Changes in ultrastructural characteristics of endolymphatic sac ribosome-rich cells of the rat during development.

It has recently been demonstrated that endolymphatic sac (ES) ribosome-rich (dark) cells respond to induced endolymph changes and are thus likely to be involved in endolymph homeostasis. Therefore, we studied the ultrastructural characteristics of rat ES ribosome-rich cells during development in order to determine the cellular distribution of organelles involved in protein metabolism, secretion and absorption, indicative for their contribution to endolymph homeostasis. During embryonal stages ribosome-rich cells contain a limited number and variety of organelles and are predominantly involved in the production of components for cell growth and differentiation. In the young adult stage (P60) three different states of ribosome-rich cells may be distinguished. State A resembles a cell with only limited metabolic activities whereas state B is characterized by numerous different intracellular organelles and is considered to be involved in production and secretion as well as absorption and degradation of complex proteins. A third cellular state, state C, is filled with phagolysosomes and contains very few other organelles. This is considered to be a final (pre)apoptotic state. Autoradiography data suggest that ES ribosome-rich cells are capable of synthesis and secretion of tyrosine-containing proteins and may thus be involved in regulation of the osmolarity of endolymph based on the capacity to bind cations as well as water molecules. In addition, ES ribosome-rich cells appear to synthesize and secrete fucosylated glycoproteins into the endolymph. In conclusion, the present data suggest that ES ribosome-rich cells are actively involved in endolymph homeostasis through secretion and absorption of complex proteins and it is hypothesized that they are able to adapt their function or activities in response to changes in endolymph composition.

Differences in endolymphatic sac mitochondria-rich cells indicate specific functions.

OBJECTIVE/HYPOTHESIS: The purpose of the study was to examine the specific involvement of endolymphatic sac mitochondria-rich cells in endolymph homeostasis. STUDY DESIGN: Transmission electron microscopy and immunohistochemistry were performed on the endolymphatic sac of young adult rats, and two important developmental stages were also investigated. METHODS: Ultrastructural characteristics of endolymphatic sac mitochondria-rich cells were studied more concisely and compared with renal mitochondria-rich cells (i.e., the intercalated cells). In addition, expression of cytokeratins 7 and 19 was determined. RESULTS: Until birth, only one type of mitochondria-rich cell is observed in the rat endolymphatic sac. In young adult animals, distinct differences in mitochondria-rich cell ultrastructure in the endolymphatic sac enables classification into subtypes or configurations. Comparison of endolymphatic sac mitochondria-rich cells with renal intercalated cells reveals striking similarities and provides additional information on their specific function in endolymph homeostasis. Furthermore, differences in cytokeratin expression are determined in endolymphatic sac mitochondria-rich cells. CONCLUSIONS: Differences in morphology of endolymphatic sac mitochondria-rich cells develop after birth and may reflect a distinct functional or physiological state of the cell. In analogy to renal intercalated cells, the distribution patterns of H+-adenosine triphosphatase and Cl-/HCO3- exchanger may differ between subtypes. We propose that subtype A mitochondria-rich cells, from which protruding A mitochondria-rich cells are the activated state, are involved in proton secretion (apical H+-adenosine triphosphatase) and thus are potential candidates for hearing loss accompanying renal tubular acidosis. Subtype B mitochondria-rich cells are the most likely candidates to be affected in Pendred syndrome because of the assumed function of pendrin as apical Cl-/HCO3- exchanger.