Week – Structure and types
of mammalian placenta.
Books: Mc Geady, T.A., Quinn, P.J.,
FitzPatrick, E.S. and Ryan, M.T. 2013.
Veterinary Embryology, Blackwell
Synergy, Chap- 2.
1. Describe placenta and what is its function?
2. List down the four types of placenta and describe each type.
3. Define: Cotyledon, Caruncle and Placentome.
4. What are the three classifications of placentation types found in
animals Based on Layers Between Fetal and Maternal Blood?
5. There are how many placentomes present in sheep, goats and
cows?
6. Implantation is the attachment of the placenta in the uterine
wall, when does implantation occur in the Dogs and cats?
1.
2.
Developmental stages of the avian embryo.
Illustration of the different developmental
stages of chicken embryo from day one to
day 21 of incubation.

The placentas of all eutherian (placental) mammals
provide common structural and functional features, but
there are striking differences among species in gross and
microscopic structure of the placenta.
Two characteristics are particularly divergent and form
bases for classification of placental types:
1. The gross shape of the placenta and the
distribution of contact sites between fetal membranes and
endometrium.
2. The number of layers of tissue between maternal
and fetal vascular systems.

Differences in these two properties allow classification
of placentas into several fundamental types.
The placenta (also known as afterbirth)
is an organ that connects the
developing fetus to the uterine wall to allow
nutrient uptake, waste elimination, and gas
exchange via the mother's blood supply,
fights against internal infection and produces
hormones to support pregnancy.
Placentation
refers to the formation, type and structure, or
arrangement of placentas.
Placentation occurs inside the uterus.
The function of placentation is to transfer
nutrients from maternal tissue to a
growing embryo.
 Placentation
occurs after the implantation of
the embryo into the uterine wall and
involves the remodeling of blood vessels in
order to supply the needed amount of blood.
 The
developing fetus is connected to it via
an umbilical cord.

In humans, placentation takes place 7–8
days after fertilization.
Implantation in Ruminants

Implantation in ruminants is non-invasive and
some authors prefer to use the term attachment.

There is close attachment between embryonic
membranes and the endometrium overlying
caruncles at 5 weeks in cattle and 3 weeks in
sheep.

Shortly thereafter, the placenta is established.
Ruminants have a cotyledonary placenta.
= Instead of having a single large
area of contact between maternal and fetal
vascular systems, these animals have numerous
smaller placentae.
The terminology used to describe ruminant
placentation is:
 Cotyledon: the fetal side of the placenta
 Caruncle: the maternal side of the placenta
 Placentome: a cotyledon and caruncle
together

Caruncles
- are oval or round thickenings in the
uterine mucosa resulting from proliferation of
subepithelial connective tissue.
caruncles are readily visible in the non-pregnant
uterus. They are the only site in the uterus to form
attachments with fetal membranes.
Patches of chorioallantoic membrane become
cotyledons by developing villi that extend into
crypts in the caruncular epithelium.
The image below shows caruncles in an incised non-pregnant sheep
uterus (left) and cross sections through placentomes from a
midgestation sheep pregnancy (right). Bovine placentomes looks
similar, but have a convex appearance rather than the concave
shape seen in sheep.
Pregnant sheep, goats and cattle have between 75 and 125 placentomes.
Deer also have a cotyledonary placenta, but only 4 to 6 placentomes which
are correspondingly larger.

The image shows an incised uterus from a pregnant sheep,
roughly 50 days of gestation. The numerous button-shaped
structures are placentomes, and the surfaces in view are actually
cotyledons - the fetal side of the placentome. The slightly milkylooking membrane covering and between placentomes is the
chorioallantois. The fetus is clearly visible inside the amnion.
Implantation in Dogs and Cats

Dogs and cats are litter bearing species, and prior to
fixation and implantation, the blastocysts become
evenly spaced throughout the uterine horns.

In dogs, implantation occurs roughly 18 to 20 days
after the preovulatory LH surge (about diestrus day 8
to 10).

In cats, implantation has been reported to occur 12
to 14 days after mating.

Implantation is centric and antimesometrial.
Gross Structure of the Placenta
 The
zonary placenta takes the form of a
band that encircles the fetus. In dogs and
cats, it is complete, while in species like
ferrets and raccoons, it is incomplete (i.e.
two half bands)
The images above show dissection of a
near-term cat uterus removed surgically.
 In
1604, Fabricius introduced a placental
classification scheme based upon the
macroscopic structure of the sites where
attachment occurs between the embryo
and the endometrium of the uterus.
 He listed four main placental types. These
are now referred to as the diffuse,
cotyledonary, zonary, and discoidal
placentas.
Classification Based on Placental Shape and
Contact Points
Examination of placentae from different
species reveals striking differences in their
shape and the area of contact between fetal
and maternal tissue:
1. Diffuse:
2. Cotyledonary:
3.Zonary:
4. Discoid
1. In diffuse placentae, seen in horses, pigs,
camels, lemurs, opossums, kangaroos, and
whales, the chorionic sac meets the uterine
endometrium over its entire surface.

The villi of the chorion are distributed evenly
throughout the surface of the chorion, and they
extend into processes in the uterine
endometrium. (total coverage)

Almost the entire surface of the allantochorion is
involved in formation of the placenta.
2. Cotyledonary placentae,

common to ungulates such as cows, deer,
goat, and giraffe, have their villi clumped
together into circular patches called
cotyledons.
 The
fetal cotyledon meets with maternal
regions called caruncles to form the
placentome where maternal-fetal exchanges
take place. ("buttons")
2. Cotyledonary placentae,
 Multiple,
discrete areas of attachment
(cotyledons) are formed by interaction of
patches of allantochorion with endometrium.
 The
fetal portions of this type of placenta are
called cotyledons, the maternal contact sites
(caruncles), and the cotyledon-caruncle
complex a placentome.
3. The zonary placenta
is characteristic of carnivores, the chorionic villi
have aggregated to form a broad band that circles
about the center of the chorion.
 The placenta takes the form of a complete or
incomplete band of tissue surrounding the fetus.

Such zones may be complete circles (such as those
in dogs and cats) or incomplete (such as those in
bears and seals).
3. The zonary placenta (circular or partially circular band)

It is thought that zonary placentae form from diffuse placentae in
which the villi at the ends regress, leaving only those in the center
to function.

At the edges of the zonary placenta is the hemophagous organ, which
is green.

The color is due to the degradation of hemoglobin into bilivirdin.
This green organ provides iron for the developing fetus.

Seen also in seals, bears, and elephants.
4. The discoid placenta
is seen in numerous groups -- humans, mice,
insectivores, rabbits, rats, and monkeys.
 In
such placentae, part of the chorion
remains smooth, while the other part
interacts with the endometrium to form the
placenta. The maternal blood cells are in
direct contact with the fetal chorion. (disc
shaped)

Just prior to formation of the placenta, there
are a total of six layers of tissue separating
maternal and fetal blood.
There are three layers of fetal extraembryonic
membranes in the chorioallantoic placenta of all
mammals, all of which are components of the
mature placenta:
1. Endothelium lining allantoic capillaries
2. Connective tissue in the form of chorioallantoic
mesoderm
3. Chorionic epithelium, the outermost layer of
fetal membranes derived from trophoblast


There are also three layers on the maternal side,
but the number of these layers which are
retained - that is, not destroyed in the process
of placentation - varies greatly among species.
The three potential maternal layers in a
placenta are:
1. Endothelium lining endometrial blood vessels
2. Connective tissue of the endometrium
3. Endometrial epithelial cells

The placentation types found in animals are:
1. endotheliochorial placentation
 In this type of placentation, the chorionic villi are in
contact with the endothelium of maternal blood
vessels. (e.g. in most carnivores like cats and dogs)

2. epitheliochorial placentation
Chorionic villi, growing into the apertures of uterine
glands ( epithelium). (e.g.
in ruminants, horses, whales, lower primates)
3. hemochorial placentation (e.g. in higher order
primates, including humans, and also
in rabbits, guinea pigs, mice, and rats)