AN78.1-5 | Second week of development — Summary & Reflection

REFLECT

Before reading the summary, test yourself:

1. Draw a cross-section of the conceptus at Day 14, labelling: syncytiotrophoblast, cytotrophoblast, somatic extraembryonic mesoderm, chorionic cavity, amnion, amniotic cavity, epiblast, hypoblast, yolk sac, splanchnic extraembryonic mesoderm, connecting stalk, lacunae with maternal blood. (If you can draw and label this from memory, you understand the second week.)

1. A woman's urine pregnancy test is positive, but transvaginal ultrasound shows no gestational sac in the uterus. What are the two most important diagnoses to consider, and what blood test would you order to distinguish them?

1. Explain to a first-year student why the second week is called the "Week of Twos" — give at least four examples.

1. A child is born with a membrane covering the back of the pharynx that must be surgically removed to open the oral cavity. Which embryological structure persisted abnormally, and which Week 2 structure defines its cranial boundary?

1. Why does the syncytiotrophoblast not divide mitotically? Where does it get new cells from?

KEY TAKEAWAYS

Key Takeaways — Second Week of Development

Implantation (AN78.3):
- Normal site: posterior wall of uterine fundus
- Three phases: apposition → adhesion → invasion (Days 6–12)
- Implantation window: Days 20–24 of menstrual cycle
- Ectopic pregnancy: 95% tubal (ampulla most common); tubal rupture is a surgical emergency
- Placenta praevia: low implantation near internal os → third-trimester bleeding

Trophoblast (AN78.2):
- Cytotrophoblast (inner): divides, feeds cells to the outer layer
- Syncytiotrophoblast (outer): invades, does NOT divide, produces hCG, forms lacunae
- Lacunae → primitive uteroplacental circulation (haemochorial placenta)
- hCG rescues the corpus luteum → maintains progesterone → maintains pregnancy

Bilaminar Disc & Blastocyst Events (AN78.1):
- Embryoblast → epiblast (dorsal, all germ layers) + hypoblast (ventral, extraembryonic)
- Amniotic cavity from epiblast; primary → secondary yolk sac from hypoblast
- Prochordal plate = cranial end marker, site of future mouth

Extraembryonic Mesoderm (AN78.4):
- Forms by Day 12, splits into somatic (lines trophoblast → chorion) and splanchnic (covers yolk sac)
- Extraembryonic coelom = chorionic cavity
- Connecting stalk = future umbilical cord
- Two body axes established: dorsoventral + craniocaudal

Clinical Correlates (AN78.5):
- Decidual reaction: stromal cells → decidual cells (glycogen + lipid storage)
- Decidua basalis (under embryo → placenta), capsularis (over embryo), parietalis (rest)
- hCG detectable in blood by Day 8–10, urine by Day ~14 post-fertilisation
- Doubling time of hCG: every 48–72 hours (slower rise → suspect ectopic)
- ~50–60% of fertilised eggs fail to implant → most pregnancy losses are subclinical

The "Week of Twos": two trophoblast layers, two embryonic disc layers, two cavities, two extraembryonic mesoderm layers, two nutrition modes.