Aberrant death in dark chondrocytes of the avian growth plate

Growth plate chondrocytes of embryonic chick femurs were
examined by electron microscopy, cytophotometry and
autoradiography. Apart from the well-described `light'
chondrocyte, a different `dark' type of chondrocyte was
present, comprising 10 ± 35% of the cell population. They were
found at all stages of chondrocyte differentiation and in all
ages of the femurs studied. Well developed rough endoplasmatic reticulum and Golgi complex, many secretory vesicles,
energetically active mitochondria and a lot of glycogen,
indicating high activity of the cytoplasm, were combined with
low RNA synthesis, gentle margination and scattered
compaction of the chromatin. DNA cytometry revealed that
most of dark cells were diploid, but 15 ± 30% were tetraploid,
with the absence of an S-phase. Substantial loss of DNA was
found in about 10% of dark chondrocytes. The TUNEL reaction
demonstrated a limited number of DNA strand breaks.
Advanced dark cells possessed the nuclear features of both
apoptosis and necrosis. Besides chromomeric-chromonemic
compaction, a chromatin arrangement similar to that of
prometaphase and metaphase, as well as amitotic nuclear
segregation, all of them degenerative, were found. Our
interpretation is that the dark chondrocytes undergo an
aberrant type of cell death which may be combined with
aberrant cell cycle. Cell death of dark chondrocytes is
preceded by a pre-mortal burst of secretion.