The availab~lity of inorganic nitrogen in the surf-zone of Nahant Bay, Massachusetts, USA, was studied in relation to the distribution and decomposition of a free-living population of the filamentous brown alga Pilayella littoralis. Slight ammonium enrichment was evident in the surf zone of one beach location and in the immediate vicinity of a sewage treatment plant outfall. Profiles of sediment ammonium were determined over an annual cycle at 4 mid-intertidal beach sites. Most sites contained sediment ammonium from 0.05 to 0.1 pm01 cm-l, while one site with chronic algal deposition and burial showed greatly elevated sediment ammonium levels (5 6 pm01 cm-3). Efflux of ammonium from deep and shallow subtidal sediments and from intertidal surf-zone sediments was typically 5 1 0 0~m o l m-' h-', except at the site with chronic algal burial and decomposition, where flux of ammonium ranged from 200 to 2000 pm01 m-' h ' Sed~ment concentrations and benthic flux of ammonium from sandy subtidal and surf-zone sediments are generally low in Nahant Bay; however, beaches with chronic deposition and burial of free-living P. littoralis have very high ammonium content and flux owing to the decomposition of algae that have effectively concentrated nitrogen from a much larger area of the bay.

All too frequently, courses and clinical experiences are introduced in dental education merely becausse they seem to be the popular things to do. Unfortunately, however, just as frequently such experiences are inappropriately planned and developed; they fail to achieve the basic purpose behind their introduction. This study reports such a failure in dental education. A course in dental care for the special patient was hastily and inadequately conceived and implemented. Thecourse did not achieve its objective'in fact, had it been allowed to continuein subsequent years, it could have done more harm than good. The means exist for evaluating whatever id done, and the educator should be willing to accept failure as well as success. The recognition and identification of failure can infact serve a worthwhile purpose, to the advantage of the system, if the elementscontributing to the failure are identified and proper changes are introduced.

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To investigate the potential role of platelets in the inhibition of megakaryocytopoiesis, freeze-thawed extracts of human platelets were added to serumless liquid cultures of murine marrow. When acetylcholinesterase (AchE), a marker of megakaryocytic differentiation in mice, was assayed, a significant inhibition of enzymatic activity was noted in cultures containing the equivalent of greater than 5 X 10(6) solubilized platelets per milliliter. Freeze-thawed extracts of granulocytes had significantly less inhibitory effect than did platelets. Transforming growth factor beta (TGF-beta), a growth factor known to be inhibitory to some cell lineages and to be found at relatively high concentrations in platelets, was then added to liquid marrow cultures. A similar inhibition of AchE activity was detected when cultures were stimulated with mitogen-stimulated conditioned medium. The effect was potent with 50% inhibition of AchE activity observed at 4 pmol TGF-beta/L. To determine if TGF-beta inhibited specifically one aspect of megakaryocytic differentiation, the factor was added to isolated single megakaryocytes in serumless culture induced by interleukin 3 (IL3) to increase in size. The number of megakaryocytes increasing in size in response to IL 3 exposure was reduced from 68% to 20% when both factors were simultaneously added to cultures. Colony assays showed that megakaryocytic and granulocyte- macrophage colony detection was inhibited at picomolar concentrations of the factor. These data suggest that TGF-beta is a potent in vitro inhibitor of the murine megakaryocytic lineage, although its effects are not limited to this lineage.