7 public questions tagged with this topic.
Following fertilization, the zygote undergoes cleavage, forming a morula before further development into a blastocyst. 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: Biology - Zoology portion covering relevant system and function.
Mammalian embryo displays highly regulative development: early blastomeres retain remarkable flexibility and equipotency. Murine four-cell embryo blastomeres remain totipotent as demonstrated by ability of isolated single cell to contribute to both trophectoderm and inner cell mass compartments and to support full-term development when aggregated with carrier cells or transplanted into foster mothers. Single-nucleus RNA sequencing shows equal expression of Oct4 and Cdx2 potentiality. Potency restriction occurs at compaction and blastocyst stage when Cdx2 and Oct4 become mutually exclusive, sep
Ref: Tarkowski, Nature 1959; Tarkowski & Wroblewska 1967: Mouse 4-cell blastomeres totipotent contribution.
P-granules, ribonucleoprotein assemblies marking germline, segregate to posterior blastomere P1 during first division and subsequently to germline precursors P2-P4. Their posterior migration along cell cortex before division requires microtubule motor dynein transporting granule components along cortical microtubules toward centrosome at posterior pole. PAR-1 dependent microtubule organization creates flow. Mutants in dynein heavy chain dhc-1 or its regulator dynactin cause equal distribution of P-granules to both AB and P1, losing germline restriction. Myosin II and kinesin contribute to cort
Ref: Updike & Strome 2010; Gilbert Chapter 4: Dynein motor responsible for P-granule migration in early embryo.
During early Drosophila development before cycle 14, zygotic genome remains largely quiescent with only minor early zygotic genes transcribed. Embryo relies entirely on maternal products loaded during oogenesis by nurse cells, including mRNAs for bicoid, nanos, dorsal, gurken, and histones supporting rapid mitoses. These maternal determinants drive syncytial divisions, nuclear migrations, axis establishment and positional information. Activation of gap genes, pair-rule genes and cellularization requires maternal gradient interpretation. Therefore early patterning, cleavage and axis specificati
Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 22: Maternal control of early development.
Drosophila exhibits superficial cleavage where nuclear division occurs without cytoplasmic division. Zygote nucleus undergoes rapid synchronous mitoses within central cytoplasm, producing multinucleate syncytium. Around nuclear cycle 10, nuclei migrate peripherally into cortical cytoplasm, arranging beneath plasma membrane. Actin caps form above each nucleus, but no cell membranes separate nuclei, defining syncytial blastoderm. Only at cycle 14 does membrane invagination occur simultaneously around every cortical nucleus, converting syncytium into cellular blastoderm of epithelial monolayer su
Ref: NCBI, Molecular Biology of the Cell, Chapter 22: Syncytial blastoderm in Drosophila.
Compaction to form morula requires establishment of strong cell-cell adhesion mediated by E-cadherin, a calcium-dependent adhesion molecule. Extracellular calcium rigidifies cadherin repeats enabling trans-homophilic binding between blastomeres, intracellularly binding beta-catenin which links to alpha-catenin and actin cytoskeleton. Activation and clustering of this cadherin-catenin complex drives blastomere flattening, polarization, tighter apposition and induction of tight junction formation. Blockage of cadherin or removal of calcium prevents compaction, while calcium ionophores alone with
Ref: Maître JL et al., Nature Cell Biology 2015: E-cadherin beta-catenin regulation of compaction and Hippo mediated cell fate.
Pharyngeal gill slits are present in chordates during early development and aid in respiration in aquatic forms (NCERT, Chapter 4, page 55). Lungs are in terrestrial vertebrates, tracheae and spiracles in arthropods.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Vertebrata - Cyclostomata and Pisces
