Cells are the fundamental structural and functional units within living organisms. All living organisms consist of one or more cells. With the exception of bacteria, all organisms are made of eukaryotic cells, which have a membrane-enclosed nucleus and organelles (e.g., mitochondria, endoplasmic reticulum, and ribosomes). The nucleus within each cell contains the hereditary information for the entire organism, encoded within DNA.

In multi-cellular organisms, cells differentiate and specialize. Specialized cells organize into tissues (e.g., muscle, blood, bone, fat, nerve), which make up organs (e.g., kidneys, heart, stomach, lung), which, in turn, comprise organ systems (e.g., respiratory, digestive, excretory). Genes that do not pertain to the functioning of each individual cell become inactive, or "turn off." For example, a kidney cell uses only the DNA needed to be a kidney cell. The remaining information is "turned off," but it is still present. There are more than 200 different types of cells (nerve cells, muscle cells, epithelial cells, blood cells, bone cells, etc.) among the human body's estimated 100,000,000,000,000 total cells.

The development of colon cancer from a normal cell to a malignant tumor is a multi-step process. Colon epithelial cells are only one layer deep and they are arranged in patterns called “crypts.” Stem cells are located at the bottom of the crypts and they give birth to daughter cells, which migrate up the side of the crypt, dividing several times while in transit. As they migrate, they differentiate, and towards the top of the crypt, they undergo apoptosis – or programmed cell death – and are exfoliated into the fecal stream. We are able to recapture these colon cells from fecal material and analyze the mRNA in the cells to track changes in colon gene expression patterns. We can do this at any stage of the carcinogenic process, from normal healthy cells, to cells that have become dysplastic, to the development of what are called aberrant crypts (shown here), to colon polyps and through to carcinomas.

All vertebrates share the same basic body plan, with tissues and organs functioning in a similar manner. Here, we will focus on the human body, studying form (anatomy) and function (physiology). The two go hand-in-hand and are an extension of one of biology's central themes, evolution.

Animals are made of complex systems of cells, which must be able to perform all of life's processes and work in a coordinated fashion to maintain a stable internal environment. Early in a human's development, groups of cells specialize into three fundamental embryonic or germ layers: endoderm, mesoderm, and ectoderm. These embryonic layers differentiate into a number of specialized cells and tissues. Tissues are groups of cells similar in structure and function and may be held together by some sort of matrix. There are four primary groups of tissues: epithelial, connective, muscular, and nervous.

Different tissues functioning together for a common purpose are called organs (eg, stomach, kidney, lung, heart).

Organ systems are composed of individual organs working together to accomplish a coordinated activity. For example, the stomach, small intestine and large intestine all play a role in digestion.

Tissues are groups of cells similar in structure and function. The four primary categories of tissues are epithelial, connective, muscular, and nervous.

• Epithelial tissues cover the outer and inner surfaces of the body, serving as "gatekeepers" for the passage of materials. Epithelial cells have glandular roles, secreting substances such as mucous and hormones. Some simple epithelial tissues consist of only one layer, while stratified epithelial tissues are comprised of multiple layers. The shape of the cell on the outer surface (columnar, cuboidal, or squamous) facilitates identification of epithelial tissues.
• Connective tissues help to bind, support, and join other tissues. The cells of connective tissues are sparsely arranged in a profuse extracellular matrix which can be fluid (as in blood), flexible (as in cartilage) or hard and crystalline (as in bone). 
• Muscle tissues, composed of cells called fibers, are unique in the abundance and arrangement of actin and myosin filaments, which enable muscle to contract. In humans, there are three types of muscle tissues: smooth, cardiac, and skeletal.
• Nerve tissues are comprised primarily of neurons (nerve cells), which detect stimuli and send responses in the form of electrochemical charges called impulses. Nerve tissues include a variety of supporting cells called neuralgia and Schwann cells. These cells support, insulate, and eliminate foreign materials in and around neurons.

Cells are the fundamental, structural, and functional units within living organisms. All living organisms consist of one or more cells. With the exception of bacteria, organisms are made of eukaryotic cells, which have a membrane enclosed nucleus and organelles (mitochondria, endoplasmic reticulum, and ribosomes). The nucleus within each cell contains the hereditary information for the entire organism, encoded within DNA.

In multi-cellular organisms, cells differentiate and specialize. Specialized cells organize into tissues (muscle, blood, bone, fat, nerve), which make up organs (kidneys, heart, stomach, lung), which, in turn, make up organ systems (respiratory, digestive, excretory). Genes that do not pertain to the functioning of each individual cell "turn off." For example, a kidney cell uses only the DNA needed to be a kidney cell. The remaining information is "turned off," but it is still present. There are more than 200 different types of cells (nerve cells, muscle cells, epithelial cells, blood cells, bone cells, etc.) among the human body's estimated 100,000,000,000,000 total cells.

Different cells have dramatically distinct shapes and sizes. For example, muscle cells (also called myoblasts), which are required for movement, are long and thin, and contain contractile elements. Meanwhile, bone cells (osteoblasts), which are responsible for bone maintenance, are very small and round, and must live embedded in hard calcified bone. Epithelial cells, which make up the nephron in the kidney, filter waste products. These epithelial cells must be able to retain nutrients and materials needed by the body and return them to circulation, while leaving wastes, toxins, and excess water to be excreted. These cells are thin and dense.