Download The Umbilical Cord and Body- stalk. The umbilical cord (Fig. 28

The Umbilical Cord and Bodystalk.
The umbilical cord (Fig. 28)
attaches the fetus to the placenta;
its length at full time, as a rule, is
about equal to the length of the
fetus, i.e., about 50 cm., but it may
be greatly diminished or increased.
The rudiment of the umbilical cord is
represented by the tissue which connects the
rapidly growing embryo with the extraembryonic area of the ovum.
Included in this tissue are the body-stalk and
the vitelline duct —the former containing the
allantoic diverticulum and the umbilical
vessels,
the
latter
forming
the
communication between the digestive tube
and the yolk-sac.
• The body-stalk
• is the posterior segment of the embryonic area,
and is attached to the chorion. It consists of a
plate of mesoderm covered by thickened
ectoderm on which a trace of the neural groove
can be seen, indicating its continuity with the
embryo.
Running through its mesoderm are the two
umbilical arteries and the two umbilical
veins, together with the canal of the
allantois—the last being lined by entoderm
(Fig. 31).
The body-stalk
Its dorsal surface is covered by the amnion,
while its ventral surface is bounded by the extraembryonic celom, and is in contact with the
vitelline duct and yolk-sac.
• With the rapid elongation of the embryo and the
formation of the tail fold, the body stalk comes
to lie on the ventral surface of the embryo (Figs.
27 and 28), where its mesoderm blends with
that of the yolk-sac and the vitelline duct.
The cord is covered by a layer of ectoderm which
is continuous with that of the amnion, and its
various constitutents are enveloped by embryonic
gelatinous tissue, jelly of Wharton.
• The vitelline vessels and duct, together with
the right umbilical vein, undergo atrophy and
disappear; and thus the cord, at birth, contains
a pair of umbilical arteries and one (the left)
umbilical vein.
The Decidua
• Before the fertilized ovum reaches the uterus, the
mucous membrane of the body of the uterus
undergoes important changes and is then known as
the decidua.
• The thickness and vascularity of the mucous
membrane are greatly increased; its glands are
elongated and open on its free surface by funnelshaped orifices, while their deeper portions are
tortuous and dilated into irregular spaces.
• The interglandular
tissue is also increased
in quantity, and is
crowded with large
round, oval, or polygonal
cells, termed decidual
cells.
• These changes are well
advanced by the second
month of pregnancy,
when the mucous
membrane consists of
the following strata (Fig.
33):
• (1) stratum compactum, next the
free surface; in this the uterine
glands are only slightly expanded,
and are lined by columnar cells;
• (2) stratum spongiosum, in which
the gland tubes are greatly dilated
and very tortuous, and are
ultimately separated from one
another by only a small amount of
interglandular tissue, while their
lining cells are flattened or cubical;
• (3) a thin unaltered or boundary
layer, next the uterine muscular
fibers, containing the deepest parts
of the uterine glands, which are not
dilated, and are lined with columnar
epithelium; it is from this epithelium
that the epithelial lining of the uterus
is regenerated after pregnancy.
stratum compactum
stratum spongiosum
unaltered or boundary layer
• Distinctive names are applied to different
portions of the decidua.
• The part which covers in the ovum is named
the decidua capsularis;
• the portion which intervenes between the
ovum and the uterine wall is named the
decidua basalis or decidua placentalis; it is
here that the placenta is subsequently
developed.
• The part of the decidua which lines the
remainder of the body of the uterus is known
as the decidua vera or decidua parietalis.
decidua placentalis
decidua vera
decidua capsularis
• Coincidently with the growth of the embryo, the
decidua capsularis is thinned and extended and the
space between it and the decidua vera is gradually
obliterated, so that by the third month of pregnancy
the two are in contact. By the fifth month of pregnancy
the decidua capsularis has practically disappeared,
while during the succeeding months the decidua vera
also undergoes atrophy, owing to the increased
pressure.
• The glands of the stratum compactum are obliterated,
and their epithelium is lost.
• In the stratum spongiosum the glands are
compressed and appear as slit-like fissures, while
their epithelium undergoes degeneration.
• In the unaltered or boundary layer, however, the
glandular epithelium retains a columnar or cubical
form.
the process of Implantation or Imbedding of
the Ovum.—
The fertilization of the ovum occurs in the lateral
or ampullary end of the uterine tube and is
immediately followed by segmentation. On
reaching the cavity of the uterus the
segmented ovum adheres like a parasite to
the uterine mucous membrane, destroys the
epithelium over the area of contact, and
excavates for itself a cavity in the mucous
membrane in which it becomes imbedded.
•
and the ovum is then completely surrounded
by the uterine mucous membrane.
The structure actively concerned in the process
of excavation is the syncytiotrophoblast of the
ovum, which possesses the power of dissolving
and absorbing the uterine tissues. The
trophoblast proliferates rapidly and forms a
network of branching processes which cover the
entire ovum and invade and destroy the maternal
tissues and open into the maternal blood
vessels, with the result that the spaces in the
trophoblastic network are filled with maternal
blood; these spaces communicate freely with
one another and become greatly distended and
form the intervillous space.