Storage & Mobilization of Lipids

• Dietary fats brought to the cells via the circulation as chylomicrons,

• Lipids synthesized in the liver and transported in blood with very low-density lipoproteins (VLDLs), and

• Free fatty acids and glycerol synthesized by the adipocytes.

Chylomicrons (Gr. chylos, juice + micros, small) represent particles of variable size, up to 1200 nm in diameter, formed from ingested lipids in epithelial cells lining the small intestine and transported in the blood and lymph. They consist of a core containing mainly triglycerides, surrounded by a stabilizing monolayer of phospholipids, cholesterol, and several apolipoproteins.

VLDLs appear as much smaller complexes (30–80 nm, providing a greater surface-to-volume ratio) of similar lipid and protein composition to chylomicrons, but undergo synthesis and release in liver cells. Clinical tests for circulating levels of lipoproteins routinely measure blood lipids after fasting to allow depletion of chylomicrons. Varying levels of apoproteins and triglycerides in the complexes allow their categorization according to density, from VLDL to high-density lipoprotein (HDL).

In adipose tissue both chylomicrons and VLDLs become hydrolyzed at the luminal surfaces of blood capillaries by lipoprotein lipase, an enzyme synthesized by the adipocytes and transferred to the capillary cell membrane (Figure 6–2). Free fatty acids then enter the adipocytes by both active transport and diffusion. Within the adipocytes the fatty acids combine with glycerol phosphate, supplied by glucose metabolism, to again form triglycerides, which then get deposited in the growing lipid droplet. Insulin stimulates glucose uptake by adipocytes and accelerates its conversion into triglycerides, and the production of lipoprotein lipase.

Figure 6–2 Lipid storage and mobilization from adipocytes. Triglycerides are transported by blood and lymph from the intestine and liver in lipoprotein complexes known as chylomicrons (Chylo) and VLDLs. In the capillary endothelial cells of adipose tissue, these complexes are partly broken down by lipoprotein lipase, releasing free fatty acids and glycerol. The free fatty acids diffuse from the capillary into the adipocyte, where they are reesterified to glycerol phosphate, forming triglycerides that are stored in the lipid droplet until needed. Norepinephrine from nerve endings stimulates the cyclic AMP (cAMP) system, which activates hormone-sensitive lipase to hydrolyze the stored triglycerides to free fatty acids and glycerol. These substances diffuse into the capillary, where the fatty acids bind albumin for transport throughout the body for use as an energy source. Abundant caveolae in the adipocyte plasmalemma are rich in cholesterol and other lipids and appear to mediate endocytosis of fatty acids necessary for growth of the lipid storage droplet.

Upon adipocyte stimulation by nerves or various hormones, stored lipids become mobilized and cells release fatty acids and glycerol. Norepinephrine released in the adrenal gland and by postganglionic sympathetic nerves in adipose tissue activates a hormone-sensitive lipase that breaks down triglycerides at the surface of the stored lipid droplets (Figure 6–2). Growth hormone (GH) from the pituitary gland also stimulates this lipase activity. The free fatty acids diffuse across the membranes of the adipocyte and the capillary endothelium and bind the protein albumin in blood for transport throughout the body. The more water-soluble glycerol remains free in blood for uptake in the liver. Insulin inhibits the hormone-sensitive lipase, reducing fatty acid release, and stimulates enzymes for lipid synthesis. Besides insulin and GH, other peptide hormones also cooperate in regulating lipid synthesis and mobilization in adipocytes.

Hormonal activity of white adipocytes themselves includes production of the 16-kDa polypeptide hormone leptin (Gr. leptos, thin), a “satiety factor” with target cells in the hypothalamus, other brain regions, and peripheral organs, which helps regulate the appetite under normal conditions and participates in regulating the formation of new adipose tissue.

Although white adipose tissue associated with different organs appears histologically similar, differences in gene expression have been noted between visceral deposits (in the abdomen) and subcutaneous deposits of white fat. Such differences may have importance for medical risks of obesity; it is well established that increased visceral adipose tissue raises the risk of diabetes and cardiovascular disease, whereas increased subcutaneous fat does not. The release of visceral fat products directly to the portal circulation of the liver may also influence the medical relevance of this form of obesity.

In response to body needs, lipids undergo mobilization rather uniformly from white adipocytes in all parts of the body, although adipose tissue in the palms, soles, and fat pads behind the eyes resists even long periods of starvation. During starvation, adipocytes can lose nearly all their fat and become polyhedral or spindle-shaped cells with only very small lipid droplets.

Histogenesis of White Adipose Tissue

Figure 6–3 Development of white and brown fat cells. Mesenchymal stem cells differentiate as progenitor cells for all types of connective tissue, including preadipocytes. These are initially of at least two types. Preadipocytes developing within the lateral mesoderm of the embryo produce large number of white adipocytes (forming white adipose tissue) and a smaller number of so-called “beige” adipocytes with cytological features and gene expression patterns of both white and brown adipocytes. White adipocytes are unilocular, with one large lipid droplet occupying most of the cytoplasm. The white adipocyte is usually much larger than that shown here in relation to the other cell types. Brown adipocytes differentiate from another population of preadipocytes located in paraxial embryonic mesoderm and remain multilocular (having many small lipid droplets) with numerous mitochondria (not shown here). Mitochondrial metabolism of lipid in brown adipocytes releases heat rather than ATP. Cells functioning as brown adipocytes can also develop from beige adipocytes during adaptation to cold temperatures.

As shown in Figure 6–3, white adipocytes develop together with a smaller population of cells termed beige adipocytes, which remain within white adipose tissue and have histological and metabolic features generally intermediate between white and brown adipocytes. With adaptation to cold temperatures beige adipocytes change reversibly, forming many more small lipid droplets, adopting a gene expression profile more like that of brown fat and begin to release heat as described below.

At birth humans have stores of white adipose tissue, which begin to accumulate by the 14th week of gestation. Both visceral and subcutaneous fat becomes well developed after this time. Proliferation of progenitor cells diminishes by late gestation, and adipose tissue increases mainly by the filling of existing adipocytes until around age 10, followed by a period of new fat cell differentiation that lasts through adolescence. New adipocyte formation occurs around small blood vessels, where undifferentiated mesenchymal cells are most abundant.

Excessive adipose tissue accumulation, or obesity, occurs when nutritional intake exceeds energy expenditure, an increasingly common condition in modern, sedentary lifestyles. Although adipocytes can differentiate from mesenchymal stem cells throughout life, adult-onset obesity mainly involves increasing the size of existing adipocytes (hypertrophy). Childhood obesity, in contrast, often involves increases in both adipocyte size and numbers due to the differentiation of more preadipocytes from mesenchymal cells (hyperplasia). Weight loss after dietary changes results from reductions in adipocyte volume, but not their overall number.

Embryology

Figure 11–1 The embryonic development of adrenergic cells and tumors that develop from them (in parentheses). Sympathogonia are primitive cells derived from the neural crest. Neuroblasts are also called sympathoblasts; ganglion cells are the same as sympathocytes; and pheochromocytes are mature chromaffin cells.

At 6 weeks of gestation, groups of these primitive cells migrate along the central vein and enter the fetal adrenal cortex to form the adrenal medulla, which is detectable by the eighth week. The adrenal medulla at this time is composed of sympathogonia and pheochromoblasts, which then mature into pheochromocytes. The cells appear in rosette-like structures, with the more primitive cells occupying a central position. Storage granules can be found in these cells at 12 weeks. The adrenal medullas are very small and amorphous at birth but develop into recognizable adult form by the sixth month of postnatal life.

Pheochromoblasts and pheochromocytes also collect on both sides of the aorta to form the paraganglia. These cells collect principally at the origin of the mesenteric arteries and at the aortic bifurcation where they fuse anteriorly to form the organ of Zuckerkandl, which is quite prominent during the first year of life. Pheochromocytes (chromaffin cells) also are found scattered throughout the abdominal sympathetic plexi as well as in other parts of the sympathetic nervous system.

Gross Structure

Microscopic Structure

Nerve Supply

Blood Supply

Norepinephrine is converted to epinephrine by the enzyme phenylethanolamine-N-methyltransferase (PNMT). In mammals, the expression of PNMT is induced by cortisol. Chromaffin cells that produce epinephrine are exposed to higher concentrations of cortisol from capillaries draining adrenocortical cells, whereas chromaffin cells that produce norepinephrine are supplied by arteries that course directly to the adrenal medulla (see sections on biosynthesis and secretion, discussed later).

The central vein of the right adrenal is short and drains directly into the vena cava with about 5% having multiple veins. About 5% of right adrenal veins drain into the hepatic vein. For the left adrenal gland, the vein is somewhat longer and drains into the left renal vein.

Alterations in Thyroid Function Tests

Despite the similarities in serum thyroid hormone levels between HIV and the euthyroid sick syndrome, significantly ill HIV-infected patients often do not demonstrate the elevated reverse T₃ levels characteristic of the euthyroid sick syndrome. The significance of this difference is unknown.

Thyroxine-binding globulin (TBG) levels are increased in HIV-infected patients, rising progressively with advancing immunosuppression. TBG levels correlate inversely with the CD4 lymphocyte count. The increase in TBG does not appear to be due to generalized changes in protein synthesis, increases in sialylation and altered clearance of TBG, or changes in estrogen levels. The significance of the increased TBG found in HIV-infected patients is unknown. However, increases in TBG affect total T₄ and T₃ measurements, which should be considered when interpreting these tests in HIV-infected patients.

Subtle alterations in TSH dynamics have been reported in stable HIV-infected patients. Despite normal TSH and free T₄ levels, these individuals demonstrate significantly higher TSH values and lower free T₄ values than uninfected controls. In circadian studies, HIV-infected individuals have higher TSH pulse amplitudes with unchanged pulse frequency as well as a higher peak TSH in response to thyrotropin-releasing hormone (TRH) stimulation. These studies are consistent with a subtle state of compensated hypothyroidism; the mechanisms underlying these alterations have not been elucidated.

The typical pattern of alterations in thyroid function tests in HIV-infected patients is outlined in Table 25–1.

Opportunistic Infections and Neoplasms

A few of these pathogens are noteworthy because they have been associated on occasion with both hyper- and hypothyroidism. P. carinii has been associated with inflammatory thyroiditis accompanied by hypothyroidism in seven cases, hyperthyroidism in three cases and normal thyroid function in one case. Antithyroid antibodies were negative in all six cases in which they were measured. Radionuclide scanning in seven cases revealed poor visualization of the entire thyroid gland in patients with bilateral disease and nonvisualization of the affected lobe in patients with unilateral disease. Two patients with hyperthyroidism had normalization of thyroid function after treatment of the P. carinii infection. Kaposi’s sarcoma has also been reported to infiltrate the thyroid gland and resulted in significant destruction and hypothyroidism in at least one case. In two cases, lymphoma was associated with thyroid infiltration, causing thyroidal enlargement. HCV coinfection may increase the prevalence of thyroid disease and perhaps cancer.

Medication Effects

Osteoblasts

Figure 8–3 Osteoblasts, osteocytes, and osteoclasts. (a) Diagram showing the relationship of osteoblasts to the newly formed matrix called “osteoid,” bone matrix, and osteocytes. Osteoblasts and most of the larger osteoclasts are part of the endosteum covering the bony trabeculae. (b) The photomicrograph of developing bone shows the location and morphologic differences between active osteoblasts (Ob) and osteocytes (Oc). Rounded osteoblasts, derived from progenitor cells in the adjacent mesenchyme (M), cover a thin layer of lightly stained osteoid (Os) on the surface of the more heavily stained bony matrix (B). Most osteoblasts that are no longer actively secreting osteoid will undergo apoptosis; others differentiate either as flattened bone lining cells on the trabeculae of bony matrix or as osteocytes located within lacunae surrounded by bony matrix. (×300; H&E)

During the processes of matrix synthesis and calcification, osteoblasts occur as polarized cells with ultrastructural features denoting active protein synthesis and secretion. Secreting matrix components at the cell surface in contact with existing bone matrix, osteoblasts produce a layer of unique collagen-rich material called osteoid between the cuboidal cell layer and the preexisting bone surface (Figure 8–3). Deposition of calcium salts into this newly formed matrix completes this process of bone appositional growth.

Figure 8–4 diagrams basic aspects of the bone matrix mineralization process. Prominent among the noncollagen proteins secreted by osteoblasts is the vitamin K-dependent polypeptide osteocalcin, which together with various glycoproteins binds Ca²⁺ ions and concentrates this mineral locally. Osteoblasts also release membrane-enclosed matrix vesicles rich in alkaline phosphatase and other enzymes whose activity raises the local concentration of PO₄³⁻ ions. In the resulting microenvironment with high concentrations of both these ions, matrix vesicles serve as foci for the formation of hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂] crystals, the first visible step in calcification. These crystals grow rapidly by accretion of more mineral and eventually produce a confluent mass of calcified material embedding the collagen fibers and proteoglycans (Figure 8–4).

Figure 8–4 Mineralization in bone matrix. From their surfaces adjacent to the bone matrix, osteoblasts secrete type I collagen, several glycoproteins, and proteoglycans. Some of these factors, notably osteocalcin and certain glycoproteins, bind Ca²⁺ with high affinity, raising the local concentration of these ions. Osteoblasts also release very small membrane-enclosed matrix vesicles containing alkaline phosphatase and other enzymes. These enzymes remove PO₄⁻ ions from various matrix macromolecules, creating a high concentration of these ions locally. The high Ca²⁺ and PO₄⁻ ion concentrations cause calcified nanocrystals to form in and around the matrix vesicles. The crystals grow and mineralize further with formation of small growing masses of calcium hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂], which surround the collagen fibers and all other macromolecules. Eventually, the masses of hydroxyapatite merge as a confluent solid bony matrix as calcification of the matrix is completed.

Osteocytes

Figure 8–5 Osteocytes in lacunae. (a) TEM showing an osteocyte in a lacuna and two dendritic processes in canaliculi (C) surrounded by bony matrix. Many such processes are extended from each cell as osteoid is being secreted; this material then undergoes calcification around the processes, giving rise to canaliculi. (×30,000) (b) Photomicrograph of bone, not decalcified or sectioned, but ground very thin to demonstrate lacunae and canaliculi. The lacunae and canaliculi (C) appear dark and show the communication between these structures through which nutrients derived from blood vessels diffuse and are passed from cell to cell in living bone. (×400; Ground bone) (c) SEM of nondecalcified, sectioned, and acid-etched bone showing lacunae and canaliculi (C). (×400) (Reproduced with permission from Dr Matt Allen, Indiana University School of Medicine, Indianapolis.)

Exchange of metabolites between osteocytes and blood vessels occurs through the small amount of interstitial fluid in the lacunae and canaliculi between the bone matrix and the osteocytes and their processes. Osteocytes also communicate with one another and ultimately with nearby osteoblasts and bone lining cells via gap junctions at the ends of their processes. These connections between osteocyte processes and nearly all other bone cells form an extensive lacunar-canalicular network that allows osteocytes to serve as mechanosensors detecting the mechanical load on the bone as well as stress- or fatigue-induced microdamage and to trigger remedial activity in osteoblasts and osteoclasts.

Normally the most abundant cells in bone, osteocytes exhibit significantly less RER, smaller Golgi complexes, and more condensed nuclear chromatin than osteoblasts (Figure 8–5a). Osteocytes maintain the calcified matrix and their death causes progressive resorption of adjacent matrix. While sharing most matrix-related activities with osteoblasts, osteocytes also express many different proteins, including factors with paracrine and endocrine effects that help regulate bone remodeling.

Osteoclasts

Figure 8–6 Osteoclasts and their activity. Osteoclasts represent large multinucleated cells derived by the fusion in bone of several blood-derived monocytes. (a) Photo of bone showing two osteoclasts (Ocl) digesting and resorbing bone matrix (B) in relatively large resorption cavities (or Howship lacunae) on the matrix surface. An osteocyte (Oc) in its smaller lacuna is also shown. (×400; H&E) (b) Diagram of an osteoclast, including its circumferential sealing zone where integrins tightly bind the cell to the bone matrix. The sealing zone surrounds a ruffled border of microvilli and other cytoplasmic projections close to this matrix. The sealed space between the cell and the matrix is acidified to ∼pH 4.5 by proton pumps in the ruffled part of the cell membrane and receives secreted matrix metalloproteases and other hydrolytic enzymes. Acidification of the sealed space promotes dissolution of hydroxyapatite from bone and stimulates activity of the protein hydrolases, producing localized matrix resorption. The breakdown products of collagen fibers and other polypeptides undergo endocytosis by osteoclasts and degradation in lysosomes, while Ca²⁺ and other ions are released directly and taken up by the blood. (c) SEM of a single osteoclast after a few hours culture on a flat substrate of bone. A trench forms on the bone surface by the slowly migrating osteoclast. (×5000) (Figure 8–6c, reproduced with permission from Alan Boyde, Institute of Dentistry, Queen Mary, University of London.)

In an active osteoclast, the membrane domain that contacts the bone forms a circular sealing zone that binds the cell tightly to the bone matrix and surrounds an area of membrane with many surface projections, called the ruffled border. This circumferential sealing zone allows a specialized microenvironment to form between the osteoclast and the matrix in which bone resorption occurs (Figure 8–6b).

Into this subcellular pocket the osteoclast’s ruffled border pumps protons to acidify and promote local dissolution of hydroxyapatite and secretes matrix metalloproteinases and other hydrolytic enzymes for the localized digestion of collagen and other matrix proteins. These activities occur with regulation by signaling factors from other nearby bone cells. Osteoblasts activated by parathyroid hormone produce M-CSF, RANKL, and other factors that play important roles controlling formation and activity of osteoclasts.