<P> Often associated with aging and age - related diseases in vivo, senescent cells can be found in many renewable tissues, including the stroma, vasculature, hematopoietic system, and many epithelial organs . Resulting from accumulation over many cell divisions, senescence is often seen in age - associated degenerative phenotypes . Senescent fibroblasts in models of breast epithelial cell function have been found to disrupt milk protein production due to secretion of matrix metalloproteinases . Similarly, senescent pulmonary artery smooth muscle cells caused nearby smooth muscle cells to proliferate and migrate, perhaps contributing to hypertrophy of pulmonary arteries and eventually pulmonary hypertension . </P> <P> During skeletal myogenesis, cycling progenitor cells known as myoblasts differentiate and fuse together into non-cycling muscle cells called myocytes that remain in a terminal G phase . As a result, the fibers that make up skeletal muscle (myofibers) are cells with multiple nuclei, referred to as myonuclei, since each myonucleus originated from a single myoblast . Skeletal muscle cells continue indefinitely to provide contractile force through simultaneous contractions of cellular structures called sarcomeres . Importantly, these cells are kept in a terminal G phase since disruption of muscle fiber structure after myofiber formation would prevent proper transmission of force through the length of the muscle . Muscle growth can be stimulated by growth or injury and involves the recruitment of muscle stem cells--also known as satellite cells--out of a reversible quiescent state . These stem cells differentiate and fuse to generate new muscle fibers both in parallel and in series to increase force generation capacity . </P> <P> Cardiac muscle is also formed through myogenesis but instead of recruiting stem cells to fuse and form new cells, heart muscle cells--known as cardiomyocytes--simply increase in size as the heart grows larger . Similarly to skeletal muscle, if cardiomyocytes had to continue dividing to add muscle tissue the contractile structures necessary for heart function would be disrupted . </P> <P> Of the four major types of bone cells, osteocytes are the most common and also exist in a terminal G phase . Osteocytes arise from osteoblasts that are trapped within a self - secreted matrix . While osteocytes also have reduced synthetic activity, they still serve bone functions besides generating structure . Osteocytes work through various mechanosensory mechanisms to assist in the routine turnover over bony matrix . </P>

What type of cell enters the g0 phase