FIGURE 2.57 • Association of the centrosome duplication and primary cilium formation with the cell cycle. After a cell emerges from mitosis, it possesses a single centrosome (MTOC) surrounded by amorphous pericentriolar material. The primary cilium formation frst occurs during G1 phase in which the centrosome migrates toward the cell membrane and initiates the process of ciliogenesis. Necessary structural and transport proteins are acquired and activated to build primary cilium axoneme (9 0) directly on the top of the mature centriole. During the end of G1 phase, as well as in GO, the primary cilium functions as an external receiver antenna sensing and interpreting signals from the extracellular environment. Duplication of centrioles begins near the transition between the G1 and S phases of the cell cycle, and the two centrioles are visible in S phase. During the late G2 phase, centrioles reach their full maturity, whereas the primary cilium is disassembled. This allows centrioles to migrate away from the cell membrane and participate in the mitotic spindle formation. Once cell division is complete, the centrioles can proceed to ciliary reassembly in G1 phase. (Based on  Santos N, Reiter JF. Building it up and taking it down: the regulation of vertebrate ciliogenesis. Dev Dyn 2008;237:1972–1981.) ume, compressing the other formed organelles into a thin rim at the margin of the cell. Lipid droplets are usually extracted by the organic solvents used to prepare tissues for both light and electron microscopy. What is seen as a fat droplet in light microscopy is actually a hole in the cytoplasm that represents the site from which the lipid was extracted. In individuals with genetic defects of enzymes involved in lipid metabolism, lipid droplets may accumulate in abnormal locations or in abnormal amounts. Such diseases are classified as lipid storage diseases.
 Crystalline inclusions contained in certain cells are recognized in the light microscope. In humans, such inclusions are found in the Sertoli (sustentacular) and Leydig (interstitial) cells of the testis. With the TEM, crystalline inclusions have been found in many cell types and in virtually all parts of the cell, including the nucleus and most cytoplasmic organelles. Although some of these inclusions contain viral proteins, storage material, or cellular metabolites, the significance of others is not clear.
The cytoplasmic matrix is a concentrated aqueous gel consisting of molecules of different sizes and shapes.
The cytoplasmic matrix (ground substance or cytosol) shows little specific structure by light microscopy or conventional TEM and has traditionally been described as a concentrated aqueous solution containing molecules of different size and shape (e.g., electrolytes, metabolites, RNA, and synthesized proteins). In most cells, it is the largest single compartment. The cytoplasmic matrix is the site of physiologic processes that are fundamental to the cell’s existence (protein synthesis, breakdown of nutrients). Studies with high-voltage EM (HVEM) of 0.25to 0.5-m sections reveal a complex three-dimensional structural network of thin microtrabecular strands and cross-linkers. This network provides a structural substratum on which cytoplasmic reactions occur, such as those involving free ribosomes, and along which regulated and directed cytoplasmic transport and movement of organelles occur.