Slices or sections through the bursa cloacalis and thymus of chick embryos at 7–21 days of incubation were observed by light and electron microscopy to determine whether major differences existed in the surface morphologies of lymphoid cells in these organs, and whether the surface morphologies of these cells changed during ontogeny. These organs were fixed concurrently and identically at each stage.
The thymus was packed at all stages with spherical cells having fine structures characteristic of those of lymphoid cells. Many irregularly shaped, epithelial cell processes were present between lymphoid cells.
The bursa contained many irregularly shaped stromal cells as well as spherical cells. The latter were few in number during early development, but became the predominant type of cell near the end of incubation. Spherical cells in the bursa consisted of three types based on fine structure: lymphoid cells, granulocytic cells, and cells which were probably precursors of granulocytic cells. Spherical cells in the bursa could not be classified into these three types by their surface morphologies, however, because the latter at any one stage of development were similar. At 7–8 days of incubation, spherical cells in the bursa could not be differentiated consistently from neighboring stromal cells by scanning electron microscopy alone, but by 9 days, spherical cells could be identified routinely by this method.
At 9–10 days of incubation, only minor differences existed in the surface morphologies of the spherical cells in the bursa and thymus: Bursal cells displayed long, ridgelike processes, whereas thymic cells exhibited fine surface undulations and large blebs. At 11 days, the surfaces of the spherical cells in the bursa were covered by numerous short microvilli, but the surfaces of thymic cells were unchanged. Bursal cells retained their microvilli through 14 days of incubation, but between 15 and 21 days progressively lost their microvilli, becoming essentially bald near the end of this period. Likewise, thymic cells gradually lost their surface wrinkles and blebs. Near the time of hatching, both types of cells were smoothsurfaced and tightly packed, with individual cells assuming polyhedral configurations.