stem cells

Question 1

what is the definition of stem cells?

Answer 1

a stem cell is an unspecialised cell from the embryo, fetus, or adult that under certain conditions:
1. has self-renewal capability by reproducing itself for long periods, or in the case of adult stem cells, throughout the life of the organism
2. can differentiate whereby unspecialised cells give rise to specialised cells that make up the tissues and organs of the body, generating cells that travel down various differentiation pathways

Question 2

what are the unique features of all stem cells?

Answer 2

1. stem cells are unspecialised, and do not have any tissue-specific structures that allow it to perform specialised functions
2. stem cells are capable of dividing and renewing themselves for long periods by mitotic divisions, through a process known as proliferation, provided that the resulting cells continue to be unspecialised
3. stem cells can give rise to specialised cells through a process called differentiation, triggered either by internal or external signals. during differentiation, differential gene expression occurs when certain genes become activated and other genes become inactivated in an intricately regulated fashion. thus, a differentiated cell develops specific structures and performs certain functions. meanwhile, external signals for cell differentiation include: chemicals secreted by other cells, physical contact with neighbouring cells, and certain molecules in the microenvironment such as growth factors and nutrients
4. stem cells have stem cell potency, which refers to the range of cell types to which a stem cell can differentiate into, and is determined by the number of possible pathways that it can take in its subsequent development through specialisation

Question 3

what are the characteristics of totipotent stem cells?

Answer 3

1. they have the ability to give rise to all cell types that make up an organism
2. they are formed immediately upon fertilisation, up to the first 5 days after fertilisation
3. they consist only of zygotic stem cells that exist as the zygote and the morula, which are the first few cells produced when the zygote divides rapidly by mitosis to form a compact ball of cells
4. these cells have the potential to differentiate into any cell type in the adult body and any cell of the extra-embryonic membranes
5. totipotent stem cells occur at the earliest stage of embryonic development, before the blastocyte stage, where they have the potential to develop into a fetus when they are placed into the uterus via IVF

Question 4

what are the characteristics of pluripotent stem cells?

Answer 4

1. these cells descend from totipotent stem cells, are derived from the inner cell mass of the blastocyst, and have the ability to give rise to types of cells that develop from the mesoderm, endoderm and ectoderm, from which all the cells of the body arise
2. they do not have the potential to make differentiated cells that form the extra-embryonic membranes
3. these cells are formed from 5-7 days after fertilisation
4. the only known sources of human pluripotent stem cells are those isolated and cultured from inner cell mass of early human embryos and from fetal tissue that was destined to be part of the gonads

Question 5

what are the characteristics of multipotent stem cells?

Answer 5

1. multipotent stem cells descend from pluripotent stem cells and can differentiate into many cell lines within a specific type of tissue, but are limited to a number of cell types as a result
2. they are more specialised than totipotent and pluripotent stem cells
3. they are formed after 7 days upon fertilisation
4. multipotent stem cells are found in adult animals, perhaps most organs in the body where they can replace dead or damaged cells
5. multipotent adult stem cells may also be the cells that upon accumulation of sufficient mutations, can produce a clone of cancer cells

Question 6

what are the characteristics of unipotent stem cells?

Answer 6

1. unipotent stems are capable of differentiating along only one cell lineage
2. adult stem cells in many differentiated, undamaged tissues are typically unipotent and give rise to just one cell type under normal condition, allowing for a steady state of self renewal
3. if the tissue becoms damaged and the replacement of multiple cell types is required, multipotent stem cells may become activated to repair the damage

Question 7

what is the process of differentiation in pluripotent stem cells?

Answer 7

1. during differentiation, the cells acquire a particular set of characteristics, enabling them to perform a specific function
2. pluripotent cells give rise to more pluripotent stem cells, and can generate committed stem cells, which give rise to a small population of cells, which usually involves formation of an intermediate progenitor cell
3. progenitor cells can divide a number of times and have a very limited capacity for self-renewal. however, with each successive divison, the cells usually become progressivley more differentiated

Question 8

what are the characteristics of committed stem cells?

Answer 8

commited stem cells are those that have a more limited pathway of development compared to pluripotent cells and are destined to produce a specific group of cells

Question 9

what are the characteristics of progenitor cells?

Answer 9

1. progenitor cells are partly differentiated cells that divide and give rise to differentiated cells
2. these cells are regarded as ‘committed’ to differentiating along a limited number of cellular developmental pathways, although this characteristic may not be as definitive as once thought
3. progenitor cells usually show some evidence of differentiation, although the process is not complete until the fully differentiated cell has been formed

Question 10

how do stem cells undergo self-renewal and and differentiation?

Answer 10

1. asymmetric replicating and differentiating divison is the classic self-renewal process, where the stem cell divides to give rise to two daughter cells, one remaining identical to itself, and the other responding to subtle changes in the local environment, going on to differentiate.
2. symmetric differentiating division and symmetric replication divison also occurs as our bodies age, marked by a decreasing capacity to repair our tissues and the fact that we are more prone to developing tumours
3. asymmetric divison occurs, where the progenitor cells become progressively more differentiated with each division, and eventually stops dividing, having acquired the characteristics of specific differentiated cell type. this allows for the variation of the number and rate of cell divisions, as well as the range of differentiated cell types formed within different tissues. this is preceded by localisation of regulatory molecules or differentiated segregation of cell membrane proteins

Question 11

what are the differences between embryonic and adult stem cells?

Answer 11

1. embryonic stem cells are obtained from the inner cell mass of blastocyte of donated fertilised eggs, of the produce of somatic nuclear transfer, while adult stem cells are obtained from adult tissues and organs
2. embryonic stem cells are pluripotent while adult stem cells are multipotent
3. embryonic stem cells can become most cells/tissues in the body, is easier to culture in the lab and has great potential for developing future therapies to cure diseases, while adult stem cells are hard to culture in the lab and most are limited to become specific tissue types
4. embryonic stem cells are potentially ethically problematic, as blastocysts must be destroyed when cells are removed, while adult stem cells are less ethical problematic and are already used in therapies

Question 12

what are the characteristics of the three types of embryonic stem cells

Answer 12

1. embryonic stem cells can isolated from the inner cell mass of the blastocyte. they can also be extracted from frozen surplus 2-5 day pre-implantation embryos fertilised in vitro
2. embryonic germ cells can be isolated from the precursor to the gonads in aborted fetuses
3. embryonic carcinoma cells can be isolated from teratocarcinomas, which is a tumour that occasionally occurs in a gonad of a fetus. these cells are usually aneuploid

Question 13

what are the defining properties of an embryonic stem cell?

Answer 13

1. capable of undergoing an unlimited number of symmetrical divisions without differentiating
2. exhibit and maintain a stable, diploid, normal complement of chromosomes.
3. pluripotent ESCs can give rise to differentiated cell types that are derived from all three primary germ layers of the embryo
4. ESCs are capable of developing into all fetal tissues during development.
5. ESCs are clonogenic, where a single ESC can give rise to a colony of genetically identical cells, or clones which have the same properties as the original cell
6. ESCs are easy to obtain pure and can be cultivated in large numbers

Question 14

what are the fates of ESCs in a developing embryo?

Answer 14

1. in a developing embryo, the formation of the inner cell mass is considered the first event that distinguishes cells from each other in terms of their relative positions, other than the inside and outside of the morula
2. the cells of the ICM will continue developing to form the embryo, to form a third layer.
3. the layer nearest the amniotic cavity is the ectoderm, which is the outer layer
4. closer to the blastocyst cavity is the endoderm
5. the mesoderm, which is the third layer, eventually forms in the middle
6. this three layered structure is called the primordial embryo, or the gastrula. after the formation of the primary germ layers, the fates of many cells are determined, which means that they are destined to develop as specific cell types

Question 15

what are the properties of adult stem cells?

Answer 15

1. adult stem cells can make identical copies of themselves for long periods of time, and this ability to proliferate is referred to as long-term self-renewal
2. adult stem cells are able to give rise to fully differentiated cells that have mature phenotypes, are fully integrated into the tissue and are capable of specialised functions that are appropriate for the tissue
3. adult stem cells are clonogenic, as a single adult stem cell can give rise to a colony of genetically identical cells, which then gives rise to all the appropriate, differentiated cell types of the tissue in which it resides

Question 16

why is hematopoiesis a particularly well-studied example of cell differentiation

Answer 16

1. blood cells at all stages are relatively accessible
2. blood cells can be grown in culture
3. the hematopoietic system can thus be considered as a hierarchical system with the multipotent hematopoietic stem cell at the top, which generates committed precursors that become committed to the main lineages
4. the stem-cell niche that they inhabit is provided by the bone marrow stromal cells, and secreted signal proteins together with other signals from stomal cells seem necessary to maintain stem-cell proliferation
5. stem cell activity is also regulated by external signals that are provided by the stomal cells in the form of hematopoietic growth factors and other cytokines, regulating the numbers of the different types of cells

Question 17

what are the characteristics and roles of hematopoietic stem cells?

Answer 17

1. only an estimated 1 in 10000-15000 cells in the bone marrow is a HSC
2. HSCs are constantly being generated in the bone marrow where they differentiate into mature types of blood cells.
3. the primary role of HSCs is to replace blood cells, as all the blood cells in the adult human originate from a population of multipotent HSCs located in the bone marrow.
4. HSCs are self-renewing and are the precursors of progenitor cells that are committed to one of the hematopoietic lineages at a later stage
5. HSCs are derived from the mesoderm in the embryo, and colonise a number of definitive blood-forming sites that include the fetal liver, thymus, spleen and bone marrow

Question 18

describe the lymphoid lineage

Answer 18

1. the lymphoid linage produces lymphocytes, which include the two antigen-specific cell types of the immune system, the B and T lymphocytes
2. B lymphocytes are WBCs that come from the bone marrow, and make antibodies that help fight infections
3. T lymphocytes are WBCs that attach virus-infected cells, foreign cells and cancer cells. T lymphocytes also produce a number of substances that regulate the immune respose. they develop in the thymus from precursors that originate from the multipotent stem cells in the bone marrow
4. both B and T lymphocytes undergo a further terminal differentiation after they encounter antigens

Question 19

describe the myeloid lineage

Answer 19

1. the myeloid lineage gives rise to the rest of the WBCs which all derive from the bone marrow in adults, including eosinophils, neutrophils, basophils, mass cells and monocytes
2. WBCs function to defend the body against pathogenic invasion by phagocytosis, identification and destruction of cancer cells that arise within the body, and to phagocytise debris from dead or injured cells during wound healing or tissue repair

Question 20

describe the erythroid lineage

Answer 20

1. the erythroid lineage yields the RBCs and the magakaryocytes, which give rise to blood platelets
2. erythrocytes function mainly to transport gases in the blood
3. blood platelets are for blood clotting functions

Question 21

what are the three main sources of human embryonic stem cell lines?

Answer 21

1. cell lines that already exist
2. spare embryos left over from fertility treatment
3. custom-made embryos created by somatic cell nuclear transfer (SCNT)