A human fibroblast—the most common cells of connective tissue—undergoes mitosis or cell division. Cell components include actin (gray/brown), myosin II (green) and DNA (magenta).

Image courtesy of Nilay Taneja.

A micrograph depicts the hippocampal region of the human brain. Located in the medial temporal lobe, it acts as a crucial component of the limbic system for learning, memory and spatial navigation.

Image courtesy of Sarah Moghadam and Ahmad Salehi.

Developing mouse nerve cells displaying nuclei (yellow) surrounded by cell bodies, with long extensions called axons and thin branching structures called dendrites. Electrical signals travel from the axon of one cell to the dendrites of another.

Image courtesy of Torsten Wittman, UC San Francisco.

Proteins in the neural tissues of a zebrafish embryo direct cells to line up and form the neural tube, which will become the spinal cord and brain. Zebrafish embryonic development research may help pinpoint the underlying cause of common neural tube defects, such as spina bifida, which occur in about 1 in 1,000 newborn children.

Image courtesy of Alexander Schier, Harvard University.

Colorized scanning electron micrograph of Vibrio bacteria. Some species of the rod-shaped bacteria cause cholera in humans.

Image courtesy of Tina Weatherby Carvalho, University of Hawaii at Manoa.

Colorized scanning electron micrograph of a blood clot, with red blood cells tangled in fibrin, a tough, insoluble fibrous protein that acts like a molecular glue and is essential for clotting and wound healing.

Image courtesy of Thomas Deerinck, UC San Diego.

False color scanning electron micrograph of a single human ovum or egg cell. Human ova are among the largest cells in the human body and are visible to the naked eye, roughly the size of a single, small grain of sand.

Image courtesy of SPG.

Colorized scanning electron micrograph of surface of human skin. The outermost layer of skin is called the stratum corneum and is composed primarily of keratinocytes, living and dead. Dead cells form horny scales, and release peptides called defensins, which are part of the first line of immune defense mechanisms.

Image courtesy of Eye of Science/SPL.

Colorized scanning electron micrograph depicts normal red blood cells (red, concave) with elongated blood cells indicative of sickle-cell disease (pink).

Image courtesy of Eye of Science/SPL.

Colorized scanning electron micrograph of human lung tissue, depicting alveoli and bronchus. Lung alveoli are tiny, grape-like sacs, numbering in the millions, where essential gas exchange (oxygen/carbon dioxide) occurs. The bronchus is a major airway that branches from the trachea to conduct air into the lungs.

Image courtesy of Dennis Kunkel/SPL.