Characteristic Protrusions Research Articles

The Beak of Eukaryotic Ribosomes: Life, Work and Miracles.

Ribosomes are not totally globular machines. Instead, they comprise prominent structural protrusions and a myriad of tentacle-like projections, which are frequently made up of ribosomal RNA expansion segments and N- or C-terminal extensions of ribosomal proteins. This is more evident in higher eukaryotic ribosomes. One of the most characteristic protrusions, present in small ribosomal subunits in all three domains of life, is the so-called beak, which is relevant for the function and regulation of the ribosome's activities. During evolution, the beak has transitioned from an all ribosomal RNA structure (helix h33 in 16S rRNA) in bacteria, to an arrangement formed by three ribosomal proteins, eS10, eS12 and eS31, and a smaller h33 ribosomal RNA in eukaryotes. In this review, we describe the different structural and functional properties of the eukaryotic beak. We discuss the state-of-the-art concerning its composition and functional significance, including other processes apparently not related to translation, and the dynamics of its assembly in yeast and human cells. Moreover, we outline the current view about the relevance of the beak's components in human diseases, especially in ribosomopathies and cancer.

Properties in Langmuir Monolayers and Langmuir-Blodgett Films of a Block Copolymer Based on N-Isopropylacrylamide and 2,2,3,3-Tetrafluropropyl Methacrylate

The amphiphilic block copolymer poly(N-isopropylacrylamide)-Ge(C6F5)2-poly(2,2,3,3-tetrafluoropropyl methacrylate) was prepared by the reaction of chain transfer to bis-(pentafluorophenyl)germane during the polymerization of N-isopropylacrylamide and the subsequent postpolymerization of isolated functional polymers in 2,2,3,3-tetrafluoropropyl methacrylate. The conversion of the block copolymer was 68% and the molecular weight of the sample was 490,000 g/mol. The colloidal chemical properties of Langmuir monolayers and Langmuir-Blodgett films of synthesized block copolymer have been studied. For comparison, a functional polymer, namely, poly-N-isopropylacrylamide with terminal -Ge(C6F5)2H group, was synthesized and studied. The concentrations of spreading solutions were selected and the effect of subphase acidity on the formation of monolayers of macromolecules of the block copolymer was studied. It was found that regardless of the acidity of the subphase, high pressure of fracture of films are characteristic of monolayers of collapse pressures πmax = (48-61) mN/m. The morphology of the Langmuir-Blodgett films of functional polymer exhibit isolated elongated micelles with high densities in the form of "octopus" on the periphery of which there are terminal hydrophobic groups. For the Langmuir-Blodgett film of block copolymer, a comb-like structure is observed with characteristic protrusions.

Abstract 4141: Hypoxia-induced transition from collective to amoeboid single cell dissemination in epithelial cancer cells

Abstract Tumor hypoxia, by elevating hypoxia-inducible factors (HIFs), is an established inducer of epithelial-to-mesenchymal transition (EMT) and subsequent cancer cell invasion and metastasis. Using epithelial carcinoma spheroids in 3D fibrillar collagen, we characterized the invasion patterns as well as cellular and molecular mechanisms of hypoxia-induced cancer cell migration modes. While epithelial cancer cells show collective invasion under normoxic conditions, hypoxia or pharmacological stabilization of HIF-1 using the prolyl hydroxylase inhibitor dimethyloxalylglycine (DMOG) induced EMT-like detachment and migration of single cells. Besides mesenchymal movement, most epithelial cells converted to amoeboid migration with characteristic actin-rich filopodal or distinctive blebby protrusions towards the direction of migration. Whereas mesenchymal migrating cells moved with low velocities in a directionally persistent manner, amoeboid migrating cells generated a broad spectrum of low to high velocities, but with less persistent invasion paths. Sub-cellular molecular analysis showed typical amoeboid features particularly in hypoxic blebby single cell migration, including low to lacking collagen degradation along the migration path and insensitivity to broad-spectrum matrix metalloproteinase (MMP)-inhibitor GM6001, non-focalized cortical actin cytoskeleton within blebs that interacted with collagen structures and low-beta1 integrin expression with lack of integrin focalization. Thus, tumor hypoxia induces a diversity of single-cancer cell invasion modes, including blebby amoeboid migration, thereby enhancing predominantly MMP- and integrin-independent amoeboid dissemination in parallel to EMT induction. Citation Format: Steffi Lehmann, Veronika A M te Boekhorst, Julia Odenthal, Liying Jiang, Sjoerd van Helvert, Peter Friedl. Hypoxia-induced transition from collective to amoeboid single cell dissemination in epithelial cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4141. doi:10.1158/1538-7445.AM2015-4141

Ultrastructure and distribution of chromatophores in the skin of the leopard gecko (Eublepharis macularius)

AbstractThe aim of this study was to describe the ultrastructure and arrangement of pigment cells in the leopard gecko (Eublepharis macularius) skin to explain how wild‐type coloration is formed. The study also attempted to explain, on a morphological level, how skin colour changes occur. Samples of leopard gecko skin were collected from wild‐type coloration adult specimens. The morphology of pigmented cells was determined using light microscopy on haematoxylin and eosin (H&E) stained sections and in transmission electron microscopy. These studies indicate that skin of E. macularis contains xanthophores and melanophores but lacks iridophores and that this is probably related to nocturnal activity. The number and distribution of xanthophores and melanophores determines the skin colour and pigmentation pattern. The colour changes depend on the arrangement of characteristic protrusions of melanophores and the degree of filling them with melanosomes.

An instability at the edge of a tissue of collectively migrating cells can lead to finger formation during wound healing

In wound healing assays, a monolayer of epithelial cells is allowed to migrate onto empty surface area. When the motile cells close the artificial wound, the edge of the tissue does usually not move uniformly but characteristic fingerlike protrusions are observed. We model the collectively moving cells as a system of self-propelled particles using the Toner-Tu equations for an active fluid. A linear stability analysis of perturbations at the tissue edge reveals an instability in the disordered nonmoving state. The instability is purely due to spontaneous motility and velocity alignment between cells. It can account for finger formation in wound healing experiments.

Constancy of Head Turning Recorded in Healthy Young Humans

This study evaluated the 'constancy' of head turning as recorded two-dimensionally by accelerometers. Fourteen healthy participants turned the head with his/her natural and comfortable speed. Maximum inclination angles (MIA) during head turning were measured in four (anterior, posterior, right, and left) directions of clockwise (CW) and counter-clockwise (CCW) conditions. Three indices were used for the evaluation: (1) standard deviations of MIA as an index of 'spatial constancy,' (2) anterior/posterior and right/left ratios from intervals among four MIA as indices of 'temporal constancy,' and (3) first derivatives from head turning trajectories as an index of 'angular velocity.' The spatial index varied from 0.15 degrees to 9.96 degrees (CW condition) and from 0.56 degrees to 10.6 degrees (CCW condition). The temporal index in the anterior-posterior direction varied from 0.711 to 1.103 (CW condition) and from 1.071 to 1.905 (CCW condition). The index in the right-left direction varied from 0.773 to 2.081 (CW condition) and from 0.842 to 1.226 (CCW condition). Characteristic hollows or protrusions were detected from the first derivatives of head turning trajectories and were regarded as abrupt changes in angular velocity during head turning. The results suggest that these three indices are appropriate tools for evaluation of the constancy of head turning.

Cytologic Features of Ascitic Fluid Complicated by Small Cell Variant T-cell Prolymphocytic Leukemia -A Case Report-

T-cell prolymphocytic leukemia (T-PLL) is a rare, mature T-cell lymphoproliferative disorder with a post-thymic mature T-cell phenotype. The disease is characterized by rapidly rising lymphocytosis, lym-phadenopathy, and splenomegaly. The clinical course is usually aggressive and progresses with frequent skin lesions and serous effusions. In 25% of cases, leukemic cells are small and tumor cells may not have a discrete nucleolus under light microscopy. Although the presence of characteristic cytoplasmic protrusions or blebs in tumor cells is a common morphologic finding in the peripheral blood film irrespective of the nuclear features, small cell variants lacking the typical nuclear features can cause diagnostic problems in clinical cytology. Furthermore, the small leukemic cells can share some cytologic findings with lymphocyte-rich serous effusions caused by non-neoplastic reactive lymphocytosis as well as other small lymphocytic lymphoproliferative disorders. Here, we describe the cytological findings of ascitic fluid complicated by small cell variant T-PLL in a 54-year-old man, the cytology of which was initially interpreted as small lymphocytic malignancy such as small lymphocytic lymphoma/chronic lymphocytic leukemia.

Variability in Apical Elongation of Wood Fibres in Lonchocarpus Sericeus

Morphological variability in wood fibres of Lonchocarpus sericeus (Poir.) Kunth ex DC. (Leguminosae), a tropical hardwood tree with doublestoreyed cambium, was examined in thin tangential and transverse sections as well as in macerations of wood tissue. Occurrence of characteristic protrusions (lateral expansions) was detected on the extended part of the main fibre body. Distance between the adjacent protrusions corresponded to the height of a storey (horizontal tier) of the cambial initials. Rays were shorter in height than the neighbouring fusiform initials and therefore unable to reach the boundary of the storey. This situation facilitated the lateral expansion of the adjoining fibres during their apical elongation by intrusive growth. The presence of the characteristic protrusions on the fibre body thus indicated that the given fibre was associated with a double-storeyed cambium having rays shorter than the length of fusiform initials. The ultimate shape of fibres was thus correlated to the height of storeys and the height and width of rays.

Two-dimensional Averaged Images of the Dynactin Complex Revealed by Single Particle Analysis

The dynactin complex interacts with dynein and numerous other proteins to provide for a wide range of subcellular transport functions. A detailed understanding of the structure and subunit organization of dynactin should yield new insights into its function. In the present study, we used single particle analysis to obtain a two-dimensional averaged image of dynactin isolated from chick embryo brains and visualized by negative stain electron microscopy (EM). Each individual image, consisting of the shoulder/sidearm and the rod, closely resembled the previously published quick-freeze deep-etch rotary-shadow electron micrographs. However, the averaged image revealed novel structural features that may have functional significance. The bulky shoulder complex has a triangular shape and is 13 nm wide and 8 nm high. The rod, with an overall length of 40 nm, consists of clearly defined lobes that are apparently grouped into three parts, the pointed-end complex, the middle segment, and the extra lobes at the barbed end. The pointed-end complex reveals the characteristic protrusions and clefts that were previously observed only in the isolated pointed-end complex. In the middle segment, the seven lobes are fitted to the helical symmetry of F-actin. A narrow but prominent gap separates the previously unidentified extra three lobes at the barbed end from the middle segment. The averaged image we obtained contrasts dramatically with the simple Arp1 polymer that was previously reported by single particle analysis of bovine brain dynactin. These apparent structural differences are probably due to the greater stability and integrity of the chick embryo brain dynactin preparation. We propose a new structural model for dynactin, based on our observations.

Oogenesis in phthirapterans (Insecta: Phthiraptera). I. Morphological and histochemical characterization of the oocyte nucleus and its inclusions

We characterize morphological and histochemical changes occurring within the oocyte nucleus (germinal vesicle) during oogenesis in two phthirapteran species: the pig louse, Haematopinus suis (Anoplura) and the pigeon louse, Columbicola columbae (Mallophaga). In previtellogenic oocytes, within the oocyte nucleus the chromatin condenses and forms the karyosome. In contact with the karyosome numerous dense and highly heterogeneous nuclear bodies occur. We demonstrate that these nuclear inclusions have complex structure and contain RNA and argyrophilic proteins of nucleolar organizer (Ag-NOR proteins). Results of immunogold electron microscopy experiments are also presented. The obtained results suggest that the phthirapteran nuclear bodies are assemblages of ribonucleoproteins that are stored in the oocyte nucleus and might be utilized during early stages of embryonic development. In the investigated species, the nuclear envelope of the germinal vesicle is equipped with characteristic protrusions. Ultrastructural analysis revealed striking similarity of these structures to the initial stages of the formation of accessory nuclei. Based on these results, we speculate on the possible evolutionary origin of the accessory nuclei in phthirapterans.

Influence of dietary concentrate to forage ratio on the development of rumen mucosa in calves

Effects of structural and non‐structural carbohydrates on the development of rumen fermentation and ruminai mucosa in calves were examined during the weaning period. Barley/soybean meal (SBM) group was fed a concentrate starting from 2 weeks of age, whereas alfalfa group received a mixture of concentrate and alfalfa hay in which the proportion of the latter was gradually increased from 20% to 70% between weeks 2 and 9 of age. The total volatile fatty acid concentration in rumen fluid of calves increased with age, but at 9 weeks there were no significant differences between the two diets (barley/SBM group 153 mmol/1, alfalfa group 150 mmol/1). Rumen papillae at 9 weeks of age, as compared to 6 weeks of age, were longer and fewer in number per square centimetre mucosa, with larger cut surface. This resulted in a higher surface of papillae per square centimetre mucosa at 9 weeks (barley/SBM group 286 mm2/cm2, alfalfa group 245 mm2/cm2) than at 6 weeks of age (barley/SBM group 217 mm2/cm2, alfalfa group 198 mm2/cm2). At 9 weeks of age, the pH (barley/SBM 5.0, alfalfa 5.7), the acetate to propionate ratio (barley/SBM 2.2, alfalfa 3.2) as well as the length of the papillae in the ventral ruminai sac (barley/SBM 1.96 mm, alfalfa 2.37 mm) were increased in the alfalfa group when compared to the barley/SBM group (P<0.1). In the former group, the proportion of butyrate revealed significantly increased values at 4 and 6 weeks of age. In animals of the barley/SBM group at 9 weeks of age, characteristic protrusions with proliferated thick epithelium occurred on the papillae and increased the surface for absorption. On the epithelium (Stratum corneum) desquamating cells with parakeratosis could be observed. In the alfalfa group the papillae of the ventral ruminai sac were longer, without protrusions. The morphotypes of the adhering rumen microflora differed between the groups. It can be concluded that feeding greater amounts of non‐structural carbohydrates increases the surface for absorption of the rumen epithelium in calves. The absence of hyperkeratosis and rumenitis in the barley/SBM group indicated that there is no reason to limit high starch diets in the early weaning period of calves.

The effect of topically applied autologous serum on corneal epithelial healing following alkali wounds

The dynactin complex interacts with dynein and numerous other proteins to provide for a wide range of subcellular transport functions. A detailed understanding of the structure and subunit organization of dynactin should yield new insights into its function. In the present study, we used single particle analysis to obtain a two-dimensional averaged image of dynactin isolated from chick embryo brains and visualized by negative stain electron microscopy (EM). Each individual image, consisting of the shoulder/sidearm and the rod, closely resembled the previously published quick-freeze deep-etch rotary-shadow electron micrographs. However, the averaged image revealed novel structural features that may have functional significance. The bulky shoulder complex has a triangular shape and is 13 nm wide and 8 nm high. The rod, with an overall length of 40 nm, consists of clearly defined lobes that are apparently grouped into three parts, the pointed-end complex, the middle segment, and the extra lobes at the barbed end. The pointed-end complex reveals the characteristic protrusions and clefts that were previously observed only in the isolated pointed-end complex. In the middle segment, the seven lobes are fitted to the helical symmetry of F-actin. A narrow but prominent gap separates the previously unidentified extra three lobes at the barbed end from the middle segment. The averaged image we obtained contrasts dramatically with the simple Arp1 polymer that was previously reported by single particle analysis of bovine brain dynactin. These apparent structural differences are probably due to the greater stability and integrity of the chick embryo brain dynactin preparation. We propose a new structural model for dynactin, based on our observations.

Hormonal dependence of pinopode formation at the uterine luminal surface

The uterine luminal epithelium during the period of receptivity for nidation displays characteristic protrusions of the apical surface named pinopodes. The effects of oestradiol and progesterone, singly or in combination, on the formation and regression of pinopodes were investigated using scanning electron microscopy. The appearance of pinopodes was found to be strictly progesterone dependent. When given together with progesterone, before the development of pinopodes, high doses of oestradiol (plasma level approximately 300 pmol/l) inhibited pinopode formation; on the contrary, low doses of oestradiol (nidatory doses) did not interfere with the process until the 4th day of treatment. When oestradiol was given as a single injection, after pinopode formation, both doses were equivalent in inducing their regression 48-72 h later. It appears that the hormonal conditioning for pinopode formation and for the development of uterine receptivity for egg implantation is the same. These observations support the hypothesis that pinopodes could be an extremely useful tool to estimate uterine receptivity. The experiments we describe here, together with observations made a few years ago, in stimulated cycles in the human, suggest that implantation failure as a result of a hormonal imbalance during the time intervening between ovulation and nidation, may be a general phenomenon.

Distribution of filipin‐sterol complexes in the plasma membrane of stallion spermatozoa during the epididymal maturation process

The presence and distribution of cholesterol in mature and immature epididymal spermatozoa was analyzed using filipin as a cytochemical tool in freeze-fracture replicas and thin section preparations. The polyenic-antibiotic filipin formed complexes with 3, beta -OH sterols, producing characteristic protrusions, or pits, that were heterogeneously distributed in the plasma membrane of stallion spermatozoa, revealing a specific organization in a functionally specialized area of the gamete. The acrosomal region of the sperm head presented a significantly higher density of filipin sterol complexes than the postacrosomal region, which was usually free of these complexes. The plasma membrane of the flagellum also showed filipin sterol complexes randomly distributed in freeze-fracture replicas. The strong filipin labeling observed in the membrane of spermatozoa obtained from the caput region of the epididymis decreased significantly during epididymal passage. The significance of these changes is not completely understood, but they might contribute to establishing the molecular organization necessary for sperm transit and storage in the epididymis as well as to development of motile spermatozoa that are able to fertilize the oocyte and induce normal embryonic development.

Immunohistochemical localization of keratin in human prostate

The localization of keratin was investigated in normal human prostate, benign prostatic hyperplasia (BPH), and prostatic adenocarcinoma using immunohistochemical technique on frozen tissue sections. The purpose of this study was to identify changes in the distribution patterns of keratin due to malignancy. In normal and benign tissue specimens, keratin was detected in the cytoplasm of basal cells and glandular epithelial cells. In the glandular epithelial cells keratin was found as a deposit of fine granules. In the basal cells, the positive staining for keratin had a uniform distribution in the scanty cytoplasm. In specimens of prostatic adenocarcinoma, the basal cells retained a strong positive reaction for keratin. The shapes and the distributions of basal cells were markedly different in malignant specimens. Basal cells formed a discontinuous layer and surrounded the population of neoplastic cells in tissue sections containing the cribriform patterns. The cells expressed characteristic protrusions into extracellular spaces between the cancer cells. Keratin-positive granules were demonstrated in the adenocarcinoma cells as well. These granules had slightly smaller sizes and were distributed randomly. The study demonstrates that the immunohistochemical localization of keratin provides a refinement for the characterization of cells in tissue sections of prostate.

Filipin-sterol complexes in the plasma membrane of zebrafish spermatozoa.

The presence and distribution of filipin-sterol complexes in the plasma membrane of zebrafish (Brachydanio rerio) sperm was investigated. The zebrafish sperm plasma membrane, treated with freeze-fracture techniques, is seen to contain a multitude of intramembranous particles that, in a specific region of the posterior part of the sperm head, are organized into unusual particle arrays that appear as simple hexagons or parallelograms. The polyene antibiotic filipin forms complexes with 3-beta-OH sterols to produce characteristic protrusions and pits in membranes that are readily observable in freeze-fracture replicas. Numerous filipin-sterol complexes were found to populate the sperm plasma membrane, and the complexes exhibited variability in their distribution in different sperm. This appears to be the first illustrated example of an acrosomeless sperm that exhibits a high concentration of filipin-sterol complexes. In contrast, the unique grating formed by the intramembranous particles as well as variable amounts of membrane surrounding the unusual particle arrays were always free of the filipin-sterol complexes. Thus, while cholesterol appears to be present in the plasma membrane of the zebrafish sperm, it is not apparent in the highly differentiated region of the membrane based on the observed distribution of the filipin-sterol complexes.

Uncoupling of the membrane skeleton from the lipid bilayer. The cause of accelerated phospholipid flip-flop leading to an enhanced procoagulant activity of sickled cells.

We have previously reported that the normal membrane phospholipid organization is altered in sickled erythrocytes. More recently, we presented evidence of enhanced transbilayer movement of phosphatidylcholine (PC) in deoxygenated reversibly sickled cells (RSC) and put forward the hypothesis that these abnormalities in phospholipid organization are confined to the characteristic protrusions of these cells. To test this hypothesis, we studied the free spicules released from RSC by repeated sickling and unsickling as well as the remnant despiculated cells. The rate of transbilayer movement of PC in the membrane of deoxygenated remnant despiculated cells was determined by following the fate of 14C-labelled PC, previously introduced into the outer monolayer under fully oxygenated conditions using a PC-specific phospholipid exchange protein from beef liver. The rate of transbilayer movement of PC in the remnant despiculated cells was significantly slower than in deoxygenated native RSC and was not very much different from that in oxygenated native RSC or irreversibly sickled cells. The free spicules had the same lipid composition as the native cells, but were deficient in spectrin. These spicules markedly enhanced the rate of thrombin formation in the presence of purified prothrombinase (Factor Xa, Factor Va, and Ca2+) and prothrombin, indicating the exposure of a significant fraction of phosphatidylserine (PS) in the outer monolayer. This effect was not observed when the spicules in this assay were replaced by normal erythrocytes, deoxygenated native RSC, or a deoxygenated sample of RSC after repetitive sickling/unsickling. The results are interpreted to indicate that the destabilization of the lipid bilayer in sickled cells, expressed by the enhanced flip-flop of PC and the exposure of PS in the outer monolayer, occurs predominantly in those parts of the membrane that are in spicular form.

Muscle cell differentiation in the ascidian heart

The simple tubular heart of tunicates consists of a single layer of striated muscle cells which display distinct electrical properties at the luminal and extraluminal surfaces. We have investigated heart morphogenesis and cytodifferentiation in the ascidian, Botryllus schlosseri. Myocardium is formed by invagination from the wall of the heart primordium. Cell polarity is clearly apparent in the undifferentiated cells of the heart primordium and is maintained throughout the whole course of cardiac muscle differentiation. Myocardium cells are initially cubic in appearance, then undergo a progressive flattening with the formation of characteristic protrusions at the luminal surface. The first sign of muscle cell differentiation is the formation of close associations between sarcoplasmic reticulum cisterns and the plasma membrane at the luminal and junctional surface. Myofibrillogenesis also occurs near the luminal surface, whereas the cell portion facing the pericardial cavity maintains an undifferentiated structure. The findings support the hypothesis that membrane changes precede and influence myofibril formation in developing muscle cells.

Studies on the progestational endometrium of the rabbit. II. Electron microscopy, day 0 to day 13 of gonadotrophin-induced pseudopregnancy.

The fine structure of the endometrial epithelium of the pseudo-pregnant rabbit from the day of induced ovulation (day 0) to the 13th day is here correlated with previously defined light microscopic phases. In Phase 1 (0-1 day), in which there is a presumed "priming" of the endometrium by ovarian steroidal hormones, no changes were observed. In Phase 2 (1-3 days), in addition to mitotic activity, the epithelium showed a disappearance of the mucification and lymphocytic migration typical of Phase 1 and also of the non-pregnant or "estrous" phase, and showed other nuclear and cytoplasmic changes which probably reflect endogenous growth and protein synthesis. In Phase 3 (4-6 days), two distinct populations of reacting cells were present: (1) surface and cryptal cells investing the now folded mucosal surface, and (2) glandular cells. The first group showed characteristic dome-like protrusions of the cytoplasm into the lumen, and also showed distinct cytoplasmic and nuclear changes which appear to be a prelude to the succeeding phase of fusion but are not necessarily secretory in type. The glandular cells, in contrast, showed cytoplasmic changes which appear to reflect active secretory activity (hypertrophy of the Golgi area, cytoplasmic vacuoles containing electron-opaque material, etc.). This phase coincides with the maximal secretion of uterine-specific proteins, and electron-opaque material is abundant within the endometrial lumen. In Phase 4 (6-8 days), the surface and cryptal epithelium undergoes a transformation into multinucleated cells, the result of a process of lysis of intervening plasma membranes, the precise mechanism of which (i.e., with or without initial membrane fusion) was not determined. Cell fusion proceeded earlier and more actively mesometrially than antimesometrially. The glandular cells showed evidence of reduced secretory activity, but did not at any stage undergo multinucleate-cell transformation. In Phase 5 (8-13 days) there was progressive fusion, and the number of nuclei per cytoplasmic sac appeared increased, presumably due to the continued action of progesterone which is maximal during this phase. Glandular cells showed further reduced secretory activity but remained columnar. Ciliation of the epithelium was sporadic in the pre-secretory phases and rare or absent in the secretory and fusion phases; it became widespread during the phase of decline after day 14, a period which will not be included in this study. The fine structure of the ciliated cells was the same at all stages; there was no evidence for their origin from a reserve population; it is possible that they arise by modification of the multinucleated cells. Cytoplasmic crystals and intramitochondrial densities or lamellae were observed during the secretory and fusion stages, the former only in the glands, the latter in the surface and cryptal epithelium. They appear to be associated with rising or maximal progesterone secretion.