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Biomedical subjects

D Bok

Publications and source records attributed to D Bok.

121 records · Page 7Linked to original sources
PubMed1971

The osmotic behavior of rod photoreceptor outer segment discs.

The permeability properties of frog rod photoreceptor outer segment discs were investigated in preparations of purified, dark-adapted, outer segment fragments by the techniques of direct volume measurement and electron microscopy. Outer segment discs were found to swell and contract reversibly in response to changes in the osmotic pressure of the bathing medium in accordance with the Boyle-van't Hoff law. By use of the criterion of reversible osmotic swelling, the disc membrane is impermeable to Na(+), K(+), Mg(+2), Ca(+2), Cl(-), and (PO(4))(-3) ions, whereas it is freely permeable to ammonium acetate. The disc membrane is impermeable to sucrose, although its osmotic behavior towards this substance is different from its behavior towards impermeable ions. Electron microscopy showed that the osmotic effects on the rod outer segment fragments represent changes in the intradiscal volume. Fixation with glutaraldehyde did not abolish the permeability properties of the disc membrane, and fixed membranes were still capable of osmotic volume changes. It is concluded from this study that the frog's rod photoreceptor outer segment discs are free-floating membranous organelles with an inside space separate and distinct from the photoreceptor intracellular space.

Acetates↗
PubMed1971

The role of the pigment epithelium in the etiology of inherited retinal dystrophy in the rat.

Visual cell outer segment renewal was studied in eyes of mutant Royal College of Surgeons (RCS) and Sprague-Dawley (control) rats by a combination of microscopy and radioautography with the light and electron microscopes. RCS and control rats were injected with amino acids-(3)H at 11 days of age. Radioactive rod outer segment discs were assembled at the outer segment base from radioactive proteins synthesized in the rod inner segments. In controls, all radioactive discs assembled at 11 days of age were displaced the length of the outer segments, removed from outer segment tips, and phagocytized by the pigment epithelium by 8 days after injection. In the RCS rats, disc assembly and displacement resembled controls for the first 3 days after injection. However, as disc assembly continued for some time thereafter, a layer of labeled, disorganized, lamellar debris accumulated between the outer segment tips and the pigment epithelium. The buildup of debris was accompanied by visual cell death. At no time during the study was there evidence for phagocytic activity by the pigment epithelium. 61 days after injection, the layer of debris was the only heavily radioactive component in the retina. In the retina of RCS rats, the outer segment renewal mechanism malfunctions because the pigment epithelium does not fulfill its normal phagocytic role. The end result is visual cell death and blindness.

Age Factors↗
PubMed1969

Participation of the retinal pigment epithelium in the rod outer segment renewal process.

The disposal phase of the retinal rod outer segment renewal process has been studied by radioautography in adult frogs injected with tritiated amino acids. Shortly after injection, newly formed radioactive protein is incorporated into disc membranes which are assembled at the base of the rod outer segment. During the following 2 months, these labeled discs are progressively displaced along the outer segment owing to the repeated formation of newer discs. When the labeled membranes reach the end of the outer segment, they are detached from it. They subsequently may be identified in inclusion bodies within the pigment epithelium by virtue of their content of radioactivity. These inclusions have been termed phagosomes. Disc membrane formation is a continuous process, but the detachment of groups of discs occurs intermittently. The detached outer segment fragments become deformed, compacted, undergo chemical changes, and are displaced within the pigment epithelium. Ultimately, the material contained in the phagosomes is eliminated from the cell. In this manner the pigment epithelium participates actively in the disposal phase of the rod outer segment renewal process.

Animals↗
PubMed1968

Visual pigment renewal in the mature frog retina.

It has been demonstrated by autoradiography that radioactive amino acids serve as precursors for proteins which are subsequently incorporated into retinal rod outer segment discs in mature animals. By the isolation and purification of visual pigment from retinas of adult frogs after injection of tritiated leucine and phenylalanine, it has been shown that at least part of this labeled protein consists of visual pigment (rhodopsin).

Animals↗
PubMedDate not supplied

A report card on diversity: lessons for business from higher education.

Institutions of higher learning in the United States have long played a disproportionate role in supplying leadership talent to the world's business and professional organizations. For 30 years, the most selective schools have been working to increase diversity in their student bodies. New research by the former presidents of Princeton and Harvard suggests that the experiences and initiatives of these academic institutions can provide business leaders with insight into how to create diverse organizations that succeed. The first insight has to do with clarity of mission. It is not enough to pursue diversity because it is "the right thing to do." In an insert, Raymond Gilmartin, the CEO of Merck, echoes that view, discussing the relationship between diversity and Merck's competitiveness. The second insight concerns recruiting. The authors challenge what they call "the myth of pure merit," the notion that recruiting is a precise science based only on grades and test scores. Instead, they argue, merit is about assembling a team by deciding which applicants, considered individually and collectively, will contribute most to achieving the company's goals. The third insight concerns how organizations help employees perform to their potential. Of the factors contributing to high graduation rates at the most selective schools, higher expectations and the efforts of mentors stand out as most important. Finally, the fourth insight is about how to achieve accountability in a corporate setting. Boards must ask: Are our recruiting policies working? and How are recruited employees doing?

Administrative Personnel↗