Involvement of vesicle coat material in casein secretion and surface regeneration.

Abstract

The ultrastructure of the apical zone of lactating rat mammary epithelial cells was studied, with emphasis on vesicle coat structures. Typical 40-60-nm ID "coated vesicles" were abundant, frequently associated with the internal filamentous plasma membrane coat or in direct continuity with secretory vesicles (SV) or plasma membrane proper. Bristle coats partially or totally covered membranes of secretory vesicles identified by their casein micelle content. This coat survived SV isolation. Exocytotic fusion of SV membranes and release of the casein micelles was observed. Frequently, regularly arranged bristle coat structures were identified in those regions of the plasma membrane that were involved in exocytotic processes. Both coated and uncoated surfaces of the casein-containing vesicles, as well as typical "coated vesicles," were frequently associated with microtubules and/or microfilaments. We suggest that coat materials of vesicles are related or identical to components of the internal coat of the surface membrane and that new plasma membrane and associated internal coat is produced concomitantly by fusion and integration of bristle coat moieties. Postexocytotic association of secreted casein micelles with the cell surface, mediated by finely filamentous extensions, provided a marker for the integrated vesicle membrane. An arrangement of SV with the inner surface of the plasma membrane is described which is characterized by regularly spaced, heavily stained membrane to membrane cross-bridges (pre-exocytotic attachment plaques). Such membrane-interconnecting elements may represent a form of coat structure important to recognition and interaction of membrane surfaces.

Rescue of mice homozygous for lethal albino deletions: implications for an animal model for the human liver disease tyrosinemia type 1.

“…Washing, postfixation in osmium tetroxide (2% in the same buffer), dehydration, embedding, and ultrathin sectioning were done as described previously (Franke et al 1976). …”

Serous Cells in the Parotid Glands of Two Species of Tamarins: Polarized Secretory Granules

“…It nonetheless is true that, in many species, spherules, sometimes lenticular in form, adhere to the bounding membrane of their encompassing granule (Tandler and Phillips, 1993). In the rat mammary gland, spherules-here called micelles-which consist of casein, are attached to the inner aspect of the granule membrane by fine filaments of unknown nature (Franke et al, 1976).…”

“…In the lactating rat mammary gland, secretory granules are partially or completely coated with bristles (Franke et al, 1976). Most of the granules in early stages of exocytosis illustrated by these workers have a partial bristle coat and it is this membrane zone that is in contact with the plasmalemma, strongly suggesting that these granules are in fact polarized.…”

Mammary gland secretion: hormonal coordination of endocytosis and exocytosis

“…The secretory vesicles of the MEC contain casein micelles, soluble proteins, lactose, ions, minerals and numerous biologically active molecules (Vilotte et al, 2003 and references therein). The morphological features of the association between secretory vesicles and the plasma membrane of the MEC have been extensively described (Franke et al, 1976). However, it is still not known whether the secretory vesicles localized in the vicinity of the apical membrane are docked and then readily releasable under stimulation.…”