Plasma-membrane diversity in a highly polarized cell.

Abstract

In their classical experiments on membrane fluidity, Frye and Edidin (1) triggered further research on the fluid mosaic model of membrane structure (2) in numerous other laboratories . Since then, investigators in this field have also emphasized the planar, functionally specialized mosaics within the generally fluid bilayer of the membrane. These mosaics comprise areas where specific groups of lipids, proteins, glycoconjugates, and sterols aggregate to mediate specific functions . To cite several different facets of the membrane-domain concept, we can recall the now familiar work of Goldstein et al. (3) on the distinctive participation of coated pits in receptor-mediated endocytosis. Other thought-provoking membrane regionalities are the rosette-particle arrays instrumental in mucocyst secretion in Tetrahymena (4), the need for specific lipids for the complete activity of cytochrome c oxidase and other membrane-related enzymes (5), and the variations of phospholipid content in continuous membrane systems . (Consider the nuclear envelope and the rough-surfaced endoplasmic reticulumThe Symposium on Plasma-Membrane Diversity (see footnote) involved the existence and topography of such polysaccharide, lipid, and protein mosaics in the plasma membrane . We addressed ourselves to the broad questions : Do our methods and techniques verify the presence of domains in living cells? Where these membrane domains apparently exist, are their structure and composition pertinent to specific membrane-functions? How are domains sustained in a supposedly fluid environment? And when they reside in the depth as well as the plane of the bilayer, can we manipulate them to modify plasma-membrane and cell function?Plasma-membrane Diversity in a Highly Polarized CellSince motile sperm were first observed by Leowenhok via light microscopy more than 300 years ago, we have been aware that the spermatozoon is a highly polarized cell . Austin (7) and Bedford and Cooper (8), among others, authoritatively demonstrated that the various regions of this cell served different This work was presented in a Symposium

Ultrastructural description and cytochemical study of the spermatozoon of Crotallus durissus (Squamata, Serpentes)

“…In addition, sperm membrane cholesterol efflux regulates sperm capacitation during residence the female reproductive tract ( Travis and Kopf, 2002). The polyenic antibiotic filipin has been largely employed, mainly in freeze-fracture studies, to allow visualizing biological membrane sterols, based on the formation of filipin-sterol complexes ( Elias et al, 1979 andFriend, 1982). The presence of these complexes was identified in all the C. durissus spermatozoan regions.…”

Lipid lateral diffusion and membrane organization

“…sperm maturation and capacitation, early embryogenesis, tight junction formation in epithelia, highly polarized cells are able to overcome the randomizing effect of lateral diffusion and to localize certain membrane components to specific regions of the cell surface [26,32,33].…”

“…Thus, spermatozoa exhibit membrane regionalization, with different lipid and protein composition between the anterior and posterior head, midpiece and tail regions [26,33,46]. Correlatively, the lateral motion of the lipid probe Crh-di1 inserted into the plasma membrane of ejaculated ram sperm vary from one cell surface region to the other with respective D and M values of 6.3 u and 58% in the anterior head, 5.9 u and 71% in the posterior head, 9.3 u and 44% in the midpiece and 4.8 u and 58% in the tail [47].…”

Localized surface antigens of guinea pig sperm migrate to new regions prior to fertilization.

“…11 of these antigens have been identified as proteins. Evidence from other investigators indicates that these surface domains also differ in their lipid composition (5).…”

“…The arrangement of molecules into discrete domains on the surface of cells is now known to be of widespread occurrence (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11). Membrane topography may reflect the functional geometry of the cell; for example, in intestinal epithelial cells digestive enzymes are enriched on the apical surface (12).…”