The Development in vitro of the Mammalian Gonad. Ovary and Ovogenesis

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The Development in vitro of the Mammalian Gonad. Ovary and OvogenesisAuthor(s): P. N. MartinovitchSource: Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 125, No.839 (Apr. 27, 1938), pp. 232-249Published by: The Royal SocietyStable URL: .Accessed: 07/05/2014 16:42Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . .JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact .The Royal Society is collaborating with JSTOR to digitize, preserve and extend access to Proceedings of theRoyal Society of London. Series B, Biological Sciences. This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions E. C. Dodds and W. Lawson Mills, W. H. and Nixon, I. G. 1930 J. Chem. Soc. p. 2520. Norris, J. F., Thomas, R. and Brown, B. M. 1910 Ber. dtsch. chem. Ges. 43, 2958. Robson, J. M. and Schonberg, A. 1937 Nature, Lond., 140, 196. Russanow, A. 1889 Ber. dtsch. chem. Ges. 22, 1944. Schiff, H. 1874 Liebigs Ann. 172, 357. Schlenk, W. 1909 Liebigs Ann. 368, 303. Schmidlin, J. and Garcia-Banus, A. 191a Ber. dtsch. chem. Ges. 45, 3199. Schmidlin, J. and Massini, P. 1909 Ber. dtsch. chem. Ges. 42, 2381. Schneider, F. 1899 Ber. dtsch. chem. Ges. 32, 689. Schopf, M. 1894 Ber. dtsch. chem. Ges. 27, 2324. Serini, A. and Steinruck, K. 1937 Naturwissenschaften, 25, 682. Smith, A. 1893 Ber. dtsch. chem. Ges. 26, 68. Stoermer, R. and Kippe, B. 1903 Ber. dtsch. chem. Ges. 36, 4009. Thiele, J. and Schleussner, K. 1899 Liebigs Ann. 306, 198. Ullmann, F. and Mourawiew-Vinigradoff, A. 1905 Ber. dtsch. chem. Ges. 38, 2218. Ullmann, F. and Stein, A. 1906 Ber. dtsch. chem. Ges. 39, 624. Ullmann, F. and Wurstemberger, R. 1905 Ber. dtsch. chem. Ges. 38, 4105. Werner, G. 1895 Ber. dtsch. chem. Ges. 28, 1999. Wislicenus, W. and Endres, A. 1903 Ber. dtsch. chem. Ges. 36, 1194. Zincke, T. and Munch, S. 1904 Liebigs Ann. 335, 184. Zincke, T. and Muhlhausen, G. 1903 Ber. dtsch. chem. Ges. 36, 131. 611.013.i6:6ii.013.9 The development in vitro of the mammalian gonad. Ovary and ovogenesis* By P. N. Martinovitch Strangeways Research Laboratory, Cambridge {Communicated by F. H. A. Marshall, F.E.S.?Received 15 December 1937) [Plates 9-11] Introduction Several investigators have tried to cultivate in vitro the mammalian and avian gonad, usually with the object of obtaining the growth and differenti? ation of the germ cells under in vitro conditions. The results so far obtained, however, have been disappointing. Champy (1920) is the only * This work has been made possible by a fellowship granted by the Rockefeller Foundation. This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions development in vitro of the mammalian gonad 233 author, so far as we know, who has definitely claimed to observe some differentiation of the germ cells in vitro. He states that in testicular ex- plants from the mature rabbit he observed the transformation of spermato- gonia into spermatocytes, but the process did not advance beyond the leptotene phase of meiosis. Champy's findings, however, have not been confirmed by other investigators, probably owing to the difficulty of cultivating mature testicular tissue. In birds, progressive differentiation of the germ cells in vitro has not been recorded. Cultures of ovarian and testicular tissue of the embryonic fowl have been studied by Fano and Garofolini (1927 and 1928). The explants formed an epithelial outgrowth which was soon replaced by what appeared to be fibroblasts. Dantchakoff (1932) cultivated the undif- ferentiated gonad of 3|-day fowl embryos and described three types of cells in her cultures: (a) large amoeboid cells?Waldeyer's "Ureier" or " Urkeimzellen"?which in vitro developed pseudopodia and showed active amoeboid movement, (b) connective tissue strands, (c) epithelium? like tissue. Wermel (1933) made cultures of the undifferentiated gonad and of embryonic and adult testes and observed growth of both the somatic and germinal tissue. Muratori (1935 and 1937) cultivated gonad tissue from embryonic and newly hatched fowls, but although he describes the nuclear pattern of the germ cells emigrating from the explants, he makes no mention of any differentiation occurring in the explanted gonocytes. Several workers have grown mammalian gonad tissue in vitro. Maccabruni (1913) studied cultures of the human foetal ovary and observed proliferation from the explant of connective tissue only. Wolff and Zondek (1925) obtained proliferation of both connective tissue and epithelium from explants of human foetal ovaries and from the ovary of a 45-year old woman. Olivo (1934) obtained successful cultures of the cumulus oophorus tissue containing the egg from a human embryo, but failed to cultivate the granulosa tissue without the egg, from the same ovary. He also noted that the egg enlarged somewhat during cultivation and two large nuclei appeared in it. Mjassojedoff (1925) found that the follicular cells of the rabbit ovary when growing in vitro, quickly lose their epithelial character and in 6-8 days become very like fibroblasts. Champy (1920, 1926, 1927) and Champy and Morita (1928) explanted ovarian and testicular tissue from mammals (rabbit, rat) and from some of the lower vertebrates, and observed that the germinal tissue proliferated in vitro in an undifferentiated form. Champy regarded this observation as evidence in support of his dedifferentiation theory. As stated above, Champy also described one This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions P. N. Martinovitch stage of spermatogenesis in vitro which took place in a 9-day culture of adult rabbit testis after all the spermatocytes present at the time of explantation had degenerated. Esaki (1928) studied the fate of interstitial and Sertoli cells in explants of rabbit and guinea-pig testis. He stated that the interstitial cells are derived from the mesenchyme and that in vitro they become transformed into spindle-shaped elements indistinguishable from ordinary fibroblasts; the Sertoli cells became rounded and phago- cytic in culture. Mihailoff (1937) recorded the survival in vitro, for about 70 days, of the seminiferous tubules of the immature rabbit. A few primitive germ cells (gonocytes) persisted in a more or less normal state during this period, but failed to differentiate. Both mesenchyme derivative and Sertoli cells grew and migrated actively. The object of the present investigation was to cultivate the entire ovary of embryonic and new-born rats and mice and to study its behaviour in vitro with special reference to the growth and differentiation of the germ cells. It was clear that if the germinal tissue could be made to differentiate in vitro, it would provide a new method for analysing the complex factors responsible for the growth and development of the ovary (Martinovitch 1937)- Of recent years something has been learned of the influence of the gonadotropic hormones on the normal functions of the ovary. Little is known, however, of the possible influence of the organ as a whole on the germinal tissue which it contains or of the inherent capacity of the germ cells for independent growth and differentiation. The results of the present experiments shed some light on both these problems and also on the vexed question of the length of survival of the mammalian ovum. Material and technique Forty ovaries from rats and mice ranging in age from the 16th day of embryonic life to the 4th day after birth were cultivated in vitro by the watch-glass method (Fell and Robison 1929). The culture medium was composed of equal parts of chicken embryo extract prepared in Pannett and Compton's solution and chicken plasma. In some experiments 1 % glucose was added to the medium. One ovary from each animal was explanted in vitro and the corresponding ovary was fixed as a control. In order to compare the rate of growth of the ovum in vitro and in vivo, the normal ovaries of 8-, 5- and 2-day-old rats were also fixed and sectioned. The explants and some of the controls were fixed in Allen's modification of Bouin's fluid and serially sectioned. The remaining controls were fixed This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions development in vitro of the mammalian gonad 235 in unmodified Bouin's fluid. Sections were stained with Meyer's haemat- oxylin, Heidenhain's haematoxylin or by Feulgen's method counterstained with light green. Methods of preparation of the ovaries employed in this work were not of the nature to reveal any of the various cytoplasmic constituents. Observations The results showed that the entire process of oogenesis can take place in vitro under appropriate conditions of cultivation. Oogenesis in vitro does not differ materially from the normal process in vivo, and the series of development of explanted ova are therefore described in chronological order, beginning with a stage of development just preceding the appearance of the first meiotic figures. For the sake of convenience, the rat ovaries only are considered, although the mouse ovaries behave in a similar way. The development in vitro of ovaries from 15-16-day embryonic rats. Normal (control) 15-16-day embryonic ovary (fig. 1, Plate 9) This ovary was taken from one of the same batch of embryos as the explanted ovaries B XI2 to B XI6 inclusive, and was fixed at the beginning of the culture period as a control. The dense mass of tissue composing the ovary contains two main types of cell: (a) the oogonia, large cells with big spherical nuclei and (b) the somatic ovarian cells which are smaller and contain round or oval nuclei. The oogonia are most abundant towards the centre of the ovary and are very scarce in the ovarian epithelium. In some regions of the ovary there is no visible sign of a developing albuginea, the germinal epithelium being continuous with the underlying tissue, but elsewhere, especially near the site of attachment to the body wall, some of the mesenchyme cells immediately beneath the epithelium have begun to elongate. Similar elongated mesenchyme cells occur in the interior of the ovary where they appear in section as single rows indicating the boundaries of the future germinal nests and cords. The nuclei of the oogonia contain numerous very fine, dust-like particles of chromatin and several larger, usually peripheral, knots of intensely staining chromatin. Although the particles seem to be connected by very fine protoplasmic bridges, no sign of that formation of chromatin threads which heralds the onset of meiosis, could be detected. In general appear? ance the oogonia resemble the spermatogonia?"gonies poussiereuses " of Regaud?which occur in the testis of a new-born rat, except that the fine Vol. CXXV. B. 16 This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions P. N. Martinovitch particles of chromatin are slightly coarser and the chromatin knots less distinct than in the spermatogonia. Many germ cells are undergoing mitotic division, metaphase being much the most numerous stage. The cells of the mesenchyme and of the germinal epithelium are also dividing actively. A few germ cells are degenerating. The ovary contains many nucleated red blood cells which in the explanted organs sometimes persisted for as long as 10 or 12 days. Explanted ovary after 19 hr. in vitro (explant No. B XIX) The most conspicuous change in the general structure of the ovary, as compared with that of the control described above, is the increase in the number of spindle-shaped cells which now form well-defined parallel single cell rows running in a dorso-ventral direction with regard to the gonad's original attachment to the body wall. The future nests and cords are thus more clearly outlined. The mesenchyme cells beneath the germinal epithelium, though not yet spindle-shaped, are more numerous, but a typical stroma has not so far differentiated in this region. The cells of the germinal epithelium are irregular in shape and arrange? ment, some being only loosely attached to the surface of the explant. The injured surface of the organ is covered by a single, discontinuous layer of epithelial cells. Many white blood cells adhere to the ovarian wall, but there is no migration of the ovarian tissue into the medium. The germ cells, which are very abundant, differ from those of the control. The powdery particles of chromatin have increased both in number and staining intensity, and the large chromatin knots have either partially or completely disappeared. In a few of the germ cells the chromatin granules have a linear arrangement which foreshadows the thread-like formations of the leptotene phase. The number of degenerate germ cells is about the same as in the control. No central necrotic area ever forms in cultures of 15-16-day ovaries. Explanted ovary after 42 hr. in vitro (explant No. B XI2) The first stages of the meiotic prophase?leptotene and synaptene?- appear at this period of cultivation. The leptotene phase is represented by numerous fine threads either scattered evenly through the nucleus (fig. 2, Plate 9) or slightly polarized (fig. 3, Plate 9). Cells at a stage inter? mediate between the preceding "dusty" stage and the leptotene are still fairly common. Synaptene figures are already numerous. The chromatin threads have begun to contract towards one end of the nucleus. A double structure was This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions development in vitro of the mammalian gonad 237 not observed before the contraction stage, although some of the threads were distinctly looped forming the familiar bouquet figure. In later stages of contraction, the closely packed threads appear as an almost uniform mass of chromatin to one side of the otherwise clear nucleus, with merely a few loops protruding at the margin to indicate the filamentous nature of the mass. The number of dividing oogonia, mostly in metaphase, is still very great (fig. 4, Plate 9). The degenerating germ cells appear as an intensely stained, ellipsoid mass of nuclear material capped with a deeply staining crescent of cyto? plasm. The connective tissue of the stroma has not differentiated much further, although in the region of the hilum a narrow band of elongated cells clearly demarcates the ovarian tissue proper from a tiny strip of mesovarian tissue included in the explant. The germinal epithelium consists of a single layer of flattened cells, separated from the underlying germ cells by a sheet of rather flattened connective tissue cells which represent the developing albuginea. Ova do not occur either in the germinal epithelium or in the albuginea. Cells have now begun to migrate from the surface of the ovary. Explanted ovary after 50 hr. in vitro (explant No. B XI3) Structures resembling Pfliiger's cords make their first appearance at this stage of cultivation (fig. 5, Plate 9). The mesenchyme separating groups of germ cells has become compressed and flattened to form well-marked partitions, thus producing a cord-like structure in the ovarian tissue. The cords are packed with oocytes and Sertoli cells, the former being the more numerous. Only a few cells of the oogonial type are present, the proportion of oocytes having greatly increased. Leptotene and synaptene figures are about equally represented. The healthy and normal appearance of the explanted ovary at this stage is striking. Fewer germ cells are dividing than before, and the mitotic wave has receded towards the periphery of the organ. It is noteworthy that some of the metaphase figures are abnormal; the chromosomes are clumped together in certain germ cells, and in others spherical pycnotic masses of chromatin of different sizes are scattered throughout the cell body. It is most unlikely that such cells complete division. On the other hand, mitotic figures in the somatic cells, both in the germinal epithelium and in the cords, appear normal (fig. 6, Plate 9) (see Rauh 1926). 16-2 This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions P. N. Martinovitch The single-layered germinal epithelium contains an unusually large number of dividing cells. In this explant there is no true albuginea, and the nests of germ cells lie close to the surface. A few cells of the germinal epithelium and underlying mesenchyme have emigrated from the organ. In all cultures of 15-16-day embryonic ovaries the zone of outgrowth was found to be small, although its area could be increased by adding glucose to the medium and was of the familiar fibroblastic type. In explants from embryos of this age the emigration of germ cells was never, and the formation of epithelial sheets only occasionally, observed in the zone of growths, though it should be emphasized that no special study was made of the nature of the outgrowth in these cultures. Explanted ovary after 71 hr. in vitro (explant No. B XI4) At this stage the vast majority of the oocytes are at the advanced contraction phase of meiosis (synizesis) (fig. 7, Plate 10). In preparations stained by Feulgen's method, the chromatin appears as a deeply stained, almost impenetrable mass, but in those stained with Meyer's haemat- oxylin the individual threads are sometimes clearly visible. The leptotene nuclei are distributed along a narrow zone parallel to the surface of the organ, and intermingled with them are the dividing oogonia, now greatly reduced in number. The strands and partitions of connective tissue between the nests of germ cells have increased in both number and size, and the germinal nests have become correspondingly smaller and more numerous. There is no appreciable change in the germinal epithelium or underlying stroma. Explanted ovary after 96 hr. in vitro (explant No. B XI5) At this stage many of the oocytes have emerged from the contraction phase and the chromatin threads have reappeared. The oocytes appear on the whole less healthy than in the preceding culture, as indicated by the variations in the appearance, number and distribution of the chromatin threads and in the size of the nuclei. In some cells the threads are long and fine and in others short and stubby; sometimes they appear as a continuous, highly convoluted spireme, filling the whole nucleus, but in others they are definitely polarized, the spireme-like arrangement occurs in large nuclei only, but otherwise there seems to be no correlation between the appear? ance of the chromatin threads and the size or shape of the nucleus con? taining them. This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions development in vitro of the mammalian gonad 239 It is often difficult to tell which of the various forms of chromatic thread represent successive stages in a continuous process, and which are mere abnormalities. It is also impossible to determine which of the abnormalities represent an irreparable injury to the cell, and which are merely reactions to new conditions of life and not necessarily lethal. Many of the germ cells are obviously necrotic and oocytes are de? generating at every stage of development; very few are in mitosis and these appear abnormal. The larger nests of germ cells are breaking up into smaller groups. The germinal epithelium is no longer a continuous layer, owing to the continual migration of cells into the surrounding medium and to the failure of the remaining cells to divide fast enough to fill the gaps thus produced. The underlying mesenchyme cells, many of which are in mitosis, have become definitely flattened. A conical mass of connective tissue has begun to differentiate near the original hilum of the ovary, the base of this cone coincides with the former area of attachment and the apex is thrust into the centre of the ovary. Explanted ovary after 6-6*16 days in vitro (explants No. B XI6 and B XIIM) Nearly all the germ cells have now evolved from the contraction phase. The chromatin filaments, once more distinct, now form the thick, short, polarized loops characteristic of the post-contraction "bouquet" phase of the normal embryonic ovary (fig. 8, Plate 10). The oocytes, though greatly reduced in number, look healthier than in the ovary of the preceding stage. Some diplotene nuclei have already appeared. The germinal epithelium has almost disappeared and is represented by a few scattered epithelial cells clinging to the surface of the ovary. After the virtual disappearance of the germinal epithelium, the surface of the ovary is covered by the albuginea, which is composed of three or four layers of flattened fibrocytes. There is a striking difference in appearance between the typical connective tissue cells of the albuginea and the mesenchyme-like cells of the deeper layers of the ovary. Up to this time the size of the ovary has remained unaltered. Explanted ovary after 6*21-8 days in vitro (explants Nos. B XII3 4 5 6) In vitro the pachytene phase of meiosis is brief. As stated above, a few diplotene nuclei had already appeared at the preceding stage, although most of the oocytes were in the pachytene phase. This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions P. N. Martinovitch Diplotene nuclei become much more numerous by about the 7th day of incubation (fig. 9, Plate 11). Many nuclei contain discontinuous, double threads which have been interpreted as representing the homologous chromosomes separating after their intimate contact during the zygotene phase. Such nuclei were most numerous in a 7-2-day culture, but in sister cultures obtained from the same litter and fixed a few hours before and after 7-2 days they were comparatively rare. This indicates that the separation of the homologous chromosomes occurs fairly quickly. It is interesting that in many of the oocytes which have passed through the pachytene phase, the chromatin loops much resemble a fine spireme. These loops are not so fine as the "smooth" leptotene threads and have a beaded appearance. In Feulgen preparations the nucleoli of the germ cells are faintly stained with light green. The explants on the whole seemed quite healthy, although many oocytes were degenerating. The general structure was the same as in the preceding stage. Although no accurate timing of the duration in vitro of synaptene stage has been attempted in these experiments, it has been found that on the whole it agrees fairly well with that recorded by Pratt and Long (1917) in normal ovaries. Later development of the explanted ovary up to the 30th day in vitro The stages in the development of the explanted ovary subsequent to the diplotene phase may be described together. In normal development, according to the usual view, the diplotene phase is followed by a resting stage in which the individuality of the chromatin threads seems to disappear completely. The nucleus then con? tains an achromatic network in which are scattered many small chromatin granules and from one to three larger chromatin nucleoli. Most of the oocytes remain unchanged in this condition for weeks, months or, according to some authors, even for years, but a few enlarge and eventually become full-grown ova. Somewhat similar changes occur in vitro. After the diplotene phase the nuclear material becomes less chromophil and breaks down into one to three chromatin nucleoli and a number of smaller particles. Inter? connecting achromatic strands are often extremely hard to see. The chromatin granules are coarser and less deeply staining than those of the normal oocyte. Sometimes the nuclear material consists of a nucleolus and a loosely connected, amorphous mass of particles with no apparent organization. The resting ova change little during subsequent cultivation. This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions development in vitro of the mammalian gonad 241 As in the normal ovary, some of the resting ova in the explants begin to enlarge. Enlargement of the cytoplasm is accompanied by growth of the nucleus and increase of the chromatin. In the full-grown ova, the chromatin particles are large and coarse, and one or two large, spherical plasmasomes are present. The plasmasomes differ from the normal in being "less chromophil" and more clearly visible, a difference which is due to the fact that in the explants the surface of the plasmasomes is often free from the adherent particles of chromatin by which they are observed in the normal ovum. In nearly full-grown ova the nucleus assumes an excentric position. The ova are usually spherical and, unlike the normal ovum, appear completely devoid of a zona pellucida. Abnormally shaped ova sometimes occur. Fig. 10 (Plate 11) shows one that is ring-shaped in section, the interior being occupied by a large, hollow space. A "peculiarity " of all ova cultivated in vitro is that even in the best fixed preparations they appear shrunken, so that a clear, ring-like space, sometimes bridged by fine radially distributed strands of protoplasm, intervenes between the ovum and the surrounding tissue. Whether this appearance is an artifact or whether it also occurs in the living explant has not been ascertained. Some of the ova degenerate. Large ova do so by a process of karyor- rhexis. The chromatin material breaks down into deeply stained fragments of various sizes, which are at first irregular in outline but gradually become spherical. No normal maturation division has been observed in the older cultures, but one abnormal figure was found in a 25-day culture. This dividing ovum has passed in vitro through all the stages of differentiation and growth from an oogonium to a maturing ovum. From the foregoing description it is seen that the development of the explanted ovary during synapsis differs little from that of the normal organ. After this stage, however, it shows a divergence from the normal which may be considered as peculiar to conditions in vitro. Two chief abnormalities occur in culture: (1) partial or total failure of the Sertoli cells to develop into Graafian follicles; (2) failure of the growing ovum to move towards the interior of the ovary. As a result of these abnormalities, nearly full-grown ova, either naked or surrounded by a few follicular cells, are clustered together near the surface of the ovary in what are really the original germinal nests and cords (fig. 11, Plate 10). As in vivo only a small number of oocytes at a time begin to grow, the vast majority remaining unchanged. In vitro, the first batch of growing ova attain their maximum size 18 days after explantation, a period This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions P. N. Martinovitch corresponding to 13 days of extra-uterine life. Measurements show that the largest size reached in culture is a diameter of 50-55/*. This first batch of ova soon degenerates and is replaced by others. After 18 days' cultiva? tion, ova of almost all sizes are to be found in the same ovary. Growth of the ovum in vitro is somewhat retarded as compared with the normal. The explanted ovary does not enlarge very much, and after a month's cultivation is only about double its original volume. It enlarges in three ways: (1) by the growth of the ova, (2) by the formation of a primary follicle around certain oocytes, (3) by multiplication of the stroma cells. Many connective tissue cells are lost by migration into the medium and by degeneration. The ovarian stroma undergoes very slight differentiation in vitro and the cells retain their primitive, mesenchymal appearance even after several weeks' cultivation. In the centre of the explant some typical stroma appears consisting of fairly dense fibrous tissue and a few scattered cells but elsewhere the intercellular fibres are very scanty. Medullary cords never develop in the explanted mouse ovary. The germinal epithelium, as stated above, usually disappears after a week's cultivation, partly owing to migration of cells into the medium and partly owing to loss through mechanical damage inflicted during sub- cultivation. Sometimes, however, a few clustered cells of epithelial appear? ance cling to the surface of the explant for several weeks. As already mentioned, no central area of necrosis appears in explanted ovaries from 16-day embryos, but degeneration changes affecting the entire organ begin to occur after 26-28 days in vitro. The translucency of the healthy explant gradually disappears and the organ becomes opaque. After 30 days' cultivation the whole organ is necrotic. The development in vitro of ovaries from late embryonic and new-born rats It seemed interesting to compare the development in vitro of the ovaries from 16-day embryos with that of ovaries from older embryos and newly born rats. Accordingly ovaries from 17-, 18-, 19- and 20-day embryos, new born and 4-day rats were cultivated in vitro. It was found that on the whole the more highly developed the ovary at the time of explantation, the further it will differentiate during subsequent cultivation. This is particularly true of the ovarian follicle but only up to a certain stage in its development. Thus an explanted ovary from a 19-20- day embryonic rat after 3 weeks' cultivation may contain numerous, small Graafian follicles which do not differ essentially from follicles of the same This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions development in vitro of the mammalian gonad 243 size in the normal ovary (cf. figs. 12 and 13, Plate 11). Moreover, these follicles have moved towards the centre of the explant, as in vivo, instead of retaining their original superficial position as in explants of the 16-day embryonic ovary (cf. figs. 11 and 12). The amount and arrangement of the stroma also approaches more closely that of the mature gland than does that of explants of younger embryonic ovaries. Another difference between the older and younger ovaries in culture is the appearance in the former of a central necrotic area. This degenerate region is seen shortly after explantation but disappears a few days later, the dead matter being either resorbed or cast out. The formation of the central necrotic area may be due to poor aeration or to a slow exchange of metabolites or to both these factors. Its disappearance seems to show that after an interval the ovary adjusts itself to the conditions in vitro. A necrotic area may disappear completely, to reappear a second time and again vanish. Many of the residual ova survive in vitro until the entire ovary degenerates (fig. 14, Plate 11). In the explanted ovary of a 4-day mouse, a typical maturation-division figure was observed after 9 days' cultivation (fig. 15, Plate 11). The appearance of the figure was preceded by an actual growth of the ovum, since measurements showed that in the control ovary the diameter of the largest ovum was 45/*, while the diameter of the dividing ovum in the culture was 52/*. The growth and differentiation of the ova is never seriously disturbed by cutting off the blood supply, and nearly all the young ova, at whatever stage in the early meiotic prophase they are explanted, continue their normal development in vitro. There seems to be no phase of meiosis which is particularly susceptible to injury by the process of explantation; but it is doubtful whether the different phases are of equal resistance to prolonged cultivation. Discussion Phenomena of meiosis and growth of the ovum The results recorded in this communication show that the primitive germ cells can undergo in vitro all the normal changes of differentiation and growth up to the stage of the nearly full-grown ovum. The question arises as to how far the growth and development of the ovum is controlled by factors intrinsic in the cell and how far it is influenced by external agents. Two kinds of external agent must be considered: (a) those supplied by the body as a whole, and (b) those supplied by the ovary. The distinction This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions P. N. Martinovitch between the two classes is arbitrary, however, as interaction between all the factors concerned, both extrinsic and intrinsic, may play an important part in the development of the germ cells. Explantation in vitro, while depriving the ovary of its natural environ? ment, by no means removes it completely from the influence of the body. The two constituents of the culture medium, plasma and embryo extract, might contain agents produced by the soma, which acted upon the germ cells. The somatic agents known to influence the differentiation of the gonad are: (1) gonadotropic hormones, and (2) sex hormones. With regard to the possible influence of gonadotropic hormones in the medium on the development of the explanted ovary, Pincus (1936) quotes extensive experimental evidence showing fairly conclusively that the Graafian follicles can only be activated by these hormones after attaining a certain stage of maturity and that growth of the ovum and of the early follicle are independent of the hypophysis. Even if the gonadotropic hormones do stimulate oogenesis under normal conditions, however, it is unlikely that the plasma and embryo extract contain enough of these substances to influence the explanted ova (see Robson 1934). On these grounds, therefore, it seems fairly safe to exclude the gonadotropic hormones as possible somatic agents stimulating oogenesis in the explants. Whether the sex hormones in the culture medium influence oogenesis in vitro and if so, in what way, is difficult to determine. The recent experiments of Dantchakoff (1936) and Willier, Gallagher and Koch (1937) on the embryonic fowl, show that small doses of oestrin cause the histo- logically undifferentiated gonad of a zygotically determined male embryo to develop into an ovary and the genetically determined male germ cells, that it contains, to be transformed into female germ cells; whereas the male sex hormone causes the undifferentiated female gonad to become an ovo- testis. But these authors make no clear statement whether the sex hormones may influence the process of gametogenesis under the conditions of their experiments. It should be emphasized, also, that these results refer to the avian gonad only, and whether they also apply to mammalian development is at present unknown. The part that may, possibly, be played by the ovarian soma in inducing ovogenesis has received very little attention, and nothing is known of the possible interrelationship between the germ and other ovarian cells. That the germinal epithelium is not essential to the growth of the ovum after synapsis, is proved by the fact that the epithelium usually disappears This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions development in vitro of the mammalian gonad 245 after a week's cultivation while the ova may continue to grow for another 20 days. The tissue culture experiments afford no evidence as to whether the stroma plays any specific part in the development of the ova, as this tissue persists, though in an undifferentiated form, throughout the culture period. They also give no information about the epithelial cells of the cortex in relation to oogenesis. Very little is known of the ovum-follicle relationship in the early stages of the normal development of the ovum. In the explants nearly full-grown ova, 50-55/* in diameter, developed completely devoid of follicle cells. This showed (a) that the early growth of the ovum is not dependent on the follicular epithelium, and (b) that in culture, growth of the follicle is not conditioned by growth of the ovum. Whether the ovum can complete its growth in the absence of a follicle is doubtful. The only full- sized ovum found in the explants, a binucleate egg 68/* in diameter, occurred in an ovary from a 19-20 day embryo rat and was covered by a follicular envelope several cell layers thick. Why some growing ova should develop a follicle and others fail to do so is quite obscure. It was noted that ovaries explanted at birth formed more and larger follicles than those removed from embryos, suggesting that the capacity for follicular development increases with age. With regard to the influence of the soma on oogenesis, the explantation experiments seem to indicate that in normal development the organism as a whole merely nourishes the germ cells and has no specific effect on their differentiation. What influence, if any, the ovary itself exerts upon the germ cells remains obscure, because although the explanted ovary develops a rather simplified structure in vitro, it is still too complex to allow of any definite conclusions being drawn. Survival in vitro of the ovum and the problem of neo-formation of the germ cells Whether the primitive germ cells persist, stored in the ovary, throughout post-embryonic life or whether they are continuously (Kingery 1917; Arai 1920; Butcher 1927, 1932; Hargitt 1930 a) or periodically (Allen 1923) renewed, are controversial questions. The average life of the ovary in vitro is about one month, and in some explants many, and in others, a few, residual ova persist until the end of this period. The residual ova succumb only when the entire organ degenerates. In the explants, new ova are formed only by the mitotic division of pre-existing oogonia and not by neo-formation from the germinal epithelium. In the explanted ovaries of late embryos, but never in those This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions P. N. Martinovitch of 15-16 days' gestation, a few ova occur in the germinal epithelium but they are at the same stage of development as those in the deeper parts of the ovary, and there is no evidence that such cells differentiate from the germinal epithelium. The germinal epithelium is usually completely lost after a week's cultivation and is thus finally eliminated as a possible source of new germ cells during the last three weeks of the culture period. All the germ cells in older cultures are embryonic in origin, i.e. they are cells which have passed through the early phases of meiosis. But even though tissue culture experiments show that neo-formation of germ cells from the germinal epithelium does not take place in vitro, they do not disprove the possibility that such proliferation might occur in normal post-embryonic life. With the exception of Firket (1920), who claims that the primary germ cells of the albino rat degenerate 15 days after birth, those who support the view that the embryonic ova degenerate early agree that they normally survive for a period which exceeds the maximum culture period in the present experiments. In order to test the storage theory more completely it would be highly desirable, therefore, if a method was found of keeping the resting ova in vitro for longer periods of time than it has been possible heretofore. On the other hand, these experiments destroy one of the arguments used by Hargitt and others against the storage theory. Hargitt (1930a) argues that "if an assumption be made that many latent primary follicles are stored in the mammalian ovary, a serious difficulty is encountered in explaining the stimulation of a few to renewed development, while others remain quiescent. Likely enough, some hormone might stimulate renewed activity, but why of a few not all of the same age?" In the tissue culture experiments the ovaries were explanted before the ova had begun to enlarge and, from the evidence given above, the explants throughout the culture period, were known to contain embryonic ova only. Nevertheless, as in the normal post-embryonic organ, only a few ova enlarged at any one time and many residual ova persisted unchanged until the end of cultivation. That only a few out of a large number of apparently similar oocytes should enlarge at one time, is indeed a surprising fact for which no adequate explanation is at present forthcoming. The factors which cause the growth of any given ovum must be sought in the ovary itself. The position of the ovum is not concerned, since in the explants nearly full-grown ova often occur at the periphery of the organ (fig. 11, Plate 10) while on the other hand resting ova have been observed in the centre of a mouse ovary which had recovered from permanent ligation (unpublished data). Nor does the This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions development in vitro of the mammalian gonad 247 size of the ovary seem to be important in this connexion, since the volume of the explants is only about one-tenth that of the normal ovary of corresponding age. I wish to express my thanks to Dr H. B. Fell, Director of the Strange- ways Laboratory, for her help and advice throughout the investigation. I am also indebted to Mr V. C. Norfield, chief assistant at the laboratory, who was responsible for the photomicrographs. Summary of results 1. The ovaries of embryonic and new-born rats and mice when explanted in vitro continue to live and develop for about 4 weeks. 2. Growth and differentiation of the germ cells occur in 100 % of cultures. 3. In explants of 16-day embryonic ovaries the oogonia undergo all but the final stages of normal differentiation and growth, and form nearly full-sized ova 50-55/* in diameter. The ova in explants obtained from 2-5- day rats reach full-size and may undergo apparently normal maturation division. 4. The oocytes in the cultures only enlarge if they pass through the synaptic phases of meiosis. 5. Growth of the follicle in the explants is partly, and sometimes com? pletely, suppressed. 6. The egg may enlarge in the absence of a follicular envelope. 7. The formation of a zona pellucida was never observed in ova growing in vitro. 8. Growth of the ovum is somewhat retarded in vitro. 9. Oocytes in culture multiply by the mitotic division of already differen? tiated oogonia and not by neo-formation from the germinal epithelium. 10. The germinal epithelium usually persists for about a week and then disappears owing to the emigration of its component cells into the sur? rounding culture medium. 11. Emigration of germ cells into the medium was not observed. 12. After 4 weeks' cultivation the embryonic rat ovary has about doubled its volume. References Allen, E. 1923 Amer. J. Anat. 31, 439-70. Arai, H. 1920 Amer. J. Anat. 27, 405-62. Butcher, E. O. 1927 Anat. Bee. 37, 13-25. ? 1932 Anat. Bee. 54, 87-98. This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions P. N. Martinovitch Champy, Ch. 1920 Arch. Zool. exp. gen. 60, 461-500. ? 1926 C.B. Soc. Biol., Paris, 94, 1082. ? 1927 C.B. Soc. Biol., Paris, 96, 597. Champy, Ch. and Morita, J. 1928 Arch. exp. Zellforsch. 5, 308-40. Dantchakoff, W. 1932 Z. Zellforsch. mikr. Anat. 15, 581-644. ? 1936 Bull. Biol. 70, 241-307. Esaki, S. 1928 Z. mikr. anat. Forsch. 15, 368-404. Fano, G. and Garofolini, L. 1927 J. Physiol. 63, vi-vii. ? ? 1928 Arch. sci. biol. 12. Fell, H. B. and Robison, R. 1929 Biochem. J. 23, 767-84. Firket, J. 1920 Anat. Bee. 18, 309-21. Hargitt, Geo. T. 1930 J. Morph. Physiol. 49, 277-321. ? 1930 a J. Morph. Physiol. 49, 333-47. Kingery, H. M. 1917 J. Morph. Physiol. 30, 261-305. Maccabruni, F. 1913 Ann. ostet. ginec. 1, 57-65. Martinovitch, P. N". 1937 Nature, Lond., 139, 413. Mihailoff, W. 1937 Z. Zellforsch. 26, 174-201. Mjassojedoff, S. W. 1925 Arch. mikr. Anat. 104, 1-24. Muratori, G. 1935 Arch. ital. anat. embriol. 35, 397-412. ?? 1937 Contr. Embryol. 154, Carnegie Inst. Wash., Publ. 479, 59-69. Olivo, M. O. 1934 Arch. ital. anat. embriol. 33, 718-25. Pincus, G. 1936 "The Eggs of Mammals." New York: The Macmillan Co. Pratt, B. H. and Long, J. A. 1917 J. Morph. Physiol. 29, 441-56. Rauh, W. 1926 Z. Anat. Entwicklungsm. 78, 637-68. Robson, J. M. 1934 "Recent advances in Sex and Reproductive Physiology.'1 London: J. and A. Churchill, Ltd. Wermel, E. M. 1931 Z. Zellforsch. 13, 545-65. ? 1933 Arch. exp. Zellforsch. 14, 554-73. Willier, B. H., Gallagher, T. F. and Koch, F. C. 1937 Physiol. Zool. 10, 101-22. Wolff, E. K. and Zondek, B. K. 1925 Virchows Arch. 254, 1-16. Description of plates All figures are photographs of stained preparations Plate 9 Fig. 1. Normal ovary of a 16-day rat embryo. Control to figs. 2, 3, 4, 5, 6, 7, 8, 9, 10. All the germ cells are of the oogonial type. Bouin-Allen and Feulgen. x 1200. Fig. 2. Section through a 16-day embryonic rat ovary cultivated in vitro for 42 hr., showing the leptotene stage of meiosis. Note the fine strands of the chromatin. Bouin-Allen and Feulgen. x 1000. Fig. 3. Section through the same ovary as that shown in fig. 2. The leptotene and synaptene stages of the meiotic prophase are seen, x 1000. Fig. 4. Dividing oogonia in a 16-day embryonic rat ovary fixed after 42 hr. in vitro. Bouin-Allen and Feulgen. x 1000. Fig. 5. Section through a 16-day embryonic rat ovary after 50 hr. cultivation showing nests and cords of cells which have developed in vitro. Note the healthy appearance of the ovary. Bouin-Allen and Feulgen. x 140. This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions Proc. Roy. Soc, B$ vol. 125, Plate 9 i-p * f .'. \r -if ^ rs \ ? *^ ^*?i fr 5MW^: ;**?:*??>& r;v^.V/;.3? ...?* -A? A*. :4-.,??,# JT w*, V * * 0?b i >> ft \j2& (FaMng p. 248) This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions Moy. Soc, B, vol. 125, Plate 10 ^ ^: ., r This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions Proc. Boy. Soc, B, vol. 125, Plate 11 j|HB& dp 13" This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions development in vitro of the mammalian gonad 249 Fig. 6. The abnormal division of two adjacent germ cells (a) is seen (lower clumped metaphase figure in a different plane of visibility), and the normal division of a somatic cell (b) in the germinal epithelium. From a 16-day embryonic rat ovary fixed after 50 hr. in vitro. Bouin-Allen and Feulgen. x 1000. Plate 10 Fig. 7. Section through a 16-day embryonic rat ovary after 71 hr. cultivation showing the oocytes in the deep contraction stage (synizesis) of the meiotic prophase. Bouin-Allen and Feulgen. x 1000. Fig. 8. Section through a 15-16-day embryonic rat ovary after 6-16 days in vitro. Numerous oocytes in the pachytene phase of the meiosis. Bouin-Allen and Feulgen. x 1200. Fig. 11. Section through a 16-day mouse ovary after 19 days' cultivation. Nearly full grown ova are closely packed in the region of the original nests and cords. An incomplete single layer of cells surrounds some of the ova; the other ova are naked. Bouin-Allen and Meyer's haematoxylin. x 400. Plate 11 Fig. 9. Section through a 15-16-day embryonic rat ovary after 7-2 days in vitro. Oocytes in the diplotene phase of the meiosis are shown. Bouin-Allen and Feulgen. x 1200. Fig. 10. A large ring-like ovum in a 16-day embryonic rat ovary grown in vitro for 24 days. Bouin-Allen and Feulgen. x 1000. Fig. 12. Section through the ovary of a 19-20-day embryonic rat after 21 days in vitro. Typical Graafian follicles have developed during cultivation and the nuclei of the oocytes have passed into the resting stage. Bouin-Allen and Meyer's haematoxylin. x 180. See fig. 13. Fig. 13. Normal 19-20-day embryonic rat ovary. Control to fig. 12. x 290. Fig. 14. Section through the same explant as that shown in fig. 12. Numerous residual ova are seen, x 100. Fig. 15. Section through the ovary of a 4-day (post-embryonic) rat ovary after 9 days' cultivation in vitro. Bouin-Allen and Meyer's haematoxylin. x 640. This content downloaded from on Wed, 7 May 2014 16:42:34 PMAll use subject to JSTOR Terms and Conditions Contentsp. 232p. 233p. 234p. 235p. 236p. 237p. 238p. 239p. 240p. 241p. 242p. 243p. 244p. 245p. 246p. 247p. 248[unnumbered][unnumbered][unnumbered]p. 249Issue Table of ContentsProceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 125, No. 839 (Apr. 27, 1938), pp. 187-290Polyphenol Oxidase. Purification, Nature and Properties [pp. 187-204]The Coagulation of Plasma by Trypsin [pp. 204-213]Studies on the Hypophysectomized Ferret. X. Growth and Skeletal Development [pp. 214-221]Molecular Structure in Relation to Oestrogenic Activity. Compounds without a Phenanthrene Nucleus [pp. 222-232]The Development in vitro of the Mammalian Gonad. Ovary and Ovogenesis [pp. 232-249]The Receptive Mechanism of the Background Response in Chromatic Behaviour of Crustacea [pp. 250-263]Chromatic Behaviour of Elasmobranchs [pp. 264-282]The Polarization of a Calomel Electrode [pp. 283-290]