Overview of cell structures

The nucleus is usually the most prominent structure in the cell and, in non-dividing cells, the nucleus is well demarcated from the cytoplasm by the nuclear envelope. The nucleus houses the genomic DNA, transcribes and processes messenger RNA and contains the nucleolus, the site of ribosomal RNA production and the initial assembly of ribosomes.

The nucleolus is the site of ribosomal RNA transcription and the initial assembly of ribosomes. Nuclei generally have one nucleolus; however, multiple nucleoli are common in highly active cells, such as liver cells.

The endoplasmic reticulum (ER) is an interconnected system of intracellular unit membranes. Regions of the endoplasmic reticulum with ribosomes are called rough endoplasmic reticulum (RER) and are arranged as flattened sacs. Regions lacking ribosomes are termed smooth endoplasmic reticulum (SER), appear more tubular.

The Golgi apparatus, usually located near the nucleus, consists of flattened, membranous sacs. These sacs receive newly synthesized proteins from the RER via transport vesicles. The vesicles fuse with the forming face of the Golgi, and their proteins are post-translationally modified, e.g., glycosylated or phosphorylated, and packaged on the maturing face of the Golgi for transport throughout the cell.

Proteins for export are synthesized by the RER and transferred to the Golgi via transport vesicles. The Golgi modifies these proteins and packages them as secretory vesicles/granules for transport to the plasma membrane, where they are released from the cells by exocytosis. Secretory granules consist of a single unit membrane surrounding the contents of the the granule.

Lysosomes, single-membrane organelles, are the major degradative organelle, containing acid hydrolase enzymes. Lysosomes form by the fusion of pre-lysosomal vesicles from the Golgi with endosomes, phagosomes, or autophagosomes. Lysosomal contents appear amorphous and highly variable, reflecting contents and stages of degradation.

Mitochondria, the major site of ATP synthesis, contain enzymes of the citric acid (Krebs) cycle, oxidative phosphorylation and electron transport systems. A mitochondrion is formed by two unit membranes, the inner of which has folds, or cristae, that project into the mitochondrion. The number, shape and distribution of mitochondria vary with cellular activity and type of the cell.

Lipid droplets are cellular inclusions that are not surrounded by a unit membrane. Lipid provides energy for cell metabolism and may be present as multiple droplets (shown here) or as a single, large droplet, such as seen in an adipocyte. Lipid is extracted during routine tissue processing techniques, so the location of the lipid often appears as an empty space.

A centriole is composed of nine triplets of microtubules arranged as a cylinder. A pair of centrioles, oriented at right angles to each other and often located near the nucleus, form the diplosome. Centrioles are the site of origin of microtubules for the spindle apparatus during cell division. They also form basal bodies located at the bases of cilia and flagella, where they organize the microtubules of these structures.

Polysomes or polyribosomes are groups of ribosomes in the cytoplasm attached to a single messenger RNA molecule. Polysomes synthesize protein mostly for internal use by the cell.

Cells store glucose in the form of glycogen granules, which can be present individually or in small clusters. In liver cells, which store large amounts of glycogen, these granules are abundant. 

Junctional complexes are present in the apical regions of epithelia and consist of three components: (1) Zonula occludens, a band encircling each cell, sealing the lumen from the intercellular space; (2) Zonula adherens, forming a band that attaches adjacent cells to each other; and (3) Desmosomes (maculae adherens), disc-shaped attachment points that are also present in other regions of cell contact.

A hemidesmosome is a specialization of the basal plasma membrane of an epithelial cell and resembles one-half of a desmosome. Hemidesmosomes serve to attach the basal surface of the cell to the underlying basal lamina.

Microvilli are short (less than one micron), non-motile extensions of the apical plasma membrane. Microvilli greatly increase the absorptive surface area of a cell.

Cilia are hair-like, highly motile extensions, greater than 1 micron in height, that project from the apical surface of some cells. A basal body is present at the base of cilium. Each ciliated cell possesses numerous cilia, which beat in concert to propel materials along the epithelial surface.