Page 325 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

292 SECTION | II Organ Toxicity

VetBooks.ir Placentation exchange of gases, nutrients, metabolic wastes and xeno-
biotics between the fetal and the maternal circulations.
Most mammalian species are “eutherian” and, during preg-
Although the term “implantation” is frequently used to
nancy, form a placenta comprised of both fetal and mater-
nal components, which acts as an attachment between the describe the appropriately timed attachment of the placen-
fetal and the maternal systems, functions as a transient tal membranes to the endometrium, only the conceptuses
endocrine organ and plays essential roles in the exchange of of rodent and primate species undergo true implantation
gases, nutrients and metabolic wastes between the maternal (Senger, 2003). Placental exchange involves the processes
of simple (passive) diffusion, facilitated diffusion and
and the fetal circulations (Ginther, 1992; Senger, 2003).
active transport, as well as pinocytosis and phagocytosis
The yolk sac, chorion, amnion and allantois are the extra-
of some nutrients (Senger, 2003).
embryonic membranes formed by the pre-attachment mam-
The passage of materials across the placenta has been
malian embryo (Senger, 2003). While the yolk sac in most
traditionally thought of as primarily a function of the inti-
mammalian species normally undergoes regression, the
macy (i.e., number of tissues layers) between the maternal
allantois and chorion generally fuse to form the allantochor-
and the fetal circulations, especially with respect to mater-
ion, and the fluid-filled amnion provides a shock absorbing,
nal immunoglobulins which cross hemo- and endothelio-
aquatic environment to facilitate fetal development and
chorial placentas but not those types of placentae having
transport (Ginther, 1992; Senger, 2003). The allantochorio-
more layers. However, since most xenobiotics cross the
nic membrane is the fetal contribution to the placenta and
placenta by simple diffusion, it is currently thought that
the chorionic villi are the structures which interdigitate with
molecular size and solubility are the most important
the maternal endometrium (Senger, 2003).
determinants of the ability of potential teratogens in the
maternal circulation to cross the placenta into the fetal
Types of Placentas
circulation (Senger, 2003; Lehman-McKeeman, 2008).
Mammalian placentation can be classified according to
Some toxic xenobiotics can be actively transported by
the degree of intimacy between the maternal and the fetal
mechanisms intended for structurally similar endogenous
circulations (i.e., the number of tissue layers separating
molecules (Lehman-McKeeman, 2008), and it is thought
maternal and fetal blood) and by the pattern of distribu-
that transplacental transport of lead can mimic that of
tion of the chorionic villi on the surface of the placenta calcium (Evans et al., 2003).
facing the maternal endometrium (Senger, 2003).
Epitheliochorial placentas (placentae) have a total of six
The “Placental Barrier”
layers separating the maternal and fetal circulations and
are observed in a variety of species, including equids and Because the placenta “blocks” the ability of very large
swine. Ruminant placentation is described as syndesmo- molecules to cross from the maternal circulation into the
chorial because of the transient erosion and regrowth of fetal circulation, the term “placental barrier” has been
the maternal epithelium, which results in the intermittent used to describe this protective function of the placenta.
exposure of maternal endothelium (capillaries) to Given that a large number of potential toxicants diffuse
chorionic epithelium (Senger, 2003). Canine and feline across the placenta and reach the fetus, the term “barrier”
placentas are classified as endotheliochorial, and the might be somewhat of a misnomer. However, multidrug
hemochorial placentation observed in rodents and pri- resistance protein and enzymes involved in biotransforma-
mates has essentially only chorionic epithelium separating tion of xenobiotics have been found in the placenta
the maternal blood from that of the fetus. The placenta of (Lehman-McKeeman, 2008), The placenta is multifunc-
each species is associated with a typical distribution of tional, and regardless of its relative inefficacy as a
the chorionic villi, classified as being either diffuse (e.g., “barrier” between the maternal and fetal circulations, with
equids and swine), cotyledonary (e.g., ruminants), zonary respect to at least some xenobiotics, the nutritional and
(e.g., dogs and cats) or discoid (e.g., rodents and pri- endocrine functions of the placenta are essential for suc-
mates). Some species, such as the rabbit, have variations cessful completion of the mammalian pregnancies.
in their placentation over the course of gestation, and Placental toxicity has been discussed in much greater
what begins as an epitheliochorial placenta has trans- detail elsewhere (Gupta and Gupta, 2017).
formed into a hemoendothelial type of placentation by the
end of pregnancy (Lehman-McKeeman, 2008).
Normal Embryonic and Fetal Development
Placental Function Blastocyst Formation and Differentiation
In multitocous species, each fetus has its own placenta, of the Germ Cell Layers
with the previously described endocrine functions, which In order for a zygote to develop into a viable offspring,
“attaches” the fetus to the endometrium and facilitates the multiple steps involving cellular division, migration,