Implantation is a crucial step in mammalian reproduction, as it is a gateway to further embryonic development and successful pregnancy. Successful implantation requires an intricate succession of molecular and genetic interactions.
These reciprocal interactions between the embryo and the uterus must be executed within a limited time (Psychoyos, 1986) known as the “window of implantation” or “window of receptivity”. Any breach in the communication between the endometrium and the embryo during this time leads to implantation failure. Implantation failure is an unsolved problem in reproductive medicine and is considered to be an important cause of infertility.
The early embryo enters the uterine cavity as an 8 cell morula and becomes a 30-200 cell blastocyst before implantation. The process of implantation begins with apposition and adhesion of the blastocyst to the uterine epithelium. This occurs 2-4 days after the morula enters the uterine cavity. The process is mediated by cytokines and involves adhesion molecules (integrins) that interact with the extracellular components eg. Laminin and firbronectin.
The embryo hatches from the zona pellucida 1-3 days after the morula enters the endometrial cavity. The endometrium is prepared for implantation by the complex activity of cytokines, growth factors and lipids. These factors are modulated by the sex hormones (mainly progesterone). The endometrium has a receptivity window of only a few days.
Hatching leaves the blastocyst free of the zona pellucida. It should have occured approximately at the end of the uterine tube or in the body of the uterus. The embryo floats in the uterine glands rich mucus secretion and able to directly access this nutrition for continued growth.
The blastocyst initially weakly adheres to the endometrial wall rolling across its surface. Increased adhesion may lead to attachment, adplantation, on the inner cell mass side of the blastocyst. This will be the site where implantation will begin and the placenta will develop.
Trophoblast cells at the site of adplantation proliferate and form an additional layer the syncitiotrophoblast layer. This layer of cells rapidly divide, secrete enzymes that degrade the endometrial extracellular matrix and secrete human Chorionic Gonadotropin (hCG). Trophoblastic invasion rapidly follow adhesion of the blastocyst. This is mediated by proteinase degradation of the extracellular matrix. The placenta is formed in the second week after ovulation.
• When is a urinary bhcg first positive? : 3 days before the period is due –Timing (early morning or evening etc.) affects result –Sensitive to about 25units/litres using a monoclonal antibody to the beta subunit of HCG.
• When is the blood test bhcg positive? : 8 days after ovulation –1 day after implantation –A positive test by the time of the expected period will be 98% correct.
• When is the trigger hcg out of the patients system? : By day 10 post injection.
• Which day of the cycle is the first blood test usually done on? : Day 11 after blastocyst (Day 5) transfer.
• What is the usual range of the first blood test? : 50-200
• When is the second blood test done, and what is the expected level? : 48 hours after the initial blood test. –The bhcg doubles every 48hours in a normally developing pregnancy
Implantation in a nonuterine site occurs at a rate of about 0.25 – 1%. The ampulla of the uterine tube is the most common ectopic implantation site.
In vitro fertilization
The process by which one or more eggs (oocytes) are fertilised outside the body. Fertilisation is either achieved by placing a droplet of washed sperm (~50,000) onto each egg or if the sperm count is low a single sperm can be injected into each egg (Intra-Cytoplasmic Sperm Injection – ICSI).
Embryos can be placed into the uterus at the 6-8 cell stage (3-days culture) but some clinics culture the embryos for 5 or 6 days to ensure healthy blastocyst stage embryos are placed into the uterus.
IVF and Preimplantation Genetic Diagnosis
All cells in the early embryo (until about the 8-cell stage) are said to be totipotent. That means that each cell is capable of forming a complete human. So couples using IVF can use genetic screening of their embryos by having a cell removed from their embryo(s) and tested for its genotype. The embryo will still develop normally. More than 100 diseases can be detected including hemophilia A, muscular dystrophy, Tay-Sachs disease, cystic fibrosis and Down syndrome.
Stem Cell Research
Stem cells are unspecialized cells that can renew themselves for long periods through cell division. Under certain experimental conditions, they can be induced to become cells with special functions such as the beating cells of the heart muscle or the insulin-producing cells of the pancreas.
Human embryonic stem cells are obtained from the inner cell mass – the early embryo. In the USA it is forbidden to destroy a human embryo to obtain stem cells. So they are not allowed to remove the inner cell mass or totipotent cells. There is interest in the possibility that at the 8-cell stage the cells are no longer totipotent but are still pluripotent (i.e. can form the 240-/+ human cell types).
Multiple Gestations- TWINS
About 1:80 human births (0.8%) is a twin pregnancy. About 2/3 of twins are fraternal (dizygotic) – derived from 2 eggs. One third (1:250) are identical twins (monozygotic). 35% of MZ twins divide between 2-8 cell stage, get two babies with two amnions, two chorions, and either one fused or two separate placentas cannot tell difference between these twins and fraternal until genetic testing is done.
65% of MZ twins occur by division of the inner cell mass after first week. At this point two embryos will develop with two amniotic sacs, one chorionic sac and a common placenta. IVF has greatly increased the number of fraternal (dizygotic) twins and in 2001 the rate of twins in the USA was 3%.
Separation at the two blastomere stage can lead to twinning. Splitting of the inner cell mass is the most common cause of identical twins. Incomplete separation of the inner cell mass can lead to conjoined twins.