented into different cell types, with the oocyte lying at the center. Cells can generate extensions to increase the range of messaging by reaching out to more distant cells the so-called protrusion-based intercellular communication exemplified by tunneling nanotubes (TNTs), MT-nanotubes (MTNTs), microvilli, filopodia, and cytonemes. In mammalian oocytes, the filopodia from follicular cells that grow towards the oocyte is known as transzonal projections (TZPs). They accumulate large cargos, such as EVs, proteins and RNAs, at the tip creating the gametic synapses. These projections keep the oocyte and follicular cells connected and exchanging cytoplasmic content despite the apparent physical separation caused by the oocyte's zona pellucida. The protective and nurturing nature of follicular cells towards the oocyte, due to the presence of cellular interconnectivity within follicular cells and the oocyte, is conserved between species and is essential for communication, playing a crucial role in the development of a high-quality oocyte.
Understanding human cerebellar development and pathology remains challenging due to its complexity, unique protracted developmental timeline, and notable species-specific differences. Advances in human stem cell technologies have opened new windows of opportunity for exploring these processes through improved in vitro models, providing insights into human-specific mechanisms that traditional animal models may fail to recapitulate. Here, we review recent advancements in modelling physiological and pathological processes of human cerebellar development. We first outline key features of cerebellar development with an emphasis on human-specific aspects. We then systematically summarize and evaluate various strategies for generating human stem cell-derived cerebellar models in vitro, starting with foundational studies using mouse embryonic stem cells, followed by methods using human pluripotent stem cells to generate cerebellar cells in two-dimensional cultures and three-dimensional cerebellar organoids. Subsequently, we review the application of these models to study cerebellar diseases, highlighting translational opportunities and critical challenges that remain.
The cerebellum is a critical structure for both motor and cognitive behaviors. Although the majority of human neuroimaging research has examined the cerebellum in adults, this "little brain" may be especially pivotal during development. Indeed, the cerebellum has a unique developmental timecourse relative to the cerebral cortex, suggesting that it may play a unique role in the development of motor and cognitive functions. Here, we review the literature on cerebellar development with a focus on neuroimaging studies in humans. We discuss several structural and functional MRI studies that examine the cerebellum across typical development from infancy through adulthood. We also discuss the links between the developmental time course of various cerebellar sub-regions and the emergence of core motor and cognitive functions. Finally, we investigate the consequences of cerebellar differences (and disorders that implicate the cerebellum) on motor control, language and communication, executive function, attention, and affective regulation.
During mammalian evolution, the cerebellum has diversified in size and morphology and broadened its roles from motor control to higher-order cognitive and emotional processing. To understand how cerebellar form and function have evolved, it is critical to investigate the underlying molecular and developmental mechanisms. Recent advances in single-cell genomics now allow high-resolution tracking of cellular dynamics, gene expression, and regulatory programs in complex tissues. This presents an opportunity to integrate genome-wide data to the long tradition of research on development and evolution of the cerebellum, started by the foundational comparative histological work of Ramón y Cajal on brain cell types and Olof Larsell on cerebellar anatomy. Focusing on single-cell genomics approaches, this chapter explores emerging insights into how cerebellar cell types and their developmental programs have evolved across mammals. While the emphasis is mammalian, key insights from other vertebrates are included to provide evolutionary context.
The cerebellum is a hindbrain structure that houses the majority of neurons in the mammalian brain and is involved in motor coordination, balance and higher cognitive processes. Compared to the rest of the brain, cerebellar development is protracted, with neurogenesis continuing postnatally. While the local circuitry of the cerebellum is relatively simple, having the correct proportions of its various cell types is essential for cerebellar function. Remarkably, the neonatal mouse cerebellum is highly regenerative upon injury at birth, utilizing context- and age-dependent regenerative strategies to ensure robust and efficient repair. Here, we provide an overview of how different lineages within the cerebellum interact during development and discuss the mechanisms of scaling, where the earlier-born neurons regulate the survival and proportional expansion of other cell types. We give examples of disorders where the scaling could be disrupted, perinatal injuries that affect the cerebellum and potential regenerative mechanisms in the brain. Furthermore, we discuss how the neonatal cerebellum regenerates the correct cell types in the appropriate proportions. Finally, we summarize the cell-intrinsic and -extrinsic mechanisms that regulate progenitor behaviors during postnatal cerebellar development and regeneration, specifically focusing on the nestin-expressing progenitors of the postnatal cerebellum.
With its limited diversity of neuronal types and stereotyped cellular organisation, the cerebellum is an excellent model for complex brain development. It exemplifies how simple patterning rules can give rise to complex neural circuits. The entirety of populations of excitatory and inhibitory neurons is characterised by the transient expression of either Atonal1 (Atoh1) or Ptf1a, respectively, and derived from a spatially defined population of Sox2-positive precursors. We present a model where the decision to make Atoh1 over Ptf1a lineage neurons is dictated by inductive cell-cell interactions at the posterior boundary with non-neural roof plate cells at the rhombic lip. The type of Atoh1+ve or Ptf1a+ve cell generated is dictated by a shared temporal code invested in the Sox2-expressing progenitor pool in the ventricular zone of dorsal rhombomere 1. An additional long-lived pool of Sox2 progenitors in the prospective white matter gives rise to glial cells (astrocytes) and later born interneurons, the latter of which also transiently express Ptf1a. Temporal patterning of progenitors generates neuronal diversity and offers a potent substrate for adaptation. In particular, fine-grained temporal patterning of progenitors feeding early rhombic lip derivatives dictates the connections of the cerebellum through specifying cerebellar nucleus output neurons which influence the scaling of the cortex of the cerebellum. In the human cerebellum, scaling involves species-specific adaptations that co-evolved within the human cerebral cortex.
Late-onset neurodegenerative diseases have long been conceptualized as disorders arising from cumulative cellular stress and age-related decline, with pathology emerging at the time of symptom onset. However, emerging evidence challenges this view, suggesting that developmental perturbations may establish early vulnerabilities that predispose specific neuronal populations to degeneration later in life. In the cerebellum, mutations causing spinocerebellar ataxias (SCAs) such as SCA1 and SCA6 affect genes involved in normal circuit formation, resulting in subtle early abnormalities in Purkinje cell activity and connectivity. These alterations seem to be initially buffered by compensatory mechanisms, and the eventual breakdown of homeostatic resilience during midlife may thus be the trigger for disease onset and progression. This developmental perspective reframes late-onset neurodegeneration as a lifelong process shaped by the interplay between early developmental wiring, adaptive compensation, and age-dependent vulnerability. Understanding these early developmental alterations provides critical insight into disease mechanisms and opens new avenues for pre-symptomatic intervention and prevention.
Mossy fiber and climbing fiber inputs coming into the cerebellum through the precerebellar system feed into a stereotyped circuit module at the heart of cerebellar computations. Building this circuitry requires exquisite coordination, from generating the diverse populations, to guiding axons to the appropriate targets, and finally establishing the correct synapses. In this chapter, we take a closer look at the development of the spinocerebellar system and then bring in aspects of the hindbrain precerebellar system to ask how these pieces fit together during development. We also emphasize how little we know about spinocerebellar development and suggest ways the field can proceed to fill these gaps. Overall, we hope this chapter not only provides a useful reference but also sparks new ideas for future studies of the spinocerebellar system.
This chapter reviews the role of Astrotactin 2 (ASTN2) in cerebellar development and its implications for neurodevelopmental disorders, particularly autism spectrum disorder (ASD). ASTN2 is identified as a critical gene influencing the function of the cerebellum, a brain region traditionally associated with motor control but now recognized for its roles in social cognition and emotional processing. ASTN2 mutations, including deletions and copy number variations, have been linked to increased risks of ASD and other psychiatric conditions. Studies highlight ASTN2's involvement in neuronal migration, synaptic modulation, protein trafficking, and behavior, as evidenced by mouse models displaying ASD-like behaviors when ASTN2 expression is perturbed. Additionally, ASTN2 affects the structure and function of the sole output neuron of the cerebellum, the Purkinje cell, including changes in spine density and synaptic transmission. To bridge the knowledge gap regarding the role of ASTN2 in human neurodevelopmental disorders, human induced pluripotent stem cells are being employed to further investigate ASTN2's function in human neuronal development and physiology. These studies position ASTN2 as a potential target for future therapeutic interventions in ASD and other neurodevelopmental disorders.
