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Photoperiod is an important factor of mammalian seasonal rhythm. Here, we studied morphological differences in the Harderian gland (HG), a vital photosensitive organ, in male striped dwarf hamsters (Cricetulus barabensis) under different photoperiods (short photoperiod, SP; moderate photoperiod, MP; long photoperiod, LP), and investigated the underlying molecular mechanisms related to these morphological differences. Results showed that carcass weight and HG weight were lower under SP and LP conditions. There was an inverse correlation between blood melatonin levels and photoperiod in the order SP > MP > LP. Protein expression of hydroxyindole-O-methyltransferase (HIOMT), a MT synthesis-related enzyme, was highest in the SP group. Protein expression of bax/bcl2 showed no significant differences, indicating that the level of apoptosis remained stable. Protein expression of LC3II/LC3I was higher in the SP group than that in the MP group. Furthermore, comparison of changes in the HG ultrastructure demonstrated autolysosome formation in the LP, suggesting the lowest autophagy level in under MP. Furthermore, the protein expression levels of ATP synthase and mitochondrial fission factor were highest in the MP group, whereas citrate synthase, dynamin-related protein1, and fission1 remained unchanged in the three groups. The change trends of ATP synthase and citrate synthase activity were similar to that of protein expression among the three groups. In summary, the up-regulation of autophagy under SP and LP may be a primary factor leading to loss of HG weight and reduced mitochondrial energy supply capacity., Competing Interests: The authors have declared that no competing interests exist.

Posterior lingual glands consist of two sets of minor salivary glands that serve important functions in oral physiology. To investigate the hypothesis that the hypoglossal nerve provides sympathetic innervation to the posterior lingual glands, we examined ultrastructural changes in the glands following hypoglossal denervation. In the posterior deep lingual glands (of von Ebner), the serous acinar cells showed a decrease in the number of secretory granules and an increase in lipofuscin accumulation. The ratios of cells containing lipofuscin granules were 11.39, 36.49 and 50.46%, respectively, of the control, 3- and 7-day post-axotomy glands (P < 0.001). Intraepithelial phagocytotic activity was increased. The mucous acinar cells in the posterior superficial lingual glands (of Weber) also showed degenerative changes after hypoglossal denervation. One week after nerve transection, marked cytoplasmic vacuolation and fragmentation of organelles were frequently observed. Degenerative changes were also found in unmyelinated axons associated with the glands. We provide the first evidence of the structural and functional connections between the sympathetic component of the hypoglossal nerve and posterior lingual glands.

Previous studies have shown the existence of a sympathetic component in some cranial nerves including the hypoglossal nerve. In this study, the horseradish peroxidase (HRP) tract-tracing retrograde technique and experimental degeneration method were used to elucidate the possible neuroanatomical relationship between the superior cervical ganglion (SCG) and the hypoglossal nerve of hamsters. About 10% of the SCG principal neurons were HRP positive following the tracer application to the trunk of hypoglossal nerve. Most of the HRP-labelled neurons were multipolar and were randomly distributed in the ganglion. When HRP was injected into the medial branch of the hypoglossal nerve, some of the SCG neurons were labelled, but they were not detected when HRP was injected into the lateral branch. The present findings suggest that postganglionic sympathetic fibres from the SCG may travel along the hypoglossal nerve trunk via its medial branch to terminate in visceral targets such as the intralingual glands. By electron microscopy, the HRP reaction product was localised in the neuronal somata and numerous unmyelinated fibres in the SCG. In addition, HRP-labelled axon profiles considered to be the collateral branches of the principal neurons contained numerous clear round and a few dense core vesicles. Besides the above, some HRP-labelled small myelinated fibres, considered to be visceral afferents, were also present. Results of experimental degeneration following the severance of the hypoglossal nerve showed the presence of degenerating neuronal elements both in the hypoglossal nucleus and the SCG. This confirms that the hypoglossal nerve contains sympathetic component from the SCG which may be involved in regulation of the autonomic function of the tongue.

Intracellular recording and HRP injection were used to characterize retinocollicular axons in the hamster. Eighteen retinotectal axons were filled with HRP. The type of axon recovered most often (N = 10) had a receptive field with center-surround organization, gave sustained responses to flashed spots, had linear spatial summation, and responded only to more slowly moving stimuli. These fibers had a mean axonal diameter of 0.58 microns (SD = 0.21) and an average conduction latency of 3.4 msec (SD = 1.2) to optic chiasm (OX) stimulation. They gave rise to circular or vertically elongated arbors with an average cross-sectional area of 33,238 microns2 (SD = 12,763) and were almost completely restricted to the stratum griseum superficiale (SGS). We recovered 6 fibers with very different structural and functional properties. The receptive fields of these axons also had center-surround organization, but gave phasic responses to flashed stimuli, had nonlinear spatial summation, and responded to rapidly moving stimuli. These axons had an average diameter of 0.84 micron (SD = 0.09) and an average latency of 2.1 msec (SD = 0.40) to OX stimulation. They had elongated arbors with an average cross-sectional area of 78,045 microns2 (SD = 14,252) and innervated the SGS, the stratum opticum (SO), and upper stratum griseum intermediale (SGI). We also recovered 2 fibers that had thin (0.3 and 0.6 micron) axons and latencies of 4.2 and 3.6 msec to OX stimulation. They both gave rise to horizontally oriented arbors in the stratum zonale (SZ) and upper SGS.

Histomorphology of the cheek pouch was studied in 14 hamsters by light and transmission electron microscopy. The cheek pouch wall was devoid of any lymphatic tissue and dense subepithelial tissue (i.e. the lamina propria) would render lymph drainage almost impossible and might constitute impermeable morphological barrier for non-recognition of transplants evoking a host immune response. Because in the literature it was reported that there is absence of any arteriovenous anastomoses on the pouch wall, and interruption of arterial supply failed to alter the growth rate of tissue grafts, we speculated that epidermal growth factors present in the saliva could play a role in maintaining the growth of tissue transplants.

The onset and evolution of the meiotic prophase were assessed by histological and cytological techniques in fetal and neonatal ovaries obtained from day 13 postcoitum to day 11 postpartum in the golden hamster (Mesocricetus auratus). The histological technique based on the Feulgen stain and the cytological technique based on the Giemsa stain of isolated nuclei were judged to be specific for staining chromatin and chromosomal DNA. The results obtained with both techniques were quite similar and confirmed most of the meiotic prophase takes place during the neonatal period. However, at variance with other reports, it was found that the meiotic prophase starts on day 14 postcoitum. This discrepancy is attributed to the greater specificity of techniques and time span covered in the present study.

The ultrastructure of the macula densa of the hamster kidney is, in general, similar to that reported for the few other species which have been studied. Their structure indicates that macula densa cells differ in a number of important respects from those of the rest of the thick ascending limb and the distal convoluted tubule. Among these differences may be mentioned the more widely distributed mitochondria with their circular and elliptical profiles; the extensive subsurface vacuolation and the irregular and shallow folds of the basal plasma membrane. Unlike some reports on other species, however, the Golgi complexes were not restricted to an infranuclear position, but were observed in a variety of situations. It was also observed that T-H glycoprotein characteristically associated with the plasma membranes of the cells of the thick ascending limb and the distal convoluted tubule was invariably absent in the macula densa. The possible physiological implications of this and the other observations are discussed.

This study provides quantitative information on the testes of seasonally breeding golden hamsters during active and regressed states of gonadal activity. Seminiferous tubules occupied 92.5% of testis volume in adult gonadally active animals. Leydig cells constituted 1.4% of the testicular volume. The mean volume of an individual Leydig cell was 1092 microns 3, and each testis contained about 25.4 million Leydig cells. The volume of an average Sertoli cell nucleus during stage VII-VIII of the cycle was 502 microns 3. A gram of hamster testis during the active state of gonadal activity contained 44.5 million Sertoli cells, and the entire testis contained approximately 73.8 million Sertoli cells. Testes of the hamsters exposed to short photoperiods for 12-13 wk displayed a 90% reduction in testis volume that was associated with a decrease in the volume of seminiferous tubules (90.8% reduction), tubular lumena (98.8%), interstitium (72.7%), Leydig cell compartment (79.3%), individual Leydig cells (69.7%), Leydig cell nuclei (50.0%), blood vessels (85.5%), macrophages (68.9%), and Sertoli cell nuclei (34.1%). The diameter (61.1%) and the length (36.8%) of the seminiferous tubules were also decreased. Although the number of Leydig cells per testis was significantly lower (p less than 0.02) after short-photoperiod exposure, the number of Sertoli cells per testis remained unchanged. The individual Sertoli cell in gonadally active hamsters accommodated, on the average, 2.27 pre-leptotene spermatocytes, 2.46 pachytene spermatocytes, and 8.17 round spermatids; the corresponding numbers in the regressed testes were 0.96, 0.20, and 0.04, respectively. The striking differences in the testicular structure between the active and regressed states of gonadal activity follow photoperiod-induced changes in endocrine function and suggest that the golden hamster may be used as a model to study structure-function relationships in the testis.

The testes of the spiny mice showed asymmetry, the left being significantly heavier than the right (P = 0.025). Histological studies indicated that spermatozoa were first present in the testes of animals 35--45 days of age but the maturation of the accessory glands, especially the lateral prostates and coagulating glands, occurred later. The highest fructose concentration in the adult was in the lateral prostates (126.97 +/- 22.23 mg fructose/100 g, n = 5) and coagulating glands (99.38 +/- 17.65 mg fructose/100 g gland weight, n = 5). Coagulation tests of mixtures of extracts of seminal vesicles and coagulating glands from spiny mice and rats indicated that the vesiculase of the spiny mouse was active on rat substrates and vice versa. Cross-reactions of extracts of house mouse (Mus musculus), hamster (Mesocricetus auratus), gerbil (Meriones unguiculatus), and guinea-pig (Cavia porcellus) seminal vesicles (substrate) and coagulating glands (vesiculase) with those of rats and spiny mice showed that although the substrates of rat and spiny mouse were readily coagulated by vesiculase from all the other species, rat and spiny mouse vesiculase were not equally active on substrates of the other species.

The autonomic outflow and sensory structures in the ovary and accessory reproductive organs of the hamster are described by means of specific fluorescence and enzyme histochemical techniques for the demonstration of catecholamine and acetylcholinesterase (AChE), respectively. Sympathetic nerves accompany branches of the major blood vessels in the mesentery of the ovary, oviduct and tubal uterine horn and invest the vascular bed in each of these organs. Vasomotor fibers predominate in the ovary and oviduct, though occasional adrenergic axons supply thecal and interstitial tissues in the ovary and the longitudinal smooth muscle of the oviduct. Fluorescent myomotor axons run in the suspensory ligament and outer myometrial layer of the uterus, but most of the numerous sympathetic and AChE-fibers in the tubal third of the horn supply the intramural and submucosal vascular plexuses. A limited electron microscopic study of the central spiral (preplacental) arteries of the endometrium indicates that the surrounding terminal AChE-fibers are identical to the fluorescent and granular vesicle-bearing adrenergic axons which form neuromuscular junctions with these vessels. Based on the discovery of specialized sensory endings in the walls of the large collecting veins which drain the hamster uterus, a mechanism is proposed to account for the regulation of blood flow through maternal placental vessels which are devoid of an arteriolar neuromuscular apparatus.

Relative length is a constant and distinctive characteristic for each autosomal SC, despite variations in absolute length from cell to cell. Arm ratio is distinctive for each SC except for two of the three sub-acrocentrics, and serves, together with relative length, for identification. The constancy of relative length and arm ratios indicates biological stability and lack of physical distortion in these spread preparations. There is a 1:1 relationship between relative lengths of autosomal SCs and mitotic autosomes: their arm ratios are similar. These close parallels provide strikingly similar SC and somatic karyotypes. Variability was observed in sub-acrocentric arm ratios and in lengths of unpaired X and Y axes, correlated with the presence of constitutive heterochromatin. - Utilizing progressive differentiations of the X and Y chromosomes for staging, it is demonstrated that autosomal SCs decrease in length from late zygotene to mid-pachytene, and then increase at late pachytene. Within a nucleus, synchrony of length changes is maintained. It is concluded that the factors governing autosomal SC length are regular for any given bivalent from cell to cell and may be related to those that control somatic autosome length relationships. - The X and Y axes differ quantitatively as well as qualitatively from autosomal SCs. The SC portion of the X and Y is constant in length through most of pachytene; the unpaired axes shorten and lengthen, but not in proportion to autosomal SCs. X and Y relative lengths ard arm ratios vary throughout pachytene and do not maintain proportionality with somatic values. The evidence suggests, but does not prove, that the long arm of the X is paired with the short arm of the Y.-Twists occur in autosomal SCs at increasing frequencies throughout pachytene but cannot account for length changes. The number of twists per SC is directly proportional to SC length. Intertwining of SCs is random and proportional to SC length. End-to-end associations of autosomal SCs appear to be random; however, the ends of the X and Y are less often involved in such connections. - The length of axial material in all chromosomes at pachytene, expressed as an equivalent length of DNA double helix, represents 0.013% of the diploid DNA complement.

Alterations occur predominantly in cells of the zona reticularis in retired breeder Mongllian gerbils and golden hamsters. In gerbils, the size and number of lipid droplets increased and numerous large lipid-lysosomal complexes were observed in zona reticularis parenchymal cells or in macrophages, which were more numerous than in younger animals. Abundant lipofuchsin pigment was observed in zona reticularis parenchymal cells, and mitochondria contained electron-opaque inclusions. In the hamster, lipid droplets were present in inner zona reticularis cells and there was an increased amount of pigment within macrophages. Some of the alterations in gerbils and hamsters may at least in part reflect a stimulation of zona reticularis cells in retired breeders, attributable to increased prolactin secretion.

The structure of nexuses in the atrioventricular node of the golden hamster was studied with the transmission electron microscope, using thin sections and freeze-fracture replicas, and was compared with that of nexuses in the working myocardium of the right ventricular wall. Whereas ventricular myocardium contained macular nexuses only, nodal tissue contained annular and linear configurations as well as maculae of varying size. The significance of such variations in nexus pattern is not clear although several hypotheses are discussed in the literature. Measurements made on electron micrographs, after allowing for tilt of the specimen, yielded a particle diameter of 10.59 nm for nodal myocardium and 10.95 nm for ventricular myocardium, both measurements being substantially higher than figures generally cited in the literature. In each area the measurements had a normal distribution suggesting a single type of particle. The small but significant difference in particle size between the two areas is more likely to be caused by dissimilarities in packing arrangement rather than by differences in intrinsic structure or in functional state.

Because, in previous investigations on the rat pineal gland, karyometric studies of pinealocytes from cortical and medullary regions had yielded contradictory results, experiments were carried out to resolve this problem. In immersion-fixed, paraffin-embedded pineal glands, nuclear size of cortical regions was invariably larger than that in the medulla, the nuclear size clearly depending on the plane of sectioning. The differences between cortex and medulla were abolished in (a) pineal glands sagittally bisected prior to immersion fixation and (b) perfusion-fixed, epoxy resin-embedded pineal tissue, suggesting that unequal pinealocyte nuclear size in cortex and medulla is artefactual. In the hamster pineal gland, cortical and medullary regions are separated by a narrow band of interstitial cells, indicating that different structural features account for this subdivision in different species.

Hamster lung morphometry and histology have been studied in an attempt to determine differences between hamster and human lungs which may have relevance for lung carcinogenesis studies. Morphometric measurements were made on fresh lungs, lung casts, and histological sections. Cell type and frequency measurements were determined from frozen, paraffin, 1 micron plastic (glycol methacrylate) and electron microscopic sections. A standard terminology for hamster lung histology is established, and differences between hamster and human lung morphometry and histology are discussed.

Changes in apparently healthy hamsters, consistent with proteinuria, are reported, but no IgG deposits or amyloid in the glomeruli were detected. Further investigation is required into the significance and the aetiology of these, as yet, obscure alterations.

Electron microscope studies of the myocardium of the golden hamster revealed that the primary T-tubules in ventricular and atrial myocytes are substantially wider than their counterparts in most other mammalian species. Another feature is the presence of numerous lipid bodies but these are not as closely apposed to mitochondria as in seal myocardial cells. General atrial myocardial cells in the hamster contain numerous electron-dense spherical granules but these are absent from cells of the nodal and conducting system; in regard to these latter features in hamster resembles most other mammals.

Neural processes from mammalian pinealocytes have been discovered in several brain areas. These processes were visualized immunocytochemically in the Djungarian hamster, Phodopus sungorus, with an antiserum against bovine retinal S-antigen and traced as far as the region of the posterior commissure and habenular nuclei. This result indicates that pineal-to-brain connections exist in the mammal, and that the mammalian pineal gland, currently thought of only as a neuroendocrine organ, may communicate directly with select brain regions by way of these projections. The existence of mammalian pinealocyte projections is consistent with the view that these cells are not of glial origin but are derivatives of photoreceptor cells of the pineal complex of lower vertebrates that transmit signals to the brain by neural projections.

The dorsal motor nucleus of the vagus nerve of the hamster was studied electron microscopically. Two types of neuron distinguished by their ultrastructural features were identified. Type I cells, which had an average dimension of 20 X 12 microns, were rich in cytoplasmic organelles. The smaller Type II cells (15 X 9 microns) had scanty cytoplasm and a deeply indented nucleus. Type I cells were labelled by horseradish peroxidase (HRP) following its injection into the vagus nerve, whereas Type II cells were not. Seven days after cervical vagotomy, Type I neurons underwent typical retrograde degeneration. By the fourteenth postoperative day, the density of the cells was greatly enhanced, an early sign of cell death. Type II cells were not affected by vagotomy. With the electron microscope, three types of axon terminals were identified in the neuropil of the DMN: (1) boutons containing round agranular vesicles, (2) boutons containing round and elongated dense-cored vesicles, and (3) boutons containing pleomorphic agranular vesicles, a few of which were dense-cored. All of them showed synaptic contacts with HRP-labelled dendrites. Occasional boutons containing pleomorphic vesicles showed degenerative changes after vagotomy. Moreover, a few of them were seen to contain HRP granules. The observations made in the present study strongly suggest that the Type I neurons send efferent fibres to the vagus nerve. The Type II cells which are not labelled by HRP nor affected by vagotomy, probably serve as interneurons, or they may be cells projecting to other areas of the brain.

In addition to annulospiral sensory endings and fusimotor endings, multiaxonal nerve endings consisting of a bundle of naked axons were frequently found in the muscle spindles of the adult Chinese hamster. The multiaxonal endings were mainly distributed in the equatorial region of the nuclear bag fibers, adjacent to the annulospiral sensory endings. Terminal axons composing the endings were less than 1.5 microns in diameter and contained a significant number of clear synaptic vesicles and large granulated vesicles. Some axons formed synaptic contacts with each other and with the muscle cell surface. These structural features suggest that the multiaxonal endings are efferent in nature. They may possibly represent a temporal structure of fusimotor endings, or belong to the autonomic nerves.

On Day 9 of pregnancy (day of mating = Day 1), the number of corpora lutea in the right ovary was greater than that in the left (mean +/- s.e.m. 9.3 +/- 0.1 and 6.5 +/- 0.3 respectively; N = 70). Although the percentages of ova fertilized on the left and right side were not different (82% and 94%), the percentage wastage was higher on the left side (20%) than the right (14%). A significant difference in sperm numbers in the right (2.8 X 10(6] and the left (0.5 X 10(6] uterine horns were found 1.5 h after mating in 51 females. Morphometric measurements of the lower uterine luminal size showed that the right side (103.9 mm3) was larger than the left side (88.9 mm3; N = 5). It is obvious that there is structural and functional asymmetry in the ovary and uterus in the golden hamster.

The histomorphology of the terminal bronchiolar region of the mouse, rat, hamster, guinea pig, gerbil and rabbit was studied. Although some general structural similarities existed in the progressive intrapulmonary branching pattern of the airway tree between species, there were conflicting accounts in the literature about the presence of the respiratory bronchioles in common laboratory mammals. In our light microscopy study we failed to detect the existence of typical respiratory bronchioles with characteristic interruptions on their walls projecting into the alveoli. Frequently in these species the terminal bronchioles were short and abrupt, opening directly into several alveolar ductules.

The morphology of the convoluted testicular artery and the pampiniform plexus of the golden hamster was studied by light microscopy and corrosion cast techniques combined with scanning electron microscopy. The artery was found to be totally enclosed by the pampiniform plexus, except for minor superficial areas where the artery was exposed. Although no direct connection between the artery and the vein was found in the area of apposition, the arterial and venous walls reduced their thickness by sharing a single tunica adventitia, which seemed well suited to the transfer of substances by diffusion. Many band-like structures of the venous walls were found in the deep part of the spermatic cord, suggesting that these may act as barriers to slow down the venous blood velocity. The venous wall here and there showed a stick-like endothelial bridge, suggesting that it may prevent the veins from over distension. In addition to the close relation between the artery and the vein, lymphatic vessels and mast cells were distributed widely within the connective tissue of the arterio-venous walls and venous walls. Mast cells were situated mainly in the area of apposition, especially at the base of the protruding venous wall. These morphological findings suggest that mast cells may be involved in the counter-current transfer mechanism in the spermatic cord of the golden hamster.

The membrane systems of hamster general atrial and ventricular myocytes were studied and their reactivity with the sterol probes, filipin and tomatine, compared with that of atrioventricular nodal myocytes. The sarcolemma of atrial and atrioventricular nodal myocytes was considerably more reactive to sterol probes than the sarcolemma of general ventricular myocytes. Atrial myocyte sarcolemma was also more reactive than previously reported in rat atrial myocytes. Particle-free regions of annular nexuses were consistently unreactive with all combinations of sterol probes whereas much smaller particle-free regions of macular nexuses and desmosomes were reactive with the same probes. Intracellular membrane systems of all three types of cardiac myocyte reacted in a similar manner with sterol probes. These results indicate variation in comparable sarcolemma composition of cardiac myocytes between the rat and hamster and between cardiac regions within the same species.

The ultrastructure of adult hamster (Mesocricetus auratus) testis was studied by means of light and electron microscopes during fourteen months after bilateral vasectomy. In all the vasectomized animals there was marked degeneration of the semniiferous tubules as well as reduction of spermatogenesis. The thickening of the basal membrane was quite evident and showed extensive infolding. In the Sertoli cells, the presence of spherical or oval membrane-bound granules, vacuoles and degenerating cytoplasmic organelles were observed in the vasectomized group. Degenerative changes of sub-cellular organelles were quite evident in the process of spermatogenesis and spermiogenesis. The number of spermatogonia, spermatocytes, spermatids and mature sperms were considerably reduced; in some cases, almost completely degenerated. Only one type of spermatogonia (Type A) remained intact in the germinal epithelium. Large numbers of multi-nucleate spermatids were observed eight months after vasectomy. These cells were identifiable at all stages of spermatogenesis but were readily detected in cap phase spermatids. There was no evidence of any regeneration occurring in the seminferous tubules examined eight months after vasectomy.

The fine structure of the filiform papillae on the hamster tongue is described level by level from the basal layer to the surface. We did not observe two distinct types of cells with different morphology or components which could be held responsible for the production of two different types of keratin as have been described in other animals, but rather a uniformity of cell structures in each layer and only the so-called "smooth" type of keratin. However, keratin granules were more abundant in the anterior part of the papilla.

Cell abutments in the ovarian bursal epithelium of the golden hamster included tight junctions, desmosome-like junctions, gap junctions and zonulae adhaerentes. The plasma membranes of adjacent epithelial cells were often closely apposed forming a zonula adherens at the apex of the intercellular space, but these did not fuse along the entire length of the intercellular space. Gap junctions and other intercellular junctions did not allow diffusion of ferritin into the junctional areas, but failed to prevent ferritin from filling the intercellular space basal to the junction. This suggested that these junctions were not continuous around the entire circumference of the cell. In general, intercellular connections in the bursal epithelium may be labile and this lability, as well as periodic distention of the bursa, may give rise to separation of epithelial cells. Ferritin injected into the bursal cavity reached lymphatics via three routes: (1) via stomata; (2) via gaps and pores; (3) via intercellular spaces. Transcellular transport did not make a major contribution to permeation of ferritin across the bursal epithelium, and ferritin particles did not enter fenestrated blood vessels within 30 minutes after injection. The variety of routes may facilitate rapid and complete drainage of fluid and cellular components from the bursal cavity.

The plug is an eosinophilic mass, partly homogenous and partly porous, filling the proximal urethra in rats and occasionally extending into the bladder. Its average weight in 131 adult rats was 0.063 g. These plugs are normally present in the urethra of adult male rats, and this seems to be the case for all laboratory Muridae and Cavidae. The absence of a plug in an adult male may be a sign of abnormality associated with failing health. There is an interesting similarity between the amino acid composition of the content of seminal vesicles, that of the urethral plug, and that of the copulatory vaginal plug in female rodents.

During meiosis and espermiogenesis the Golgi apparatus shows the greatest morphological and physiological changes. During the second meiotic division it appears as a very large organelle, formed by prominent dictyosomes and by two types of vesicles: one in the external part of the body with diameters that range from 40--100 nm, and the other in the central part of the organelle, larger in size, from 200 to 500 nm. The acrosome, once formed, is spheric (1600 nm in diameter) with the glucoproteins forming a round and dense body occupying its central zone. Later the acrosome moves against the nuclear membrane. These morphological changes occur within a very short time, while the spermatid practically continues occupying the same position in the cellular association of the seminiferous tubule.

Male Djungarian hamsters were raised from birth in either long (16L:8D) or short (8L:16D) photoperiods. In 16L:8D, testes were large (about 400 mg) and showed full spermatogenesis from 31 days of age, the cauda epididymidis contained motile spermatozoa from 35 days and the accessory glands were large and reached their final size at about 60 days of age. In 8L:16D, testes remained small (about 15 mg) and undeveloped up to an age of about 130 days; they then developed to reach normal size and full spermatogenesis. In 16L:8D body weight increased steadily up to an age of about 160 days, but in 8L:16D body weight increase was slower and reached normal values, after a plateua at about 80 days, at about 190--260 days. Most animals in 8L:16D moulted into winter pelage, and then into the summer pelage; those in 16L:8D kept the summer pelage throughout.

A subpopulation of basal epithelial cells which retains tritiated thymidine label for extended periods was previously demonstrated in skin and oral mucosae of mice. The present study examined the presence of similar cells in hamsters. Five-day-old hamsters were labeled with tritiated thymidine and the rate at which label was diluted from the basal cells observed. A small percentage of basal cells was found to retain label for up to 69 days. The location of such label-retaining cells ( LRCs ) in the palatal epithelium and in tongue papillae was examined. Thirty and 69 days after labeling, approximately 80% of LRCs in palate were located in the proximal halves of papillae and 80% of LRCs in tongue were positioned basally with approximately 30% of such LRCs occupying positions previously suggested to be stem cell locations. The finding that slowly cycling keratinocytes are related to patterns of tissue architecture is compatible with a function of these cells as epithelial stem cells.

Stereological studies on the adrenal cortex of unilaterally adrenalectomised hamsters have revealed differences in adrenal compensatory hypertrophy in male and female animals. The operation evokes an increase in the average volume of the zona fasciculata cells in the male and of the zona reticularis cells in the female.

The transformation of fibroblasts into decidual cells was studied ultrastructurally in golden hamsters pregnant for 3, 3 1/2, 4, 4 1/2, 5, or 5 1/2 days (post-ovulation). Cells within the endometrial stroma were generally separated from one another and spindle-shapted before blastocyst contact with the uterine epithelium. Once the epithelium enclosed the blastocyst (3 1/2 days), stromal cells adjacent to the blastocyst antimesometrially began to exhibit more extensive Golgi complexes with increased secretory activity. As differentiation proceeded the amount of cellular contact increased and various types of junctions formed between the cells. Throughout the period examined, intercellular space progressively decreased, while the cisternal width of the granular endoplasmic reticulum continually increased. Special organelles, whose functions remain unknown, first appeared in differentiating cells at 4 days (fibrils) and 5 1/2 days (crystalloid and 'dumb-bell' structures).