Implantation (Days 6 - 10)

Soon after the morula enters the uterine cavity, uterine fluid enters the compact ball of cells to create a fluid-filled cavity within the ball. This space is called the blastocystic cavity and it pushes all of the inner cells to one side, creating an inner cell mass called the embryoblast (which develops into the embryo) along with the spherical shell called the trophoblast (which develops into the placenta). This process occurs roughly 4 days after fertilisation, and once the cavity is formed the morula is called a blastocyst. Over the next couple of days floating in the uterine cavity the zona pellucida surrounding the blastocyst degenerates, allowing it to increase rapidly in size using uterine gland secretions for nourishment.

The blastocyst settles on and attaches to the endometrial epithelium of the uterus 6 days after fertilisation. Once attached, the trophoblast rapidly proliferates and differentiates into an inner layer of cells called the cytotrophoblast and an outer layer of multi-nucleated cells called the syncytiotrophoblast. The inner cytotrophoblast proliferates continuously, the cells of which migrate to the syncytiotrophoblast. The outer syncytiotrophoblast layer possesses finger-like projections which invade the endometrial tissue to anchor the blastocyst to the uterine wall and seek nourishment for the embryo. The cells in this layer secrete proteolytic enzymes which erode the endometrial tissue to enable the blastocyst to slowly embed itself into the endometrium, as well as digesting decidual cells in the endometrium which are loaded with glycogen and lipids for embryonic nutrition. The syncytiotrophoblast also produces human chorionic gonadotrophin (hCG), which prevents degeneration of the corpus luteum and therefore maintains a steady production of progesterone – the hormone responsible for maintaining the uterine lining and preventing spontaneous abortion. Human chorionic gonadotrophin is the hormone which is detected in pregnancy tests.

Day 8Day 9

A = implanting blastocyst at Day 6, B = implanting blastocyst at Day 7

By the 8th day after fertilisation the inner embryoblast starts to differentiate into a flattened bilaminar embryonic disc. This disc is comprised of two layers: an upper thicker single layer of cells called the epiblast, and a lower thinner single layer of cells called the hypoblast. As this occurs a second small cavity appears in the blastocyst between the epiblast and the embedded cytotrophoblast. This is known as the amniotic cavity, surrounded by the amnion. The blastocystic cavity becomes the exocoelomic cavity surrounded by the exocoelomic membrane, which itself is surrounded by the cytotrophoblast.

By the 9th day the exocoelomic cavity and membrane are modified to become the primary umbilical vesicle, formerly known as the yolk sac. A layer of loose connective tissue forms between the cytotrophoblast layer surrounding the blastocyst and the primary umbilical vesicle/amniotic cavity, known as extraembryonic mesoderm. This mesoderm is formed by cells from the hypoblast which have migrated and differentiated into mesoderm, and later forms the chorionic sac. Whilst the extraembryonic mesoderm is developing, small cavities form in the invading syncytiotrophoblast called lacunae which are filled with a mixture of cellular debris from assimilated uterine glands and maternal blood from eroded endometrial capillaries. These lacunae provide nutrition for the blastocyst via diffusion.

Implantation 2Implantation 3

A = an implanting blastocyst on Day 9 B = an implanting blastocyst on Day 9
A = an implanted blastocyst on Day 10 B = an implanted blastocyst on Day 12

By the 10th day the blastocyst is fully embedded in the endometrium. There is a small hole left in the endomedtrium behind the blastocyst from where it began to burrow in, which is corked by a closing plug which is essentially a blood clot. After 2 days the plug disappears and the endometrial epithelium is regenerated.

Implantation is now fully complete. During the rest of the second week of development the lacunae fuse to form lacunar networks, the extraembryonic mesoderm divides into 2 to form the chorionic cavity, and primary chorionic villi begin to form. See the timeline for more information on these events.

An animation of the events occurring during implantation