Embryology: Early Development and Cleavage

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Questions and Answers

Given the constraints of yolk presence on cleavage patterns, which of the following scenarios would MOST strongly support and exemplify meroblastic cleavage?

A microlecithal egg undergoing complete and equal division, resulting in blastomeres of approximately equal size and cytoplasmic composition.
A mesolecithal egg exhibiting a moderate amount of yolk concentrated in the vegetal pole, leading to slightly unequal cleavage.
A macrolecithal egg with a dense concentration of yolk obstructing complete cleavage, confining division to a small disc of cytoplasm at the animal pole. (correct)
An isolecithal egg, uniformly provisioned with yolk distributed across its cytoplasm, facilitating holoblastic and synchronous cell divisions.

Holoblastic cleavage, characterized by complete division of the zygote, is exclusively observed in microlecithal eggs, and does not occur at all in mesolecithal eggs due to their moderate yolk content.

False (B)

Describe how the architecture of a blastula fundamentally differs in species exhibiting holoblastic versus meroblastic cleavage, and elaborate on the implications of these structural differences for subsequent gastrulation movements.

In holoblastic cleavage, the blastula typically exhibits a blastocoel surrounded by cells of roughly equal size, permitting coordinated and extensive cell migrations during gastrulation. In meroblastic cleavage, the blastula may lack a distinct blastocoel, and cell movements during gastrulation are often more limited and localized due to the presence of a large yolk mass.

During amphibian development, the ______ lip of the blastopore is crucial as the primary organizer, initiating the complex cell movements of gastrulation.

dorsal Signup and view all the answers

Match the cell type with its developmental origin:

Epidermis = Ectoderm Notochord = Mesoderm Gut Lining = Endoderm Neural Crest Cells = Ectoderm Signup and view all the answers

Within the context of neurulation, what distinguishes primary neurulation from secondary neurulation at cellular and morphogenetic levels?

Primary neurulation is characterized by the folding of the neural plate to form the neural tube, while secondary neurulation involves the formation of a solid cord of cells that subsequently cavitates to form the neural tube. (C) Signup and view all the answers

The archenteron, a primitive gut formed during gastrulation, directly gives rise to the coelom in all deuterostomes via enterocoely.

False (B) Signup and view all the answers

Formulate a scenario in which disruption of the organizer region during amphibian gastrulation leads to predictable developmental abnormalities. Detail the molecular mechanisms involved.

Disrupting the organizer region by ablation or ectopic transplantation could lead to axis duplication or severe axis truncation. Molecularly, this involves interfering with the expression or function of key signaling molecules like Chordin, Noggin, and Follistatin, which normally antagonize BMP signaling to establish the dorsal-ventral axis. Signup and view all the answers

The process by which the mesoderm splits to form the coelom is termed ______, contrasting with enterocoely where the coelom originates from outpockets of the archenteron.

schizocoely Signup and view all the answers

Match each germ layer with a structure that it forms:

Ectoderm = Epidermis Mesoderm = Heart Endoderm = Digestive tract lining Neural Crest = Peripheral neurons Signup and view all the answers

If the expression of Fibroblast Growth Factor 8 (FGF8) is selectively ablated in the anterior neural ridge during early vertebrate brain development, which phenotype would MOST accurately represent the resulting developmental outcome?

Substantial truncation or absence of the forebrain structures, attributable to impaired anterior neural plate induction. (D) Signup and view all the answers

The organizer region in amphibian embryos is functionally equivalent to the node in avian and mammalian embryos, serving identical roles in axis formation and germ layer induction, regulated by the same set of molecular signals without evolutionary divergence.

False (B) Signup and view all the answers

Evaluate the evolutionary implications of heterochrony in the context of amphibian metamorphosis, and explain how temporal shifts in developmental events can lead to significant morphological diversity.

Heterochrony, or changes in the timing of developmental events, allows for significant morphological diversity. In amphibians, shifts in the timing of metamorphosis can lead to paedomorphosis (retention of larval features in adults) or acceleration of adult characteristics, profoundly impacting species diversification. Signup and view all the answers

During neural tube formation, the ______ cells undergo an epithelial-to-mesenchymal transition, migrating away from the dorsal neural tube to give rise to diverse cell types including neurons, glia, and pigment cells.

neural crest Signup and view all the answers

Match components with a cleavage characteristic:

Holoblastic = complete cleavage Meroblastic = incomplete cleavage Microlecithal = small yolk Macrolecithal = large yolk Signup and view all the answers

Within a developing vertebrate limb bud, if Sonic hedgehog (Shh) signaling is constitutively activated across the entire anterior-posterior axis, bypassing its normal regulation by the Zone of Polarizing Activity (ZPA), which developmental anomaly is MOST likely to manifest?

Mirror-image duplication of digits along the anterior-posterior axis, leading to a limb with symmetrical digit patterns. (A) Signup and view all the answers

In avian embryos, Koller's sickle is functionally analogous to the Spemann-Mangold organizer in amphibian embryos, yet resides in the epiblast, and performs comparable axis-inducing functions via nodal signaling and subsequent expression of goosecoid and chordin though it is structurally dissimilar.

True (A) Signup and view all the answers

Contrast the processes of epiboly and involution during amphibian gastrulation, detailing their specific contributions to the final arrangement of germ layers and their interdependence in the context of axial elongation.

Epiboly involves the spreading of ectodermal cells over the embryo's surface to enclose the inner germ layers, while involution is the inward movement of the mesoderm and endoderm at the blastopore lip. Epiboly ensures coverage, and involution positions the layers correctly for axial elongation. Signup and view all the answers

The avian hypoblast, though not directly contributing to definitive embryonic structures, is critical for establishing the ______, a signaling center responsible for initiating gastrulation.

primitive streak Signup and view all the answers

Matching components with what they might form

Epimere = somites Mesomere = kidney Hypomere = lining of body cavity Notochord = structure of the spinal column Signup and view all the answers

Flashcards

Fertilization

Fusion of sperm and egg that initiates embryonic development.

Cleavage

Rapid cell division of the zygote into many cells called blastomeres.