Expression and distribution of intermediate-filament proteins and laminin during the development of the bovine Müllerian duct

The expression pattern of several intermediate-filament proteins (vimentin, cytokeratin 8, 18 and 19) and the basal lamina component laminin was investigated in the Wolffian and the Müllerian ducts of bovine embryos and fetuses. The material studied comprised sexually undifferentiated stages [crown-rump length (CRL) 0.9 cm/1.0 cm/1.2 cm/1.9 cm/2.5 cm] and female stages (CRL 3.0 cm/4.2 cm/5.1 cm). Laminin could be demonstrated in the basal lamina of the developing Wolffian and Müllerian duct as well as in the stroma surrounding the Müllerian duct. The intermediate-filament protein vimentin was expressed in the mesothelium of the funnel field and in the epithelium of the Müllerian duct in all studied specimens, whereas the epithelial cells of the Wolffian duct only showed vimentin expression from a CRL of 2.2 cm onwards. In the cranial part of the Müllerian ducts only a few cells stained with pan-cytokeratin antibodies, whereas mesothelium and epithelium of the Wolffian duct showed as distinct immunostaining in all investigated stages. Both genital ducts showed no immunostaining with the antibody against cytokeratin 19 at any time of development. We conclude from our immunohistochemical results that the epithelial cells of the Wollfian duct do not contribute cells to the developing Müllerian duct.     

Prenatal differentiation of bovine oviductal epithelium: an electron microscopic study

In this study, we investigated the ultrastructural changes during the prenatal differentiation of oviductal epithelium in 16 bovine embryos and fetuses from CRL of 18.0 cm to a CRL of 94.0 cm. Ciliated and secretory cells of bovine uterine tube, a derivative of the Müllerian duct, differentiate to distinct development stages in the prenatal period. The typical cellular pattern, which is generally characteristic for the adult bovine oviduct, is also obtained during fetal life. In the early stages (CRL 18.0/20.4 cm), the bovine oviductal epithelium appears mostly undifferentiated. The epithelial cells show only a few mitochondria, some cisternae of rough endoplasmic reticulum (rER) and a small Golgi-complex. Most of the cytoplasm is filled with a large amount of glycogen, which decreases during later development. Interspersed between the undifferentiated epithelial cells, a few cells undergoing ciliogenesis can be observed. Ciliogenesis increased significantly during the later prenatal developmental stages. At a CRL of 55.0 cm, ciliated cells appear fully differentiated with mature cells covering their luminal surface. Formation of cilia usually use the acentriolar pathway. Fibrous granules occurred initially in association with the Golgi-apparatus and r(ER) in the supranuclear cytoplasm. Fibrous granules later fuse with deuterosomes and give rise to procentrioles, which are translocated to the luminal plasma membrane. There they become arranged in a line just beneath the apical cell membrane and further differentiate to basal bodies from which the formation of cilia and striated rootlets take place. Clear signs of differentiation of secretory cells were first seen in our material in fetuses with a CRL of 51.0 cm and 64.0 cm. These cells contain a well developed rER and Golgi-apparatus with dilated cisterns. In the supranuclear cytoplasm, the number of secretory granules continuously increases during later development and the cells adapt to the morphology of mature secretory cells at the CRL 94.0 cm.     

Origin of müllerian duct and its later development in relation to Wolffian duct and anogenital distance in the rat

On day 14 of fetal life in the rat, Müllerian (paramesonephric) duct in both sexes was first observed close to Wolffian (mesonephric) duct. Its caudal end joined Wolffian duct at about the level of the middle of gonadal anlage. Thereafter, Müllerian duct became independent of Wolffian duct through its whole length. Therefore, the main part of Müllerian duct may be formed by budding from Wolffian duct. On day 16, the anogenital distance in male started to increase with thickening of Wolffian duct and disappearance of Müllerian duct. Castration of male fetuses on day 19 stopped later masculinizing development. Therefore, it is clear that the fetal testis is an organ crucial for the masculinization of the urogenital tract as it activates the secretion of masculinizing hormone(s).     

3D Reconstruction of the Mouse's Mesonephros

The present work reports on the three-dimensional reconstruction of the segmented mesonephros during the embryonic development of the mouse. With a light microscope and an automatic reconstruction of surfaces, aspects of the mesonephros are described. These surfaces are obtained by using digitized contour lines. A new interpolation method called DSI (Discrete Smooth Interpolation) enables correction of the distortion induced by microtomy in paraffin sections. After a tri-angulation step, this method uses a smoothing algorithm, which implies a spatial redistribution of the vertices of the triangles to correct the rotational and translational misalignment. The use of this 3D program improves the understanding of the development patterns and helps us to appreciate changes in the rebuilt mesonephros. By 10.5 embryonic days, tubules emerge from the Wolffian body and begin their formation, then between 11.5 and 13.5 embryonic days, tortuous mesonephric tubules bound to the Wolffian duct form small curls, which grow and finally unwind. At the same time, mesonephric tubules unbound to the Wolffian duct appear, and on 13.5 embryonic days, the Müllerian duct is visible. After 14.5 embryonic days, the segmented mesonephros keeps its general aspect but decreases in size. At this time, each gonad is provided with both Wolffian and Müllerian ducts. Later, the Wolffian duct differentiates into the definitive male duct system, whilst the Müllerian duct regresses. Conversely, the paramesonephric duct differentiates into the definitive female duct system, whilst the mesonephric duct in turn degenerates. By this time degeneration has begun in the cranial portion of the mesonephros and this process progresses caudally. The spatial organization of the mesonephric tubules and the precise organization of all connections between these elements and the ducts may be well defined. Such approach can allow for a high definition of the normal pattern of mesonephros differentiation.     

Scanning electron microscopic study of the functional anatomy of the porcine oviductal mucosa

A three-dimensional model has not been clearly established for the porcine oviductal mucosa. The oviducts of 12 cyclic sows were examined by Scanning Electron Microscopy to study the structure and nature of the mucosal arrangement of the oviduct. Epithelial cyclic changes were also studied. The oviductal infundibulum is an asymmetric funnel-shaped structure surrounding the ostium, in which a wide and a narrow side can be distinguished. The mucosa is more complex in the narrow side, showing numerous and tortuous longitudinal primary folds, while the mucosa becomes simpler in the wide side. Around the ostium abdominale wide secondary folds form cul-de-sacs, with their opening pointing in ovarian direction. Areas between folds throughout the lumen of the oviduct show a high-degree of complexity. Inter-fold spaces are occupied by a system of irregular grooves and pockets, with the presence of basal crypts in the caudal oviduct. Marked variations were observed in the oviductal epithelium depending on the oviductal segment, basal or apical areas of the folds, and phase of the oestrous cycle. Cyclical changes were observed in the infundibulum and ampulla, so that prominent and numerous ciliated cells lined apical areas of the folds in the follicular phase; whereas secretary cells were predominant throughout all areas of epithelial surface in the luteal phase. The oestrous cycle phase appeared do not affect the epithelial population cells of the caudal segments of the oviduct: ciliated and secretory cells uniformly lined apical and basal areas of the folds. The topography of the oviduct provides a complex system of regulation, which may influence not only the passage of cells, but also movement of fluid within the oviductal canal.     

Entwicklung und Zelldifferenzierung des Nucleus nervi hypoglossi beim Rind

Based upon light- and electron-microscope examinations, the ontogenetic development of the nucleus of cranial nerve XII is documented. At 1-cm crown-rump length (CRL), the caudal pole of the nucleus nervi hypoglossi forms a uniform cell column with the cornu ventrale of the spinal cord. During this period, its caudal area shows signs of cellular degeneration. From 3.5 cm CRL onward, all nuclear groups can be identified. At 53 cm CRL, they correspond to the pattern as described in the adult brain. Electron-optically, at 2.5 cm and 3.6 cm CRL, the nucleus of cranial nerve XII exhibits a close relationship to the matrix layer which consists of elements of dark nuclei. The hypoglossus nucleus is composed of dark and light cell types. It is the latter type that represents the presumptive neuron; it shows an increased ultrastructural differentiation from 2.5 cm CRL onward.     

Hydrometra in two cats

Chronic abdominal distension in two female cats 2½ and 13 years old respectively was due to bilateral hydrometra. In both cases the condition resulted from absence of an internal cervical os with consequent accumulation of normal uterine secretions. The most likely cause was a segmental failure in the development of the Müllerian duct system.     

A scanning electron microscopic study of the peritoneal mesothelium covering the genital tract and its ligaments in the cow

This study was undertaken to describe the surface features of the peritoneal mesothelium covering the genital tract and adjacent ligaments of the cow during the oestrous cycle. The relationship between mesothelial surface and spermatozoa was also evaluated after intra-uterine and intraperitoneal insemination. Surface features of mesothelial cells from 25 cyclic cows were examined by scanning electron microscopy and by image analysis. Presence of spermatozoa was evaluated by scanning electron microscopy in seven additional cows. In the external side of the infundibulum, the oviductal mucosa exceeds the free margin, forming a continuous band measuring 2.5-10 mm in width. This oviductal epithelium shows cyclical variations with a predominance of ciliated cells during the follicular phase. In respect of the mesothelium, no clear morphological differences were observed associated with the side of ovarian bursa (internal versus external), or with the phase of the oestrous cycle. Mesothelial cells covering the uterus and mesometrium have a higher microvilli density and length and a smaller cell surface area than in the oviduct and adjacent structures. The presence of solitary cilia in the mesosalpinx and mesotubarium superius (infundibulo-cornual ligament) of some specimens was also observed. When samples were processed without postfixation in osmium tetroxide, a layer of amorphous material covered all surfaces. After intra-uterine insemination of five cows, no spermatozoa were found on their peritoneal mesothelium. Numerous spermatozoa were found after intraperitoneal insemination being attached throughout mesothelial surfaces. These results indicate that there are morphological differences between regions, but no cyclic changes, in the surface features of mesothelial cells covering the genital tract and adjacent ligaments of the cow, and that spermatozoa can bind to mesothelial surfaces after intraperitoneal insemination.     

On the Development of the Feline Skeleton

Light microscopic observations revealed that in camel foetuses of 25 mm crown-to-rump length (CRL) the primordial tubular system of the prospective lung was formed of several tubules lined by undifferentiated columnar epithelium. Intra-epithelial neuroendocrine cells were the first elements to be differentiated in the lining epithelium of the primordial tubular system of the prospective lung as early as 25 mm CRL. On reaching 50–67 mm CRL, the primordial tubular system started to differentiate into two systems of primordial tubules, the prospective bronchial or light tubules and the future respiratory or dark tubules. The lining epithelium of the prospective bronchial tubules revealed a clear evidence of ciliogenesis as early as 80 mm CRL. From 800 mm CRL onwards, the bronchial epithelium demonstrated ciliated and non-ciliated secretory cells. The non-ciliated secretory cells of the bronchial epithelium of fetal camel lung showed moderate reaction to AB/PAS technique, for the first time, in fetuses reaching 600 mm CRL.     

Prevalence of the AMHR2 mutation in Miniature Schnauzers and genetic investigation of a Belgian Malinois with persistent Müllerian duct syndrome

Persistent Müllerian duct syndrome (PMDS) is a sex‐limited disorder in which males develop portions of the female reproductive tract. Important consequences of PMDS are cryptorchidism and its sequelae of infertility and increased risk of testicular cancer. Anti‐Müllerian hormone (AMH) and its receptor (AMHR2) induce the regression of the Müllerian ducts in male embryos. In Miniature Schnauzer dogs, the genetic basis has been identified as an autosomal recessive nonsense mutation in AMHR2, but the allele frequency of the mutation is unknown. Thus, the primary objective of this study was to estimate the prevalence of the AMHR2 mutation in North American Miniature Schnauzers, in order to ascertain the value of genetic testing in this breed. An additional objective was to determine whether mutations in AMH or AMHR2 were responsible for PMDS in a Belgian Malinois; this would aid development of a genetic test for the Belgian Malinois breed. Genomic DNA from 216 Miniature Schnauzers (including one known PMDS case) was genotyped for the AMHR2 mutation, and DNA from a single PMDS‐affected Belgian Malinois was sequenced for all coding exons of AMH and AMHR2. The Miniature Schnauzer cohort had an AMHR2 mutation allele frequency of 0.16 and a carrier genotypic frequency of 0.27. The genetic basis for PMDS in the Belgian Malinois was not determined, as no coding or splicing mutations were identified in either AMH or AMHR2. These findings support a benefit to AMHR2 mutation testing Miniature Schnauzers used for breeding or with cryptorchidism.     

Morphological Studies on the Development of the Bronchial Epithelium of the Fetal Camel Lung

Light microscopic observations revealed that in camel foetuses of 25 mm crown‐to‐rump length (CRL) the primordial tubular system of the prospective lung was formed of several tubules lined by undifferentiated columnar epithelium. Intra‐epithelial neuroendocrine cells were the first elements to be differentiated in the lining epithelium of the primordial tubular system of the prospective lung as early as 25 mm CRL. On reaching 50–67 mm CRL, the primordial tubular system started to differentiate into two systems of primordial tubules, the prospective bronchial or light tubules and the future respiratory or dark tubules. The lining epithelium of the prospective bronchial tubules revealed a clear evidence of ciliogenesis as early as 80 mm CRL. From 800 mm CRL onwards, the bronchial epithelium demonstrated ciliated and non‐ciliated secretory cells. The non‐ciliated secretory cells of the bronchial epithelium of fetal camel lung showed moderate reaction to AB/PAS technique, for the first time, in fetuses reaching 600 mm CRL.     

Histological and histochemical study of the proventriculus (Ventriculus glandularis) of common starling (Sturnus vulgaris)

The histological and histochemical structures of the proventriculus of starling (Sturnus vulgaris) were examined using haematoxylin and eosin and special staining, that is periodic acid schiff (PAS), Masson's trichrome, Alcian blue, Orcein and Reticulin. All three cranial, middle and caudal parts of the proventriculus were also studied. The study results showed that the wall of the proventriculus consisted of mucosal, submucosal, muscular and serosal tunics. The mucosal tunic presented folds and sulci on its luminal surface. In the first third of the proventriculus, the tunica mucosa characterized by presence of folds lined by stratified squamous epithelium and presence of simple tubular glands in the lamina propria. In the middle and caudal thirds of the proventriculus, the surface was covered by a columnar epithelium, and the branched tubular glands were extended through the lamina propria. From the base of the branched tubular glands, the deep proventricular glands were observed that were compound tubuloalveolar lobules. The surface epithelium of the tunica mucosa and the cells lining the proventricular glands showed a positive reaction to PAS and Alcian blue stainings. In addition, the epithelial cells of the tubular and branched tubular glands showed Masson's trichrome-positive reaction. The submucosal tunic was thin in the proventricular wall. The tunica muscularis was formed by a thin inner layer of longitudinal smooth muscle fibres and a thick outer layer of circular fibres. The serosa consisted of loose connective tissue, rich in blood vessels and covered by mesothelium.     

Surface alterations in the bovine pelvic peritoneum following rectal examination of reproductive organs: a scanning electron microscopy study

The aim of the present study was to evaluate possible injury provoked by an intensive reproductive programme based on rectal palpation in the cow. Alterations to the mesothelial surface of the peritoneum and to the oviductal mucosa were explored by scanning electron microscopy (SEM). Nine multiparous Friesian cows were selected from each of two commercial dairy herds. Cows in herd 1 were maintained on a weekly reproductive health programme, while those in herd 2 were not subjected to gynaecological exploration. Surface features of the mesothelial cells of the reproductive tract and adjacent ligaments, and the oviductal mucosa were explored by SEM observation of tissue specimens from each animal. Six cows in the intensive reproductive programme showed microscopic alterations of the peritoneal surface. Strands were observed in the oviducts and adjacent ligaments in five of these cows. These strands, composed of filaments covered with mesothelial cells, were probably adhesions. Cells with abundant vacuoles on their surface were detected in the mesothelium corresponding to three animals, two of which also showed microscopic strands. No peritoneal surface alterations were observed in the cows from herd 2. No oviductal mucosal modifications were detected in either group. These findings suggest that intensive reproductive programmes based on rectal palpation may increase the risk of peritoneal surface alterations in the cow, but effects on reproductive physiology are unlikely.     

Uterine malignant mixed Müllerian tumor (Metaplasic carcinoma) in the cat: clinicopathologic features and proliferation indices

A homologous malignant mixed Müllerian tumor of the uterus occurring in an 8-year-old Persian cat was described with regard to its clinical and pathologic features. A polypoid multinodular mass of the right uterine horn was shown by an ultrasound examination. Grossly, the right uterine horn was enlarged because of a vegetative and infiltrating tumor, grayish-white in color, that penetrated the uterine wall to the level of the perimetrium. Many metastatic nodules were found in abdominal and thoracic cavities. Histologically, the neoplasm had both carcinomatous and sarcomatous components and was diagnosed as an uterine malignant mixed Müllerian tumor. This is the fourth case reported in cats. The histologic features and proliferation rate of this tumor were similar to the corresponding human neoplasms, which occur mainly in postmenopausal women. The possible hormone dependence of the tumor is briefly discussed.     

Three‐Dimensional Architecture of the Ovine Oviductal Mucosa

The objective of this work was to establish for the first time a complete three‐dimensional model of the ovine oviductal mucosa. The oviducts of 15 cyclic ewes were examined combining the direct examination of the mucosa, by scanning electron microscopy (SEM) and histology, with the SEM observation of resin moulds of the oviductal lumen. Around the ostium abdominale, all longitudinal primary folds and wide secondary are seen to form cul‐de‐sacs, with their opening pointing in the ovarian direction were observed. At the connection of the ampulla to the isthmus, there is a sharp change in the morphology, from a high folded structure to a smoother one. In the utero‐tubal junction, the primary folds broaden and become more voluminous, the lumen has a slit‐like appearance, and secondary folds form cul‐de‐sacs with their opening oriented towards the uterus. The areas between the folds throughout the lumen of the oviduct show a high degree of complexity. The presence of crypts was observed in all the regions studied, branched in the ampulla and spiniform in the isthmus. Marked variations were observed in the oviductal epithelium depending on the oviductal segment and the basal or apical areas of the folds. The variations found regarding the phase of the oestrous cycle were similar to those described in studies of other species. The anatomy of the oviductal mucosa provides a complex system that seems to be designed to regulate the movement of fluids and the passage of cells within the oviductal canal.     

Anatomical and Histological Organization of the Testes of the Inseminating Catfish Trachelyopterus striatulus (Steindachner, 1877) (Siluriformes: Auchenipteridae)

The testicular morphology, spermatogenesis and occurrence of sperm in the ovarian lumen of Trachelyopterus striatulus were studied using anatomical, histological and biometric techniques. A total of 50 catfish (T. striatulus) were captured, measuring 14.9 ± 2.5 cm of standard length, body weight was 81.2 ± 34.5 g and their testes weighed 16.9 ± 6.1 g. The testes of T. striatulus are paired organs, showing two distinct regions: cranial, which shows a compact medial part and with fringes ventrally, and caudal region, which is formed of the seminal vesicle with fringes laterally and two saculiform expansions. The testes presented a length of 35.2 ± 6.9 mm, and the fringes showed a cranial length of 12.1 ± 3.8 mm and caudal length of 6.4 ± 2.6 mm. Histologically, the cranial fringes are spermatogenic and showed cells with significantly different nuclear diameters, ranging from 8.2 ± 1.5 μm (primary spermatogonia) to 1.88 ± 0.3 μm (spermatid). The seminal vesicles and saculiform expansions showed tubules with a simple prismatic secretory epithelium containing spermatozoa and secretion into the lumen. The caudal fringes are exclusively for secretory flow, consisting of tubules with a simple cuboidal epithelium. The common spermatic duct showed a simple cuboidal epithelium and contained spermatozoa with secretion into the lumen. The secretion of the caudal region is acidophilic, with neutral glycoproteins and sialomucin. T. striatulus ovaries showed free spermatozoa or were organized in spermatozeugmata into the ovarian lumen and between the ovuligers lamellae.     

Development of the mesonephros in camel (Camelus dromedarius)

The study of the development of the mesonephros in the camel (Camelus dromedarius) was carried out on 16 embryos ranging from 0.9 to 8.6 cm crown vertebral rump length (CVRL). At 0.9 cm CVRL, the mesonephros is represented by a narrow strip along the roof of the thoracolumbar part of the vertebral column. At 1.4 cm CVRL, some of the mesonephric tubules are canalized but others are still solid. The mesonephric corpuscles are well developed at 1.9 cm CVRL and occupy almost the entire abdominal cavity in between the liver and the gut. Histologically, the glomeruli occupy the ventromedial aspect of the mesonephros while the mesonephric tubules become numerous, larger and more coiled. At 3 cm CVRL, the metanephros is invaginated in the caudal pole of the mesonephros, and the mesonephric tubules in some areas are differentiated into secretory and collecting tubules. At 3.5 cm CVRL the mesonephros is related dorsally to the postcardinal vein and ventrally to the subcardinal vein. At 4.7 cm CVRL continuous regression of the mesonephros from cranialwards to caudalwards is observed. At 5.3-5.5 cm CVRL, the cranial part of the mesonephros is divided into medial and lateral regions, and later the medial region completely disappears and is replaced by the primordium of the adrenal gland. At 8.6 cm CVRL, the caudal part of the mesonephros completely disappears.     

In Vivo and In Vitro Sperm Interaction with Oviductal Epithelial Cells of Llama

Sperm reservoirs in South American Camelids would be crucial for successful fertilization. Since ovulation occurs approximately 36 h after mating, the maintenance of the sperm viability in the oviduct waiting for the ovum is a critical reproductive event. Our study aimed at determining whether the isthmus or the utero tubal junction (UTJ) could function as a sperm reservoir in llama by means of in vivo and in vitro experiments. For the in vivo experiments, the oviducts of adult females with a dominant follicle bigger than 7 mm were examined for the presence of sperm at 6, 18, 24, 28 and 35 h after mating. The results using scanning and transmission electron microscopy showed ultrastructural differences between isthmus and UTJ with respect to (1) predominance of secretory cells in the UTJ and ciliated cells in the isthmus epithelium and (2) cytoplasmic bulbous projection of the secretory cells in the UTJ. Sperm adhered by a mucus‐like substance were seen only in the UTJ at 6, 18, 24 and 28 h post‐mating. Lack of sperm adhered to oviductal mucosa was observed around ovulation (35 h). In vitro experiments demonstrated higher ability of UTJ epithelial cell explants with respect to isthmus explants to bind sperm in a co‐cultured system. The anatomical features and the presence of a sperm bonding agent in the UTJ together with the in vitro differential binding of sperm to UTJ explants strongly suggest that both may be feasible mechanisms that facilitate sperm storage in this oviductal region in llama.     

Development of the glandular epithelium of the bovine parotid gland during ontogenesis

The development of the parotid gland was examined in 36 bovine embryos and foetuses with a crown-rump-length (CRL) from 28 up to 1000 mm by light, transmission electron microscopical and actin-immunohistochemical methods. The anlage of the parotid gland in an embryo with 28 mm CRL can be found at the lateral angle of the primitive oral cavity as a local thickening of the epithelium. During the second month, the differentiation of primary ducts and endbuds starts and a lumen develops in the primary ducts. At the end of the second month a lumen appears in the terminal endbuds. In the immature endpiece cells first secretory granules can be seen from a CRL of 240 mm. In the third month differentiation between intra- and inter-lobular ducts is possible. Immature myoepithelial cells present as a basal layer of flattened cells between the epithelial cells and the basement membrane at the end of the second month. During further development they increase in number, become more flattened and form long cellular processes. At the end of the fourth month isolated actin filament bundles are formed, which were also detected by an antibody against smooth muscle actin. The actin filaments condense continuously until they fill the cell processes completely at the end of foetal development.     

Morphological Changes and Proliferative Activity in the Oviductal Epithelium during Hormonally Defined Stages of the Oestrous Cycle in the Bitch

The aim of the present study was to investigate morphological changes and proliferative activities in the epithelium of the canine oviduct with regard to the part of the oviduct – possibly indicating the existence of a locally restricted sperm reservoir – and the stage of the oestrous cycle. Nine healthy adult nulliparous bitches were submitted to ovariohysterectomy at three stages of the cycle: anoestrus (n = 3), late follicular phase (n = 3) and mid‐luteal phase (n = 3). The whole oviduct ranging from the utero‐tubal junction (UTJ) to the infundibulum (IN) was collected, divided into UTJ, IN plus six segments of equal length, i.e. eight oviductal specimens per animal were studied by light microscopy. Morphological characteristics of ovaries and endometrium were recorded macroscopically and verified histologically. The height of oviduct epithelial cells and percentage of ciliated cells (CC) were assessed and the respective data analysed statistically. Proliferative activity was immunohistochemically visualized by means of Ki‐67 antigen detection. Blood was collected and concentrations of oestradiol‐17β and progesterone (P₄) were measured. Within the IN and five of the six tissue samples collected from the ampulla and isthmus in anoestrous bitches, the oviductal surface epithelium consisted of low cuboidal cells demonstrating a uniform dark staining intensity. Only a very few scattered lighter staining CC could be detected. Under the influence of oestrogens during late follicular phase, the oviductal epithelium was highly differentiated. Lighter stained CC with apically located nuclei were easily distinguishable from basophilic secretory cells with apical cytoplasmic protrusions. Cell height and percentage of CC were significantly higher than in anoestrus (p ≤ 0.05). During mid‐luteal phase, high levels of P₄ were associated with differentiated and dedifferentiated cells as well as cells in regression seen in the mucosal folds of all samples. The percentage of CC and cell height were significantly lower than during late follicular phase (p ≤ 0.05). Further signs of dedifferentiation consisted of a loss of cilia, a pinching off of the apical cytoplasm as well as the presence of debris and macrophages within the oviductal lumen. In the oviductal part of UTJ and the caudal isthmus hormone‐dependent variations in cellular morphology were less distinct. Changes in cell height were minimal and did not differ significantly throughout the oestrous cycle. Hypertrophic cells with large nuclei were predominantly present at these sites, but did not consistently demonstrate signs of ciliation or secretion. Sporadic proliferating activity, visualized by means of Ki‐67 antigen, was mainly seen in some cells of the late follicular phase samples. Thus, overall proliferative activity is generally very low or may occur within a relatively short period of time. It therefore cannot be excluded, that periods exhibiting higher mitotic rates are not included in the present study. It should, however, be mentioned that cells demonstrating morphological signs of apoptosis can only be seen very sporadically within a few specimens during mid‐luteal phase, thus, reflecting low proliferative capacities and minimal cellular turnover found during this study. The results of the present study strongly indicate that oestrogens cause hypertrophy and differentiation, whereas P₄ induces gradual dedifferentiation or regression of the oviductal epithelium. Furthermore, they reveal clearly visible changes in the morphology of the tubal epithelium during the oestrous cycle. Depending on the tubal segment, these are, however, variably expressed. Whether the low degree of cellular variation of the UTJ and caudal isthmus is caused by specific hormone concentrations at these sites or specific regulatory mechanisms and may be associated with specific functional properties such as the formation of a locally restricted sperm reservoir needs further investigations.