Paper - Development of the Mouse Gonads 1

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Brambell FWR. The development and morphology of the gonads of the mouse. Part I. The morphogenesis of the indifferent gonad and of the ovary. (1927) 101: 391-407.

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This historic 1927 paper by Brambell is the first in a series investigating the development of the mouse gonad.

See also: Brambell FWR. The development and morphology of the gonads of the mouse. Part I. The morphogenesis of the indifferent gonad and of the ovary. (1927) 101: 391-407.
Brambell FWR. The development and morphology of the gonads of the mouse. Part II. The development of the Wolffian body and ducts. (1927) : 206-219.
Brambell FWR. The development and morphology of the gonads of the mouse. Part III. The growth of the follicles. (1928) 103: 259-272.
Rowlands IW. and Brambell FWR. The development and morphology of the gonads of the mouse. Part IV. The post-natal growth of the testis. (1932) : 200-213.

Modern Notes: ovary | mouse

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Table References  

O'Rahilly R. (1979). Early human development and the chief sources of information on staged human embryos. Eur. J. Obstet. Gynecol. Reprod. Biol. , 9, 273-80. PMID: 400868
Otis EM and Brent R. Equivalent ages in mouse and human embryos. (1954) Anat Rec. 120(1):33-63. PMID 13207763

Theiler K. The House Mouse: Atlas of Mouse Development (1972, 1989) Springer-Verlag, NY. Online
OTIS EM & BRENT R. (1954). Equivalent ages in mouse and human embryos. Anat. Rec. , 120, 33-63. PMID: 13207763

Witschi E. Rat Development. In: Growth Including Reproduction and Morphological Development. (1962) Altman PL. and Dittmer DS. ed. Fed. Am. Soc. Exp. Biol., Washington DC, pp. 304-314.
Pérez-Cano FJ, Franch À, Castellote C & Castell M. (2012). The suckling rat as a model for immunonutrition studies in early life. Clin. Dev. Immunol. , 2012, 537310. PMID: 22899949 DOI.

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The Development and Morphology of the Gonads of the Mouse — Part I. The Morphogenesis of the Indifferent Gonad and of the Ovary

Francis William Rogers Brambell (1901 – 1970)

By F. W. Rogers Brambell, Fellow of the International Education Board.

(Communicated by Prof. J. P. Hill, F.R.S.— Received February 15, 1927.)

From the Department of Anatomy (Embryology and Histology, University College, London.

Plates 23-31.

  • The expenses of this research were defrayed by a. Grant from the Government Grants Committee of the Royal Society, for which I wish to express my thanks.
James Peter Hill (1873 - 1954)
The author desires to express his thanks to Prof. J. P. Hill, F.R.S., for his advice and criticism, and to Dr. A. S. Parkes for the animals used in this research.

1. Introduction

The Work described in this series of papers was carried out with a View to furnishing controls of the experiments on the mouse undertaken in collaboration with Dr. A. S. Parkes and others. This paper deals with the development of the indifferent gonad of the mouse, up to the stage at which it differentiates into an ovary or a. testis. This takes place on the 12th day post. coitum, when the testis can first be distinguished. The ovaries remain in the inrlifferelit stage for some time longer, but can be identified by a process of elimination.

The present paper is concerned also with the development of the ovary from the time when sex can be distinguislied until sexual maturity (8 weeks post pa-rtum). It is not proposed to consider here the structure of the maturing follicle, the process of atresia, or the formation of the corpus luteum. These problems will be reserved for a subsequent paper on the adult ovary of the mouse.

Details of the breeding, &c., of the colony of mice are recorded elsewhere (Parkes). Both copulation and parturition take place at night, the vaginal plug or litter, as the case may be, being detcc.ted the next morning. The average period between the findings of the plug and of the litter is 19 days and this may be taken as the normal gestation period for this colony.

The ages of the embryos and young are calculated in the present series from the date of finding the plug or litter respectively, and several hours must; be added on to arrive at the true age. Thus a 13-day embryo is from the uterus of a mouse killed on the morning of the 13th day after the of the plug, and 13%-day embryo from one killed on the evening of the 13th day. Actually, a few hours must be added on to each of these to arrive at the true age, if it is desired to do so. It was, however, found more convenient in practice to omit to do this. The young mouse is weaned at 3 weeks old. The young female attains sexual maturity at about 8 weeks old, and the young male at about the same age, but they continue to grow for some time afterwards.

2. Technique and Material

The embryos were fixed entire, with, in the larger ones, the abdomen opened to allow the fluid to penetrate, or the gonads were dissected out separately or attached to a portion of the lumbar body-wall. The ovaries were dissected out before fixing in the young animals. Complete serial sections were made of the region of the gonads of the smaller embryos, and of the ovaries themselves of the larger embryos and young animals.

The specimens were in the fixative in all cases before they had been dead more than a few minutes. The animals were killed by decapitation, without the use of an anaesthetic. Bouin’s fluid was used, followed by Ehrlich’s haematoxylin and eosin, or by Pasini’s staining, or Champy’s fluid followed by Heidenhain’s iron-haematoxylin.

The material available consisted of 37 embryos from 18 pregnant mice, killed at daily or half-daily intervals from 9 to 18 days 11.0. This material has been arranged in 17 Stages, as the variation in the state of development of the earlier embryos (9 to 12 days) necessitated a seriation not strictly in accordance with their ages. The variation in the stages of development between healthy embryos from the same uterus is always very small. The ovaries of 13 young animals, ranging from birth to maturity, were also available, and comprised Stages 18 to 28. In the dorsal margins of the epithelial nucleus, at first anteriorly. These extend posteriorly. This process results in the separation of the gonad from the

3. Description

The formation of the gonad.——The first indication of the formation of the germinal ridges is found in embryos 9 days p.c. (Stage 1), where a few of the so-called “ primordial” germ-cells occur beneath the peritoneal epithelium along the ventral border of the Wolffian body. At this time the epithelium in this region is similar to that of the rest of the peritoneum and consists of a single layer of cells, but a few cells beneath its surface indicate the beginning of proliferation. This is soon followed by the thickening of the epithelium to form the germinal ridge (Stage 2, 9 days 19.0.). The epithelium becomes two cells- thick at first and continues proliferating to form a solid plate several cells thick along the ventral aspect of the Wolffian body. In section, this plate of cells is slightly concave on its dorsal side and convex on its ventral surface. It is first formed anteriorly and gradually develops in a posterior direction. It is formed from the covering epithelium by a proliferation of epithelial cells continuous over its surface, and not in the form of separate cords, blocks or chains of cells. The primordial germ—cells are included in this epithelial mass or “ nucleus,” which constitutes the germinal ridge. The peritoneal epithelium covering it is the germinal epithelium. Mitoses are numerous in Stages 2 to 9 in the cells of the germinal epithelium and epithelial nucleus.

The proliferation from the germinal epithelium continues, so that the resulting epithelial mass or nucleus changes in cross section from a flattened (Plate 30, fig. 5) to a broad oval and then to a circle (Stage 5, 10 days 11.6.). This change begins anteriorly and gradually extends posteriorly. As soon as the epithelial nucleus has become circular in Cross section at the anterior end the nipping ofi of the germinal ridge to form the Wolffian body and gonad begins. This nipping ofi is more apparent than actual, and results from the growth of the Wolffian body and gonad rather than from an actual deepening of the grooves which delimit them. The separation of the Wolffian body, with the gonad attached to it, is affected first (Plate 30, figs. 2 and 4). gradually posteriorly (Stages, 6, 7 and 8 ; 11, 12 and 11% days p.c. respectively) along the line Where the dorso-lateral and ventro-medial grooves delimit the mesonephros. The Wolfiian body is attached to the body-Wall only by a relatively narrow mesentery (Plate 30, figs. 1 and 3) by the time sexual differentiation is definitely established (Stage 9, 12 days p.c.). The rapid growth of the gonad at this time results in the formation of two grooves along

It begins anteriorly and extends

Wolffian body, except for the hilar attachment. Nothing is included in the gonad but the epithelial nucleus, with its germ-cells and capillary loops, except a few mesenchymal elements in the angle between the dorsal aspect of the epithelial nucleus and the germinal epithelium on the median border at the extreme anterior end. The transformation of the germinal ridge into the definitive gonad is thus efiected in a manner similar to that described in the human embryo by Felix (6).

The epithelial nucleus remains in complete continuity with the germinal epithelium until sexual differentiation takes place. The vascular supply is established from the beginning from the capillary loops which extend into the forming germinal ridge.

The d1fl'erentiation of ovary and tes£is.—The differentiation of the indifierent gonad into 21. testis begins as soon as it is nipped off. The first indications of this process are seen in Stage 7 (12 days 12.0.), and appear to set in between 11% and 12 days 32.0. They consist in the male (Plate 30, fig. 2) of a downgrowth of mesenchymal tissue from the base of the gonad between the germinal epithelium and the epithelial nucleus which begins anteriorly and gradually extends ventrally and posteriorly until it forms a continuous sheath beneath the epithelium. This is the primordium of the tunica albuginea of the testis. It becomes thickest along the ventral side of the gonad where the ends of the capillary loops extend into it. Other mesenchyme cells invade the epithelial nucleus, simultaneously with the formation of the albuginea, and break it up into thick twisted finger-shaped spermatic cords. The mesenchyme exhibits many mitoses. The downgrowth of mesenchyme results in the primordial germ-cells being confined to the spermatic cords. Numerous mitotic figures are present in the newly formed cords (Stage 9, 12 days 12.6.) whichgrow rapidly. The formation of these spermatic cords from the proliferated epithelial cells is, therefore, secondary in the mouse. The germinal epithelium, isolated outside the newly formed albuginea, does not stop proliferating immediately, but continues for a short time (Plate 30, fig. 3), and becomes several cells thick I (Stage 9, 12 days 12.6.). The cells proliferated, however, never penetrate the tunica albuginea, but remain in S‘il'll. and appear to become re-arranged as a single layer by growth and stretching.

The ovaries (Plate 30, fig. 4) do not develop characteristic structures until some time after the differentiation of the testes. In Stage 9 (12 days 19.6.) they are still only negatively distinguishable by lacking any downgrowth of mesenchyme tissue or the formation of a tunica albuginea (Plate 30, fig. 1). The ovary (Stages 7, 8 and 9; 12, 11; and 12 days 11.0. respectively) remains in the indifierent condition with a solid epithelial nucleus everywhere continuous with the germinal epithelium. The epithelial nucleus undergoes no secondary rearrangement into cords by the downgrowth of connective tissue, as it does in the male. The germinal epithelium continues adding to it by proliferating cells. This proliferation in the female is an unbroken continuation of that which too]; place from the earliest formation of the germinal ridge.

Sex cannot be determined by dissection under a binocular dissecting microscope at 12 days 12.0. (Stage 9). At 12} days p.c. the testes appear rather thicker than the ovaries and this difference is more marked at 12; days pm. At all subsequent stages sex can easily be determined by dissection.

The primordial germ-celI.9.—-The so-called primordial or primitive germ-cells of other authors occur in the mouse. A few are present in Stage 1 (9 days p.c.) in the region of the germinal ridge, but are not found elsewhere. These cells (Plate 28,figs. 1, 6 to 8) are characterised by their large size and clear appearance, owing to staining less densely than the surrounding cells. The nuclei are large and oval, not irregular in shape, and measure 8-5 p. in average diameter. They contain two or three large acidophil nucleoli and a small amount of chromatin, chiefly scattered around the nuclear membrane and nucleoli in small lumps and granules of irregular size and shape. Linen threads, with fine chromatin granules strung on them, radiate from the largest nucleolus, which is situated near the centre of the nucleus, to the larger lumps of chromatin around the periphery. It is impossible to determine whether these threads are attached to the nucleolus itself or to the fragments of chromatin clustered close around it. In general, the nucleus is large and clear, with little chromophil material compared with the other cells of the embryo.

The cells of the germinal epithelium (Plate 28, fig.2) at the stages until sexual differentiation is definitely established, resemble in many points the primordial germ-cells (Plate 28, fig. 7). Like them, the nuclei contain two, three or more large acidophil nucleoli. The chromatin is rather more evenly distributed, but strings of small granules can sometimes be seen radiating from the nucleoli to the larger peripheral lumps of chromatin. The nuclei probably contain about the same amount of chromophil material as those of the primordial germ- cells, but they appear much denser, owing to their smaller size and the con- sequent greater concentration of the chromatin. The cytoplasm is also rather more deeply staining than in the primordial germ—cells. The cell-body and the nucleus are smaller, and the latter is less regular in shape. Cells, similar to those in the germinal epithelium, are plentiful in the epithelial nucleus (Plate 28, fig. 3).

Every gradation of cell intermediate in character (Plate 28, figs. 3, 5) between these and the primordial germ-cells is also present in the epithelial nucleus (Plate 29, fig. 2) of this and other stages prior to sexual differentiation. It is impossible in many cases to decide whether a given one of these cells should be considered a primordial germ-cell or an epithelial cell. Other cells in the epithelial nucleus of the gonad are more irregular in shape and size with irregular nuclei (Plate 28, fig. 4). These nuclei contain several scattered acidophil nucleoli and are rich in chromatin, which is scattered throughout, making them appear very dense and in marked contrast to those of the germ-cells. These cells are obviously of epithelial origin also, and stages intermediate between them and the cells of the germinal epithelium are present.

Some of the primordial germ-cells, especially about Stage 7 (12 days 32.6.), ‘ are unusually large (Plate 28, fig. 6). Mitoses occur in the primordial germ-cells * from Stage 2 (9 days 12.0.) ; they are frequent especially from Stage 5 (10 days p.c.) onwards.

It is noteworthy that the primordial germ-cells are confined to the germinal ridge and are absent from the mesentery of the gut and other parts of the embryo in Stage 1 (9 days 11.0.). In Stage 9 (12 days gm.) and all subsequent stages they are also confined to the gonad. In Stages 2 to 8 inclusive they occurred outside the germinal ridge (Plate 29, fig. 1). The distribution of these extra-regional germ-cells is summarised in Table I.

The development of the ovary.— The germinal epithelium :— The early proliferation of cells from the germinal epithelium continues for some time without intermission in the ovary after sexual differentiation. This is not so in the male, in which the proliferation ceases soon after sexual differentiation and the establishment of the tunica. albuginea. The proliferation is virtually at an end in the 12-day 31.0. male, but it is still actively progressing in the 12%-day 1 female. In the female from this time onwards the proliferation becomes less active, but continues until puberty. The cells are not proliferated from the epithelium in the form of separate strands or cords, but as single cells more or less continuously over the surface until the septa ovarii have extended to the periphery. From this time the cells are proliferated between the septa, in the form of thick cords. This corded arrangement of the proliferation is secondary and is imposed by the development of the septa. The cells, as they become separated from the epithelium, push their way through the thin tuniea albuginea where it is present and merge with the other elements of the ovarian cortex. The proliferation appears rather more active at first on the side of the ovary away from the mesovarium. After birth the proliferation becomes less evident, and after puberty is attained at about 7 weeks p.p., it has apparently ceased altogether. The small amount of post-partu-m prolifera- tion that seems to occur is limited to the regions between the larger follicles and is more active in the neighbourhood of the hilum.

Table I.

Stage. Distfibufi(;:::1_E:fi:-regional Stage of development of gonads.

1 None first rudiment of germinal ridge (9 days p.c.) forming.

2 Some in immediate vicinity of the Germinal ridge formed. (9 days p.c.) base of t-he germinal ridge.

3 Near base of germinal ridge and one Germinal ridge formed. (11 days 131.6.) in the upper part of the mesentery.

4 Plentiiul near the base of the germinal Lateral grooves beginning to deepen (12 days 13.0.) ridge anteriorly.

5 Plentiful near the base of the germinal Nipping ofi begun, but is not complete

(10 days 11.0.) ridge, many in the mesentery and even at the anterior end. some far down it.

6 Near the base of the germinal ridge Nipping off more advanced. (11 days p.c.) only.

7 Near the base of the germinal ridge at Nipping of incomplete at the anterior (12 days 32.0.) the posterior and only. end and proceeding at the posterior end. B An occasional one near the base of the Nipping ofi almost complete, even

014 days p.c.) germinal ridge. at the posterior end.

9 None Nipping off of the gonads complete.

(12 days p.c.)

The epithelial proliferation in the mouse is thus one single process continuing without intermission from the time when the germinal ridge is first formed at 9 days 11.0. until about the seventh week pp. when puberty is attained. The present material affords no evidence justifying the division of this process in the mouse into medullary, cortical and definitive proliferations, such as de Winiwarter and Sainmont have demonstrated in the ovary of the cat (16).

The epithelial nucleus.— The epithelial nucleus of the ovary is at first composed entirely of epithelial cells and germ-cells. Numerous mitoses are observable in both,and the growth of the whole is contributed to by the continued proliferation of more epithelial cells from the germinal epithelium. Subsequently to the downgrowth of mesenchymal elements into the epithelial nucleus, which soon after sex is established, the cells of epithelial origin remain distinguishable as such. They continue to increase, but more slowly, by division

within the ovary and by further proliferation from the germinal epithelium. These epithelial elements are observable in the cortex of the young ovary and are an important constituent of the inter-follicular tissue. The intermediate stages described above in the indifierent gonad, suggestive of the formation of germ-cells from the epithelial cells, are present in the ovary after sexual differentiation and are observable until all the germ-cells have entered on the prophase changes about the 13th or 14th day past coitum.

The germ-cells.— The “primordial” germ-cells found in the gonad from the time of its formation exhibit many mitoses until the 12th day 32.0., after which they multiply less rapidly, and by the 13th or 14th day p.c. have ceased dividing. At this time all the germ-cells in the gonad enter upon the prophase changes and subsequently none can be found with testing nuclei. There is little degeneration of germ-cells observable in the gonad before sexual differentia- tion or in the ovary until about the 16th day 12.0. All the germ-cells are similar and there are no cytological grounds for distinguishing the so-called “primordial” from the later formed germ-cells. It would appear, therefore, leaving aside the question of their origin, that the, so-called “primordial” germ-cells in the mouse persist and enter on the reduction stages.

Some early prophase stages can be distinguished as early as Stage 9 (12 days 12.0.), but it is not till a day or two later (Stages 12 and 13, 13% and 14 days 13.0.) that all the germ-cells exhibit them (Plate 31, fig. 4). Deutobroque stages are most plentiful at this time. Synaptic stages become most plentiful about 15 or 16 days 12.0. (Stages 14 and 15). In the new-born young (Plate 31, fig. 3) there are no stages earlier than pachytene, which are scarce. Many of the nuclei are diplotene, but the majority are already dictyate. In the 4-day old young (Stage 19), all the oocytes have reached the dictyate condition. From this time on, no reduction stages earlier than the dictyate are observable in any of the oocytes of the mouse.

Germ-cells are found actually in the germinal epithelium of the ovary at all stages after sex is differentiated and in the adult. Prior to the appearance of the prophase changes the nuclei of these are in the testing stage or in mitosis. From the 13th or 14th day p.c. onwards they always exhibit prophase stages like the germ-cells in the deeper parts of the ovary. ‘In the young mouse, four or more days old, and throughout later life these oocytes in the epithelium are always in the dictyate condition and never exhibit the earlier prophase stages.

The tunica albuginea and septa ovarii.—The ovary is composed, as already described, of an epithelial nucleus consisting of germ-cells and epithelial elements alone, which is everywhere continuous with the germinal epithelium at 12 days

p.c. (Stage 9). No primitive tunica albuginea is developed in t-he mouse ovary, such as has been described in other mammals about this stage of development. The only mesenchymal tissue present in the mouse ovary at this time is a slight downgrowth between the epithelial nucleus and the germinal epithelium along the median dorsal border at the extreme anterior end. This band of mesenchyme is never extensive and appears to have been included in the gonad by the pro- cesses resulting in its separation, with the Wolffian body, from the body wall. It is impossible to say whether or not this structure represents the rudiments of the primitive tunica albuginea of other forms.

The downgrowth of mesenchymal elements from the hilum, which gives rise to the connective tissue of the ovary and which results in the formation of the definitive tunica albuginea and septa ovarii, begins shortly after the difierentia- tion of sex. The first indications of this downgrowth can be detected at 1211; days 11.0. (Stage 10), especially at the anterior end. They consist of the penetration of a few elements from the mcsenchyme of the hilum between the germinal epithelium and the epithelial nucleus along the medial and lateral dorsal borders of the ovary, and of others into the epithelial nucleus itself along the mid-dorsal line. The peripheral downgrowths constitute the primordium of the tunica albuginea, and the trabeculae extending into the epithelial nucleus represent the primordia of the septa ovarii.

The tunica albuginea gradually extends ventralwards and at the same time develops posteriorly. About the 16th day p.c. (Stage 15) it becomes continuous over the ovarian surface under the germinal epithelium. It is, however, very thin at this time and consists of a single layer of mesenchyme cells. It gradually becomes more definite and thicker and its cells develop into connective tissue.

At the same time, the septa ovarii extend radially from the hilum towards the periphery of the ovary (Plate 31, fig. 4). Some reach the periphery and fuse with the tunica albuginea about Stage 12 (13% days p.c.), and, by the time the tunica albuginea ispcontinuous, they are completely established. They consist at first of thin trabeculae of mesenchyme cells growing down from the hilum. Later these transform into connective tissue. The septa ovarii break up the epithelial nucleus, previously a solid mass, into thick finger-shaped cords radiating from the hilar region to the periphery (Rate 31, figs. 2 and 3). At the time the rate tubules become united with the hilar insertions of these epithelial cords, when they are being formed by the development of the septa ovarii, and the urogenital union is effected. The bases of the septa ovarii, in con- sequence, form sheaths around the connections of the rete and epithelial cords.

These sheaths, which become augmented by the further downgrowth of mesenchymatous elements from the hilum, constitute the mediastinum ovarii.

The medulla.——The formation of the medulla results from two simultaneous processes: (a) the degeneration of the germ-cells in the deepest parts of the epithelial cords, and (b) the downgrowth of connective tissue from the hilum. The first indications of medulla formation are observed at 16 days p.c. (Stage 15), when a number of the central germ-cells exhibit signs of degeneration and the connective tissue forming the mediastinum and the bases of the septa ovarii is becoming thickened. At the next stage (17 days 12.6.) the medulla. forms a well marked knob of tissue projecting into the ovary from the hilum (Plate 31, fig. 2). The downgrowth of connective tissue from the hilum and the degeneration of the oocytes in the deeper layers of the epithelial nucleus are well marked. The development of the medulla continues and in the neW—born young is more or less complete (Plate 31, fig. 3).

The medulla is therefore formed around the rete connections and the blood- vessels of the ovary. The rete tubules remain in it as more or less constant remnants of the normal ovary throughout life.

The extensive degeneration of large follicles, occurring in the ovary at weaning time, results in an extension and consequent loosening of the medullary zone. The loose medulla of the adult ovary contains many lacunae and enlarged blood vessels.

The fall-icles.—The larger oocytes, situated in the inner ends of the epithelial cords, first develop a follicular epithelium. Some of these have three or four flattened epithelial cells grouped around them at 18 days 11.0. (Stage 17). These follicle cells are derived from the undifferentiated epithelial elements of the cords and they constitute the primordium of the membrane granulosa. The number of these primitive follicles increases rapidly until at 6 days 13.1). (Stage 20) all the oocytes have a follicular epithelium. The formation of the follicles in speedily followed by the growth of the deeper ones. The oocytes of those having dictyate nuclei enter upon the growth stage and the follicular epithelium grows into a layer of cubical cells, at first single, then two or three cells thick. Numerous mitoses are observable in the cells of the growing follicular epithelium. Many such follicles are already apparent at 6 days pp. (Stage 20). Meanwhile: each of these first formed follicles has acquired a primitive theca. The primitive theca is forming around some of the follicles at 4 days pp. (Stage 19}- It is composed of a thin coat of elements, like connective tissue cells, arranged in a circular manner aroimd the follicular epithelium. It is diflicult to be sure whether these elements are derived from the epithelial cells of the cords or from the connective tissue cells of the septa ovarii and medulla. Each follicle, by the time the membrane. granulosa consists of a single layered cubical epithelium, is provided with such a primitive thecal coat.

A very large number of follicles enter on the growth stage immediately after their formation. Many in process of rapid growth can be seen at 6 days 12.3). (Stage 20). The oocyte grows to mature size and the membrane. granulosa becomes many cells thick (Stages 21 and 22; 8 and 14 days 12.12.). Traces of an antrum are seen in some of the larget follicles at 14 days 12.11. (Stage 22). These growing follicles reach their maximum development at Weaning time (3 weeks p.p., Stage 23). At this stage these follicles compose the major part of the ovary (Plate 31, fig. 1), which is as large or even larger than the adult organ, having increased enormously in size, owing to the growing follicles, during the previous fortnight. As many as fifty such precocious follicles can be counted in a single section at this stage. The largest follicles measure about 0-3 mm. in diameter and have a distinct antrum. The oocytes in the majority of these follicles exhibit obvious signs of degeneration: they are vacuolated and slightly shrunken and often exhibit spindles, polar bodies and even cleavage stages ; phenomena well recognised in the oocytes of degenerating follicles. They sometimes contain crystalloidal bodies, obviously also degeneration products. The membrane granulosa of the follicles also exhibits signs of degeneration in the large number of pycnotic cells and the invasion of leucocytes.

The appearance at the next stage (4 weeks p.p., Stage 24) is very different. The majority of the precocious follicles have degenerated and disappeared, in consequence of which the ovary has shrunken considerably in size and its surface has become somewhat folded. The rnedullary zone has extended, and become loosened in texture, doubtless owing to the sudden slackening of the internal pressure exerted by the follicles. The degenerate remains of some of the oocytes and follicles are present and an occasional full-grown oocyte, with only a. single layered follicular epithelium stretched thin over its surface. How such a follicle could have supplied sufficient nourishment for the oocytc to attain mature dimensions is a problem, yet this abnormality is by no means uncommon in the ovary of the mouse one month or more old. In this, as in all subsequent stages, a number of growing follicles of various sizes occurs, although they are never as numerous as inthe previous stage (23 ; 21 days p. 12.). Some of thee are apparently normal, others obviously degenerating. The size of the largest follicles present increases until maturity is attained, usually about the 8th week post partum (Stage 28), when ovulation occurs and the oestrous cycle starts.

I have not observed follicular atresia resulting in the formation of corpora lutea atretica in the mouse ovary prior to the attainment of sexual maturity. This may be associated with the fact that there is no “interstitial” tissue composed of large vacuolated cells, such as occurs in the rabbit and the cat, and which has been said to originate from the cells of the corpora lutea atretica by some authors, in the normal ovary of the mouse.

The degeneration and absorption of the oocyte results in a cavity being formed in the follicle around the shrunken zona pellucida, the most persistent part of the oocyte. In many cases the membrane granhlosa breaks down into this cavity and degenerates completely and the theca shrinks in size. This process results in the former follicle being only represented by a small cavity, containing the crumpled zona pellucida of the oocyte, surrounded by a fibrous wall, the old theca. In some follicles, however, after the breakdown of the oocyte, fibroblasts from the theca appear to penetrate the membrana granulosa and form a wall Within it around the cavity containing the remnants of the oocyte. In this case the cells of the membrane granulosa may persist and add to the epithelial elements composing the interfollicular tissue of the ovarian cortex. Such cells, however, do not form large vacuolated interstitial cells like those found in the ovary of the cat and the rabbit.

4. Discussion.

The "primordial" germ cells.——In 1870 VValdeyer (14) expressed the View that the germ cells arose from the germinal epithelium. five years later Uoette (7) suggested the extra-regional origin of the germ-cells in the toad and their possible migration. Many authors, including Rubaschkin (13), have supported this view, but Dustin (5) suggested a dual origin for the germ-cells, and almost simultaneously de Winiwarter and Sainmont (16) maintained that the primordial germ-cells all degenerate in the cat, and that the definitive germ-cells arise later from the germinal epithelium. De Winiwarter (17) later insisted on a similar fate for the primordial germ-cells in the human being. Felix (6), expressed the view that in man the primordial germ-cells disappeared and that the secondary germ-cells arose from the germinal epithelium. In the mouse, Jenkinson (8) figured primordial germ-cells and described them as forming in the yolk-sac and migrating to the genital ridge. He was cautious in expressing an opinion as to their "fate, and admitted the possibility of an epithelial origin for some of the germ-cells. I

There can be no doubt that the so-called “ primordial” germ-cells of other authors occur in the mouse. They are present in the primordium of the germinal ridge before the outset of the epithelial proliferation (Stage 1; 9 days p.(:,), and are found in the mesentery of the gut and in the mesenchyme near the germinal ridge, as well as in it, until the nipping off of the gonad is completed. Their cytological characteristics render them easily distinguishable from the other cells of the body and they are indistinguishable from the germ-cells that, in later stages, undergo the prophase changes of the reduction division. Further, there is no evidence, in the material studied, that these undergo degeneration ; on the contrary, they would appear to persist and to undergo the maturation phases.

The occurrence of primordial germ-cells outside the germinal ridge, from the time when it is formed until the gonad is nipped off, is certain also. In the mouse they are found first in the forming germinal ridge (Stage 1; 9days p.c.). Subsequently, they are found in the mesenchyme at the base of the germinal ridge (Plate 29, fig. 1), between it and the base of the mesentery of the gut and even some distance from the latter. They appear to be absent, however, from all other parts of the embryo, and in the regions referred to their frequency greatly increased as the distance from the germinal ridge decreased. They were not found outside the gonad shortly after the latter was completely nipped off (Stage 9; 12 days 13.0.). It would appear probable that the primordial germ-cells are (as) either formed at a distance from the germinal ridge and migrate up the mesentery and through the mesenchyme of the dorsal body-wall to it, as described by many authors, or are (b) formed in the germinal ridge and wander out of it. The distribution described in this paper (Table I) is in favour of either hypothesis. The apparent formation of germ-cells from epithelial cells in the germinal ridge, and the failure to find a yolk-sac origin for the extra-regional germ—cells, however, support the latter suggestion, although the material described is insufficient to arrive at a conclusion.

It remains to consider the evidence for the formation of germ-cells within the germinal ridge. The cytological description indicates that the cells of the germinal epithelium which are proliferated into the germinal ridge may difierentiate in two different directions: (a) into germ—cells; (b) into cells with irregular—sl1aped dense nuclei which will presumably form the folliclar epithelium. Every intermediate stage between the imdoubted germ-cells and the undifierentiated germinal epithelial cells can be seen in a single section and typical examples are figured (Plate 28 and Plate 29, fig. 2). The cytological evidence for the formation of the “ primordial” germ-cells in the germinal ridge is, therefore, satisfactory. Furthermore, even if it were established that the primordial germ-cells migrate into the germinal ridge from elsewhere it would still be impossible to deny that others, in every way indistinguishable from them, were difierentiated from the epithelial cells. It would also be diflieult to imagine a sufficient number of germ-cells migrating to account for the rapid increase in the number found in the gonad between 10 and 12 days p.c., although many of those in the gonads are in mitosis. It is worthy of note that no marked degeneration of germ-cells in the gonads is observable up to the time when sex is definitely established.

The laterformat-ion. of germ-cells.—De Winiwarter and Sainmont (16) find that all the germ-cells of the medullary and cortical cords degenerate in the cat, and that the definitive oocytes are formed from about 4 months pp. They describe at great length the nuclear changes observable in the ovules of the second proliferation or cortical cords. They state that they pass successively through the protobroque, poussieroide, deutobroque, leptotene, synaptene, pachytene, diplotene and dictyae stages. They state at the end that none of these cells ever produce the definitive ova, all of which are derived from the third proliferation.

Kingery finds that in the mouse (9) all the germ-cells formed before birth degenerate. The second formation of germ-cells, the definitive oocyte, from the germinal epithelium is most rapid from 3 to 25 days post partum, but goes on slowly almost until sexual maturity at 40-45 days after birth. The definitive germ—cells pass in from the epithelium, through the tunica, singly, not in cords. Kingery states that all the oocytes which will be difierentiated are present in the ovaries when sexual maturity is attained. He says that “ in the develop- ment of Ike definitive germ-cells, which areformed after birth, there is not the slightest indication of synizesis.”

Germ-cell counts made at difierent ages would be, obviously, the most direct method of solving these problems. The enormous labour involved in making a sufficient number of counts of each stage to produce significant figures, in view of the large individual variation and the inaccuracy of the counts, renders this almost impossible,

The material described in this paper shows that the nuclei of all the oocytes have reached the dictyate stage shortly after birth (4 days 12.19.). Subsequently the earlier reduction figures are never found in any of the germ-cells. Therefore if any of the germ-cells are formed after birth it is clear that they do not pass through these prophase changes. The occurrence of small oocytes in the germinal epithelium at all stages does not necessarily prove, although it suggests, their formntion 1‘v]161'e- Such Oocytes may have remained there during the development of the ovary, or they may have been pushed into the epithelium from

the cortex by the internal pressure exerted by the growth of the deeper-lying follicles.

The present material also appears to show that many of the earlier formed germ-cells do persist until adult life. Although a certain number of oocytes degenerate before birth and a large number enter on a precocious growth stage after birth and degenerate about 3 weeks post pm-tmn-_. many oocytes in various stages remain and appear to persist. The degeneration of oocytes about weaning time corresponds to the post-partmn degenerations noticed by de Winiwarter and Sainmont (16) and by Kingery (9), and which they believed involved all the earlier-formed germ—cells. It appears from some experimental results recorded elsewhere in collaboration with Dr. Parkes and Miss fielding (4) that germ-cells present in the ovary before birth can persist until adult life. In these experiments mice were X-rayed in utero and at birth. The majority of the oocytes in these animals degenerated speedily, but some remained and developed a follicular epithelium. These follicles grew and persisted until about the time when the animals became mature, when they ultimately became atretic. It is noteworthy that the ovaries of these mice sterilized at or before birth, as well as of others sterilized by X-rays at weaning time (3), never regenerated any germ-cells although they remained functional in regulating the oestrous cycle. On the other hand, regeneration of ovarian tissue, co-ntarinireg germ-cells, after complete double ovariotomy has been shown to occur (12) in about 10 per cent. of cases in the mouse. In view of these apparently conflicting results. it is impossible to arrive at a definite conclusion as to whether there is a post partum formation of germ- cells in the ovary of the mouse.

The differentiation of sc:c.—The gonads of the mouse differentiate into ovaries or testes very early. It is possible to distinguish histologically the testes at 11§ or 12 days p.c. The ovaries can then be distinguished by a process of elimination, but do not exhibit characteristic ovarian structures until‘ some hours later. This differentiation of the testes before the ovaries has been observed in the cat by de Winiwarter and Saiumont (16) and in the cow by Van Beak (2)_ Sex can be distinguished microscopically at 12% days p.c. Kirl-{ham (11)., however, failed to distinguish sex earlier than 13 days 31.0. in the mouse embryo.

The difierentiating testis develops a tunica albuginea, which separates the Spermatic cords from the epithelium, as described in the cat by de Winiwarter and Sainmont (16). The ovary, however, develops no corresponding primitive tunica albuginea and the epithelial nucleus remains continuous with the germinal epithelium. De Winiwarter and Sainmont (16) in the cat, and Van Beck (2) in the cow, describe the formation of a transient tunica albuginea, and Felix (6) in the human being finds a separation of the epithelial nucleus from the superficial epithelium at the time of the differentiation of sex. The epithelial p9'olzferatz'ons.—The proliferation from the epithelium which , forms the epithelial nucleus of the indifferent gonad of the mouse continues without interruption, although more slowly, in the ovary throughout embryonic life and after birth until weaning time or even later. The fact that, in the male, the germinal epithelium continues proliferating for a brief period after the establishment of a continuous tunica albuginea, is further evidence that the proliferation is one continuous process. The proliferation takes place over the Whole surface of the ovary and is not, at first, in the form of separate cords. Later the formation of the septa ovarii breaks it up into thick cords. De Winiwarter and Sainmont in the cat (16), and the former author in the rabbit (15) and man (17), find three distinct epithelial proliferations in the form of cords : the medullary, cortical and third or definitive proliferations. Kingsbury in the cat (10) and Kingery in the mouse (9) fail to find any distinction between the medullary and cortical cords, but both distinguish a. post-pa-rm-m formation of germ-cells. Allen (1) describes the formation from the epithelium of medullary and Pfliiger cords in the ovaries of embryo rabbits and pigs. Van Beck (2) in the cow finds that the medullary and cortical cords do not arise from a separate epithelial proliferation, but he also distinguishes a separate postvpa-rtum formation of germ-cells. The present results are, therefore, most nearly in agreement with those of Kingery, but do not show any distinction between the embryonic and the post-part-mn proliferation. The lack of a primitive tunica albuginea in the ovary, like that described in other mammals, is probably correlated with the absence of a break between medullary and Cortical proliferations. These peculiarities of the ovary of the mouse may be due to the small size and extremely rapid development of the organ. 5.


1. The first traces of the germinal ridge are found in the 9-day p.c. mouse embryo. 2. The growth of the germinal ridge is effected by proliferation from the epithelium. This results in the formation of a solid epithelial nucleus, which is not in the form of separate cords.

3. The testes differentiate from 11} to 12 days pm. The differentiation is efiected by the downgrowth of connective tissue from the hilum forming a continuous tuniea albuginea and breaking the epithelial nucleus up into twisted spermatic cords. The formation of the spermatic cords is therefore secondary.

4. The ovaries remain in the indifierent condition after the testes have difierentiated. There is no downgrowth of connective tissue, no primitive tunica albuginea. and the epithelial nucleus remains a solid mass not broken up into cords. The epithelium continues proliferating without intermission.

5. “ Primordia ” germ-cells are found in the mouse. Evidence is brought forward, suggesting that they originate from the cells of the germinal epithelium and that some, wandering out of the gonad, constitute the extra-regional primordial germ-cells.

6. In the ovary the epithelial proliferation continues, without intermission, although more slowly, until the 4th or 5th week post partum. This proliferation cannot be divided into medullary, cortical and definitive proliferat-ions, as described by de Winiwarter and Sainmont in the cat (16).

7. Mesenchymal elements growing down from the hilum form the tuniea albuginea. and septa ovarii. These are completely established by 16 days 11.0. The development of the septa ovarii imposes a secondary corded arrangement on the epithelial nucleus of the ovary.

8. The medulla is formed by a downgrowth of connective tissue from the hilum at the time when the deeper situated germ-cells are degenerating.

9. Some oocytes can be seen degenerating at all stages after the diflerentiation of the ovary. Shortly after birth a large number of follicles enter upon a. precocious growth stage. These reach their maximum development and then degenerate about weaning time (3 weeks p.p.).

10. No conclusion was arrived at as to Whether there was a post-part-um formation of oocytes in the young mouse. No germ-cells exhibiting the synaptic stages were observed in the stages more than four days old.


Allen BM. The embryonic development of the ovary and testis of the mammals. (1904) J. Anat. 3(2): 89-154.

(1) Allen, B. M.. ‘ Am. Jour. Anat.,’ vol. 3 (1904).

(2) Van Beek, W. 16., ‘Zeits. fiir die Gesamte Ans.t.,’ Abt. 1. (1924).

(3) Brambell, Parkes, A. S., and fielding. U., ‘ R.S. Proc.,' B, vol. 101, p. 29 (1927).

(4) IbI'd., p. 95.

(5) Dustin, A. P., ‘ Arch. de Biol.,’ vol. 23 (1907).

(0) Felix, W., Keibel & Mall, ‘ Manual of Human Embryology,‘ vol. 2 (1912).

(7) Goette, A., ‘ Die Entwicklungsgeechichte der Unke (Bombinator s7gneu.s),' Leipzig

(1875). (8) Jenkinson, J. W., ‘ Vertebrate Embryology,’ Oxford (1913).


[O 0-: ‘fiG. 6.—Very large primordial germ-cell from a. 12-day embryo. X 2340.

(9) Kingery, H. M., ‘ Jour. Morph.,’ vol. 30 (1917). (10) Kingsbury, B. ]:‘., ‘ Amer. Jour. Anat..’ vol. 15 (1913). ‘ (ll) Kirkhem, W’. B., ‘ Anet. Rem,’ vol. 10 (1916). (12) Perkes, A. S., fielding, U., and Brambell, ‘ Roy. Soc. Prod.‘ (13) Rubaschkin, W., ‘ Anat. Hefte,’ vol. 29 (1909). (14) Waldeyer, W., ' Eierstock und Ei,’ Lepizig (1870). (15) de Winiwarter, H., ‘ Arch. do Biol.,’ vol. 17 (1900). (16) de Winiwarter and Sainmont, G., ibid., vol. 24 (1909). (17) de \Vim'warter, H., ibid., vol. 25 (1910).

Description of Plates

GULDE Ln'1'r1:3s:——B.I-"., Blood vessel. G.E., G-erminal epithelium. G.E. 1-3, Stages in transformation of germinal epithelial cells into germ-cells. G.M., Germ-oells in M., Medulls. M.T., Mesonephric tubule. M .D., Miillerian duct. P.G.. Primordial germ-cell. R.’['., Rete tubule. S.0., Septa ovarii. T.A., Tunica slbuginea. U.E._. Undiflerentiated epithelial cell in epithelial nucleus. W.D., Wolfliian duct. The arrow points to the mid-line of the embryo in all cases.

Plate 28

Brambell1927a plate28.jpg

Fig. 1. Primordial germ-cell from 11.5-day embryo. X 2340.

Fig. 2. Three cells of the germinal epithelium of 11.5-day embryo. X 2340.

Fig. 3. Cell in epithelial nucleus of 1 14.5-day embryo illustrating first stage in transformation of an epithelial cell into a germ-cell. X 2340.

Fig. 4. Two undifierentiated epithelial cells in the epithelial nucleus of 11.5-day embryo. x 2340.

Fig. 5. Two cells in the epithelial nucleus of ll.5-day embryo, illustrating the second ~ stage in the transformation of an epithelial cell into a germ-cell. X 2340.

Fig. 7. Primordial germ-cell from 9-day embryo. X 2340.

Fig. 8. Primordial germ-cell and mesenohyme cell from a 9-day embryo. X 2340.

Plate 29

Brambell1927a plate29.jpg

Fig. l. Six primordial germ-cells in the body-wall above the germinal ridge of a 10-day embryo. x 1000.

Fig. 2. Germinal ridge of a 10-day embryo showing cells proliferated from the epithelium in the undifierentiated condition and in various stages of differentiation into germ-cells. x 1000.

Plate 30

Brambell1927a plate30.jpg

Fig. l. Ovary of 12-day embryo (Stage 9), showing epithelial nucleus everywhere continuous with the germinal epithelium. x 240.

Fig. 2. Testis of 12-day embryo (Stage 7), showing developing tuniea albuginea. and germinal epithelium thickened outside it. X 280.

Fig. 3. Testis of 12-day embryo (Stage 9), showing spermatic tubules cut oif from 17110 germinal epithelium by a well developed tunica. albuginea.. X 170.

Fig. 4. Ovary of 12-day embryo (Stage 7), showing epithelial nucleus still continual‘ with the germinal epithelium. x 280.

Fig. 5. Germinal ridge of 12-day embryo (Stage 4), showing formation of epithelial nucleus. X 265.

Plate 31

Brambell1927a plate31.jpg

Fig. 1. Ovary of young mouse three weeks old, showing large number of follicles which have developed precociously and are about to degenerate. X 42.

Fig. 2. 0vary of 17-day embryo showing the formation of the medulla. X 140.

Fig. 3. 0vary of new-born mouse showing the structure of the ovary before the growth of the follicles (same fig. as fig. 5 in Plate 7 (4) ). X 140.

Fig. 4. Ovary of 13,1;-day embryo showing many of the germ-cells in synopsis. X 150.

We are indebted to Mr. F. J. Pittock for the photomicrographs reproduced in Plates 29-31. The drawings in Plate 28 were made with the aid of a camera lucida.

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