Structural reorganization of the rough endoplasmic reticulum without size expansion accounts for dexamethasone-induced secretory activity in AR42J cells

Identification of the physiological substrates of PDIp, a pancreas-specific protein-disulfide isomerase family member

About 20 members of the protein-disulfide isomerase (PDI) family are present in the endoplasmic reticulum of mammalian cells. They are thought to catalyze thiol-disulfide exchange reactions within secretory or membrane proteins to assist in their folding or to regulate their functions. PDIp is a PDI family member highly expressed in the pancreas and known to bind estrogen in vivo and in vitro However, the physiological functions of PDIp remained unclear. In this study, we set out to identify its physiological substrates. By combining acid quenching and thiol alkylation, we stabilized and purified the complexes formed between endogenous PDIp and its target proteins from the mouse pancreas. MS analysis of these complexes helped identify the disulfide-linked PDIp targets in vivo, revealing that PDIp interacts directly with a number of pancreatic digestive enzymes. Interestingly, when pancreatic elastase, one of the identified proteins, was expressed alone in cultured cells, its proenzyme formed disulfide-linked aggregates within cells. However, when pancreatic elastase was co-expressed with PDIp, the latter prevented the formation of these aggregates and enhanced the production and secretion of proelastase in a form that could be converted to an active enzyme upon trypsin treatment. These findings indicate that the main targets of PDIp are digestive enzymes and that PDIp plays an important role in the biosynthesis of a digestive enzyme by assisting with the proper folding of the proenzyme within cells.

Identification of a functional enhancer variant within the chronic pancreatitis-associated SPINK1 c.101A>G (p.Asn34Ser)-containing haplotype

The haplotype harboring the SPINK1 c.101A>G (p.Asn34Ser) variant (also known as rs17107315:T>C) represents the most important heritable risk factor for idiopathic chronic pancreatitis identified to date. The causal variant contained within this risk haplotype has however remained stubbornly elusive. Herein, we set out to resolve this enigma by employing a hypothesis-driven approach. First, we searched for variants in strong linkage disequilibrium (LD) with rs17107315:T>C using HaploReg v4.1. Second, we identified two candidate SNPs by visual inspection of sequences spanning all 25 SNPs found to be in LD with rs17107315:T>C, guided by prior knowledge of pancreas-specific transcription factors and their cognate binding sites. Third, employing a novel cis-regulatory module (CRM)-guided approach to further filter the two candidate SNPs yielded a solitary candidate causal variant. Finally, combining data from phylogenetic conservation and chromatin accessibility, cotransfection transactivation experiments, and population genetic studies, we suggest that rs142703147:C>A, which disrupts a PTF1L-binding site within an evolutionarily conserved HNF1A-PTF1L CRM located ∼4 kb upstream of the SPINK1 promoter, contributes to the aforementioned chronic pancreatitis risk haplotype. Further studies are required not only to improve the characterization of this functional SNP but also to identify other functional components that might contribute to this high-risk haplotype.

ER sheet persistence is coupled to myosin 1c-regulated dynamic actin filament arrays

The endoplasmic reticulum (ER) comprises a dynamic three-dimensional (3D) network with diverse structural and functional domains. Proper ER operation requires an intricate balance within and between dynamics, morphology, and functions, but how these processes are coupled in cells has been unclear. Using live-cell imaging and 3D electron microscopy, we identify a specific subset of actin filaments localizing to polygons defined by ER sheets and tubules and describe a role for these actin arrays in ER sheet persistence and, thereby, in maintenance of the characteristic network architecture by showing that actin depolymerization leads to increased sheet fluctuation and transformations and results in small and less abundant sheet remnants and a defective ER network distribution. Furthermore, we identify myosin 1c localizing to the ER-associated actin filament arrays and reveal a novel role for myosin 1c in regulating these actin structures, as myosin 1c manipulations lead to loss of the actin filaments and to similar ER phenotype as observed after actin depolymerization. We propose that ER-associated actin filaments have a role in ER sheet persistence regulation and thus support the maintenance of sheets as a stationary subdomain of the dynamic ER network.

Identification of discrete sites in Yip1A necessary for regulation of endoplasmic reticulum structure

The endoplasmic reticulum (ER) of specialized cells can undergo dramatic changes in structural organization, including formation of concentric whorls. We previously reported that depletion of Yip1A, an integral membrane protein conserved between yeast and mammals, caused ER whorl formation reminiscent of that seen in specialized cells. Yip1A and its yeast homologue Yip1p cycle between the ER and early Golgi, have been implicated in a number of distinct trafficking steps, and interact with a conserved set of binding partners including Yif1p/Yif1A and the Ypt1/Ypt31 Rab GTPases. Here, we carried out a mutational analysis of Yip1A to obtain insight into how it regulates ER whorl formation. Most of the Yip1A cytoplasmic domain was dispensable, whereas the transmembrane (TM) domain, especially residues within predicted TM helices 3 and 4, were sensitive to mutagenesis. Comprehensive analysis revealed two discrete functionally required determinants. One was E95 and flanking residues L92 and L96 within the cytoplasmic domain; the other was K146 and nearby residue V152 within the TM domain. Notably, the identified determinants correspond closely to two sites previously found to be essential for yeast viability (E76 and K130 in Yip1p corresponding to E95 and K146 in Yip1A, respectively). In contrast, a third site (E89) also essential for yeast viability (E70 in Yip1p) was dispensable for regulation of whorl formation. Earlier work showed that E76 (E95) was dispensable for binding Yif1p or Ypt1p/Ypt31p, whereas E70 (E89) was required. Collectively, these findings suggest that the ability of Yip1A to bind its established binding partners may be uncoupled from its ability to control ER whorl formation. In support, Yif1A knockdown did not cause ER whorl formation. Thus Yip1A may use the sites identified herein to interact with a novel binding partner to regulate ER membrane organization.

Yip1A structures the mammalian endoplasmic reticulum

Yip1A depletion leads to reorganization of the ER into stacked and concentrically whorled membranes as well as a slowing of cargo export. The network dispersal function of Yip1A depends on a conserved residue. Thus, a conserved Yip1A-mediated ER network dispersal mechanism may regulate the protein export function of the organelle.

The structure of the endoplasmic reticulum (ER) undergoes highly regulated changes in specialized cell types. One frequently observed type of change is its reorganization into stacked and concentrically whorled membranes, but the underlying mechanisms and functional relevance for cargo export are unknown. Here, we identify Yip1A, a conserved membrane protein that cycles between the ER and early Golgi, as a key mediator of ER organization. Yip1A depletion led to restructuring of the network into multiple, micrometer-sized concentric whorls. Membrane stacking and whorl formation coincided with a marked slowing of coat protein (COP)II-mediated protein export. Furthermore, whorl formation driven by exogenous expression of an ER protein with no role in COPII function also delayed cargo export. Thus, the slowing of protein export induced by Yip1A depletion may be attributed to a proximal role for Yip1A in regulating ER network dispersal. The ER network dispersal function of Yip1A was blocked by alteration of a single conserved amino acid (E95K) in its N-terminal cytoplasmic domain. These results reveal a conserved Yip1A-mediated mechanism for ER membrane organization that may serve to regulate cargo exit from the organelle.

How and why does the endoplasmic reticulum move?

The ER (endoplasmic reticulum) is a fascinating organelle that is highly dynamic, undergoing constant movement and reorganization. It has many key roles, including protein synthesis, folding and trafficking, calcium homoeostasis and lipid synthesis. It can expand in size when needed, and the balance between tubular and lamellar regions can be altered. The distribution and organization of the ER depends on both motile and static interactions with microtubules and the actin cytoskeleton. In the present paper, we review how the ER moves, and consider why this movement may be important for ER and cellular function.

Intracellular autoactivation of human cationic trypsinogen mutants causes reduced trypsinogen secretion and acinar cell death

Mutations in the activation peptide of human cationic trypsinogen have been found in patients with chronic pancreatitis. Previous biochemical studies demonstrated that mutations p.D19A, p.D22G, and p.K23R strongly stimulate trypsinogen autoactivation. In the present study, we characterized the cell biological effects of these mutants using human embryonic kidney 293T and AR42J rat acinar cells. We found that relative to wild-type trypsinogen, secretion of the mutants from transfected cells was markedly decreased. This apparent secretion defect was completely rescued by inhibition of autoactivation via (1) inclusion of the small molecule trypsin inhibitor benzamidine in the growth medium; or (2) cotransfection with the physiological trypsin inhibitor SPINK1; or (3) by mutation of the catalytic Ser(200) residue in trypsinogen. In contrast, extracellularly added SPINK1 or other nonpermeable proteinaceous trypsin inhibitors did not restore normal secretion of the mutants, indicating that intracellular autoactivation is responsible for the observed secretion loss. Acinar cells expressing the p.D22G mutant detached from the culture plate over time, became terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive, and exhibited elevated levels of the proapoptotic transcription factor CHOP. The observations indicate that activation peptide mutants of human cationic trypsinogen undergo autoactivation intracellularly, which leads to decreased trypsinogen secretion and eventual acinar cell death. The results thus define a novel pathological pathway for parenchymal injury in hereditary chronic pancreatitis.

The plant endoplasmic reticulum: a cell-wide web

The ER (endoplasmic reticulum) in higher plants forms a pleomorphic web of membrane tubules and small cisternae that pervade the cytoplasm, but in particular form a polygonal network at the cortex of the cell which may be anchored to the plasma membrane. The network is associated with the actin cytoskeleton and demonstrates extensive mobility, which is most likely to be dependent on myosin motors. The ER is characterized by a number of domains which may be associated with specific functions such as protein storage, or with direct interaction with other organelles such as the Golgi apparatus, peroxisomes and plastids. In the present review we discuss the nature of the network, the role of shape-forming molecules such as the recently described reticulon family of proteins and the function of some of the major domains within the ER network.

Role of kinesin-1 and cytoplasmic dynein in endoplasmic reticulum movement in VERO cells

Generating the extended endoplasmic reticulum (ER) network depends on microtubules, which act as tracks for motor-driven ER tubule movement, generate the force to extend ER tubules by means of attachment to growing microtubule plus-ends and provide static attachment points. We have analysed ER dynamics in living VERO cells and find that most ER tubule extension is driven by microtubule motors. Surprisingly, we observe that approximately 50% of rapid ER tubule movements occur in the direction of the centre of the cell, driven by cytoplasmic dynein. Inhibition of this movement leads to an accumulation of lamellar ER in the cell periphery. By expressing dominant-negative kinesin-1 constructs, we show that kinesin-1 drives ER tubule extension towards the cell periphery and that this motility is dependent on the KLC1B kinesin light chain splice form but not on KLC1D. Inhibition of kinesin-1 promotes a shift from tubular to lamellar morphology and slows down the recovery of the ER network after microtubule depolymerisation and regrowth. These observations reconcile previous conflicting studies of kinesin-1 function in ER motility in vivo. Furthermore, our data reveal that cytoplasmic dynein plays a role in ER motility in a mammalian cultured cell, demonstrating that ER motility is more complex than previously thought.

Climp-63-mediated binding of microtubules to the ER affects the lateral mobility of translocon complexes

Microtubules are frequently seen in close proximity to membranes of the endoplasmic reticulum (ER), and the membrane protein CLIMP-63 is thought to mediate specific interaction between these two structures. It was, therefore, of interest to investigate whether these microtubules are in fact responsible for the highly restricted lateral mobility of the translocon complexes in M3/18 cells as described before. As determined by fluorescence recovery after photobleaching, the breakdown of microtubules caused by drug treatment or by overexpression of the microtubule-severing protein spastin, resulted in an increased lateral mobility of the translocons that are assembled into polysomes. Also, the expression of a CLIMP-63 mutant lacking the microtubule-binding domain resulted in a significant increase of the lateral mobility of the translocon complexes. The most striking increase in the diffusion rate of the translocon complexes was observed in M3/18 cells transfected with a siRNA that effectively knocked down the expression of the endogenous CLIMP-63. It appears, therefore, that interaction of microtubules with the ER results in the immobilization of translocon complexes that are part of membrane-bound polysomes, and may play a role in the mechanism that segregates the rough and smooth domains of the ER.

Active relocation of chromatin and endoplasmic reticulum into blebs in late apoptotic cells

Plasma membrane blebbing is a defining characteristic of apoptosis, but its significance is not understood. Using live-cell imaging we have identified two phases of apoptotic blebbing. The early phase is restricted to adherent cells, and is prevented by the Rho-activated kinase inhibitor Y27632. The late phase is partially resistant to Y27632, and generates morphologically distinct membrane protrusions that are likely precursors to apoptotic bodies. Late blebbing is observed in all apoptotic cells tested. It occurs at a fixed period before phosphatidyl serine exposure, indicating that it is a universal and important feature of apoptosis. Late blebs contain a cortical layer of endoplasmic reticulum that often surrounds condensed chromatin, while other organelles are excluded. The appearance in some apoptotic cells of partially formed sheets of endoplasmic reticulum suggest that these cortical layers are newly formed by the remodelling of the endoplasmic reticulum of interphase cells. Formation of endoplasmic reticulum and chromatin-containing blebs requires both actin and microtubules, and is prevented by the caspase-6 inhibitor zVEID.fmk.

Expression of pro-Muclin in pancreatic AR42J cells induces functional regulated secretory granules

It is not clear how protein cargo is sorted to and retained in forming regulated secretory granules (RSG). Here, the sulfated mucin-type glycoprotein pro-Muclin was tested for its ability to induce RSG in the poorly differentiated rat pancreatic cell line AR42J. AR42J cells express RSG content proteins, but they fail to make granules. Adenovirus-pro-Muclin-infected AR42J cells store amylase, accumulate RSG, and respond to hormonal stimulation by secreting the stored protein. Expression of pro-Muclin combined with the inducing effect of dexamethasone resulted in a significant enhancement of the efficiency of regulated secretion. The effect of pro-Muclin was a strong decrease in constitutive secretion compared with dexamethasone-induction alone. A pro-Muclin construct missing the cytosolic tail domain was less effective at improving the efficiency of regulated secretion compared with the full-length construct. Increased expression of cargo (using adenovirus amylase) also modestly enhanced regulated secretion, indicating that part of pro-Muclin's effect may be due to increased expression of cargo protein. Overall, the data show that pro-Muclin acts as a sorting receptor that can induce RSG, and that its cytosolic tail is important in this process.

Active translocon complexes labeled with GFP-Dad1 diffuse slowly as large polysome arrays in the endoplasmic reticulum

In the ER, the translocon complex (TC) functions in the translocation and cotranslational modification of proteins made on membrane-bound ribosomes. The oligosaccharyltransferase (OST) complex is associated with the TC, and performs the cotranslational N-glycosylation of nascent polypeptide chains. Here we use a GFP-tagged subunit of the OST complex (GFP-Dad1) that rescues the temperature-sensitive (ts) phenotype of tsBN7 cells, where Dad1 is degraded and N-glycosylation is inhibited, to study the lateral mobility of the TC by FRAP. GFP-Dad1 that is functionally incorporated into TCs diffuses extremely slow, exhibiting an effective diffusion constant (Deff) about seven times lower than that of GFP-tagged ER membrane proteins unhindered in their lateral mobility. Termination of protein synthesis significantly increases the lateral mobility of GFP-Dad1 in the ER membranes, but to a level that is still lower than that of free GFP-Dad1. This suggests that GFP-Dad1 as part of the OST remains associated with inactive TCs. Our findings that TCs assembled into membrane-bound polysomes diffuse slowly within the ER have mechanistic implications for the segregation of the ER into smooth and rough domains.

Caveolin-1 binding to endoplasmic reticulum membranes and entry into the regulated secretory pathway are regulated by serine phosphorylation. Protein sorting at the level of the endoplasmic reticulum

Caveolin-1 serves as the main coat protein of caveolae membranes, as an intracellular cholesterol shuttle, and as a regulator of diverse signaling molecules. Of the 12 residues conserved across all caveolin isoforms from all species examined to date, only Ser(80) and Ser(168) could serve as phosphorylation sites. We show here that mimicking chronic phosphorylation of Ser(80) by mutation to Glu (i.e. Cav-1(S80E)), blocks phosphate incorporation. However, Cav-1(S168E) is phosphorylated to the same extent as wild-type caveolin-1. Cav-1(S80E) targets to the endoplasmic reticulum membrane, remains oligomeric, and maintains normal membrane topology. In contrast, Cav-1(S80A), which cannot be phosphorylated, targets to caveolae membranes. Some exocrine cells secrete caveolin-1 in a regulated manner. Cav-1(S80A) is not secreted by AR42J pancreatic adenocarcinoma cells even in the presence of dexamethasone, an agent that induces the secretory phenotype. Conversely, Cav-1(S80E) is secreted to a greater extent than wild-type caveolin-1 following dexamethasone treatment. We conclude that caveolin-1 phosphorylation on invariant serine residue 80 is required for endoplasmic reticulum retention and entry into the regulated secretory pathway.

Enhanced binding to the molecular chaperone BiP slows thyroglobulin export from the endoplasmic reticulum

To examine how binding of BiP (a molecular chaperone of the hsp70 family that resides in the endoplasmic reticulum) influences the conformational maturation of thyroglobulin (Tg, the precursor for thyroid hormone synthesis), we have developed a system of recombinant Tg stably expressed in wild-type Chinese hamster ovary (CHO) cells and CHO-B cells genetically manipulated for selectively increased BiP expression. The elevation of immunoreactive BiP in CHO-B cells is comparable to that seen during the unfolded protein response in the thyrocytes of certain human patients and animals suffering from congenital hypothyroid goiter with defective Tg. However, in CHO-B cells, we expressed Tg containing no mutations that induce misfolding (i.e. no unfolded protein response), so that levels of all other endoplasmic reticulum chaperones were normal. Increased availability of BiP did not accelerate Tg secretion; rather, the export of newly synthesized Tg was delayed. Tg detained intracellularly was concentrated in the endoplasmic reticulum. By coimmunoprecipitation, BiP exhibited enhanced binding to Tg in CHO-B cells. Moreover, two-dimensional gel analysis showed that BiP associated especially well with intracellular Tg containing mispaired disulfide bonds, thought to represent early Tg folding intermediates. An endoplasmic reticulum chaperone of the hsp90 family, GRP94, was also associated in Tg-chaperone complexes. The results suggest that increased binding of BiP to Tg leads to its delayed conformational maturation in the endoplasmic reticulum.

Thyroglobulin transport along the secretory pathway. Investigation of the role of molecular chaperone, GRP94, in protein export from the endoplasmic reticulum

GRP94 serves as a molecular chaperone in the endoplasmic reticulum (ER). In normal thyrocytes, GRP94 interacts transiently with thyroglobulin (Tg), and in thyrocytes of animals suffering from congenital hypothyroid goiter with defective thyroglobulin, GRP94 and thyroglobulin associate in a protracted fashion. In order explore possible consequences of GRP94 binding, we have studied recombinant nonmutant thyroglobulin expressed in control Chinese hamster ovary (CHO) cells in comparison to that produced in CHO cells genetically manipulated for selectively increased GRP94 expression. Levels of ER chaperones other than GRP94 did not detectably differ, and thyroglobulin achieved transport competence in both kinds of CHO cells. However, increased availability of GRP94 caused the residence time of Tg in the ER to be remarkably prolonged. This was accompanied by a major increase in Tg directly associated with GRP94 and an increase in the ER pool size of Tg. Importantly, co-immunoprecipitation analysis revealed disulfide-linked Tg complexes (previously reported as an early Tg-folding intermediate) especially associated with GRP94. Indeed, non-native Tg, GRP94, and a 78-kDa protein likely to be BiP, appeared in ternary complexes. Under these conditions, GRP94 association appears directly involved in prolongation of Tg folding and export, consistent with a role in quality control in the ER.

Formation of crystalloid endoplasmic reticulum in COS cells upon overexpression of microsomal aldehyde dehydrogenase by cDNA transfection

When rat liver microsomal aldehyde dehydrogenase (msALDH) was overexpressed in COS-1 cells by cDNA transfection, large granular structures containing both msALDH and endogenous protein disulfide isomerase appeared (Masaki et al. (1994) J. Cell Biol. 126, 1407–1420). Confocal laser microscopy revealed that these granular structures are dispersed throughout the cytoplasm. Electron microscopy showed that the structures are composed of regularly arranged crystalloid smooth endoplasmic reticulum (ER). The formation of the crystalloid ER was accompanied by a remarkable proliferation of smooth ER, which appeared occasionally continuous to the rough ER. We suggest that the smooth ER, proliferated from the rough ER, is transformed and assembled into the crystalloid ER by head-to-head association of the msALDH molecules on the apposed smooth ER membranes. In order to understand the molecular mechanism of the crystalloid ER formation, we asked which portions of the msALDH molecules are needed for the crystalloid ER formation by expressing deletion mutants or chimera protein of msALDH in COS-1 cells. The overexpression of msALDH molecules lacking the stem region preceding the membrane spanning region, although they were exclusively localized in the ER, did not induce the formation of crystalloid ER. More detailed analysis showed that the amino acid sequence FFLL, located in the stem region, is necessary to form the crystalloid ER. The chimera protein containing the last 35 amino acids of msALDH at the carboxyl terminus of chloramphenicol acetyltransferase was localized to the ER, but did not induce the formation of the crystalloid ER. These results suggest that at least two regions, the bulky amino-terminal region and the FFLL sequence in the stem region of msALDH molecules are required for the formation of the crystalloid ER.

Corticosteroids increase procollagen gene expression, synthesis, and secretion by human intestinal smooth muscle cells

BACKGROUND & AIMS

Collagen synthesis by smooth muscle cells plays an important role in intestinal fibrosis. Corticosteroids inhibit collagen synthesis in fibroblasts. The aim of this study was to examine the effect of corticosteroids on the expression of collagen by human intestinal smooth muscle (HISM) cells in vitro.

METHODS

Collagen synthesis was determined by the sensitivity of radiolabeled protein to collagenase. Secretion was determined by polyacrylamide gel electrophoresis of radiolabeled procollagen in the medium. Procollagen messenger RNA was determined by Northern blot analysis.

RESULTS

Collagen synthesis by confluent HISM cells was not affected by corticosteroids at 10(-10) to 10(-5) mol/L but, in subconfluent cultures, was nonspecifically increased 50% at 10(-5) mol/L. Procollagen secretion was nonspecifically increased 60% at 10(-6) mol/L dexamethasone without any effect on the type III/I ratio. Procollagen I and III messenger RNA levels responded in a biphasic manner: a 45%-65% increase at 10(-10) mol/L and a 15% and 30% decrease at 10(-8) and 10(-6) mol/L. In fibroblasts, collagen synthesis was inhibited 85% by dexamethasone, procollagen secretion was decreased 70%, the type III/I ratio decreased from 70:1 to 18:1, and procollagen messenger RNA was inhibited 25% and 60% (types I and III).

CONCLUSIONS

Collagen expression by HISM cells is refractory to corticosteroids and, at certain concentrations, is augmented.

Membrane traffic motors

There is a wealth of data suggesting that microtubules and associated motor proteins play important roles in orchestrating membrane traffic within higher eukaryotes, with myosins and actin filaments fulfilling similar functions in organisms such as fungi, algae and plants. In addition, evidence is accumulating that both cytoskeletal systems can co-operate within one cell. Recent studies have highlighted how individual motor proteins can act at multiple steps in the membrane-traffic pathways, and in contrast, how more than one motor type may be involved in each transport step and in generating organelle morphology.

Cytological changes related to maturation of the organ of Corti and opening of Corti's tunnel

Maturation of the organ of Corti in the gerbil was analyzed between 2 and 16 days after birth (DAB) by electron microscopy and immunostaining for beta-tubulin. At 2 DAB, the organ of Corti consisted of stratified epithelium bearing immature sensory hair cells (HCs) and supporting cells. Maturation of OHCs and Deiters cells progressed in a medial-to-lateral direction and cytoskeletal development in inner pillar cells preceded that in outer pillar cells at the single location studied along the frequency-place map. Pillar cell differentiation progressed through a unique stage characterized by the appearance and stratification of structural features apparently concerned with opening of Corti's tunnel and subsequently showed other structural changes related to maturity toward the adult form. Development of the microtubule cytoskeleton occurred first in the cell's apex and proceeded basally. Ruffling of a middle region of the cell surface by microvilli appeared to promote separation between inner and outer pillar cells and initiate tunnel opening at 4 DAB. Proliferation of distended cisternae of granular reticulum evidenced proteinaceous secretion by these cells between 4 and 8 DAB. Subsequent tunnel expansion at about 14 DAB coincided with appearance in outer pillar cells of tubulocisternal endoplasmic reticulum and associated Golgi complexes that are thought to mediate fluid and ion secretion. Sixteen days postnatally after disappearance of granular and tubulocisternal reticula and Golgi complexes and at the time of clearing of tunnel fluid, lysosomes interpreted as mediating catabolism of endocytosed protein congregated beneath the apical and apicolateral plasmalemmae of inner pillar cells. As with pillar cells, development of the microtubule system in Deiters cells proceeded from the cell's apex to base. Following differentiation of their microtubule system by 8 DAB, Deiters cells showed expansion of Golgi cisternae between 10 and 15 DAB and development of tubulocisternal endoplasmic reticulum at 15 DAB. Hair cells possessed abundant, distinctively large mitochondria from 4 to 10 DAB. The subsurface cisternae matured earlier in medial as opposed to lateral outer hair cells. Vesicles budding from underlying cisternae appeared associated with development of subsurface cisternae and at 16 DAB were still observed in third row but not in more mature first row HCs.