The permeability of the pineal gland to lanthanum was investigated by means of transmission electron microscopy and X-ray microanalysis. Different pineal cell populations were studied. Light pinealocytes as well as glial cells were not permeated by the tracer lanthanum, which normally stays extracellularly. Dark pinealocytes showing different degrees of degenerative changes are permeated by lanthanum. The loss of plasma membrane integrity explains the entry of lanthanum into these cells. We conclude: (1) the lanthanum tracer can be used as an indicator of an alteration in membrane permeability, (2) dark pinealocytes appear as cells in the state of degeneration and represent a different physiological state than light pinealocytes.
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