Implantation

Introduction

The term used to describe process of attachment and invasion of the uterus endometrium by the blastocyst (conceptus) in placental animals. Abnormal implantation is where this process does not occur in the body of the uterus (ectopic) or where the placenta forms incorrectly.

--Mark Hill 19:10, 5 August 2009 (EST) Page under development - notice removed when completed.

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

Week 1 and 2 Human Development Overview

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

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

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Cite this page: Hill, M.A. (2024, May 6) Embryology Implantation. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Implantation

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G