Implantation (nidation) entails the interaction (apposition and adhesion) of the trophoblast with the uterine epithelium. The success of implantation depends on a stringent harmony of endometrial and embryonic development.

Classifications. Types of implantation have been categorized based on the degree to which an adherent embryo invades the uterine lining. Embryos of some species are intrusive and penetrate the epithelial layer of the endometrium. In primates the embryo makes its way into the endometrial interstitium (Figure 5-22). Rodent embryos become lodged in a uterine fold. In farm animals embryos simply attach to the uterus by placental outgrowth, and therefore can remain free-floating for an extended period. Farm animals are classified as superficial/central implanters (Table 5-5); in the strictest sense, their embryos do not actually implant.

Biochemical basis. Initial attachment of the trophoblast to the endometrium involves formation of junctional complexes between complementary cell-surface glycoproteins (eg., integrins). Localized production of estrogens by the embryo sensitizes the uterus for implantation. Embryos that implant intrusively secrete proteolytic enzymes that degrade the uterine mucosa. Release of immune/inflammatory agents by the uterus and(or) embryo, such as histamine, PGE2, and IL-1 have been implicated in nidation.

Preimplantation embryos of several species secrete PAF coincident with a mild transitory decrease in maternal blood platelets (thrombocytopenia). Besides a proposed autocrine action of PAF in embryonic metabolism and growth, it may participate in the process of implantation and maternal recognition of pregnancy.

Marsupials and monotremes. Unlike eutherian mammals, marsupials are born very early in embryonic development; they only loosely implant and do not form a placenta (other than a simple yolk sac type). The newborn must crawl to the maternal pouch (marsupium), wherein it will attach to a nipple during further maturation (Figure 5-23).

Offspring of the oviparous monotremes also are raised in maternal pouches. An egg is deposited from the cloaca directly into the pouch for incubation (Figure 5-24) (after hatching, the young receives nourishment from milk that is secreted from a nippleless mammary gland).

Delayed implantation. In certain animals the embryo does not implant during the immediate period following fertilization, but remains in a state of suspended growth (blastocyst stage) - diapause. A diapause allows for birth to occur under the most favorable of conditions. Metabolic and(or) environmental cues are critical in determining the length (which can be variable within species) of the embryonic diapause - the true gestational stage (ie., beyond uterine attachment) is generally stable. A few exemplary cases of delayed implantation are mentioned below.

If conception were to take place soon after parturition, and those offspring were born before the previous young were weaned, then milk available to the newborn would be severely limited and the chance of survival unlikely; this predicament occurs in mice and rats conceiving to the first postpartum estrus - to cope, embryos become developmentally detained until the lactational demand imposed by the previous litter wanes (pregnancy with lactation is ~ 40 days). A similar quandary is encountered when a kangaroo mates with a joey already in pouch and at heel - gestation is extended to about 235 days to compensate.

In some species embryonic diapause is a mandatory component of all pregnancies. Seals come ashore to whelp once a year and breed at this time before returning to the sea - the embryo remains in dormancy for several months before implanting and progressing to term about one year later.

Ovulation may or may not coincide with the breeding season in bats. If fertilization does occur in conjunction with mating, then development of the embryo will be arrested during torpor/hibernation - if not, ovulation is delayed until awakening, when stored sperm cells fertilize the ova and gestation ensues.

Other species in which delayed implantation is known to occur includes the armadillo, badger, bear, mink, roe deer, sea lion, shrew, and skunk.

There is a paucity of information concerning regulatory physiological mechanisms of delayed implantation. The uterus plays an important role in mediating the diapause - dormant embryos promptly resume development when removed from the uterus and placed in culture. Steroidogenic activity of the CL is temporarily diminished during the diapausal hiatus (a threshold of progesterone might act via the uterus to control the timing of implantation).