Download Fetal membranes

Implantation
The term implantation is used to describe process of attachment and invasion of
the uterus endometrium by the blastocyst in placental animals. Approximate implantation times
are: one week (human); two weeks (dog, cat, sheep), 3-5 weeks (cattle), 3-8 weeks horse.
(adhesion of the blastocyst to the wall of uterus)
The implantation has three stages
Apposition Stage: The trophoblast comes into contact with the endometrium. Under the
influence of a trypsin-like enzyme from the blastocyst the endometrium increases mitosis and
produces pinopods (protrusions from the endometrial surface) which withdraw fluid from the
lumen by pioncytosis. As apposition is completed the microvilli of both trophoblast and
endometrial surface interdigitate and the pinopods are withdrawn.
Adhesion Stage: The microvilli disappear and the production of sticky glycoprotein
leads to contact over a large surface area.
Penetration Stage: Contraction of microfilaments in the trophoblast permits the
blastocyst to migrate between endometrial cells. At the same time a syncytiotrophoblast forms
and synthesis of trophoblast-specific proteins begins. Decidual cells form in the endometrial
stroma. These are large cells rich in glycoprotein and lipid.
Placentation
The placenta is the area of apposition between uterine lining and fetal membranes where
metabolites are exchanged for sustaining pregnancy.
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.
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:

Diffuse: Almost the entire surface of the allantochorion is involved in formation of the
placenta. Seen in horses and pigs.

Cotyledonary: Multiple, discrete areas of attachment called cotyledons are formed by
interaction of patches of allanto-chorion 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. This type of placentation is observed in
ruminants.
Placentome: Is a discrete area of interdigitation between a maternal caruncle and a fetal
cotyledon.

Zonary: The placenta takes the form of a complete or incomplete band of tissue
surrounding the fetus. Seen in carnivores like dogs and cats, seals, bears, and elephants.

Discoid: A single placenta is formed and is discoid in shape. Seen in primates and
rodents.
Classification Based on Layers Between Fetal and Maternal Blood
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
Placentas may also be classified according to the tissue layers separating fetal and maternal
blood. Uterine epithelium, uterine connective tissue and uterine endothelium may be eroded,
giving rise to four placental types:
epitheliochorial (swine, equine, cattle); synepitheliochorial, formerly called syndesmochorial,
(sheep, goats); endothelialchorial (carnivore); and hemochorial (primates & rodents).
Type of Placenta
Common Examples
Diffuse, epitheliochorial
Horses and pigs
Cotyledonary, epitheliochorial
Ruminants (cattle, sheep, goats, deer)
Zonary, endotheliochorial
Carnivores (dog, cat, ferret)
Discoid, hemochorial
Humans, apes, monkeys and rodents
Fetal membranes:
Four fetal membranes develop in a conceptus. Two arise from the trophoblast layer of the
blastocyst (and are continuous with the somatopleure of the embryo). Two arise from the inner
cell mass of the blastocyst (and are continuous with splanchnopleure of the embryo); these two
splanchnopleure membranes are vascular. The four fetal membranes are:
1. Chorion — from trophoblast, forms the outer boundary of the entire conceptus.
2. Amnion — from trophoblast, is formed by folds of chorion in domestic animals
(in
humans, amnion forms by caviation deep to a persistent trophoblast). The amnion encloses the
embryo within a fluid filled amnionic cavity.
3 . Allantois — from the inner cell mass, develops as an outgrowth of hindgut splanchnopleure.
The allantois grows to fill the entire extra-embryonic coelom, with fluid-filled allantoic cavity.
The outer surface of allantois binds to the inner surface of chorion and the outer surface of
amnion. The allantois is highly vascular and provides the functional vessels of the placenta, via
umbilical vessels.
4 . Yolk sac — from the inner cell mass, develops early (with hypoblast formation) and is
continuous with midgut splanchnopleure. Supplied by vitelline vessels, yolk sac is most
important in egg laying vertebrates. It forms an early temporary placenta in the horse and dog.
Note: The term conceptus refers to the embryo or fetus plus its fetal membranes.