What are synovial joints surrounded by?

Synovial joints are characterized by a structure that includes a joint capsule, which is essential for their function and stability. The joint capsule is a fibrous connective tissue that surrounds the synovial joint, providing support and protection to the joint structure. This capsule encloses the joint space and is lined with a synovial membrane, which secretes synovial fluid. This fluid lubricates the joint, allowing for smooth movement between the articulating bones.

The joint capsule not only helps to maintain the integrity of the joint but also plays a crucial role in containing synovial fluid, ensuring that the joint functions properly during movement. The presence of this capsule also differentiates synovial joints from other types of joints, such as fibrous or cartilaginous joints, which do not have such a structure.

In contrast, while muscle tissue and bone tissue are associated with joints, they do not encapsulate or surround the joint itself. Fluid sacs may refer to structures like bursae that reduce friction, but they are not the main component that surrounds the joint. The defining feature of synovial joints is indeed the joint capsule, making it the correct answer.