Pigs have a non-invasive placenta classified as being epitheliaochorial and diffuse.
Pig blastocysts elongate extensively beginning about day 11 and become evenly spaced throughout both uterine horns. As the allantois develops and that cavity expands, the embryos become apposed to the uterine surface and being to attach.
Attachment is initially associated with distinctive changes in endometrial epithelial cells. Their apical surfaces become rounded with bulbous protruberances around which chorionic epithelial cells become molded. Processes from chorionic epithelial cells push between endometrial epithelial cells but do not penetrate the basement membrane. Attachment is strengthend by intertwinning of microvilli from chorionic and endometrial epithelial cells and interlocking ridges that act like "tongue and groove" fasteners between the two epithelia. These attachments soon cover essentially the entire endometrial surface and the chorionic ridges become substantially larger as pregnancy progresses.
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