Abstract Homeostasis of the vertebrate retina is supported by two non-neuronal cell types, Müller glia and the retinal pigment epithelium (RPE). Their apical domains delineate the subretinal space, where photoreceptor outer segments reside. While the individual interactions between the supporting cells and photoreceptors are well studied, Müller glia and RPE are presumed to remain physically separate in a healthy retina. Here, we describe a revised model of the subretinal space architecture based on detailed FIB-SEM and confocal microscopy imaging of the zebrafish and mouse outer retina. Long apical processes extend from both Müller glia and the RPE, interacting with one another while also encircling cone outer segments. These contact regions, at the interface of the three cell types are highly elaborated in fish and, although sparser, are still present in mouse. The growth and overlap of Müller glial and RPE apical processes occurs subsequent to the emergence of the outer segment in both species, and coincides with the onset of visual function in mouse. Although enriched at cone sites, this specialized interaction does not appear to depend solely on cone localization in either species, pointing to a general hallmark of the vertebrate subretinal space. Together, our work provides major insights into the architecture of the subretinal space and the interactions between Müller glia, RPE and photoreceptors, while demonstrating conservation across species.

Phosphoinositides (PIs) are a family of seven low abundance membrane lipids, each with distinct signaling functions. The phosphoinositide kinase PIKfyve generates phosphoinositide-3,5-bisphosphate (PI(3,5)P2) and PI5P. Emerging evidence implicates PIKfyve in key cellular processes, including autophagy, phagocytosis, endosomal trafficking, lysosomal maintenance, and melanosome formation. Complete loss of PIKfyve function is embryonic lethal in model organisms. In humans, heterozygous mutations in PIKFYVE are associated with Fleck corneal dystrophy and congenital cataracts. In this study, we investigate the role of PIKfyve in photoreceptors and the adjacent retinal pigment epithelium (RPE), host to dynamic endolysosomal pathways required for enduring the high oxidative stress environment, transporting metabolites and phototransduction components, and the breakdown of outer segment discs. To assess PIKfyve function in the retina and RPE in our zebrafish model, we employed CRISPR/Cas9-mediated gene editing and pharmacological inhibition using the specific PIKfyve inhibitor apilimod. Loss of PIKfyve activity leads to RPE expansion characterized by the accumulation of LC3- and LAMP1-positive vacuoles, along with defects in phagosome degradation and minor changes to melanosome biogenesis. Photoreceptors deprived of PIKfyve function develop a single large vacuole in the inner segment, while the OS remains largely intact over the timespan analyzed. Electroretinography (ERG) recordings revealed complete visual impairment in pikfyve crispant larvae and significantly reduced visual function in larvae treated with apilimod post embryogenesis. These findings highlight the critical role of PIKfyve in the development and homeostasis of the RPE and retina.

Müller glia are retinal support cells that play crucial roles in tissue structure, waste management, and repair. A challenge for the field has been to find Müller glia markers that detect non-reactive cells or work well in non-mammalian models. We introduce two novel markers for identifying reactive and non-reactive Müller glia in vertebrate retinas: a modified enzyme lectin (GFP-EndoNDM) and a monoclonal antibody (mAb735). These markers recognize polysialic acid (polySia), which is a highly conserved glycosylation modification in humans and vertebrates. In the retina, polySia is present on Müller glia predominantly in the form of polySia-NCAM. We used GFP-EndoNDM and mAb735 to investigate polySia distribution in Müller glia of fish, amphibians, reptiles, birds, rodents, retinal organoids, and humans. In adult retinas of most species, polySia was localized to the Müller glia and spanned outer to inner retina, with inner plexiform layer (IPL) sublaminae ramifications. Gliosis was also detectable in degenerating murine retinas. Notable species differences were that only outer retinal regions of Müller glia were labelled in adult zebrafish, whereas the outer Müller glia body up to the first IPL sublamina was labelled in adult turquoise killifish. There was no significant retinal polySia labeling in larval zebrafish, but it was present in the brain. Larval turquoise killifish have polySia throughout the retina, similar to other adult vertebrates. Labelling polySia expands the scientific toolbox for Müller glia markers, and offers a versatile way to visualize and monitor structural changes in non-reactive and reactive Müller glia across most vertebrate species. Highlights ### Competing Interest Statement The authors have declared no competing interest. BrightFocus Foundation, https://ror.org/03cvfxv40, M2021001F, M2024011N Fighting Blindness Canada University of Alberta, https://ror.org/0160cpw27, Startup Funds

Craniofacial development involves the concerted action of several different tissue types and cellular processes that must be intricately regulated. Here, we describe the role of the BMP ligand Growth and Differentiation Factor 6a ( gdf6a ) in the development of the zebrafish craniofacial skeleton. Larval gdf6a mutant zebrafish have malformations in the midline of the craniofacial skeleton that correlate with the expression of gdf6a in the embryonic pharyngeal arches. We show that Gdf6a has arch-specific roles in craniofacial morphogenesis; Gdf6a promotes chondrogenesis and alignment of midline craniofacial elements in the posterior pharyngeal arches (arches 2 and 3) and regulates morphogenesis within the mandibular symphysis of pharyngeal arch 1. We demonstrate that Gdf6a regulates craniofacial development through activation of canonical BMP signaling, likely acting cooperatively with additional BMP ligands. Taken together, this work elucidates how Gdf6a / BMP signaling directs development of the craniofacial skeleton, specifically along the ventral midline. Summary Statement Gdf6a regulates the development of the midline structures in the zebrafish craniofacial skeleton in an arch-specific manner via canonical BMP signaling. ### Competing Interest Statement The authors have declared no competing interest. Natural Sciences and Engineering Research Council, https://ror.org/01h531d29, RGPIN-2022-03658, RGPIN-2016-04682

Vertebrate photoreceptors detect light through a large cilium-based outer segment, which is filled with photopigment-laden membranous discs. Surrounding the base of the outer segment are microvilli-like calyceal processes (CPs). Although CP disruption has been associated with altered outer segment morphology and photoreceptor degeneration, the role of the CPs remains elusive. Here, we used zebrafish as a model to characterize CPs. We quantified CP parameters and report a strong disparity in outer segment coverage between photoreceptor subtypes. CP length is stable across light and dark conditions, yet heat-shock inducible expression of tagged actin revealed rapid turnover of the CP actin core. Detailed imaging of the embryonic retina uncovered substantial remodeling of the developing photoreceptor apical surface, including a transition from dynamic tangential processes to vertically oriented CPs immediately prior to outer segment formation. Remarkably, we also found a direct connection between apical extensions of the Müller glia and retinal pigment epithelium, arranged as bundles around the ultraviolet sensitive cones. In summary, our data characterize the structure, development and surrounding environment of photoreceptor microvilli in the zebrafish retina.

Although previous studies have reported fatty infiltration of the gastrocnemius-soleus complex, little is known about the volumetric distribution and patterns of fatty infiltration. The purpose of this anatomical study was to document and quantify the frequency, distribution, and pattern of fatty infiltration of the gastrocnemius-soleus complex. One hundred formalin-embalmed specimens (mean age 78.1 +/- 12.3 years; 48F/52M) were serially dissected to document the frequency, distribution, and pattern of fatty infiltration in the medial and lateral heads of gastrocnemius and soleus muscles. Fatty infiltration was found in 23% of specimens, 13 unilaterally (8F/5M) and 10 (5M/5F) bilaterally. The fatty infiltration process was observed to begin medially from the medial aspect of the medial head of gastrocnemius and medial margin of soleus and then progressed laterally throughout the medial head of gastrocnemius and the marginal, anterior, and posterior soleus. The lateral head of gastrocnemius remained primarily muscular in all specimens. Microscopically, the pattern of infiltration was demonstrated as intramuscular with intact aponeuroses, and septa. The remaining endo-, peri-, and epimysium preserved the overall contour of the gastrocnemius-soleus complex, even in cases of significant fatty replacement. Since the external contour of the calf is preserved, the presence of fatty infiltration may be underdiagnosed in the clinic without imaging. Myosteatosis is associated with gait and balance challenges in the elderly, which can impact quality of life and result in increased risk of falling. The findings of the study have implications in the rehabilitation management of elderly patients with sarcopenia and myosteatosis. Fatty infiltration of the gastrocnemius-soleus complex is common in the elderly and displays a medial to lateral progression. As the calf contour is preserved, the condition is not visible externally.image

Abstract Vertebrate photoreceptors detect light through a large cilium-based outer segment, which is filled with photopigment-laden membranous discs. Surrounding the base of the outer segment are calyceal processes (CPs), actin-based microvilli-like protrusions extending from the apical surface of the adjacent inner segment. While CP disruption has been associated with altered outer segment morphology and photoreceptor degeneration, the role of the processes remains elusive. Here, we used zebrafish as a model to characterize CPs. We quantified CP length, number, and diameter, and report a strong disparity in outer segment coverage between different photoreceptor subtypes. CP length was stable across light and dark conditions, while heat shock inducible expression of tagged actin revealed rapid turnover of the CP actin core and of the actin roots extending into the inner segment. Detailed imaging of the larval zebrafish retina revealed the presence of dynamic tangential processes extending from the photoreceptor progenitor apical surface up until the emergence of an actin dome over the growing inner segment. As the tangential processes retracted, CPs emerged just prior to outer segment formation. CPs did not accompany the emerging outer segment cilium but immediately aligned with the nascent outer segment discs. Remarkably, we also found a direct connection between apical extensions of the Müller glia and retinal pigment epithelium, arranged as bundles around the CP encircled outer segments of the ultraviolet sensitive cones. In summary, our data characterizes CP structure, development, and surrounding environment in the zebrafish retina.

Motile and non-motile cilia are associated with mutually-exclusive genetic disorders. Motile cilia propel sperm or extracellular fluids, and their dysfunction causes primary ciliary dyskinesia. Non-motile cilia serve as sensory/signalling antennae on most cell types, and their disruption causes single-organ ciliopathies such as retinopathies or multi-system syndromes. CFAP20 is a ciliopathy candidate known to modulate motile cilia in unicellular eukaryotes. We demonstrate that in zebrafish, cfap20 is required for motile cilia function, and in C. elegans , CFAP-20 maintains the structural integrity of non-motile cilia inner junctions, influencing sensory-dependent signalling and development. Human patients and zebrafish with CFAP20 mutations both exhibit retinal dystrophy. Hence, CFAP20 functions within a structural/functional hub centered on the inner junction that is shared between motile and non-motile cilia, and is distinct from other ciliopathy-associated domains or macromolecular complexes. Our findings suggest an uncharacterised pathomechanism for retinal dystrophy, and potentially for motile and non-motile ciliopathies in general.

Photoreceptor dysfunctions and degenerative diseases are significant causes of vision loss in patients, with few effective treatments available. Targeted interventions to prevent or reverse photoreceptor-related vision loss are not possible without a thorough understanding of the underlying mechanism leading to disease, which is exceedingly difficult to accomplish in the human system. Cone diseases are particularly challenging to model, as some popular genetically modifiable model animals are nocturnal with a rod-dominant visual system and cones that have dissimilarities to human cones. As a result, cone diseases, which affect visual acuity, colour perception, and central vision in patients, are generally poorly understood in terms of pathology and mechanism. Zebrafish (Danio rerio) provide the opportunity to model photoreceptor diseases in a diurnal vertebrate with a cone-rich retina which develops many macular degeneration-like pathologies. Zebrafish undergo external development, allowing early-onset retinal diseases to be detected and studied, and many ophthalmic tools are available for zebrafish visual assessment during development and adulthood. There are numerous zebrafish models of photoreceptor disease, spanning the various types of photoreceptor disease (developmental, rod, cone, and mixed photoreceptor diseases) and genetic/molecular cause. In this review, we explore the features of zebrafish that make them uniquely poised to model cone diseases, summarize the established zebrafish models of inherited photoreceptor disease, and discuss how disease in these models compares to the human presentation, where applicable. Further, we highlight the contributions of these zebrafish models to our understanding of photoreceptor biology and disease, and discuss future directions for utilising and investigating these diverse models.

Gut microbial products direct growth, differentiation, and development in animal hosts. However, we lack system-wide understanding of cell-specific responses to the microbiome. We profiled cell transcriptomes from the intestine, and associated tissue, of zebrafish larvae raised in the presence or absence of a micro biome. We uncovered extensive cellular heterogeneity in the conventional zebrafish intestinal epithelium, including previously undescribed cell types with known mammalian homologs. By comparing conventional to germ-free profiles, we mapped microbial impacts on transcriptional activity in each cell population. We revealed intricate degrees of cellular specificity in host responses to the microbiome that included regulatory effects on patterning and on metabolic and immune activity. For example, we showed that the absence of microbes hindered pro-angiogenic signals in the developing vasculature, causing impaired intestinal vascularization. Our work provides a high-resolution atlas of intestinal cellular composition in the developing fish gut and details the effects of the microbiome on each cell type.

Sensory neurons use specialized apical processes to perceive external stimuli and monitor internal body conditions. The apical apparatus can include cilia, microvilli, or both, and is adapted for the functions of the particular cell type. Photoreceptors detect light through a large, modified cilium (outer segment), that is supported by a surrounding ring of microvilli-like calyceal processes (CPs). Although first reported 150 years ago, CPs remain poorly understood. As a basis for future study, we therefore conducted a review of existing literature about sensory cell microvilli, which can act either as the primary sensory detector or as support for a cilia-based detector. While all microvilli are finger-like cellular protrusions with an actin core, the processes vary across cell types in size, number, arrangement, dynamics, and function. We summarize the current state of knowledge about CPs and the characteristics of the microvilli found on inner ear hair cells (stereocilia) and cerebral spinal fluid-contacting neurons, with comparisons to the brush border of the intestinal and renal epithelia. The structure, stability, and dynamics of the actin core are regulated by a complement of actin-binding proteins, which includes both common components and unique features when compared across cell types. Further, microvilli are often supported by lateral links, a glycocalyx, and a defined extracellular matrix, each adapted to the function and environment of the cell. Our comparison of microvillar features will inform further research into how CPs support photoreceptor function, and also provide a general basis for investigations into the structure and functions of apical microvilli found on sensory neurons.

Gastrocnemius is a superficial calf muscle with two muscular heads that contribute fibres to the Achilles tendon. While gastrocnemius functions during movement of the knee and ankle joints, the muscle is of particular clinical interest as a target for treatments to alleviate lower limb spasticity and pain. Here, we describe abnormal calf musculature discovered incidentally during cadaveric dissection in an anatomy teaching laboratory. In particular, the medial head of the gastrocnemius in the left leg exhibited almost complete replacement by adipose tissue, while the lateral head and soleus muscles were unaffected. On the right side, the medial head again showed fatty infiltration, but only in the distal part of an otherwise substantial muscle belly. While the nerve to the left medial head was intact, a cyst was discovered in the sacral vertebral column. The full, but isolated replacement of a muscle belly by fat is a highly unusual finding. Potentially, muscle degeneration followed by fatty infiltration was secondary to compression of the S1, S2 nerve roots and denervation of the muscle, but other possible causes are reviewed in the discussion.

The tumor microenvironment significantly contributes to tumor initiation, progression, neo-angiogenesis, and metastasis, and a better understanding of the role of the different cellular players would facilitate the development of novel therapeutic strategies for cancer treatment. Towards this goal, intravital imaging is a powerful method to unravel interaction partners of tumor cells. Among vertebrate model organisms, zebrafish is uniquely suited for in vivo imaging studies. In recent years zebrafish has also become a valuable model in cancer research. In this chapter, we will summarize, how zebrafish has been used to characterize cells of the tumor microenvironment. We will cover both genetically engineered cancer models and xenograft models in zebrafish. The majority of work has been done on the role of innate immune cells and their role during tumor initiation and metastasis, but we will also cover studies focusing on adipocytes, fibroblasts, and endothelial cells. Taken together, we will highlight the versatile use of the zebrafish model for in vivo tumor microenvironment studies.

Photoreceptor disease results in irreparable vision loss and blindness, which has a dramatic impact on quality of life. Pathogenic mutations in RP1L1 lead to photoreceptor degenerations such as occult macular dystrophy and retinitis pigmentosa. RP1L1 is a component of the photoreceptor axoneme, the backbone structure of the photoreceptor’s light-sensing outer segment. We generated an rp1l1 zebrafish mutant using CRISPR/Cas9 genome editing. Mutant animals had progressive photoreceptor functional defects as determined by electrophysiological assessment. Optical coherence tomography showed gaps in the photoreceptor layer, disrupted photoreceptor mosaics, and thinner retinas. Mutant retinas had disorganized photoreceptor outer segments and lipid-rich subretinal drusenoid deposits between the photoreceptors and retinal pigment epithelium. Our mutant is a novel model of RP1L1-associated photoreceptor disease and the first zebrafish model of photoreceptor degeneration with reported subretinal drusenoid deposits, a feature of age-related macular degeneration.

Purpose The electroretinogram (ERG) is a powerful approach for investigating visual function in zebrafish ocular disease models. However, complexity, cost, and a literature gap present as significant barriers for the introduction of this technology to new zebrafish laboratories. Here, we introduce a simplified and effective method to obtain zebrafish ERGs. Methods In-house assembled recording electrodes and a custom 3D-printed platform were used to gather high-quality and consistent ERG data from zebrafish at 3 developmental timepoints—larval, juvenile, and adult. Fish were tested under both scotopic (dark-adapted) and photopic (light-adapted) conditions to differentiate between the rod and cone systems, respectively. Results Robust ERG waveforms across all developmental timepoints were obtained using the methodology presented here. We observed an overall increase in signal amplitude as development progressed, reflecting maturation of the zebrafish retina. Oscillatory potentials could also be isolated from the generated waveforms. Conclusions This simplified approach to the zebrafish ERG can generate waveforms comparable to the existing approaches and helps reduce barriers for zebrafish laboratories studying ocular development and disease.

Purpose:Analysis of photoreceptor morphology and gene expression in mispatterned eyes of zebrafish growth differentiation factor 6a (gdf6a) mutants.Methods:Rod and cone photoreceptors were compared between gdf6a mutant and control zebrafish from larval to late adult stages using transgenic labels, immunofluorescence, and confocal microscopy, as well as by transmission electron microscopy. To compare transcriptomes between larval gdf6a mutant and control zebrafish, RNA-Seq was performed on isolated eyes.Results:Although rod and cone photoreceptors differentiate in gdf6a mutant zebrafish, the cells display aberrant growth and morphology. The cone outer segments, the light-detecting sensory endings, are reduced in size in the mutant larvae and fail to recover to control size at subsequent stages. In contrast, rods form temporarily expanded outer segments. The inner segments, which generate the required energy and proteins for the outer segments, are shortened in both rods and cones at all stages. RNA-Seq analysis provides a set of misregulated genes associated with the observed abnormal photoreceptor morphogenesis.Conclusions:GDF6 mutations were previously identified in patients with Leber congenital amaurosis. Here, we reveal a unique photoreceptor phenotype in the gdf6a mutant zebrafish whereby rods and cones undergo abnormal maturation distinct for each cell type. Further, subsequent development shows partial recovery of cell morphology and maintenance of the photoreceptor layer. By conducting a transcriptomic analysis of the gdf6a larval eyes, we identified a collection of genes that are candidate regulators of photoreceptor size and morphology.

Congenital ocular coloboma is a genetic disorder that is typically observed as a cleft in the inferior aspect of the eye resulting from incomplete choroid fissure closure. Recently, the identification of individuals with coloboma in the superior aspect of the iris, retina, and lens led to the discovery of a novel structure, referred to as the superior fissure or superior ocular sulcus (SOS), that is transiently present on the dorsal aspect of the optic cup during vertebrate eye development. Although this structure is conserved across mice, chick, fish, and newt, our current understanding of the SOS is limited. In order to elucidate factors that contribute to its formation and closure, it is imperative to be able to observe it and identify abnormalities, such as delay in the closure of the SOS. Here, we set out to create a standardized series of protocols that can be used to efficiently visualize the SOS by combining widely available microscopy techniques with common molecular biology techniques such as immunofluorescent staining and mRNA overexpression. While this set of protocols focuses on the ability to observe SOS closure delay, it is adaptable to the experimenter's needs and can be easily modified. Overall, we hope to create an approachable method through which our understanding of the SOS can be advanced to expand the current knowledge of vertebrate eye development.

We present a cadaveric case study of an 88-year-old woman with an unusual posterior perineal hernia containing small bowel, rectum, and mesentery. Dissection revealed several loops of the small bowel occupying the presacral space and displacement of the rectum into a large perineal evagination. The intestinal mucosa appeared to have been healthy at the time of death, and we did not find any indication of rectal prolapse. There was also no evidence of past surgery, suggestive of a primary hernia. We conclude this patient had a posterior enterorectal perineal hernia. Suggestions for surgical repair are described.

The eye primordium arises as a lateral outgrowth of the forebrain, with a transient fissure on the inferior side of the optic cup providing an entry point for developing blood vessels. Incomplete closure of the inferior ocular fissure results in coloboma, a disease characterized by gaps in the inferior eye and recognized as a significant cause of pediatric blindness. Here, we identify eight patients with defects in tissues of the superior eye, a congenital disorder that we term superior coloboma. The embryonic origin of superior coloboma could not be explained by conventional models of eye development, leading us to reanalyze morphogenesis of the dorsal eye. Our studies revealed the presence of the superior ocular sulcus (SOS), a transient division of the dorsal eye conserved across fish, chick, and mouse. Exome sequencing of superior coloboma patients identified rare variants in a Bone Morphogenetic Protein (Bmp) receptor (BMPR1A) and T-box transcription factor (TBX2). Consistent with this, we find sulcus closure defects in zebrafish lacking Bmp signaling or Tbx2b. In addition, loss of dorsal ocular Bmp is rescued by concomitant suppression of the ventral-specific Hedgehog pathway, arguing that sulcus closure is dependent on dorsal-ventral eye patterning cues. The superior ocular sulcus acts as a conduit for blood vessels, with altered sulcus closure resulting in inappropriate connections between the hyaloid and superficial vascular systems. Together, our findings explain the existence of superior coloboma, a congenital ocular anomaly resulting from aberrant morphogenesis of a developmental structure.