Implantation

Notice - Mark Hill

Currently this page is only a template and will be updated (this notice removed when completed).

Introduction

The term 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 and continues through the second week of development. 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.

Links: Week 2 | Placenta Development | Lecture - Week 2 Development | Category:Implantation

Some Recent Findings

Implantation failure: molecular mechanisms and clinical treatment.^[1] "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 20708167 "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

Pinopods

(uterodomes) Cellular feature seen on the apical uterine epithelium surface, the presence of these structures is thought to be a marker for endometrial receptivity. These transient microprotrusions inter-digitate with microvilli on the apical syncytiotrophoblast surface of the blastocyst during initial adplantation and implantation process.

In humans, receptivity occurs 6 days after the post-ovulatory progesterone surge and lasts about 2 to 4 days. A similar "receptivity window" occurs following fertilization in other species: rat day 5 and mouse day 4.5.

Implantation Cartoon

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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

Links: Quicktime version | Flash Version | Quicktime movie

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

Decidual Reaction

During pregnancy, following implantation, the endometrium is altered in response to the implanting conceptus and renamed the decidua. The decidual reaction 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
deposition of fibrinoid and glycogen and epithelial plaque formation (at anchoring villi)
presence of decidual cells are indicative of pregnancy

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

Links: Ectopic Pregnancy | Movie - Ectopic pregnancy ultrasound

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

↑ <pubmed>20729534</pubmed>

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Cite this page: Hill, M.A. (2024, May 5) Embryology Implantation. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Implantation

What Links Here?

[1] <pubmed>20729534</pubmed>

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