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AN78.1-5 | Second week of development — Part 1

Implantation — How the Embryo Buries Itself (AN78.3)

Implantation is the process by which the blastocyst attaches to and embeds itself within the uterine endometrium. It begins on Day 6–7 and is essentially complete by Day 10–11. Understanding implantation requires knowing three things: where it happens, when it happens, and how it happens.

Figure: Implantation — How the Embryo Buries Itself (AN78.3)

Where: The Normal Site

The normal site of implantation is the posterior wall of the body (fundus) of the uterus, slightly closer to the midline. Why here? Because the blood supply is richest in this region, and the endometrium is thickest. The embryo is not randomly landing — the endometrial surface expresses adhesion molecules (integrins, selectins) and secretes chemokines that guide the blastocyst to the optimal site. This is called the implantation window, a brief period (Days 20–24 of the menstrual cycle, corresponding to Days 6–10 post-fertilisation) during which the endometrium is receptive.

Outside this window, the endometrium actively rejects attachment. This is why timing is everything — in IVF clinics, embryo transfer is meticulously timed to coincide with the implantation window.

When: The Timeline

• Day 6: The blastocyst, having shed its zona pellucida, makes initial contact with the endometrial epithelium. This is called apposition — the blastocyst loosely sits on the surface. The embryonic pole (the side with the inner cell mass) faces the endometrium.
• Day 7: Adhesion — the trophoblast cells at the embryonic pole firmly attach to the endometrial epithelial cells via integrin receptors. This is the point of no return.
• Days 7–9: Invasion — the trophoblast actively digests its way through the epithelium and into the underlying stromal tissue. The syncytiotrophoblast (which we'll discuss in the next section) is the invasive front, secreting enzymes (matrix metalloproteinases) that break down the extracellular matrix.
• Days 9–10: The blastocyst is completely embedded within the endometrial stroma. The entry point on the surface is sealed by a fibrin plug called the closing plug (or coagulation plug).
• Days 10–12: The endometrial epithelium regenerates over the closing plug, and the blastocyst is now entirely enclosed within the endometrium — interstitial implantation is complete.

How: The Three Phases

Implantation proceeds through three overlapping phases:

1. Apposition — The blastocyst orientates itself so that the embryonic pole (inner cell mass side) faces the endometrium. The trophoblast at this pole will be the first to invade. Finger-like projections on the endometrial surface called pinopodes (from Greek: pino = drink, podes = feet) help anchor the blastocyst.

2. Adhesion — Molecular cross-talk between trophoblast and endometrium: L-selectin on the trophoblast binds to carbohydrate ligands on the endometrium (similar to how white blood cells stick to blood vessel walls). Then integrins (especially alpha-v-beta-3) form firm attachments.

3. Invasion — The syncytiotrophoblast extends finger-like processes between endometrial epithelial cells, pushes through the basement membrane, and invades the stroma. It erodes maternal capillaries and endometrial glands, creating pools of maternal blood called lacunae (Latin: lakes) — these are the earliest precursors of the placental blood spaces.

Clinical Significance: Abnormal Implantation Sites

When the blastocyst implants anywhere other than the uterine body, it is called an ectopic pregnancy (from Greek: ek = out, topos = place). The most common abnormal sites are:

• Tubal (95% of ectopics) — usually in the ampulla of the fallopian tube. The tube cannot expand like the uterus, so the growing embryo causes the tube to rupture, leading to life-threatening internal haemorrhage. This is a surgical emergency.
• Cervical — rare but dangerous; heavy bleeding risk during attempted removal.
• Ovarian — the embryo implants on the surface of the ovary.
• Abdominal — the rarest; the embryo implants on the peritoneum, bowel surface, or omentum. Occasionally, abdominal pregnancies can progress to advanced stages, but they are always high-risk.

Placenta praevia — even when implantation occurs inside the uterus, if it happens too low (near the internal os of the cervix), the resulting placenta can cover the cervical opening, causing painless but torrential bleeding in the third trimester. This is placenta praevia — one of the most important obstetric emergencies you'll encounter.

Trophoblast Differentiation — The Two Layers That Build the Placenta (AN78.2)

The trophoblast is the outer cell layer of the blastocyst — the shell that surrounds the embryo. During the second week, it differentiates into two distinct layers, and understanding these layers is the key to understanding how the placenta forms.

Figure: Trophoblast Differentiation — The Two Layers That Build the Placenta (AN78.2)

The Two Layers

As the blastocyst begins to implant (Day 7–8), the trophoblast at the embryonic pole differentiates into:

1. Cytotrophoblast (inner layer) — a layer of distinct, well-defined cells with clear cell boundaries. Think of these as the stem cells of the trophoblast — they are mitotically active (they keep dividing) and continuously feed new cells into the outer layer. The prefix cyto- means cell, reflecting that these are clearly individual cells.

2. Syncytiotrophoblast (outer layer) — a continuous, multinucleated mass without cell boundaries. Individual cytotrophoblast cells fuse together to form this syncytium (from Greek: syn = together, cytium = cell). Imagine it as a living, expanding carpet of fused cells. The syncytiotrophoblast is the invasive front — it is the tissue that actually burrows into the endometrium.

Here is the critical relationship: the cytotrophoblast divides and feeds cells outward. Those cells fuse into the syncytiotrophoblast, which invades the maternal tissue. The syncytiotrophoblast itself does not divide — it grows only by incorporating new cells from the cytotrophoblast. This is an important exam point.

What the Syncytiotrophoblast Does

The syncytiotrophoblast is remarkably aggressive and multifunctional:

a) Invasion — It secretes matrix metalloproteinases (MMPs) that digest the extracellular matrix of the endometrium. It pushes between and through endometrial cells, eroding everything in its path — epithelium, stroma, blood vessel walls, and gland walls.

b) Lacunar formation — By Days 8–9, small spaces called lacunae (singular: lacuna) appear within the syncytiotrophoblast. These are initially filled with tissue fluid and secretions from eroded endometrial glands. By Days 11–12, the syncytiotrophoblast has eroded maternal capillaries and venous sinusoids, and maternal blood now flows into the lacunae. This creates the lacunar stage — the earliest form of the uteroplacental circulation. Blood flows in from eroded arteries, bathes the lacunae, and drains out through eroded veins. This is the beginning of the haemochorial type of placenta (where maternal blood directly contacts the trophoblast — no endothelial barrier between them).

c) Hormone production — The syncytiotrophoblast produces human chorionic gonadotropin (hCG) starting around Day 8–9. This hormone enters the maternal blood through the lacunae and signals the corpus luteum in the ovary to keep producing progesterone. Without hCG, the corpus luteum would degenerate, progesterone would fall, and the endometrium would shed — ending the pregnancy. This is the rescue signal that maintains the pregnancy through the first trimester.

hCG is also the basis of all pregnancy tests. Urine pregnancy tests detect hCG, which becomes detectable in maternal urine around Day 10–12 (roughly when the next period would have been expected). Blood tests (serum beta-hCG) can detect it even earlier.

The Trophoblast Shell

By the end of the second week, the syncytiotrophoblast completely surrounds the embryo, and a network of lacunae is established. The cytotrophoblast forms a complete inner lining. Together, these two layers — with their lacunae — constitute the primitive trophoblastic shell. This shell will later develop finger-like projections called chorionic villi (in the third week), which massively increase the surface area for nutrient exchange.

Why Does the Trophoblast Not Get Rejected?

The embryo is genetically half-paternal — it should, in theory, be rejected by the mother's immune system as foreign tissue. But the syncytiotrophoblast expresses a unique set of HLA molecules (HLA-G instead of HLA-A and HLA-B) that suppress maternal immune attack. It also secretes immunosuppressive cytokines. This is an active area of research in reproductive immunology. When this tolerance fails, it may contribute to recurrent miscarriage and pre-eclampsia.

From Blastocyst to Bilaminar Disc — The Day-by-Day Sequence (AN78.1)

The NMC competency AN78.1 asks you to describe "cleavage and formation of blastocyst" — you covered cleavage and morula formation in the first week. Here, we complete the story by tracing what happens to the blastocyst after it begins to implant, day by day.

Figure: From Blastocyst to Bilaminar Disc — The Day-by-Day Sequence (AN78.1)

Day 7–8: Implantation Begins, Embryoblast Reorganises

As the trophoblast begins invading the endometrium (covered above), the inner cell mass (embryoblast) — the cluster of cells that will become the actual embryo — undergoes its own transformation. The cells reorganise into two distinct layers:

• Epiblast — a layer of tall columnar cells facing the trophoblast (dorsal side). These cells will give rise to all three germ layers of the embryo during gastrulation in the third week. The epiblast is the source of the entire embryo proper.
• Hypoblast (also called the primitive endoderm) — a layer of small cuboidal cells facing the blastocyst cavity (ventral side). The hypoblast will contribute to extraembryonic membranes but does NOT contribute to the embryo itself.

Together, the epiblast and hypoblast form the bilaminar embryonic disc — a flat, two-layered plate. This is the "disc" in "bilaminar germ disc," and it is the structural plan from which the entire body will develop.

Day 8: The Amniotic Cavity Appears

Small spaces appear within the epiblast and coalesce to form the amniotic cavity — a fluid-filled space between the epiblast and the overlying cytotrophoblast. The epiblast cells that line this cavity become the amnioblasts, forming the amnion — the membrane that will eventually surround the entire fetus as the "bag of waters." When a woman's "waters break" before delivery, it is the amnion that ruptures.

Day 9: The Blastocyst is Fully Embedded

The blastocyst is now completely buried within the endometrial stroma. The surface defect is sealed by the closing plug. The trophoblast is well-differentiated into cytotrophoblast and syncytiotrophoblast, with lacunae forming in the syncytiotrophoblast.

Meanwhile, the hypoblast begins to spread. Cells from the hypoblast migrate along the inner surface of the cytotrophoblast, forming a thin membrane called Heuser's membrane (or the exocoelomic membrane). This membrane, together with the hypoblast, lines the blastocyst cavity, converting it into the primary yolk sac (also called the exocoelomic cavity).

Day 10–11: The Prochordal Plate and the Body Axes

At one end of the bilaminar disc, the epiblast and hypoblast cells are particularly adherent — they are tightly fused without any intervening space. This region is the prochordal plate (also called the prechordal plate). Its significance is enormous:

• It marks the future cranial (head) end of the embryo — the very first indication of head-to-tail polarity
• It is the site where the buccopharyngeal membrane will form (the future mouth)
• It is a region where mesoderm will NOT form during gastrulation (remaining bilaminar), creating the oropharyngeal membrane

The prochordal plate therefore establishes the craniocaudal axis of the embryo before any other axial structure appears.

Day 12: Lacunar Circulation Established

Maternal blood enters the lacunae as the syncytiotrophoblast erodes spiral arteries. The primitive uteroplacental circulation begins — maternal blood flows through the lacunar networks. This is when the embryo transitions from histotrophic nutrition (absorbing secretions from eroded glands) to haemotrophic nutrition (receiving nutrients from maternal blood).

Days 13–14: The "Grand Rearrangement"

Several major events happen almost simultaneously:

• The primary yolk sac is pinched off and replaced by the smaller secondary (definitive) yolk sac
• The remnants of the primary yolk sac persist briefly as exocoelomic cysts before degenerating
• The extraembryonic mesoderm forms (covered in detail in the next section)
• The chorionic cavity appears (also next section)
• Primary chorionic villi begin to form — columns of cytotrophoblast pushing into the syncytiotrophoblast

By the end of Day 14, the embryo is a bilaminar disc suspended by a stalk within a fluid-filled cavity, surrounded by trophoblast that is bathed in maternal blood. It is ready for the dramatic events of the third week — gastrulation.