Three-dimensional mammalian tooth development using diceCT

Sharp and fully loaded: 3D tissue reconstruction reveals how snake fangs stay deadly during fang replacement

Snake venom is produced, transported and delivered by the sophisticated venom delivery system (VDS). When snakes bite, the venom travels from the venom gland through the venom duct into needle-like fangs that inject it into their prey. To counteract breakages, fangs are continuously replaced throughout life. Currently, the anatomy of the connection between the duct and the fang has not been described, and the mechanism by which the duct is reconnected to the replacement fang has not been identified. We examined the VDS in 3D in representative species from two families and one subfamily (Elapidae, Viperidae, Atractaspidinae) using contrast-enhanced microCT (diceCT), followed by dissection and histology. We observed that the venom duct bifurcates immediately anterior to the fangs so that both the original and replacement fangs are separately connected and functional in delivering venom. When a fang is absent, the canal leading to the empty position is temporarily closed. We found that elapid snakes have a crescent-shaped venom reservoir where venom likely pools before it enters the fang. These findings form the final piece of the puzzle of VDS anatomy in front-fanged venomous snakes. Additionally, they provide further evidence for independent evolution of the VDS in these three snake taxa.

Postnatal development in a marsupial model, the fat-tailed dunnart (Sminthopsis crassicaudata; Dasyuromorphia: Dasyuridae)

Marsupials exhibit unique biological features that provide fascinating insights into many aspects of mammalian development. These include their distinctive mode of reproduction, altricial stage at birth, and the associated heterochrony that is required for their crawl to the pouch and teat attachment. Marsupials are also an invaluable resource for mammalian comparative biology, forming a distinct lineage from the extant placental and egg-laying monotreme mammals. Despite their unique biology, marsupial resources are lagging behind those available for placentals. The fat-tailed dunnart (Sminthopsis crassicaudata) is a laboratory based marsupial model, with simple and robust husbandry requirements and a short reproductive cycle making it amenable to experimental manipulations. Here we present a detailed staging series for the fat-tailed dunnart, focusing on their accelerated development of the forelimbs and jaws. This study provides the first skeletal developmental series on S. crassicaudata and provides a fundamental resource for future studies exploring mammalian diversification, development and evolution.

Multiscale imaging of the rat brain using an integrated diceCT and histology workflow

Advancements in tissue visualization techniques have spurred significant gains in the biomedical sciences by enabling researchers to integrate their datasets across anatomical scales. Of particular import are techniques that enable the interpolation of multiple hierarchical scales in samples taken from the same individuals. In this study, we demonstrate that two-dimensional histology techniques can be employed on neural tissues following three-dimensional diffusible iodine-based contrast-enhanced computed tomography (diceCT) without causing tissue degradation. This represents the first step toward a multiscale pipeline for brain visualization. We studied brains from adolescent male Sprague-Dawley rats, comparing experimental (diceCT-stained then de-stained) to control (without diceCT) brains to examine neural tissues for immunolabeling integrity, compare somata sizes, and distinguish neurons from glial cells within the telencephalon and diencephalon. We hypothesized that if experimental and control samples do not differ significantly in morphological cell analysis, then brain tissues are robust to the chemical, temperature, and radiation environments required for these multiple, successive imaging protocols. Visualizations for experimental brains were first captured via micro-computed tomography scanning of isolated, iodine-infused specimens. Samples were then cleared of iodine, serially sectioned, and prepared again using immunofluorescent, fluorescent, and cresyl violet labeling, followed by imaging with confocal and light microscopy, respectively. Our results show that many neural targets are resilient to diceCT imaging and compatible with downstream histological staining as part of a low-cost, multiscale brain imaging pipeline.

Timing of Mouse Molar Formation Is Independent of Jaw Length Including Retromolar Space

For humans and other mammals to eat effectively, teeth must develop properly inside the jaw. Deciphering craniodental integration is central to explaining the timely formation of permanent molars, including third molars which are often impacted in humans, and to clarifying how teeth and jaws fit, function and evolve together. A factor long-posited to influence molar onset time is the jaw space available for each molar organ to form within. Here, we tested whether each successive molar initiates only after a minimum threshold of space is created via jaw growth. We used synchrotron-based micro-CT scanning to assess developing molars in situ within jaws of C57BL/6J mice aged E10 to P32, encompassing molar onset to emergence. We compared total jaw, retromolar and molar lengths, and molar onset times, between upper and lower jaws. Initiation time and developmental duration were comparable between molar upper and lower counterparts despite shorter, slower-growing retromolar space in the upper jaw, and despite size differences between upper and lower molars. Timing of molar formation appears unmoved by jaw length including space. Conditions within the dental lamina likely influence molar onset much more than surrounding jaw tissues. We theorize that molar initiation is contingent on sufficient surface area for the physical reorganization of dental epithelium and its invagination of underlying mesenchyme.

Enhancing CT imaging: A safe protocol to stain and de-stain rare fetal museum specimens using diffusible iodine-based staining (diceCT)

Computed tomography (CT) scanning is being increasingly employed in the study of natural history, particularly to investigate the internal anatomy of unique specimens in museum collections. Different techniques to enhance the contrast between tissues have been developed to improve the quality of the scans while preserving the integrity of these rare specimens. Diffusible iodine-based contrast enhanced computed tomography (diceCT) was found to be particularly effective and reversible for staining tissues in formalin preserved specimens. While it can also be effectively employed to stain ethanol-preserved specimens of small size, the reversibility of this process and the applicability to large-bodied animals has never been thoroughly tested. Here, we describe a novel diceCT protocol developed to stain and de-stain ethanol-preserved prenatal specimens of baleen whales (Mysticeti, Cetacea). These large (10-90 cm in length only considering early fetal stages) specimens present unique challenges as they are rare in collections and irreplaceable, therefore it is imperative to not damage them with the staining process. Before trying this protocol on baleen whales' specimens, we conducted a pilot study on commercially available fetal pigs using the same parameters. This allowed us to optimize the staining time to obtain the best results in CT scanning and to test first-hand the effect of de-staining on ethanol-based specimens. External coloration of the specimens is also a concern with iodine-staining, as stained specimens assume a bright red color that needs to be removed from both internal and external tissues before they can be stored. To test the reversibility of the stain in ethanol-preserved specimens with fur, we also conducted a small experiment using commercially acquired domestic mice. After these trials were successful, we applied the staining and de-staining protocol to multiple fetal specimens of mysticetes up to 90 cm in length, both ethanol- and formalin-preserved. Specimens were soaked in a solution of 1% pure iodine in 70% ethanol for 1-28 days, according to their size. After scanning, specimens are soaked in a solution of 3% sodium thiosulfate in 70% ethanol that is able to completely wash out the iodine from the tissues in a shorter time frame, between a few hours and 14 days. The same concentrations were used for formalin-preserved specimens, but DI water was used as solvent instead of ethanol. The staining technique proved particularly useful to enhance the contrast difference between cartilage, mineralized bone, teeth, and the surrounding soft tissues even when the specimens where scanned in medical-grade CT scans. The specimens did not suffer any visible damage or shrinkage due to the procedure, and in both the fetal samples and in the mice the color of the stain was completely removed by the de-staining process. We conclude therefore that this protocol can be safely applied to a variety of ethanol-preserved museum specimens to enhance the quality of the CT scanning and highlight internal morphological features without recurring to dissection or other irreversible procedures. We also provide tips to best apply this protocol, from how to mix the solutions to how to minimize the staining time.

Tooth agenesis: What do we know and is there a connection to cancer?

Like all developmental processes, odontogenesis is highly complex and dynamically regulated, with hundreds of genes co-expressed in reciprocal networks. Tooth agenesis (missing one or more/all teeth) is a common human craniofacial anomaly and may be caused by genetic variations and/or environmental factors. Variants in PAX9, MSX1, AXIN2, EDA, EDAR, and WNT10A genes are associated with tooth agenesis. Currently, variants in ATF1, DUSP10, CASC8, IRF6, KDF1, GREM2, LTBP3, and components and regulators of WNT signaling WNT10B, LRP6, DKK, and KREMEN1 are at the forefront of interest. Due to the interconnectedness of the signaling pathways of carcinogenesis and odontogenesis, tooth agenesis could be a suitable marker for early detection of cancer predisposition. Variants in genes associated with tooth agenesis could serve as prognostic or therapeutic targets in cancer. This review aims to summarize existing knowledge of development and clinical genetics of teeth. Concurrently, the review proposes possible approaches for future research in this area, with particular attention to roles in monitoring, early diagnosis and therapy of tumors associated with defective tooth development.