Placental Structure And Function
The
human placenta is the sole interface between the mother and her developing
embryo/fetus. Humans differ from most other mammals in that maternal blood
comes into direct contact with fetally derived placental tissues. This
organization characterizes the hemochorial placenta through which all maternal
nutrients and fetal wastes must pass. The placenta is a very active organ that
has specialized mechanisms to promote fetal growth and survival. These include,
but are not limited to, efficient gas exchange, active transport of energy
substrates, immunologic tolerance of the fetal allograft and fetal acquisition
of maternal immunity.
Placental morphology
After it enters the uterus, the
human blastocyst resides within the uterine cavity for 2–3 days prior to
implantation into the decidualized uterine endometrium (Chapter 16).
Implantation can be divided into three distinct processes: apposition of the
blastocyst to the endometrial epithelium at the site of implantation, a brief period
of stable adhesion of the blastocyst to this epithelium and invasion of the
developing embryo into the uterine decidua. The signals governing these processes
are complex and involve active maternal and fetal participation. Apposition requires
the secretion of soluble mediators by uterine epithelia and the blastocyst that
include interleukins, prostaglandins and leukemia inhibiting factor (LIF). Adhesion
is promoted by the expression of ligands on the surface of the developing
embryo that specifically bind to receptors expressed on the uterine lining at
the site of implantation. One receptor–ligand pair that has been implicated in embryo adhesion is heparin-binding epidermal
growth factor and heparin sulfate proteoglycans. Also important in the adhesion
process is a family of adhesion molecules expressed on uterine epithelia in a
time-specific and hormone-dependent fashion: the integrins. Invasion of
the blastocyst into the maternal uterine decidua requires an alteration in the
expression of embryonic surface molecules, from those promoting adhesion to the
endometrium to others that stimulate invasion of vascular structures. During
invasion, the embryo also begins to secrete proteases that digest between the
cells of the decidua and allow invasion to areas deep within the uterine
lining.
The blastocyst is comprised of
two populations of cells (see Fig. 16.1): one will become the fetus, the other,
the placenta. At the blastocyst stage, the embryo is characterized by a
fluid-filled cavity (the blastocele) surrounded by a layer of trophectoderm
cells. The trophectoderm will develop into the placenta. Within the
trophectoderm shell is a collection of cells called the inner cell mass. All
nonplacental fetal tissues will arise from the inner cell mass.
During implantation,
trophectoderm cells begin to differentiate into cellular subtypes that will
characterize the mature placenta. The mature placenta is comprised of a mass of
tree-like placental cotyledons called villae, which are bathed in maternal
blood (Fig. 17.1). Blood enters the space between the villae through
low-resistance, high-flow vessels that branch from the maternal uterine spiral
arteries. Fetal vessels are located within the core of each placental
cotyledon. Loose connective tissue and layers of trophoblast cells surround
each fetal vessel. The inner layers of the trophoblast shell around the fetal.
