#inner cell mass

Stem Cell -ll

The inner cell mass of a blastocyst is composed of:

Mouse blastocyst at embryonic day 3.5 consists of outer trophectoderm surrounding fluid cavity and inner cell mass clustered at embryonic pole expressing Oct4 and Nanog. Inner cell mass cells express Oct4, Nanog, Sox2, and are functionally pluripotent, capable of forming all somatic lineages and germ cells when tested by chimera contribution and teratoma assays, but do not generate trophoblast under normal development. After implantation, ICM diverges into epiblast and primitive endoderm. Cultured in LIF and 2i inhibitors, ICM outgrowths establish embryonic stem cell lines, confirming pluripotency not totipotency or lineage-restricted multipotency.

Ref: Evans & Kaufman, Nature 1981; Martin, PNAS 1981; Gilbert, Chapter 6: ICM pluripotency and ESC derivation.

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Stem Cell -lll

The inner cell mass (ICM) of a blastocyst contains cells that are:

Inner cell mass arises inside blastocyst at embryonic day 3.5 mouse, day 5 human, isolated from trophectoderm outer epithelium forming blastocoel. ICM cells express Oct4, Nanog, Sox2 pluripotency network, capable of differentiation into all three embryonic germ layers contributing to fetus but not efficiently to placenta, meeting pluripotency definition. Cultured ICM generates embryonic stem cells retaining pluripotency demonstrable via chimera and teratoma formation. Totipotent forms extraembryonic as well seen in zygote, multipotent limited to subset like blood, unipotent single lineage such as epidermal basal. ICM potency explains ES derivation.

Ref: Evans and Kaufman, Nature 1981, Pluripotency of Inner Cell Mass and ES Cells.

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Fertilization in mammals-ll

Inner cell mass primarily gives rise to:

Blastocyst comprises outer trophectoderm epithelium and inner cell mass cluster at embryonic pole. Inner cell mass cells remain apolar, maintain active Hippo signaling with cytoplasmic YAP, express Oct4 Nanog Sox2, and retain pluripotency. During implantation ICM splits into epiblast forming fetus plus extraembryonic mesoderm and primitive endoderm forming yolk sac. Thus ICM provides progenitors of all fetal tissues and some extraembryonic membranes. Its preservation is essential for embryonic stem cell derivation, representing source of embryonic development proper, regenerative medicine, lineage plasticity and developmental potential studies.

Ref: Moore & Persaud, The Developing Human, 11th ed., Chapter 2: Inner cell mass origin of embryo proper.

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Fertilization in mammals-lll

Inner cell mass predominantly forms:

Mammalian blastocyst consists of outer trophectoderm and cluster of cells at embryonic pole termed inner cell mass. Trophectoderm progenitors express Cdx2 and differentiate into placenta and trophoblast giant cells contacting uterus. Inner cell mass cells retain expression of Oct4, Nanog, Sox2, Klf4 maintaining pluripotency to generate all three germ layers of fetus, plus primitive endoderm giving yolk sac and amnion. Polar body elimination and zona pellucida are not embryonic lineages. Consequently inner cell mass is source of embryonic stem cells and exclusively forms embryo proper, while trophectoderm provides extraembryonic support essential for implantation and nutrient exchange.

Ref: Marikawa & Alarcon, Dev Biol 2009: Inner cell mass pluripotency and trophectoderm differentiation in blastocyst.

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