Murine Adipose Organ Development
The adipose organ of fetal rats is dissectible. In this plate, the fetal adipose organ of a rat at gestational day 20 is shown in the upper panel. It is visually evident that most of the organ is represented by the anterior subcutaneous depot and that all
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© Springer International Publishing Switzerland 2018 S. Cinti, Obesity, Type 2 Diabetes and the Adipose Organ, https://doi.org/10.1007/978-3-319-40522-3_12
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Murine Adipose Organ Development
12.1 Murine Adipose Organ Development PLATE 12.1
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The adipose organ of fetal rats is dissectible. In this plate, the fetal adipose organ of a rat at gestational day 20 is shown in the upper panel. It is visually evident that most of the organ is represented by the anterior subcutaneous depot and that all other sites are poorly developed at this gestational stage. The posterior depot is poorly developed and mainly located in the inguinal area. Very poorly developed visceral fat is visible in the mesenteric, mediastinal-periaortic, and perirenal sites. The color of the organ is mainly brown or brownish with the whitest parts represented by the mesenteric fat (see scheme). These data suggest that the adipose organ development starts with the formation of different independent depots located in subcutaneous and visceral areas of the body. The anatomy of adult murine adipose organ is formed by a unitary structure, likely deriving from a progressive fusion of all different independent depots. The gross anatomy of progressive enlargement of the organ during the first twelve postnatal days is shown in the lower panel. Development is greatly prevalent in subcutaneous fat where BAT (IS: interscapular, SS: subscapular, DC: deep cervical) is prevalent over WAT (SC: superficial cervical) in the anterior depot and WAT (DL: dorsolumbar, I: inguinal, G: gluteal) is prevalent over BAT in the posterior depot. Visceral depots are poorly developed, but it is visually evident that the most developed is the mediastinal-periaortic (MP) BAT. Mesenteric (M), retroperitoneal (R), and epididymal (E) depots are mainly WAT and inter-renal (IR) mainly BAT. Note that most of the anterior subcutaneous depot, until postnatal day 2, is formed only by BAT; then (see days 6 and 12) WAT develops into this depot that therefore develops into a mixed depot in adult mice.
Gross Anatomy
Plate 12.1 Gross anatomy of fetal and newborn rats adipose organ 387
Murine Adipose Organ Development
PLATE 12.2
The first signs of development in the adipose organ of fetal rats are observed around the 15th day of intrauterine life in the dorsal upper thoracic region, dorsolaterally to the spinal cord among the dorsal muscles. The anlage of the adipose organ in the perinatal period produces mainly brown adipocytes and has therefore been referred to as brown adipose tissue anlage. Brown adipocytes developing in this area share with muscle cells the transcription factor Myf5, a master regulator of skeletal muscle differentiation and myogenesis. Interestingly retinoblastoma protein (Rb) seems to play an important role determining white versus brown adipocyte differentiation. Dilated capillaries (CAP) and “mesenchymal” cells in a loose matrix limited by skeletal muscles characterize histologically the anlage at this stage of development (left top panel). “Mesenchyma
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