Early Embryonic Development: Morula to Trilaminar Disc

Fifty Q&A flashcards covering tight junctions, morula fluid dynamics, twin formation, osteopontin and MMP-9 roles in implantation, end of the ovum phase, and detailed steps of mammalian gastrulation including the formation of the bilaminar and trilaminar embryonic discs.

What is the primary function of tight junctions between outer morula cells?

To create an impermeable seal that prevents fluid loss from the developing embryo.

Tight junctions are composed of what type of molecules?

Large transmembrane proteins that link adjacent cell membranes and anchor to the cytoskeleton.

Why must the morula form tight junctions before fluid accumulation begins?

The sealed barrier allows fluid to collect internally without leaking, enabling cavity formation.

Which two distinct cell populations arise after the morula begins to accumulate fluid?

The inner cell mass (ICM) and the trophoblast.

The inner cell mass will eventually develop into what?

The future fetus.

The trophoblast cells will ultimately form which supporting structure?

The placenta.

Which ion is actively pumped by outer morula cells to draw water inward by osmosis?

Sodium (Na⁺).

What is the name of the endocytic process by which cells take in extracellular fluid to help fill the blastocoel?

Pinocytosis.

Despite water influx, why don’t morula cells disperse randomly?

Embryonic cells are highly organized and possess positional information guiding their arrangement.

Early versus late splitting of the ICM primarily affects which structure in identical twins?

The amniotic sac (separate sacs early, shared sac late).

Do twins produced by early ICM splitting share an amniotic sac?

No, each twin forms its own amniotic sac.

Do twins produced by late ICM splitting share an amniotic sac?

Yes, both twins occupy the same amniotic sac.

Are twins created by ICM splitting genetically identical or fraternal?

Genetically identical (clones).

Is the mechanism behind spontaneous ICM splitting fully understood?

No; genetics and epigenetics are implicated, but the exact cause remains uncertain.

Which uterine-secreted glycoprotein is crucial for blastocyst hatching and adhesion?

Osteopontin (OPN).

Name the three main functions of osteopontin in early pregnancy.

Induces blastocyst hatching, promotes blastocyst–uterine adhesion, and stimulates MMP-9 expression.

Osteopontin forms a molecular bridge between which two cell types?

Trophoblast cells and uterine endometrial epithelial cells.

Which enzyme does osteopontin stimulate the blastocyst to produce for tissue digestion?

Matrix metalloproteinase-9 (MMP-9).

What is the role of MMP-9 during implantation?

It digests the zona pellucida and uterine epithelium, creating a wound that enables implantation and vascularization.

Which embryonic cell population secretes MMP-9?

Trophoblast (trophectoderm) cells.

Implantation establishes what vital connection between embryo and mother?

A functional maternal-embryonic blood supply.

Completion of implantation marks the end of which developmental phase?

The ovum phase.

How many primary germ layers are formed during gastrulation?

Three: ectoderm, mesoderm, and endoderm.

The ectoderm eventually forms which general body structures?

Outer layers such as skin, nervous system, and hide/exoskeleton.

The mesoderm gives rise to which types of organs or tissues?

Primary organs and structures including heart, musculature, bone, and body cavities.

The endoderm primarily forms which internal structures?

Inner linings and open-cavity organs such as the gut and lungs.

List the five fundamental morphogenetic movements that create germ layers across species.

Invagination, involution, ingression, delamination, and epiboly.

Define invagination in embryology.

An infolding of a cell sheet into the embryo to form a new internal layer.

Define involution in embryology.

Inward rolling of a cell sheet that spreads over the inner surface, like turning a sock inside-out.

Define ingression in embryology.

Individual cells detach, migrate inward, and proliferate to form a new layer.

Define delamination in embryology.

Synchronous division of a cell sheet producing a parallel second layer.

Define epiboly in embryology.

Expansion and thinning of an outer cell sheet that spreads to enclose deeper layers.

Which morphogenetic movement initially creates the bilaminar disc in mammals?

Delamination.

What two layers compose the bilaminar embryonic disc?

The epiblast (upper layer) and the hypoblast (lower layer).

Formation of the bilaminar disc coincides with the appearance of which protective cavity?

The amniotic sac (amniotic cavity).

After delamination, what serves as the template for further embryonic organization?

The embryonic disc.

During the trilaminar stage, which epiblast-derived structures establish embryonic axes?

The primitive streak and primitive node.

The primitive streak first appears at which end of the embryonic disc?

The caudal (tail) end and extends toward the cranial (head) end.

Epiblast cells migrating through the primitive streak replace the hypoblast to form which germ layer?

The endoderm.

Subsequent migration of epiblast cells between endoderm and epiblast forms which germ layer?

The mesoderm.

Once the mesoderm forms, the remaining epiblast is renamed as what?

The ectoderm.

In mammals, does the hypoblast contribute to definitive adult tissues?

No; it serves as a temporary template and is replaced by definitive endoderm.

Why is a yolk sac unnecessary in mammalian embryos after implantation?

Maternal blood supply provides nutrients, eliminating the need for a large yolk reserve.

Which axis descriptors become established by the primitive streak and node?

Cranial–caudal and left–right (median axis) orientation.

What does it mean when developmental biologists say cells "know what time it is"?

Each cell follows an intrinsic developmental clock and positional cues to coordinate morphogenesis.

Which outer embryonic cell layer actively engages with maternal tissues during implantation?

The trophoblast (trophectoderm).

In the nude mouse ear graft example, what key principle of implantation is illustrated?

Successful revascularization and tissue integration following creation of a wound.

Tissue trauma during implantation stimulates nearby blood vessels to do what?

Branch and revascularize the implanting embryo or graft.

Which early embryonic event immediately precedes the onset of gastrulation in mammals?

Completion of implantation and formation of the bilaminar disc.

What is the first specialized tissue type to arise after trilaminar germ layer formation (topic for future study)?

Nervous tissue, initiated during neurulation.