Placentation — the placenta is the area of apposition between the uterine lining and fetal membranes
where metabolites are exchanged for sustaining pregnancy
• placenta types based on apposition area are:
- diffuse (pig) – broad surface contact
- zonary (carnivore) – apposition area is a cylindrical band
- placentomes (ruminant) – rows of discrete interdigitating contacts involving a maternal caruncle
and fetal cotyledon
- (equine) – contact appears diffuse but consists of microscopic placentomes (microplacentomes )
- discoid (primates & rodents) – disc-shaped apposition area

• another placenta classification is based on the tissue layers separating maternal and fetal blood:
- six intervening layers are described:
fetal blood — fetal endothelium | fetal connective tissue | chorion ||
uterine epithelium | uterine connective tissue | uterine endothelium — maternal blood

• four placental types are recognized based on intervening tissue layers:
- epitheliochorial (swine, equine, cattle) – all six layers are present
- synepitheliochorial or syndesmochorial (sheep, goats) – five layers (loss of uterine epithelium)
- endotheliochorial (carnivore) – four layers (uterine epithelium & connective tissue lost)
- hemochorial (primates, rodents) – three layers (all three uteine layers lost)
Note: only maternal tissue layers are lost

Fetal membranes — four fetal membranes develop in a conceptus
- two originate from the trophoblast layer and
- two originate from the inner cell mass of the blastocyst:
these latter two membranes involve splanchnic mesoderm and are the vascular layers
Note: fetal membranes are hollow and filled with liquid, the cavity originates from the blastocoele
of the blastocyst or from the hollow gut

The four fetal membranes are:
1] chorion – forms outer boundary of the entire conceptus (embryo + fetal membranes),
originates from trophoblast
2] amnion – encloses the embryo, originates from trophoblast via folds of chorion in domestic
mammals (in primates & rodents the amnion forms by cavitation deep to a
persistent trophoblast layer)
3] allantois – conveys umbilical vessels to the placenta, originates as an outgrowth of hindgut
splanchnopleure; the fluid-filled allantois grows to fill the extraembryonic coelom and line
the chorion, bringing blood vessels
4] yolk sac – serves briefly as a temporary placenta in some mammals (dog, horse), originates
as the hypoblast layer of the blastocyst and becomes continuous with midgut splanchnopleure;
supplied by vitelline vessels

Implantation — adhesion of the mammalian conceptus to the uterine wall leading to placenta formation
- following zona pellucida rupture, the blastocyst is initially unattached and nourished
by uterine gland secretions
- in time, the mobile blastocyst attaches to the uterine lining (human 1 week; dog/cat 2 weeks;
cattle/horse 3 or more weeks; deer/bear delayed up to 4 months)
Note: in human and Guinea Pig, implantation involves invasion into the uterine wall
vs adhesion to the uterine lining