<P> Properties of stem cells can be illustrated in vitro, using methods such as clonogenic assays, in which single cells are assessed for their ability to differentiate and self - renew . Stem cells can also be isolated by their possession of a distinctive set of cell surface markers . However, in vitro culture conditions can alter the behavior of cells, making it unclear whether the cells shall behave in a similar manner in vivo . There is considerable debate as to whether some proposed adult cell populations are truly stem cells . </P> <P> Embryonic stem (ES) cells are the cells of the inner cell mass of a blastocyst, an early - stage embryo . Human embryos reach the blastocyst stage 4--5 days post fertilization, at which time they consist of 50--150 cells . ES cells are pluripotent and give rise during development to all derivatives of the three primary germ layers: ectoderm, endoderm and mesoderm . In other words, they can develop into each of the more than 200 cell types of the adult body when given sufficient and necessary stimulation for a specific cell type . They do not contribute to the extra-embryonic membranes or the placenta . </P> <P> During embryonic development these inner cell mass cells continuously divide and become more specialized . For example, a portion of the ectoderm in the dorsal part of the embryo specializes as' neurectoderm', which will become the future central nervous system . Later in development, neurulation causes the neurectoderm to form the neural tube . At the neural tube stage, the anterior portion undergoes encephalization to generate or' pattern' the basic form of the brain . At this stage of development, the principal cell type of the CNS is considered a neural stem cell . These neural stem cells are pluripotent, as they can generate a large diversity of many different neuron types, each with unique gene expression, morphological, and functional characteristics . The process of generating neurons from stem cells is called neurogenesis . One prominent example of a neural stem cell is the radial glial cell, so named because it has a distinctive bipolar morphology with highly elongated processes spanning the thickness of the neural tube wall, and because historically it shared some glial characteristics, most notably the expression of glial fibrillary acidic protein (GFAP). The radial glial cell is the primary neural stem cell of the developing vertebrate CNS, and its cell body resides in the ventricular zone, adjacent to the developing ventricular system . Neural stem cells are committed to the neuronal lineages (neurons, astrocytes, and oligodendrocytes), and thus their potency is restricted . </P> <P> Nearly all research to date has made use of mouse embryonic stem cells (mES) or human embryonic stem cells (hES) derived from the early inner cell mass . Both have the essential stem cell characteristics, yet they require very different environments in order to maintain an undifferentiated state . Mouse ES cells are grown on a layer of gelatin as an extracellular matrix (for support) and require the presence of leukemia inhibitory factor (LIF) in serum media . A drug cocktail containing inhibitors to GSK3B and the MAPK / ERK pathway, called 2i, has also been shown to maintain pluripotency in stem cell culture . Human ES cells are grown on a feeder layer of mouse embryonic fibroblasts (MEFs) and require the presence of basic fibroblast growth factor (bFGF or FGF - 2). Without optimal culture conditions or genetic manipulation, embryonic stem cells will rapidly differentiate . </P>

Does a nerve cell have a cell wall