Implantation

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Contents

Introduction

Human conceptus fully implanted (Stage 5).
Uterine Leukemia Inhibitory Factor (LIF) Expression[1]

The term "implantation" is used to describe process of attachment and invasion of the uterus endometrium by the blastocyst (conceptus) in placental animals. In humans, this process begins at the end of week 1, with most successful human pregnancies the conceptus implants 8 to 10 days after ovulation, and early pregnancy loss increases with later implantation.[2] The implantation process continues through the second week of development.

The initial phase of the implantation process is "adplantation". This first phase requires the newly hatched blastocyst to loosely adhere to the endometrial epithelium, often "rolling" to the eventual site of implantation where it is firmly adhered. This process requires both the blastocyst adhesion interaction with the endometrium during the "receptive window".

Subsequent development of the placenta allows maternal support of embryonic and fetal development. If implantation has not proceeded sufficiently during the menstrual cycle to allow hormonal feedback to the ovary, then the next cycle may commence leading to conceptus loss. There is also evidence, from animal models, that a conceptus with major genetic does not develop or implant correctly leading to their loss during the first and second weeks of development.

In recent years with the development or Assisted Reproductive Technologies (ART or IVF) there is a growing interest in this process, with techniques that introduce the blastocyst into the uterus to allow normal implantation to occur.

Abnormal implantation is where this process does not occur in the body of the uterus (ectopic) or where the placenta forms incorrectly. In addition implantation can occur normally but with an abnormal conceptus, as in a hydatiform mole development.


Implantation Links: Introduction | Week 2 | Trophoblast | Placenta Development | Lecture - Week 1 and 2 | Implantation Timeline | Ectopic Implantation | Category:Implantation

Some Recent Findings

  • Chemokine gene silencing in decidual stromal cells limits T cell access to the maternal-fetal interface[3] "The chemokine-mediated recruitment of effector T cells to sites of inflammation is a central feature of the immune response. The extent to which chemokine expression levels are limited by the intrinsic developmental characteristics of a tissue has remained unexplored. We show in mice that effector T cells cannot accumulate within the decidua, the specialized stromal tissue encapsulating the fetus and placenta. Impaired accumulation was in part attributable to the epigenetic silencing of key T cell-attracting inflammatory chemokine genes in decidual stromal cells, as evidenced by promoter accrual of repressive histone marks. These findings give insight into mechanisms of fetomaternal immune tolerance, as well as reveal the epigenetic modification of tissue stromal cells as a modality for limiting effector T cell trafficking."
  • Human Endometrial CD98 Is Essential for Blastocyst Adhesion[4] "These results indicate that CD98, a component of tetraspanin-enriched microdomains, appears to be an important determinant of human endometrial receptivity during the implantation window." OMIM CD98
  • Implantation failure: molecular mechanisms and clinical treatment.[5] "Implantation is a complex initial step in the establishment of a successful pregnancy. Although embryo quality is an important determinant of implantation, temporally coordinated differentiation of endometrial cells to attain uterine receptivity and a synchronized dialog between maternal and embryonic tissues are crucial. The exact mechanism of implantation failure is still poorly understood."
  • Preimplantation factor promotes first trimester trophoblast invasion [6] "Preimplantation factor is a novel embryo-derived peptide that influences key processes in early pregnancy implantation, including immunity, adhesion, remodeling, and apoptosis. Herein, we explore the effects of synthetic preimplantation factor on trophoblast invasion."
  • Mouse trophoblastic cells exhibit a dominant invasiveness phenotype over cancer cells PMID20826050 "We investigated the interactions between trophoblasts and metastatic cancer cells and found the phenomenon that mouse trophoblastic cells invaded the monolayer of malignant cancer cells in vitro and appeared the general trait of invasiveness to more than 30 types of malignant cancer cell lines which were derived from different histological origins and with different invasive or metastatic potential."


Week 1 and 2 Human Development Overview

Week 1 Human Development Overview

Endometrial Receptivity

In humans, receptivity occurs 6 days after the post-ovulatory progesterone surge and lasts about 2 to 4 days (about days 20 to 24 of the menstrual cycle).

A similar "receptivity window" occurs following fertilization in other species: rat day 5 and mouse day 4.5. Many studies have looked into identifying markers for this receptivity period both to optimise and to block this process.

Carnegie Stage 5

The conceptus is shown fully implanted in the uterine wall. Uterine wall is shown on the left and the uterine cavity is shown on the right.

Early Stage 5 Late Stage 5
Stage5 bf08.jpg Stage5 bf11L.jpg


Links: Carnegie Stage 5

Implantation Animation

The media player is loading...

This animation shows the process of implantation, occurring during week 2 of development in humans.

The beginning of the animation shows: the uterus lining (endometrium epithelium), the hatched blastocyst with a flat outer layer of trophoblast cells (green), the inner cell mass which has formed into the bilaminar embryo (epiblast and hypoblast) and the large fluid-filled space (blastocoel).

green cells - trophoblast layer of the conceptus

blue cells - epiblast layer of the bilaminar embryo

yellow cells - hypoblast layer of the bilaminar embryo

white cells - uterine endometrium epithelium

red - maternal blood vessel

Implantation Movie Links: MP4 version | Quicktime version | Week 2 Chorionic Cavity Movie | Implantation | Week 2 | Trophoblast | Human Chorionic Gonadotropin | Placenta Development | Movies
Stage5 bf08.jpg


The second week of human development is concerned with the process of implantation and the differentiation of the blastocyst into early embryonic and placental forming structures.

  • implantation commences about day 6 to 7
  • Adplantation - begins with initial adhesion to the uterine epithelium
    • blastocyst then slows in motility, "rolls" on surface, aligns with the inner cell mass closest to the epithelium and stops
  • Implantation - migration of the blastocyst into the uterine epithelium, process complete by about day 9
  • coagulation plug - left where the blastocyst has entered the uterine wall day 12

Normal Implantation Sites - in uterine wall superior, posterior, lateral


Uterine Epithelium

Human uterine tube ciliated epithelium SEM

Uterine epithelial cilia are responsible for the initial movement of the ooycte and conceptus (zygote, morula, blastocyst). In humans, this is during the first week of development. Uterine epithelial microvilli are involved with the implantation process. Hormones (estrogen and progesterone) regulate both cilia and microvilli number and structure. The differences in size and shape of cilia and microvilli are shown by scanning micrographs of the lumenal surface of the epithelium lining the mammalian uterine tube.

Cilia Microvilli
  • long processes
  • epithelial cell apical membrane specializations
  • microtubule filled motile structure
  • estrogens control ciliated cells number.
  • shorter processes
  • epithelial cell apical membrane specializations
  • microfilament filled non-motile structure
  • microvilli length hormonally controlled (progesterone, short; estrogen, long thin)

Uterodomes or Pinopods

Scanning electron microscope images of the pig endometrial surface of a day 13 pregnant sow.[7]

(uterodomes) Cellular feature seen on the apical uterine epithelium surface. The presence of these structures is thought in many species to be a marker for endometrial receptivity. In humans though, recent studies have shown pinopodes are also present throughout the luteal phase of the menstrual cycle.[8] It has also been suggested that their role is not primarily pinocytotic, hence the alternate suggested name "uterodomes" based upon their appearance when imaged by electron microscopy.[9]

These transient microprotrusions inter-digitate with microvilli on the apical syncytiotrophoblast surface of the blastocyst during initial adplantation and implantation process.

Inhibitory Interactions

The epithelial surface has an associated glycocalyx, the cell surface formed by transmembrane and secreted glycoproteins. Mucins are a major components and mucin 1 (MUC1), a transmembrane protein appears to have dual roles. Firstly, acting as a barrier to both microbial infection and enzymatic attack. Secondly, its expression is altered by hormones and decreased MUC1 expression associated with receptivity. This suggests that high levels of mucin are inhibitory to implantation. Interestingly the same protein is highly expressed later in the placental amnion, where its anti-bacterial and anti-adhesive roles are again required.[10]

Adhesive Interactions

Desmosome cartoon

Cascade of endometrial/conceptus adhesive interactions[11]:

  1. Carbohydrate-mediated binding to the glycocalyx.
  2. Progressing to tighter binding involving osteopontin (OPN), members of the immunoglobulin superfamily (IgSF), integrin and cadherin families, trophinin and CD44.
  3. Activation of proteases including MMPs and ADAMs may well be important in these molecular assemblies.
  4. Lateral epithelial membrane components, including desmosomes, detach and reassemble as trophectoderm extends between maternal epithelial cells.

Implantation Factors

Implantation cartoon 01.jpg Implantation cartoon 02.jpg

Molecular Implantation and Decidualization[12]

Trophinin

  • Trophinin is a membrane protein expressed on blastocyst trophectoderm cells and on uterine endometrium epithelial cells.
  • Adhesion is thought to occur through trophinin-trophinin binding.
  • Adhesion also triggers two trophinin mediated effects:
  1. trophectoderm cells activate for implantation (proliferation, invasion)
  2. maternal endometrial epithelial cells induced programmed cell death (apoptosis).[13]


Links: OMIM

Cytokines

In mice, endometrial secretion of two IL-6 family cytokines, leukemia inhibitory factor (LIF) and Interleukin-11 (IL-11), are key requirements for implantation. A recent human study suggests that there is a similar requirement for human conceptus implantation.[14]

Uterine Leukemia Inhibitory Factor (LIF) Expression[1]

Implantation LIF.jpg

a - At day 4 of pregnancy, oestrogen E2 induces LIF expression in the endometrial glands, leading to LIF secretion into the uterine lumen. There, LIF binds to its receptors on the surface of epithelial cells. b - This makes the uterus receptive to the blastocyst, which implants by day 5 of pregnancy. Hu et al. find that LIF expression in the endometrial glands also depends on the regulatory activity of p53. In the absence of p53, insufficient LIF is produced, the uterus does not become adequately receptive, and fewer blastocysts implant.

Galectin 9

Galectin 9, a protein that binds galactosides and has many different roles, has been identified as a marker for the mid- and late-secretory phases of human endometrium and decidua. The high expression at uterodomes during the period of implantation, suggests that it may also mark endometrial receptivity.[15]


Links: OMIM - leukemia inhibitory factor | OMIM - Interleukin-11 | OMIM - Galectin 9

Day 8 to 9

Gray0032.gif

  • am. - amniotic cavity
  • b.c. - blood clot, at the site of initial implantation
  • b.s. - body-stalk, or connective stalk later forming the placental cord region with placental blood vessels
  • ect. - embryonic ectoderm that will contribute to embryonic and placental membrane development
  • ent. - entoderm (endoderm), this was the historic term for what we today call endoderm that will contribute to embryo development
  • mes. - mesoderm, consisting of both embryonic mesoderm (in the trilaminar embryonic disc) and extraembryonic mesoderm (outside the trilaminar embryonic disc)
  • m.v. - maternal vessels, spiral arteries that have been opened at their ends
  • tr. - trophoblast, relative to the embryonic disc the outer syncitiotrophoblast and inner cytotrophoblast layers that will contribute to placental development
  • u.e. - uterine epithelium, the epithelial layer that lines the unerus
  • u.g. - uterine glands, the glands that secrete nutrients to support the initial growth both before and after implantation
  • y.s. - yolk-sac, the endoderm lined and extraembryonic mesoderm covered cavity that will contribute to the gastrointestinal tract, blood and blood vessels

Maternal Immune

How does the implanting conceptus avoid immune rejection by the maternal immune system? There are a number of maternal and embryonic mechanisms that are thought to act to prevent immune rejection of the implanting conceptus.

Chemokine Gene Silencing

Remove the attraction of maternal immune cells.

A mouse study[3] has shown that the normal immune response to inflammation, accumulation of effector T cells in response to chemokine secretion does not occur during implantation. This is prevented locally by epigenetic silencing of chemokine expression in the decidual stromal cells.

Corticotropin-Releasing Hormone

Kill the maternal immune cells.

Both maternal and implanting conceptus release CRH at the embryo implantation site. This hormone then binds to receptors on the surface of trophoblast (extravillous trophoblast) cells leading to expression of a protein (Fas ligand, FasL) that activates the extrinsic cell death pathway on any local maternal immune cells ( T and B lymphocytes, natural killer cells, monocytes and macrophages).[16] This cannot be the only mechanism, as mice with dysfunctional FasL proteins are still fertile.

Decidual Reaction

Placenta anchoring villi

During pregnancy, at implantation the endometrium is altered by the maternal steroid hormones, estrogen and progesterone and in response to the implanting conceptus and renamed the "decidua". This process of signaling is called the decidual reaction or decidualization, and commences at the site of implantation and spreads throughout the uterine lining.

  • occurs within the uterus wall
  • initially at site of implantation and includes both cellular and matrix changes
  • reaction spreads throughout entire uterus, not at cervix
  • promoted by the maternal steroid hormones, estrogen and progesterone
  • extensive proliferation and differentiation of uterine stromal cells
  • deposition of fibrinoid and glycogen and epithelial plaque formation (at anchoring villi)
  • presence of decidual cells are indicative of pregnancy

In uterine stromal decidualization, bone morphogenetic protein 2 (BMP2) nonactive precursor protein is cleaved by proprotein convertase 5/6 (PC6) to produce the active form. Deletion or knockdown of either BMP2 or PC6 inhibits decidualization leads to implantation failure and female infertility.[17]

A recent human histological study has shown that endometrial stromal cell decidualization leads to a loss of lymphatics particularly apparent around the uterine spiral arteries.[18]

Links: OMIM - bone morphogenetic protein 2 | OMIM - proprotein convertase 5

Cervical Mucus Plug

Along with the decidualization, estrogen also stimulates the production of mucus from glands at the opening of the uterus, the cervix, where it joins the vagina. This secreted mucus then forms a plug/barrier (CMP) acting in a mechanical and antibacterial manner.

Abnormal Implantation

Tubal Pregnancy

Ectopic tubal pregnancy

Abnormal implantation sites or Ectopic Pregnancy occurs if implantation is in uterine tube or outside the uterus.

  • sites - external surface of uterus, ovary, bowel, gastrointestinal tract, mesentry, peritoneal wall
  • If not spontaneous then, embryo has to be removed surgically

Tubal pregnancy - 94% of ectopic pregnancies

  • if uterine epithelium is damaged (scarring, pelvic inflammatory disease)
  • if zona pellucida is lost too early, allows premature tubal implantation
  • embryo may develop through early stages, can erode through the uterine horn and reattach within the peritoneal cavity
Tubal pregnancy historic.jpg Abnormal implantation sites.jpg
Links: Ectopic Pregnancy | Movie - Ectopic pregnancy ultrasound


Hydatidiform Mole

Hydatidiform Mole

Another type of abnormality is when only the conceptus trophoblast layers proliferates and not the embryoblast, no embryo develops, this is called a "hydatidiform mole", which is due to the continuing presence of the trophoblastic layer, this abnormal conceptus can also implant in the uterus. The trophoblast cells will secrete human chorionic gonadotropin (hCG), as in a normal pregnancy, and may appear maternally and by pregnancy test to be "normal". Prenatal diagnosis by ultrasound analysis demonstrates the absence of a embryo.

There are several forms of hydatidiform mole: partial mole, complete mole and persistent gestational trophoblastic tumor. Many of these tumours arise from a haploid sperm fertilizing an egg without a female pronucleus (the alternative form, an embryo without sperm contribution, is called parthenogenesis). The tumour has a "grape-like" placental appearance without enclosed embryo formation. Following a first molar pregnancy, there is approximately a 1% risk of a second molar pregnancy.

This topic is also covered in Placenta - Abnormalities

Abnormal Placentation

Placental implantation abnormalities

Abnormalities can range from anatomical associated with degree or site of inplantation, structure (as with twinning), to placental function, placento-maternal effects (pre-eclampsia, fetal erythroblastosis) and finally mechanical abnormalities associated with the placental (umbilical) cord.

This topic is also covered in Placenta - Abnormalities

References

  1. 1.0 1.1 Wenwei Hu, Zhaohui Feng, Angelika K Teresky, Arnold J Levine p53 regulates maternal reproduction through LIF. Nature: 2007, 450(7170);721-4 PMID:18046411
  2. A J Wilcox, D D Baird, C R Weinberg Time of implantation of the conceptus and loss of pregnancy. N. Engl. J. Med.: 1999, 340(23);1796-9 PMID:10362823
  3. 3.0 3.1 Patrice Nancy, Elisa Tagliani, Chin-Siean Tay, Patrik Asp, David E Levy, Adrian Erlebacher Chemokine gene silencing in decidual stromal cells limits T cell access to the maternal-fetal interface. Science: 2012, 336(6086);1317-21 PMID:22679098
  4. Francisco Domínguez, Carlos Simón, Alicia Quiñonero, Miguel Ángel Ramírez, Elena González-Muñoz, Hans Burghardt, Ana Cervero, Sebastián Martínez, Antonio Pellicer, Manuel Palacín, Francisco Sánchez-Madrid, María Yáñez-Mó Human endometrial CD98 is essential for blastocyst adhesion. PLoS ONE: 2010, 5(10);e13380 PMID:20976164 | PMC2955532 | PLoS One
  5. Hakan Cakmak, Hugh S Taylor Implantation failure: molecular mechanisms and clinical treatment. Hum. Reprod. Update: , 17(2);242-53 PMID:20729534
  6. Christina M Duzyj, Eytan R Barnea, Min Li, S Joseph Huang, Graciela Krikun, Michael J Paidas Preimplantation factor promotes first trimester trophoblast invasion. Am. J. Obstet. Gynecol.: 2010, 203(4);402.e1-4 PMID:20708167
  7. Qian Ren, Shu Guan, Jinluan Fu, Aiguo Wang Temporal and spatial expression of muc1 during implantation in sows. Int J Mol Sci: 2010, 11(6);2322-35 PMID:20640155 | PMC2904919
  8. C E Quinn, R F Casper Pinopodes: a questionable role in endometrial receptivity. Hum. Reprod. Update: , 15(2);229-36 PMID:18997181
  9. C R Murphy Understanding the apical surface markers of uterine receptivity: pinopods-or uterodomes? Hum. Reprod.: 2000, 15(12);2451-4 PMID:11098008
  10. Ruchira Sood, James L Zehnder, Maurice L Druzin, Patrick O Brown Gene expression patterns in human placenta. Proc. Natl. Acad. Sci. U.S.A.: 2006, 103(14);5478-83 PMID:16567644
  11. Harmeet Singh, John D Aplin Adhesion molecules in endometrial epithelium: tissue integrity and embryo implantation. J. Anat.: 2009, 215(1);3-13 PMID:19453302
  12. Jeeyeon Cha, Xiaofei Sun, Sudhansu K Dey Mechanisms of implantation: strategies for successful pregnancy. Nat. Med.: 2012, 18(12);1754-67 PMID:23223073 | Nat Med
  13. Tamura N, Sugihara K, Akama TO, Fukuda MN. Trophinin-mediated cell adhesion induces apoptosis of human endometrial epithelial cells through PKC-δ. Cell Cycle. 2011 Jan 1;10(1):135-43 PMID21191175
  14. M Marwood, K Visser, L A Salamonsen, E Dimitriadis Interleukin-11 and leukemia inhibitory factor regulate the adhesion of endometrial epithelial cells: implications in fertility regulation. Endocrinology: 2009, 150(6);2915-23 PMID:19213836
  15. Yuji Shimizu, Maryam Kabir-Salmani, Mehri Azadbakht, Kazuhiro Sugihara, Keiji Sakai, Mitsutoshi Iwashita Expression and localization of galectin-9 in the human uterodome. Endocr. J.: 2008, 55(5);879-87 PMID:18506087
  16. A Makrigiannakis, E Zoumakis, S Kalantaridou, C Coutifaris, A N Margioris, G Coukos, K C Rice, A Gravanis, G P Chrousos Corticotropin-releasing hormone promotes blastocyst implantation and early maternal tolerance. Nat. Immunol.: 2001, 2(11);1018-24 PMID:11590404
  17. Sophea Heng, Sarah Paule, Belinda Hardman, Ying Li, Harmeet Singh, Adam Rainczuk, Andrew N Stephens, Guiying Nie Posttranslational activation of bone morphogenetic protein 2 is mediated by proprotein convertase 6 during decidualization for pregnancy establishment. Endocrinology: 2010, 151(8);3909-17 PMID:20555025
  18. Mila Volchek, Jane E Girling, Gendie E Lash, Leonie Cann, Beena Kumar, Stephen C Robson, Judith N Bulmer, Peter A W Rogers Lymphatics in the human endometrium disappear during decidualization. Hum. Reprod.: 2010, 25(10);2455-64 PMID:20729537

Reviews

Jeeyeon Cha, Xiaofei Sun, Sudhansu K Dey Mechanisms of implantation: strategies for successful pregnancy. Nat. Med.: 2012, 18(12);1754-67 PMID:23223073

Natalie J Hannan, Jemma Evans, Lois A Salamonsen Alternate roles for immune regulators: establishing endometrial receptivity for implantation. Expert Rev Clin Immunol: 2011, 7(6);789-802 PMID:22014020

Richard J Paulson Hormonal induction of endometrial receptivity. Fertil. Steril.: 2011, 96(3);530-5 PMID:21880274

Michiko N Fukuda, Kazuhiro Sugihara An integrated view of L-selectin and trophinin function in human embryo implantation. J. Obstet. Gynaecol. Res.: 2008, 34(2);129-36 PMID:18412772

J D Aplin Embryo implantation: the molecular mechanism remains elusive. Reprod. Biomed. Online: 2006, 13(6);833-9 PMID:17169205


Articles

Michiko N Fukuda, Kazuhiro Sugihara Signal transduction in human embryo implantation. Cell Cycle: 2007, 6(10);1153-6 PMID:17495530 | PDF

Search PubMed

Search Pubmed: Embryo Adplantation | Embryo Implantation | tubal pregnancy | Endometrial Receptivity | Placenta Abnormalities | Pinopods | decidualization


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Cite this page: Hill, M.A. (2014) Embryology Implantation. Retrieved April 20, 2014, from http://embryology.med.unsw.edu.au/embryology/index.php?title=Implantation

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Dr Mark Hill 2014, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
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