Microglia, astrocytes, and macrophages react differentially to central and peripheral lesions in the developing and mature rat whisker-to-barrel pathway: a study using immunohistochemistry for lipocortin1, phosphotyrosine, s100 beta, and mannose receptors

Adult and neonatal rats were subjected to transection of the left infraorbital nerve or ablation of the left parietal cortex. The ensuing glial reaction in the whisker-to-barrel pathway was studied with immunohistochemistry for Lipocortin1- (LC1+), phosphotyrosine- (PY+), S100 beta- (S100 beta+), and mannose receptor- (MR+) immunoreactive microglia, astrocytes, and macrophages. Four days after infraorbital nerve transection in adult rats, LC1+ and PY+ microglia were prominently increased in the trigeminal sensory brain-stem nuclei on the deafferented side compared with the intact side. Changes were negligible at the second synapse of the pathway, i.e., the thalamic ventroposterior medial nucleus. Cortical ablation in adults led to an increase in microglia in the ipsilateral ventroposterior medial nucleus that reciprocally connects with the ablated cortex. Moreover, microglial reactions occurred in the contralateral trigeminal sensory brain-stem nuclei in which corticofugal projections from the ablated cortex terminate. S100 beta+ astrocytes, in contrast, appeared unaltered after both types of lesion in adults. In neonates, LC1+, PY+, and S100 beta+ cells did not have the adult morphology of microglia or astrocytes. Four days after nerve transection, LC1+ and PY+ cells were sparse and remained unchanged. In contrast, S100 beta+ cells substantially increased in the deafferented trigeminal brain-stem nuclei. Four days after cortical ablation in neonates, LC1+, PY+, and S100 beta+ cells had accumulated in the deprived thalamus. In contrast to adults, many of these cells were MR+ macrophages. In the deprived brain-stem, only S100 beta+ cells increased and none were macrophages. Therefore, macrophages do not appear to stem from microglia, and neonatal LC1+, PY+, and S100 beta+ cells may possess functions different from those in adults.

Genetic deletion of COX-2 prevents increased renin expression in response to ACE inhibition

Cyclooxygenase-2 (COX-2) is expressed in macula densa (MD) and surrounding cortical thick ascending limb of the loop of Henle (cTALH) and is involved in regulation of renin production. We and others have previously found that selective COX-2 inhibitors can inhibit renal renin production (Cheng HF, Wang JL, Zhang MZ, Miyazaki Y, Ichikawa I, McKanna JA, and Harris RC. J Clin Invest 103: 953-961, 1999; Harding P, Sigmon DH, Alfie ME, Huang PL, Fishman MC, Beierwaltes WH, and Carretero OA. Hypertension 29: 297-302, 1997; Traynor TR, Smart A, Briggs JP, and Schnermann J. Am J Physiol Renal Physiol 277: F706-F710, 1999; Wang JL, Cheng HF, and Harris RC. Hypertension 34: 96-101, 1999). In the present studies, we utilized mice with genetic deletions of the COX-2 gene in order to investigate further the potential role of COX-2 in mediation of the renin-angiotensin system (RAS). Age-matched wild-type (+/+), heterozygotes (+/-), and homozygous null mice (-/-) were administered the angiotensin-converting enzyme inhibitor (ACEI), captopril, for 7 days. ACEI failed to significantly increase plasma renin activity, renal renin mRNA expression, and renal renin activity in (-/-) mice. ACEI increased the number of cells expressing immunoreactive renin in the (+/+) mice both by inducing more juxtaglomerular cells to express immunoreactive renin and by recruiting additional renin-expressing cells in the more proximal afferent arteriole. In contrast, there was minimal recruitment of renin-expressing cells in the more proximal afferent arteriole of the -/- mice. In summary, these results indicate that ACEI-mediated increases in renal renin production were defective in COX-2 knockout (K/O) mice and provide further indication that MD COX-2 is an important mediator of the renin-angiotensin system.

Role of p38 in the regulation of renal cortical cyclooxygenase-2 expression by extracellular chloride

We have previously shown that in renal cortex, COX-2 expression is localized to macula densa and surrounding cortical thick ascending limb of Henle (cTALH). Dietary salt restriction increases local expression of COX-2, which mediates renin production and secretion. Given that decreased luminal chloride [Cl(-)] at the level of the macula densa increases renin production and secretion, we investigated the role of extracellular ion concentration on COX-2 expression. Quiescent rabbit cTALH cells were incubated in a physiological salt solution containing high or low levels of NaCl. Immunoreactive COX-2 expression increased significantly in the low NaCl solution. COX-2 expression also increased after administration of the Na(+)/K(+)/2Cl(-) cotransport inhibitor, bumetanide. Selective substitution of chloride led to increased COX-2 expression, whereas selective substitution of sodium had no effect. The p38 MAP kinase inhibitor PD169316 decreased low NaCl-induced COX-2 expression. Low-salt or low-chloride medium induced cultured cTALH to accumulate >/= 3-fold higher levels of pp38, the activated (phosphorylated) form of p38; low-salt medium also increased pJNK and pERK levels. Feeding rats a low-salt diet for 14 days induced a significant increase in renal cortical pp38 expression, predominantly in the macula densa and cTALH. These results suggest that reduced extracellular chloride leads to increased COX-2 expression, which may be mediated by activation of a p38-dependent signaling pathway.

Nitric oxide regulates renal cortical cyclooxygenase-2 expression

We have previously shown that cyclooxygenase-2 (COX-2) is localized to the cortical thick ascending limb of the loop of Henle (cTALH)/macula densa of the rat kidney, and expression increases in response to low-salt diet and/or angiotensin-converting enzyme (ACE) inhibition. Because of the localization of neuronal nitric oxide synthase (nNOS) to macula densa and surrounding cTALH, the present study investigated the role of nitric oxide (NO) in the regulation of COX-2 expression. For in vivo studies, rats were fed a normal diet, low-salt diet or low-salt diet combined with the ACE inhibitor captopril. In each group, one-half of them were treated with the nNOS inhibitors 7-nitroinidazole (7-NI) or S-methyl-thiocitrulline. Both of these NOS inhibitors inhibited increases in COX-2 mRNA and immunoreactive protein in response to low salt and low salt+captopril. For in vitro studies, COX-2 expression was studied in primary cultures of rabbit cTALH cells immunodisssected with Tamm-Horsfall antibody. Basal COX-2 immunoreactivity expression was stimulated by S-nitroso-N-acetyl-penicillamine (SNAP), an NO donor, and intracellular cGMP concentration. The cultured cells expressed immunoreactive nNOS, and 7-NI inhibited basal COX-2 immunoreactivity expression, which could be partially overcome by cGMP. In summary, these studies indicate that NO is a mediator of increased renal cortical COX-2 expression seen in volume depletion and suggest important interactions between the NO and COX-2 systems in the regulation of arteriolar tone and the renin-angiotensin system by the macula densa.

Microglia in organotypic hippocampal slice culture and effects of hypoxia: ultrastructure and lipocortin-1 immunoreactivity

Lipocortin-1 immunocytochemistry was used to study the various cell forms of microglia that appear during organotypic hippocampal tissue culture, as well as in the in vitro toxic hypoxia model. Antibodies against lipocortin-1 identified activated and phagocytic cells that were abundant in a slice after the plating of a culture: cells of the intermediate form at the later time-points of culturing, resting ramified microglia beginning from the seventh day of culturing, as well as activated and phagocytic cells that appeared in the slice after experimental toxic hypoxia induced by potassium cyanide treatment. Lipocortin-1-positive microglia cell forms corresponded well to the description of the microglia in vivo, and the morphology of microglia corresponded to the circumstances under which these cells were observed in slice cultures. Electron microscopic studies have demonstrated, for the first time, that microglia in organotypic slice culture preserve morphological features typical of different microglial forms in vivo, as well as specific contacts and interactions with the other neural tissue elements. After experimental toxic hypoxia, rapid changes in microglial ultrastructure and localization were observed, reminiscent of in vivo models of ischaemia. In conclusion, observations of microglial morphology and behaviour allow us to suggest that microglia in the organotypic culture preserve their essential characteristic features and properties, thus providing an important model system for studying the structure and function of these cells.

Microglia and astrocytes in the adult rat brain: comparative immunocytochemical analysis demonstrates the efficacy of lipocortin 1 immunoreactivity

The distribution of glial cells (microglia and astrocytes) in different regions of normal adult rat brain was studied using immunohistochemical techniques and computer analysis. Lipocortin 1, phosphotyrosine, and lectin GSA B(4), were used for identification of microglia, while S100beta and glial fibrillary acidic protein identified astrocytes. Bioquant computerized image analysis was used to quantify and map the immunostained cells in sections from adult rat brain. If lipocortin 1 was used as a marker, more microglial cells were detected than with phosphotyrosine or lectin. The lipocortin 1-positive microglial population was most numerous (on average, 130+/-5 cells/mm(2) of the brain section area) in neostriatum, and least (51+/-4 cells/mm(2)) in cerebellum and medulla oblongata. In general, the density of lipocortin 1 microglia was higher in the forebrain, and lower in the midbrain, and the least in the brainstem and cerebellum. The number of S100beta astrocytes was two to three times larger than the number of microglial cells, and approximately two times greater than glial fibrillary acidic protein cells. A high density of astrocytes was found in the hypothalamus and hippocampus (more than 260 cells/mm(2)); they were more numerous in the white matter than in the gray matter. Fewer astrocytes were observed in the cerebral cortex, neostriatum, midbrain, medulla oblongata and cerebellum (less than 200 cells/mm(2)). Thus lipocortin 1 and S100beta were shown to be the most specific and reliable markers for microglia and astrocytes, respectively. The regional population differences demonstrated for lipocortin 1 microglia and S100beta astrocytes presumably reflect structural and functional specializations of the certain brain regions.

Cyclooxygenase-2-selective inhibitors impair glomerulogenesis and renal cortical development

BACKGROUND

Antenatal exposure to nonsteroidal anti-inflammatory drugs (NSAIDs) has been associated with renal dysgenesis in humans.

METHODS

These studies characterized cyclooxygenase-2 (COX-2) versus COX-1-selective inhibition on nephrogenesis in the rodent using histomorphometry, immunohistology, and in situ hybridization.

RESULTS

Administration of a COX-2-selective inhibitor (SC58236), started during pregnancy until weaning, significantly impaired development of the renal cortex and reduced glomerular diameter in both mice and rats. An identical phenotype was demonstrated in COX-2 -/- mice. In contrast to its effects on the developing kidney, a COX-2 inhibitor had no effect on glomerular volume in adult mice. This effect was specific for COX-2 because maternal administration of a COX-1-selective inhibitor (SC58560) did not affect renal development despite significantly inhibiting gastric mucosal prostaglandin E2 (PGE2) synthesis in pups. The expression of COX-2 immunoreactivity peaked in the first postnatal week and was localized to S-shaped bodies and the macula densa in the cortex. Treatment with a COX-2 inhibitor during this period (from postnatal day 0 to day 21) severely reduced glomerular diameter, whereas treatment limited to pregnancy did not affect glomerular size.

CONCLUSION

These data demonstrate an important role for COX-2 activity in nephrogenesis in the rodent, and define a specific time period of susceptibility to these effects.

Interactions of the renin-angiotensin system and neuronal nitric oxide synthase in regulation of cyclooxygenase-2 in the macula densa

Cyclooxygenase-2 (COX-2) expression in rat kidney is localized to the macula densa and the immediately proximal cTALH and increases after salt restriction. Either ACE inhibitors or AT1 receptor blockers increase COX-2 expression in both control and salt-restricted animals, suggesting that the RAS activation feedback inhibits renal cortical COX-2 expression. To determine whether increased COX-2 expression in response to ACE inhibition mediated increases in renin production, rats were treated with Captopril for 1 week with or without the specific COX-2 inhibitor, SC58236. Plasma renin activity increased significantly in the Captopril group. This increase was partially reversed by simultaneous treatment with SC58236. Kidney renin activity also increased in the Captopril group compared with control, which was also significantly inhibited by SC58236 treatment. Because of the localization of bNOS to MD and surrounding cTALH, the current study investigated the role of NO in the regulation of COX-2 expression. Rats were fed a normal diet, low salt diet or low salt diet combined with captopril and half of them were treated with the neuronal NOS inhibitor, 7-NI, and half with vehicle. After 7 days, mRNA was extracted and the microsome proteins purified from renal cortex. COX-2 mRNA expression was measured by Northern-blot and normalized with GAPDH. 7-NI treatment decreased COX-2 mRNA and immunoreactive COX-2 expression in each group. In summary, these studies indicate that COX-2 from macula densa/cTALH is a regulator of renin production and release. Angiotensin II may be a negative regulator of cTALH/macula densa COX-2 expression, and NO may mediate increased renal cortical COX-2 expression seen in volume depletion. These studies suggest important interactions between the NO and COX-2 systems in the regulation of arteriolar tone and the renin-angiotensin system by the macula densa.

Regulation of cyclooxygenase-2 (COX-2) in rat renal cortex by adrenal glucocorticoids and mineralocorticoids

Production of prostaglandins involved in renal salt and water homeostasis is modulated by regulated expression of the inducible form of cyclooxygenase-2 (COX-2) at restricted sites in the rat renal cortex. Because inflammatory COX-2 is suppressed by glucocorticoids, and prostaglandin levels in the kidney are sensitive to steroids, the sensitivity of COX expression to adrenalectomy (ADX) was investigated. By 2 weeks after ADX in mature rats, cortical COX-2 immunoreactivity increased 10-fold in the cortical thick ascending limb and macula densa. The constitutive isoform, COX-1, was unchanged. The magnitude of the changes and specificity of COX-2 immunoreactivity were validated by in situ hybridization histochemistry of COX-2 mRNA and Western blot analysis. Increased COX-2 activity (>5-fold) was documented by using a specific COX-2 inhibitor. The COX-2 up-regulation in ADX rats was reversed by replacement therapy with either corticosterone or deoxycorticosterone acetate. In normal rats, inhibition of glucocorticoid receptors with RU486 or mineralocorticoid receptors with spironolactone caused up-regulation of renal cortical COX-2. These results indicate that COX-2 expression in situ is tonically inhibited by adrenal steroids, and COX-2 is regulated by mineralocorticoids as well as glucocorticoids.

Angiotensin II attenuates renal cortical cyclooxygenase-2 expression

We have previously shown that in rat renal cortex, cyclooxygenase-2 (COX-2) expression is localized to cTALH cells in the region of the macula densa, and that dietary salt restriction increases COX-2 expression. Administration of the angiotensin converting inhibitor, captopril, further increased COX-2 mRNA and renal cortical COX-2 immunoreactivity, with the most pronounced expression in the macula densa. Administration of an AT1 receptor antagonist, losartan, also significantly increased cortical COX-2 mRNA expression and COX-2 immunoreactivity. Mutant mice homozygous for both Agtr1a and Agtr1b null mutations (Agtr1a-/-,Agtr1b-/-) demonstrated large increases in immunoreactive COX-2 expression inthe cTALH/macula densa. To determine whether increased COX-2expression in response to ACE inhibition mediated increases in renin production, rats were treated with captopril for one week with or without the specific COX-2 inhibitor, SC58236. Plasma renin activity increased significantly in the captropril group, and this increase was significantly inhibited by simultaneous treatment with SC58236. Thus, these studies indicated that angiotensin II inhibitors augment upregulation of renal cortical COX-2 in states of volume depletion, suggesting that negative feedback by the renin-angiotensin system modulates renal cortical COX-2 expression and that COX-2 is a mediator of increased renin production in response to inhibition of angiotension II production.

Selective increase of cyclooxygenase-2 expression in a model of renal ablation

Previous studies have suggested a possible role for prostaglandins (PGs) in mediating alterations in nephron structure and function ensuing after renal ablation. Two isoforms of cyclooxygenase (COX) have been described: constitutive (COX-1) and inducible (COX-2). We examined expression of these isoforms following subtotal renal ablation (5/6 ablation, RA) in rats. In renal cortex, COX-2 mRNA and immunoreactive protein (IP) increased progressively compared with sham-operated littermates. In contrast, there were no significant changes in COX-1 mRNA expression. In normal kidney, cortical COX-1 IP was immunolocalized predominantly to mesangial cells and collecting tubules, whereas COX-2 IP was found in a subset of cortical thick ascending limb of Henle's loop (CTAL) cells in the region of the macula densa (MD). Following RA, significantly increased COX-2 IP was detected in the MD and surrounding CTAL cells. In addition, fainter immunoreactive COX-2 was detected in scattered visceral epithelial cells and mesangial cells of the glomerulus. Immunoblotting of isolated glomeruli demonstrated a selective increase of glomerular immunoreactive COX-2 expression following RA. No change of COX-1 expression was seen. To determine COX activity, isolated glomeruli were incubated with arachidonic acid and PGE2 measured by enzyme immunoassay (EIA). Compared with sham, glomeruli from 2 wk RA produced significantly more PGs. SC-58560, a selective COX-1 inhibitor, did not inhibit PG production in the remnant glomeruli at concentrations up to 10(-4) M, whereas SC-58236, a relatively selective COX-2 inhibitor, significantly inhibited PG production by RA glomeruli. In preliminary studies, to define mechanisms of altered expression of glomerular COX-2, rat mesangial cells were incubated with serum from sham or 2 wk RA. There were significant increases in COX-2 expression in response to 2 wk RA serum. In summary, these results indicate selective increases in renal cortical COX-2 expression following renal ablation.

Prostaglandins in macula densa function

Cyclooxygenase (COX)-2 mRNA and immunoreactive protein localize to the macula densa and adjacent cortical thick ascending limb in renal cortex, and chronic NaCl restriction increases expression of this enzyme. These findings suggest an integral role for eicosanoids generated by macula densa-associated COX-2 in mediating renin release. As selective inhibitors of COX-2 become available, it will be important to assess their effects on the renin-angiotensin system and glomerular hemodynamics.

Constitutive expression of cyclooxygenase-2 in rat vas deferens

Prostaglandins, lipoid substances discovered in human semen as modulators of uterine muscle contractility, are known to play significant roles in virtually all mammalian organ systems, but their male reproductive functions are unclear. Cyclooxygenase, the rate-limiting enzyme in prostaglandin synthesis, occurs in two isoforms distinguished on the basis of constitutive (COX-1) or inducible (COX-2) expression patterns in mammalian tissues. However, in the adult rat male reproductive system, immunohistochemistry and Western and Northern analysis showed that COX-2 is the predominant isoform and is heavily localized to the epithelium of the distal vas deferens, where constitutive expression is manyfold greater than in any other organs of the body. COX-2 is not detected in the proximal one-half of the vas nor in the testis, epididymis, seminal vesicles, or prostate. Elimination of luminal sperm by vasectomy does not affect COX-2 levels, whereas castration severely depletes COX-2 and androgen replacement after castration restores COX-2, indicating that COX-2 expression in the vas is androgen dependent. Because the distal vas also comprises an extensive submucosal venous plexus connected to the penile corpora cavernosa, prostaglandins from the vas may play a role in erection.

Cyclooxygenase-2 in rat nephron development

The inducible second isoform of cyclooxygenase (COX-2) that mediates inflammation also is expressed at low levels in normal adult rat kidneys and is upregulated in response to noninflammatory stimuli (R. C. Harris, J. A. McKanna, Y. Akai, H. R. Jacobson, R. N. DuBois, and M. D. Breyer, J. Clin. Invest. 94: 2504-2510, 1994). Roles in morphogenesis are indicated by reported teratogenicity of COX inhibitors and renal dysgenesis in COX-2 knockout mice (J. E. Dinchuk, B. D. Car, R. J. Focht, J. J. Johnston, B. D. Jaffee, M. B. Covington, N. R. Contel, V. M. Eng, R. J. Collins, P. M. Czerniak, A. G. Stewart, and J. M. Trzaskos, Nature 378: 406-409, 1995; S. G. Morham, R. Lagenbach, C. D. Loftin, H. F. Tiano, N. Vouloumanos, J. C. Jennette, J. F. Mahler, K. D. Kluckman, A. Ledford, C. A. Lee, and O. Smithies. Cell 83: 473-482, 1995). Blots from developing rat kidneys demonstrated that COX-2 mRNA and immunoreactive protein were present in neonates, peaked in the 2nd and 3rd postnatal weeks and declined to adult levels by the 3rd month. Immunolocalization and in situ hybridization detected intense COX-2 immunoreactivity and mRNA in a subset of thick ascending limb epithelial cells near the macula densa in each developing nephron; after 2 wk the COX-2 gradually waned. These data demonstrate that COX-2 expression is subject to normal developmental regulation and can be sustained over extended periods; they also support the conclusion that metabolites of COX-2 play important roles in the differentiation and early functions of mammalian nephrons.

Infraorbital nerve transection and whisker follicle removal in adult rats affect microglia and astrocytes in the trigeminal brainstem. A study with lipocortin1- and S100beta-immunohistochemistry

Transections of the infraorbital nerve in adult rats resulted in progressive alterations of microglia identified by Lipocortinl immunoreactivity at the sites where the primary afferents terminate, i.e. in the trigeminal brainstem sensory nuclei. Microglia proliferated three- to four-fold. Their cell bodies enlarged and their processes thickened. Microglial responses were similar to the removal of whisker follicles. However, they were restricted to discrete nuclear subregions that matched with the known whisker somatotopy. Astrocytes identified by S100beta immunoreactivity showed minor increases in size and in population density. No microglial or astrocytic reactions were found in the second and third synaptic relays of the somatosensory pathway. Because both types of lesion reportedly lead to the reorganization of primary afferents, our results establish the two experimental designs as valuable tools to elucidate the role of microglia and Lipocortin1 in adult brain plasticity.

Gliogenesis in postnatal rat optic nerve: LC1 + microglia and S100-beta + astrocytes

Lipocortin 1 (LC1) and S100-beta, two Ca(2+)-binding proteins that serve as specific markers for microglia and astrocytes, respectively, have been used to study postnatal gliogenesis in the rat optic nerve. Computerized image analysis was used to quantify and map the stained and unstained glia in transverse sections (10 microns thick) taken 1-2 mm from the chiasm in optic nerves from rat pups at postnatal day 0 (P0), P7, P14, P21, P28, P38 and adults. The number of astrocytes was remarkably constant (100 per section) at all ages. Because the area of the nerve increases 10-fold from P0 to adult, the population density of astrocytes begins al > 5000 mm-2 and drops to 400 mm-2 in the mature nerve; however, because the nerve length increases two-fold, the number of astrocytes doubles over the same period. In contrast, the number of LC1 + cells per section initially is sparse (4 at P0), increases rapidly up to 36 at P21 and levels off at 49 in adults. The microglia population density is relatively stable throughout development (200-300 mm-2) except during the peak of oligodendroblast apoptosis (P21) when it rises to 450 mm-2. Neonatally, LC1 immunoreactivity predominantly labels spherical-ameboid cells; but by P28 they are replaced by mature ramified microglia. The number of unstained cells (putative oligodendrocytes) per section increases from 11 at P0 to a peak of 308 at P21, and declines slightly to 269 in adults. While generally confirming concepts of astrocyte and oligodendrocyte ontogeny from the literature, the present report adds considerable detail regarding microglia, which often have been ignored. Microglia identified by LC1 immunoreactivity comprise 12% of the glia in adult optic nerve near the chiasm.

Production of heparin binding epidermal growth factor-like growth factor in the early phase of regeneration after acute renal injury. Isolation and localization of bioactive molecules

We have recently reported that heparin-binding epidermal growth factor-like growth factor (HB-EGF) mRNA is induced in the rat kidney after acute ischemic injury. The present studies were designed to investigate whether bioactive HB-EGF protein is also produced in response to renal injury induced by either ischemia/reperfusion or aminoglycosides. Heparin-binding proteins were purified from kidney homogenates by heparin affinity column chromatography using elution with a 0.2-2.0 M gradient of NaCl. A single peak of proteins that eluted at 1.0-1.2 M NaCl was detected in the postischemic kidney within 6 h of injury. This eluate fraction stimulated DNA synthesis in quiescent Balb/c3T3, RIE, and NRK-52E cell lines, all of which are responsive to the epidermal growth factor family of mitogenic proteins. The EGF receptor of A431 cells was also tyrosine phosphorylated by this eluate peak. Furthermore, immunoblotting with a polyclonal antibody against rat HB-EGF indicated that the eluate peak contained immunoreactive proteins of 22 and 29 kD mol wt, consistent with the reported sizes of the secreted form and membrane anchored form of HB-EGF, respectively. Immunohistochemical studies revealed that HB-EGF was produced predominantly in distal tubules in kidneys injured either by ischemia/reperfusion or aminoglycoside administration. We also found that during metanephric development immunoreactive HB-EGF was detected in the ureteric bud as early as E14.5 and persisted in structures arising from the ureteric bud throughout embryogenesis. These results suggest that in response to acute injury, HB-EGF is produced predominantly in distal tubules and that endogenous HB-EGF may be an important growth factor involved in renal epithelial cell repair, proliferation, and regeneration in the early stages of recovery after acute renal injury, as well as in nephrogenesis.

Immunohistochemical localization of lipocortin 1 in rat brain is sensitive to pH, freezing, and dehydration

Lipocortin 1 (LC1, annexin 1) has received considerable attention as a substrate for protein kinases, as a Ca++- and phosphatidylserine-binding protein, and as a mediator of glucocorticoid anti-inflammatory effects. However, there has been confusion over localization of LC1 immunoreactivity (LC1-ir), which reportedly localizes to neurons and/or to astrocytes or microglia in rat brain. To test whether these contradictory data arise from unusual properties of the antigen, we developed a novel brain slice model to determine fixation and staining variables. The specificity of anti-LC1 sera was ensured by pre-absorption and affinity purification with immobilized recombinant LC1. Specific LC1-ir was detected in ramified microglia of brains perfused with acidified aldehydes and embedded in paraffin. However, commonly used immunohistochemical procedures have unexpected profound effects. LC1-ir was eliminated by fixation with neutral/alkaline aldehydes, by freezing before strong acid-aldehyde fixation, or by staining without partial de/rehydration before the primary serum. The sensitivity of LC1 epitopes to proton and water activities may reflect molecular properties important to LC1's roles in vivo. True LC1-ir was not detected in normal neurons or astrocytes.

Lipocortin 1 in apoptosis: mammary regression

BACKGROUND

Enigmatically, degradation of debris generated in programmed cell death (apoptosis) elicits little inflammation. Having previously detected the upregulation of lipocortin 1 (LC1), a 35-kDa protein with anti-inflammatory and immuno-suppressive properties, at sites of non-inflammatory phagocytosis in the central nervous system (J Neurosci Res 36:491-500, 1993), we sought to determine if LC1 was involved in apoptosis.

METHODS

LC1 immunoreactivity in mammary glands of adult rats was quantified in situ using video microdensitometry before and during postlactational regression.

RESULTS

LC1 is present in the mammary ducts but is absent from the alveoli during lactation. One day after weaning, however, LC1 is detected in the lactiferous cells and, as apoptosis proceeds over the ensuing 4 days, total LC1 in the gland increases > 10-fold over resting levels. LC1 remains high in both the apoptotic cells and epithelial phagocytes through day 10, but the total LC1 per gland drops as the apoptotic cells are cleared.

CONCLUSIONS

Published experiments have shown that LC1 specifically binds Ca++ and phosphatidylserine, and that these affinities are modulated by tyrosine phosphorylation and cross-linking with transglutaminase. Thus, LC1 appears to be a candidate for several putative activities in apoptosis (e.g., phagocyte recognition via phosphatidylserine binding and/or buffering intracellular Ca++) in addition to its anti-inflammatory role.

Cyclooxygenase-2 is associated with the macula densa of rat kidney and increases with salt restriction

The kidney is a rich source of prostaglandins. These eicosanoids, formed by cyclooxygenase-dependent metabolism of arachidonic acid, are important physiologic mediators of renal glomerular hemodynamics and tubular sodium and water reabsorption. Two separate isoforms of cyclooxygenase (COX) have now been identified: constitutive COX-1, encoded by a 2.8-kb mRNA, and mitogen-activated COX-2, encoded by a 4.0-4.5-kb mRNA. COX-2 expression increases during development and inflammation, but, except for brain, constitutive expression is low. It has been generally accepted that physiologic renal production of prostaglandins is mediated by COX-1. However, in the absence of inflammation, low levels of COX-2 mRNA are also detectable in the kidney. To examine the role of COX-2 in the kidney and determine its intrarenal localization, we used a 1.3-kb cDNA probe specific for the 3' untranslated region of rat COX-2 and COX-2-specific antiserum. The COX-2-specific cDNA probe hybridized with a 4.4-kb transcript in total RNA from adult rat kidney. Immunoblots of microsomes isolated from kidney cortex and papilla indicated immunoreactive COX-2 in both locations. In situ hybridization and immunohistochemistry indicated that renal cortical COX-2 expression was localized to the macula densa of the juxtaglomerular apparatus and to adjacent epithelial cells of the cortical thick ascending limb of Henle. In addition, COX-2 immunoreactivity was detected in interstitial cells in the papilla. No COX-2 message or immunoreactive protein was detected in arterioles, glomeruli, or cortical or medullary collecting ducts. When animals were chronically sodium restricted, the level of COX-2 in the region of the macula densa increased threefold (from 0.86 +/- 0.08 to 2.52 +/- 0.43/mm2) and the total area of the COX-2 immunoreactive cells in cortex increased from 34 microns2/mm2 of cortex to 226 microns2/mm2 of cortex. The intrarenal distribution of COX-2 and its increased expression in response to sodium restriction suggest that in addition to its proposed role in inflammatory and growth responses, this enzyme may play an important role in the regulation of salt, volume, and blood pressure homeostasis.

Lipocortin I immunolocalization in normal and hydropic guinea pig ears

Annexins are a family of eight highly conserved proteins that bind phospholipids in the presence of calcium. One of these proteins, lipocortin I, has restricted distribution in adult and developing tissues, suggesting regulatory function. Among the protean roles theorized are regulation of inflammation by influencing eicosanoid production, participation in endo- and exocytosis, and control of membrane permeability. Such processes could have important roles in the inner ear; therefore, we investigated the patterns of lipocortin I expression in the normal guinea pig ear. Lipocortin I appeared at high levels in nonsensory, endolymph-facing tissues. Perilymph-facing cells had little lipocortin I activity. Lipocortin I was minimally expressed, or is absent, in sensory cells of the cochlea and vestibular systems. In the kidney, changes in quantity and distribution of lipocortin I have been seen during recovery from acute tubular necrosis. In an attempt to gain insight into the role of lipocortin I, the authors investigated its response to an experimental insult. Surgically created endolymphatic hydrops was chosen as an insult involving the endolymph-producing epithelia that richly express lipocortin I. Comparing unilaterally created hydrops, for up to 3 weeks' duration, to contralateral control ears demonstrated no quantitative or distribution changes in lipocortin I.

Lipocortin 1 immunoreactivity identifies microglia in adult rat brain

Immunohistochemical localization of two Ca(++)-binding proteins, Lipocortin 1 (LC1) and S100-beta, demonstrates two distinct classes of primitive glia in the floor plate of rat embryos. With proper fixation (formalin-lysine-periodate-acetic acid), dendritic glia in the CNS of adult rats also apparently stain for either LC1 or S100-beta in the ratio of 1:3. In order to further distinguish and identify these two glial classes, we have examined their population density, topography, and responses to localized neuron death. Neurons of the ipsilateral thalamus undergo apoptosis following cortical ablation; the contralateral thalamus serves as control. By eight days post-lesion, the number of LC1 cells in the ipsilateral thalamus has increased > 4-fold, the increase comprising primarily activated phagocytes adjacent to degenerating neurons. The S100-beta glia in the same region are virtual- ly indistinguishable from control; but background staining (apparently representing extra-cellular S100-beta) is increased. Thus, the responses of dendritic LC1 glia resemble these previously described for microglia and are quite different from the astrocytes identified by S100-beta immunoreactivity. Both dendritic and activated forms of LC1 glia stain with the microglial marker, Griffonia simplicifolia iso-lectin B4. However, before the correspondence of LC1 glia and microglia can be confirmed, two anomalies require resolution: (1) the LC1 glia are greater in number and more evenly distributed than microglia marked with other methods; (2) the dendritic LC1 glia apparently are progeny of primitive glia that form the midline raphe of the embryonic floor plate. The participation of LC1 glia in the removal of CNS debris supports the hypothesis that LC1 plays anti-inflammatory and/or immunosuppressive roles in phagocytes.

Localization of p35 (annexin I, lipocortin I) in normal adult rat kidney and during recovery from ischemia

The 35-kDa protein (p35, lipocortin I, annexin I), originally discovered as a Ca(++)-dependent substrate for the EGF receptor tyrosine kinase, binds Ca++ and phospholipids, is developmentally regulated in embryos and has restricted expression in adults. Immunohistochemistry of normal rat kidney shows that p35 is enriched in epithelia of Bowman's capsule, the macula densa, and medullary/papillary collecting ducts, suggesting that p35 is related to specialized renal functions. Light staining is observed in the thick ascending limb; elsewhere, immunoreactivity is nil. Since renal recovery from ischemia involves both hyperplasia and hypertrophy and reportedly is accelerated by EGF, we examined p35 distribution during this process. After 48 hours of recovery, both the distribution and amount of renal p35 are altered. Immunoblots show p35 levels increased at least threefold in whole-kidney homogenates. The expression of p35 is still highly restricted in recovering kidneys; however, the thick ascending limb now stains heavily. This is the first documentation of alterations in annexin levels during a pathophysiologic response. However, our attempts to discern effects of exogenous EGF on the recovery from ischemia were negative for both mitotic index and renal function assays.

Optic chiasm and infundibular decussation sites in the developing rat diencephalon are defined by glial raphes expressing p35 (lipocortin 1, annexin I)

p35, a Ca(++)-phospholipid-binding protein that serves as a substrate for the EGF receptor tyrosine kinase, is expressed by primitive glial ependymal cells to define a raphe occupying the ventral midline in the spinal cord and hindbrain of rat embryos (McKanna and Cohen, Science 243:1477-1479, 1989). p35 appears transiently in the median one-third (80 microns) of the floor plate at precisely the time and place where axons cross to form the ventral commissure. We postulated that if p35 is involved with commissure development, homologous p35 raphes might be found at decussation sites rostral to the floor plate, including the optic chiasm. The present report describes two developmentally regulated p35 raphes in the diencephalon. One raphe is present for 2-3 days at the rostral lip of the nascent infundibulum, the reported decussation site of axons running from the supraoptic nucleus to the neurohypophysis; the other raphe appears in the rostral two-thirds of the optic chiasm, the site traversed by the optic axons. p35 is never expressed in the caudal one-third of the chiasm that accommodates non-retinal axons. To the best of our knowledge, this is the first identification of a specific marker for the retinal component of the optic chiasm. Because the p35 is gone by embryonic day 18.5, it is absent during final stages of chiasm formation when axons from the temporal retina decussate. Thus, p35 also may contribute to the "barrier" perceived by fibers that remain ipsilateral. Our data suggest that the p35 raphe contributes to the midline's role in commisure morphogenesis. Putative lipocortin activities including regulating PLA2, eicosanoids, or intracellular Ca++ could be involved in altering cue specificity as decussating axon growth cones traverse the p35 compartment.

A computer-assisted direct-imaging system to obtain numerical densities of neurons in human cortex

Studies of the numerical density of microscopic items in brain tissue is a time-consuming endeavor. However, such information is important for numerous issues such as the relationship between structure and function in the normal brain, individual differences, and studies of brains of neuropsychiatric patients. A computer-assisted imaging system specifically devised to obtain estimates of numerical densities in human cortex is described here. Its main advantage is that the microscopist can analyze the original image directly under the microscope, and most aspects of data acquisition and quantitative analysis are accomplished by the computer. The key features of the system are a Microvid (an electronic camera lucida) and the use of X, Y, and Z stage encoders in conjunction with three-dimensional computer software. The complete system is relatively inexpensive and is simple to set up and use. The reliability and validity of the numerical densities obtained using this system are documented.

Morphologic changes in human carcinoma cells (A-431) stimulated by epidermal growth factor: effect of cholesterol and low-density lipoproteins on the ruffling response

Stimulation of A-431 carcinoma cells with epidermal growth factor (EGF) causes dramatic morphologic responses including ruffling, rounding, and bulk-phase pinocytosis. In attempts to explore the mechanisms responsible for changes in plasmalemma topography, we have investigated the effects of exogenous sterols thought to alter membrane fluidity. Light and scanning electron microscopy revealed a time- and concentration-dependent inhibition of ruffling (greater than 90%) by cholesterol. This effect could be duplicated by preincubation of the cells with comparable levels of low-density lipoproteins (LDL). EGF-stimulated bulk-phase endocytosis also is inhibited by treatment with cholesterol. No alteration of EGF binding, kinase stimulation, or internalization was detected in cells incubated in cholesterol-enriched medium (175 micrograms/ml in 0.5% ethanol), nor did cholesterol or LDL have any effect on EGF-stimulated rounding. Morphometry of electron micrographs from cholesterol-treated cells revealed a selective depletion of interdigitating lateral surface membrane that normally appears to be recruited to generate apical ruffles. Thus, the sterol inhibition of ruffling may be due to redistribution of plasmalemma rather than to changes in membrane viscosity. Together with previous observations, these data suggest that EGF-stimulated ruffling and bulk-phase pinocytosis are related phenomena, whereas EGF-stimulated cell rounding is an independent process.

The EGF receptor kinase substrate p35 in the floor plate of the embryonic rat CNS

P35 is a calcium- and phospholipid-binding protein that was originally isolated as a substrate for the epidermal growth factor (EGF) receptor tyrosine kinase and later was found to be related to lipocortin I. Immunohistochemistry was used to localize p35 to a raphe of primitive glial ependymal cells in the median one-third of the floor plate in the central nervous system (CNS) of rat embryos. The p35 appears by embryonic day 12 before the arrival of pioneering ventral commissural axons. The unexpected, discrete distribution of this protein during development opens the question of its role in neural morphogenesis.

Early changes in experimental hydrocephalus

In acute obstruction of the cerebral spinal fluid (CSF) absorption pathways, fluid is produced more rapidly than it is absorbed, and the ventricles enlarge proximal to the obstructions. Communicating hydrocephalus results from a difference between the rates of production and absorption of cerebrospinal fluid. In animals with chronic communicating hydrocephalus, the initial pathologic changes appear to involve the periventricular tissue near the angles of the lateral ventricles. The present investigation was designed to identify the various changes associated with the production of communicating hydrocephalus in acutely hydrocephalic preparations and to relate these findings to those found in experimental animals with chronic communicating hydrocephalus. The results of this study seem to confirm that the changes noted in the chronically hydrocephalic animals occur as early as 12 hours after the restriction of the normal flow of CSF.

A luteinizing hormone-releasing hormone agonist for the prevention of chemotherapy-induced ovarian follicular loss in rats

In an attempt to prevent chemotherapy-induced ovarian follicular loss, [D-Leu6, des-Gly10-NH2]-luteinizing hormone-releasing hormone ethylamide (LHRHa) was given subcutaneously to Sprague-Dawley cycling female rats in two daily doses of 2.5 micrograms starting 2 days prior to and concomitant with cyclophosphamide (CTX) (5 mg/kg/day for 21 days). Four groups of female cycling rats (10 in each) received either no treatment, CTX alone, CTX + LHRHa, or LHRHa alone. One ovary from each animal was serially sectioned, stained, and examined for the number and size of follicles. CTX produced a significant reduction in the total number of follicles. The pool of growing follicles (medium to large, greater than 30 microns in diameter) appeared to be vulnerable to the cytotoxic effect of CTX. LHRHa resulted in a significant reduction in the number of medium-to-large follicles and an increase in the number of small follicles. When given in combination with CTX, LHRHa significantly further reduced the number of medium-to-large follicles, significantly increased the number of small follicles, and resulted in an increase in the total number of follicles. Chronic LHRHa treatment resulted in functional deprivation of follicles from gonadotropins, thus halting the process of recruitment from the quiescent pool of primordial follicles into the CTX sensitive pool and thereby preserving the functional potential of the ovary.

Visualization of epidermal growth factor receptors in human epidermis

The localization of epidermal growth factor (EGF) receptors in normal human epidermis was examined with two independent experimental methods. The distribution of EGF receptor sites was studied using light microscopic autoradiography with [125I]EGF and direct immunocytochemical techniques with EGF receptor antibodies and protein A-colloidal gold complexes. Direct visualization by autoradiography indicated that the concentration of EGF receptors was greatest in the lower epidermal layers. Ultrastructural morphometric analysis of protein A-gold complexes showed that EGF receptors were primarily associated with the plasma membranes although intranuclear and cytoplasmic localization was also evident. This postembedment immunolocalization method also confirmed the relative differences in the number of EGF receptors found in individual epidermal layers (basalis greater than spinosum greater than granulosum greater than corneum layers). This inverse relationship between numbers of EGF receptors and the degree of epidermal differentiation and/or keratinization may suggest a physiologic role for EGF in these processes in human epidermis.

Rapid rounding of human epidermoid carcinoma cells A-431 induced by epidermal growth factor

Epidermal growth factor (EGF) induces rapid rounding of A-431 human epidermoid carcinoma cells in Ca(++)-free medium. Cell rounding is not induced by a variety of other polypeptide hormones, antiserum to cell membranes, local anesthetics, colchicine, cytochalasin B, or cyclic nucleotides. However, trypsin, like EGF, induces rounding of A- 431 cells in the absence of Ca(++). Both trypsin- and EGF-induced rounding are temperature dependent, appear to be energy dependent, and are inhibited by cytochalasins, suggesting that the active participation of microfilaments in cell rounding. However, a medium transfer experiment suggests that EGF-induced rounding is not attributable to secretion of a protease, and a number of serine protease inhibitors have no effect on the EGF-induced rounding process. Cell rounding is not attributable to the slight stimulation by EGF of the release of Ca(++) that is observed in the Ca(++)-free medium, as stimulation of such release by the ionophore A23187 neither induces cell rounding nor blocks EGF-induced rounding. Cells that have rounded up after treatment with EGF or trypsin spread out upon addition of Ca(++) to the medium, even in the continuing presence of EGF or typsin. Like the cell-rounding process, the cell-spreading process is temperature dependent, appears to be energy dependent, and is inhibited by cytochalasin B. Thus, EGF does not destroy the ability of the cell to spread; rather, in the presence of the EGF (or trypsin), cell spreading and the maintenance of the flattened state become dependent on external Ca(++). Because untreated cells remain flattened in the absence of Ca(++), the data suggest that EGF may disrupt Ca(++)-independent mechanisms of adhesion normally present in A-431 cells.

Phase separation in alcohol/liquid carbon dioxide solvent systems facilitates critical point drying

Carbon dioxide and methanol or ethanol, although miscible, form alcohol/CO2 solutions that do not easily mix with additional pure liquid CO2. If the CO2 inlet is situated at the top of a critical point drying apparatus chamber, pure CO2 will entirely displace the alcohol/CO2 phase (which is more dense) while keeping the chamber filled with liquid. This unexpected phenomenon is invaluable in critical point drying delicate biological tissues which remain continuously immersed, avoiding surface or convection currents. By providing an objective criterion for intermediate solvent displacement, the protocol also eliminates ambiguous 'flushing' steps.

Hormone receptor topology and dynamics: morphological analysis using ferritin-labeled epidermal growth factor

Previous studies using a biologically active 1:1 conjugate of EGF and ferritin (F-EGF) have traced the binding and internalization of the hormone molecules. In the present report, we develop ultrastructural criteria for identification of the F-EGF.receptor complex, and, thereby, enable utilization of the F-EGF as an indirect marker to localize the receptor for this peptide hormone. The ferritin cores of bound F-EGF are situated 4-6 nm from the extracellular surface of the membrane. When cells were incubated for up to 30 min at 37 degrees C, this characteristic spatial relationship was observed in all uptake stages (surface clustering, endocytosis, and incorporation into multivesicular bodies), indicating that the hormone.receptor complex remains intact through these steps. However, when incubation was continued for periods sufficient to allow hormone degradation (30-60 min), pools of free ferritin were observed in lysosomes. In the presence of various amine inhibitors of hormone degradation, internalization and multivesicular body incorporation proceeded, but hormone.receptor degradation was blocked as evidenced by preservation of the ferritin-membrane relationship; i.e., no pools of free ferritin were seen after 60 min. These data provide morphological support for the hypothesis that down-regulation of surface receptors involves internalization of intact hormone.receptor complexes. In addition, we have developed a method for viewing the surface of intact cells en face, allowing closer scrutiny of the clustering of F-EGF.receptor complexes in the plane of the membrane prior to internalization. The particles in the F-EGF clusters observed by this method are spaced at 12 nm center-to-center, serving to set upper limits on the packing dimensions of the EGF.receptor complex.

Rapid induction of morphological changes in human carcinoma cells A-431 by epidermal growth factors

The morphological effects of epidermal growth factor (EGF) on human carcinoma cells A-431 have been examined by scanning electron microscopy. These flat polygonal cells normally exhibit only small membrane folds, but show extensive ruffling and extension of filopodia within 5 min of exposure to EGF at 37 degrees C. This ruffling activity is transient, subsiding within another 5--15 min, but several other changes in surface morphology follow. Within the first hour of exposure to the hormone, the cell surface becomes exceedingly smooth and the nuclei seem to protrude above the plane of the otherwise thin monolayer, giving the cells a "fried egg" appearance. Cells at the edges of colonies gradually retract from the substrate, leading to reorganization, by 12 h, of the monolayer into multilayered colonies. EGF thus induces both rapid and long-term alterations in the morphology of these epidermoid cells.

Rapid stimulation of pinocytosis in human carcinoma cells A-431 by epidermal growth factor

Horseradish peroxidase (HRP) uptake was used to measure fluid-phase pinocytosis in monolayers of human epithelioid carcinoma cells (A-431). Histochemistry confirmed that cell-associated HRP was restricted to intracellular vesicles. Biochemical methods showed that HRP uptake in control cultures was directly proportional to the duration of exposure. The addition of low concentrations of epidermal growth factor (EGF) to the incubation media produced a 10-fold increase in the initial rate of pinocytosis. The EGF effect was rapid (within 30 s) but transient; the rate of pinocytosis returned to control levels within 15 min. Metabolic inhibitors reduced the EGF-stimulated rate of pinocytosis by greater than 90%. A conjugate of EGF and ferritin (F:EGF) was used to simultaneously compare the intracellular locations of EGF and HRP. Much of F:EGF was internalized in approximately 100-nm vesicles, while most of the HRP was located in much larger vesicles (range 0.1--1.2 micrometer) which also contained F:EGF. The tumor-promoter 12-0-tetradecanoyl-phorbol-13-acetate, which shares several biological activities with EGF, was also effective in stimulating an increase in the rate of pinocytosis.

Direct visualization of the binding and internalization of a ferritin conjugate of epidermal growth factor in human carcinoma cells A-431

We have prepared a conjugate of epidermal growth factor (EGF) and ferritin that retains substantial binding affinity for cell receptors and is biologically active. Glutaraldehyde-activated EGF was covalently linked to ferritin to produce a conjugate that contained EGF and ferritin in a 1:1 molar ratio. The conjugate was separated from free ferritin by affinity chromatography using antibodies to EGF. Monolayers of human epithelioid carcinoma cells (A-431) were incubated with EGF:ferritin at 4 degrees C and processed for transmission electron microscopy. Under these conditions, approximately 6 X 10(5) molecules of EGF:ferritin bound to the plasma membrane of each cell. In the presence of excess native EGF, the number of bound ferritin particles was reduced by 99%, indicating that EGF:ferritin binds specifically to cellular EGF receptors. At 37 degrees C, cell-bound EGF:ferritin rapidly redistributed in the plane of the plasma membrane to form small groups that were subsequently internalized into pinocytic vesicles. By 2.5 min at 37 degrees C, 32% of the cell-bound EGF:ferritin was localized in vesicles. After 2.5 min, there was a decrease in the proportion of conjugate in vesicles with a concomitant accumulation of EGF:ferritin in multivesicular bodies. By 30 min, 84% of the conjugate was located in structures morphologically identified as multivesicular bodies or lysosomes. These results are consistent with other morphological and biochemical studies utilizing 125I-EGF and fluorescein-conjugated EGF.

Contractile vacuoles in cells of a fresh water sponge, Spongilla lacustris

Forty or more independently functioning contractile vacuoles (CVs) occupy the central region of fresh water sponge pinacocytes. Eash CV undergoes a cycle of enlargement by fusion, movement, shape change, rounding up, and emptying over the course of 5--30 min. Diameter at discharge varies between 1 and 13 micron. CVs in all cell types are associated with submicroscopic coated vesicles. Filled CVs are bounded by an unmodified trilaminar membrane, but vacuoles with excess membrane frequently show coated evaginations. These evaginations are thought to pinch off as coated vesicles, providing an avenue for membrane recycling in the CV system.

Cyclic Membrane Flow in the Ingestive-Digestive System of Peritrich Protozoans

As peritrich food vacuoles condense during the initial stage of digestion, excess membrane pinches off as cup-shaped vesicles which exhibit a structured coat on the non-cytoplasmic surface of the membrane. As the membrane cycles from cup-shaped vesicles to diskoidal vesicles to cytopharynx to food vacuoles, the coat undergoes structural transformations from the condensed form (5 x 16 nm peg-shaped elements) to an extended form (long thin filaments). Review of the literature reveals morphologically similar coats which undergo similar transformations in the digestive organelles of flagellate protozoa, Hydra absorptive cells, insect pericardial cells, ileal absorptive cells of suckling rats, cells of the guinea-pig placenta, mammalian Langerhans cells, and macrophages. The similar functional situation in which these coated membranes occur suggests that the coat is important to the recognition and binding of macromolecules.

Cyclic Membrane Flow in the Ingestive-Digestive System of Peritrich Protozoans

Food vacuoles in peritrichs form by pinching off the distal half of the cytopharynx; and thus the pharyngeal membrane must be renewed during feeding. Correlation of light- and electron-microscopic observations indicates that the membranes of the ingestive-digestive system recycle. As the young food vacuoles enter the first stage of digestion (condensation of the vacuole), they pinch off cup-shaped coated vesicles which, in association with the microtubular post-oral fibres, return to the pharynx. In the peripharyngeal region, the 0.25-µm diameter cups flatten into 0.42-µm diameter disks, which then fuse with the pharyngeal membrane, thereby providing membrane for future food vacuoles. Ultrastructural evidence for similar patterns of cyclic membrane flow and the functional implications of the association of the pharyngeal membrane with the microtubules of the pharyngeal ribs are considered for other protozoa, including suctorians.

Membrane recycling: vesiculation of the amoeba contractile vacuole at systole

Ultrastructural data on the protozoan Amoeba proteus support a model of membrane recycling. At systole the amoeba contractile vacuole fuses with the cell surface and expels its contents. Observations by electron microscopy indicate that, as the vacuole empties, its bounding membrane transforms into tiny (35 nanometers in diameter) vesicles, identical to the vesicles that segregate fluid and contribute to the diastolic vacuole.