THE TIOAlOLOCY O F THE VESICULAR OVARIAN FOIJLICLES O F THE MAMMALIAN OVARY

WITTI TIIF: COELOM *

TT. W. MOSSMAN Ucpar tnant of Anatomy, Univwsity of Wisconsin

ONE TEXT P'IGURE A N D O N E P1,A'L'E (F,IGIIT FtGUKIiS)

For many years the most widely accepted theory of the origin of the coclom has been the gonocoel theory. I n this paper tlic problcm considered is in a sense the reverse of this. It is the theory of the liomology of the cavities of the ovarian folliclcs of mammals with the coclom, and it is largely based on tlie ontogciictic fact of the origin of the follicular epi- thclhim from tlic peritoneal covering of the ovary, although certain other accessory evidence will be presented. I n brief the cvidence indicates that the follicular cavities of mammals arc isolated port ions of tlie coelom, and that the follicular epithelium and lutciri cells are specialized portions of the coelomic epithelium. For convenience and clarity each sup- porting fact is discussed separately in t<he following six paragraphs.

1. Piimordial ova occasioiially f o i m partly developed pri- mary oocytes while still lying in the surface epithelium of the ovary or just within the mouth of an egg tube. This con- dition has been observed several times. It is especially well shown in the ovary of the porcupine (figs. 4, 5, 6, 7 and 8), and occasionally in the pocket gopher and the grizzly bear. These surface eggs sometimes enlarge considerably but eventually uiidergo hyalin degericration (fig. 8). Clark ( '23)

'This work was aidcd by a grant from the Wisconsin Alumni Research Foundation.

G43

644 11. W. MOSSMAN

described and figured ova in the germinal epithelium of a 3-month-old guinea pig. She suggested the possibility that these ova liad migrated to the surface from the interior of the ovary, but thought this unlilrely. I n this she was surely cor- rect, fo r in the porcupine early stages of diflferentiation of these ova from the surface epithelium arc easily found (fig. 7) . Such surface eggs might also be considered to havc developed from residual primordial germ cells, but there is certainly no cvidcnce of this aiid it seems much more probable thilt they cliffereiitiate from the generalized ovarian epithelium. The ones illustrated in figure 7 were in an animal at least 6 nioiiths old althougli not an adult. ‘I’hcy havc been found in adults, even pregnant animals, but less commonly than in the younger individuals. The pi*eseiicc of such surf ace eggs illustratm the generally accepted fact that the surface or germinal epi- thelium of the ovary contains cells which have the potcnti- alities of germ cells. Of course this brings up the question of whether the epithelial covering of the ovary should be thought of as merely a specialized portion of the coclomic lining or as a distinctly dif’fcrent tissue, the ‘germinal epi- thelium.’ There is little in favor of the latter view, and so I shall considcr the ovarian surface epithelium homologous to the peritoneurn. Onc might carry thc homology even farther and comparc the tunica albnginca and tlieca intcriia and ex- tcrna with the fibrous layer of the peritoneum.

2. Ovarian follicles in all stages from fetus to adult are derivcd from tubes or cords of cells invaginated from the surface epithelium of the ovary. These often retain their connection with the surface cpithclium until after primordial ova and even primary folliclcs a rc dcveloped within thcm. Figures 4 and 5 show the invagination of tubes from the sur- face epitlicliurn in. the porcupine ovary. Figure 2 shows similar tubes in the ovary of an adult grizzly bear, and figure 3 shows numerous more or less solid cords in the ovary of a young bitch. I n most mammals these ‘tubes’ are rcally cords but that docs not alter the fact that thcy arc sprouts from the coelomic lining. ‘I’his indicates that the follicular cavities

OVARJAN FOLLlC1,E AXD COELOM 645

Iatct. tllevclopctl in them may be xgardcd as homologous to the pei*itoneal cavity, since their lining cells are derived di- rectly fiwm a portion of t,lic pei*itoneal epithelium.

3. Tlic follicular epitheliuni is a serous epithelium like that of thc peritonrum. Tt is t rue that the granulosa cells are not of the squamous type as arc those of the general peritoneum, for tlicy have i*etaiiied their enihryonic more-or-less columnar shape and also their embryonic capacity for mitosis. 111 the older folliclcs they form '1 stratified layer which is also not characteristic of coelornic epithelium in general. 0 1 3 tlie other liaiid in cystic follicles where, having lost their specialized Pniiction, they persist for moiitlis or years beyond their normal life span they do form a single layer of epithelial cells more clia~-acteristic of the adult peritoneum. Thus because of its derivation from the coelornic epitlielium of the ovary and be- eiiuSC of the embryonic nature of its cells the ovarian epi- tlieliuni may be regarded as a portion of the coelomic lining serving a peculiar function as the lining of an egg-bearing follicle. Fo r some reason, perhaps the necessity for rapid follicular growth, tlie persistence of the embryonic charac- teristics of tlie follicular epithelium is denianded. It may pass from this embryonic condition in either of three direc- tions. 1) Normally, through follicular rupture to the forma-. tion of specialized endocrine cells, the lutein cells, and then cveiitnally t o degeneration. 2) Normally (in the sense that it is of uiiiveixd occurrence) tliroug-h f ollieular atiaesia to dc- geiiei*atioii. 3) Abnormally, through loss of its special fuiic- tion, to form a cystic follicle where the epithelium seems to mature aiid bccoine more like the normal, adult coelomic lining. 4. Follicular fluid is essentially a sct'ous fluid although the

albuminous material in it must be more concentrated than in ordinary peritoneal fluid. Studies of Hill, Allen and ICramer ( '35), and McReiizie and Terrill ( '37) on folliculai. rupture have shown that it may he iii par t gelatinous. The follicular hormone is found in it, but not in greater concentration than in extracts of the whole ovary, indicating that the hormone probahly diffuses iiito tlie follicular. fluid from the endocrine

646 IX. W. MOSSMAN

cells of the thcca iiiterna and that its presence in the fluid is more o r less fortuitous. To my knowledge there is no pub- lished account of the chemical nati1i.c of the follicular fluid, nor is there an analysis of normal peritoneal fluid of a mammal. Petei*s ('35) cites some work on the chemistry of ascitic fluid. The concensus of opinion seems to he that the coelomic fluid is formed from the blood serum by ultrafiltra- tion and contains relatively little protein. There is no doubt that it contains some protein as it will coagulate quickly when a Irace of blood is added. An accurate chemical analysis of peritoneal and follicular fluid from the same individual seems to be needcd. Follicular rupture might possibly depend in par t upon osmotic pi*cssure dine to passage of water from the p r i tonca l fluid to the undoubtedly more concentrated fol- licular liquor.

5. Ovarian follicles rupture inio the peritoneal cavity at ovulation thus reestablishing coriiiection with the coelom.

6. The corpus luteum in some species is occasionally everted to form a plaque on tlic ovarian surface so that the glandular tissue occupies the same relation to the coelom that its aricestral cells did before their invagination to fo1.m the egg tuhcs. This OCCUI'S frequently in the pocket gopher (fig. 9) . The soui*ce of the ncw epithelium which covers the region of these everted corpora after they degenerate, or which over- grows the rupture points of normal corpora, is not known. I t may regenerate from the normal epithelium surrounding the lutein area or rullture point, or may be differentiated di- rectly from the mescnchymatous connective tissue cells which migrate into the cavity of the corpus luteum and which spread 0\7er the exposed surface of the rupture point or that of the everted corpus. My observations lead me to favor the latter process as the one of greatest importance, although un- doubtedly there is some regeneration from the edges of the surrounding normal epithelium.

T'o summarize, it can be said that in mammalian ovaries the follicular epithelium is to be regarded as a modified coelomic epithelium, and the follicular cavities as isolated portions of

0TA4FL1AX FOLLICLE AND COELON 647

the peritoneal cavity filled with slightly modified peritoneal fluid. Figure 1 illustrates this conception of tlie follicle.

Such a relation of the follicle to the coeloni can be traced pb~~logciicticrtlly as well a s ontogenetically. TThile it is not the purpose here to disciiss this phase of thc problem brief mention may be made of certain facts. lii practically all verte- brates the germinal epithelium of the ovary ei thm lies entirely on thc ovarian surface or develops at an early time from tlie

Fig. 1 Diagram of ni:mnialiaii OT ary t o illustrate tlic tlieoretical hoiiiology of the follieular e~iithelium and lutein tissue t o the coeloitm liiiiiiy aitd of the follieulxr earitp to the cot~loiii. The only conditiori indicated on this diagram which has not been shown to occur riornially is the ronnrc.tion of the ca l i tp of n vesicular follicle by an opeii cgg tube to the peritoneal carit)-. 111 SOIIIL: animals a distinct, narrow prolongation of the follimlar carity toward thc surfare is indira- ti,, o i this rclalionuhip. e.c.l., e r t~ i ted corpus liiteum with fibroblastic layer covering it wliicli ~vould ordiiierilp be eii~losed within thc corpus except in the region of the hca l i i i~ rupture point (conipnrc fig. 9 ) . c.t., egg tubes. n.c.l., iiornial corpus luteum with cavity fillrd b~ fibroblastic tissue which is taking part 111 the healing of the rupture point. p f . , primary follicle connected t o surface by cgg tubr. p,g.e., piimordial germ cells or surface o i a (eoiitpare figs. 4, 6, 7 and 4). r f . , riipturcd folliele iriimeiliatv!y a f te r nvulation. \.f., vc ' s~~ula i follitle.

648 H. W. MOSSBIAS

surface epithelium. The la r is by far the more usual condition. Also in practically all vertebrates the eggs develop either in the surface epitlielinm or in the deeper tissues, and upon ripening pass from, o r through, the ovarian surfacc into the general body coelom or a cavity that is obviously a specialized portion of it (certain teleosts). These facts and similar ones were used long ago in developing the ‘goiiocoel’ theory of the origin of the coelom. Hatschek (1878) is iisually given credit for having started this line of reasoning when he suggested that the ‘pole cells, which form the coelomic lining of annelids and molluscs might be primitive germ cells. How- ever Bcrgh in 3885 on the basis of studies on the gonads of flat-worms first put the theory to the forefront as an ex- planation of the phylogenetic origin of the coelom throughout the animal kingdom. The eiiterocoel theory, first suggested by the work of Leuckart (1848) on coelenterates; Agassiz (1864) on echinodcrms ; Ililctschnikoff (1869) 011 echinoderms and Balanoglossus, was actually introduced as a theory of de- velopment of thc coelom by Metsclinikoff (1874). Lankester (1875) also proposed the idea of an eiiterocoel in connection with his ‘planula’ theory and clabarated upon it greatly later (1877). Huxley (1875) accepted the entcrocoel theory for a few phyla, including echinoderms, Sagitta and Balanoglossus, but introduced the idea of a ‘schizocoel’ f o r molluscs and annelids, and of an ‘epicoel’ for tunicates a id possibly verte- brates. The conception of the entcrococlous origin of the coelum was in general accepted by Balfour (1880) and by 0. and R. Hertwig (1882). Therefore the rapid rise in favor of Bergh’s ‘gonocoel theory’ after he introduced it in 1885, in spite of an impressive array of ‘enterocoelists,’ was testi- mony to its value. It was elaborated by Goodrich (1 895) , by E. Meyer (’01) and by Lang (’04). The latter made it the basis for his ‘hemocoel’ theory of the vascular system, and Goodrich (’30) still accepts it as tlic. most logical and useful theory of phylogenetic origin of the coelom. The ‘nephrocoel’ theory introduced by Ziegler (1898) has never gained wide acceptance.

There are some facts on the basis of which thc enterocoel, g onoco el and neph r oc o el t 11 e o 1- i c s ma J- 1) e p ii r t i r? 11 y h a rnio- nized. This is particularly true of thc first two. The ‘entero- coel’ theory mas based 011 si ich forms as Amphioxus where the coelomic pouches are apparently evaginations of the archenteron. Even in annelids the pole cells temporarily iorm par t of the lining of tlie archeiiteron and in coelenterates tlie gonads are definitely pouches of the enteron. The homology of the coelomic cavities of ,4niphioxus with those of higher choi+dates is reasonably clear, arid in all the liiglier verte- bratc-is, from fishes lip, the geim cells seem to arise from, or a t least to lie in, tlic coelomic lining, thus strongly indicating a very close l)liylogeiietic relation hetmecn the gomcls and tlie coelom.

Of course if one looks a l tlie serons irieiiibimies from the functional stanclpoint most of the evidence, at least from the vertchrales, bet’iiis to point t o tlieir lieiiig aclaptatioiis witliin the mesoderm to allow freedom of movement between parts. Such a n explanation ~ o u l d explain tlie early development of the pericardial cavity and the relatively late differentiation 01’ the pleural cavities in vertebrate embryos. It would also explain other fniictionally similar cavities such as tlie synovial cavities of movable joints and the subcutaneous synovial bursae of tlie human body as well as the even more beautifully adapted synovial tendon sheathes. WoodJones ( ’13) in his cli scussion of the functional history of the coelom considers this and several other functions in detail.

A somewhat similar teleological explanation for the sepa- ration of the ovogenie coelomic epithelium from tlie general cavity may also be advanced. Bray it not be that the obvious disadvantages of the exposed coelomic lining as a site f o r the development of eggs led by adaptation to the carrying of this process beneath the ovarian surface t I n such an environ- ment the delicate ova may develop to full size and have a relatively tough vitelline membrane, or zona pellucida iii the case of mammalv, formed aloout them before they are finally thrown into the body cavity. Perhaps the inammalian fol-

licular antruni can also be explaiiied as a necessary aclapta- tion because of the small size of the inainmalian egg. It is difficult to see how such a small particle could be set free with sureness, eren though it developed on the surface of the ovary, if it did not have the body of follicular liquor to wash it out into the furiiiel of the oviduct. Such esplanations however are of little scientific value, merely giving some of us a certain amount of mental satisfaction.

Whalever one proposes in regard to the fundamental struc- ture and evoliition of the ovary should also be applicable in a broad way a t least to the testis. The male and female germ cells are certainly homologous. In mammals both testis and ovary pass through an iiidiffcrent stage which consists cs- sentially of a mass of epithelial cords or tubules derived from the surface cpilheliuni and separated 1))- so-called ‘inter- mediate’ tissue probably of a mesenchymal instead of meso- lhelial origin. Thus the seminiferous tubules, and part at least of the ductiili efferentes are dei*ivatives of the eoelomic epithelium. Certainly on tlie basis of ontogenetic evidence we can go still further and consider the mesonephric tubules and duct as also mesothelial in origin, but this takes us into a consideration of the phylogeiiy of these structures which is entirely beyorid the piirpose of this paper.

However there is still one highly debatable point. Are me justified in making the assumptiori that the follicular aiitra of mammalian ovaries are either pliylogenetically or onto- genetically derived from the eoelom, or should we simply say that they arc homologous to the coelom? There is no good reason to doubt that the follicular epithelium is both phylo- genetically and ontogenetically derived from the coelomic lining. However, the cavities of the egg tubes are not simply hroken into segments and then enlarged to form the antra of the follicles. Instead they disappear at the time the primary iollicles are formed, and no follicnlar cavity is present until sometime later when the cells of the follicular epithelium separate again giving rise to the early antra. The phylo- genetic history of the follicular eal-ity seems to be rsseiitiallj-

0VAI:IAX FOLLICLE A S D C O E L O M 6.71

similar. That is to say, although egg tubes or cords form from the surface epitlielium in many lower vertebrates, the follicles never retain or develop cavities, but hare throughout their growth and maturity essentially the strnctiire of a primary follicle of a rrmrimalian ovaq-, except that the oocyte is larger because of its greater accumulation of yolk. This condition is equally true of the egg-laping mammals as well as of the iiiiniecliate ancestral group, the rcptilcs. Therefore, both the ontogcn~tic arid phylogeuetic evidence seems to point to the ovariaii follicles of mammals as a secondary cavitation in coelomic epithelium homologous to the coelomic cavity but not directly der~vcd from it either duriiig development of the individual or the race.

S U N Y A R Y

The theory is proposed that the follicular epithelium and luteiii cells of mammalian ovaries are both ontogenetically and phylogenetically specialized portions of the coelomic epi- thelium, and tlittt the follicular cavities are homologous to the coelom although both their ontogenetic and phylogenetic histories indicate that they are not direct outpouehings of the coelom but develop secondarily, probably to meet the specialized conditions attendant upon the ovulation of the microscopically small mammalian ova.

LITERATURE CITED

AGASSIZ, L O ~ I S 1864 Embrjology of the staifish. Cuntrib. Nat. Hist. t'. S .

BALFOUR, I?. M. 1880 On the structure and honiologies of the gerrniiial layers

EERGH, R. S. 1885 Die Exkretionsorgane der Wurmcr. Eine Ucbersicht.

CLARK, ESTHER €3. Observations on the o t a and oparies of the guinea pig,

Goonxrc~. E. 8. 1895 Coelorn, genital duets and nepltridia. Qaart. J. h11e. Sci.,

1930 Studies in the structure and development of vertebrates, Marmillan, London, pp. 715-719.

HATSCHEK, B. 1878 Studien ubcr EntwiekIungYgescliichtp der Anneliden. Ein Beitrag zur lforphologie der Bilaterim. Arbeit. Zool. Inst. Wien, vol. 1.

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of the embryo.

Kosmos. Zeitschr. f . d. ges. Entwickluiigslehre, Bd. 17, S. 97-122. 1923

Cavia cobnga. Anat. Rec., vol. 25, I J ~ . 313-331.

Quart. J. Mic. Sci., vol. 20, pp. 247-273.

~ 0 1 . 37, pp. 477-510.

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I~ERJWIG. O., AND K. HER IXTG 1882 Die ("olo~iitheorie. Yersuch einer l k k - larung des mittleren Keiniblattes. Zeitsclir. Nntnrwiss., Ril. 15, S. 1-150.

HILL. R. T., EDGAR ALLEN A s i ) T. C . KRAXBK 1935 Cinemicrographic studies of labbit o\ulation.

HITXLEP, THOMAS 1875 On the rlausificatioii of the aiiinial kingdom. Quart. J . Mic. Sci., vol. 15, pp. 92-56.

L~NG. A. 1904 Britragc zu riner Trophotiiltheorie. Zeitrscli. f . Naturmjus., Bd. 38, 8. 1-376.

LAKIWSTER, E. RAY 1875 On thr inlaginate planula of Palutlinn. Quart. J. Rfk. Sci., \ol. 15, pp. 159-166.

- - - -_ -- S o t c s 011 the cnibryologp and classificntioii of the animal kingdoni : vuiiipiisiriq a re\ ision of the s p ~ ~ i ~ l a t i o i l u ~ c l a t i + c t o the origin and signifitanee (if t f i ~ germ Iw?cis. Quiut. J. Mic. Sei., vol. 17,

U e ber die hl orpli nl ogie iind Verwen dt ecliaf tsrerhal tnisse der wirl~elloscn Thicre. Ein Reituag 7131 C'hnrnktcristih und Classification der tierisclieii F o i men. Hraiinschweig.

1937 Estrus, o\ulation and related phcuornena in the em!. nliseouri Agric. Elup. Sta. Eull., no. 364,

?JFThCHNIKOw, E l . l A s 1 8 W fitiidieii iiber die >htwicklung der F:chinodermes RIPnioires de 1 'Acad. inip6riale d. sci. de St. Peters-

Studien uber die Entwicklunq der Meduxen iind Siphonophoren.

METER, EDOUAKD 1901 Studien 5lm Korperban der Annelidcn. Mittheilungen

PETERS, Jonx P. 1935 Body Water. The Erchaiige of Fluids hi Man. Thomas.

U'OOTJ-JOXES, FREDERIC 1913 The funrtional history of the coelom and the

, H. E. 1898 Ueber den clerzeitigen Stand der Ciiloinfrage. Verhandl.

h u n t . Rec., 101. 63, pp. 239-245.

1877

pp. 399-454. LEV CKAR'I, H. 1 X I 8

NcI<EYZlE , FEF .~ F., A V I ) (" IAIHF K . TFRRII I ,

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und Keiiiertinen. bourg. 7th ser., vol. 14, no. 8.

18 i4 Zcitsclir. f . wissenschaft. ZOO~. , Ed. 24, 8. 15-83.

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PLATE

PLATE 1

EbPL 9EATIOn’ O F FIGURES

f Egg tubes in thr ovarj of the adult g r iu l ) bvar. It can b(’ s h o w ~ i 1 ~ ) follow ing the series of sections tliat the two primary follicles a t the left are coiincrted by their epithelium to the egg tube leading toward them from the ovarian suifase directly above.

Egg cords in the w a r y of a near13 full grown bul sexuall) ininiatuie bitch. Such follicles as the largest one (in this vase hiovular), as well a 4 the smaller ones ;tbove, can be shown t o be connected t o egg cords which occasionally may be traced all the way to the surface. The cords in the dog ovary a re much more tortuous than in the bear. Longitudinal scctioiiv of the cordv often show a row of three o r more OVR in them.

of a n jniinature porcupine, 111 ohably about G months old, showing egg tubes, surface eggs developing in the epi- thelium. x 98.

High powcr kiew of ovary shown in figure 4 showing detail of the egg tube penetrating the dense tunica dbuginea. Erethizon clorsatus 92, shde C,, section 4, l o p . x 250.

High power view of ovary shown in figure 4 showing at the left the detail of a surface egg and an egg nest imuiediately deep t o the tunica albuginea probably derived from an egg tube or cord whirh penetratrd the tunic in tha t region. A t the right are two o t a undergoing enlargement hetwpen the fibers of the tunic. A t the bottom of the indentation of the epithelium near the renter nre two lightly stained cells which are probably early stagcs in the differentiation of surface eggs from the epitheliurn. Erethizon dorsatus 9 2 , slide C., section 4, l o p . X 050.

IIigh power view of ovary shown in figuie 4 slioaing a surface egg and several enlarged and more lightly stained surface epithelial eells which seem to be i n the prncess of differentiating into ova. Ercthizon dorsatus 92, slide C,, section 5, 10 p. x 250.

High power view of ovarr sho\rn in figure 4 showing surface egg with i ts thin capsule of flattened rpithelial trlls. The appearance of this egg suggests that i t is being extruded €rum the epithelium. I n the epithelium at the extreme

in mass characteristic of the later stages of degeneration of on dorsatus $2, slide C‘:, wctioii I , 7 0 p. x 2.50.

Everted corpus luteum in the oiary of a pocket gophei (Geomyx bursarius). The interior of a normal rorpus is represented by the fibroblastic tissue coiering Ilie convex surfare of the ovary between the two marks. The outer surface of a normal cwrpus is represented by the inner concave boundary of the lutein tissue. Embr?-os in the early embryonic disc stage were just implanted j n the uterus (if this animal. Geomps bursarius Pia, slide 10, stctinn 33, 8 p.

Ursus liorribilis El, sljrlr 2, section 3, 10 p. X 51) 3

Cams familiaris 94, blicle 7, seetioil 3 , 10 p. X 110. 4 Surface and cortical portion of the o

Erethizon dorsatus of. slide c,, sertion 4, 10 p. 3

6

7

8

9

X 45.

654

OVARIAN FOI>LICLE AND COEIAOM I€. W. MOSSMAN

PLATE 1

655