Animal Developmental Biology Questions and Answers

Answer: D

The statement not true of spermatogenesis is differentiation of the gamete occurs while diploid, during first meiotic prophase.

That describes oogenesis: the oocyte grows and differentiates while still diploid, arrested in the first meiotic prophase. In spermatogenesis, differentiation (spermiogenesis, forming the acrosome and flagellum) happens after meiosis, when the cells are haploid spermatids.

The other three are true of spermatogenesis. Each meiosis gives four sperm, against one egg in oogenesis. The X and Y form a condensed sex body in first meiotic prophase, excluded from recombination outside the pseudoautosomal region and silenced. And meiosis begins continually from a mitotically renewing stem cell population.

Answer: D

In the chick, cells move inward through the Primitive streak.

The primitive streak forms as a thickening in the posterior epiblast of the chick blastoderm. Epiblast cells migrate towards it and pass through its central groove into the interior (ingression), forming endoderm and mesoderm. Hensen's node at its anterior end plays the role that the dorsal lip of the blastopore plays in amphibians.

The blastoderm is the whole disc of cells sitting on the yolk, not the opening cells pass through. The yolk is not cellular and is not traversed during gastrulation; chick cleavage is meroblastic and confined to the blastodisc. Cleavage furrows divide cells during cleavage, before gastrulation begins.

Answer: D

The keyed origin of squamous cell carcinoma is Ectoderm.

Squamous cell carcinoma is a malignant tumour of squamous (flat, stratified) epithelium. Its classic and commonest site is the skin, whose epidermis develops from surface ectoderm, and in that sense it is grouped with ectodermal tumours.

Carcinomas in general are tumours of epithelia, and squamous carcinoma can also arise in epithelia of other origins, such as the lungs after squamous metaplasia, but it is never a tumour of mesoderm or mesenchymal tissue: malignancies of those tissues are sarcomas. The key takes the skin tumour as the type case.

Answer: B

The keyed answer is Cholesterol.

Hedgehog proteins undergo autoprocessing: the C-terminal domain cleaves the precursor and attaches a cholesterol molecule covalently to the C-terminus of the N-terminal signalling fragment. This cholesterol tethers the protein to membranes, controls its release from the producing cell (with the transporter Dispatched) and shapes its spread through the tissue, so it is the modification developmental biology textbooks link with forming the Hedgehog gradient.

The signalling fragment also carries a palmitoyl group at its N-terminus, which is needed for full activity, but the modification that defines Hedgehog is the cholesterol adduct. Linolenic acid and a generic carboxylic acid play no such role.

Answer: C

They are all mechanisms of morphogenesis.

Morphogenesis is the generation of form: how tissues and organs acquire their shape. Gilbert's developmental biology lists its cellular mechanisms as changes in cell shape and the folding of epithelial sheets (as in neural tube formation), cell migration (as in neural crest cells), cell division, cell death by apoptosis (as in separating the digits), and changes in the extracellular matrix.

Cleavage is the series of early divisions that turn the zygote into many cells. Pattern formation is the setting up of the spatial organisation of cell fates, such as the body axes. Differentiation is the process by which a cell becomes a specialised type with its own proteins, not the movement or folding of cells.

Answer: C

In Drosophila early fate is set by Syncytial specification.

The early Drosophila embryo is a syncytium: nuclei divide without cell walls and share one cytoplasm. Gradients of maternal and zygotic transcription factors, such as Bicoid at the anterior and Nanos at the posterior, spread through this common cytoplasm, and each nucleus adopts a fate according to the concentrations it is exposed to. Only after this pattern is laid down do cell membranes form.

Autonomous specification (as in tunicates and molluscs) depends on cytoplasmic determinants inherited by individual blastomeres. Conditional specification, misspelled 'Conductional' here, depends on interactions between cells, as in vertebrates. 'Maternal specification' is not one of the three recognised modes, although maternal gene products feed into the syncytial gradients.

Answer: D

The membrane keyed as unique to terrestrial vertebrates is the amnion.

Reptiles, birds and mammals are called amniotes because the embryo develops inside the amnion, a fluid-filled sac that keeps it moist and cushioned. This adaptation freed reproduction from water and is the textbook defining feature of terrestrial vertebrates.

The yolk sac is the oldest extra-embryonic membrane and is found in fishes and amphibians too, so it is clearly wrong. The chorion and allantois also arise only in amniotes, but it is the amnion that names the group and is singled out as its key innovation, which is the reading the key follows.

Answer: A

Anterior-posterior patterning of the neural tube relies on retinoic acid.

Retinoic acid, made in the paraxial mesoderm, forms a gradient that is higher posteriorly. It acts through nuclear receptors to set the expression of Hox genes, giving the hindbrain and spinal cord their positional identities, and excess retinoic acid posteriorises anterior structures.

BMP4 works along the dorsal-ventral axis, specifying dorsal fates in opposition to ventral Sonic hedgehog. Wnt4 is best known in kidney and gonad development, and FGF4 in early embryonic and limb development rather than neural A-P patterning as its key role.

Answer: D

Epiblast formation and migration in the zebrafish needs E-cadherin.

During epiboly and gastrulation, zebrafish cells move as coherent sheets held together by E-cadherin. Mutants lacking E-cadherin, such as half baked, fail in epiboly because the deep cells cannot stay attached and move outward, so the blastoderm does not spread over the yolk.

N-cadherin is typical of neural tissue and mesenchyme, P-cadherin of placenta and epidermis, and R-cadherin of retina; none is the sheet-forming adhesion molecule of the zebrafish epiblast.

Answer: B

Pluripotency is maintained by Oct 4, Nanog and SOX 2.

These three transcription factors form the core pluripotency network of embryonic stem cells. They bind together at many target genes, activate genes that keep the cells self-renewing and pluripotent, and repress genes of differentiation. Oct4 and Sox2 are also two of the four Yamanaka factors used to make induced pluripotent stem cells.

Stat3 carries the LIF signal in mouse ES cells but is not part of the core network. Cdx2 and Eomesodermin specify the trophectoderm lineage, so they mark the loss of pluripotency.