📚 Stem Cells | 干细胞
Stem cells are undifferentiated biological cells that can both self-renew and differentiate into specialised cell types. In A-Level Biology, understanding stem cells is fundamental to grasping how complex organisms develop from a single fertilised egg and how tissues are maintained and repaired. This article explores the types, properties, and applications of stem cells, along with the ethical debates surrounding their use.
干细胞是未分化的生物细胞，既能够自我更新，又能够分化成特化的细胞类型。在 A-Level 生物学中，理解干细胞是掌握复杂生物体如何从单个受精卵发育而成、以及组织如何维持和修复的基础。本文探讨干细胞的类型、特性和应用，以及围绕其使用的伦理争议。
1. What are Stem Cells? | 什么是干细胞？
Stem cells are defined by two key abilities: self-renewal, which is the capacity to divide repeatedly while remaining undifferentiated, and potency, which refers to the potential to differentiate into one or more specialised cell types. Unlike most somatic cells that have a fixed identity, stem cells serve as a cellular reservoir from which new functional cells can be generated. They are found in embryos, where they give rise to the entire organism, and in adult tissues, where they replace damaged or worn-out cells.
干细胞由两项关键能力定义：自我更新，即在保持未分化状态的同时反复分裂的能力；以及分化潜能，指分化为一种或多种特化细胞类型的潜力。与大多数具有固定身份的体细胞不同，干细胞充当着可以产生新功能细胞的细胞储库。它们存在于胚胎中，产生整个生物体，也存在于成体组织中，替换受损或衰老的细胞。
2. Characteristics of Stem Cells | 干细胞的特性
All stem cells share three fundamental properties. First, they are unspecialised cells that lack tissue-specific structures, although they still contain a complete genome. Second, they can divide for long periods while maintaining the stem cell pool; this is often achieved by asymmetric division, where one daughter cell remains a stem cell and the other becomes a progenitor cell destined for differentiation. Third, under appropriate signals, they can differentiate into cells with specialised functions, such as neurons, muscle cells, or insulin-secreting beta cells. The balance between self-renewal and differentiation is tightly controlled by the stem cell niche – the microenvironment that provides physical support and molecular signals.
所有干细胞都具有三个基本特性。首先，它们是非特化细胞，缺乏组织特异性结构，但仍然含有完整的基因组。其次，它们能够长时间分裂并维持干细胞池；通常通过不对称分裂实现，即一个子细胞保持干细胞状态，另一个成为注定要分化的祖细胞。第三，在适当的信号下，它们能够分化成具有特化功能的细胞，如神经元、肌肉细胞或分泌胰岛素的 β 细胞。自我更新和分化之间的平衡由干细胞龛（微环境）严格调控，该微环境提供物理支持和分子信号。
3. Potency: Totipotent, Pluripotent, Multipotent, and Unipotent | 分化潜能：全能、多能、专能和单能
The differentiation potential of stem cells is categorised into four levels of potency. Totipotent stem cells, such as the zygote and the first few blastomeres, can give rise to all embryonic and extraembryonic tissues (including the placenta). Pluripotent stem cells, like the inner cell mass of the blastocyst, can form all three germ layers (ectoderm, mesoderm, endoderm) but not extraembryonic structures. Multipotent stem cells, such as haematopoietic stem cells in bone marrow, can produce a limited range of cell types within a specific lineage. Unipotent cells, like skin stem cells, can only generate one cell type but still possess the property of self-renewal, distinguishing them from non-stem cells. Understanding potency is critical when evaluating therapeutic applications.
干细胞的分化潜能分为四个能级。全能干细胞，如受精卵和早期卵裂球，能够产生所有胚胎和胚外组织（包括胎盘）。多能干细胞，如囊胚的内细胞团，能够形成三个胚层（外胚层、中胚层、内胚层），但不能形成胚外结构。专能干细胞，如骨髓中的造血干细胞，能够产生特定谱系内有限范围的细胞类型。单能细胞，如皮肤干细胞，只能产生一种细胞类型，但仍具备自我更新特性，从而区别于非干细胞。在评估治疗应用时，理解细胞潜能至关重要。
4. Embryonic Stem Cells | 胚胎干细胞
Embryonic stem cells (ESCs) are derived from the inner cell mass of a blastocyst, typically around day 5 after fertilisation in humans. They are pluripotent and can proliferate indefinitely in culture while maintaining a normal karyotype. Under the influence of specific growth factors and transcription factors, ESCs can be directed to differentiate into virtually any somatic cell type. Key pluripotency factors include Oct4, Sox2, and Nanog, which form a regulatory network that suppresses differentiation genes and activates self-renewal genes. However, the use of ESCs raises ethical concerns because the process involves the destruction of a developing embryo, which some consider a potential human life.
胚胎干细胞 (ESC) 源自囊胚的内细胞团，在人类中通常为受精后约第 5 天。它们具有多能性，能够在培养中无限增殖，同时保持正常核型。在特定生长因子和转录因子的影响下，ESC 可被引导分化成几乎任何体细胞类型。关键多能性因子包括 Oct4、Sox2 和 Nanog，它们形成一个抑制分化基因并激活自我更新基因的调控网络。然而，ESC 的使用引发了伦理担忧，因为该过程涉及破坏发育中的胚胎，一些人认为这可能是一个潜在的人类生命。
5. Adult Stem Cells | 成体干细胞
Adult stem cells, also known as somatic or tissue-specific stem cells, are found in many differentiated tissues, including bone marrow, skin, gut, and brain. They are generally multipotent or unipotent, with a more restricted differentiation capacity than ESCs. For example, haematopoietic stem cells (HSCs) can give rise to all types of blood cells, while mesenchymal stem cells can produce bone, cartilage, and fat cells. Adult stem cells play a vital role in tissue homeostasis and repair. Their isolation is less ethically controversial since it does not require embryo destruction, but they are often present in very low numbers and can be difficult to expand in vitro without losing stemness.
成体干细胞，也称为体细胞或组织特异性干细胞，存在于许多已分化的组织中，包括骨髓、皮肤、肠道和脑。它们通常为专能或单能，分化能力比 ESC 更为受限。例如，造血干细胞 (HSC) 能产生所有类型的血细胞，而间充质干细胞能产生骨、软骨和脂肪细胞。成体干细胞在组织稳态和修复中起关键作用。其分离伦理争议较小，因为不需要破坏胚胎，但它们在体内数量通常极少，且难以在体外扩增而不丧失干性。
6. Induced Pluripotent Stem Cells (iPSCs) | 诱导性多能干细胞 (iPSCs)
Induced pluripotent stem cells are produced by reprogramming adult somatic cells, such as skin fibroblasts, back to a pluripotent state. This is achieved by introducing a defined set of transcription factors – typically Oct4, Sox2, Klf4, and c-Myc (the Yamanaka factors) – using viral vectors or other delivery methods. The resulting iPSCs closely resemble embryonic stem cells in morphology, gene expression, and differentiation potential. iPSC technology circumvents many ethical issues associated with ESCs and enables the generation of patient-specific cell lines for disease modelling, drug screening, and potentially autologous cell therapy without immune rejection. However, challenges remain, including the risk of tumour formation due to residual undifferentiated cells and the use of oncogenic factors like c-Myc.
诱导性多能干细胞是通过将成体体细胞（如皮肤成纤维细胞）重编程回多能状态而产生的。这通过使用病毒载体或其他递送方法导入一组确定的转录因子——通常为 Oct4、Sox2、Klf4 和 c-Myc（山中因子）来实现。由此产生的 iPSC 在形态、基因表达和分化潜能方面与胚胎干细胞非常相似。iPSC 技术规避了与 ESC 相关的许多伦理问题，并且能够生成患者特异性细胞系，用于疾病建模、药物筛选，以及潜在的不会引起免疫排斥的自体细胞治疗。然而，挑战仍然存在，包括因残留未分化细胞而导致的肿瘤形成风险，以及使用如 c-Myc 等致癌因子。
7. Stem Cells in Development and Differentiation | 发育与分化中的干细胞
The first stem cell in mammalian development is the totipotent zygote. After several cleavage divisions, the morula forms, and subsequent compaction and cavitation lead to the blastocyst stage. At this point, cells segregate into the trophectoderm (which forms the placenta) and the inner cell mass (ICM), which is pluripotent and will form the embryo proper. As development proceeds, ICM cells undergo gastrulation, generating the three germ layers through a cascade of differential gene expression orchestrated by morphogen gradients. Each germ layer gives rise to progressively more restricted progenitor cells, demonstrating the hierarchical nature of stem cell differentiation. Understanding these developmental pathways helps scientists mimic differentiation protocols in vitro to generate desired cell types from pluripotent stem cells.
哺乳动物发育中的第一个干细胞是全能的受精卵。经过数次卵裂，形成桑葚胚，随后的压实和空腔化导致囊胚阶段。此时，细胞分化为滋养外胚层（形成胎盘）和内细胞团 (ICM)，后者具有多能性并将形成胚胎本体。随着发育进行，ICM 细胞经历原肠胚形成，通过形态发生素梯度协调的差异基因表达级联产生三个胚层。每个胚层产生逐渐更多限制性的祖细胞，展示了干细胞分化的层级性质。理解这些发育途径有助于科学家在体外模拟分化方案，从多能干细胞产生所需细胞类型。
8. The Role of Transcription Factors and Epigenetics | 转录因子与表观遗传的作用
Stem cell fate is controlled by a complex interplay of transcription factors and epigenetic modifications. Master regulators like Oct4, Sox2, and Nanog bind to specific DNA sequences to activate pluripotency-associated genes and repress lineage-specific genes. Epigenetic mechanisms – such as DNA methylation, histone modification, and chromatin remodelling – can lock cells into a differentiated state by making certain genes inaccessible. Reprogramming somatic cells to iPSCs involves erasing these epigenetic marks and re-establishing a pluripotent epigenetic landscape, a process that is often incomplete and contributes to variability among iPSC lines. The reversibility of differentiation demonstrated by nuclear transfer and iPSC technology revolutionised our understanding of cellular plasticity and opened new avenues in regenerative medicine.
干细胞的命运由转录因子和表观遗传修饰之间复杂的相互作用控制。主调控因子如 Oct4、Sox2 和 Nanog 结合特定 DNA 序列，激活多能性相关基因并抑制谱系特异性基因。表观遗传机制——如 DNA 甲基化、组蛋白修饰和染色质重塑——可以通过使某些基因不可接近而将细胞锁定在分化状态。将体细胞重编程为 iPSC 的过程包括擦除这些表观遗传标记并重建多能性表观遗传景观，这一过程往往不完整，导致 iPSC 系之间的异质性。通过核移植和 iPSC 技术所展示的分化可逆性，彻底改变了我们对细胞可塑性的理解，并为再生医学开辟了新途径。
9. Therapeutic Uses of Stem Cells | 干细胞的治疗应用
Stem cells hold immense promise for treating degenerative diseases and injuries. One well-established therapy is bone marrow transplantation, which uses haematopoietic stem cells to reconstitute the blood and immune system in patients with leukaemia or lymphoma. Researchers are also exploring the use of pluripotent stem cell-derived cells for conditions such as Parkinson’s disease (dopaminergic neurons), type 1 diabetes (insulin-producing beta cells), and spinal cord injury (oligodendrocytes). iPSCs enable the creation of ‘disease-in-a-dish’ models, allowing scientists to study pathologies and test drugs in a patient-specific context. However, before widespread clinical use, issues of safety, efficiency, and cost must be resolved. Ensuring complete differentiation and eliminating residual pluripotent cells is vital to prevent teratoma formation.
干细胞在治疗退行性疾病和损伤方面前景广阔。一种成熟的疗法是骨髓移植，利用造血干细胞重建白血病或淋巴瘤患者的血液和免疫系统。研究人员也在探索使用多能干细胞衍生细胞治疗帕金森病（多巴胺能神经元）、1 型糖尿病（胰岛素生成 β 细胞）和脊髓损伤（少突胶质细胞）等疾病。iPSC 能够创建“皿中疾病”模型，使科学家能够在患者特异性背景下研究病理机制并测试药物。然而，在广泛临床应用之前，必须解决安全性、效率和成本问题。确保完全分化并消除残留的多能细胞对于防止畸胎瘤形成至关重要。
10. Ethical and Social Considerations | 伦理与社会考量
The use of embryonic stem cells is ethically contentious because it involves the destruction of human embryos, which some believe have moral status from conception. Different countries have adopted varying regulatory frameworks; for instance, UK legislation permits ESC research under strict licensing, while other nations have tight restrictions. The advent of iPSCs has alleviated some ethical concerns, but new issues arise, such as the potential for creating human-animal chimeras, the commodification of human tissues, and the long-term risks of genetic modification. Informed consent, donor anonymity, and equitable access to treatments are additional societal challenges. A balanced view acknowledges the potential benefits of stem cell research while advocating for rigorous ethical oversight and public dialogue.
胚胎干细胞的使用在伦理上存在争议，因为它涉及破坏人类胚胎，而一些人认为胚胎从受孕起就拥有道德地位。不同国家采取了不同的监管框架；例如，英国立法在严格许可下允许 ESC 研究，而其他国家则有严格限制。iPSC 的出现缓解了一些伦理担忧，但也产生了新的问题，如制造人-动物嵌合体的可能性、人体组织的商品化以及基因修饰的长期风险。知情同意、供者匿名和公平获得治疗是额外的社会挑战。平衡的观点承认干细胞研究的潜在益处，同时倡导严格的伦理监督和公众对话。
11. Comparison of Stem Cell Types | 干细胞类型比较
The table below summarises the key features of the main stem cell types encountered in the Cambridge A-Level syllabus. Understanding these distinctions is essential for exam questions that ask you to compare sources, potency, advantages, and limitations.
下表总结了剑桥 A-Level 教学大纲中主要干细胞类型的关键特征。理解这些区别对于要求比较来源、潜能、优点和局限性的考试问题至关重要。
| Feature (English) | 特征 (中文) | Embryonic Stem Cells | Adult Stem Cells | Induced Pluripotent Stem Cells |
|---|---|---|---|---|
| Source (来源) | 来源 | Inner cell mass of blastocyst | Various differentiated tissues (e.g. bone marrow, skin) | Reprogrammed adult somatic cells |
| Potency (潜能) | 潜能 | Pluripotent | Mostly multipotent or unipotent | Pluripotent |
| Self-renewal (自我更新) | 自我更新 | Unlimited in vitro | Limited; may senesce in culture | Unlimited in vitro |
| Ethical concerns (伦理问题) | 伦理问题 | Destruction of embryo | Few; requires informed consent | Fewer than ESCs; safety issues (tumorigenicity) |
| Clinical use (临床应用) | 临床应用 | Clinical trials for retinal disease, etc. | Established: bone marrow transplant, skin grafts | None yet approved; used for disease modelling and drug testing |
12. Exam Tips and Common Misconceptions | 考试技巧与常见误区
In Cambridge A-Level exams, questions on stem cells often require precise use of terminology. Never confuse ‘totipotent’ with ‘pluripotent’: totipotent cells can form a whole organism, including extraembryonic membranes; pluripotent cells cannot. Remember that adult stem cells are not pluripotent under normal physiological conditions – only via artificial reprogramming can they acquire pluripotency. Another common pitfall is assuming that all stem cell therapies are widely available; be specific about which treatments are established (e.g., HSC transplants) and which are still experimental. When discussing ethics, present both sides: mention the potential to cure disease alongside concerns about embryo destruction. Use the term ‘iPSC’ only when referring to induced pluripotent stem cells, and know the Yamanaka factors. Practice comparing the sources, properties, and limitations of different stem cell types using a table or bullet points.
在剑桥 A-Level 考试中，关于干细胞的问题通常要求准确使用术语。永远不要混淆“全能”和“多能”：全能细胞可以形成整个生物体，包括胚外膜；多能细胞则不能。记住，在正常生理条件下成体干细胞不是多能的——只有通过人工重编程才能获得多能性。另一个常见误区是假设所有干细胞疗法都已广泛应用；要具体说明哪些疗法是成熟的（例如，造血干细胞移植），哪些仍处于实验阶段。讨论伦理时，要呈现两面：既要提及治愈疾病的潜力，也要提及对胚胎销毁的担忧。仅当指诱导性多能干细胞时使用“iPSC”一词，并熟知山中因子。练习使用表格或要点来比较不同干细胞类型的来源、特性和局限性。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply