#tissue repair

Stem Cell -lll

What is the function of mesenchymal stem cells?

Mesenchymal stem cells, also termed mesenchymal stromal cells, originate perivascularly in bone marrow and adipose tissue. Defined by CD73, CD90, CD105 positivity and absence of hematopoietic markers CD45 and CD34, they exhibit multipotency limited to mesodermal lineages: osteoblasts synthesizing bone matrix via Runx2, chondrocytes producing cartilage collagen II and aggrecan via Sox9, and adipocytes via PPAR-gamma. Differentiation is driven by Wnt and BMP cues. Unlike hematopoietic or neural stem cells, they do not generate neurons, blood, or gametes under physiological conditions inside the body.

Ref: Caplan, J Orthop Res 1991; Gilbert, Chapter 6: Mesenchymal stem cell multipotency Runx2 Sox9 lineage markers.

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Stem Cell -lll

Which tissue does NOT contain adult stem cells?

Adult stem cells are identified in many regenerative tissues including bone marrow hematopoietic and mesenchymal compartments, intestinal crypts, hair follicle bulge, basal epidermis, liver oval cells, satellite cells of muscle, and subventricular zone of brain. They require vascular support, stromal signals, and basement membrane attachment for survival. Mature erythrocytes are highly specialized, enucleated, organelle-free cells lacking DNA, transcriptional capacity, and mitotic machinery. Therefore they cannot house self-renewing reserve, de-differentiate, or contribute to regeneration, distinguishing terminal differentiation from stem cell reservoirs essential for homeostasis.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 18: Adult stem cell reservoirs and terminal RBC anucleation.

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Stem Cell -lll

Adult stem cells are primarily responsible for:

Adult stem cells persist postnatally within specialized niches such as bone marrow, intestinal crypts, and hair follicle bulge. Unlike embryonic stem cells that construct the entire organism, these tissue-resident populations maintain homeostasis by replacing short-lived differentiated cells lost to turnover, injury, or mechanical stress. Through balanced self-renewal and lineage commitment governed by Wnt, Notch, BMP and local growth factors, they generate transit-amplifying progenitors that restore epithelia, blood, muscle, and neural lineages, ensuring long-term repair without forming extraembryonic tissues or whole embryos during adulthood.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 6: Adult stem cells, niche maintenance and tissue repair mechanisms.

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Regeneration -l

Which process occurs first during regeneration?

Regeneration timeline begins with immediate wound healing response essential to close exposed tissues, prevent infection, and establish signaling milieu. Within minutes epidermal cells migrate and spread over wound, forming wound epithelium that becomes signaling center secreting ROS, Wnt inhibitors, and growth factors. Immune cells release cytokines activating JNK, ERK, and apoptotic signals inducing wound-induced genes. Only after epithelium sealed can neoblast recruitment, blastema formation, and proliferation start; differentiation and repatterning follow later days. Failure to close wound blocks subsequent steps, highlighting wound healing temporally and mechanistically as first obligatory process preceding repatterning, differentiation, or extensive mitosis.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Wound healing as earliest step preceding blastema formation.

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Regeneration -l

What is the effect of inhibiting β-catenin in regenerating planaria?

Beta-catenin protein level translates Wnt signal strength into transcriptional fate choices. Inhibition via double-stranded RNA or small molecule tankyrase inhibition prevents nuclear accumulation and removes activation of posterior target genes. Posterior blastema deprived of Wnt output defaults to anterior genetic program expressing sfrp-1, notum, and prep forming ectopic head complete with brain and eyes. Anterior wound also forms head leading to animal with two heads, duplicated central nervous systems pointing outward. This multiple-head phenotype demonstrates binary switch where beta-catenin threshold determines identity. Stem cell maintenance, muscle overgrowth, or wound closure defects not observed as primary consequence of beta-catenin loss.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: β-catenin inhibition leads to two-headed planaria via anteriorization.

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Regeneration -l

Which of the following is an example of stem cell-mediated regeneration?

Stem cell-mediated regeneration relies on persistent adult stem cells that continuously supply differentiated derivatives. In planarians, population of clonogenic neoblasts, pluripotent adult stem cells expressing piwi-1 and prog-1, migrates toward wounds, proliferates massively, and differentiates into everything including epidermis, gut, muscle, and central nervous system. Single cNeoblast can rescue lethally irradiated animals. Zebrafish fin regeneration uses dedifferentiation-based blastema classified as epimorphosis, Hydra budding combines stem cell activity with morphallactic remodeling, mammalian liver regeneration uses compensatory proliferation of differentiated hepatocytes rather than dedicated pluripotent pool, making planarian example textbook stem cell-mediated.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Planaria neoblasts as pluripotent stem cells for regeneration.

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Regeneration -l

Which of the following describes regeneration in vertebrates?

Vertebrate regeneration is not limited to embryonic stages but re-engages developmental programs via integrated genetic and cellular mechanisms operating asexually without gamete formation. It involves wound epidermis formation, immune modulation, dedifferentiation of mature cells into progenitors, activation of resident stem cells, and proliferation forming blastema or compensatory growth. Gene regulation is central: Wnt, FGF, BMP, Hedgehog, and retinoic acid pathways are redeployed with epigenetic remodeling and positional memory. This distinguishes regeneration from reproduction, which produces new organism, and explains why mammals regenerate liver and digit tips while salamanders regenerate whole limbs.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Vertebrate regeneration - genetic regulation and cellular mechanisms.

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Regeneration -lll

Compensatory regeneration differs because it:

Compensatory regeneration represents distinct strategy where organ mass restored without blastema formation and without loss of functional differentiation. Hepatocytes continue synthesizing plasma proteins, metabolizing, while dividing after partial hepatectomy driven by IL-6, TNF-alpha, HGF signaling. No dedifferentiation to progenitor occurs, lineage remains restricted, organ architecture restored by hypertrophy and hyperplasia of functional cells. This contrasts epimorphosis requiring dedifferentiation and morphallaxis involving repatterning with transdifferentiation. Maintaining differentiated function during proliferation ensures physiological demands met throughout restoration process, defining hallmark of compensatory hyperplasia in liver and pancreatic islets.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Compensatory regeneration maintaining differentiated function.

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Regeneration -lll

Regeneration of mammalian hair follicles exemplifies:

Mammalian hair follicle exemplifies stem-cell mediated regeneration where dedicated resident stem cells fuel cyclical turnover rather than forming blastema or using compensatory division of differentiated cells. Bulge region harbors slow-cycling multipotent stem cells activated by dermal papilla Wnt signals at anagen, producing transient amplifying progeny differentiating into hair lineage. Inhibitory BMP maintains quiescence during telogen. After wounding, bulge cells migrate to epidermis contributing to repair. This paradigm illustrates niche-regulated stem population sustaining tissue renewal, distinguishing stem-cell mediated regeneration from epimorphosis and morphallaxis and highlighting mammalian regenerative strategy for continuously renewing epithelia.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Hair follicle example of stem-cell mediated regeneration.

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Cell Communication and ECM

Fibroblasts are primary producers of ECM in

Connective tissue, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cell Communication and ECM

Fibronectin mainly functions in

Cell migration and adhesion, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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