[Structural and functional characterization of cyst cells in Sarcocystis sp. ( Sporozoa, Apicomplexa)]. 
By means of light and electron microscopy, the structural pattern of muscle cysts (sarcocysts) was examined for the four species of the genus Sarcocystis: S. muris (from murine skeletal muscles), Sarcocystis sp.  and S. fusiformis (from, respectively, heart and skeletal muscles of buffalo), and S. ovifelis (from ovine tong muscles).  The orderly fashion of the interior of the cyst is attained by partitition of its space into numerous compartments with the involvement of the intermediate filaments.  These, in their turn, are bound to each other by thin filaments to make eventually a common filamentous net.  The net limits separate groups of cells referred to as cyst zoites.  The common net of filaments and microtubules (when present) may be regarded not only as the organizer of the cyst interior cytoskeleton, but also as the main mechanism of substance transportation in various directions: from the host cell to the sarcocyst, and within or outside the cyst.  The role of dedifferentiation, proliferation and differentiation processes is suggested in the establishment of the fixed sequence of events throughout the unidirectional development of cyst cells and their interaction, from precystic meronts to cyst merozoites (gamonts).  Special attention is paid to metrocyte morphogenesis and functioning.  In the present work, metrocytes subjected to apoptosis were recognized.  It is suggested that phenomenon of programmed cell death in metrocytes may be associated with the control of cell number in mature and ageing sarcocysts.