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.
