Xenopus and Chick Embryogenesis: Key Developmental Stages and Signaling Pathways

Where is the oocyte nucleus positioned in Xenopus?

At the animal pole.

What establishes the dorsal-ventral axis in Xenopus embryos?

Sperm entry at the ventral side.

What is the grey crescent in Xenopus development?

A region formed by cortical rotation that marks the future dorsal side.

What key maternal factors are involved in mesoderm induction in Xenopus?

VegT and Vg-1.

What role does X-Wnt-11 signaling play in Xenopus embryos?

It stabilizes β-catenin in dorsal cells, activating genes crucial for organizer formation.

What happens when the Nieuwkoop Center is transplanted to the ventral side of a host embryo?

It can induce a secondary axis, resulting in a twinned embryo.

What is the function of the Nieuwkoop Center in Xenopus development?

It induces the Spemann organizer, which orchestrates neurulation and axial patterning.

How do chick embryos differ from Xenopus in terms of germ layer formation?

Chick embryos do not have predetermined germ layers; cells mix and proliferate before gastrulation.

What initiates germ layer specification in chick embryos?

The formation of the primitive streak as epiblast cells ingress at Hensen's node.

What is the role of the yolk balancer in chick embryos?

It helps maintain embryo orientation.

What structures are contained in a laid chick egg?

Blastoderm, yolk, egg white (albumen), air sac, vitelline membrane, and shell membranes.

What is the timeline for cleavage in chick embryos?

Cleavage begins immediately post-fertilization.

When does gastrulation start in chick embryos?

Around 6 hours after laying.

What forms by 16 hours in chick embryogenesis?

The primitive streak and Hensen's node.

When does organogenesis begin in chick embryos?

By Stage 4.

How many somites are formed by Stage 14 in chick development?

22 somites.

When does hatching occur in chick embryos?

Around Day 21.

What is the subgerminal space in chick embryos?

A space that forms beneath the blastoderm.

What is the area pellucida in chick embryos?

The part of the blastoderm that becomes the embryo.

What is the area opaca in chick embryos?

The part of the blastoderm that surrounds the area pellucida.

What are the two layers of the chick blastoderm?

The epiblast and hypoblast layers.

What does the epiblast give rise to during gastrulation?

All three germ layers: ectoderm, mesoderm, and endoderm.

What are Koller's sickle and the posterior marginal zone (PMZ) responsible for?

They are key signaling centers that initiate primitive streak formation.

What is the role of Hensen's node in embryonic development?

It acts as an organizer for anterior-posterior axis development and head structures.

What are the two distinct regions formed in the chick blastoderm?

The area pellucida and area opaca.

How is the dorsal-ventral axis determined in the chick embryo?

By the epiblast's position relative to the yolk; dorsal is away from the yolk, ventral is adjacent.

What do transplantation experiments reveal about PMZ cells?

They can induce a secondary primitive streak in a host blastoderm, confirming their organizer role.

Which signaling molecules do epiblast cells express to activate Nodal signaling?

Vg1 and Wnt8c.

What role does Koller's sickle play in signaling?

It contributes FGF signals that promote Nodal expression.

What does the hypoblast secrete that acts as a Nodal antagonist?

Cerberus.

What are TGF-β family members like Activin and Nodal responsible for?

They activate Smad2/3 and Smad4 transcription factor complexes, regulating gene expression for germ layer specification.

Which BMPs signal through Smad1/5/8?

BMPs 2, 4, 6, 7, 9, and 10.

What happens to Hensen's node during early development?

It progresses anteriorly and then regresses posteriorly, laying down the notochord and head process.

What follows the regression of Hensen's node?

Somite formation occurs in a rostro-caudal sequence.

What extraembryonic structures develop during embryogenesis?

The amnion, chorion, yolk sac, and allantois.

What type of cleavage occurs in human embryogenesis?

Holoblastic and radial cleavage.

When does implantation occur in human embryogenesis?

Around day 14.

When does gastrulation begin in human development?

At week 3.

What significant developmental processes occur between weeks 4-8 in humans?

Folding and organogenesis.

When does significant growth and maturation of the fetus occur?

By week 20.

When does birth typically occur in human development?

Around week 38.

What is the initial stage of human development called?

Fertilization, forming a zygote.

What process occurs after fertilization to produce a morula?

Cleavage, resulting in a 2-cell, 4-cell, and eventually an 8-cell morula.

What occurs by Day 4 of human development?

Compaction, forming a compacted morula.

What is formed by Day 5 in human development?

The blastocyst, consisting of trophectoderm and inner cell mass (ICM).

When does hatching from the zona pellucida occur?

Around Days 6-7.

What happens during implantation?

The blastocyst implants into the uterine endometrium by Days 8-9.

What layers does the inner cell mass (ICM) differentiate into?

Epiblast and hypoblast layers, forming the embryonic disc and amniotic cavity.

What are monozygotic twins?

Identical twins that occur when a single embryo splits.

What is the occurrence rate of monozygotic twins?

Approximately 3 out of 1000 births.

What is the outcome if the embryo splits before Day 5?

Twins typically have separate placentas and amniotic sacs (33%).

What happens when splitting occurs between Days 5-9?

Twins share a chorion but have separate amnions (66%).

What can result from late splitting after Day 9?

Conjoined twins.

What are dizygotic twins?

Fraternal twins that arise from two separate fertilized eggs.

How do dizygotic twins differ from monozygotic twins?

Dizygotic twins always have separate placentas and sacs.

What happens during trophoblast differentiation during implantation?

It differentiates into cytotrophoblast and syncytiotrophoblast layers.

What is the role of extravillous trophoblasts?

They anchor the placenta and remodel maternal spiral arteries to increase blood flow.

What is the significance of trophoblast invasion in normal pregnancy?

It supports fetal development by enhancing placental perfusion.

What can impaired trophoblast invasion lead to?

Poor placental perfusion and maternal hypertension, as seen in severe preeclampsia.

Why are mouse embryos used as a model for developmental studies?

They have rapid generation times, large litters, and genetic tractability.

What key developmental stages occur in mouse embryos?

Cleavage, compaction, blastocyst formation, and implantation.

What forms by 5.5 days in mouse development?

The egg cylinder, comprising trophectoderm, extraembryonic ectoderm, visceral and parietal endoderm, and epiblast.

When does gastrulation begin in mouse development?

Around Day 6.

What follows gastrulation in mouse development?

Turning and organogenesis.

What role does the distal visceral endoderm (DVE) play in early mouse development?

The DVE secretes Wnt and Nodal antagonists such as Lefty-1 and Cerberus-like 1 to restrict Nodal signaling.

What happens by embryonic day 6 (E6) in mouse development?

The anterior visceral endoderm (AVE) migrates to establish anterior identity.

What initiates primitive streak formation and gastrulation in mouse embryos?

The formation of the node at the posterior.

What emerges as cells ingress through the primitive streak during gastrulation?

The embryonic endoderm.

What do experiments suggest about the movement of the node in axis formation?

The node's movement is necessary and instructive for patterning.

From where do the germ layers arise during mouse development?

Ectoderm from animal pole regions, mesoderm from marginal zones, and endoderm from vegetal regions.

What characterizes the blastula and gastrula stages in mouse development?

The blastula features a blastocoel and yolk plug, while the gastrula develops a gut tube with defined mouth and anus openings.

What governs cell migration during gastrulation?

Adhesion molecules and cytoskeletal dynamics.

What does the differential adhesion hypothesis propose?

Cells sort based on adhesive strength, involving cadherins, CAMs, and integrins.

What are some adhesion structures involved in cell migration?

Adherens junctions, desmosomes, and calcium-independent interactions via the immunoglobulin superfamily.

How do migrating cells generate cortical tension?

By utilizing actomyosin networks.

What structures do cells extend to probe and adhere to substrates?

Filopodia and lamellipodia.

What facilitates epithelial-to-mesenchymal transition (EMT) during cell migration?

Apical constriction driven by actin and myosin.

What additional structures support dynamic morphogenesis in migrating cells?

Contractile rings and focal contacts.

Invagination

Inward folding of a cell sheet.

Involution

Inward rolling of cells over a surface.

Ingression

Migration of individual cells from an epithelium into the embryo.

Epiboly

Expansion and thinning of epithelial sheets.

Intercalation

Insertion of cells between neighbors to elongate tissues.

Convergent extension

Narrowing and lengthening of tissues through coordinated cell rearrangement.

Sea Urchin Gastrulation

Gastrulation begins with invagination of the vegetal plate to form the archenteron.

Primary mesenchyme cells

Ingress into the blastocoel during sea urchin gastrulation.

Secondary mesenchyme cells

Extend filopodia to guide gut elongation in sea urchin gastrulation.

Drosophila Mesoderm Formation

Involves mesodermal invagination driven by transcription factors Snail and Twist.

Snail

Suppresses E-cadherin to initiate epithelial-to-mesenchymal transition (EMT) in Drosophila.

Twist

Induces N-cadherin to stabilize mesodermal identity in Drosophila.

Xenopus Morphogenetic Movements

Exhibit involution at the blastopore lip, where bottle cells initiate mesodermal internalization.

Polonaise movements

Coordinated cell flows that precede streak formation in amniotes.

Hensen's node

Acts as an organizer, guiding head process and notochord formation in amniotes.

Chick, Mouse, and Human Gastrulation

Gastrulation begins at the primitive streak, where epiblast cells ingress to form mesoderm and endoderm.

What are the four classes of genes involved in Drosophila segmentation?

Maternal coordinate genes, gap genes, pair-rule genes, and segment polarity genes.

What role do maternal coordinate genes play in Drosophila development?

They establish the anterior-posterior (A/P) axis.

Name two examples of gap genes in Drosophila.

Krüppel and tailless.

What is the function of pair-rule genes in Drosophila segmentation?

They divide the embryo into alternating parasegments.

How do segment polarity genes contribute to Drosophila segmentation?

They refine segment boundaries and polarity within each parasegment.

What is the developmental progression from a syncytial blastoderm to a segmented larva in Drosophila?

Parasegments give rise to adult segments, such as C1-C3 for the head, T1-T3 for the thorax, and A1-A8 for the abdomen.