Electron microscopical studies on the genesis of white adipocytes: differentiation of immature pericytes into adipocytes in transplanted preadipose tissue

Adipose Tissue Development Relies on Coordinated Extracellular Matrix Remodeling, Angiogenesis, and Adipogenesis

Despite developing prenatally, the adipose tissue is unique in its ability to undergo drastic growth even after reaching its mature size. This development and subsequent maintenance rely on the proper coordination between the vascular niche and the adipose compartment. In this review, the process of adipose tissue development is broken down to explain (1) the ultrastructural matrix remodeling that is undertaken during simultaneous adipogenesis and angiogenesis, (2) the paracrine crosstalk involved during adipose development, (3) the mechanical regulators involved in adipose growth, and (4) the proteolytic and paracrine oversight for matrix remodeling during adipose development. It is crucial to gain a better understanding of the complex relationships that exist between adipose tissue and the vasculature during tissue development to provide insights into the pathological tissue expansion of obesity and to develop improved soft-tissue reconstruction techniques.

Elucidating the Preadipocyte and Its Role in Adipocyte Formation: a Comprehensive Review

Adipogenesis is a complex process whereby the multipotent adipose-derived stem cell is converted to a preadipocyte before terminal differentiation into the mature adipocyte. Preadipocytes are present throughout adult life, exhibit adipose fat depot specificity, and differentiate and proliferate from distinct progenitor cells. The mechanisms that promote preadipocyte commitment and maturation involve numerous protein factor regulators, epigenetic factors, and miRNAs. Detailed characterization of this process is currently an area of intense research and understanding the roles of preadipocytes in tissue plasticity may provide insight into novel approaches for tissue engineering, regenerative medicine and treating a host of obesity-related conditions. In the current study, we analyzed the current literature and present a review of the characteristics of transitioning adipocytes and detail how local microenvironments influence their progression towards terminal differentiation and maturation. Specifically, we detail the characterization of preadipocyte via surface markers, examine the signaling cascades and regulation behind adipogenesis and cell maturation, and survey their role in tissue plasticity and health and disease.

Perivascular adipose tissue (PVAT) in atherosclerosis: a double-edged sword

Perivascular adipose tissue (PVAT), the adipose tissue that surrounds most of the vasculature, has emerged as an active component of the blood vessel wall regulating vascular homeostasis and affecting the pathogenesis of atherosclerosis. Although PVAT characteristics resemble both brown and white adipose tissues, recent evidence suggests that PVAT develops from its own distinct precursors implying a closer link between PVAT and vascular system. Under physiological conditions, PVAT has potent anti-atherogenic properties mediated by its ability to secrete various biologically active factors that induce non-shivering thermogenesis and metabolize fatty acids. In contrast, under pathological conditions (mainly obesity), PVAT becomes dysfunctional, loses its thermogenic capacity and secretes pro-inflammatory adipokines that induce endothelial dysfunction and infiltration of inflammatory cells, promoting atherosclerosis development. Since PVAT plays crucial roles in regulating key steps of atherosclerosis development, it may constitute a novel therapeutic target for the prevention and treatment of atherosclerosis. Here, we review the current literature regarding the roles of PVAT in the pathogenesis of atherosclerosis.

Heterogeneity of adipose tissue in development and metabolic function

Adipose tissue is a central metabolic organ. Unlike other organs, adipose tissue is compartmentalized into individual depots and distributed throughout the body. These different adipose depots show major functional differences and risk associations for developing metabolic syndrome. Recent advances in lineage tracing demonstrate that individual adipose depots are composed of adipocytes that are derived from distinct precursor populations, giving rise to different populations of energy-storing white adipocytes. Moreover, distinct lineages of energy-dissipating brown and beige adipocytes exist in discrete depots or within white adipose tissue depots. In this Review, we discuss developmental and functional heterogeneity, as well as sexual dimorphism, between and within individual adipose tissue depots. We highlight current data relating to the differences between subcutaneous and visceral white adipose tissue in the development of metabolic dysfunction, with special emphasis on adipose tissue expansion and remodeling of the extracellular matrix. Moreover, we provide a detailed overview of adipose tissue development as well as the consensus and controversies relating to adult adipocyte precursor populations.

Modelling physiological and pathological conditions to study pericyte biology in brain function and dysfunction

BACKGROUND

Brain pericytes ensheathe the endothelium and contribute to formation and maintenance of the blood-brain-barrier. Additionally, pericytes are involved in several aspects of the CNS immune response including scarring, adhesion molecule expression, chemokine secretion, and phagocytosis. In vitro cultures are routinely used to investigate these functions of brain pericytes, however, these are highly plastic cells and can display differing phenotypes and functional responses depending on their culture conditions. Here we sought to investigate how two commonly used culture media, high serum containing DMEM/F12 and low serum containing Pericyte Medium (ScienCell), altered the phenotype of human brain pericytes and neuroinflammatory responses.

METHODS

Pericytes were isolated from adult human brain biopsy tissue and cultured in DMEM/F12 (D-pericytes) or Pericyte Medium (P-pericytes). Immunocytochemistry, qRT-PCR, and EdU incorporation were used to determine how this altered their basal phenotype, including the expression of pericyte markers, proliferation, and cell morphology. To determine whether culture media altered the inflammatory response in human brain pericytes, immunocytochemistry, qRT-PCR, cytometric bead arrays, and flow cytometry were used to investigate transcription factor induction, chemokine secretion, adhesion molecule expression, migration, phagocytosis, and response to inflammatory-related growth factors.

RESULTS

P-pericytes displayed elevated proliferation and a distinct bipolar morphology compared to D-pericytes. Additionally, P-pericytes displayed lower expression of pericyte-associated markers NG2, PDGFRβ, and fibronectin, with notably lower αSMA, CD146, P4H and desmin, and higher Col-IV expression. Nuclear NF-kB translocation in response to IL-1β stimulation was observed in both cultures, however, P-pericytes displayed elevated expression of the transcription factor C/EBPδ, and lower expression of the adhesion molecule ICAM-1. P-pericytes displayed elevated phagocytic and migratory ability. Both cultures responded similarly to stimulation by the growth factors TGFβ₁ and PDGF-BB.

CONCLUSIONS

Despite differences in their phenotype and magnitude of response, both P-pericytes and D-pericytes responded similarly to all examined functions, indicating that the neuroinflammatory phenotype of these cells is robust to culture conditions.

Organization of nuclear architecture during adipocyte differentiation

Obesity is a serious health problem worldwide since it is a major risk factor for chronic diseases such as type II diabetes. Obesity is the result of hyperplasia (associated with increased adipogenesis) and hypertrophy (associated with decreased adipogenesis) of the adipose tissue. Therefore, understanding the molecular mechanisms underlying the process of adipocyte differentiation is relevant to delineate new therapeutic strategies for treatment of obesity. As in all differentiation processes, temporal patterns of transcription are exquisitely controlled, allowing the acquisition and maintenance of the adipocyte phenotype. The genome is spatially organized; therefore decoding local features of the chromatin language alone does not suffice to understand how cell type-specific gene expression patterns are generated. Elucidating how nuclear architecture is built during the process of adipogenesis is thus an indispensable step to gain insight in how gene expression is regulated to achieve the adipocyte phenotype. Here we will summarize the recent advances in our understanding of the organization of nuclear architecture as progenitor cells differentiate in adipocytes, and the questions that still remained to be answered.

Convertible visceral fat as a therapeutic target to curb obesity

New therapeutic and preventative strategies are needed to address the growing obesity epidemic. In animal models, brown adipose tissue activation and the associated heat produced contribute to countering obesity and the accompanying metabolic abnormalities. Adult humans also have functional brown fat. Here, we present and discuss the concepts of murine and human white adipose tissue plasticity and the transdifferentiation of white adipocytes into brown adipocytes. Human visceral adipocytes - which are crucial contributors to the burden of obesity and its complications - are particularly susceptible to such transdifferentiation. Therefore, we propose that this process should be a focus of anti-obesity research. Approved drugs that have browning properties as well as future drugs that target molecular pathways involved in white-to-brown visceral adipocyte transdifferentiation may provide new avenues for obesity therapy.

Intimal pericytes as the second line of immune defence in atherosclerosis

Inflammation plays an essential role in the development of atherosclerosis. The initiation and growth of atherosclerotic plaques is accompanied by recruitment of inflammatory and precursor cells from the bloodstream and their differentiation towards pro-inflammatory phenotypes. This process is orchestrated by the production of a number of pro-inflammatory cytokines and chemokines. Human arterial intima consists of structurally distinct leaflets, with a proteoglycan-rich layer lying immediately below the endothelial lining. Recent studies reveal the important role of stellate pericyte-like cells (intimal pericytes) populating the proteoglycan-rich layer in the development of atherosclerosis. During the pathologic process, intimal pericytes may participate in the recruitment of inflammatory cells by producing signalling molecules and play a role in the antigen presentation. Intimal pericytes are also involved in lipid accumulation and the formation of foam cells. This review focuses on the role of pericyte-like cells in the development of atherosclerotic lesions.

Pericytes at the intersection between tissue regeneration and pathology

Perivascular multipotent cells, pericytes, contribute to the generation and repair of various tissues in response to injury. They are heterogeneous in their morphology, distribution, origin and markers, and elucidating their molecular and cellular differences may inform novel treatments for disorders in which tissue regeneration is either impaired or excessive. Moreover, these discoveries offer novel cellular targets for therapeutic approaches to many diseases. This review discusses recent studies that support the concept that pericyte subtypes play a distinctive role in myogenesis, neurogenesis, adipogenesis, fibrogenesis and angiogenesis.

Gene expression and protein secretion during human mesenchymal cell differentiation into adipogenic cells

Background

Mesenchymal stromal cells (MSC) can be obtained from potentially any tissue from the human body, but cells purified from different sources are undoubtedly different, and for each medical application, the MSC with the best regenerative potential should be chosen.

Results

Bone marrow-derived mesenchymal stromal cells (BM-MSC), adipose tissue-derived mesenchymal stromal cells (AT-MSC) and Wharton’s Jelly-derived mesenchymal stromal cells (WJ-MSC) were isolated from human tissues and were cultured under differentiation media supplemented with fetal bovine serum. We quantified the expression of stem cell and adipocyte genetic markers using quantitative real time PCR, as well as the secretion of cytokines, extracellular matrix components and growth factors using Luminex and ELISA. All three MSC differentiated into adipogenic cells. AT-MSC showed the highest shift in ADIPOQ, CEBPA and PPARG mRNA expression. BM-MSC kept high expression levels of stem-cell markers SOX2 and POU5F1. WJ-MSC showed the lowest increase in mRNA expression when cells were induced to differentiate into adipocytes. Regarding protein secretion, adipocyte-like cells generated from WJ-MSC secreted the highest chemokine levels. AT-MSC-derived adipocyte-like cells secreted the lowest cytokine amounts and the highest quantity of collagen types I and III. Adipocyte-like cells obtained from BM-MSC secreted high amounts of most angiogenic factors, growth factors TGF-β1 and TGF-β2, collagens type II and IV, heparan sulfate, laminin and aggrecan.

Conclusion

Mesenchymal stromal cells purified from different tissues have a different behavior when induced to differentiate into adipocyte-like cells.

White, brown and pink adipocytes: the extraordinary plasticity of the adipose organ

In mammals, adipocytes are lipid-laden cells making up the parenchyma of the multi-depot adipose organ. White adipocytes store lipids for release as free fatty acids during fasting periods; brown adipocytes burn glucose and lipids to maintain thermal homeostasis. A third type of adipocyte, the pink adipocyte, has recently been characterised in mouse subcutaneous fat depots during pregnancy and lactation. Pink adipocytes are mammary gland alveolar epithelial cells whose role is to produce and secrete milk. Emerging evidence suggests that they derive from the transdifferentiation of subcutaneous white adipocytes. The functional response of the adipose organ to a range of metabolic and environmental challenges highlights its extraordinary plasticity. Cold exposure induces an increase in the 'brown' component of the organ to meet the increased thermal demand; in states of positive energy balance, the 'white' component expands to store excess nutrients; finally, the 'pink' component develops in subcutaneous depots during pregnancy to ensure litter feeding. At the cell level, plasticity is provided not only by stem cell proliferation and differentiation but also, distinctively, by direct transdifferentiation of fully differentiated adipocytes by the stimuli that induce genetic expression reprogramming and through it a change in phenotype and, consequently function. A greater understanding of adipocyte transdifferentiation mechanisms would have the potential to shed light on their biology as well as inspire novel therapeutic strategies against metabolic syndrome (browning) and breast cancer (pinking).

Hyaline globules in mammary myofibroblastoma: a case report

A 52-year-old otherwise healthy woman presented with a solitary firm mass in the right breast. Histopathological evaluation of the 1.5-cm mass showed a mammary myofibroblastoma of the conventional spindle-cell type. High-power examination of hematoxylin-eosin-stained sections showed round, eosinophilic, intracytoplasmic, as well as extracellular, hyaline globules. These 5- to 20-µm globules appeared gray with a pinkish rim on Masson's trichrome stain. Immunohistochemically, the hyaline globules were strongly reactive with smooth muscle myosin heavy chain, desmin, and caldesmon. Histologically similar inclusion bodies have been reported in phylloides tumors--including those with myoid differentiation. To our knowledge, this is the first description of hyaline globules, a peculiar histological curiosity with no known clinical significance, in mammary myofibroblastoma.

Role of pericytes in skeletal muscle regeneration and fat accumulation

Stem cells ensure tissue regeneration, while overgrowth of adipogenic cells may compromise organ recovery and impair function. In myopathies and muscle atrophy associated with aging, fat accumulation increases dysfunction, and after chronic injury, the process of fatty degeneration, in which muscle is replaced by white adipocytes, further compromises tissue function and environment. Some studies suggest that pericytes may contribute to muscle regeneration as well as fat formation. This work reports the presence of two pericyte subpopulations in the skeletal muscle and characterizes their specific roles. Skeletal muscle from Nestin-GFP/NG2-DsRed mice show two types of pericytes, Nestin-GFP-/NG2-DsRed+ (type-1) and Nestin-GFP+/NG2-DsRed+ (type-2), in close proximity to endothelial cells. We also found that both Nestin-GFP-/NG2-DsRed+ and Nestin-GFP+/NG2-DsRed+ cells colocalize with staining of two pericyte markers, PDGFRβ and CD146, but only type-1 pericyte express the adipogenic progenitor marker PDGFRα. Type-2 pericytes participate in muscle regeneration, while type-1 contribute to fat accumulation. Transplantation studies indicate that type-1 pericytes do not form muscle in vivo, but contribute to fat deposition in the skeletal muscle, while type-2 pericytes contribute only to the new muscle formation after injury, but not to the fat accumulation. Our results suggest that type-1 and type-2 pericytes contribute to successful muscle regeneration which results from a balance of myogenic and nonmyogenic cells activation.

Adipose tissue plasticity from WAT to BAT and in between

Adipose tissue plays an essential role in regulating energy balance through its metabolic, cellular and endocrine functions. Adipose tissue has been historically classified into anabolic white adipose tissue and catabolic brown adipose tissue. An explosion of new data, however, points to the remarkable heterogeneity among the cells types that can become adipocytes, as well as the inherent metabolic plasticity of mature cells. These data indicate that targeting cellular and metabolic plasticity of adipose tissue might provide new avenues for treatment of obesity-related diseases. This review will discuss the developmental origins of adipose tissue, the cellular complexity of adipose tissues, and the identification of progenitors that contribute to adipogenesis throughout development. We will touch upon the pathological remodeling of adipose tissue and discuss how our understanding of adipose tissue remodeling can uncover new therapeutic targets. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

The vascular endothelium of the adipose tissue gives rise to both white and brown fat cells

Adipose tissue expansion involves the enlargement of existing adipocytes, the formation of new cells from committed preadipocytes, and the coordinated development of the tissue vascular network. Here we find that murine endothelial cells (ECs) of classic white and brown fat depots share ultrastructural characteristics with pericytes, which are pluripotent and can potentially give rise to preadipocytes. Lineage tracing experiments using the VE-cadherin promoter reveal localization of reporter genes in ECs and also in preadipocytes and adipocytes of white and brown fat depots. Furthermore, capillary sprouts from human adipose tissue, which have predominantly EC characteristics, are found to express Zfp423, a recently identified marker of preadipocyte determination. In response to PPARγ activation, endothelial characteristics of sprouting cells are progressively lost, and cells form structurally and biochemically defined adipocytes. Together these data support an endothelial origin of murine and human adipocytes, suggesting a model for how adipogenesis and angiogenesis are coordinated during adipose tissue expansion.

Adipose tissue stem cells meet preadipocyte commitment: going back to the future

White adipose tissue (WAT) is perhaps the most plastic organ in the body, capable of regeneration following surgical removal and massive expansion or contraction in response to altered energy balance. Research conducted for over 70 years has investigated adipose tissue plasticity on a cellular level, spurred on by the increasing burden that obesity and associated diseases are placing on public health globally. This work has identified committed preadipocytes in the stromal vascular fraction of adipose tissue and led to our current understanding that adipogenesis is important not only for WAT expansion, but also for maintenance of adipocyte numbers under normal metabolic states. At the turn of the millenium, studies investigating preadipocyte differentiation collided with developments in stem cell research, leading to the discovery of multipotent stem cells within WAT. Such adipose tissue-derived stem cells (ASCs) are capable of differentiating into numerous cell types of both mesodermal and nonmesodermal origin, leading to their extensive investigation from a therapeutic and tissue engineering perspective. However, the insights gained through studying ASCs have also contributed to more-recent progress in attempts to better characterize committed preadipocytes in adipose tissue. Thus, ASC research has gone back to its roots, thereby expanding our knowledge of preadipocyte commitment and adipose tissue biology.

Transplantation of adipose tissue and stem cells: role in metabolism and disease

Humans and other mammals have three main adipose tissue depots: visceral white adipose tissue, subcutaneous white adipose tissue and brown adipose tissue, each of which possesses unique cell-autonomous properties. In contrast to visceral adipose tissue, which can induce detrimental metabolic effects, subcutaneous white adipose tissue and brown adipose tissue have the potential to benefit metabolism by improving glucose homeostasis and increasing energy consumption. In addition, adipose tissue contains adipose-derived stem cells, which possess the ability to differentiate into multiple lineages, a property that might be of value for the repair or replacement of various damaged cell types. Adipose tissue transplantation has primarily been used as a tool to study physiology and for human reconstructive surgery. Transplantation of adipose tissue is, however, now being explored as a possible tool to promote the beneficial metabolic effects of subcutaneous white adipose tissue and brown adipose tissue, as well as adipose-derived stem cells. Ultimately, the clinical applicability of adipose tissue transplantation for the treatment of obesity and metabolic disorders will reside in the achievable level of safety, reliability and efficacy compared with other treatments.

Fighting fat with fat: the expanding field of adipose stem cells

We are in the midst of a dire, unprecedented, and global epidemic of obesity and secondary sequelae, most prominently diabetes and hyperlipidemia. Underlying this epidemic is the most hated of cells, adipocytes, and their inherent dynamic ability to expand and renew. This capacity highlights a heretofore undefined stem compartment. Recent in vivo studies, relying upon lineage tracing and flow cytometry methods, have begun to unravel the identity of adipose stem cells, their niche, and the dynamism central to adipose expansion. Thus, the field is moving in a direction that may allow us to manipulate adipose stem cells to beneficial therapeutic ends.

White fat progenitor cells reside in the adipose vasculature

White adipose (fat) tissues regulate metabolism, reproduction, and life span. Adipocytes form throughout life, with the most marked expansion of the lineage occurring during the postnatal period. Adipocytes develop in coordination with the vasculature, but the identity and location of white adipocyte progenitor cells in vivo are unknown. We used genetically marked mice to isolate proliferating and renewing adipogenic progenitors. We found that most adipocytes descend from a pool of these proliferating progenitors that are already committed, either prenatally or early in postnatal life. These progenitors reside in the mural cell compartment of the adipose vasculature, but not in the vasculature of other tissues. Thus, the adipose vasculature appears to function as a progenitor niche and may provide signals for adipocyte development.

Differences in stem and progenitor cell yield in different subcutaneous adipose tissue depots

BACKGROUND

Human adipose tissue has been shown to contain multipotent cells with properties similar to mesenchymal stromal cells. While there have been many studies of the biology of these cells, no study has yet evaluated issues associated with tissue harvest.

METHODS

Adipose tissue was obtained from the subcutaneous space of the abdomen and hips of 10 donors using both syringe and pump-assisted liposuction. Tissue was digested with collagenase and then assayed for the presence of different stem and progenitor cell types using clonogenic culture assays, including fibroblast colony-forming unit (CFU-F) and alkaline phosphatase-positive colony-forming unit (CFU-AP). Paired analysis of samples obtained from the same individual was used to compare harvest method and site.

RESULTS

Syringe suction provided significantly greater recovery of adipocytes and a non-significant trend towards improved recovery of cells in the adipocyte-depleted fraction. There was considerable donor-to-donor variation in stem cell recovery. However, paired analysis of tissue obtained from different subcutaneous sites in the same donor showed that tissue harvested from the hip yielded 2.3-fold more CFU-F/unit volume and a 7-fold higher frequency of CFU-AP than that obtained from the abdomen. These differences were statistically significant.

DISCUSSION

Harvest site influences the stem and progenitor cell content of subcutaneous adipose tissue.

Lipomatous Variant of Angiomyofibroblastoma: Report of Two Cases and Review of the Literature

Angiomyofibroblastoma (AMF) is a rare, benign, mesenchymal tumor occurring mainly in the female genital tract. Some cases contain scattered mature adipocytes, but the lipomatous variant in which mature adipose tissue is prominent or striking is rare. Only five cases have been reported in the English literature. We report two more such cases that were composed of 70 to 80% and 30 to 40% adipose tissue, respectively. Immunohistochemical analysis showed that the tumor cells were positive for estrogen receptor, progesterone receptor, vimentin, and Bcl-2, and negative for cytokeratin AE1/1, EMA, and CD117. Tumor cells in the first case were positive for CD34 but not desmin and muscle-specific actin. The opposite expression profile of these three markers was observed in tumor cells in the second case: positive for desmin and muscle-specific actin and negative for CD34. Rare cells were positive for S-100 in adipose-rich areas in the first case. Our findings indicate that the tumor cells in the lipomatous variant have similar immunoprofile to those of usual AMF and support the concept that the lipomatous variant probably represents an extreme end of a wide spectrum of differentiation in AMF.

Lipomatous hemangiopericytoma of the head and neck: immunohistochemical and DNA ploidy analyses

BACKGROUND

Lipomatous hemangiopericytoma (LHPC) is a newly described rare soft tissue tumor with unpredictable biologic behavior and is difficult to diagnose by conventional histologic parameters. The molecular analyses of this entity to date are sparse. Only a few cases of LHPC have been reported. Although one case of LHPC in the sinonasal region was briefly reported, this is the first case in the head and neck region with detailed clinicopathologic features and molecular analysis of this entity.

METHODS

We reported a case of LHPC in a 55-year-old woman with a slowly growing lesion in the occipital area that was diagnosed by CT and MRI and removed surgically. Immunohistochemical and DNA ploidy analyses were performed.

RESULTS

A panel of 16 markers was included for immunohistochemical analysis. Diffuse immunopositivity of CD57 in our case provides supportive evidence that LHPC is linked with HPC because this marker is also present in approximately 50% of conventional HPCs. CD57 should be used in the immunohistochemical panel in any lesion suspected to be LHPC. Furthermore, CD57 along with CD34 and XIIIa is thought to stain for primitive mesenchymal stem cells, suggesting a bimodal/multimodal differentiation of LHPC. By flow cytometry, we found that tumor cells were 100% diploid with the S-phase fraction (SPF) being 3.21%. A significant positive correlation was detected between nuclear proliferating index and SPF (p < 0.001, by Spearman analysis). These findings provide molecular evidence indicating a benign nature of LHPC.

CONCLUSIONS

Contrary to the old belief that HPC has an aggressive nature, this variant of tumor looks less aggressive. The patient was followed for 1 year without any evidence of recurrence, supporting our pathologic hypothesis.

Adipose tissue angiogenesis

A review of adipose tissue angiogenesis includes the morphological and cytochemical development of adipose tissue vasculature and the concept of primitive fat organs. Spatial and temporal relationships between fetal vascular and fat cell development are discussed, including depot- and genetic-dependent arteriolar differentiation. The relationship between connective tissue deposition and elaboration of adipose tissue vasculature is discussed with respect to regulating adipocyte development in a depot-dependent manner. In vitro studies indicated that depot-dependent vascular traits may be attributable to intrinsic growth characteristics of adipose tissue endothelial cells. These studies indicate that adipogenesis may be regulated by factors that drive angiogenesis. Fundamental aspects of angiogenesis, including basement membrane breakdown, vasculogenesis, angiogenic remodeling, vessel stabilization, and vascular permeability were reviewed. Critical angiogenic factors include vascular endothelial growth factor (VEGF), VEGF receptors, angiopoietins (Ang), ephrins, matrix metalloproteinases, and the plasminogen enzymatic system. Vascular endothelial growth factor is the most critical factor because it initiates the formation of immature vessels and disruption of a single VEGF allele leads to embryonic lethality in mice. Expression of VEGF is influenced by hypoxia, insulin, growth factors, and several cytokines. Angiogenic factors secreted and/or produced by adipocytes or preadipocytes are discussed. Vascular endothelial growth factor expression and secretion by adipocytes is regulated by insulin and hypoxia, and is associated with adipose tissue accretion. Vascular endothelial growth factor accounts for most of the angiogenic activity of adipose tissue. The proposed role of leptin as an adipogenic factor is reviewed with respect to efficacy on various aspects of angiogenesis relative to other angiogenic factors. The VEGF and leptin genes are both hypoxia inducible, but potential links between VEGF and leptin gene expression have not been examined. Finally, several studies including a study of mice treated with antiangiogenic factors indicate that adipose tissue accretion can be controlled through the vasculature per se.

Lipomatous hemangiopericytoma: a rare variant of hemangiopericytoma that may be confused with liposarcoma

We describe the clinicopathologic features and biologic behavior of 16 cases of histologically benign hemangiopericytoma containing a variable amount of mature fat as an intrinsic part of the neoplasm. These so-called lipomatous hemangiopericytomas occurred primarily in men (12 men and 4 women) with a mean age of 54 years (range, 33-74 years). All occurred in deep soft tissue and had an average size of 10 cm when first detected. All were characterized by a relatively sharp border and typical histologic features of hemangiopericytomas, including oval to round cells surrounding a sinusoidal and staghorn vasculature often with perivascular hyalinization. Mature fat varied in amount but usually occupied approximately one quarter to three quarters of the area of tumor. Mitotic activity was low, with more than half the cases having no mitotic activity. Five cases showed moderate nuclear atypia. In four cases, the pericytic regions had sclerotic zones. In contrast to liposarcoma, neither lipoblasts nor isolated atypical hyperchromatic cells within mature fat, as are seen in well-differentiated liposarcoma, were present. Immunohistochemistry performed in four cases showed factor XIIIa in tumor cells and an intricate pattern of immunoreactivity around cells for type IV collagen. CD34 and smooth-muscle actins were identified in two of four cases. Follow-up in seven cases showed no recurrences or metastases within the follow-up period of 1 to 7 years. Because these lesions are located in deep soft tissue and contain large amounts of mature fat, they could be mistaken for well-differentiated liposarcomas in limited biopsy material, although the distinction is easily made in examining the entire specimen. The lipomatous hemangiopericytoma represents yet another example of a bimodal mesenchymal tumor containing mature fat and raises the question of whether a common cytogenetic abnormality can explain the emergence of two clonal populations in this hybrid tumor.

Lipomatous hemangiopericytoma: a morphologically distinct soft tissue tumor

Hemangiopericytoma of soft tissue is a controversial pathologic entity. The relative nonspecificity of the characteristic branching capillary pattern and cytologic features of the constituent cells, in addition to the lack of a distinct immunohistochemical staining profile, has resulted in uncertainty and a lack of consensus regarding this subgroup of tumors. Notwithstanding the doubt surrounding this entity, a morphologically unique variant, designated lipomatous hemangiopericytoma, was reported in 1995. To our knowledge, there have been no further reports of these tumors since the original description. We describe a lipomatous hemangiopericytoma that arose within the thigh of a 41-year-old woman. The tumor presented as a slowly enlarging, minimally tender, pulsatile mass. The tumor was completely excised and was found to be composed of an admixture of typical hemangiopericytoma and predominantly mature adipose tissue. Unlike previous descriptions of this entity, the current example exhibited a full range of adipocyte differentiation, including many multivacuolated adipocytes of variable size with characteristic nuclear scalloping (lipoblast-like cells). The mitotic count was less than 2 per 10 high-power fields examined. The clinical course has been benign during the short follow-up period. We discuss the pathologic features, including the immunohistochemical staining profile and ultrastructural appearance of this distinctive tumor, and briefly discuss the relationship between hemangiopericytoma and solitary fibrous tumor of soft tissue, a neoplasm with many clinical and pathologic similarities.

Extravulvar subcutaneous cellular angiofibroma

Cellular angiofibroma is a rare distinctive mesenchymal neoplasm of the vulva or perineal region. We report here one unique extravulvar case. A 43-year-old woman presented with an asymptomatic tumor, 7 cm in diameter, located in the subcutaneous tissue of the chest below the left submammary sulcus. Histologically, the lesion was composed of uniform spindled stroma cells, numerous thick-walled vessels, and scarce mature adipocytes. An additional feature was the presence of prominent perivascular lymphoid aggregates. The stromal cells were positive for vimentin and negative for CD34 and muscle, epithelial, myoepithelial, or neural markers. Although nasopharyngeal angiofibroma or the group of acral angiofibromas have a concurrent heading, cellular angiofibroma should not be mistaken with them. Differential diagnosis of this distinctive tumor especially includes aggressive angiomyxoma, angiomyofibroblastoma, superficial angiomyxoma, vascular myxolipoma, and other tumors with spindle cells reminiscent of those in angiofibroma.

Continuous subendothelial network formed by pericyte-like cells in human vascular bed

The localization of pericyte-like cells in adult human vascular bed was studied immunocytochemically using anti-pericyte antibody--3G5. In large, medium- and small-size arteries, pericyte-like cells were identified in the inner intimal layer, predominantly in the subendothelium, in the outer layer of the media and in the adventitia in vasa vasora. In veins, pericytes were found in the subendothelial layer of the intima, in the media and in the adventitia where they were located in vasa vasora. These cells were revealed in the subendothelium of arterioles, venules and capillaries of the pia mater encephali. Examination of intimal en face preparations of various size blood vessels showed that subendothelial pericyte-like cells form a network contacting with their processes. Thus we found a continuous subendothelial network formed by pericyte-like cells in the human vascular bed.

Angiomyofibroblastoma of the female genital tract: analysis of 17 cases including a lipomatous variant

The clinicopathological and immunohistochemical profile of 17 cases of angiomyofibroblastoma (AMF) arising in the genital tract of females is reported. The lesions usually presented as painless masses and were located in the superficial vulvar region (15 cases), canal of Nuck (one case), and perineum (one case) in women ranging in age from 38 to 60 years (median, 46 years). The tumors were well delineated and ranged in size from 2 to 8 cm in greatest dimension. Microscopically, they were composed of spindled and epithelioid mesenchymal cells arranged in cords and nests preferentially arrayed around numerous small to medium-sized vessels. Mitotic activity ranged from 0 to 7 mitoses per 50 high-power fields (HPF) with no abnormal mitotic figures. Minimal nuclear atypia was appreciated. Intralesional fat was present in 12 cases and in two of these cases constituted most of the tumor (lipomatous variant of AMF). Tumor cells expressed vimentin (five of five cases), estrogen receptor protein (six of six cases), progesterone receptor protein (five of six cases), desmin (six of eight cases), CD34 (one of six cases), and smooth muscle actin (one of seven cases). None of the eight women with follow-up of up to 25 years (mean, 7.8 years) after simple excision developed a recurrence. This study confirms the benign nature of AMF, broadens its morphological spectrum to include a lipomatous variant, and proposes an origin from a perivascular stem cell that is capable of myofibroblastic and fatty differentiation.

Ultrastructure and immunohistochemistry of the embryonic type of fat identified in the human infant breast

In this paper we describe the light and electron microscopic appearance of the embryonic type of fat in human infant breast, together with immunocytochemical findings. This fat tissue was composed of numerous capillaries surrounded by a mixed population of undifferentiated mesenchymal cells and preadipocytes at various stages of differentiation. The preadipocytes were characterised by a number of cytoplasmic processes, varying numbers of lipid droplets, and an envelope of electrondense material outside the cell membrane. Immunocytochemistry showed a characteristic distribution of collagen type IV adjacent to and vimentin and S100 protein within the preadipocytes. This is the first report of the ultrastructure of the human mammary embryonic type of fat. The possible role of the embryonic type of fat in the development and growth of the human breast is discussed.

The behaviour of liposarcoma in tissue culture. A specific growth pattern

There have been few studies of liposarcoma in tissue culture. We report a study of 27 cases of liposarcoma including examples of all subtypes in tissue culture. All the cases showed a uniform growth pattern of glass adherence by polygonal cells with short processes distributed in a random fashion. The cytoplasm of these cells became progressively loaded with glycogen followed by lipid droplets. These lipid droplets tended to fuse progressively to form a single or dominant vacuole. At the same time, the cells tended to adopt a rounded shape. This pattern of growth, which was seen in all subtypes of liposarcoma with minimal variation, is different from that shown by other soft tissue tumours and is similar to the growth of embryonal fat in tissue culture.

Capillary pericytes: perspectives and future trends

A complete understanding of the microcirculation requires full knowledge of the structure and function of each of the constituent cells, including pericytes. Vascular endothelium and smooth muscle cells have been investigated intensively during the last two decades, but much less is known about the metabolism and function of capillary pericytes. However, the development of new electron microscopy techniques and the application of new cell culture and molecular biology techniques should allow for the rapid elucidation of the cellular biochemistry and the microvascular function and pathology of this ubiquitous capillary cell.

In vivo differentiation of brown adipocytes in adult mice: an electron microscopic study

The differentiation of brown adipocyte precursor cells was studied in interscapular brown adipose tissue of adult mice by electron microscopy. Different stages of cell differentiation were characterized in situ. Previous autoradiographic studies suggested that interstitial cells represent the precursor cells of fully differentiated brown adipocytes. The present observations provide morphological evidence for a progressive differentiation of interstitial stem cells into mature brown adipocytes. Four typical stages of development were identified: (1) interstitial cells, (2) protoadipocytes, (3) preadipocytes, and (4) mature brown adipocytes. Interstitial stem cells were small spindle shaped cells, situated between brown adipocytes and characterized by a high nuclear-cytoplasmic ratio, the scarcity of organelles, and the absence of lipid inclusions. Protoadipocytes resembled interstitial cells except that they contained a few tiny lipid droplets in their cytoplasm. Preadipocytes had a larger cytoplasm enclosing many mitochondria and lipid droplets; the smooth endoplasmic reticulum was well developed surrounding the lipid droplets, and was closely associated with the mitochondria. Preadipocytes had the typical structure of growing cells, developing long cytoplasmic processes between and around blood capillaries. Mature brown adipocytes represented the final stage of differentiation. Almost all their cellular volume was occupied by lipid droplets and numerous mitochondria with very dense cristae. Brown adipocytes were also characterized by a tight association with blood capillaries, as expected from metabolically active cells requiring oxygen and substrates. These observations provide direct ultrastructural evidence for a progressive differentiation of interstitial cells into brown adipocytes with a continuum of intermediate cellular types.

In vitro induction of the fat-storing phenotype in a liver connective tissue cell line-GRX

Liver connective tissue cells have been characterized as perisinusoidal myofibroblasts and hepatic lipocytes (Ito cells, fat-storing cells). A concept of a single mesenchymal cell population that may be modulated between these two phenotypes has been postulated. We have previously established a continuous murine cell line, GRX, obtained from fibrotic granulomatous lesions induced by schistosomal infection in mouse liver. This cell line is considered to represent liver myofibroblasts. In the present study we have induced the conversion of these cells into lipocyte (fat storing) phenotype by treatment with insulin and indomethacin. We have quantified the lipid synthesis and the increase of activity of involved enzymes during the induction of the fat-storing phenotype and described modifications of cell organization along this modulation of cell functions.

S-100 protein in white preadipocytes: an immunoelectronmicroscopic study

Differentiation of adipocytes from their precursor cells (preadipocytes) is an important problem in the study of the pathogenesis of obesity. Unfortunately, among the immature stages of adipocytes, only relatively differentiated forms can be identified by their fine structure; because early preadipocytes cannot be distinguished from fibroblasts solely on the basis of their morphology, it is impossible to assess the size of the preadipocyte population. S-100 protein has been identified in various mammalian tissues and recently mature adipocytes have been shown to be positive for this protein. Because fibroblasts are negative for S-100 protein, the present study tested the S-100 immunoreactivity of preadipocytes by the peroxidase-antiperoxidase (PAP) preembedding method at the ultrastructural level both in vivo and in culture. Mature adipocytes and early preadipocytes, including fibroblast-like cells devoid of lipid droplets, were positive both in vivo and in culture. Endothelial cells and pericytes were negative; but flattened, lipid-free, fibroblast-like cells surrounding the pericytes were positive. True fibroblasts both in vivo and in culture were negative. Therefore, S-100 protein can be a useful biochemical marker in distinguishing fibroblasts from early preadipocytes.

Epididymal white adipose tissue after cold stress in rats. I. Nonmitochondrial changes

Epididymal adipose tissue in the rat is generally considered to be "pure" white adipose tissue (WAT) with a characteristic structure and function. Previous studies in cats have, however, indicated that adipose tissue with the morphological appearance of WAT could be converted into a tissue with the morphological appearance of brown adipose tissue (BAT) by intermittent cold stress. The present electron microscopic and morphometric study describes the effect of intermittent cold stress on the epididymal WAT of young rats. The tissue volume decreased markedly as did the lipid content. The mitochondrial volume increased dramatically. The extracellular matrix was vastly reduced as was the thickness of the plasma membrane, and the number of gap junctions between adipocytes increased markedly. Indications of neoinnervation and neovascularization were observed. A great abundance of preadipocytes indicated proliferative activity of the endothelium. The low amount of lipid droplets and a relative abundance of smooth and rough endoplasmic reticulum. Golgi apparatus, and lysosomes in the epididymal WAT of cold-stressed rats gave the cells the morphological appearance of young adipocytes or preadipocytes whereas the hypertrophic and hyperplastic mitochondria, the relative paucity of ribosomes on lipid droplet membranes, and the increased innervation and vascularization gave the cells the morphological characteristics of brown adipose tissue.

Tritiated thymidine autoradiographic study on postnatal development of epididymal adipose tissue in the normal mouse

Cell proliferation was studied in the developing epididymal adipose tissue of mice by means of 3H-thymidine autoradiography. 4 mice in each age group were killed 30 min after a single injection of 3H-thymidine given at the 6th, 10th, 16th, 21st, 28th, 35th, 42nd, 49th or 56th postnatal day. The labeling index of the adipose tissue was high for several postnatal days but it was highest on the 6th day. The labeling index decreased thereafter, and cell proliferation appeared to almost cease after 56 days. Another 16 mice were given a single injection of 3H-thymidine at 6 days old; pairs of animals were killed at 30 min, 2, 4, 6, 8, 10, 14 and 18 days later. From the 2nd to 6th days of pulse labeling, the number of labeled fat cells increased significantly, whereas the number of labeled poorly differentiated mesenchymal cells decreased reciprocally. The labeling index of fat cells remained unchanged from the 6th to the 18th day. These results suggest that there is a population of fat cell-precursor cells in the mesenchymal tissue of the new born mouse, and that it takes 2-6 days for the precursor cells to become mature fat cells in the epididymal fatty tissue.

Proliferation and differentiation of brown adipocytes from interstitial cells during cold acclimation

The mechanisms of brown adipocyte proliferation and differentiation during cold acclimation (and/or adaptation to hyperphagia) have been studied by quantitative photonic radioautography. [3H]thymidine was injected to warm-acclimated (25 degrees C) rats and to animals exposed to 5 degrees C for 2 days. Samples of interscapular brown adipose tissue were collected for quantitative analysis of mitotic frequencies at various periods of time (4 h-15 days) after the injection of [3H]thymidine, the rats being maintained at the temperatures to which they were initially exposed. Confirming our previous results [Bukowiecki et al., Am. J. Physiol. 242 (Endocrinol. Metab. 5): E353-E359, 1982], it was found that cold exposure for 2 days markedly enhanced mitotic activity in endothelial cells, interstitial cells, and brown preadipocytes rather than in fully differentiated brown adipocytes. The total tissue labeling index (percent of labeled nuclei) increased approximately 70 times over control values. We now report that cellular labeling progressively increased in mature brown adipocytes during cold acclimation, whereas it correspondingly decreased in interstitial cells and brown preadipocytes. This indicates that the sequence of events for cellular differentiation is interstitial cells----brown preadipocytes----mature brown adipocytes. Remarkable, labeling frequency did not change in endothelial cells during cold acclimation demonstrating that these cells cannot be considered as progenitors of brown adipocytes. It is suggested that brown adipocyte proliferation and differentiation from interstitial cells represent the fundamental phenomena explaining the enhanced capacity of cold-acclimated and/or hyperphagic rats to respond calorigenically to catecholamines.

Lipoblastomatosis: ultrastructure of two cases and relationship to human fetal white adipose tissue

Two cases of lipoblastomatosis were examined by electron microscopy. In each there was a spectrum of adipose cell differentiation from mature adipocytes to undifferentiated mesenchymal cells. The cells had various shapes and were loosely related to one another and to small blood vessels in a stroma containing sparse collagen and proteoglycan granules. Adipocytes and lipoblasts had a discrete basal lamina but this was absent in immature mesenchymal cells, which had prominent intermediate (10-nm) microfilaments and few or no lipid droplets. Neither case had foci resembling brown adipose tissue or hibernoma (large polygonal cells closely apposed to one another and to capillaries with densely packed pleomorphic mitochondria and focal undulating plasmalemmal invaginations). Both the ultrastructure of lipoblastomatosis and its characteristic loose lobular arrangement by light microscopy closely resemble human fetal "white" adipose tissue.

Hibernoma: distinctive light and electron microscopic features and relationship to brown adipose tissue

Four hibernomas and samples of developing human brown and white adipose tissue were observed. Distinctive features of hibernomas were 1) lobules of closely apposed large polygonal cells and capillaries; 2) three principal cell types (granular eosinophilic, multivacuolated, and univacuolated) varying in prominence from case to case; 3) investment of each tumor cell by basal lamina; 4) an inverse relationship between lipid droplet size and the number of mitochondria per unit of cytoplasm; 5) pleomorphic mitochondria with dense matrixes or large, round mitochondria with transverse lamellar cristae; 6) undulating plasmalemmal invaginations; 7) micropinocytotic vesicles; 8) periodic short plasmalemmal densities; and 9) a conspicuous lack of cytoplasmic membrane systems. The frequent association of micropinocytotic vesicles and undulating plasmalemmal invaginations in proximity to capillaries strongly suggests that the invaginations represent a localized cell membrane adaptation for efficient endocytosis. In human fetal brown adipose tissue, which is first recognizable in fetuses of 21 weeks' gestational age, the most characteristic cell was the polygonal multivacuolated cell. Univacuolated cells were present in brown adipose tissue of older fetuses, and in infants and adults entire lobules containing univacuolated cells coexisted with lobules of multivacuolated cells and granular eosinophilic cells. The ultrastructure of human brown adipose tissue resembled that of hibernomas and was similar to previously described features of this tissue in animals. Developing white adipose tissue differed from brown adipose tissue by its loose plexiform arrangement of capillaries and spindle-shaped cells in less circumscribed lobules and by the absence of polygonal multivacuolated cells. The authors did not identify centripetal lobular maturation in white adipose tissue, but peripheral growth "caps" were a common finding in maturing brown adipose tissue. They consider brown adipose tissue to be a special form of adipose tissue, the variable cytologic composition of which is reflected in the histologic spectrum of hibernomas.

Brown adipose tissue hyperplasia: a fundamental mechanism of adaptation to cold and hyperphagia

Cold acclimation (4 degrees C) and "cafeteria diets" increased the thermic response of rats to catecholamines. This phenomenon was accompanied by six- to eightfold increases of interscapular brown adipose tissue (IBAT) weight, total tissue cytochrome oxidase activity, and total number of brown adipocytes. Quantitative radioautographic experiments using [3H]thymidine disclosed that cold exposure markedly enhanced the mitotic activity in blood capillaries and small-venule endothelial cells, adipose tissue interstitial cells, and preadipocytes rather than in fully differentiated brown adipocytes. IBAT mitotic index increased 70 times over control values after only 2 days of cold exposure. Thereafter, the proliferative activity progressively decreased. IBAT cell composition was modified during cold acclimation as the percentage of interstitial cells and preadipocytes increased over the other cellular types. Because brown adipose tissue is the principal site of norepinephrine-induced thermogenesis in homeothermal animals, it is suggested that brown adipocyte proliferation from precursor cells represents the fundamental phenomenon explaining the increased capacity of cold-acclimated animals to respond calorigenically to catecholamines.

Clonal preadipocyte cell lines with different phenotypes derived from murine marrow stroma: factors influencing growth and adipogenesis in vitro

We have isolated continuously growing cell lines derived from mouse bone marrow stroma. These cell lines were independently obtained, and though they showed morphologies ranging from the epithelioid to the fibroblastoid patterns, they all differentiated into adipocytes. Subclones obtained from two cell lines had a very high frequency (90-100%) of differentiation into adipocytes after two or three weeks of arrested growth. Though extensive accumulation of lipid often mechanically impaired mitosis, the cells committed to adipocytes did not suffer an irreversible loss of proliferative capacity. Adipogenesis was obtained in conditions similar to those required for fat cell formation in long-term bone marrow culture. The cell lines were found to be insensitive to insulin as a signal of adipocyte differentiation. The ultrastructural characteristics of the preadipocytes and fat cells are also similar to those of the fat cells developing in long-term bone marrow culture. As such, these cell lines should prove useful for analysing cell/cell interactions in haemopoiesis.

Malignant fibrous histiocytoma of bone associated with focal hemangiopericytomatous pattern

A case of malignant fibrous histiocytoma of the tibia of a 49-year-old Japanese farmer was presented. The patient was diagnosed as malignant hemangiopericytoma by biopsy, whereas the amputated material showed a marked pleomorphism representing features of malignant fibrous histiocytoma. Ultrastructurally, undifferentiated mesenchymal cells, intermediate cells between the undifferentiated mesenchymal cells and fibroblastic cells, fibroblastic cells and histiocytoid cells including bizarre giant cells were confirmed in a broad modulating spectrum. The hemangiopericytomatous lesion was predominated by intermediate cells, the ultrastructure of which bore a close resemblance to that of pericytes, and was regarded to be resulted from a differentiation of undifferentiated mesenchymal cells towards the pericyte.

Nevus lipomatosus cutaneous superficialis. An electron microscopic study of four cases

Four cases of nevus lipomatosus cutaneous superficialis were studied under the electron microscope. The results showed the vascular origin of the fat cell and differentiation of young fat cells into mature fat cells. The findings are comparable to those observed in fetal adipogenesis.