Book - A Text-book of Embryology 6
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.
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.
| Historic Disclaimer - information about historic embryology pages |
|---|
 |
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
- meanbloal
- 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.
