The morphology and differentiation of stromal cells in the cortex of follicles in the bursa of Fabricius of the chicken

Mesenchymal reticular cells (MRCs) form a supporting system in the cortex of the bursal follicle. The stellate‐shaped MRCs exhibit a low electron density, which is helpful for their identification. A remarkable feature of MRC is the formation of multiple blebs in the nuclear envelope. The large, irregularly shaped blebs—which are perinuclear spaces—may be detached from the nuclear membrane, creating a sac‐like granular endoplasmic reticulum (GER). Inside the bleb, membrane‐bound bodies originate from cytoplasmic impressions. The cytoplasm contains a few round mitochondria, in which the internal membranes form either ovoid vesicles or the entire internal structure is indistinct. These mitochondria may be associated with the blebs. The classical Golgi complex with cis and trans faces cannot be recognized, but the accumulation of very small vesicles occurs around two or three stacked flat cisterns. The MRC forms a continuous layer along the corticomedullary basal lamina (CMBL), and during cell migration between the cortex and medulla, it may contribute to the temporary closure of the gap in the CMBL. At the outer surface of the cortex, transitory cells between the MRC and fibrocytes of the interfollicular connective tissue are present, and both cells can produce GER by blebbing. This finding suggests that MRCs and fibrocytes may have a common origin. The other stromal cell is the macrophage (Ma), which may fuse together to form multinucleated giant cells. The definition of histological classification of the third type of stromal cell is questionable, but certain morphological features may be referred to as progenitors of MRCs.