Terminal Filament Research Articles

Ovariole Structure of the Cochineal Scale Insect,Dactylophis coccus

The ovaries of the adult cochineal scale insect, Dactylopius coccus Costa (Hemiptera: Coccoidea: Dactylopiidae) are made up of more than 400 short ovarioles of the telotrophic type. The ovarioles develop asynchronously. The ovarioles consist of a germarium with six or seven nurse cells, a vitellarium with an oocyte, and pedicel. A terminal filament is lacking. A maturing oocyte was attached to the trophic core by the trophic cord during previtellogenesis and most of vitellogenesis.

Structure of ovaries and oogenesis in dermapterans. I. Origin and functioning of the ovarian follicles

The ovaries of the studied earwig species ( Forficula auricularia, Chelidurella acanthopygia, Doru lineare and Opisthocosmia silvestris) are meroistic–polytrophic and composed of numerous short ovarioles that consist of a terminal filament, germarium and vitellarium. The germaria of adult females comprise meiotic (pachytene) and postmeiotic (differentiating) germ cell clusters, as well as small prefollicular cells. All germ cell clusters consist of two cells that are connected by a single intercellular bridge. In the vitellarium there are usually 2 ovarian follicles only. The individual follicle consists of a transcriptionally dormant oocyte and a single, polyploid nurse cell and is surrounded by a layer of somatic follicular cells (FCs). During previtellogenesis the nurse cell enlarges and becomes highly transcriptionally active. Concurrently its nucleus attains a characteristic, irregular shape. In the nurse cell nucleus of one studied species, F. auricularia, in addition to chromatin aggregations and RNA- and Ag-NOR-positive nucleoli, a single compact DNA-positive body is present. During advanced vitellogenesis the molecules synthesized in the nurse cells (RNAs, proteins, as well as nurse cell organelles) are transferred to the ooplasm via the intercellular bridge. During this transfer the nurse cell nucleus is retained in the cell centre and does not occlude the intercellular bridge. The results of our studies indicate that such position of the nurse cell nucleus is maintained solely by its extended shape. In other words, the rigid extensions keep the nucleus in the cell centre while the cytoplasm flows, in between these extensions, towards the intercellular bridge connecting the nurse cell with the oocyte.

Molecular basis of the C-terminal tail-to-tail assembly of the sarcomeric filament protein myomesin

Sarcomeric filament proteins display extraordinary properties in terms of protein length and mechanical elasticity, requiring specific anchoring and assembly mechanisms. To establish the molecular basis of terminal filament assembly, we have selected the sarcomeric M-band protein myomesin as a prototypic filament model. The crystal structure of the myomesin C-terminus, comprising a tandem array of two immunoglobulin (Ig) domains My12 and My13, reveals a dimeric end-to-end filament of 14.3 nm length. Although the two domains share the same fold, an unexpected rearrangement of one beta-strand reveals how they are evolved into unrelated functions, terminal filament assembly (My13) and filament propagation (My12). The two domains are connected by a six-turn alpha-helix, of which two turns are void of any interactions with other protein parts. Thus, the overall structure of the assembled myomesin C-terminus resembles a three-body beads-on-the-string model with potentially elastic properties. We predict that the found My12-helix-My13 domain topology may provide a structural template for the filament architecture of the entire C-terminal Ig domain array My9-My13 of myomesin.

Ultra-estrutura dos ovaríolos de Tropidacris collaris (Stoll) (Orthoptera: Romaleidae) submetido a três fotoperíodos

The research evaluated the ultrastructure of the ovarioles of Tropidacris collaris (Stoll), submitted to photoperiods 10L:14D, 12L:12D and 14L:10D. Sixty nymphs (30 males and 30 females) in the last stage of development were paired in ten couples in each treatment. Thirty days after adult emergence, the females were immobilized with ethylic ether and dissected under stereomicroscope. The ovarioles were transferred to Karnovsky fixative (2.5% glutaraldehyde, 4% paraformaldehyde and 0.1 M sodium cacodylate buffer) and analyzed in transmission and scanning electron microscopes. The different photoperiods had no effect on the ovarioles' ultrastructure. Each ovariole is covered by a thick sheath constituted by a homogeneous and filamentous material. In the terminal filament, there are cells with large nuclei, some with scarce cytoplasm and projections cytoplasmatic, besides filamentous structures assuming characteristic of conjunctive tissue. In the germarium, the germ cells are big with large nuclei, scarce cytoplasm and plasma membrane containing interdigitations. The follicular cells are small with a small nucleus, yet presenting cytoplasmatic projections. In the vitellarium the follicular cells suffer modifications in their morphology varying from cubic to flat.

Efficacy of molecular DNA methods for confirming species identifications on morphologically variable populations of toxin-producing Anabaena (Nostocales)

Algal samples were analyzed from 3 lakes, Crane Prairie Reservoir and Odell Lake in Oregon and an Anonymous North East System, using both standard taxonomic criteria for identification and DNA sequencing techniques. Two toxin-producing Anabaena populations, one with consistent akinete structure and another with variable akinete structure, were investigated. Samples were characterized based on several genetic markers (nifH, cpcBA-IGS, ITS1), toxins (anatoxin-a, saxitoxin, and microcystin) and morphological variation. Taxonomy within the Nostacales is based on vegetative and terminal cell structure, filament type and aggregation, and position and structure of heterocysts and akinetes. Many taxonomists rely heavily on akinete structure for microscopic identification. Identification from material preserved with Lugol's solution is challenging due to the breakup of colonies, cell distortion, and masking of pigment color. Based on morphological variation, the Crane Prairie and Odell populations were identified as A. flos-aquae, A. circinalis, or A. lemmermannii, and toxin analysis detected the presence of microcystin. These populations were most similar to A. lemmermannii (cpcBA-IGS) or Anabaena sp. (ITS1) by DNA sequence analysis. The Anonymous North East System population was identified as A. flos-aquae, A. circinalis or A. spiroides based on morphological variation, and both microcystin and anatoxin-a were detected in these samples. Sequences most similar to A. cylindrica (nifH), A. planktonica (cpcBA-IGS), A. spiroides or Aphanizomenon flos-aquae (ITS1) were identified in the Anonymous North East System samples, but there were no definitive matches. Although molecular methods can be useful tools for confirming identification based on field material, their ability to resolve issues of taxonomic identification are dependent on the comprehensiveness of the sequence database. Taxonomic keys based on cell morphology and identification based on current DNA sequence databases are subject to similar levels of variation and uncertainty.

A TUBULAR MASTIGONEME‐RELATED PROTEIN, OCM1, ISOLATED FROM THE FLAGELLUM OF A CHROMOPHYTE ALGA,OCHROMONAS DANICA1

The phylogenetic group stramenopiles refers to the systematic groups that possess tripartite tubular hairs (stramenopiles) on their flagella. There have been a number of studies describing the fine structure of these mastigonemes and a few studies isolating the component proteins; however, these proteins and their gene sequences have not yet been identified. In the present study, we identified a mastigoneme protein (Ocm1) of the chrysophycean algaOchromonas danicaPringsh. (UTEX LB1298). Its corresponding gene,Ocm1, was identified by using degenerate primers that correspond to the partial amino acid sequences of a protein (85 kDa) obtained from a mastigoneme‐rich fraction of isolated flagella. The polypeptide encoded byOcm1has four cysteine‐rich, epithelial growth factor (EGF)–like motifs, potentially involved in protein–protein interactions. It lacks obvious hydrophobic regions characteristic of transmembrane domains, suggesting that this polypeptide is not likely a protein for anchoring the mastigoneme. In addition, a polyclonal antibody against Ocm1 labeled the area where the tubular shafts of the mastigonemes are located, but not the basal portion or the terminal filaments.

Structure of the ovaries and oogenesis in Cixius nervosus (Cixiidae), Javesella pellucida and Conomelus anceps (Delphacidae) (Insecta, Hemiptera, Fulgoromorpha)

Ovary organization in representatives of two families of Fulgoromorpha, Cixiidae ( Cixius nervosus) and Delphacidae ( Javesella pellucida and Conomelus anceps), was examined by light and transmission electron microscopy. Ovaries of studied fulgoromorphans consist of telotrophic ovarioles. From apex to base individual ovarioles have four well defined regions: a terminal filament, tropharium (trophic chamber), vitellarium and pedicel (ovariolar stalk). Tropharia are not differentiated into distinct zones and consist of syncytial lobes containing multiple trophocyte nuclei embedded in a common cytoplasm. Lobes are radially arranged around a branched, cell-free trophic core. Early previtellogenic (arrested) oocytes and prefollicular cells are located at the base of the tropharium. The vitellarium houses linearly arranged developing oocytes each of which is connected to the trophic core by a broad nutritive cord. Each oocyte is surrounded by a single layer of follicular cells that become binucleate at the beginning of vitellogenesis.

Genetic dissection of a stem cell niche: The case of the Drosophila ovary

In this work, we demonstrate a powerful new tool for the manipulation of the stromal component of a well-established Drosophila stem cell niche. We have generated a bric-a-brac 1 (bab1)-Gal4 line that drives UAS expression in many somatic ovary cell types from early larval stages. Using this Gal4 line, we could effectively induce FLP/FRT-mediated recombination in the stromal cells of the ovarian germline stem cell niche. Mutant clones were observed in the developing ovary of larvae and pupae, including in somatic cell types that do not divide in the adult, such as the cap cells and the terminal filament cells. Exploiting the ability of bab1-Gal4 to generate large clones, we demonstrate that bab1-Gal4 is an effective tool for analyzing stem cell niche morphogenesis and cyst formation in the germarium. We have identified a novel requirement for engrailed in the correct organization of the terminal filaments. We also demonstrate an involvement for integrins in cyst formation and follicle cell encapsulation. Finally using bab1-Gal4 in conjunction with the Gal80 system, we show that while ectopic dpp expression from stromal cells is sufficient to induce hyperplastic stem cell growth, neither activation nor inactivation of the BMP pathway within stromal cells affects germline stem cell maintenance.

Sarcomeric Alpha‐actinin is a Myotendinous Junction Component in Neonatal Mice

Myotendinous junctions (MTJs) are cell adhesion sites where new sarcomeres are added in series during longitudinal muscle growth. Currently, little is known about the developmental dynamics of this specialized region of the muscle cell. This study compared the ultrastructure and composition of MTJs from rapidly-growing neonatal mice, and adult mice whose muscles have reached a steady-state length. At neonatal MTJs membrane exhibits greatly reduced folding, myofibrils are immature and interspersed with unorganized cytoplasm, and the tendon is densely populated with mononucleated cells, most likely fibroblasts, many of which are in close association with the muscle cell. Immunolocalization of the nebulin-related protein N-RAP, which is localized exclusively to the terminal cytoskeletal filament bundles at MTJs in adult skeletal muscle, is also restricted to these sites in neonatal muscle. Vinculin and dystrophin were co-localized subjacent to the membrane, also at both adult and neonatal MTJs. Interestingly, sarcomeric α-actinin was localized to MTJ adhesion plaques in neonates, but was not detected at the adult MTJ. The variable presence of sarcomeric α-actinin at MTJs suggests that muscle fibers differentially regulate actin-membrane interactions during postnatal growth, either in response to developmental stage-specific differences in mechanical stress, or to accommodate serial sarcomere addition in neonates. Supported by a grant from the American Heart Association, and by the Dean of the UMKC School of Biological Sciences.

The nymphs of the genus Camelobaetidius Demoulin (Ephemeroptera : Baetidae) in Brazil : new species, new records, and key for the identification of the species

Three new species of Camelobaetidius Demoulin (Ephemeroptera : Baetidae), C. hamadae sp. n., C. lassance sp. n., and C. maranhensis sp. n., are described based on nymphs from Brazil. Some of the diagnostic characters of the new taxa are : C. hamadae sp. n., two thoracic gills at base of forelegs and claws with 36-37 denticles ; C. lassance sp. n., claws with approximately 60 denticles and posterior margin of terga with few, large and elevated spines ; C. maranhensis sp. n., pronotum with medial pair of tubercles and terminal filament shorter than 10th abdominal segment. Camelobaetidius cayumba (Traver & Edmunds), C. matilei Thomas & Peru, and C. tuberosus Lugo-Ortiz & McCafferty, are recorded for the first time from Brazil. A key to the species of Camelobaetidius recorded from Brazil based on nymphs is presented.

Hedgehog signaling controls Soma‐Germen interactions during Drosophila ovarian morphogenesis

The genetic analysis of Drosophila adult oogenesis has provided insights into the molecular mechanisms that control cell proliferation, differentiation, migration, and intercellular signaling. However, little is known about the larval and pupal cellular events leading to the formation of the highly organized adult ovary, which is composed of ovarioles each containing germline cells enveloped by specialized somatic cells. We describe here the presence of ovarioles devoid of any germ cells in adult females mutant for fused, which encodes a Hedgehog signal transducing serine/threonine kinase. We show that this phenotype corresponds to a requirement for fused function for the organization of germ cells with respect to ovarian somatic cells during ovariole formation specifically during pupal stages and provide some evidence by means of clonal analysis suggesting that fused function may be necessary in the germline. hedgehog is expressed specifically in somatic terminal filament cells in pupal ovaries, and females bearing hedgehog strong loss-of-function mutations also exhibit aberrant germ cell distribution and formation of agametic ovarioles. These results indicate a positive role for Fused in the transduction of somatic Hedgehog signaling instructing ovariole morphogenesis. We also provide evidence for the use of noncanonical Hedgehog signal transducer(s) within germline cells.

Structure of the ovary and the differentiation of follicular epithelium in the pig louse, Haematopinus suis (Insecta: Phthiraptera)

The female reproductive system of the pig louse, Haematopinus suis (Insecta: Phthiraptera) is composed of paired ovaries, lateral oviducts, and a common oviduct that leads into a vagina. Clusters of mycetocytes (= cells filled with symbiotic organisms) are associated with lateral oviducts. Each ovary is composed of five loosely arranged ovarioles of the polytrophic-meroistic type. An individual ovariole is covered by a basal lamina and is composed of a terminal filament, germarium, and vitellarium. The terminal filament is composed of large, disc-shaped cells that are orientated perpendicularly to the long axis ofthe ovariole. The basal part of the terminal filament is separated from the germarium by a well-developed transverse septum. The germarium is short and filled with clusters of oogonial cells. In each cluster the cells arejoined by intercellular bridges, filled with fusomal material. Within the cluster, only one cell, the future oocyte, enters the prophase of the first meiotic division; the other cells differentiate into nurse cells. The basal part ofthe germarium is filled with the somatic prefollicular cells. The boundary between the germarium and the vitellarium is not distinct. The vitellarium contains linearly arranged ovarian follicles in subsequent stages of oogenesis (previtellogenesis, vitellogenesis and choriogenesis). Each follicle consists of an oocyte and 7 nurse cells and is surrounded by follicular cells. During oogenesis the follicular cells diversify, so that ultimately, five morphologically distinct subpopulations of these cells can be distinguished: (1) cells in contact with the nurse cells, (2) anterior cells, (3) mainbody cells, (4) posterior cells, and (5) interfollicular cells. Interestingly, the follicular cells associated with the anterior part of the oocyte, i.e. located in space at the oocyte/nurse cell border (fold cells) are mitotically active throughout previtellogenesis. It might be suggested, in this context, that the separation of the oocyte from the nurse cell compartment is brought about by mitotic divisions, consequent multiplication and centripetal migration of these cells.

Ultrastructural studies of the ovary of Palaeococcus fuscipennis (Burmeister) (Insecta, Hemiptera, Coccinea: Monophlebidae)

Ovaries of Palaeocoocus fuscipennis are composed of about 100 telotrophic ovarioles that are devoid of terminal filaments. In the ovariole a tropharium ( = trophic chamber) and vitellarium can be distinguished. The tropharium contains 7 trophocytes. A single oocyte develops in the vitellarium. The oocyte is surrounded by follicular cells that do not undergo diversification into subpopulations. The obtained results are discussed in a phylogenetic context.

External Morphology of Eggs of <I>Tipula</I> (<I>Lunatipula</I>) <I>decolor</I>, <I>Tipula</I> (<I>Lunatipula</I>) <I>dedecor</I>, and <I>Tipula</I> (<I>Acutipula</I>) <I>latifurca</I> (Diptera: Tipulidae)

External morphology of eggs of Tipula (Lunatipula) decolor Mannheims, Tipula (Lunatipula) dedecor Loew, and Tipula (Acutipula) latifurca Vermoolen were studied with both light and scanning electron microscopy. Mating pairs of these species were collected and maintained under laboratory conditions. Eggs were laid singly and were usually well separated from each other in cotton fiber batting. Eggs of these species are oval and black. The surface of the egg of T. decolor was covered by chorionic protrusions of similar size, and there was a single micropylar opening at the anterior end of egg. The egg surface of T. dedecor was covered with low protrusions with sharp tips, and there were numerous micropylar openings at the anterior end of egg. The egg surface of T. latifurca was smooth and lacked chorionic protrusions, and the anterior end of egg has a disc-shaped structure formed by coiled terminal filament.

Morphology of female reproductive tract of the predator Podisus nigrispinus (Dallas) (Heteroptera: Pentatomidae) fed on different diets

The morphology of the reproductive tract of Podisus nigrispinus (Dallas) females fed with Alabama argillacea (Hübner) larvae, artificial diet, Tenebrio molitor L. larvae or Musca domestica L. larvae were studied. The reproductive tract of females of this species presented yellow coloration and independent of the diet, each ovary had seven ovarioles joined through terminal filaments and forming a bunch shape structure. The histological data revealed that the ovary of P. nigrispinus was of meroistic telotrophic type, with each individual ovariole divided in a terminal filament, a tropharium (trophic chamber), a vitellarium, and a pedicel. The prey type affected the development and morphometry of these structures. Females of P. nigrispinus fed with 3rd or 5th instar larvae of cotton leafworm (A. argillacea) presented developed ovaries with ovarioles showing a great number of oocytes in advanced stages of development. Females fed with artificial diet presented atrophic ovaries and ovarioles practically without oocytes. Females fed with T. molitor or M. domestica showed ovaries in intermediary stage of development. The central ovariole was longer in females fed with 5th instar larvae of cotton leafworm and shorter in those fed with artificial diet. Most developed oocytes were observed in ovaries of females fed with 5th or 3rd instar larvae of cotton leafworm, and the majority of atrophic oocytes were found in females fed with artificial diet.

The initial stages of oogenesis and their relation to differential fertility in the honey bee ( Apis mellifera) castes

Neither the overall differences in ovariole number nor the caste-specifically modulated expression of vitellogenin can fully explain the striking caste differences in honey bee reproduction, in particular the mechanisms that block oogenesis in virgin queens and in workers kept in the presence of a queen. For this reason we investigated the initial stages of oogenesis in queens in relation to mating status and in workers exposed to different social conditions. A striking feature in ovarioles of both castes was a considerably elongated terminal filament which consisted not only of normal terminal filament cells but also contained apparently undifferentiated cells that were tentatively considered as stem cells. BrdU incorporation was detected in the upper germarium, as well as in the terminal filament. Cytoskeleton analysis by TRITC-phalloidin labeling for F-actin, and immunofluorescence detection for β-tubulin did not reveal structural differences in the early oogenesis steps between queens and queenless workers. In contrast, queenright workers showed signs of a disorganized microtubule and microfilament system that could explain the histological evidence for progressive cell death observed in the germaria. In addition to cytoplasmic tubulin we also detected marked intranuclear foci indicating the presence of nuclear β II-tubulin.

The apical disposition of the Caenorhabditis elegans intestinal terminal web is maintained by LET-413

We wish to understand how organ-specific structures assemble during embryonic development. In the present paper, we consider what determines the subapical position of the terminal web in the intestinal cells of the nematode Caenorhabditis elegans. The terminal web refers to the organelle-depleted, intermediate filament-rich layer of cytoplasm that underlies the apical microvilli of polarized epithelial cells. It is generally regarded as the anchor for actin rootlets protruding from the microvillar cores. We demonstrate that: (i) the widely used monoclonal antibody MH33 reacts (only) with the gut-specific intermediate filament protein encoded by the ifb-2 gene; (ii) IFB-2 protein accumulates near the gut lumen beginning at the lima bean stage of embryogenesis and remains associated with the gut lumen into adulthood; and (iii) as revealed by immunoelectron microscopy, IFB-2 protein is confined to a discrete circumferential subapical layer within the intestinal terminal web (known in nematodes as the “endotube”); this layer joins directly to the apical junction complexes that connect adjacent gut cells. To investigate what determines the disposition of the IFB-2-containing structure as the terminal web assembles during development, RNAi was used to remove the functions of gene products previously shown to be involved in the overall apicobasal polarity of the developing gut cell. Removal of dlg-1, ajm-1, or hmp-1 function has little effect on the overall position or continuity of the terminal web IFB-2-containing layer. In contrast, removal of the function of the let-413 gene leads to a basolateral expansion of the terminal web, to the point where it can now extend around the entire circumference of the gut cell. The same treatment also leads to concordant basolateral expansion of both gut cell cortical actin and the actin-associated protein ERM-1. LET-413 has previously been shown to be basolaterally located and to prevent the basolateral expansion of several individual apical proteins. In the present context, we conclude that LET-413 is also necessary to maintain the entire terminal web or brush border assembly at the apical surface of C. elegans gut cells, a dramatic example of the so-called “fence” function ascribed to epithelial cell junctions. On the other hand, LET-413 is not necessary to establish this apical location during early development. Finally, the distance at which the terminal web intermediate filament layer lies beneath the gut cell surface (both apical and basolateral) must be determined independently of apical junction position.

Morphological identification of nematode larvae of small ruminants and cattle simplified

A simplified system has been developed to facilitate identification of nematode larvae of the common nematodes of cattle, sheep and goats. Firstly, in addition to the characteristics conventionally used for identification (such as the shape of the cranial extremity and numbers of intestinal cells), the lengths of the infective sheath tails of infective larvae of each genus/species are related to that of Trichostrongylus spp. instead of using measurements for differentiation. For instance, if the mean length of the sheath tail (the distance the sheath extends caudad beyond the caudal tip of the larva) of Trichostrongylus spp. is assumed to be “ X”, then that of Haemonchus contortus is 2–2.7“ X”, and that of Oesophagostomum spp. from sheep is 4–7“ X”, etc. Secondly, by estimating the proportion of the sheath tail of a larva comprised of a terminal thin whip-like filament, identification is aided, particularly in those L3 of species that resemble one another closely, such as Chabertia ovina and Oesophagostomum venulosum or Oesophagostomum columbianum. After some practice with the system it is usually necessary to measure only one or two sheath tails of L3 in a mixed population, whereupon the identity of most of the remaining L3 can be estimated in relation to those measured, without a need for further measurements. The keys were found to facilitate differential larval identification and are particularly useful for training.

Structure of the ovary in Steingelia (Sternorrhyncha: Coccinea), and its phylogenetic implications

The paired ovaries of Steingelia gorodetskia are composed of about 100 telotrophic ovarioles devoid of terminal filaments (scale insect autapomorphy). In structure they resemble those of other scale insects, but differ in the following details: (a) all ovarioles develop synchronously, (b) they are suspended to the lateral oviducts by means of long stalks, (c) the tropharium is tubular (unique in scale insects) and (d) consists of 15–35, trophocytes, 2–4 previtellogenic oocytes that further develop, and numerous somatic prefollicular cells, (e) the vitellarium houses 2–4 linearly arranged vitellarial oocytes (versus one in most scale insects). Most of these features must be considered as plesiomorphic corresponding with the conditions in the most primitive Heteroptera. Bacterial endosymbionts have been found in some somatic cells, trophocytes, oocytes and in the nutritive cord. Present results support the opinion, based on external morphology, that the Steingeliidae are closely related to the Ortheziidae, Xylococcidae and Matsucoccidae.

Wingless signaling regulates the maintenance of ovarian somatic stem cells in Drosophila.

Identifying the signals involved in maintaining stem cells is critical to understanding stem cell biology and to using stem cells in future regenerative medicine. In the Drosophila ovary, Hedgehog is the only known signal for maintaining somatic stem cells (SSCs). Here we report that Wingless (Wg) signaling is also essential for SSC maintenance in the Drosophila ovary. Wg is expressed in terminal filament and cap cells, a few cells away from SSCs. Downregulation of Wg signaling in SSCs through removal of positive regulators of Wg signaling, dishevelled and armadillo, results in rapid SSC loss. Constitutive Wg signaling in SSCs through the removal of its negative regulators, Axin and shaggy, also causes SSC loss. Also, constitutive wg signaling causes over-proliferation and abnormal differentiation of somatic follicle cells. This work demonstrates that wg signaling regulates SSC maintenance and that its constitutive signaling influences follicle cell proliferation and differentiation. In mammals, constitutive beta-catenin causes over-proliferation and abnormal differentiation of skin cells, resulting in skin cancer formation. Possibly, mechanisms regulating proliferation and differentiation of epithelial cells, including epithelial stem cells, is conserved from Drosophila to man.