Book - A Text-book of Embryology 6

From Embryology

Chapter VI. The Deciduae And The Embedding Of The Ovum - The Placenta - The Umbilical Cord

Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

Heisler 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs

Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.

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The Deciduae And The Embedding Of The Ovum

The decidnn (deciduous ur cadncons membranes) are the hypertrophied mucosa i»f the uterus so dcvelojwd as to form not only a lining for the uterine cavity, liut also an envelope enclosing the ovum, and a specially all^reil part which serves as a bond of connection between the ovum and the womb.

During tlic four or five days preceding lucnslruation. the socalled constructive stage of the mensiruiil eytle, the miienus membrane of the womb liecomes mnch thickened and unusually vascular, the purpose of these changes being evidently the preparation of the uterus for the reception of the ovum in the event of impregnation. If impregnation has not occurred, the thickcueil mucosa, the decidua menstmalis, is in great part easi off ns a purt i^f the menstrual discharge; if, on the other I, conception has taken place, ^e raucous membrane undergoes I greater hyj^ertrophy. On scc, it is seen lo consist of a super1 compact stratum and a deeper b^'™"""*' ^nrr layer rcjKising directly upon the muscular wall of the {.Uterus, In the compact layer are the necks of the much enlarged uterine glands, while in the spongy layer are their greatly branched and often tortuous bodies (Fig. 51). The tortuosity and division of the deeper extremities of the glands produce the characteristic appearance of a section of the spongy stratum.

jl.— Crnan Beellon through iici.iia nicttibrane of Iho Hi the begliiiilng nf preg^ (aner Kiindral knd Bn

The alterations necessary to convert the menstrual decidua into the decidusB of pregnancy take place in part while the ovum is still in the Fallopian tube ; when it reaches the uterus it becomes attached to the mucous membrane of the latter, usually along the upper part of the posterior wall. A portion of the mucous membrane eventually comes to enclose the ovum as in a distinct envelope (Plate V., Fig. 1). The part of the uterine mucosa which thus surrounds the ovum is the decidua reflexa; the part still lining the cavity of the womb 13 the decidua vera ; the part that is in contact with the chorion frondosum is the decidua serotina. The decidua sor\>tina afterward becomes the maternal part of the placenta, iMtimatoly uniting with the chorion frondosum.

r«til recontlv it was believed that the ovum became imnUnUnl ti/K)n the surface of the mucosa, and that the latter irr\^\v up nr\>und and over it to form the decidua reflexa. VUi^ tluH^ry has Ik^ou completely set aside by the recent m\\v<turHtions of IVters of Vienna, whose results have been v\^«Knm>i by Webster of Chicago. Peters' observations \iv*v uu^\io u|H\n the gravid uterus of a suicide, the ovum ts^a^ v^^nln^UUnl in a triangular i>rominence on the upi)er »Hxx^<%»* ^\<:<\m of the i>osterior uterine wall. The ovum »Mxs«.xut^>J »u tlmv diameters respectively 1.6, 0.8, and 0.9 »wu». »w vN^^mKAUnl HS^^ iHMUg three or four days.

tX^ ■im* tt i t lft'if ^Iht OTum (Plate IV.), or its sinking into

Ik u,4x>s.^*, X v^uK^kly mHH>mplished by the c-osion of the

. , s ;4w ;;<: 'u>> t^ xv|^ iho hitter, presumably by the phagocytic ^. u .. i »iK M^»^4^UxU*t. Actual erosion is evident from xnH^ivv epithelium at this place. The .»..^ KUH*^^at vM^x ivlation with the deeper layers of excavation are undermined, jmhK ww^hI by the mucosa, the area

i. . v.;vvl \iu^ v\vu|^\s) by an organized blood clot.

the tisane ftmgns (Plate IV.). The overhanging edges of the excavation constitute the beginning of the reflexa, which is obviously, therefore, not produced by the upgrowth of a circular fold of mucosa. The trophoblast strands or early villi extend toward and into the serotina, to which some of them become attached. It is thought by Webster that they may absorb fluid and imtriment, and that by phagocytic action they open up the blood-spaces of the serotina, thus bringing them into communication with the lacuna? of the trophoblast.

The blood-lacunse of the trophoblast form a system of intercommunicating spaces, the beginnings of the later intervillous spaces of the placenta ; they are filled with maternal blood from the serotina, and are lined with syncytium (Fig. 63), the latter being thin in places and resembling an endothelium. There is no extension, however, of the endothelium of the serotinal vessels, either upon the villi or into the spaces (Peters and Webster).

The ssmcytiiun is the more or less irregular layer of nucleated protoplasm which appears upon the surface of the ovum toward the end of the first week, lining the trophoblast lacunae, and later |>enetrating as irregular masses into the serotina, where it is found until the end of pregnancy. No traces of it are found on the veni after the sixth week. The origin of the syncytium has long been in dispute. Peters has shown that it results from the tninsformation of the superficial part of the trophoblast, probably from contact of the latter maternal blood, which, he thinks, exercises a blending influence upon the trophoblast cells, so that as individual cells they disappear, the result being a non-cellular but nucleated protoplasm. Peters also believes that corpuscles of the maternal blood are appro])riated by the syncytium, and that the latter, covering villi and chorion as it does, has something to do with the interchange of nutriment and waste products between the maternal blood and the ovum. What remains of the early trophoblast after the formation of the svncvtium is the layer of Langhans.

The ciliated epithelium of the uterine mucous membrane disappears by the end of the first month of pregnancy (Minot) ; somewhat later, that of the uterine glands is also lost. By the end of the fifth month the fetus and its appendages have increased in size to such an extent that they completely fill the cavity of the womb, and the space between the vera and the reflexa is obliterated. After the second month the vera becomes progressively thinner and the reflexa undergoes degenerative changes to such a degree that by the end of pregnancy merely remnants of it are present.* By the sixth month the vera is intimately blended with the chorion.

The Placenta

The placenta, in certain groups of mammals, including man, is the organ of nutrition for the fetus during about the latter two-thirds of the period of gestation. In man, it is a discoid structure, attached by one surface to the wall of the womb, and connected on its opiH)site aspect with the fetus through the medium of the umbilical cord.

The human placenta represents the highest specialization of an apparatus for bringing the fetal blood into intimate relation with the blood of the mother. In eggs that develop otit^ide of the body of the mother, such as those of reptiles, bink, and the lowest mammals, the Monotremata, the ^nviiuj ombrvo necessarily acquires no connection with tl^* uterine mucosa, but draws upon its original dower of outriuu^nt, the deutoplasm, until its development is compU»t\xL wht^n it bn»aks through the stiell and seeks its own Kkh{ : in ilu>i** srr*>U[>s the chorion develops no villi. In Uu> Miar^ipwl^ a srn>up of mammals higher than the monouvMuvx^ \\w ovum* although developing in the uterus, forms IK* V U»^t> v^>nmvtJo« with it, but obtains its nourishment bv viiMjsv utiMiMttort tr\mi the uterine mucosa. On the other i,i.«xi, u u\ iKUtuuHls higher than monotremes and marsu>,;.^ \w iK'i-^oii is vlistinguished by the presence of villi

ii. vvHx.» »K *»uuM» pUwnta and the non-villous chorion svi^tniit ^rn^lations exist; for example, in

va,<.v^ uKi '^i.Hm^ vnUvrs^ there is no proper placenta,

expiiUinii of the fetus. In the Cariiivora tbe pliiuenia has the form of a zone or ring — placenta zonaria — while In man anil wrtain iilliwl in.iinmals, as opes, rodents, and some others, it is iliseoid in shape — placenta diacoidea.

The human placenta is formed in the third month of pregnancy; since it rrsiilts from the union of the chorioa frondosum with tlic decidua serotina, it consists of a fetal and a matemal part.

Our conceptions of the development of the placenta must be modified to accord with recent investigations. The e

lx?<]<liii;rof thcnviim in the uterine mucosa and the modifications iM-curni)<; in the <-liiirion, including the };ru\vtli of iti villi and its ditli-ri'ntlution into the ehoriiiii fnmdusun) and thf cliorion leve, have Ik-cii ci)nsidered nlwve. It will be r<;(-:tlli.-d iliiit the uvuni e:its ii.s way, aH it were, into the iiiwir-n, tliu-i <-aii>inf: th<' sii|terfieiiil layers of the latter to di.s»|i|iear at ihe >\w of implantation. It is [tossihiy this jiroeos of i-rosion n|>oii the jKirt of the fend trophoIilaKt thai o|ieii~ u]) llie :di-<-:idy ililati'd caiiillarie!; of the s<.'n)tinu — the siniiseB — and idlows the nialeriiiil hliMHl aeeesK to the hlo.-1-hi.-iuia- of the li-oi.hol.lasl. wlure it thus Imthes the jiriniiiive villi. Since the entire trojilmhlaiit is vaeiiolated,

Fig. TA— fthcnwIlFiidfithi'lliim :

lln- niati-mal hUfKl sit this time — the lirsl week — is hroiitrht iiilo relation with the whole surface of the <'horii>n. lii the lail-f l.ulf of llie first month the distiiuMiou li.twcen ehoiion li'.i.'l'»-iitij and ehurion leve Itejiins to lie matiifcst, the villi •/ lt» miKT irmdiiaUy retn^rading until, in the sixth week, vui- i,Ti j^HAilv degenerated, many of iheni W\n\t without

irion frondosnm inei-ease in si/e. nnmlier, M.,)»n,.f wntu- of thorn acquiring jitlaelnucnt to the .re ui- tt.truW.mteringthei*<'n)tina] hhioil sinnti^'s. .li vtntinues tliroughont im'unaney. in ibe oecond week, these heing

extensions of the allantoic blood-vessels. The syncytium of the chorionic laciinie, now the intervillous sp&cti.s, increases in quantity, and not only tines the spaces, but exists in the form of masses, some of which become attached to the semtina between the villi, while others penetrate into it, many l>eing found at the fourth month in the serotinal connective-tissue sjiaces. the decidua serotina (basal decidual in the first month is edematous ami hypercmic, presenting dilated capillaries and blood-spaces, many of which communicate with the intervillous spaces of the chorion. By the sixth weelt its surface epithelium is entirely lost, and the jiarts of its glanils contained in the compacta are to a great cxtutit tiblilerated. In

c leprvsenWdon of the dcvclopiui'iil <if Itii: placcnu. (altfir iTopholilut: <!i,. lyncyllum; £>i., cndulhelium ; Va^ eUl cBplllariea; d.>., ileclilual septum; F.b.. Qbrln;

the fourth month it is thinner, more irregular in thickness, contains less sinuses, and shows degeneration in the compacta, with many masses of syncytium. Toward the end of pregnancy the sinuses increase in sine, and the irregularity in thickness and the degeneration are more marked. The placenta at term is u discoid mass, in ttilu, but less flattened after its expulsion from the uterus. Its diameter is from 15 to 20 and its thickness from 3 to 4 centimeters. The uterine surface is convex and irregular, and is imperfectly divided into tnfts or cotyledons. The somewliut ooncave fetal surface, rather mottled, is covered by the loosely adherent amnion, and (iresents, usually near its center, the attachment of the umbilical cord. The maternal part of the placenta, the decidua serotma, is of varying thickness, in some places being absent. Its compact layer shows fibrinous degeneration, very few traces of glands, and no epithelium. The blood-spaces, lined with endothelium and representing greatly dilated capillaries, are in communication with the intervillous spaces of the fetal placenta (Plate IV., a). Scattered throughout the serotina are masses of syncytium. In the shed placenta there is very little of the serotina, since separation takes place through the compa^ta, the spongiosa and a part of the compactii remaining upon the uterine wall.

The fetal part comprises almost the entire thickness of the cast-off placenta. It is made up of villi* of all ages and sizes springing from the chorion (some attaclied distally to the serotina, others projecting free into the intervillous spaces), and of masses of syncytium attached to both villi and chorion (Plate IV., a). The intervillous spaces are a system of intercommunicating cavities through which the maternal blood circulates and which are in communication with the blood-spaces of the serotina. Elevations of the serotina between the villi constitute the septa placentae. The so-called marginal sinus at the periphery of the placenta is merely a system of intervillous spaces that intercommunicate more freelv because of the relative paucity and small size of the villi in this region.

The site of attachment of the placenta to the uterus is usually the upper part of the posterior wall. Under certain circumstances it may become attached lower down, even extending partly or wholly over the mouth of the womb, constituting then the condition known as placenta prsevia.

The Umbilical Cord

The blood-vessels through which the fetal blood finds its way from the fetus to the placenta and back again to the fetus, together with the atrophic vestiges of certain structures associated with the development of these vessels, constitute the structure known as the umbilical cord. In considering the growth of the human allantois it was pointed out that the

  • For structure of villi, see pages 93, 94.

Diagrammatic reprcsenlatlon or ri hkltuf drat week; 2, ■ few dayi lalet; dcBned (WeL»lcrj : n. feUI meiDblut, i iolo Irophoblut (talks in I. nctual e reduced In i and.eonnltatlng ben th In l.enlirgad in

, a tew month owing Indication

Ui diTidiia: I. in latter

. when pli [>r beginning extension

3: b, trophciblaiit. being larei or l.angh«nii; c. tmphoblul lacuna ■pace ; d. lyncytlum, §etin in its

e. dfcidua: /. maturnal blmdnlnua: g, cndotbcUain lining matumal t. cplblMtlc covering of eord; f. amniotic eplbliut; J. nmbUieal

m. extension a/ decldua an unde

Sf obarlon at edge ur piocvnta ; n, la

latter structure, as it grows from the ventral wall of the guttract into the so-called allantoic or alxloniinal stalk, bci'omcs the seat of development of the two allantoic arteries and of an equal number of allantoic veins. With the metamorphosis of the chorion frondosum into the fetal placenta, the abdominal stalk becomes more slender and at the same time; much elongated, and the allantoic blo<Ml-v(*ss(>ls are henceforth the Tnnli11ic4il vasBels. The two umbilic^il veins fuse, so that, at birth and for somc^ time before, there is but one vein, though there are still two arteries. The nmbilical vein, entering the body of the fetus through the umbilicus, passes diro<'tly to the under surface of the liver, when^ it uuitt^s with the fetal portal vein and gives oif* a bnineh of eoiumunieation, the dnctos vanosiis, to the inferior vena <*ava, aft<*r wlii<*li it enters the liver through the transverse fissure. The umbilical arteries, whose intni-embryoni<' jxirtions are (^iIIcmI the hypogastric arteries, are the direct coutinuatious of tli<; su]MTior vesical arteries of adult anatomy. They l(*avc the binly of the fetus at the umbilicus.

The umbilical conl, while e<»nsisting essentially of the three blood-vessels nu^ntioned, continus also the remnant of the allantoic stalk and of the umbilical vesicle, these stnu^tures being surrounded an«l h(;l<l together by a <juantity of embryonic connective tissue, tli(» jelly of Wharton, whi<*li makes up the chief ])art of the mass of the eord ; upon the surface is a Iay<'r of epitlu'lium, <M)ntiiiuous, at tlu^ <Iistal end of the cord, with tli<; e])ithelium of tlu; amnion.

The umbiliejil eonl has an average l(»ngth of 5.") cni., or 22 inches, but varies Ix'tween the extremes of lo ('in. (<> in('hes) and IGO em. (G4 inches); its thickness is about 1.5 em. (J inch). The eord ])res(jnts tin* ap])earan(u^ of l)eing sj>irally twisted; it is prol)able, however, that the ap{>eanincc of torsion is conferred l)y tlu^ sj>inil or coiled arrangement of its arteries, due to their excessive growth, rather than by a twist of its entire mass. There mav be one or more true knots in the cord, produced by the slipping of the f(»tus through a loop.

The position of attachment of the (^ord to the placenta is usually near, but seldom exactly in, the center of the fetal surface of that organ ; rarely it may be found attached to its edge, and still more rarely to the fetal membranes themselves at some little distance from the edge of the placenta, with which, in the latter case, it is connected by its bloodvcwhjIh.

The great length of the human umbilical cord is thought to Im5 du(i to the relatively large quantity of amniotic fluid preH<*nt in the human subject.

After birth, the portions of the hypogastric arteries extending from (h(? upper ])art of the lateral wall of the bladder to the unibilic'UH undergo atrophy, becoming impervious fibrous nirdn ; the intni-abdominal part of the umbilical vein likewine becomes atrophic and impervious, constituting the M)*<«itlled round ligament of the liver.

Relations of the Fetal Membranes at Birth

When the amniotic fluid attains its maximum bulk — at about the end of the sixth month — it requires so much space thiit it. preHM(>H the amniotic membrane closely against the ehoHou, which IntttT, covered by the remnants of the reflexa, \n in turn f<»r<H'<l into intimate relation with the vera (IMute V.)' At t(*rm the vem and chorion have become pmctieully one menil)rane. The amnion, while adhering to the inner nurfurc of the chorion, is so loosely associated with the hitter that it may be peeled ott* from it. The membnines, which (M)nstitute a fluid-filled sac surrounding the fetuH, are rupture«l by the contractions of the uterus at some time during parturition. Through this rent the /;hild is forc'ed chn'ing birth, the placentii and the membranes remaining behind. After the expulsion of the child, the vera and the phieenta <h'tac'h themselves from the uterine wall, and, with the <^horiou and the amnion, constitute the afterbirth, whieii is expelled shortly after the expulsion of the child. The separation of the; after-birth takes place in the compact layer resjM'etively of the dwidua vera and of the utt»rine phieenta. The s|M)ngy layer and what remains of the com|)acta serve for the regeneration of the uterine mucosa.