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 a system of abundant, intracellular unit membranes.  Regions of the endoplasmic reticulum studded with ribosomes are called rough endoplasmic reticulum (RER), while regions lacking ribosomes are termed smooth endoplasmic reticulum (SER).

The Golgi, 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 through 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 granules for transport to the plasma membrane, where they are released from the cells by exocytosis.  Secretory granules consist of a single 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 the cellular activity 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 common cell processing techniques, so lipid usually appears as an empty space in a cell image.

A centriole is composed of nine triplets of microtubules arranged as a cylinder.  Centrioles occur in pairs oriented at right angles to each other near the nucleus.  The pair is called a diplosome.  Centrioles are the site of origin of microtubules for the mitotic spindle and, in the form of basal bodies, are also located at the bases of cilia and flagella, where they organize the microtubules of these structures.

Polysomes are groups of ribosomes attached to a single messenger RNA molecule that synthesize protein 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 and fill the cytoplasm.  Glycogen granules are highly electron dense.

Junctional complexes, found at luminal regions of epithelia to maintain epithelial integrity, have three components.  Zonula occludens, a band encircling each cell, seals the lumen from the intercellular space.  Zonula adherens, also a band, attaches adjacent cells to each other.  Anchoring desmosomes (maculae adherentes) are spot welds randomly scattered between cells.

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 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.  Each ciliated cell possesses numerous cilia, which beat in concert to propel materials along the epithelial surface.