32 public questions tagged with this topic.
Thyroid hormones play a crucial role in the metamorphosis of tadpoles into adult frogs. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.
Ref: NCERT Biology Textbook for Class XI and XII (Zoology section), Chapter: Structural Organisation in Animals and Frog, Topic: Tissues, skeletal and organ systems.
Differential gene expression explains how genetically identical cells achieve phenotypic diversity. Every somatic nucleus contains complete genome, yet only subset of genes transcribed in any given cell type due to combinatorial control. Enhancers, promoters, transcription factors such as MyoD in muscle, Ngn3 in pancreas, epigenetic marks including H3K27ac activation and DNA methylation silencing, dictate lineage-specific transcriptomes. Post-transcriptional regulation further refines proteome. This model refutes notion DNA differs among cells or mutations drive specialization, instead emphasizing regulated transcription as driver of cellular identity, tissue function, and developmental progression from zygote to adult.
Ref: Davidson, Genomic Regulatory Systems; Gilbert, 12th ed., Chapter 2: Gene regulation.
Bone morphogenetic protein signaling must be tightly modulated by extracellular antagonists conserved from flies to mammals. Noggin, Chordin, Sclerostin, Gremlin bind BMP ligands BMP2/4-like DBL-1 and SMA-6 receptor ligands preventing receptor engagement with ALK-SMAD cascade. In C. elegans, homologous antagonists shape dorsal-ventral patterning indirectly via sma-9. Noggin exemplifies inhibition by sequestering BMPs with cysteine-rich domain creating low BMP zones allowing neural induction. This mirrors Spemann organizer secreting Chordin to dorsalize ectoderm during gastrulation, conserving mechanism for tissue specification and extracellular modulation of morphogen activity.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: BMP antagonists Noggin and Chordin dorsalize organizer and inhibit signaling.
P2 blastomere provides inductive Wnt and Src signals to EMS at four-cell stage essential for endoderm. MOM-2 Wnt ligand from P2 activates MOM-5 Frizzled, WRM-1 beta-catenin asymmetry, LIT-1 Nemo kinase cascade in EMS, polarizing division into anterior MS and posterior E daughters. POP-1 TCF nuclear levels reduced posteriorly, permitting MED/END expression. If P2 is ablated surgically or genetically, extrinsic signal disappears, EMS defaults to MS-like fate, both daughters become muscle-pharynx precursors, endoderm marker ges-1 and elt-2 fail. Thus no intestine forms, demonstrating conditional specification.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: P2-EMS interaction - Wnt signaling induces endoderm in C. elegans.
SKN-1 is bZip transcription factor related to mammalian Nrf2, maternally provided and enriched in EMS and P2 via post-translational regulation and Wnt asymmetry. In EMS blastomere, SKN-1 directly activates MED-1,2 GATA factors and END-1,3 endoderm determinants leading to mesendoderm specification and tbx-35 muscle regulator. Nuclear entry depends on phosphorylation and redox sensing. Loss abolishes MS-derived pharynx, anterior body muscle, plus E-derived intestine causing embryonic lethality. It integrates maternal polarity, MET-2, and P2 inductive inputs ensuring EMS produces both mesoderm and endoderm lineages.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: SKN-1 transcription factor specifying EMS mesendoderm fate.
LIN-3 is epidermal growth factor family ligand synthesized in anchor cell under control of EGL-38 Pax2 and lag-2 gonadal inputs. The gene encodes transmembrane protein cleaved by rhomboid protease SUP-17 and ADAM10 metalloprotease, releasing EGF domain that diffuses short distance as morphogen. Binding receptor LET-23 on underlying vulval precursors activates tyrosine kinase signaling, Ras activation, MAPK phosphorylation. Dosage matters critically: low expression fails to induce, high dose causes multivulva phenotype beyond P6.p. Thus LIN-3 functions as primary inductive cue coordinating uterine-hypodermal development.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: LIN-3 as EGF-like ligand from anchor cell inducing vulval precursors.
Germline derives exclusively from continuous P lineage through sequential asymmetric divisions preserving totipotency. P0 divides into AB and P1, P1 into EMS and P2, P2 into C and P3, P3 into D and P4. P4, the smallest blastomere at 24-cell stage, divides symmetrically into Z2 and Z3 primordial germ cells quiescent until larval feeding. P-granules, maternally deposited ribonucleoprotein complexes containing GLH helicases and PGL scaffold, segregate into P cells, repress somatic transcription via PIE-1 inhibition of P-TEFb, maintaining germ potential throughout embryogenesis.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: Germline specification - P lineage and P-granule segregation in C. elegans.
Disheveled (Dsh) is key intracellular transducer of Wnt signaling vegetally localized in sea urchin egg cortex during oogenesis. It binds and inhibits Axin-GSK-3β-APC destruction complex responsible for phosphorylating β-catenin at N-terminus and marking it for ubiquitin-proteasome degradation. By preventing phosphorylation, Disheveled stabilizes β-catenin allowing cytoplasmic accumulation and subsequent nuclear import in vegetal blastomeres. Nuclear β-catenin partners with TCF/LEF to activate endomesodermal gene regulatory network and Pmar1. Hence Disheveled action preserves β-catenin, establishing vegetal polarity and micromere specification early in embryogenesis before zygotic transcription.
Ref: Weitzel et al., Development 2004, Disheveled localization; Gilbert Chapter 8: Wnt/β-catenin regulation by Dsh.
After initial isolation experiments showing totipotency, Driesch performed recombination tests reassembling separated blastomeres or stacking isolated animal halves together in different orientations. Recombined cells interacted dynamically and compensated to produce normal albeit smaller pluteus larvae, not mosaics of partial parts expected under mosaic theory. He concluded cell fate depends intimately on relative position within whole system rather than prelocalized autonomous determinants, defining harmonious equipotential system where each part potential governed by community. This recombination proved conditional development, where intercellular signaling and position determine destiny. Findings overturned Roux's mosaic model and established embryonic field concept.
Ref: Driesch, 1891-1908 sea urchin recombination; Gilbert Chapter 3: Evidence for conditional development.
Micromeres provide inductive organizing cues to adjacent Veg2 macromere descendants specifying germ layers. They synthesize and secrete Wnt8 as short-range paracrine factor activating β-catenin signaling in neighbors and present Delta ligand on surface activating Notch receptor on Veg2 cells. Delta-Notch interaction triggers nuclear translocation of Suppressor of Hairless nic, turning on Gcm and secondary mesenchyme programs, while Wnt8 and early β-catenin drive endoderm specification via FoxA and GataE in Veg1/Veg2. These juxtacrine and paracrine signals together convert conditionally specified cells to endomesoderm. Without induction, animal cells remain ectodermal. Thus micromeres actively specify neighboring cells toward endomesodermal fates.
Ref: Sherwood & McClay, Development 1999, Delta-Notch induction of endomesoderm; Gilbert Chapter 8.
When micromeres surgically removed from 16-cell stage sea urchin embryo, remaining animal hemisphere and macromeres lack sufficient vegetal Wnt/β-catenin and Delta-Notch signals needed for endomesoderm induction. Without these inductive cues, archenteron never forms, no primary or secondary mesenchyme ingresses, and gut differentiation fails. Instead embryo becomes permanently ciliated, hollow epithelial ball composed predominantly of expanded apical ectoderm and ciliary band, termed dauerblastula or animalized embryo. It swims but never gastrulates, analogous to β-catenin inhibition phenotype. This demonstrates conditional specification of animal cells requiring micromere signals; replacement of micromeres rescues gastrulation and normal pluteus formation.
Ref: Gilbert, Developmental Biology, Chapter 8: Animalization after micromere removal - dauerblastula phenotype.
Micromeres function as embryonic organizer in sea urchins comparable to amphibian dorsal lip. Horstadius in 1935 demonstrated transplanting fluorescently labeled micromeres from 16-cell embryo to animal pole of otherwise intact host embryo causes adjacent animal cells, normally fated to ectoderm, to change destiny, invaginate and form second archenteron with associated pigment cells and secondary mesenchyme. Induction requires Delta-Notch juxtacrine presentation and Wnt8 plus early β-catenin paracrine signals that reprogram host ectoderm toward endomesoderm. Resulting twinned gastrulation mimics Spemann organizer experiment. Transplant thus induces secondary archenteron formation, proving conditional specification of animal hemisphere under micromere influence.
Ref: Hörstadius 1935 organizer experiment; Gilbert, Developmental Biology, Chapter 8: Micromere transplantation and induction.
