Implantation is an event that occurs early in pregnancy in which the embryo adheres to the wall of uterus. At this stage of prenatal development, the embryo is a blastocyst. It is by this adhesion that the fetus receives the oxygen and the nutrients from the mother to be able to grow.

The implantation window

There are many conditions that must be satisfied in order for a successful implantation to take place. There is only a specific period of time during which implantation is possible,^[1] this is the "implantation window". A reason for this window is that if implantation does not occur at a certain time, then it signifies that something is wrong. And when there is a risk that something is wrong, there will most likely be a miscarriage rather than the continued gestation of a malformed fetus.

The implantation window is started by preparations in the endometrium of the uterus, both structurally and in the composition of its secretions.

Adaption of uterus

To enable implantation, the uterus goes through changes in order to be able to receive the embryo.

Predecidualization

P The endometrium increases in thickness, becomes more vascularized and its glands grow to be tortuous and boosted in their secretions. These changes reach their maximum about 7 days after ovulation.

Furthermore, the surface of the endometrium produces a kind of rounded cells, which cover the whole area toward the uterine cavity. This happens about 9 to 10 days after ovulation.^[1] These cells are called decidual cells, which emphasises that the whole layer of them is shed off in every menstruation if no pregnancy occurs, just as leaves of deciduous trees. The uterine glands, on the other hand, decrease in activity and degenerate already 8 to 9 days^[1] after ovulation in absence of pregnancy.

The stromal cells originate from the stromal cells that are always present in the endometrium. However, the decidual cells make up a new layer, the decidua. The rest of the endometrium, in addition, expresses differences between the luminal and the basal sides. The luminal cells form the zona compacta of the endometrium, in contrast to the basalolateral zona spongiosa, which consists of the rather spongy stromal cells.^[1]

Decidualization

Decidualization succeeds predecidualization if pregnancy occurs. This is an expansion of it, further developing the uterine glands, the zona compacta and the epithelium of decidual cells lining it. The decidual cells become filled with lipids and glycogen and take the polyhedral shape characteristic for decidual cells.

Trigger

It is likely that the blastocyst itself makes the main contribution to this additional growing and sustaining of the decidua. An indication of this is that decidualization occurs at a higher degree in conception cycles than in nonconception cycles.^[1] Furthermore, similar changes are observed when giving stimuli mimicking the natural invasion of the embryo.^[1]

Parts of decidua

The decidua can be organized into separate sections, although they have the same composition.

Decidua basalis

This is the part of the decidua which is located basalolateral to the embryo after implantation.

Decidua capsularis

Decidua capsularis grows over the embryo on the luminal side, enclosing it into the endometrium. It surrounds the embryo together with decidua basalis.

Decidua parietalis

All other decidua on the uterine surface belongs to decidua parietalis.

Decidua throughout pregnancy

After implantation the decidua remains, at through least the first trimester.^[1] However, its most prominent time is during the early stages of pregnancy, during implantation. Its function as a surrounding tissue is replaced by the definitive placenta. However, some elements of the decidualization remain throughout pregnancy.^[1]

The compacta and spongiosa layers are still observable beneath the decidua in pregnancy. The glands of the spongiosa layer continue to secrete during the first trimester, when they degenerate. However, before that disappearance, some glands secrete unequally much. This phenomenon of hypersecretion is called the Arias-Stella phenomenon,^[1] after the pathologist Javier Arias-Stella.

Pinopodes

Pinopodes are small, finger-like protrusions from the endometrium. They appear between day 19 and day 21^[1] of gestational age. This corresponds to a fertilization age of approximately 5 to 7 days, which corresponds well with the time of implantation. They only persist for 2 to 3 days.^[1] The development of them is enhanced by progesterone but inhibited by estrogens.

Function in implantation

Pinopodes endocytose uterine fluid and macromolecules in it. By doing so, the volume of the uterus decreases, taking the walls closer to the embryoblast floating in it. Thus, the period of active pinocytes might also limit the implantation window.^[1]

Function during implantation

Pinopodes continue to absorb fluid, and removes most of it during the early stages of implantation.

Adaption of secretions