The intermediate filament system of the keratinizing mouse forestomach epithelium: Coexpression of keratins of internal squamous epithelia and of epidermal keratins in differentiating cells

Summary The internal epithelium of mouse forestomach represents a fully keratinized tissue that has many morphological aspects in common with the integumental epidermis. In the present study we have, therefore, analyzed keratin expression in the total epithelium, in subfractions of basal cells and in living and dead suprabasal cells that were obtained by Percoll density gradient centrifugation of trypsin-dissociated forestomach keratinocytes. The keratin analysis revealed that basal forestomach keratinocytes synthesize the same keratin types as basal epidermal cells (60 000, 52 000 and 47 000 daltons), whereas differentiating cells contain both the epidermal suprabasal keratin pair (67 000 and 59 000 daltons) and the suprabasal keratin pair characteristic for other internal squamous epithelia (57 000 and 47 000 daltons). Indirect immunofluorescence using an antibody recognizing the members of the epidermal-type suprabasal keratin pair and in-situ-hybridization experiments using specific cDNA probes for the members of the internal-type keratin pair showed that the two keratin pairs are uniformly coexpressed in living suprabasal forestomach keratinocytes. Furthermore, it could be shown that distinct cells in the basal cell layer acquire the ability to express both the 67 000/59 000 dalton and the 57 000/47 000 dalton keratin pair and that some basal cells apparently lose the ability to synthesize mRNAs for basal keratins.     

Quantitative electron microscopic analysis of the stratified epithelium of normal human buccal mucosa

Summary The epithelium of normal human buccal mucosa was subjected to stereologic analysis. Ten biopsies were selected from a total of 20 specimens collected from 10 to 15 year old females, and processed for lightand electron microscopy. At two levels of magnification, electron micrographs were sampled from four strata in epithelial ridges and from three strata in regions over connective tissue papillae. Stereologic point counting based on a recently improved system for analyzing stratified epithelia was employed to analyze a total of 1820 electron micrographs. Buccal epithelium was found to be 0.48 mm thick, interdigitated by long, slender connective tissue papillae, and comprised of a narrow basal and suprabasal, and a broad, homogeneously structured spinous and surface compartment. From basal to surface layers, the epithelium displayed a differentiation pattern different from that of keratinizing epithelia. This pattern was a function mainly of a drastic density increase of cytoplasmic filaments of a constant 80 Å diameter, a corresponding decrease of the cytoplasmic ground substance, the appearance of dark-cored membrane coating granules and individually varying amounts of glycogen deposition. It is suggested that the dense meshwork of filaments which fill 70% of the epithelial cytoplasm in a broad subsurface and surface layer, serves as the functional matrix for epithelial distensibility.This investigation was performed while Dr. Landay was on leave from the Department of Periodontology, Temple University, School of Dentistry.     

The bovine desmocollin family: a new gene and expression patterns reflecting epithelial cell proliferation and differentiation

We have discovered a third bovine desmocollin gene, DSC3, and studied expression of all three desmocollin genes, DSC1, 2, and 3, by Northern blotting, RT-PCR and in situ hybridization. DSC1 is strongly expressed in epidermis and tongue papillae, showing a "skin"-type pattern resembling that previously described for keratins 1 and 10. Expression is absent from the epidermal basal layer but appears in the immediate suprabasal layers and continues uniformly to the lower granular layer. In tongue epithelium, expression is suprabasal and strictly localized to papillae, being absent from interpapillary regions. In other epithelial low level DSC1 expression is detectable only by RT-PCR. The distribution of Dsc1 glycoproteins, detected by an isoform-specific monoclonal antibody, closely reflects mRNA distribution in epidermis and tongue. DSC2 is ubiquitously expressed in epithelia and cardiac muscle. In stratified epithelia, expression appears immediately suprabasal, continuing weakly to the lower granular layer in epidermis and to just above half epithelial thickness in interpapillary tongue, oesophageal, and rumenal epithelia. DSC3 expression is restricted to the basal and immediately suprabasal layers in stratified epithelia. In deep rete ridges DSC expression strikingly resembles the distribution of stem, transit-amplifying, and terminally differentiating cells described by others. DSC3 expression is strongly basal, DSC2 is strong in 5-10 suprabasal layers, and then weakens to be superseded by strong DSC1. These results suggest that desmocollin isoform expression has important functional consequences in epithelial proliferation, stratification, and differentiation. The data also provide a standard for nomenclature of the desmocollins.     

A three-dimensional organotypic culture of the human uterine exocervix for studying mucosal epithelial differentiation and migrating leukocytes

We report on a three-dimensional organotypic culture in vitro of explants from the human uterine exocervix. Exocervical fragments (2-3 mm3) from pre-menopausal women were cultured on sponges submerged in Dulbecco's Modified Eagle's Medium containing p-nonylphenol and 10% fetal bovine serum for up to 3 weeks and the viability and cellular responses were assayed. The fragments were analyzed by immunohistochemistry for the expression and distribution of a broad spectrum of cellular markers: p63, Ki-67, involucrin, high molecular weight cytokeratins, estrogen receptor-alpha, vimentin, CD45, and CD31. The fragments preserved their tissue architecture and cellular heterogeneity comparable to that observed in exocervical tissue in vivo. Prior to culture, the original epithelium was composed of stratified multilayered keratinocytes with integrated monocyte/dendritic-like cells in the basal and suprabasal layers. The epithelium began to exfoliate in culture and within 4 days appeared to have lost its differentiated high-zone layers of keratinocytes. After 10 days a new epithelium, slightly different from the original one, was formed; it displayed an increasing prominence of basal and suprabasal keratinocyte layers, containing infiltrating leukocytes that had probably migrated from the submucosa. The epithelium subsequently lost its organization, concomitant with a progressive involution of the stroma. Subepithelial capillaries appeared to be well maintained throughout the culture period. Aside from the maintenance of cellular heterogeneity within the fragments of exocervix, these culture systems are a valuable tool for studying the mechanisms of epithelial regeneration, and may prove to be a useful model for studying mucosal immunity.     

Involucrin synthesis and tissue assembly by keratinocytes in natural and cultured human epithelia

Different stratified squamous epithelia, whether they bear a stratum corneum or not, are shown by immunofluorescence to possess the precursor protein of the cross-linked envelope that is characteristic of epidermal s. corneum. This protein, involucrin, is not present in the deepest epithelial cells but appears in the course of their outward migration. The boundary at which involucrin first appears can sometimes by correlated with a visible boundary between zones of large and small cells. Cultured keratinocytes, derived from all stratified squamous epithelia (epidermal, corneal, conjuctival, esophageal, lingual, and vaginal), form colonies that grow together to form a stratified epithelium. The cells of the basal layer are nearly always free of detectable involucrin, but, in contrast to the natural epithelium, this protein usually makes its appearance in the cells immediately above the basal layer. When a cultured epithelium derived from epidermal keratinocytes is detached and applied as a graft to animals, the cells flatten and the distinctness of the basal layer is at first reduced; but with time the organization of the epithelium becomes more characteristic of epidermis. Cell size and shape become more orderly along the cell migration pathway, and involucrin first appears at some distance from the basal layer, instead of in immediately suprabasal cells, as in the cultured epithelium. The progeny of dissociated and cultured keratinocytes are therefore able, when grafted, to reassemble an epidermis in which the timing of specific gene expression is restored to that of the original tissue.     

Suprabasal expression of a 64-kilodalton keratin (no. 3) in developing human corneal epithelium

We have previously shown that a basic 64-kilodalton (no. 3 in the catalog of Moll et al.) and an acidic 55-kilodalton (no. 12) keratin are characteristic of suprabasal cell layers in cultured rabbit corneal epithelial colonies, and therefore may be regarded as markers for an advanced stage of corneal epithelial differentiation. Moreover, using an AE5 mouse monoclonal antibody, we showed that the 64-kilodalton keratin marker is expressed suprabasally in limbal epithelium but uniformly (basal layer included) in central corneal epithelium, suggesting that corneal basal cells are in a more differentiated state than limbal basal cells. In conjunction with previous data implicating the centripetal migration of corneal epithelial cells, our data support a model of corneal epithelial maturation in which corneal epithelial stem cells are located in the limbus, the transitional zone between the cornea and conjunctiva. In the present study, we analyzed the expression of the 64-kilodalton keratin in developing human corneal epithelium by immunohistochemical staining. At 8 weeks of gestation, the presumptive corneal epithelium is composed of a single layer of cuboidal cells with an overlying periderm; neither of these cell layers is AE5 positive. At 12-13 weeks of gestation, some superficial cells of the three- to four-layered epithelium become AE5 positive, providing the earliest sign of overt corneal epithelial differentiation. At 36 weeks, although the epithelium is morphologically mature (four to six layers), AE5 produces a suprabasal staining pattern, this being in contrast to the adult epithelium which exhibits uniform staining.(ABSTRACT TRUNCATED AT 250 WORDS)     

EEDA: a protein associated with an early stage of stratified epithelial differentiation

Using suppressive subtractive hybridization, we have identified a novel gene, which we named early epithelial differentiation associated (EEDA), which is uniquely associated with an early stage of stratified epithelial differentiation. In epidermis, esophageal epithelium, and tongue epithelium, EEDA mRNA, and antigen was abundant in suprabasal cells, but was barely detectable in more differentiated cells. Consistent with the limbal location of corneal epithelial stem cells, EEDA was expressed in basal corneal epithelial cells that are out of the stem cell compartment, as well as the suprabasal corneal epithelial cells. The strongest EEDA expression occurred in suprabasal precortical cells of mouse, bovine, and human anagen follicles. Developmental studies showed that the appearance of EEDA in embryonic mouse epidermis (E 15.5) coincided with morphological keratinization. Interestingly, EEDA expression is turned off when epithelia were perturbed by wounding and by cultivation under both low and high Ca2+ conditions. Our results indicate that EEDA is involved in the early stages of normal epithelial differentiation, and that EEDA is important for the "normal" differentiation pathway in a wide range of stratified epithelia.     

The fine structure of atypical ciliated cells in the human gastric epithelium

Gastric mucosa obtained from the body and pyloric portions of the human stomach were observed by light and transmission electron microscopy. Ciliated cells were found in two of 18 subjects examined, one patient with gastric ulcer and the other one with gastric adenocarcinoma. The ciliated cells were found in epithelia at sites away from the main lesions. The tissues containing ciliated cells showed intestinal metaplasia combined with mild chronic gastritis in both cases. The epithelial layer facing the gastric lumen was composed of columnar cells with numerous uniform microvilli and goblet cells. This epithelium extended to the superficial parts of the tubules surrounded by the lamina propria. The deeper portions of the tubules were composed of mucous secretory, endocrine, and rarely ciliated cells. These ciliated cells were provided with numerous cilia the numbers of which varied considerably from cell to cell. This was in contrast to the primary cilium which is usually single. The central part of the apical cell membrane was sometimes concave in the area from where cilia tended to arise. It was also observed that numerous basal bodies as well as mucus-like granules were contained in the same cell. The axonemal pattern was different from that of ordinary cilia and showed 9 + 0 and 8 + 1 patterns. In longitudinal sections it was found that one peripheral doublet was displaced to the center of the axoneme as it left the basal body.     

Ultrastructural studies on the epithelia of the olfactory organ of cyprinodonts (teleostei,cyprinodontoidea)

Summary The epithelia of the olfactory organ of two cyprinodontoid fish species were studied both by transmission and scanning electron microscopy. The relatively flat floor of the organ is covered by sensory and nonsensory epithelia. The latter is distributed in the form of bands or ridges separating distinct areas of sensory epithelium. Differences between the olfactory organs of the two species investigated related only to the topography and quantitative distribution of the epithelia. Their ultrastructural features are very similar. The nonsensory stratified squamous epithelium contains numerous goblet cells and surface cells provided with microridges. A hypothetical function of the microridges is discussed. The sensory epithelium consists mainly of basal, supporting, and two types of sensory cells, i.e., ciliated and microvillous receptor cells. The cilia exhibit a predominant 9+0 microtubule pattern. Both epithelia are covered by a mucus layer in which all surface structures seem to be embedded. The possible nature, origin, and movement mechanisms of the mucus are discussed.This work was supported by the Deutsche ForschungsgemeinschaftDedicated to Prof. Dr. med. W. Bargmann on the occasion of his 70th birthday     

Cytokeratin patterns of human oral epithelia: Differences in cytokeratin synthesis in gingival epithelium and the adjacent alveolar mucosa

Human oral mucosa includes various epithelia that are commonly classified as lining, masticatory, and specialized epithelia. Although adjacent tissues, the gingiva and alveolar mucosa represent two different types of epithelia: the gingiva is cornified and exhibits high rate ridges, whereas the mucosa does not normally cornify and exhibits a relatively smooth-contoured borderline between the epithelium and the underlying connective tissue. We examined the cytokeratin patterns of both epithelia using one- and two-dimensional gel electrophoresis. The gingiva expresses a great complexity of cytokeratins, including significant amounts of components nos. 1, 2, 5, 6, 10, 11, 13, 14, 16, and 17, as well as traces of cytokeratins nos. 4 and 15, i.e., a pattern similar to those of vaginal mucosa and epidermis containing proliferative keratinocytes. In contrast, the alveolar mucosa contains only two major cytokeratins, i.e., nos. 4 and 13, together with two minor amounts of cytokeratins nos. 5, 6, 14, and 17, thus resembling the patterns of certain other stratified, noncornified epithelia, such as the esophagus. Immunofluorescence microscopy using monoclonal antibodies to cytokeratins nos. 4 and 13 revealed the presence of these proteins in the suprabasal layers of alveolar mucosa, whereas in the gingiva, only certain small, suprabasal clusters of cells appeared to contain these cytokeratins. The cytoskeletal differences between gingival and alveolar mucosa are discussed in relation to the differences in their morphology and function, and with respect to pathological processes characteristic of these epithelia.     

Structure and function of intercellular junctions in human cervical and vaginal mucosal epithelia

The mucosal epithelium is a major portal for microbial invasion. Mucosal barrier integrity is maintained by the physical interactions of intercellular junctional molecules on opposing epithelial cells. The epithelial mucosa in the female reproductive tract provides the first line of defense against sexually transmitted pathogenic bacteria and viruses, but little is known concerning the structure and molecular composition of epithelial junctions at this site. In the present study, the distribution of tight, adherens, and desmosomal junctions were imaged in the human endocervix (columnar epithelium) and ectocervix (stratified squamous epithelium) by electron microscopy, and permeability was assessed by tracking the penetration of fluorescent immunoglobulin G (IgG). To further define the molecular structure of the intercellular junctions, select junctional molecules were localized in the endocervical, ectocervical, and vaginal epithelium by fluorescent immunohistology. The columnar epithelial cells of the endocervix were joined by tight junctions that excluded apically applied fluorescent IgG. In contrast, the most apical layers of the ectocervical stratified squamous epithelium did not contain classical cell-cell adhesions and were permeable to IgG. The suprabasal and basal epithelial layers in ectocervical and vaginal tissue contained the most robust adhesions; molecules characteristic of exclusionary junctions were detected three to four cellular layers below the luminal surface and extended to the basement membrane. These data indicate that the uppermost epithelial layers of the ectocervix and vagina constitute a unique microenvironment; their lack of tight junctions and permeability to large-molecular-weight immunological mediators suggest that this region is an important battlefront in host defense against microbial pathogens.     

A quantitative electron microscopic analysis of the keratinizing epithelium of normal human hard palate

Summary The epithelium of normal human hard palate was subjected to stereologic analysis. Ten biopsies were selected from a total of twenty specimens collected from 9 to 16 year old females, and processed for light- and electron microscopy. At two levels of magnification, electron micrographs were sampled from three strata (basale, spinosum, granulosum) in two locations (epithelial ridges and portions over connective tissue papillae). Stereologic point counting procedures were employed to analyse a total 1560 electron micrographs. In general, the thickness of the palate epithelium was 0.12 mm (over papillae) and 0.31 mm (in ridges), the epithelium is distinctly stratified, and homogeneously ortho-keratinized. From basal to granular layers, the composition of strata revealed decreasing densities of nuclei, mitochondria, membrane-bound organelles and aggregates of free ribosomes. Keratohyalin bodies and membrane coating granules increased, and cytoplasmic filaments with a constant diameter of about 85 Å increased from 14 to 30% of cytoplasmic unit volume. The cytoplasmic ground substance occupied a stable 50% of the epithelial cytoplasm in all strata. The composition of basal layers in ridges differed from that over connective tissue papillae. The data are discussed in relation to the observations that (1) an increasing gradient of filament density is not the most characteristic feature of ortho-keratinizing oral epithelium and (2) differences in the degree of differentiation in cells of the stratum basale coincided with the comparable frequency distribution pattern of dividing cells.The authors are thankful to Miss K. Rossinsky for excellent technical assistance, to Mrs. M. Graf-de Beer for competent data computation and to Mrs. S. Münzel-Pedrazzoli for help in morphometric analysis. This study was in part supported by Grants Nos. 51 and 106 of the Hartmann Müller Foundation and by a Grant from the Foundation of Scientific Research at the University of Zürich.     

The ciliated human keratinocyte

Human keratinocytes were investigated for the presence of single cilia. Almost all basal keratinocytes were found to carry a single cilium in normal, occluded, and psoriatic skin. The ciliary structure was progressively reduced in keratinocytes approaching the surface. No remnants of the ciliary apparatus were found in the granular layer. In one case of nickel-allergic dermatitis (patch test), the keratinocytes had lost their cilia; the significance of this surprising finding remains to be elucidated.     

Differentiation-related expression of a major 64K corneal keratin in vivo and in culture suggests limbal location of corneal epithelial stem cells

In this paper we present keratin expression data that lend strong support to a model of corneal epithelial maturation in which the stem cells are located in the limbus, the transitional zone between cornea and conjunctiva. Using a new monoclonal antibody, AE5, which is highly specific for a 64,000-mol-wt corneal keratin, designated RK3, we demonstrate that this keratin is localized in all cell layers of rabbit corneal epithelium, but only in the suprabasal layers of the limbal epithelium. Analysis of cultured corneal keratinocytes showed that they express sequentially three major keratin pairs. Early cultures consisting of a monolayer of "basal" cells express mainly the 50/58K keratins, exponentially growing cells synthesize additional 48/56K keratins, and postconfluent, heavily stratified cultures begin to express the 55/64K corneal keratins. Cell separation experiments showed that basal cells isolated from postconfluent cultures contain predominantly the 50/58K pair, whereas suprabasal cells contain additional 55/64K and 48/56K pairs. Basal cells of the older, postconfluent cultures, however, can become AE5 positive, indicating that suprabasal location is not a prerequisite for the expression of the 64K keratin. Taken together, these results suggest that the acidic 55K and basic 64K keratins represent markers for an advanced stage of corneal epithelial differentiation. The fact that epithelial basal cells of central cornea but not those of the limbus possess the 64K keratin therefore indicates that corneal basal cells are in a more differentiated state than limbal basal cells. These findings, coupled with the known centripetal migration of corneal epithelial cells, strongly suggest that corneal epithelial stem cells are located in the limbus, and that corneal basal cells correspond to "transient amplifying cells" in the scheme of "stem cells----transient amplifying cells----terminally differentiated cells."     

Evaluating alternative stem cell hypotheses for adult corneal epithelial maintenance

In this review we evaluate evidence for three different hypotheses that explain how the corneal epithelium is maintained. The limbal epithelial stem cell(LESC)hypothesis is most widely accepted. This proposes that stem cells in the basal layer of the limbal epithelium, at the periphery of the cornea, maintain themselves and also produce transient(or transit) amplifying cells(TACs). TACs then move centripetally to the centre of the cornea in the basal layer of the corneal epithelium and also replenish cells in the overlying suprabasal layers. The LESCs maintain the corneal epithelium during normal homeostasis and become more active to repair significant wounds. Second, the corneal epithelial stem cell(CESC) hypothesis postulates that, during normal homeostasis, stem cells distributed throughout the basal corneal epithelium, maintain the tissue. According to this hypothesis, LESCs are present in the limbus but are only active during wound healing. We also consider a third possibility, that the corneal epithelium is maintained during normal homeostasis by proliferation of basal corneal epithelial cells without any input from stem cells. After reviewing the published evidence, we conclude that the LESC and CESC hypotheses are consistent with more of the evidence than the third hypothesis, so we do not consider this further. The LESC and CESC hypotheses each have difficulty accounting for one main type of evidence so we evaluate the two key lines of evidence that discriminate between them. Finally, we discuss how lineage-tracing experiments have begun to resolve the debate in favour of the LESC hypothesis. Nevertheless, it also seems likely that some basal corneal epithelial cells can act as long-term progenitors if limbal stem cell function is compromised. Thus, this aspect of the CESC hypothesis may have a lasting impact on our understanding of corneal epithelial maintenance, even if it is eventually shown that stem cells are restricted to the limbus as proposed by the LESC hypothesis.     

Expression of simple epithelial type cytokeratins in stratified epithelia as detected by immunolocalization and hybridization in situ

Multi-layered ("stratified") epithelia differ from one-layered ("simple") polar epithelia by various architectural and functional properties as well as by their cytoskeletal complements, notably a set of cytokeratins characteristic of stratified tissue. The simple epithelial cytokeratins 8 and 18 have so far not been detected in any stratified epithelium. Using specific monoclonal antibodies we have noted, in several but not all samples of stratified epithelia, including esophagus, tongue, exocervix, and vagina, positive immunocytochemical reactions for cytokeratins 8, 18, and 19 which in some regions were selective for the basal cell layer(s) but extended into suprabasal layers in others. In situ hybridization with different probes (riboprobes, synthetic oligonucleotides) for mRNAs of cytokeratin 8 on esophageal epithelium has shown, in extended regions, relatively strong reactivity for cytokeratin 8 mRNA in the basal cell layer. In contrast, probes to cytokeratin 18 have shown much weaker hybridization which, however, was rather evenly spread over basal and suprabasal strata. These results, which emphasize the importance of in situ hybridization in studies of gene expression in complex tissues, show that the genes encoding simple epithelial cytokeratins can be expressed in stratified epithelia. This suggests that continual expression of genes coding for simple epithelial cytokeratins is compatible with the formation of squamous stratified tissues and can occur, at least in basal cell layers, simultaneously with the synthesis of certain stratification-related cytokeratins. We also emphasize differences of expression and immunoreactivity of these cytokeratins between different samples and in different regions of the same stratified epithelium and discuss the results in relation to changes of cytokeratin expression during fetal development of stratified epithelia, in response to environmental factors and during the formation of squamous cell carcinomas.     

The adoral sense organ in protobranch bivalves (Mollusca): comparative fine structure with special reference to Nucula nucleus

Abstract. The adoral sense organ in bivalve molluscs is a paired ridge of specialized epithelium positioned laterally at the base of the labial palps near the mouth opening, clearly distinguishable from the surrounding epithelia. Six species of the protobranch order Nuculoida, and one species of Solemyoida, were investigated by light microscopy concerning presence, gross anatomy, and innervation of the adoral sense organ. The organ was described in detail for Nucula nucleus and N. nitidosa by transmission electron microscopy; in these two species, the organ was characterized as a pseudostratified epithelial thickening with specialized cells bearing a specialized microvillar border and a basal matrix with a lamellar layer. Three types of bipolar primary receptor cells were recognized and these were reconstructed for N. nucleus. Most of the receptor cells had 2 cilia orientated parallel to the cell surface; in addition, there were 2 types of supporting cells and 1 type of basal cell. The surrounding epithelial cells were narrow with short microvilli and lacked cilia. The homology of the organ within protobranch bivalves was suggested by a character-complex of (1) position, (2) dimension of the epithelium, (3) innervation, (4) pseudostratified construction, (5) dimension of the specialized microvillar border, (6) thickening of the basal matrix, and (7) presence of specialized cell types such as receptor cells. Despite the estimated high number of protobranch species there is only scant information available on the adoral sense organ from 24 species of 8 genera. Structures and receptor types that are similar to those found in the adoral sense organ are widespread in molluscs and other invertebrate groups; this may indicate a plesiomorphy of these characters rather than an apomorphy for the protobranch clade. Therefore, the adoral sense organ may be of minor phylogenetic value above the level of protobranch orders.     

Observations on the solitary cilium of rabbit oviductal epithelium: its motility and ultrastructure

Solitary cilia have been observed on rabbit oviductal epithelial cells. In tissue cultures of fimbrial epithelium of 3- and 4-day-old animals observed by phase microscopy, most of these single cilia exhibited a vortical or funnel-type movement while others had the usual to-and-fro motility. Primary cilia are usually considered immotile. Transmission electron microscopy of specifically identified single cilia revealed differences between the ciliary shafts and basal bodies of the single cilia as compared to those of mature oviductal ciliated cells. The basal body of the solitary cilium often had at least two triangular, striated, basal foot processes, lacked electron-dense satellite material around its basal end, and occasionally had striated rootlets. In contrast, the cilia of mature ciliated cells had only one basal foot, exhibited much electron-dense satellite material, and lacked rootlets. Cross sections of the single cilia showed patterns of microtubules different from the usual 9 + 2 axonemal complexes of normal cilia and included 9 + 0, 10 + 2 singlets, 7 + 2 doublets, and 8 + 1 doublet and 2 singlets; one did have the usual 9 + 2 arrangement. We postulate that the presence of more than one basal foot process may be responsible for the vortical motility observed. The primary cilia are shorter than normal cilia; the longest one measured was 1.86 micron in length, 0.28 micron in width at its base, and 0.14 micron at its tip. Based on the light-microscopic, scanning-electron-microscopic and transmission-electron-microscopic observations, such solitary cilia were observed more frequently in the oviductal tissues of the 3- to 4-day postnatal rabbits grown in tissue culture and in ovariectomized and ovariectomized/progesterone-treated adult animals than in estrous, ovulatory, or ovariectomized/estradiol-treated rabbits.