The placental barrier

The placenta regulates the transport of nutrients from
the maternal to the fetal circulation and also the transport
of fetal metabolic waste products to the maternal
circulation. Transport may be regulated intrinsically
by cellular proteins or extrinsically by circulating hormones.
Communication between the microvillous
and basal membranes of the syncytiotrophoblast (see
below) allow stimuli originating in the mother or
fetus to regulate solute transport across the maternalor
fetal-facing membranes.
Cellular structure of the trophoblast
By the time the fertilized ovum has entered the uterus
after its journey down the Fallopian tube, several cell
divisions have occurred, and it consists of a mass of
cells. A cavity appears within the cellular mass, after
which it is called a blastocyst. The blastocyst wall is
composed of a single layer of cells called the trophoblast,
and within the cavity is an inner cell mass that
is destined to form the embryo. After implantation
of the blastocyst within the endometrium, the trophoblast
proliferates over its entire surface and, by
the eleventh day after ovulation, it consists of two
distinct layers. The thicker outer layer consists of a
multinucleated protoplasmic mass, the syncytiotrophoblast,
from whose outer surface project numerous
primary villi. Connective tissue containing fetal blood
capillaries extends into the villi which are now termed
secondary or chorionic villi. The inner layer of cells,
the cytotrophoblast, is composed of cells with clearly
defi ned cell boundaries. The growing chorionic villi
destroy endometrial tissue in their locality, leaving
intervillous spaces. With further enlargement of villi,
the spaces become interconnecting and contain blood
liberated by penetration of the maternal vessels by the
trophoblast. After the tenth week of pregnancy, the
cytotrophoblast progressively disappears until at parturition
only isolated clumps of its cells remain.
The functional unit of the placenta concerned with
exchange of materials between mother and fetus is
the chorionic villus (Fig. 3.14). Passage of dissolved
substances occurs between the maternal blood in the
intervillous spaces and the fetal blood in the capillaries
of the chorionic villi. The placental barrier is imposed
by the syncytiotrophoblast, the discontinuous
cytotrophoblast, the basal lamina of the trophoblast,
loose connective tissue, the basal lamina of the fetal
capillaries, and the capillary endothelium. Most sites
of known mechanisms of placental transport are
localized to the microvillous and basal plasma membranes
of the syncytiotrophoblast.