implantation (embryology) implantation (embryology) implantation, also known as nidation is the stage in the embryonic development of mammals in which the blastocyst hatches, attaches, adheres, and invades into the wall of the female's uterus. implantation is the first stage of gestation, and, when successful, the female is considered to be pregnant. in a woman, an implanted embryo is detected by the presence of increased levels of human chorionic gonadotropin (hcg) in a pregnancy test. the implanted embryo will receive oxygen and nutrients in order to grow. implantationimplantation as one of the early stages of human embryonic developmentdetailscarnegie stage3days5–9gives rise togastrulaanatomical terminology there is an extensive variation in the type of trophoblast cells, and structures of the placenta across the different species of mammals. of the five recognised stages of implantation including two pre-implantation stages that precede placentation, the first four are similar across the species. the five stages are migration and hatching, pre-contact, attachment, adhesion, and invasion. the two pre-implantation stages are associated with the pre-implantation embryo. in humans, following the stage of hatching that takes place around four to five days after fertilization, the process of implantation begins. by the end of the first week, the blastocyst is superficially attached to the uterine endometrium. by the end of the second week, implantation has completed. implantation stages there are five recognized stages of implantation in mammals, including two pre-implantation stages that precede the formation of the placenta. they are: migration and hatching, pre-contact, attachment, adhesion, and invasion. the first four stages are similar across the species with the process of invasion being variable. these three stages of apposition, attachment, and invasion are also alternatively termed contact (apposition), adhesion (attachment), and penetration (invasion), and can only take place during a limited timeframe known as the window of implantation when the uterus is at its most receptive. migration and hatching diagram of stages of developing blastocyst in the human until implantation there are two stages of migration involved in implantation, the first is the migration of the zygote, and the second is the migration of the trophoblast. fertilization of the oocyte, takes place in the ampulla of the fallopian tube. cilia on the lining of the tube move the zygote in its migration towards the uterus. during this migration the zygote undergoes a number of cell divisions that creates a ball of 16 compacted blastomeres called a morula. the morula enters the uterus after three or four days, and as it does a cavity called the blastocoel is formed in the morula to produce the blastocyst. the blastocyst contains the inner cell mass that will go on to develop into the embryo proper, and an outer cell layer of trophoblasts that will develop into the extraembryonic membranes (fetal membranes). the blastocyst is still enclosed in the egg-coat known as the zona pellucida, and for it to be able to implant into the uterine wall it must rid itself of this covering. this stage is known as zona hatching, and when there is sufficient dissolution the blastocyst is able to initiate the apposition stage of implantation. lytic factors in the uterine cavity, as well as factors from the blastocyst itself are essential for the breakdown of the egg-coat. mechanisms in the latter are indicated by the fact that the zona pellucida remains intact if an unfertilized egg is placed in the uterus under the same conditions. among the known molecular regulators that promote hatching are predominantly proteases that are stimulated by various growth factors. the blastocyst also produces cytokines, both pro-inflammatory and anti-inflammatory, that have crucial roles during implantation and other stages of pregnancy. both types of cytokines modulate the activity of proteases, including mmps, plasminogen activators, and cathepsins. it is unknown whether the cytokines involved in hatching are pro-inflammatory or anti-inflammatory, or which proteases are involved. however, it is well accepted that the pro-inflammatory cytokines are dominant during implantation. cytokines are also present in the uterine milk which might regulate the development and function of the blastocyst but there is no evidence to support their involvement in hatching. leukemia inhibitory factor (lif) is a pro-inflammatory cytokine expressed in the endometrium during the luteal phase of the menstrual cycle, with the highest expression seen during the window of implantation. lif plays a role in adhesion and invasion. assisted zona hatching may take place in assisted reproduction, where the zona pellucida may be artificially pierced to facilitate hatching. apposition following zona hatching, the very first loose connection or contact between the blastocyst and the endometrium is called apposition. apposition is usually made where there is a small crypt in the endometrium, and also where there has been enough breakdown of the zona pellucida to allow the blastocyst trophoblast to directly contact the underlying endometrium. ultimately, the inner cell mass (also embryoblast), inside the trophoblast layer, is aligned closest to the decidua. if the inner cell mass is not aligned with the decidua at apposition, it has the ability to freely rotate within the trophoblast and achieve this alignment. apposition is only a weak interaction of the trophectoderm with the uterine epithelium that is unstable to shear stress. apposition is also reversible allowing repositioning of the blastocyst in the uterus. adhesion adhesion is a much stronger attachment to the endometrium than the loose apposition. the trophoblasts adhere by penetrating the endometrium, with protrusions of trophoblast cells. this adhering activity is by microvilli that are on the trophoblast. the trophoblast have binding fiber connections, laminin, collagen type iv, and integrins that assist in this adhesion process. mucin-16 is a transmembrane mucin expressed at the apical surface of uterine epithelia. this mucin prevents the blastocyst from implanting in an undesired located on the epithelium. thus, muc-16 inhibits cell-cell adhesion. its removal during pinopode formation has been shown to facilitate trophoblast invasion in vitro. the identity of the molecules on the trophoblast and the endometrial epithelia that mediate the initial interaction between the two remain unidentified. however, a number of research groups have proposed that muc1, a member of the mucin family of glycosylated proteins, is involved. muc1 is a transmembrane glycoprotein expressed at the apical surface of endometrial epithelial cells during the window of implantation in humans and has been shown to be differentially expressed between fertile and infertile subjects during this time. muc1 displays carbohydrate moieties on its extracellular domain that are ligands of l-selectin, a cell adhesion molecule on the surface of trophoblast cells. an in vitro model of implantation gave evidence to support the hypothesis that l-selectin mediates apposition of the blastocyst to the uterine epithelium by interacting with its ligands. invasion illustration of an implanting embryo invasion is the further establishment of the blastocyst into the endometrium. the protrusions of trophoblast cells that adhere into the endometrium continue to proliferate and penetrate into the endometrium using gelatinase a and b. trophoblasts invade the uterus attempting to reach maternal blood supply, for setting up the foundation for fetal blood flow. as these trophoblasts penetrate, they terminally differentiate into a multinucleated tissue, a syncytium known as the syncytiotrophoblast. between this layer and the blastocyst lies the cytotrophoblast. when the syncytiotrophoblast reaches the basal membrane beneath the decidual cells, it dislodges them to further invade into the uterine stroma. dislodging is accomplished by degrading the cell adhesion molecules (cams) that link the decidual cells, and the associated extracellular matrix. degradation is achieved by the secretion of tumor necrosis factor-alpha from the syncytiotrophoblast, which inhibits the expression of cams and beta-catenin. the extracellular matrix is degraded by metalloproteinases such as collagenases, gelatinases and matrix metalloproteinases, and by serine proteases. the collagenases digest types i, ii, iii, vii and x collagen. the gelatinases exist in two forms; one digesting type-iv collagen and one digesting gelatin. the extracellular matrix is degraded by serine endopeptidases and metalloproteinases. the syncytiotrophoblast can then invade into the endometrium taking the embryo with it where it becomes embedded. eventually, the syncytiotrophoblast comes into contact with maternal blood and forms chorionic villi – the beginning of placentation. following invasion, the breach in the uterine epithelium made by the blastocyst's entry is sealed by a fibrin plug. the fibrin plug is a coagulation of a blood clot and cellular debris. extravillous trophoblasts extravillous trophoblasts are cells from the invading villi that migrate into the myometrium of the mother’s uterus. these cells remodel the spiral arteries to improve and secure maternal blood flow to the growing embryo. there is also evidence that this process occurs with the uterine veins, stabilizing them to improve drainage of fetal blood and metabolic wastes. trophoblasts have also been documented to migrate into various tissues in the mother. due to this trophoblasts have been implicated in a phenomenon known as fetomaternal microchimerism where fetal cells establish cell lines in maternal tissues. secretions the blastocyst secretes factors for a multitude of purposes during invasion. it secretes several autocrine factors, targeting itself and stimulating it to further invade the endometrium. human chorionic gonadotropin is an autocrine growth factor for the blastocyst, while insulin-like growth factor 2, stimulates its invasiveness. furthermore, secretions loosen decidual cells from each other, prevent the embryo from being rejected by the mother, trigger the final decidualization and prevent menstruation. immunosuppressive the embryo differs from the cells of the mother, and would be rejected as a parasite by the immune system of the mother if it did not secrete immunosuppressive agents. such agents include platelet-activating factor, human chorionic gonadotropin, early pregnancy factor, prostaglandin e2, interleukin-1 alpha, interleukin 6, interferon-alpha, leukemia inhibitory factor and colony-stimulating factor. prevention of menstruation human chorionic gonadotropin (hcg) not only acts as an immunosuppressive, but also signals to the mother that she is pregnant, preventing luteolysis of the corpus luteum and menstruation by sustaining the function of the corpus luteum. other factors other factors secreted by the blastocyst are; embryo-derived histamine-releasing factor tissue plasminogen activator as well as its inhibitors estradiol β1-integrins fibroblast growth factor cytl1 transforming growth factor alpha inhibin preimplantation factor uterus receptivity to enable implantation, the uterus goes through changes in order to be able to receive the conceptus. receptivity includes changes to endometrial cells in the formation of pinopodes that help to absorb uterine fluid; changes in the thickness of the endometrium and its blood supply development, and the formation of the decidua. collectively these changes are known as plasma membrane transformation, and bring the blastocyst nearer to the endometrium and immobilize it. during this stage the blastocyst can still be eliminated by being flushed out of the uterus. successful implantation is co-dependent on the viability of the embryo, and the receptivity of the uterus. a critical involved factor is the developmental synchrony between the embryo and the uterus. the synchrony gives a short period of receptivity known as the window of implantation, and involves much crosstalk between the blastocyst and the endometrium at this stage. during adhesion this communication is conveyed by receptor-ligand-interactions, both integrin-matrix and proteoglycan ones. proteoglycan receptors are found on the surface of the decidua, and their counterparts, the proteoglycans, are found around the trophoblast cells of the blastocyst. this ligand-receptor system is also present just at the implantation window. the blastocyst signals to the endometrium to adapt further to its presence, for example by changes in the cytoskeleton of decidual cells. this, in turn, dislodges the decidual cells from their connection to the underlying basal lamina, which enables the blastocyst to perform the succeeding invasion. window of implantation the window of implantation is a limited timeframe for the successful attachment of the blastocyst. in humans uterine receptivity is optimum on days 20-24 of the secretory phase of the menstrual cycle when luteinizing hormone levels are at their peak. in humans the window of implantation is only available for 24–36 hours. the endometrial microbiome has been indicated as having an important role in successful implantation in controlling endometrial cell function, and the function of the local immunity system that prevents pathogen growth. this is associated with the secretion of protective substances. pinopodes main article: pinopode pinopodes are formed at the beginning of the window of implantation, and are found in many species. they are mushroom-like protrusions from the apical cell membrane of uterine epithelial cells. pinopodes are formed by the swelling of these epithelial cells, and the fusing together of a number of microvilli, to reach a