Implantation and placentation

Implantation

Implantation involves the initial attachment of the trophoblastic wall of the blastocyst to the endometrial luminal epithelium, stimulating the decidual response. The blastocyst is present within the uterine cavity for some 72 hours prior to implantation, and during this time there is an interactive dialogue between the implanting blastocyst and endometrial decidual stromal cells ( ). In vitro culture of human preimplantation embryos shows that normally developed preimplantation human blastocysts actively enhance the uterine environment for their implantation, whereas developmentally impaired embryos induce endoplasmic reticulum stress responses in decidual cells that inhibit implantation ( ). Reponses from endometrial stromal cells also inform these interactions ( , ). Uterine surgical scars may have a long-term impact on the interactive dialogue between the implanting blastocyst and endometrial decidual stromal cells ( ).

The trophoblast of the blastocyst gives rise to three main cell types in the human placenta: syncytiotrophoblast that forms the epithelial covering of the villous tree and is the main endocrine component of the placenta; villous cytotrophoblast cells that represent a germinative population that proliferates throughout pregnancy, fusing to generate syncytiotrophoblast; and extravillous trophoblast cells that are non-proliferative and invade the maternal endometrium. The first two cell lines can be seen from stages 4 and 5 onwards. The cytotrophoblast cells that form the mural and polar trophoblast are cuboidal. Following apposition of the blastocyst to the uterine mucosa, the polar trophoblast becomes covered externally with syncytiotrophoblast, a multinucleated tissue that penetrates between the epithelial cells (see Fig. 8.10 ).

Preimplantation embryos produce matrix metalloproteinases (MMPs) that mediate penetration of the maternal subepithelial basal lamina by the syncytiotrophoblast. Trophoblast cells express L-selectin (usually seen as a mediator of neutrophil rolling and tethering in inflamed endothelium), and the endometrial epithelium upregulates selectin–oligosaccharide-based ligands ( ). Thus, differentiating cytotrophoblast cells appear to use processes that also occur in vasculogenesis and during leukocyte migration from the blood into the tissues. Flanges of syncytiotrophoblast grow between the cells of the uterine luminal epithelium towards the underlying basal lamina without apparent damage to the maternal cell membranes or disruption of the intercellular junctions. Instead, shared junctions, including tight junctions, are formed with many of the maternal uterine epithelial cells.

Implantation continues with erosion of the vascular endothelium of maternal superficial capillaries and glandular epithelium and phagocytosis of the glandular secretions. There is increasing evidence that the endometrial stromal cells grow around and encapsulate the blastocyst until it occupies an uneven implantation cavity in the stroma (interstitial implantation) ( Fig. 9.1 ). In the early postimplantation phase, the uterine surface is resealed by re-epithelialization and the formation of a plug that may contain fibrin. As the blastocyst burrows more deeply into the endometrium, syncytiotrophoblast forms over the mural cytotrophoblast, but never achieves the thickness of the syncytiotrophoblast over the embryonic pole.

The syncytiotrophoblast does not express either class I or class II major histocompatibility complex (MHC) antigens, but it secretes numerous hormones: human chorionic gonadotrophin (hCG) can be detected in maternal urine from as early as postfertilization day 10 and forms the basis for tests for early pregnancy. This hCG prolongs the life of the corpus luteum, which continues to secrete progesterone and oestrogens until these essential hormones are produced by the placenta. For details of postfertilization and postmenstrual ages see Fig. 23.3 .

Menstruation ceases on successful implantation and the endometrium transforms into the decidua to form a suitable nidus for the conceptus. Decidualization of the endometrial stroma may occur without an intrauterine pregnancy, for example in the presence of an ectopic pregnancy, after prolonged treatment with progesterone, and in the late secretory phase of a non-conception cycle. Decidual differentiation is not evident in the stroma at the earliest stages of implantation, and it may not be until a week later that fully differentiated cells are present. During decidualization, the interglandular tissue increases in quantity ( ). It contains a substantial population of leukocytes (large granular lymphocytes, macrophages and T cells) distributed amongst large decidual cells.

The most numerous are uterine natural killer (NK) cells that accumulate in the endometrium during the secretory phase of the cycle and persist until mid-pregnancy; they interact with invading extravillous trophoblast cells that express human leukocyte antigens G and C (HLA-G, HLA-C).

Decidual cells are stromal cells that contain varying amounts of glycogen, lipid and vimentin-type intermediate filaments in their cytoplasm. They are generally rounded but their shape may vary, depending on the local packing density. They may contain one, two or sometimes three nuclei, frequently display rows of club-like cytoplasmic protrusions enclosing granules, and are associated with a characteristic capsular basal lamina. Decidual cells produce a range of secretory products that may be taken up by the trophoblast, including insulin-like growth factor binding protein 1 (IGF-BP1) and prolactin. Prolactin signals to the endometrial glands to increase their secretions and may play a role in the maintenance and growth of the conceptus in the early phase of postimplantational development via histotrophic nutrition ( ): it can be detected in amniotic fluid in the first trimester of pregnancy.

Extracellular matrix, growth factors and protease inhibitors produced by the decidua all probably modulate the degradative activity of the trophoblast and support placental morphogenesis and placental accession to the maternal blood supply. Once implantation is complete, distinctive names are applied to different regions of the decidua ( Fig. 9.2 ). The decidua capsularis covers the conceptus; the decidua basalis (where the placenta subsequently develops) lies between the conceptus and the uterine muscular wall; the decidua parietalis lines the remainder of the body of the uterus. Subsets of perivascular and stromal cells in distinct decidual layers probably mediate regulatory interactions that prevent harmful innate or adaptive immune responses at the maternal-fetal interface in early human pregnancy ( ).

Development of the Placenta

Formation of the human placenta requires a developmental progression that proceeds in a specific chronological order. The development of primary, secondary and anchoring villi, and the local differentiation of haemangioblast cells within them, occur concurrently with the modification of maternal blood vessels to ensure their patency. By embryonic stage 10, with the onset of the embryonic heartbeat, a primitive circulation between the embryo and the secondary yolk sac is established. Blood vessels within the walls of the allantois and connecting stalk connect the embryonic circulation to the developing placenta during postmenstrual weeks 6–12, forming a low-oxygen environment with nutrition from endometrial glands. From postmenstrual week 12, maternal blood flows into the placental intervillous spaces and a fetoplacental circulation is fully established.

As the blastocyst implants, the syncytiotrophoblast invades the uterine tissues, including the glands and walls of maternal blood vessels (see Fig. 9.1 ; Fig. 9.3 ), and increases rapidly in thickness over the embryonic pole ( Fig. 9.4 ). Microvillus-lined clefts and lacunar spaces develop within the syncytiotrophoblastic envelope (postfertilization days 9–11) and establish communications with one another. Initially, many of these spaces contain maternal blood derived from dilated uterine capillaries and veins, as the walls of the vessels are partially destroyed. As the conceptus grows, the lacunar spaces enlarge and become confluent to form intervillous spaces.

The projections of syncytiotrophoblast into the maternal decidua are the primary villi. They are invaded initially with cytotrophoblast and then with extraembryonic mesenchyme (days 13–15) to form secondary placental villi. Capillaries develop in the mesenchymal core of the villi during postfertilization week 3. The cytotrophoblast within the villi continues to grow through the invading syncytiotrophoblast and makes direct contact with the decidua basalis, forming anchoring villi. Further cytotrophoblast proliferation occurs laterally so that neighbouring outgrowths meet to form a spherical cytotrophoblastic shell around the conceptus ( Fig. 9.5 ; see Fig. 9.3B ). Lateral projections from the main stem villus form true villi. Elongation of growing capillaries outstrips that of the containing villi, causing vessels to loop: obtrusion of capillary loops and new sprouts results in the formation of terminal villi.

As secondary villi form, single mononuclear cells become detached from the distal (anchoring) cytotrophoblast and infiltrate the maternal decidua ( Fig. 9.6 ; see Fig. 9.3B ). These extravillous cells are the third lineage of the original trophoblastic cells. Extravillous trophoblast cells may be interstitial or endovascular. Interstitial extravillous trophoblast cells invade the maternal spiral arteries from their adventitia, whereas the endovascular trophoblast cells migrate along their lumens. The smooth muscle and internal elastic lamina are replaced with extracellular fibrinoid deposits (see Figs 9.3B , 9.6 ) ( ). In the central area of the basal plate, destined to become the definitive placenta, the endovascular trophoblast plugs block the maternal spiral arteries until the end of the first trimester, restricting inflow into the intervillous space. The cells then migrate antidromically, against the flow of maternal blood, along the spiral arteries as far as the inner myometrial region or junctional zone ( ).

The definitive placenta is composed of a chorionic plate on its fetal aspect and a basal plate on its maternal aspect, separated by an intervening intervillous space containing villous stems with branches in contact with maternal blood (see Fig. 9.5 ). During the first trimester, development takes place in a physiologically low-oxygen environment supported by histotrophic nutrition from the endometrial glands which discharge into the intervillous space (see Fig. 9.3A ) ( , , ). When maternal blood bathes the surfaces of the chorion bounding the intervillous space, the human placenta is defined as haemochorial. Different grades of fusion exist between the maternal and fetal tissues in many other mammals (e.g. epitheliochorial, synepitheliochorial, endotheliochorial). The chorion is vascularized by the allantoic blood vessels of the body stalk, and so the human placenta can be termed chorio-allantoic (in some mammals, a choriovitelline placenta exists either alone or supplements the chorio-allantoic variety). It can also be defined as discoidal (in contrast to other shapes in other mammals), and deciduate (because maternal tissue is shed with the placenta and membranes at parturition as part of the afterbirth).