The osteochondral ligament: a fibrous attachment between bone and articular cartilage in Rana catesbeiana

Ontogeny of the meniscus in the anuran Xenopus laevis

The anuran knee joint is subjected to the jump, one of the tetrapods' most demanding mechanical stresses. Consistent with this continuous effort, the knee of the anurans has a complex structure comparable to that of an amniote. Here, we describe the ontogeny of the Xenopus knee tissues and study the morphogenesis of the knee joint shape by performing a geometric morphometric analysis of specially selected anatomical structures: the menisci and the long bone epiphyses. A meniscus is a crescent-shaped fibrocartilaginous structure, with a triangular cross-section inserted between joints surfaces. A meniscus transmits load across the tibiofemoral joint by increasing congruity of the long bone epiphysis and decreasing the resulting stress exerted on the articular cartilage. We ask two questions: (1) what is the tissue composition along the ontogeny of the menisci of a swimming frog? (2) How do the menisci acquire the shape that will allow their adjustment? We studied the structures and tissue ontogeny of the knee of several specimens of Xenopus laevis and evaluated the congruity of the knee structures across the species ontogeny. Histological sections showed that the cavitation process responsible for separating the menisci and the epiphyses seems to be pivotal in shaping the conformity of these structures and the long bone epiphyses of the hindlimbs. The geometric morphometric analysis allowed us to interpret three phases of differentiation associated with limb functionality. The characteristic shape of the meniscus appears early in the ontogeny of the knee, simultaneously with the epiphysis contours.

Common occurrence of Sharpey's fibres in amphibian phalanges

Sharpey’s fibres are known mainly as providing anchorage between tooth and the periodontal ligament but they occur also in other types of bones. In the postcranial skeleton these fibres are usually present at the muscle or tendon attachment sites. They were reported in all major groups of extant vertebrates, as well as in putative lissamphibian ancestors—temnospondyls and lepospondyls. However, it was recently stated that their presence was very rarely described in extant amphibians. In limbs, they were reported predominantly from proximal bones. They have not yet been reported from phalanges, which are the most commonly sectioned amphibian bones. Here, we describe phalangeal histology of nine species representing most major clades of lissamphibians. These results show that Sharpey’s fibres occur commonly in lissamphibian phalanges. In shaft, they are radially oriented and occur in the periosteal bone, at sites of tendon attachment. They can also occur in the metaphysis and contact the cartilage. This may provide a basis for foot muscle reconstructions in fossil amphibians.

Further Data on Sesamoid Identity from Two Anuran Species

Considering that the identification of equivalent entities is the basis for any comparative analysis, we compare the histology, histochemistry, shape and dimensions of epiphyses, carpal and sesamoids in two anuran frogs. Our goal was to explore the morphological correspondence among these three skeletal elements in order to clarify the sesamoid identity. We studied the skeletogenesis, contour geometric morphometry and dimensions of forelimb elements of juveniles of two anurans species Leptodactylus bufonius and Rhinella arenarum. Skeletogenesis in anurans present a common trait between carpals and sesamoids: both elements exhibit endochondral ossification. A difference between these elements is the presence of fibrocartilage in the development of sesamoids. The geometric morphometry does not allow us to establish a shape pattern that can be compared either between sesamoids and epiphyses or carpals. With regard to dimensions, our data indicate that bones categorization based on these aspects is ambiguous and therefore is useless to classify of skeletal bones. The data about tissue differentiation of sesamoids provide evidence that support the idea that these elements should be considered part of the typical endowment of the vertebrate skeleton.

Hyperossification in miniaturized toadlets of the genus Brachycephalus (Amphibia: Anura: Brachycephalidae): Microscopic structure and macroscopic patterns of variation

Species of the genus Brachycephalus, have a snout-vent length of less than 18 mm and are believed to have evolved through miniaturization. Brachycephalus ephippium, is particularly interesting; because its entire skull is hyperossified, and the presacral vertebrae and transverse processes are covered by a dorsal shield. We demonstrate in this paper that, at the macroscopic level, a completely hyperossified skull and dorsal shield occur only in B. ephippium, but not in B. ferruginus, B. izechsohni, B. pernix, B. pombali, B. brunneus, B. didactylus, and B. hermogenesi. An intermediate condition, in which the skull is hyperossified but a dorsal shield is absent, occurs in B. vertebralis, B. nodoterga, B. pitanga, and B. alipioi. The microscopic structure of hyperossification was examined in skulls of B. ephippium and B. pitanga, revealing a complex organization involving the presence of Sharpey fibers, which in humans are characteristic of periodontal connections.

Postnatal bone elongation of the manus versus pes: analysis of the chondrocytic differentiation cascade in Mus musculus and Eptesicus fuscus

Bones elongate postnatally by endochondral ossification as cells of the cartilaginous growth plate undergo a differentiation cascade of proliferation, cellular hypertrophy and matrix synthesis. Interspecific comparisons of homologous bones elongating at different rates has been a useful approach for studying the dynamics of this process. The purpose of this study was to measure quantitative stereological parameters of growth plates of the third digit of the manus and pes of the laboratory mouse, and make comparisons to chondrocytic performance parameters in the homologous bones of the big brown bat, Eptesicus fuscus, where extremely rapid postnatal elongation of bones of the manus is associated with skeletal modifications for powered flight. Measurements were made across all zones of forelimb and hindlimb autopod growth plates by dividing each growth plate into strata of equal height (from thirteen 200-mum-high strata in the metacarpus to five 40-mum-high strata in phalangeal bones of the pes). Results indicate that all chondrocytic performance parameters known to quantitatively contribute to the elongation potential of a growth plate change together. A significant finding was that in growth plates of the chiropteran manus, final hypertrophic cell size and shape were achieved early in the zone of hypertrophy, indicating that interstitial expansion of the growth plate resulting from the incremental chondrocytic height increase in the direction of elongation was completed soon after the transition from the cessation of proliferation to the initiation of hypertrophy. This is unlike what has been reported in most mammalian growth plates previously analyzed, but is the situation in the proximal tibial growth plate of rapidly growing frogs and precocial birds. This suggests that a similar adaptation for stabilization of a rapidly elongating bone has evolved independently in three widely separated groups that have in common rapid growth in limbs to be used for early active, powered locomotion.

Expression profile of Xenopus banded hedgehog, a homolog of mouse Indian hedgehog, is related to the late development of endochondral ossification in Xenopus laevis

Late development of endochondral ossification occurs at the boundary between the growth cartilage and bone marrow during the formation of long bones in Xenopus laevis. Since the Indian hedgehog (Ihh) is involved in endochondral ossification in mouse, we investigated the expression of Xenopus banded hedgehog (X-bhh), which is a homolog of mouse Ihh. RT-PCR analysis demonstrated that the X-bhh mRNA was detected from an early stage of limb formation to formation of femurs in mature frogs, and it was associated with the expression of Xenopus-ptc1 (X-ptc1), Xenopus-gli1 (X-gli1), Xenopus-type II collagen (X-col II), Xenopus-runx2 (X-runx2), and Xenopus-osteocalcin (X-ocn) mRNAs. In situ hybridization revealed that chondrogenic cells observed at early limb development expressed X-bhh and X-gli1. At later stages of limb development, chondrocytes, located slightly away from the boundary between the cartilage and bone marrow, expressed the X-bhh, X-ptc1, and X-gli1 mRNAs; however, the mesenchymal cells at the boundary failed to express these mRNAs. The X-bhh, X-ptc1, and X-gli1 mRNAs as well as those of X-runx2 and X-ocn were expressed by the mesenchymal cells in the periosteal region at the tip of the cortical bone, indicating an intimate relationship between X-bhh expression and bone formation in this region. Considered collectively, the present study suggests that X-bhh evolutionally acquired the function to induce osteogenesis; however, the expression profile of X-bhh in epiphysis is closely related to the late development of endochondral ossification in X. laevis.

Development and growth of long bones in European water frogs (Amphibia: Anura: Ranidae), with remarks on age determination

Differentiation and development of long bones were studied in European water frogs: Rana lessonae, R. ridibunda, and R. esculenta. The study included premetamorphic larvae (Gosner Stage 40) to frogs that were 5 years old. Femora, metatarsal bones, and proximal phalanges of the hindlimb exhibit the same pattern of periosteal bone differentiation and the same pattern of growth. Longitudinal and radial growth of these bones was studied by examination of the diaphyses and epiphyses, particularly where the edge of periosteal bone is inserted into the epiphysis. The periosteum seems to be responsible for both longitudinal and radial growth. Investigation of the formation, length, and arrangement of lines of arrested growth reveals that the first line is present only in the middle 25-35% of the length of the diaphysis of an adult bone; therefore, only the central portion of the diaphysis should be used for age estimation in skeletochronological studies. Comparison of the shapes and histological structures of epiphyses in the femur, metatarsal bones, and phalanges revealed that epiphyseal cartilages are composed of an inner and outer part. The inner metaphyseal cartilage has distinct zones and plugs the end of the periosteal bone cylinder; its role in longitudinal growth is questioned. The outer epiphyseal cartilage is composed of articular cartilages proper, in addition to lateral articular cartilages. Differences in the symmetry of the lateral articular cartilages of distal epiphyses of the femur and toes may reflect adaptations to different kinds of movements at the knee and in the foot.