ReproductivePhysiological process

Blastocyst

Also known as: inner cell mass, embryoblast, trophoblast, blastocoel, trophectoderm

The fluid-filled stage of the early embryo (day 4–5), consisting of an outer trophoblast that will form the placenta and an inner cell mass that will form the embryo; it hatches from the zona pellucida and implants.

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Overview

Forms≈day 4–5

Hatches≈day 5–6

Once in the uterus, the trophoblast cells pump Na⁺ (Na⁺/K⁺-ATPase) into the intercellular spaces and water follows, forming a cavity — the blastocoel. The embryo is now a blastocyst: an outer epithelium, the trophoblast (trophectoderm), surrounding the cavity, and a cluster of cells at one pole, the inner cell mass (embryoblast). The side bearing the inner cell mass is the embryonic pole, which usually leads at implantation.

The trophoblast gives rise to the chorion and the fetal part of the placenta. The inner cell mass is pluripotent and gives rise to the embryo proper as well as the amnion, yolk sac, allantois and extraembryonic mesoderm; cultured inner-cell-mass cells are embryonic stem cells. By day 5–6 the blastocyst (≈100–200 cells) hatches from the zona pellucida, freeing it to adhere to the endometrium. Before implantation it is nourished by uterine gland secretions.

Location: Uterine cavity, ≈days 4–6 after fertilisation.

Parts & subdivisions

Trophoblast
Outer layer; forms the cytotrophoblast and syncytiotrophoblast, chorion and placenta.
Inner cell mass
Pluripotent embryoblast; forms the epiblast and hypoblast (bilaminar disc) in week 2.
Blastocoel
Fluid-filled cavity.
Zona pellucida
Shed at hatching (day 5–6).

Function

Establishes the embryonic and trophoblast lineages.
Hatches and initiates implantation.

Clinical significance

Blastocyst culture in IVF

Embryos are commonly cultured to day 5 and a single blastocyst transferred; trophectoderm cells can be biopsied for preimplantation genetic testing without harming the inner cell mass.

Test yourself

When does the blastocyst normally begin to implant, and where?

Day 1–2, in the ampulla
Day 3–4, in the isthmus
Day 6–7, usually in the posterior wall of the uterine body
Day 14, in the cervix

Answer: Day 6–7, usually in the posterior wall of the uterine body

After hatching from the zona pellucida, the blastocyst begins to implant ≈6–7 days after fertilisation, usually in the upper posterior (or anterior) wall of the uterine body; implantation is complete by ≈day 10–12.