Implantation- Process, Events, Significance

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The term “implantation” is used to describe the process of attachment and invasion of the uterus endometrium by the blastocyst (conceptus) in placental animals.
It begins at the end of the first week and is completed by the end of the second week of gestation.
Implantation occurs in the endometrium of the uterus superior in the body region, slightly more often on the posterior than on the anterior wall.
It involves a receptive endometrium and hormonal factors such as estrogen, progesterone, prolactin as well as cell adhesion molecules, growth factors, and HOX genes.
Further, the uterus is ready to accept the implanting embryo only during a limited period of time known as the ‘window of implantation’, outside of which the endometrium may be indifferent or even hostile to the embryo.

Image Source: Austin Community College District

Process of Implantation

Implantation is a progressive and versatile process in which the blastocyst apposes, attaches and ﬁnally invades the underlying endometrial surface.

After fertilization and cleavage, the morula reaches the uterus between three or four days of development.
By the fifth day, blastocyst hatches out of the protecting zona pellucida by enzymatically boring a hole and squeezing itself out.
Adjacent cells of the endometrial stroma respond to the presence of the blastocyst and to the progesterone secreted by the corpus luteum by differentiating into metabolically active secretory cells called decidual cells. This response is termed as the decidual reaction.
Secretions of the decidual cells and endometrial glands include metabolites that support the growth of the implanting embryo.
Subsequently, the blastocyst attaches to the endometrial epithelium.
This is the initial phase of the implantation process called “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 has firmly adhered.
As soon as it attaches, the trophoblast starts to proliferate rapidly and differentiate into:

The fingerlike processes of the syncytiotrophoblast extend through the endometrial epithelium and invade the connective tissue.
By the end of the first week, the blastocyst is superficially implanted in the compact layer of the endometrium.
Subsequently, the uteroplacental circulation develops with the formation of lacunar networks and villi.

Day 5: The zona pellucida degenerates.
Day 6: Blastocyst adheres to the endometrial epithelium.
Day 7: Trophoblast differentiates into two layers: syncytiotrophoblast and cytotrophoblast.
Day 8: Synctiotrophoblast erodes endometrial tissues, and the blastocyst starts to embed in the endometrium.
Day 9: Blood filled lacunae appear in syncytiotrophoblast
Day 10: Blastocyst sinks beneath the endometrial epithelium and defect is filled by a closing plug
Day 10 and 11: Lacunar networks form by fusion of adjacent lacunae
Day 11 and 12: The syncytiotrophoblast erodes endometrial blood vessels, allowing maternal blood to seep in and out of lacunar networks, thereby establishing a uteroplacental circulation
Days 12 and 13: The defect in the endometrial epithelium is repaired
Days 13 and 14: Primary chorionic villi develop

The implanted 12-day embryo produces a minute elevation on the endometrial surface that protrudes into the uterine lumen.

The implantation process allows the developing blastocyst to obtain nourishment and growth factors from the eroded maternal tissues.
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.
A conceptus with major genetic defects does not develop or implant correctly leading to their loss during the first and second weeks of development.
In recent years with the development of 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.

Moore, K. L., Persaud, T. V. N., & Torchia, M. G. (2008). The developing human: Clinically oriented embryology. Philadelphia, PA: Saunders/Elsevier.
Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R., Francis-West, P.H. & Philippa H. (2015). Larsen’s human embryology (5th ed.). New York; Edinburgh: Churchill Livingstone.
Sadler, T. W., & Langman, J. (2004). Langman’s medical embryology. Philadelphia, Pa: Lippincott Williams & Wilkins.
https://embryology.med.unsw.edu.au/embryology/index.php/Implantation
https://www.rbmojournal.com/article/S1472-6483(10)60771-7/pdf
https://www.studocu.com/en/document/university-of-birmingham/reproduction-and-development/lecture-notes/implantation-and-development-of-the-placenta-lecture-notes-lecture-3/577909/view