Cerebellum Lecture: the Cerebellar Nuclei-Core of the Cerebellum.

The cerebellum is a key player in many brain functions and a major topic of neuroscience research. However, the cerebellar nuclei (CN), the main output structures of the cerebellum, are often overlooked. This neglect is because research on the cerebellum typically focuses on the cortex and tends to treat the CN as relatively simple output nuclei conveying an inverted signal from the cerebellar cortex to the rest of the brain. In this review, by adopting a nucleocentric perspective we aim to rectify this impression. First, we describe CN anatomy and modularity and comprehensively integrate CN architecture with its highly organized but complex afferent and efferent connectivity. This is followed by a novel classification of the specific neuronal classes the CN comprise and speculate on the implications of CN structure and physiology for our understanding of adult cerebellar function. Based on this thorough review of the adult literature we provide a comprehensive overview of CN embryonic development and, by comparing cerebellar structures in various chordate clades, propose an interpretation of CN evolution. Despite their critical importance in cerebellar function, from a clinical perspective intriguingly few, if any, neurological disorders appear to primarily affect the CN. To highlight this curious anomaly, and encourage future nucleocentric interpretations, we build on our review to provide a brief overview of the various syndromes in which the CN are currently implicated. Finally, we summarize the specific perspectives that a nucleocentric view of the cerebellum brings, move major outstanding issues in CN biology to the limelight, and provide a roadmap to the key questions that need to be answered in order to create a comprehensive integrated model of CN structure, function, development, and evolution.

The generation of granule cells during the development and evolution of the cerebellum.

The cerebellum coordinates vestibular input into the hindbrain to control balance and movement, and its anatomical complexity is increasingly viewed as a high-throughput processing center for sensory and cognitive functions. Cerebellum development however is relatively simple, and arises from a specialized structure in the anterior hindbrain called the rhombic lip, which along with the ventricular zone of the rostral-most dorsal hindbrain region, give rise to the distinct cell types that constitute the cerebellum. Granule cells, being the most numerous cell types, arise from the rhombic lip and form a dense and distinct layer of the cerebellar cortex. In this short review, we describe the various strategies used by amniotes and anamniotes to generate and diversify granule cell types during cerebellar development.

Sox14 Is Required for a Specific Subset of Cerebello-Olivary Projections.

We identify Sox14 as an exclusive marker of inhibitory projection neurons in the lateral and interposed, but not the medial, cerebellar nuclei. Sox14+ neurons make up ∼80% of Gad1+ neurons in these nuclei and are indistinguishable by soma size from other inhibitory neurons. All Sox14+ neurons of the lateral and interposed cerebellar nuclei are generated at approximately E10/10.5 and extend long-range, predominantly contralateral projections to the inferior olive. A small Sox14+ population in the adjacent vestibular nucleus "Y" sends an ipsilateral projection to the oculomotor nucleus. Cerebellar Sox14+ and glutamatergic projection neurons assemble in non-overlapping populations at the nuclear transition zone, and their integration into a coherent nucleus depends on Sox14 function. Targeted ablation of Sox14+ cells by conditional viral expression of diphtheria toxin leads to significantly impaired motor learning. Contrary to expectations, associative learning is unaffected by unilateral Sox14+ neuron elimination in the interposed and lateral nuclei.SIGNIFICANCE STATEMENT The cerebellar nuclei are central to cerebellar function, yet how they modulate and process cerebellar inputs and outputs is still primarily unknown. Our study gives a direct insight into how nucleo-olivary projection neurons are generated, their projections, and their function in an intact behaving mouse. These neurons play a critical conceptual role in all models of cerebellar function, and this study represents the first specific analysis of their molecular identity and function and offers a powerful model for future investigation of cerebellar function in motor control and learning.

Development of the cerebellum: simple steps to make a 'little brain'.

The cerebellum is a pre-eminent model for the study of neurogenesis and circuit assembly. Increasing interest in the cerebellum as a participant in higher cognitive processes and as a locus for a range of disorders and diseases make this simple yet elusive structure an important model in a number of fields. In recent years, our understanding of some of the more familiar aspects of cerebellar growth, such as its territorial allocation and the origin of its various cell types, has undergone major recalibration. Furthermore, owing to its stereotyped circuitry across a range of species, insights from a variety of species have contributed to an increasingly rich picture of how this system develops. Here, we review these recent advances and explore three distinct aspects of cerebellar development - allocation of the cerebellar anlage, the significance of transit amplification and the generation of neuronal diversity - each defined by distinct regulatory mechanisms and each with special significance for health and disease.

Transit amplification in the amniote cerebellum evolved via a heterochronic shift in NeuroD1 expression.

The cerebellum has evolved elaborate foliation in the amniote lineage as a consequence of extensive Atoh1-mediated transit amplification in an external germinal layer (EGL) comprising granule cell precursors. To explore the evolutionary origin of this layer, we have examined the molecular geography of cerebellar development throughout the life cycle of Xenopus laevis. At metamorphic stages Xenopus displays a superficial granule cell layer that is not proliferative and expresses both Atoh1 and NeuroD1, a marker of postmitotic cerebellar granule cells. Premature misexpression of NeuroD1 in chick partially recapitulates the amphibian condition by suppressing transit amplification. However, unlike in the amphibian, granule cells fail to enter the EGL. Furthermore, misexpression of NeuroD1 once the EGL is established both triggers radial migration and downregulates Atoh1. These results show that the evolution of transit amplification in the EGL required adaptation of NeuroD1, both in the timing of its expression and in its regulatory function, with respect to Atoh1.

Independently specified Atoh1 domains define novel developmental compartments in rhombomere 1.

The rhombic lip gives rise to neuronal populations that contribute to cerebellar, proprioceptive and interoceptive networks. Cell production depends on the expression of the basic helix-loop-helix (bHLH) transcription factor Atoh1. In rhombomere 1, Atoh1-positive cells give rise to both cerebellar neurons and extra-cerebellar nuclei in ventral hindbrain. The origin of this cellular diversity has previously been attributed to temporal signals rather than spatial patterning. Here, we show that in both chick and mouse the cerebellar Atoh1 precursor pool is partitioned into initially cryptic spatial domains that reflect the activity of two different organisers: an isthmic Atoh1 domain, which gives rise to isthmic nuclei, and the rhombic lip, which generates deep cerebellar nuclei and granule cells. We use a combination of in vitro explant culture, genetic fate mapping and gene overexpression and knockdown to explore the role of isthmic signalling in patterning these domains. We show that an FGF-dependent isthmic Atoh1 domain is the origin of distinct populations of Lhx9-positive neurons in the extra-cerebellar isthmic nuclei. In the cerebellum, ectopic FGF induces proliferation while blockade reduces the length of the cerebellar rhombic lip. FGF signalling is not required for the specification of cerebellar cell types from the rhombic lip and its upregulation inhibits their production. This suggests that although the isthmus regulates the size of the cerebellar anlage, the downregulation of isthmic FGF signals is required for induction of rhombic lip-derived cerebellar neurons.

Mosaic Expression of Thyroid Hormone Regulatory Genes Defines Cell Type-Specific Dependency in the Developing Chicken Cerebellum.

The cerebellum is a morphologically unique brain structure that requires thyroid hormones (THs) for the correct coordination of key cellular events driving its development. Unravelling the interplay between the multiple factors that can regulate intracellular TH levels is a key step to understanding their role in the regulation of these cellular processes. We therefore investigated the regional/cell-specific expression pattern of TH transporters and deiodinases in the cerebellum using the chicken embryo as a model. In situ hybridisation revealed expression of the TH transporters monocarboxylate transporter 8 (MCT8) and 10 (MCT10), L-type amino acid transporter 1 (LAT1) and organic anion transporting polypeptide 1C1 (OATP1C1) as well as the inactivating type 3 deiodinase (D3) in the fourth ventricle choroid plexus, suggesting a possible contribution of the resulting proteins to TH exchange and subsequent inactivation of excess hormone at the blood-cerebrospinal fluid barrier. Exclusive expression of LAT1 and the activating type 2 deiodinase (D2) mRNA was found at the level of the blood-brain barrier, suggesting a concerted function for LAT1 and D2 in the direct access of active T3 to the developing cerebellum via the capillary endothelial cells. The presence of MCT8 mRNA in Purkinje cells and cerebellar nuclei during the first 2 weeks of embryonic development points to a potential role of this transporter in the uptake of T3 in central neurons. At later stages, together with MCT10, detection of MCT8 signal in close association with the Purkinje cell dendritic tree suggests a role of both transporters in TH signalling during Purkinje cell synaptogenesis. MCT10 was also expressed in late-born cells in the rhombic lip lineage with a clear hybridisation signal in the outer external granular layer, indicating a potential role for MCT10 in the proliferation of granule cell precursors. By contrast, expression of D3 in the first-born rhombic lip-derived population may serve as a buffering mechanism against high T3 levels during early embryonic development, a hypothesis supported by the pattern of expression of a fluorescent TH reporter in this lineage. Overall, this study builds a picture of the TH dependency in multiple cerebellar cell types starting from early embryonic development.

Consensus Paper: Cerebellar Development.

The development of the mammalian cerebellum is orchestrated by both cell-autonomous programs and inductive environmental influences. Here, we describe the main processes of cerebellar ontogenesis, highlighting the neurogenic strategies used by developing progenitors, the genetic programs involved in cell fate specification, the progressive changes of structural organization, and some of the better-known abnormalities associated with developmental disorders of the cerebellum.

The roof plate boundary is a bi-directional organiser of dorsal neural tube and choroid plexus development.

The roof plate is a signalling centre positioned at the dorsal midline of the central nervous system and generates dorsalising morphogenic signals along the length of the neuraxis. Within cranial ventricles, the roof plate gives rise to choroid plexus, which regulates the internal environment of the developing and adult brain and spinal cord via the secretion of cerebrospinal fluid. Using the fourth ventricle as our model, we show that the organiser properties of the roof plate are determined by its boundaries with the adjacent neuroepithelium. Through a combination of in ovo transplantation, co-culture and electroporation techniques in chick embryos between embryonic days 3 and 6, we demonstrate that organiser properties are maintained by interactions between the non-neural roof plate and the neural rhombic lip. At the molecular level, this interaction is mediated by Delta-Notch signalling and upregulation of the chick homologue of Hes1: chairy2. Gain- and loss-of-function approaches reveal that cdelta1 is both necessary and sufficient for organiser function. Our results also demonstrate that while chairy2 is specifically required for the maintenance of the organiser, its ectopic expression is not sufficient to recapitulate organiser properties. Expression of atonal1 in the rhombic lip adjacent at the roof plate boundary is acutely dependent on both boundary cell interactions and Delta-Notch signalling. Correspondingly, the roof plate boundary organiser also signals to the roof plate itself to specify the expression of early choroid plexus markers. Thus, the roof plate boundary organiser signals bi-directionally to acutely coordinate the development of adjacent neural and non-neural tissues.

The evolution of the vertebrate cerebellum: absence of a proliferative external granule layer in a non-teleost ray-finned fish.

The cerebellum represents one of the most morphologically variable structures in the vertebrate brain. To shed light on its evolutionary history, we have examined the molecular anatomy and proliferation of the developing cerebellum of the North American paddlefish, Polyodon spathula. Absence of an external proliferative cerebellar layer and the restriction of Atonal1 expression to the rhombic lip and valvular primordium demonstrate that transit amplification in a cerebellar external germinal layer, a prominent feature of amniote cerebellum development, is absent in paddlefish. Furthermore, expression of Sonic hedgehog, which drives secondary proliferation in the mouse cerebellum, is absent from the paddlefish cerebellum. These data are consistent with what has been observed in zebrafish and suggest that the transit amplification seen in the amniote cerebellum was either lost very early in the ray-finned fish lineage or evolved in the lobe-finned fish lineage. We also suggest that the Atoh1-positive proliferative valvular primordium may represent a synapomorphy (shared derived character) of ray-finned fishes. The topology of valvular primordium development in paddlefish differs significantly from that of zebrafish and correlates with the adult cerebellar form. The distribution of proliferative granule cell precursors in different vertebrate taxa is thus the likely determining factor in cerebellar morphological diversity.

A thematic analysis of students' discussions on death and body donation in international online focus groups.

Historically, Anatomy education is an in-person discipline involving exposure to human body donors that facilitates personal and professional growth through, in part, the initiation of reflection on the topic of death. However, during the COVID-19 pandemic the decreased exposure to cadaveric anatomy for many health professions students may have influenced the depth of their individual reflections on this topic. Accordingly, this study aimed to investigate the effect of an alternate approach-focus group discussions between peers with varying degrees of exposure to cadaveric material-that may offer one strategy to stimulate deep reflection on the topic of death. A programmatic intervention was introduced, wherein students (n = 221) from 13 international universities discussed differences in their anatomy courses during small focus group sessions as part of an online exchange program. An inductive semantic thematic analysis was conducted on responses to an open-ended text-response question on how the activity influenced students' reflections about death. Resulting themes were organized into categories that described the content and topics of the students' discussions as they grappled with this sensitive topic. The students reportedly engaged in deep reflection and expressed an increased sense of connectedness with their peers, despite their disparate exposure levels to cadaveric anatomy and being physically distanced. This demonstrates that focus groups with students experiencing different laboratory contexts can be used to help all students reflect on the topic of death and that interchanges between dissecting and non-dissecting students can initiate thoughts about death and body donation among non-dissecting students.

Health profession students' outlooks on the medical profession during the COVID-19 pandemic: a global perspective.

BACKGROUND: This article summarizes a global study of the effect of the COVID-19 pandemic on junior health professions students' outlook on medicine. The pandemic has significantly affected health professions education. There is limited understanding of how students' pandemic experiences will affect them, and what impact these events may have on their career paths or the future of the professions. This information is important as it impacts the future of medicine. METHODS: In the Fall 2020 semester, 219 health professions students at 14 medical universities worldwide responded to the question: 'Has this experience (with COVID-19) changed your outlook on medicine as a profession?'. Short essay responses were semantically coded and organized into themes and subthemes using an inductive approach to thematic analysis. RESULTS: 145 responses were submitted. Themes were identified: (1) students reflected on the interaction between politics and healthcare; (2) reported becoming more aware of the societal expectations placed on healthcare professionals, including undertaking high risks and the sacrifices that healthcare professionals must make; (3) found reassurance from the recognized importance of healthcare professionals and expressed pride to be entering the profession; and (4) reflected on the current state of healthcare, including its limitations and future. CONCLUSION: Most students, independent of the extent of the pandemic in their respective countries, noted a change in their outlook regarding medicine. An overall positive outlook was noted in most junior students. Educators need to work on nurturing these sentiments and attitudes to help young students maintain a healthy relationship towards their chosen profession.

The Anatomy Course During COVID-19: The Impact of Cadaver-Based Learning on the Initiation of Reflection on Death.

Background: During the COVID-19 pandemic, in-person cadaveric dissection laboratories for teaching anatomy were omitted by many schools around the world. While knowledge domains can be easily evaluated via remote exams, non-traditional discipline-independent skills such as those encouraged through reflection on the topic of death are often overlooked. This study investigated how different anatomy course formats played a role in initiating students' reflections on death during the COVID-19 pandemic. Method: In fall 2020, 217 medical, dental, premedical, and health sciences students from 13 international universities discussed differences in their anatomy courses online. Formats of anatomy courses ranged from dissection-based, prosection-based, hybrid (combination of dissection and prosection) to no laboratory exposure at all. Students' responses to the question, "Did/does your anatomy course initiate your thinking about life's passing?" were collected, and they self-reported themes that were present in their reflections on death using a multiple-choice prompt. Statistical analyses to detect differences between students with and without exposure to cadavers were performed using the chi-squared test. Results: When comparing students who had exposure to human anatomical specimens to those who had no exposure, the majority of students with exposure thought that the course did initiate thoughts about life's passing, compared to students without exposure (P < 0.05). Reflection themes were consistent across groups. Discussion: These findings indicate that anatomy dissection courses are important for the initiation of students' feelings about the topic of death. Omission of cadaveric dissection- or prosection-based laboratories will decrease the likelihood that students initiate reflection on this topic and gain important transferable skills.

Absence of an external germinal layer in zebrafish and shark reveals a distinct, anamniote ground plan of cerebellum development.

The granule cell layer of the cerebellum comprises the largest population of neurons in the vertebrate CNS. In amniotes, its precursors undergo a unique phase of transit amplification, regulated by Sonic hedgehog. They do so within a prominent but transient secondary proliferative epithelium, the external germinal layer, which is formed by tangential migration of precursor cells from the rhombic lip. This behavior is a hallmark of bird and mammal cerebellum development. Despite its significance for both development and disease, it is unclear whether an external germinal layer is a requirement for granule cell production or an expedient of transit amplification. Evidence for its existence in more basal vertebrates is contradictory. We therefore examined cerebellum development in the zebrafish, specifically in relation to the expression of the basic helix-loop-helix gene Atonal 1, which definitively characterizes granule cell precursors. The expression of Atoh1a-Atoh1c, in combination with patterns of proliferation and fate maps, define precursor pools at the rhombic lip and cerebellar midline but demonstrate that an external germinal layer is absent. Sonic hedgehog signaling is correspondingly absent in the zebrafish cerebellum. Sustained roof-plate-derived signals suggest that, in the absence of transit amplification, primary granule cell precursor pools are maintained throughout development. To determine whether this pattern is specific to zebrafish or reflects a more general anamniote organization, we examined the expression of similar genes in the dogfish, Scylliorhinus canicula. We show that these anamniotes share a common ground plan of granule cell production that does not include an external germinal layer.

Alternatives to Student Outbound Mobility-Improving Students' Cultural Competency Skills Online to Improve Global Health Without Travel.

Introduction: Student outbound mobility is a major element in internationalization of medical education and global health education. However, this approach is often criticized, as it is inherently inequitable. Internationalization at home is a newer concept that aims to provide students with international skills and experiences without exchange travel. We report detailed outcomes of an international online program during the COVID-19 pandemic, which aimed to include acquisition of cultural awareness and competency-similar to what the students would have obtained if they had travelled abroad. Method: Sixty-eight students from 12 international universities participated in international small peer group collaborative work, and online networking. Perceived improvement of cultural competency using Likert scale and open-ended questions was used as a measure of success. Furthermore, students' definition of cultural competency in the different countries was obtained. Results: Students improved their cultural competency skills. Data analysis supported statistically significant improvement of the above skills after the program, in comparison to the start of the program. Discussion: Internationalization of medical education can be achieved at home-via structured online peer exchanges-and can provide students with intercultural skills and networking opportunities that are typically achieved via international in-person travel. The above represents a socially just and equitable way to reach all students and can result in improvement of their cultural competency, preparing them for their work in global health, and thereby resulting in improvement of global health. Supplementary Information: The online version contains supplementary material available at 10.1007/s40670-021-01332-9.

Initiating Students' Reflections on Life's Passing in the Anatomy Course - an International Observation at 14 Universities.

BACKGROUND: Medical and dental students' feelings and thoughts about the topic of death and life's passing are often associated with learning in the gross anatomy course, when students begin working with a deceased body donor in order to study human anatomy. Little is known of whether the format of anatomy teaching has an impact on these experiences. An observational study was performed to capture the initiation of students' sentiments on the topic of life's passing during the anatomy course at 14 international universities, identify common themes regarding these thoughts, and to study the connection to variations in anatomy course formats and included elements. METHOD: Preclinical anatomy students reflected on one question (i.e., "How did your experience in the anatomy laboratory bring about your reflections on the meaning of life and human existence as well as the sanctity of one's passing?"). Written assignments were collected and anonymously coded. Information on anatomy courses was obtained via faculty questionnaires. RESULT: A variety of themes were identified at the different schools, correlated with different anatomy formats and elements. Results indicate that the courses that offer hands-on cadaveric dissections may play an important role in triggering these sentiments. DISCUSSION: The initiation of students' sentiments about the topic of death varies and includes several themes. There can be a connection to the way anatomy is taught, particularly if hands-on comprehensive cadaveric dissection or prosections are included. CONCLUSION: In summary, anatomy courses can initiate students' thinking about life's passing - particularly in schools that offer hands-on cadaveric dissections or prosections.

An Observation of Healthcare Professions Students' Perceptions During the COVID-19 Pandemic.

This study conveys preclinical healthcare professions students' sentiments at 14 universities during the 2020 COVID-19 pandemic. Essays about students' thoughts and experiences were thematically sorted and revealed a variety of sentiments spanning from positive (e.g., pride, respect) to the more negative (e.g., anxiety, guilt, disappointment, anger). Themes revealed respect for the healthcare profession, but also the realization of its limitations, sacrifices, and risks. Healthcare profession educators need to be aware that the COVID-19 pandemic has affected students emotionally and may have long-term effects on the global healthcare profession. This study can serve as a historic documentation of how this generation of students felt and adds to the literature on how the pandemic affected the healthcare profession.

An International Partnership of 12 Anatomy Departments - Improving Global Health through Internationalization of Medical Education.

Background: At a time of global interconnectedness, the internationalization of medical education has become important. Anatomy as an academic discipline, with its close connections to the basic sciences and to medical education, can easily be connected with global health and internationalization of medical education. Here the authors present an international program based on a partnership between twelve anatomy departments in ten countries, on four continents. Details of a proposed plan for the future direction of the program are also discussed. Objective: The aim is to improve global healthcare by preparing future global healthcare leaders via early international networking, international collaboration and exchange, intercultural experience, and connecting two seemingly distant academic disciplines - anatomy and global health - via internationalization of medical education. Methods: Based in the anatomy course, the program involved early international collaboration between preclinical medical and dental students. The program provided a stepwise progression for learning about healthcare and intercultural topics beyond pure anatomy education - starting with virtual small groups of international students, who subsequently presented their work to a larger international audience during group videoconferences. The above progressed to in-person visits for research internships in the basic sciences within industrialized countries. Findings: Students appreciated the international and intercultural interaction, learned about areas outside the scope of anatomy (e.g., differences in healthcare education and delivery systems, Public and Global Health challenges, health ethics, and cultural enrichment), and valued the exchange travel for basic sciences research internships and cultural experience. Conclusions: This unique collaboration of international anatomy departments can represent a new role for the medical anatomy course beyond pure anatomy teaching - involving areas of global health and internationalization of medical education - and could mark a new era of international collaboration among anatomists.

Math-Map(ic)s.

In this issue of Neuron, Wang et al. and Machold and Fishell present contrasting molecular fate maps of Math1, which redefine the derivatives of the embryonic rhombic lip and offer a conceptual overhaul of cerebellar and precerebellar development. These fate maps identify a common developmental thread linking diverse, functionally associated neurons and reveal an exquisite temporal organization in cell production within a precise spatially defined region of neuroepithelium.

Mitotic granule cell precursors undergo highly dynamic morphological transitions throughout the external germinal layer of the chick cerebellum.

The developing cerebellum of amniotes is characterised by a unique, transient, secondary proliferation zone: the external germinal layer (EGL). The EGL is comprised solely of granule cell precursors, whose progeny migrate inwardly to form the internal granule cell layer. While a range of cell morphologies in the EGL has long been known, how they reflect the cells' differentiation status has previously only been inferred. Observations have suggested a deterministic maturation from outer to inner EGL that we wished to test experimentally. To do this, we electroporated granule cell precursors in chick with plasmids encoding fluorescent proteins and probed labelled cells with markers of both proliferation (phosphohistone H3) and differentiation (Axonin1/TAG1 and NeuroD1). We show that granule cell precursors can display a range of complex forms throughout the EGL while mitotically active. Overexpression of full length NeuroD1 within granule cell precursors does not abolish proliferation, but biases granule cells towards precocious differentiation, alters their migration path and results in a smaller and less foliated cerebellum. Our results show that granule cells show a greater flexibility in differentiation than previously assumed. We speculate that this allows the EGL to regulate its proliferative activity in response to overall patterns of cerebellar growth.