This paper explores a distinction across causal relationships that has yet to receive attention in the philosophical literature, namely, whether causal relationships are reversible or irreversible. We provide an analysis of this distinction and show how it has important implications for causal inference and modeling. This work also clarifies how various familiar puzzles involving preemption and over-determination play out differently depending on whether the causation involved is reversible.

Zuk et al in 2001 identified stem and regenerative cells within the stromal vascular fraction of fat. In preclinical studies, these cells appeared to stimulate angiogenesis and reduce inflammation, and soon thereafter, clinical use of stromal vascular fraction (SVF) evolved as researchers such as Rigotti, Coleman, Mojallal, our group, and others demonstrated that fat can be used for both therapeutic and aesthetic indications. The regenerative effects of fat and its contents on facial aesthetics have been shown at the histologic and cellular level. Regeneration of elastin and collagen fibers as well as improvement in capillary density and reduction of inflammation have been reported. We review our current approach to the use of regenerative cells and different types of fat grafts in facial surgery. The fat graft is classified, both from a regenerative point of view as well as a tissue product that can be modified into different tissue characteristics, depending on the area and condition treated. Clinical use of SVF enriched fat, millifat, microfat, and nanofat grafts as well as composite fat grafts are reviewed. Based on clinical experience and evidence to date, it appears that the regenerative effects seen with the use of SVF in aesthetic surgery are modest, but there appear to be definite histologic findings of regeneration. These improvements may not be clinically apparent to a patient when cell enriched fat grafts are compared to fat grafts alone. However, the subtle changes seen in histology may be cumulative over time. Three types of fat grafts are defined: millifat (parcel size 2.4<), microfat (1.2<), and nanofat (400-600 μm). Each are characterized by their injectability ratings and emulsification parcel size as well as amount of sSVF cells. Newer concepts of periosteal fat grafting, buccal fat pad grafting, pyriform aperture fat grafting, intraorbital fat grafting, and nanofat grafting are discussed. Composite fat grafts are presented as a new concept as is biofilling and biocontouring. The use of regenerative cells in facial surgery is evolving rapidly. Our understanding of the anatomic changes that occur with aging has become more precise and our ability to target histologic changes seen with aging has become more effective. Deep fat compartment grafting, superficial fat grafting, nanofat, and SVF are becoming important components of contemporary facial rejuvenation. The use of regenerative approaches in facial rejuvenation is a logical step in changing the paradigm from surgical treatment of aging to a more proactive prevention and maintenance approach that keeps up with changes in the tissues as they age.

Over the last two decades few topics in philosophy of science have received as much attention as mechanistic explanation. A significant motivation for these accounts is that scientists frequently use the term ‘mechanism’ in their explanations of biological phenomena. Of course, biologists use a variety of causal concepts in their explanations, including concepts like pathways, cascades, triggers, and processes. Despite this variety, mainstream philosophical views interpret all of these concepts with the single notion of mechanism. In using the mechanism concept interchangeably with other causal concepts, it is not clear that these accounts well capture the diversity of causal structures in biology. This article analyses two causal concepts in biology—the notions of ‘mechanism’ and ‘pathway’—and how they figure in biological explanation. I argue that these concepts have unique features, that they are associated with distinct strategies of causal investigation, and that they figure in importantly different types of explanation. 1Introduction2Mechanisms: The Basics3The Pathway Concept 3.1Main features3.2Investigative strategy4Explanation: Pathways and Mechanisms5Conclusion: Mechanism and Pathway as Analogy

Kaplan and Craver claim that all explanations in neuroscience appeal to mechanisms. They extend this view to the use of mathematical models in neuroscience and propose a constraint such models must meet in order to be explanatory. I analyze a mathematical model used to provide explanations in dynamical systems neuroscience and indicate how this explanation cannot be accommodated by the mechanist framework. I argue that this explanation is well characterized by Batterman's account of minimal model explanations and that it demonstrates how relationships between explanatory models in neuroscience and the systems they represent is more complex than has been appreciated.

a b s t r a c tWe argue that Koch's postulates are best understood within an interventionist account of causation, in the sense described in Woodward (2003). We show how this treatment helps to resolve interpretive puzzles associated with Koch's work and how it clarifies the different roles the postulates play in providing useful, yet not universal criteria for disease causation. Our paper is an effort at rational reconstruction; we attempt to show how Koch's postulates and reasoning make sense and are normatively justified within an interventionist framework and more difficult to understand within alternative frameworks for thinking about causation.