Transport of casein submicelles and formation of secretion granules in the golgi apparatus of epithelial cells of the lactating mammary gland of the rat

Abstract

Lactating mammary glands fixed by perfusion with 5% glutaraldehyde subsequently were postfixed with potassium ferrocyanide reduced osmium or were treated with tannic acid. Stained thin sections were examined with the electron microscope and stereopairs were prepared. The distribution of casein submicelles was analyzed in the various components of the Golgi apparatus. The Golgi stacks were composed of five or six elements, all of which contained casein submicelles 20 nm in diameter. The cis-tubular network or cis-element, as well as the underlying three or four midsaccules, showed these casein submicelles either attached to their membrane or free in the lumen. The trans-most element of the stacks formed distended prosecretory granules in which both isolated or clustered casein submicelles were suspended in an electron-lucent fluid. These micellar aggregates increased in size and became progressively more compact to form spherical dense bodies or casein micelles, in which the individual 20 nm particles could easily be resolved. Casein micelles were seen in secretory granules in addition to a wispy material of low density. The numerous small spherical vesicles (80 nm or larger) seen on the cis, lateral, or trans aspects of the stacks did not appear to contain free casein submicelles. This raises questions regarding the role of these vesicles in the transport of casein macromolecules through the Golgi stacks. It was noticeable that in this Golgi apparatus a trans-Golgi network was limited to a few small residual tubules free from casein submicelles. It thus appears that the greater part of the trans-most Golgi element gives rise to the large prosecretory granules. After leaving the Golgi region and prior to exocytosis, the secretory granules often fuse to form larger granules before exocytosis.

Lipid-deprived diet perturbs O-glycosylation of secretory proteins in rat mammary epithelial cells

“…Obviously, the trans Golgi cisternae and secretory vesicles are swollen and distended in lactating MECs (e.g. see Clermont et al, 1993). The substantial increase in lactose concentration in the milk from rats receiving the lipid-deprived diet could therefore result from the decrease in the volume of milk produced by these animals, coupled with the maintenance of lactose synthesis at high level.…”

A cisternal maturation mechanism can explain the asymmetry of the Golgi stack

“…For example, in certain algae, large glycoprotein 'scales' are synthesized in the Golgi and then transported to the cell surface; these scales can be detected only within Golgi cisternae and never in the associated transport vesicles [I 3]. Similar observations were made with mammalian cells that transport lipoprotein particles, casein submicelles or virus particles through the Golgi stack [14][15][16]. Other studies indicated that Golgi enzymes are not segregated into precisely defined subcompartments, but instead form overlapping concentration gradients within the stack [17 -20].…”

“…For example, electron microscopy studies suggest that certain kinds of secretory cargo are absent from COPI vesicles, or are much less concentrated in these vesicles than in Golgi cisternae [13][14][15] (J. Bergeron, personal communication). However, some secretory proteins can be packaged into COPI vesicles [11,12,22,24].…”

Mechanisms of transport through the Golgi complex

“…4B). Indeed, for large cargo aggregates, such as collagen, a number of EM studies seemed to provide evidence for these large aggregates remaining in cisternae as they traversed the Golgi stack (Bonfanti et al, 1998;Clermont et al, 1993;Melkonian et al, 1991;Mironov et al, 2001). Proof for the idea of (B) Cisternal-maturation model.…”