Learn the gross and histological structure of the placenta and correlate structure
The placenta is the organ of exchange between maternal and fetal tissues, providing
nutrition, respiration, and excretion for the fetus. It also produces hormones
and serves as an immunological barrier between the genetically disparate maternal
and fetal tissues. At term, the placenta is usually a circular shaped structure
measuring approximately 20 cm. in diameter and 2.5 cm. in thickness. It is primarily
of fetal origin but has a small maternal component as well. The fetal portion
of the placenta consists of the chorionic plate and its villi and the peripheral
trophoblastic shell which surrounds the intervillous space and covers the maternal
tissue. Projecting toward the decidua basalis from the chorionic plate are numerous
highly branched villi. The villi are composed of fetal connective tissue and
are covered by trophoblasts. The trophoblast is the parenchyma of the placenta.
It is the tissue through which exchanges must occur, and it is involved in the
production of placental hormones, both steroids and polypeptides. Most villi
are free in the intervillous space and are bathed in blood from the maternal
vessels. Anchoring villi contact the decidua basalis. The decidua basalis (maternal
portion of the placenta) rests on the stratum basalis and is characterized by
large, polyhedral pale blue stromal (decidua) cells. Flattened slit-like glands
lie at the boundary between decidua and stratum basalis. Multinucleated giant
cells of fetal origin are present in the endometrium. Between the maternal and
fetal tissues is an irregular layer of fibrinoid.
A. Maternal Side
Slide #90 odd, the maternal side, has myometrium, stratum
basalis, decidua basalis, junctional zone, anchoring villi and cross-sections
of villi branches. Slide #90 even, the fetal side, has the
amnion, chorionic plate and stem villi and cross-sections of villi branches.
Examine both slides.
Initially examine slide #90 odd by holding the slide up to the light
and examining it with the reversed ocular. Note the maternal and fetal tissue.
Using the microscope, find the thin stratum basalis adjacent to the thick
muscle layer (myometrium). This melts into the decidua basalis.
In some slides, it is possible to separate the stratum and decidua basalis in
the region of the slit-like glands. The decidua basalis contains many enlarged
glycogen-containing stromal (decidual) cells, a few fetal giant cells,
and blood vessels. Near the surface of the decidua basalis is the junctional
zone, which includes decidual cells, fibrinoid, and the surface trophoblastic
layer (the trophoblastic layer is lost in many areas). Locate an anchoring
villus touching the maternal surface, and note the basophilic cells of
the peripheral cytotrophoblast (cell columns). In sections through some
villi observe the central core of fetal loose connective tissue containing small
blood vessels. The outermost covering is the syncytiotrophoblast, a syncytium
with many dark-staining nuclei. During the latter half of pregnancy, as in this
slide, groups of aggregated nuclei may project at the surface forming syncytial
knots. Examine the electron micrograph of the syncytiotrophoblast.
NOTE: the abundant microvilli, rough endoplasmic reticulum and multiple nuclei.
The inner trophoblastic layer, the cytotrophoblast (Langhan's layer),
is made up of distinctly separate cells which form a continuous layer in early
pregnancy. Later in pregnancy, as in this slide, this layer is reduced to a
few scattered large pale cells with pale nuclei, located between the syncytiotrophoblast
and underlying connective tissue. The mitotic cytotrophoblast provide new cells
which fuse with the syncytiotrophoblast and contribute to its growth. Examine
the electron micrograph of the cytotrophoblast and compare its ultrastructure
to that of the syncytiotrophoblast.
B. Fetal Side
In slide #90 even, under very low power, locate the chorionic
plate, a solid tissue mass to one side of the section. Note that the connective
tissue of the chorionic plate is artificially separated from the connective
tissue of the amnion. Examine the cuboidal epithelium of
the amnion. The chorionic plate contains large blood vessels surrounded
by connective tissue. The surface of the chorionic plate bordering
the intervillous space is covered with trophoblast. Find
examples of main stem villi coming from the chorionic plate. Examine
the small branches of the villi and note the characteristics of a late
placenta: large capillaries, often near the surface, syncytial knots,
thinned syncytial cytoplasm of the syncytiotrophoblast, few cytotrophoblast
cells and fibrin deposits in the intervillous space. The stroma
of the villi contains fetal fibroblasts and Hofbauer cells. Hofbauer
cells, thought to be a type of macrophage, are oval cells with an
eosinophilic cytoplasm and a round nucleus. They can be found in round
vacuolated areas of the stroma.