Q.88 Extraembryonic tissues are derived from which one of the following?
(A) Ectoderm
(B) Endoderm
(C) Trophectoderm
(D) Mesoderm
Extraembryonic tissues are derived from the trophectoderm. This outer layer of the blastocyst differentiates into structures like the placenta and chorion, supporting embryonic development without contributing to the fetus itself.
Options Explained
Ectoderm (A)
Ectoderm forms the embryonic nervous system, epidermis, and neural crest derivatives. While extraembryonic ectoderm (e.g., amnion in mice) arises later from epiblast (ectodermal), primary extraembryonic tissues like trophoblast originate earlier from trophectoderm, not ectoderm.
Endoderm (B)
Endoderm gives rise to embryonic gut, lungs, and liver. Extraembryonic endoderm (yolk sac, parietal endoderm) derives secondarily from primitive endoderm (hypoblast), which comes from inner cell mass, not the initial extraembryonic lineage.
Trophectoderm (C)
The trophectoderm is the blastocyst’s outer epithelial layer that directly forms the trophoblast lineage, producing placenta, chorion, and other placental structures. This is the primary origin of extraembryonic tissues in mammals.
Mesoderm (D)
Mesoderm contributes to extraembryonic mesoderm (e.g., allantois, blood islands, chorionic mesoderm) but arises during gastrulation from epiblast, making it secondary. Primary extraembryonic tissues precede mesoderm formation.
Extraembryonic tissues derived from trophectoderm form the placenta and membranes essential for mammalian fetal support, a key NEET biology concept.
Blastocyst Lineage Differentiation
At the morula-to-blastocyst transition, cells polarize into trophectoderm (outer, Cdx2+ for placental fate) enclosing the inner cell mass (ICM, Nanog+ for embryo). ICM further splits into epiblast (embryo proper) and primitive endoderm (yolk sac).
| Germ Layer/Lineage | Origin | Major Extraembryonic Derivatives |
|---|---|---|
| Trophectoderm | Blastocyst outer | Placenta, chorion |
| Primitive Endoderm | ICM | Yolk sac |
| Extraembryonic Mesoderm | Epiblast (gastrulation) | Allantois, blood vessels |
| Epiblast Ectoderm | ICM | Amnion |
Developmental Timeline
-
Day 3.5 (mouse): Trophectoderm specified.
-
Implantation: Trophoblast invades uterus.
-
Gastrulation: Mesoderm adds extraembryonic components.
Why Option (C) is Correct
Primary extraembryonic tissues emerge first from trophectoderm; other contributions are secondary, confirming its defining role in standard embryology texts.
