A Closer Look: Fat Bodies

Scattered all through the body cavity of the honey bee but especially in the abdomen are irregular masses of a soft, usually white tissue composed of large, loosely united cells. These cell masses are known collectively as the fat body because the cells contain, enmeshed in their cytoplasm, small droplets of oily fat (Snodgrass and Erickson 1992). Within the abdomen, fat bodies are localized subjacent to the epidermis (parietal fat body) and around the gut (visceral fat body). Fat bodies may also be found in gonads and in muscles (Martins and Bitondi 2012). Intermingled with the fat cells (trophocytes) are other cells of larger size having a pale yellowish color, known as oenocytes. These two cell types serve as the structural components of the fat body. Oenocytes are not randomly distributed among the fat cells; their occurrence is lowest near the heart and increases laterally. Between the fat body cells there are many tracheal endcells (Raes et al. 1985). The fat body is bathed in hemolymph, which is important for optimizing the secretion and uptake of molecules in spite of the basal lamina that interfaces between the tissue and the circulating fluid. Although ‘fat body’ implies an integrally functioning organ, this is not the case; all cells function as individual units (Morse and Hooper 1985).

The sheet-like abdominal fat body of the adult worker consists of a single cell layer lining the abdominal wall (Raes et al. 1985). It is segmentally arranged; within each tergite (the dorsal plate of each abdominal segment) the sheet is attached to the epidermis at the posterior end. At the anterior edge, the connection is much looser. Within the middle of the sheet, the dorsal fat body is kept in place by the heart and laterally by longitudinal muscles. The second abdominal segment contains only a very tiny piece of fat body. In the third segment the dorsal fat body is in loose connection with the heart. In the succeeding segments, this connection is gradually more firm until the heart divides the fat body into two lateral parts.

Fat bodies function as production and storage sites for food reserves, chiefly fats, glycogen and protein compounds. The energy stored within fat bodies is especially important during larval growth and other periods when feeding is restricted. Summer bees, which live only four to six weeks, have very few fat bodies. However, winter bees have large numbers of fat bodies, distributed throughout the abdomen. Presumably these fat bodies help the winter bees to live through the long periods of confinement. The development of the fat body into a thick, white organ, filled with protein granules during the Fall is determined directly by its protein intake (Maurizio 1961 as cited by Raes et al. 1985).

Worker honey bees in the Summer begin their adult life lean but develop larger lipid (fat) stores after a few days of consuming a nutrient rich diet of pollen and honey. After one to two weeks of elevated adiposity (fat stored in the fatty tissue of the body), they undergo a dramatic loss of abdominal lipid and subsequently remain lean for the remaining weeks of their life (Toth and Robinson 2005). Lipid loss is associated with thousands of gene expression changes in abdominal fat bodies. Many of these genes are also regulated in young bees by nutrition during an initial period of lipid gain. Surprisingly in older bees, when maximum lipid loss occurs, diet plays less of a role in regulating fat body gene expression (Ament et al. 2011).

Honey bees have two insulin-like peptides (Ilps) with differing spatial expression patterns in the fat body suggesting that AmIlps1 potentially functions in lipid metabolism while AmIlps2 is a more general indicator of nutritional status. Ihle et al. (2014) fed caged worker bees artificial diets high in carbohydrates, proteins or lipids and measured expression of AmIlp1, AmIlp2 and the insulin receptor substrate (IRS) to test their responses to dietary macronutrients. Worker lifespan, weight and gustatory sensitivity to sugar were measured as indicators of physical condition. The expression of AmIlp1 was affected by diet composition and was highest on a diet high in protein. Expression of AmIlp2 and AmIRS were not affected by diet. Workers lived longest on a diet high in carbohydrates and low in protein and lipids. However, bees fed this diet weighed less than those that received a diet high in protein and low in carbohydrates and lipids. Bees fed the high carbohydrate diet were also more responsive to sugar, potentially indicating greater levels of hunger. These results support a role for AmIlp1 in nutritional homeostasis.

The activities of the fat bodies change as honey bee development proceeds. The fat bodies of young larvae very quickly grow to almost the size they are in the adult bee, and they store fats and glycogen. During the final days of the larval period the fat bodies are found to contain less fat, the production of albuminoid and glycogen having increased markedly. At the onset of
pupation, the albuminoid granules increase dramatically in size, filling and swelling out the fat body cells. The cell membranes shrink and rupture, and the contents of the fat body cells flow into the body cavity. This material provides the nourishment needed for the rapid growth of the honey bee pupae (Morse and Hooper 1985).

The fat body has a primordial role (in the earliest stage of development) in the intermediary metabolism. During the larval stage, fat bodies are actively engaged in the metabolism of lipids and carbohydrates and in the synthesis and secretion of proteins, which are stored in large quantities in the hemolymph. The most abundant proteins in larval hemolymph are the hexamerins, also known as larval serum proteins or simply, as storage proteins. Hexamerins are high molecular mass molecules composed by definition of six subunits (Martins et al. 2011). Four different hexamerins have been found in honey bee hemolymph and fat body (70a, 70b, 70c and 110) (Martins et al. 2010).

All of the honey bee hexamerin genes are highly expressed in the larval fat body of workers, queens and drones (Martins et al. 2008). Hexamerins are massively synthesized by the larval fat body and secreted into the hemolymph. Following the larval-to-pupal molt, hexamerins are sequestered by the fat body via receptor-mediated endocytosis (An energy-using process by which cells absorb molecules such as proteins by engulfing them. They gain entry into a cell without passing through the cell membrane.), broken up, and used as amino acid resources for development completion during the non-feeding pupal stage and pharate adult stage (adult waiting to emerge from a cocoon) (Martins et al. 2011).

Hexamerin genes (Hex70a, Hex70b, Hex70c, Hex110) revealed distinct structural organizations and expressing patterns, suggesting that they have specialized functions in honey bee physiology (Bitondi et al. 2006b). Particularly the genomic architecture of the Hex 110 gene diverges from the other honey bee hexamerin genes. Its conceptual translation product is larger and contains exceptionally high glutamine content, 17.8%. All hexamerin genes were abundantly expressed during larval development, but Hex70a and Hex110 were also expressed in adult workers, and at a higher level in fertile than in functionally sterile workers. This denotes a function of these hexamerins in egg production.

In the fifth instar (developmental stage between each molt) feeding larva, all hexamerins exist in a larger quantity in the hemolymph than in the fat body, indicating intense secretion to the hemolymph. During the next developmental phase, when the fifth instar spinning larva prepares for the metamorphic molt, the abundance of all hexamerins increases in the fat body. Based on what is known about the exchange of hexamerins between the fat body and hemolymph, this increase may denote the resorption of hexamerins into the fat body, via sequestration from the hemolymph. From the fifth instar spinning larva to the pharate adult phases, the hexamerins still remain relatively abundant in the fat body, although HEX 110 is the less abundant. The abundance of all hexamerins in the fat body decreases to basal levels near the time of adult emergence, i.e., at the end of the last pharate adult phase. However, HEX 70a levels increase again in the adults, where this protein persists, even in foragers.

Hex 110 transcripts (first step of gene expression in which a particular segment of DNA is copied into messenger RNA (mRNA) by the enzyme RNA polymerase) were found in high levels during the larval stages, then decreased gradually during the pupal stage, and increased again in adults. HEX 110 subunits were highly abundant in larval hemolymph, decreased at the spinning-stage, and remained at low levels in pupae and adults. In 5th instar larvae, neither starvation nor supplementation of larval food with royal jelly changed the Hex 110 transcript levels or the amounts of HEX 110 subunit in hemolymph. In adult workers, high levels of Hex 110 mRNA, but not the respective subunit, were related to ovary activation and also to the consumption of a pollen-rich diet (Bitondi et al. 2006a).

Stage-specific expression profiles were determined as well as the action of hormones and of the diet in controlling the expression of hexamerin genes (Bitondi et al. 2006b). They also measured hexamerin messenger RNA levels in worker bees differing with respect to their reproductive status and in testes of developing drone pupae. Hex 70b expression was maintained at high levels for a prolonged period of time in larvae treated with juvenile hormone or 20-hydroxyecdysone thus indicating a positive hormone regulation at the transcriptional level. Adult workers responded to the lack of dietary proteins by reducing the amount of Hex110 transcripts or HEX70b subunits, thus showing that hexamerin expression depends on diet quality. Interestingly, in testes of drone pupae, Hex70a expression increases in parallel with the time reported for the final development of spermatozoa, suggesting a function in spermatogenesis. Their findings support the premise that honey bee hexamerins are involved in specialized functions related to different physiological processes in honey bee ontogenesis (origin of an individual). The transcript and protein subunit of HEX 70a, were also detected in ovaries and testes (Martins et al. 2008, 2011).

Honey bees are sensitive to earth strength magnetic fields and are reported to contain magnetite (Fe3O4) in their abdomens. Kuterbach et al. (1982) found bands of cells around each abdominal segment that contain numerous electron-opaque, iron-containing granules. The iron is principally in the form of hydrous iron oxides.

Particulate iron was found within the trophocytes of the fat body of the adult honey bee (Kuterbach et al. 1982; Kuterbach and Walcott 1986a). These iron granules differed in their structure and composition from iron granules found in other biological systems. The granules had an average diameter of 0.32 ± 0.07 µm and were composed of iron, calcium and phosphorus in a non-crystalline arrangement. The granules were apparently randomly distributed within the cytoplasm of the cells, and were not associated with any particular cellular organelle.
The trophocytes (fat cells) of larvae and pupae undergo gross cytological changes during development. The cytoplasm of the larval and pupal trophocytes was never seen to contain electron-dense iron granules, even when examined at high magnification. Iron granules are present only in the trophocytes of post-eclosion (emergence from the comb) adults and have the same elemental composition as those in foraging adults (Kuterbach and Walcott 1986b). The granules increase in both size and number during ageing. Iron levels in developing worker honey bees were measured by proton-induced X-ray emission spectroscopy. The rate of iron accumulation was directly related to iron levels in the diet, and the iron can be obtained from pollen and honey, both major food sources of the bee. In adults, the iron content of the fat body reached maximum level (2.4 ± 0.15 µg mg-1 tissue), regardless of the amount of iron available for ingestion. Maximal iron levels are reached at the time when honey bee-workers commence foraging behavior, suggesting that iron granules may play a role in orientation. Alternatively, accumulation of iron in granules may be a method of maintaining iron homeostasis. Many of the enzymes required for energy production and respiration are iron-containing enzymes (e.g. cytochrome oxidase, NADPH oxidase). These enzymes are present in large quantities in the flight muscle, where they are used for energy production during flight. Thus flight activity creates a potentially high requirement for iron.

The iron in the trophocytes is derived, in part, from internal stores. This is shown by comparing the total iron content of the fat bodies of adults raised on an iron deprived diet with the total iron levels in the prepupae. Total iron levels in fat body of iron-deprived bees increased following ecolsion from 0.037 ± 0.005 µg (0 days) to 0.312 ± 0.003 µg (six days). Between day six and day 12, the iron levels remained constant. The total body iron (estimated by summing the total fat body iron and total flight muscle iron) of adults between 0 and 12 days post-eclosion never exceeded the total body iron of the prepupae. Since the pupae do not feed, the iron accumulated by the fat body during that time must have been derived from internal stores (Kuterbach and Walcott 1986b).

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Clarence Collison is an Emeritus Professor of Entomology and Department Head Emeritus of Entomology and Plant Pathology at Miss.State University, Miss. State, MS.