ISSN:1059-910X    

DOI:10.1002/jemt.1070320502

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractSynchronous maturation of the germ cells in the seminiferous epithelium has long been recognized by microscopy, and is believed to be a consequence of a complex interaction between the germ cells and the Sertoli cells, largely driven by testosterone and its synergistic action with follicle‐stimulating hormone. Overall coordination of the cycle of the seminiferous epithelium is reviewed with regard to the known and possible actions of testosterone upon the Sertoli cells and the germ cells. With gradual refinements of optical instrumentation and development of a wide range of histological, morphometric, biochemical, and molecular techniques, coupled with selective alterations of hormonal stimulation and the cellular composition of the testis, new approaches to the question of how sperm production is regulated are becoming available. Germ cell and Sertoli cell functions are intimately related to each other via local, intratesticular, or paracrine signals which are suppressed or triggered at certain defined steps in the spermatogenic process. The coordination of germ cell proliferation and maturation is discussed in terms of the contributions made by microscopical techniques. © 1995 Wiley‐Liss,

ISSN:1059-910X    

DOI:10.1002/jemt.1070320503

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractSpermatogenesis is a process of division and differentiation by which spermatozoa are produced in seminiferous tubules. A measure of efficiency of spermatogenesis is the estimated number of spermatozoa produced per day per gram of testicular parenchyma. This measure is not influenced by species differences in testicular size; however, it is influenced by species differences in the numerical density of germ cells and in the life spans of these cells. Seminiferous tubules are composed of somatic cells (myoid cells and Sertoli cells), and germ cells (spermatogonia, spermatocytes, and spermatids). Activity of these three germ cells divide spermatogenesis into spermatocytogenesis, meiosis, and spermiogenesis, respectively. Spermatocytogenesis involves mitotic cell division to increase the yield of spermatogenesis and to produce stem cells and primary spermatocytes. Meiosis involves duplication and exchange of genetic material and two cell divisions that reduce the chromosome number and yield four spermatids. Spermiogenesis is the differentiation of spherical spermatids into mature spermatids which are released at the luminal free surface as spermatozoa. The spermatogenic Cycle is superimposed on the three major divisions of spermatogenesis. Spermatogenesis and germ cell degeneration can be quantified from numbers of germ cells in various steps of development throughout spermatogenesis, and quantitative measures are related to number of spermatozoa in the ejaculate. Germ cell degeneration occurs throughout spermatogenesis; however, the greatest impact occurs during spermatocytogenesis and meiosis. Efficiency of spermatogenesis is related to the amount of germ cell degeneration, pubertal development, season of the year, and aging of humans and animals. Number of Sertoli cells and amount of smooth endoplasmic reticulum of Leydig cells (but not Leydig cell number) are related to efficiency of spermatogenesis. In humans, efficiency of spermatogenesis is reflected in number of spermatogenic stages per cross‐section and number of missing generations within each stage; however, the arrangement of stages along the tubular length does not reflect differences in the efficiency of spermatogenesis. In short, spermatogenesis involves both mitotic and meiotic cell divisions and an unsurpassed example of cell differentiation in the production of the spermatozoon, and daily sperm production per g parenchyma is a measure of its efficiency. © 1995 Wiley‐Liss,

ISSN:1059-910X    

DOI:10.1002/jemt.1070320504

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractThe historical background to contemporary approaches to the estimation of cell/ nuclear number and volume in the testes is reviewed. The limitations of older geometric model‐based approaches to the estimation of cell/nuclear number are discussed, and the need for absolute estimates of cell number rather than ratio estimates is examined. The physical and optical disector approaches to the direct estimation of numerical density and, hence, absolute cell number are presented together with data illustrating their operational efficiency in the testis. New approaches to the direct estimation of nuclear/cell volume, using the point‐sampled intercept family of methods, are presented and illustrated, using the example of the Sertoli cell nucleus. The use of both classical transverse and the newer vertical section approaches is explored. Estimation of Sertoli cell/nuclear volume in the volume (point‐sampled intercept procedure) and number (nucleator and rotator methods) distributions on both conventional transverse and vertical sections is discussed. The use of transverse sections of the testis is shown to produce a consistent bias in the estimation of Sertoli cell nuclear volume in 120‐day‐old animals, with all the estimators. Comparison of the Sertoli cell nuclear volume (measured on vertical sections) in the volume and number‐weighted distribution suggests a coefficient of variation of volume in the number distribution of 0.4–0.5, suggesting either a random or stage‐dependent variation in Sertoli cell nuclear size which requires further exploration. © 1995

ISSN:1059-910X    

DOI:10.1002/jemt.1070320505

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractThe influence of mesonephric tissues and the extracellular matrix on mouse gonadal differentiation was examined in vitro. Gonadal ridges, with or without the adjacent mesonephric region, were removed from mouse embryos on day 12 post coitum (p.c.), and cultured in the presence or absence of reconstituted basement membrane (matrigel) for 5 days. Culturing control undifferentiated testes with mesonephric tissues induced normal testicular differentiation. When testes without mesonephric tissues were cultured in the absence of matrigel, testicular cord formation was not observed in the explants. Sertoli cells were irregularly arranged in the testicular parenchyma, and no continuous basal lamina was formed around the Sertoli cells. However, when testes without mesonephric tissues were embedded in matrigel and cultured for 5 days, the Sertoli cells were organized into testicular cord‐like structures. The Sertoli cells positioned at the base of the cord‐like structures were closely connected to the matrigel at their basal surface, and showed a polarized distribution of vimentin filaments in their basal cytoplasm. Leydig cells, on the other hand, were differentiated in all testicular explants. In all ovarian explants, germ cells normally entered meiotic prophase. Therefore, these findings indicate that the extracellular matrix permits testicular differentiation in the absence of the mesonephros, and that removal of mesonephric tissues leads to developmental failure of cord formation because the components of the extracellular matrix around pre‐Sertoli cells are incomplete. © 1995 Wiley‐L

ISSN:1059-910X    

DOI:10.1002/jemt.1070320506

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractA calibration sample for transmission electron microscopy (TEM) has been developed that performs the three major instrument calibrations for a transmission electron microscope: the image magnification calibration for measurements of images, the camera constant calibration for indexing diffraction patterns, and the image/diffraction pattern rotation calibration for relating crystal directions to features in the image. This offers an improvement over commercially available calibration standards, where up to five different samples are required to perform these three calibrations. The new calibration sample consists of an electron‐transparent cross‐sectional TEM sample made from a molecular beam epitaxy (MBE)‐grown, single‐crystal semiconductor wafer. When the calibration structure is viewed in a TEM, it appears as a series of light and dark layers where the layer thicknesses are very accurately known. The calibrated thickness measurements of these light (silicon) and dark (SiGe alloy) layers are based on careful TEM measurements of the {111} lattice spacing of silicon which is visible on the calibration sample itself, and are supported by X‐ray diffraction measurements. Furthermore, the layer thickness variation across the entire silicon wafer has been verified to be less than 1%, allowing all samples prepared from the same wafer to have errors in the given layer thickness values of less than 1%. As the sample is a single crystal of silicon, the calibrations requiring electron diffraction information such as the camera constant calibration and the image/diffraction pattern rotation calibration can also be performed easily and unambiguously. One single calibration sample can therefore be used to provide all three of the major TEM instrument calibrations at all magnifications and all camera lengths. © 1995 Government of Canada.Exclusive worldwide publication rights in the article have been transferred to Wiley‐Liss, Inc., i

ISSN:1059-910X    

DOI:10.1002/jemt.1070320507

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

ISSN:1059-910X    

DOI:10.1002/jemt.1070320508

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractA miniature vise built into a 5 mm diameter copper capsule is described that holds small pieces of prefrozen, hydrated specimens at low temperatures within the lens of the Hitachi S900 high‐resolution scanning electron microscop

ISSN:1059-910X    

DOI:10.1002/jemt.1070320509

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

ISSN:1059-910X    

DOI:10.1002/jemt.1070320501

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY