inner cell mass

**Development/timeline** - Fertilization (week 1, ampulla of the fallopian tube) -> zygote -> cleavage -> morula (16-cell solid ball, ~day 3-4) -> blastocyst (~day 4-5). - The blastocyst has a fluid-filled cavity (blastocoel), an outer cell layer (trophoblast), and an inner cell mass (embryoblast) clustered at one pole (the embryonic pole). - Hatching from the zona pellucida occurs ~day 5-6, followed by implantation at ~day 6-7 (the embryonic pole attaches to the endometrium). Implantation occurs in the posterior superior uterine wall in the normal case. - Uterine/endometrial stage: secretory phase (progesterone), with decidualization.

**Fate of the inner cell mass** - Week 2 (bilaminar disc): the inner cell mass differentiates into two layers: - Epiblast (high columnar cells, dorsal): forms the amniotic cavity floor; gives rise to all three germ layers (ectoderm, mesoderm, endoderm) via gastrulation in week 3 -> the entire embryo proper. It also gives rise to the amnion. - Hypoblast (low cuboidal cells, ventral): forms the primary (then secondary) yolk sac; contributes to extraembryonic endoderm. - Extraembryonic mesoderm, which forms the chorionic cavity lining, connecting stalk (-> umbilical cord), and others, is derived from the epiblast/yolk sac region. - Gastrulation (week 3), initiated by the primitive streak, converts the bilaminar disc into a trilaminar disc.

**Trophoblast (contrast; commonly tested)** - Outer cell layer of the blastocyst. Differentiates into: - Cytotrophoblast (inner, mitotically active, cellular) - Syncytiotrophoblast (outer, multinucleated, invasive; secretes hCG, which maintains the corpus luteum -> progesterone until the placenta takes over at ~8-10 weeks) - Forms the fetal portion of the placenta (chorionic villi). Decidua basalis is the maternal portion. - hCG is detectable in maternal blood ~1 week after fertilization (around the time of implantation) and in urine at ~2 weeks.

**Clinical significance** - Pluripotency: embryonic stem cells (ESCs) are derived from the inner cell mass. Pluripotent cells can form all three germ layers but cannot form trophoblast/placental tissue (totipotent cells of the zygote through the early morula can). - IVF/preimplantation genetic testing: trophectoderm (future trophoblast) biopsy at the blastocyst stage is preferred over biopsy of the inner cell mass, so the embryo proper is not damaged. Because trophectoderm is sampled, results may not perfectly reflect the inner cell mass (mosaicism, confined placental mosaicism). - Twinning: - Splitting of the morula (days 0-3): dichorionic/diamniotic monozygotic twins. - Splitting of the inner cell mass (days 4-8): monochorionic/diamniotic twins (most common type of monozygotic twinning, ~70-75%). - Splitting of the bilaminar disc (days 8-12): monochorionic/monoamniotic twins. - Splitting after ~day 13: conjoined twins. - Teratogen sensitivity: the "all-or-none" period (weeks 1-2) - exposure either kills the embryo or it recovers fully, because cells are still undifferentiated. Organogenesis (weeks 3-8) is the period of highest teratogenic risk. - Ectopic pregnancy: implantation outside the uterine cavity (most commonly the ampulla of the fallopian tube), often related to PID/scarring, a prior ectopic, or IUD use. Presents with abdominal pain, amenorrhea, and vaginal bleeding; confirm with beta-hCG and transvaginal ultrasound. - Hydatidiform mole: abnormal trophoblastic proliferation (complete mole: 46,XX, entirely paternal, empty egg + sperm duplication, no fetal tissue; partial mole: 69,XXY, 2 sperm + 1 egg, with fetal parts). In a complete mole, the inner cell mass does not develop (no embryo), and the mole shows a "honeycomb"/"cluster of grapes" ultrasound appearance and very high hCG.

**High-yield mnemonic** - "Embryoblast -> Embryo; Trophoblast -> Tissue that feeds (placenta)." - Epiblast = "Epi = up/above" (faces the amniotic cavity), Hypoblast = "Hypo = below" (faces the yolk sac).