Rationale: The second heart field (SHF) contains progenitors of all heart chambers, excluding the left ventricle. The SHF is patterned, and the anterior region is known to be destined to form the outflow tract and right ventricle. Objective: The aim of this study was to map the fate of the posterior SHF (pSHF). Methods and Results: We examined the contribution of pSHF cells, labeled by lipophilic dye at the 4- to 6-somite stage, to regions of the heart at 20 to 25 somites, using mouse embryo culture. Cells more cranial in the pSHF contribute to the atrioventricular canal (AVC) and atria, whereas those more caudal generate the sinus venosus, but there is intermixing of fate throughout the pSHF. Caudal pSHF contributes symmetrically to the sinus venosus, but the fate of cranial pSHF is left/right asymmetrical. Left pSHF moves to dorsal left atrium and superior AVC, whereas right pSHF contributes to right atrium, ventral left atrium, and inferior AVC. Retrospective clonal analysis shows the relationships between AVC and atria to be clonal and that right and left progenitors diverge before first and second heart lineage separation. Cranial pSHF cells also contribute to the outflow tract: proximal and distal at 4 somites, and distal only at 6 somites. All outflow tract–destined cells are intermingled with those that will contribute to inflow and AVC. Conclusions: These observations show asymmetric fate of the pSHF, resulting in unexpected left/right contributions to both poles of the heart and can be integrated into a model of the morphogenetic movement of cells during cardiac looping.

The development of the articular cartilage of the rabbit knee joint from the 17-day fetus to the 2-year adult rabbit has been examined. At 17 days, the developing femur and tibia are separated by the interzone. Cavitation occurs around 25 days; the cells of the intermediate layer flatten and move onto those of the chondogenous layers to create the articular surfaces. After birth, growth of the cartilage is mainly the result of matrix production. Ossification of the epiphyses is complete by 6 weeks postpartum. Horizontal zones can be distinguished in the articular cartilage; the superficial cells are aligned parallel to the surface, but in the deep layers the cells are in columns. The tidemark is first seen at 12-14 weeks. The matrix of the interzone in the 17-day fetus contains types I, III and V collagens, but no type II. After cavitation at 25 days, the surface layer of the articular cartilage still contains type I, but no type II collagen. From 6 weeks postnatal onwards, type II collagen is present throughout the cartilage and type I disappears. Type III collagen is initially in the inter-territorial matrix, but later it is mainly pericellular. Type V collagen is pericellular both in the chondrogenous layers and later in the articular cartilage, but is not present in the epiphyseal cartilage below. From 6 weeks onwards, types III and V collagens create a capsule around all the chondrocytes above the tidemark. The relationship of types V and XI collagens is discussed. It is concluded that the articular chondrocytes form a unique subset of cells from the earliest stages of joint formation in the fetal rabbit.