Biomedical Engineering Reference
In-Depth Information
hematopoietic stem cells, maintain relatively rapid division, to produce the large
numbers of lymphocytes and red blood cells required by the body (Huang et al.,
2007). This is true also of the rapidly cycling cells of the broad band at the middle
of the intestinal crypt (Potten, 1991). Other stem cells, such as those in the skin
and colon, maintain a slow and constant growth, replenishing the tissue (Alonso
and Fuchs, 2003; Bjerknes, 1996), and yet other stem cells in the brain and most
other tissues remain quiescent and are only activated when stimulated by tissue
damage or hormonal exposure (Schatton and Frank, 2008). When a cell is
sufficiently differentiated that it still proliferates, but gives rise to only one cell
type, one can speak of it as determined (Spemann, 1918), canalized (Waddington,
1966), or unipotent (Blanpain et al., 2007). An example would be a megakaryo-
cyte that can give rise only to platelets. The only subsequent steps in multicellular
organism development are ''terminal differentiation'' and senescence. Whether or
not de-differentiation or trans-differentiation occurs, once cells have progressed
through their developmental path, remains a matter of controversy (Leri et al.,
2005; Raff, 2003). Stem cell migration is a normal part of mammalian develop-
ment and is a particular characteristic of the early genital ridge (Gilbert et al.,
2006). The regulation of stem cell division in the adult is controlled through
epigenetic mechanisms, such as DNA methylation and histone acetylation (Do
et al., 2006). Orderly ''replenishment'' of tissues requires cell division and the
capability of repairing some damaged tissues, as in liver regeneration (Michalo-
poulos, 2007). It is generally acknowledged that the latter requires a certain
amount of cellular ''reprogramming''. A central feature of the involvement of
stem cells in cancer is the dysregulation of the epigenetic control of stem cell
proliferation (Bapat, 2007; Bengochea et al., 2008; Karpinets and Foy, 2005).
2 Breast Anatomy
In 1840 Astley Paston Cooper published The Anatomy of the Breast (Cooper,
1840). The striking plates in this classical text are based on the author's studies
of the breasts of seven previously lactating cadavers through dyed paraffin
injection. The illustrations identified numerous structures in the vascular and
ductile system and clarified, for the first time, the gross anatomy of the drainage
network. The staining techniques were apparently very advanced for that era
but crude when compared to modern imaging methods; the wax may have
changed and displaced some of the delicate ductile structures of the breast.
Moreover, studying a non-lactating breast from a cadaver does not reveal its
normal, hormone-mediated growth and development.
The gross anatomy of the human breast shows it to be one of the only organs
not fully developed at birth (Lawrence and Lawrence, 1985). The breast
changes in size, shape, and function through puberty, pregnancy, and during
and after lactation (Creasy et al., 2004). Breast growth and development involve
two distinguishable processes: organogenesis (ductile and lobular growth) and
lactogenesis (Dontu et al., 2003).
