Implantation - Anatomy & Physiology

Introduction

The term, 'implantation' is often used to describe the attachment of the placental membranes to the endometrium in most animals. True implantation is a phenomenon in rodents and humans in which the conceptus 'buries' itself in the uterine endometrium. The conceptus temporarily disappears beneath the surface. In most other species, the conceptus does not truly implant, but attaches to the endometrial surface and remains in the luminal compartment.

Growth Rates

After hatching, the conceptus undergoes massive growth.

Cow: The blastocyst is 3mm in diameter at day 13 of gestation and enlarges to 250mm in length (elongation) between day 14-16 and appears as a filamentous thread. By day 18 of gestation, the blastocyst occupies space in both uterine horns.

Pig: The blastocysts are 2mm spheres at day 10 of gestation. During the next 24-48 hours, they elongate to ~200mm in length. By day 16, the blastocyst reaches lengths of 800-1000mm.

Pre-attachment period

For most domestic mammalian embryos, the pre-attachment period within the uterus is long (several weeks). During this time, extraembryonic membranes form by a folding process that generates:

1. Amnion

2. Chorion

3. Allantochorion

In primates, the blastocyst implants very soon after it enters the uterus and extraembryonic membranes form after implantation. For further information on extraembryonic membranes, attachment, placentation and maternal recognition of pregnancy follow this link to Pregnancy and Parturition.

Implantation is critical to the survival of the embryo; the embryo must adhere and embed itself in the endometrium in order to exchange waste and nutrients. For this reason, the first differentiation of cells is into trophoblasts and syncytiotrophoblasts and the inner cell mass (ICM). Trophoblastic cells posses adhesion molecules to adhere to the endometrium, whilst ICM cells form the organism. The extent of cellular growth and division, and the time taken for implantation differs between species. Shortly after implantation, cells of the ICM differentiate into different layers:

1.Epiblast - facing towards the uterine lumen, adjacent to syncytiotrophoblasts.

2.Hypoblast - facing towards the endometrium, adjacent to the blastocyst cavity. The hypoblast gives rise to no persisting structure in the adult.

3.Amnioblasts - cells of the epiblast that migrate up and around to allow the amniotic cavity to form.

After gastrulation, the epiblast gives rise to the three germ layers:

1. Ectoderm

2. Mesoderm

3. Endoderm