Autologous vs. allogeneic transplant

In an autologous transplant, a patient receives back his or her own cells, whereas in an allogeneic transplant a patient receives someone else’s cells. The advantage of the first procedure is that a patient doesn’t run the risk of an immune system rejection.

Blastocyst

The early mammalian embryo before it implants in the uterus. In human development, the blastocyst forms on day 4-5 after fertilization; in mouse development, on day 3-4. The blastocyst’s round sphere consists of an outer layer of cells and an inner cavity that contains embryonic stem cells. The inner stem cells mostly disperse or differentiate within a few days. The blastocyst stage, therefore, is a key time for harvesting stem cells.

Cell line

A continuously dividing batch of cells that originated from one individual. Consider a cell line made from a mammal’s embryonic stem cells. One cell or dozens are removed from the blastocyst and placed in the right culture conditions, which ensures that the cells will keep dividing indefinitely. Thus the cells are “immortalized.” When a cell line isn’t being used, it is frozen; when it’s needed, it’s thawed and grown again. The advantage of a cell line is that it provides researchers with easy access to a source of unlimited numbers of cells from one individual that are all genetically similar.

Chimera

An animal composed of cells from two or more genetically different animals. Technically speaking, even a person who has received a bone marrow transplant from someone other than an identical twin can be considered a chimera. In biology, the chimeras often utilized are chimeric mice, which are made when cells from one mouse embryo are added to cells of another mouse embryo. The two sets of genes can be distinguished with markers, providing insights into how cells mature and specialize.

Cloning

Creating a genetic duplicate of an organism. In the modern sense, cloning an animal consists of transferring the nucleus of a single cell into an egg cell. This reprograms the nucleus. Its genes lose their previous identity and return to a potent state that results in the growth of a developing embryo.

Differentiate

When a cell advances from a less mature to a more mature state. Cells that differentiate, via their progeny, are headed toward specialization. In some cases a cell can dedifferentiate, or revert to a less specialized state.

Embryonal carcinoma cell—EC cells

Stem cells derived from a teratoma type of tumor. Once in culture, they can be called EC cells.

Embryonic germ cells—EG cells

Precursors to egg and sperm cells found in the developing fetus. Once in culture, they can be referred to as EG cells.

Embryonic stem cells—ES cells

Immature, unspecialized cells in the inner cell mass of an early embryo. Once in culture, they can go by the nickname ES cells.

Genome

An organism’s full set of genes; its DNA. In the case of humans, every cell contains the entire genome except for red blood cells. A cell in your kidney, for instance, contains the human genome’s approximately 30,000 genes, as does a skin cell or a heart cell.

Germ cells

Egg and sperm cells. These germinal cells in the reproductive organs populate the germ line, which is simply the lineage consisting of egg/sperm cells that perpetuate a species.

Hematopoietic stem cell

An adult stem cell that gives rise to the specialized red and white blood cells of the blood and immune systems. These cells are found in a child’s umbilical cord blood as well as an adult’s bone marrow.

Implantation

When the embryo implants in the mother’s uterus. In mice this happens around day 4 after fertilization; in humans, around day 7-8.

In vitro fertilization (IVF)

Fertilizing an egg outside of the body with sperm in order to create an embryo. The fertilized embryo may be transferred into a woman or frozen and stored.

In vitro vs. In vivo

Cells grown in vitro are cells that, having been removed from an individual, are grown in a specially prepared culture media in a dish. Cells in vivo are cells as they exist in a living system. When tissue sections are looked at through a microscope, their cells, although dead, are seen in the context of a natural in vivo environment.

Knockout mouse

A mouse that has one or more genes inactivated. This is done by altering a gene in an embryonic stem cell and then putting the altered stem cell into an early mouse embryo, whereupon the progeny cells will populate many of the developing tissues. Knockout mice serve as models that mimic many human genetic diseases.

Lineage

The pathway that cells follow, by way of countless divisions and stages of differentiation, as they advance from an immature state to a specialized state. The further down this pathway they go, the more they “commit” to a specific “fate,” or tissue.

Mesenchymal stem cell

(Also called a marrow stromal cell) An adult stem cell found in the bone marrow that gives rise to bone, fat, cartilage, and muscle cells.

Multipotent

An adult stem cell that produces more than one specialized cell type, as opposed to a unipotent stem cell that gives rise to just one type of specialized cell. For instance, a hematopoietic stem cell is multipotent; a sperm stem cell is unipotent.

Neural stem cell

An adult stem cell that resides in central nervous system tissue and generates that tissue’s cell types: neurons and glia. Glial cells consist of astrocytes and oligodendrocytes.

Neurogenesis

The birth of new neurons in the central nervous system.

Parthenote

An egg cell that begins cleavage and development like an embryo, but without fertilization by sperm. A product of parthenogenesis.

Plasticity

A cell’s ability to change its differentiation state, or even its cell type. Cells have plasticity, for instance, if they can switch be-

tween pluripotent and mature, or mature and pluripotent, or even from one lineage to another, as from blood stem cell to muscle stem cell.

Pluripotent

A cell that can generate every cell type found in the embryo and adult, but not the cells of structures that support the embryo.

Progenitor

A “child” or “grandchild” of a stem cell. A progenitor cell is more differentiated than a stem cell and yet has some stemness that can permit it to generate multiple types of mature cells. Unlike a stem cell, it lacks the unlimited ability to self-renew.

Stem cell

An unspecialized cell that is capable of generating, via its progeny, differentiated cells and/or more cells like itself (stem cells). A stem cell ostensibly got its name because of its unique ability to give rise to different cell types, sometimes throughout the life of an organism. In contrast, when a specialized cell divides, it generates more of itself for a limited number of divisions.

Stemness

A stem cell’s potential to generate multiple mature cell types.

Totipotent

A cell that has the ability to differentiate into all cell types. In mammals, only the zygote and the cells resulting from its first few divisions have this all-encompassing ability, which includes being able to generate cells that make up the umbilical cord and placenta.

Transdifferentiate

When a cell from one tissue switches lineages and becomes a cell of another tissue. An example would be a bone marrow cell changing to a skin cell.

Zygote

The cell that results from a sperm cell fertilizing an egg cell. The zygote—the fertilized egg—is the beginning of development.