(a) What is stem cell? Discuss the types of stem cells and their application in therapeutic uses in human. 20 (b) Explain the…

Developmental biology links cell fate, gamete formation and metamorphic change.

Stem cells A stem cell is a self-renewing cell that can give rise to differentiated progeny; its potency ranges from totipotency (zygote/early blastomeres forming embryo and extra-embryonic tissues), pluripotency (embryonic stem cells forming all three germ layers), to multipotency (adult stem cells restricted to a tissue lineage). Embryonic stem cells (ESCs) are isolated from the inner cell mass of the blastocyst and are pluripotent, making them valuable for modelling development and testing drugs, though ethical and tumorigenicity concerns limit routine use. Adult/somatic stem cells (ASCs) reside in tissues such as bone marrow, skin, liver and brain; haematopoietic stem cells are multipotent and already used clinically. Induced pluripotent stem cells (iPSCs) are generated by reprogramming somatic cells with factors such as Oct4, Sox2, Klf4 and c-Myc, providing patient-specific cells and avoiding embryo use. Therapeutic applications include regenerative medicine, where stem cells replace damaged cells or secrete trophic factors. The most established use is haematopoietic stem cell/bone marrow transplantation for thalassaemia and leukaemia, restoring normal blood and immune function. In diabetes, islet or pancreatic progenitor cells are investigated to restore insulin secretion; in Parkinson’s disease, dopaminergic neuron replacement is experimental and under investigation, including iPSC-derived cells, not yet routine. Limitations include ethical issues with ESCs, reprogramming inefficiency and residual oncogenic risk with iPSCs, limited expansion of ASCs, and the need for controlled differentiation, safety and regulatory approval. Indian centres such as the Centre for Cellular and Molecular Biology (CCMB) and NCRM have contributed to stem-cell biology.

Spermatogenesis In mammals, spermatogenesis begins in the seminiferous tubules after puberty with spermatogonial stem cells; primordial germ cells are embryonic precursors that migrate to the gonad and become spermatogonia. These stem cells self-renew and differentiate into type A dark, A pale and B spermatogonia. Mitotic divisions increase cell number. Type B spermatogonia enter meiosis as primary spermatocytes. Meiosis I produces two secondary spermatocytes; meiosis II yields four haploid round spermatids. Spermiogenesis then remodels spermatids: the Golgi forms the acrosome over the anterior nucleus, centrioles organise the flagellum, excess cytoplasm is shed as residual bodies, and the nucleus condenses to form the sperm head. Sertoli cells nurture germ cells, provide nutrients, phagocytose residual bodies, secrete androgen-binding protein (ABP), and form tight junctions creating the blood-testis barrier, which protects meiotic cells from immune attack and maintains a specialised microenvironment. The process is supported by FSH acting on Sertoli cells and testosterone from Leydig cells. A suitable diagram would show a seminiferous tubule in cross-section, with basal spermatogonia, intermediate primary spermatocytes, apical spermatids, Sertoli cells, lumen and the blood-testis barrier.

Amphibian metamorphosis Metamorphosis is controlled by the hypothalamic–pituitary–thyroid axis. Hypothalamic TRH stimulates pituitary thyrotrophs to release TSH, which acts on the thyroid to secrete T3 and T4. These thyroid hormones induce developmental programmes in a tissue-specific manner: tail resorption through apoptosis, limb and gill remodelling, gut shortening and adult digestion, skin changes, eye and ear development, and neural reorganisation. In frogs, rising T3/T4 converts the aquatic larva into a terrestrial adult through coordinated apoptosis and differentiation. Corticosteroids from the adrenal cortex act synergistically, supporting catabolic and stress-related aspects of the transition. In axolotls, neoteny—retention of larval traits—is not mainly due to reduced tissue sensitivity to thyroid hormones; their tissues can respond to exogenous thyroid hormones, but the hypothalamic–pituitary drive, especially TSH secretion, is deficient or reduced, so endogenous thyroid hormone levels remain low and metamorphosis is delayed or absent.

Together, stem cells, gametogenesis and metamorphosis show how regulated cell fate, differentiation and hormonal cues underpin development, reproduction and disease. For therapeutic use, the way forward is to move from experimental cell replacement to safe, regulated regenerative therapies, while comparative developmental mechanisms such as amphibian metamorphosis illuminate evolutionary plasticity and regenerative potential.