is concerned with the ABIOTIC and BIOTIC environment into which an individual or group changes is born with must meet its biological NEEDS
All the processes we categorize mainly as Development, Evolution, or Physiology occur in response to an environment that is more or less stable, but never static.
For example, At the cellular level of organization, development involves the progressive specialization or tuning of specific lineages of cells that begin with a single cell. Differentiation occurs as a result of activation of genes which operate in their respective microniches. Cells, like organisms, exist in an ecosystem in which they represent the best adapted form possible given their intrinsic possibilities and the resources available to support them. Fuchs & Blau (2020)[i] call them “architects of their own niches.” Their development—in which they manifest specialized structure and function to enable optimal accommodation depends on their local niches… but as they proliferate, they change the niche and those changes then feed back to affect further proliferation.
[i] Tissue Stem Cells: Architects of Their Niches Elaine Fuchs1,* and Helen M. Blau Cell Stem Cell. Author manuscript; available in PMC 2021 Feb 4. Published in final edited form as: Cell Stem Cell. 2020 Oct 1; 27(4): 532–556. doi: 10.1016/j.stem.2020.09.011
- Billions of cells are lost daily from our body’s tissues, which are in a perpetual state of flux throughout our lifetime. The molecular engines that drive this turnover are self-renewing tissue stem cells. The work horses are stem cell-generated, short-lived progenitors that balance proliferation and differentiation, thereby maintaining tissue homeostasis. The homeostatic requirements for cellular replacements are tissue and context specific. They are continual in blood, epidermis, and intestine, episodic in the hair follicle and lactating mammary gland, and limited in brain and muscle. However, even largely quiescent stem cells, such as those of the muscle, can be mobilized into action when their tissue is injured. To guard against infection and heal wounds, the normal homeostatic cues—termed the “milieu interieur” in 1865 by Claude Bernard—must be overridden in ways that are still being determined.
- Understanding stem cell behaviors necessitates knowledge of their local environment or “niche.” Increasing evidence shows that whether quiescent or active, stem cells are not simple passive responders to their niches; rather, they play an integral role in creating and communicating with their niches that envelope them. Regenerative signals, emanating either from a build up in crosstalk with niche factors or from marked environmental changes upon injury, alter stem cell behavior and disrupt the homeostatic equilibrium of the tissue. In part, the stem cells’ own progenies can become important niche components: while early short-lived progenitors can send transient activating cues back to their stem cell parents to fuel their self-renewal and boost tissue growth (Blau et al., 2015; Hsu et al., 2014; Mondal et al., 2014; Porpiglia et al., 2017), differentiated progeny can home back to their niche to halt further proliferation and tissue regeneration and restore homeostasis (Montarras et al., 2005; Sato et al., 2011; Yu and Scadden, 2016). In this way, tissue regeneration is orchestrated by a delicate balance of temporally coordinated cellular interactions and molecular feedback circuits in which stem cells play a central role.
- Heterologous stem cell niche components include the basement membrane, rich in extracellular matrix and stem cell growth factors, as well as blood vessels, lymphatic capillaries, nerves, stromal, adipose, and a variety of tissue-resident immune cells that function with stem cells to guard against tissue damage and pathogens. The beauty of having immune cells as integral constituents of stem cell niches is that many are mobile, able to migrate to local lymph nodes and stimulate non-resident immune cells, which can then travel through the circulation to the site of tissue damage and contribute to the inflammatory response that clears pathogens and damaged cells from the tissue (Fan and Rudensky, 2016). Reinstating homeostasis, however, relies upon tissue repair, which is incompatible with inflammation. Since the stem cells are responsible for the reparative phase of the response, there must be intricate immune-stem cell communication to ensure not only that the pathogen invasion is under control, but also that the inflammation is subsequently dampened in order to facilitate repair (Arnold et al., 2007; Burzyn et al., 2013; Fan and Rudensky, 2016). How this happens is still largely a mystery, but a few clues are beginning to emerge.
- Here we review how stem cells serve as architects of their own niche. This microenvironment envelops the stem cell and dictates its function during homeostasis while allowing it to rapidly mobilize its tissue regenerating energies when an injury occurs. We focus on two markedly different tissues—the skin epithelium and the skeletal muscle—both of which are subjected to stress and damage throughout life. We highlight features of the stem cells and their niches and discuss how they combine context-specific and universal mechanisms to maintain tissue fitness. We discuss increasing evidence that tissue stem cells sense and communicate with an amazingly diverse array of niche components. As we are beginning to learn, this complexity enables stem cells to not only deflect minor insults and maintain homeostasis, but also remain poised to sense and respond to natural regenerative stimuli and to the diverse array of tissue damage and other stresses they encounter throughout their lifetime. Given the complexity of stem cell niches, it is also perhaps not surprising that across many different tissues, including muscle and skin, aging often involves a breakdown of extrinsic niche components, rather than the intrinsic self-renewal capacity of its stem cell residents (Blau et al., 2015; Ge et al., 2020; Pentinmikko et al., 2019; Raaijmakers, 2019; Segel et al., 2019; Tierney and Sacco, 2016). An additional consequence of aging-associated changes is an increase in tissue stiffness that can disrupt homeostatic stem cell mechanosensing (Cosgrove et al., 2014; Gilbert et al., 2010; Madl et al., 2018).
Although all domains of DEEP ETHOLOGY manifest extensive interconnectedness, it is, for most of us, more apparent in the web of life described in ECOLOGY. The ENVIRONMENT in which all organisms find themselves must–in the course of natural selection–provide for all biological needs, and all organisms compete for limited resources to meet those needs. (Whatever resource(s) might be in shortest supply is termed “selection pressure.” The flexibility of an individual (or species) in coping with these pressures results in different patterns: sexual selection (selection pressure if “female choice”), ecological selection, stabilizing selection, disruptive selection and directional selection, each describing the patterns of successful coping with pressure.
[we will revisit selection pressure relative to biological NEEDS when visiting PHYSIOLOGY: the first response of individuals to the real or perceived or even threatened inability to meet a biological need evokes a STRESS RESPONSE.]