Search for: All records

Two‐toed (Choloepus sp.) and three‐toed (Bradypus sp.) sloths possess short, rounded pisiforms that are rare among mammals and differ from other members of Xenarthra like the giant anteater (Myrmecophaga tridactyla) which retain elongated, rod‐like pisiforms in common with most mammals. Using photographs, radiographs, and μCT, we assessed ossification patterns in the pisiform and the paralogous tarsal, the calcaneus, for two‐toed sloths, three‐toed sloths, and giant anteaters to determine the process by which pisiform reduction occurs in sloths and compare it to other previously studied examples of pisiform reduction in humans and orangutans. Both extant sloth genera achieve pisiform reduction through the loss of a secondary ossification center and the likely disruption of the associated growth plate based on an unusually porous subchondral surface. This represents a third unique mechanism of pisiform reduction among mammals, along with primary ossification center loss in humans and retention of two ossification centers with likely reduced growth periods in orangutans. Given the remarkable similarities between two‐toed and three‐toed sloth pisiform ossification patterns and the presence of pisiform reduction in fossil sloths, extant sloth pisiform morphology does not appear to represent a recent convergent adaptation to suspensory locomotion, but instead is likely to be an ancestral trait of Folivora that emerged early in the radiation of extant and fossil sloths.

 

Cartilage histomorphometry is often performed on decalcified, paraffin-embedded bone sections, which provide versatility in staining applications from basic morphology to immunohistochemistry. Safranin O is a cationic dye that binds to proteoglycans in cartilage and is routinely used to assess growth plate dynamics and/or fracture repair at bone–cartilage interfaces. When used with a counterstain such as fast green, safranin O can offer exquisite differentiation of cartilage from surrounding bone. However, various decalcification and processing methods can deplete proteoglycans, rendering inconsistent, weak, or absent safranin O staining with indiscriminate bone–cartilage boundaries. We sought to develop an alternative staining methodology that preserves the contrast of bone and cartilage in cases of proteoglycan depletion that can be applied when other cartilage stains are unsuccessful. Here, we describe and validate a modified periodic acid-Schiff (PAS) protocol that we developed using Weigert's iron hematoxylin and light green stains as an alternative to safranin O for discriminating bone–cartilage interfaces of skeletal tissues. This method provides a practical solution for differentiating bone and cartilage when safranin O staining is not detected after decalcification and paraffin processing. The modified PAS protocol can be useful for studies in which identification of the bone–cartilage interface is essential but may not be preserved with standard staining approaches. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research. 

Hox genes are key developmental patterning genes that impact segmental identity and skeletal patterning. Hox11 genes are known to impact wrist and ankle development and are expressed around the developing pisiform and calcaneus. These paralogous bones in the wrist and ankle are the only carpal and tarsal to form a growth plate in mammals, although humans have lost this growth plate and the associated primary ossification center in the pisiform. Loss-of-function mutations to Hoxa11 and Hoxd11 result in pisiform truncation and appear to also cause at least some disorganization of the growth plate cartilage; however, little is known about the nature of this disorganization or if ossification timing is impacted by Hox11 genes. The present study investigates the role of Hoxa11 and Hoxd11 in pisiform growth plate organization and ossification timing. We conducted histological analysis of the pisiform growth plate in juvenile mice with Hoxa11 and Hoxd11 loss-of-function mutations and compared them to ossification patterns observed in age- and genotype-matched whole-mount specimens that were cleared and stained with Alizarin red and Alcian blue to visualize bone and cartilage, respectively. Histological analysis reveals a dosage-dependent impact of Hox11 mutations on pisiform ossification to both the primary and secondary ossification center. As the number of Hox11 mutation alleles increase, less bone is present in the early primary ossification center compared to age-matched specimens. In specimens with three loss-of-function alleles, no trabeculae or growth plate organization are visible at P9, when both are well established in wild type specimens. Cleared and stained specimens indicate a possible pseudo epiphysis forming with Hoxd11 mutation, while Hoxa11 knockout specimens have not formed any visible epiphysis or calcification by P9. These results indicate that ossification timing and patterns, along with growth plate organization, are affected by Hox11 mutations during early pisiform ossification. Furthermore, Hoxa11 and Hoxd11 alter the pisiform epiphysis differently, suggesting that each plays a specific role in formation of the ossification front and epiphysis ossification either by influencing timing, ossification progression, or both. Further work is needed to understand the mechanisms by which Hox genes impact ossification patterns and timing, as well as the differential roles of Hoxa11 and Hoxd11 in growth plate organization and epiphysis formation. 

Hox11 genes are expressed around the developing wrist and ankle and are known to substantially impact pisiform shape and length in mice. The calcaneus is a tarsal bone that is paralogous to the pisiform in the wrist, but previous descriptions of mice with Hox11 mutations have suggested that little morphological change takes place unless Hoxa11 and Hoxd11 are both knocked out, at which point the calcaneus fails to form. However, these studies primarily relied on cleared and stained whole-mount specimens which limit resolution of morphological features. This study seeks to determine if calcaneus morphology is altered by three or fewer loss-of-function Hoxa11 and Hoxd11 alleles. We obtained microCT scans of 8 week old mice and compared calcaneus morphology in wild type mice and mice with one, two, and three Hoxa11 and Hoxd11 loss-of-function alleles. We used auto3dgm to conduct a 3D geometric morphometric analysis of shape variation using surface semi-landmarks. Principle components (PC) analysis indicates that calcaneus morphology is altered in mice with Hoxa11 and Hoxd11 loss-of-function mutations. PC1 accounts for 35.4% of shape variation and results from changes to the width and height of the calcaneal neck and shape of peroneal tubercle/process. PC2 accounts for 11.9% of shape variation and results from changes to the width of the calcaneal tuberosity and height of the posterior talar facet. Most specimens with either combination of three out of four Hoxa11 and Hoxd11 loss-of-function alleles cluster together. The other genotypes form a gradient of morphological change with WT, Hoxd11 heterozygotes, and Hoxd11 knockouts being most similar to each other and Hoxa11 heterozygotes, Hoxa11 knockouts, and heterozygotes for both genes being most similar to each other. These results suggest that Hox11 loss-of-function mutations result in altered calcaneus morphology and Hoxa11 and Hoxd11 loss-of-function mutations alter the shape of the calcaneus in different ways when fewer than three alleles are knocked out. 

Hox genes are key developmental patterning genes that impact segmental identity and skeletal patterning. While Hox11 genes are known to be expressed around the developing calcaneus bone of the ankle, previous studies on mice with Hox11 mutations have indicated that calcaneus morphology is not affected until both Hoxa11 and Hoxd11are knocked out, at which point the calcaneus and talus fail to form.The pisiform bone, a wrist bone that is paralogous to the calcaneus, exhibits substantial morphological and growth plate alterations with Hox11 mutations. We have previously shown that some length differences are present in the adult calcanei of mice with Hoxa11 and Hoxd11 loss-of-function mutations. The present study investigates whether or not the calcaneus growth plate is altered by Hoxa11 and Hoxd11 loss-of-function mutation. We conducted histological analysis of the calcaneus growth plate in juvenile mice with Hoxa11 and Hoxd11 loss-of-function mutations and compared them to ossification patterns observed in whole-mount specimens that were cleared and stained with alizarin red and alcian blue to visualize bone and cartilage, respectively. Histological analysis reveals that early calcaneus growth plates preserve the hypertrophic and proliferative growth plate zones. This is in contrast to the pisiform and likely a result of Hoxc gene expression in the hind limb but not the forelimb. The shape of the epiphyseal cartilage, however, differs greatly in mice with a combined three loss-of-function alleles between Hoxa11 and Hoxd11. In these mice, the calcaneus epiphyseal cartilage is conical shaped with an elongated region of reserve zone chondrocytes. The ossification front and calcaneal tendon insertion are also altered compared to wild type specimens. The first evidence of calcaneal epiphysis ossification appears at P9 in some Hox11 mutant mice, while it typically appears at P11 in wild type specimens. By P17, the epiphysis appears to be larger in specimens with both Hoxa11 and Hoxd11 mutations compared to wild type. These results indicate that the calcaneus growth plate is more resilient to Hox11 mutations than the pisiform, but that the calcaneus exhibits morphological changes and evidence of altered ossification timing with fewer loss-of-function alleles than identified by previous studies.