Cells, the smallest structures capable of maintaining life and reproducing, compose all living things, from single-celled plants to multibillion-celled animals. The human body, which is made up of numerous cells, begins as a single, newly fertilized cell.

Almost all human cells are microscopic in size. To give you an idea how small a cell is, one average-sized adult body, according to one estimate, consists of 100 trillion cells!

To learn more about cell structure and function, select a topic listed below.

Ideas about cell structure have changed considerably over the years. Early biologists saw cells as simple membranous sacs containing fluid and a few floating particles. Today's biologists know that cells are infinitely more complex than this.

There are many different types, sizes, and shapes of cells in the body. For descriptive purposes, the concept of a "generalized cell" is introduced. It includes features from all cell types. A cell consists of three parts: the cell membrane, the nucleus, and between the two, the cytoplasm. Within the cytoplasm lie intricate arrangements of fine fibers and hundreds or even thousands of miniscule but distinct structures called organelles.

Cell membrane

Every cell in the body is enclosed by a cell (Plasma) membrane. The cell membrane separates the material outside the cell, extracellular, from the material inside the cell, intracellular. It maintains the integrity of a cell and controls passage of materials into and out of the cell. All materials within a cell must have access to the cell membrane (the cell's boundary) for the needed exchange.

The cell membrane is a double layer of phospholipid molecules. Proteins in the cell membrane provide structural support, form channels for passage of materials, act as receptor sites, function as carrier molecules, and provide identification markers.

Nucleus and Nucleolus

The nucleus, formed by a nuclear membrane around a fluid nucleoplasm, is the control center of the cell. Threads of chromatin in the nucleus contain deoxyribonucleic acid (DNA), the genetic material of the cell. The nucleolus is a dense region of ribonucleic acid (RNA) in the nucleus and is the site of ribosome formation. The nucleus determines how the cell will function, as well as the basic structure of that cell.


The cytoplasm is the gel-like fluid inside the cell. It is the medium for chemical reaction. It provides a platform upon which other organelles can operate within the cell. All of the functions for cell expansion, growth and replication are carried out in the cytoplasm of a cell. Within the cytoplasm, materials move by diffusion, a physical process that can work only for short distances.

Cytoplasmic organelles

Cytoplasmic organelles are "little organs" that are suspended in the cytoplasm of the cell. Each type of organelle has a definite structure and a specific role in the function of the cell. Examples of cytoplasmic organelles are mitochondrion, ribosomes, endoplasmic reticulum, golgi apparatus, and lysosomes.

The structural and functional characteristics of different types of cells are determined by the nature of the proteins present. Cells of various types have different functions because cell structure and function are closely related. It is apparent that a cell that is very thin is not well suited for a protective function. Bone cells do not have an appropriate structure for nerve impulse conduction. Just as there are many cell types, there are varied cell functions. The generalized cell functions include movement of substances across the cell membrane, cell division to make new cells, and protein synthesis.

Movement of substances across the cell membrane

The survival of the cell depends on maintaining the difference between extracellular and intracellular material. Mechanisms of movement across the cell membrane include simple diffusion, osmosis, filtration, active transport, endocytosis, and exocytosis.

Simple diffusion is the movement of particles (solutes) from a region of higher solute concentration to a region of lower solute concentration. Osmosis is the diffusion of solvent or water molecules through a selectively permeable membrane. Filtration utilizes pressure to push substances through a membrane. Active transport moves substances against a concentration gradient from a region of lower concentration to a region of higher concentration. It requires a carrier molecule and uses energy. Endocytosis refers to the formation of vesicles to transfer particles and droplets from outside to inside the cell. Secretory vesicles are moved from the inside to the outside of the cell by exocytosis.

Cell division

Cell division is the process by which new cells are formed for growth, repair, and replacement in the body. This process includes division of the nuclear material and division of the cytoplasm. All cells in the body (somatic cells), except those that give rise to the eggs and sperm (gametes), reproduce by mitosis. Egg and sperm cells are produced by a special type of nuclear division called meiosis in which the number of chromosomes is halved. Division of the cytoplasm is called cytokinesis.

Somatic cells reproduce by mitosis, which results in two cells identical to the one parent cell. Interphase is the period between successive cell divisions. It is the longest part of the cell cycle. The successive stages of mitosis are prophase, metaphase, anaphase, and telophase. Cytokinesis, division of the cytoplasm, occurs during telophase.

Meiosis is a special type of cell

division that occurs in the production of the gametes, or eggs and sperm. These cells have only 23 chromosomes, one-half the number found in somatic cells, so that when fertilization takes place the resulting cell will again have 46 chromosomes, 23 from the egg and 23 from the sperm.

DNA replication and protein synthesis

Proteins that are synthesized in the cytoplasm function as structural materials, enzymes that regulate chemical reactions, hormones, and other vital substances. DNA in the nucleus directs protein synthesis in the cytoplasm. A gene is the portion of a DNA molecule that controls the synthesis of one specific protein molecule. Messenger RNA carries the genetic information from the DNA in the nucleus to the sites of protein synthesis in the cytoplasm.




an aggregation of similarly specialized cells united in the performance of a particular function.



accidental tissue, a tissue growing in or upon a part to which it is foreign; it may be either analogous or heterologous.
adenoid tissue, lymphoid t.
adipose tissue, connective tissue made up of fat cells in a meshwork of areolar tissue; called also fatty t..
adipose tissue, brown, a thermogenic type of adipose tissue containing a dark pigment. It arises during embryonic life in certain specific areas in many mammals, including humans, and is prominent in newborns. It remains distinct and conspicuous in adults in only certain species, especially those that hibernate. Cf. white adipose t. Called also brown fat.
adipose tissue, white, adipose tissue, yellow, the adipose tissue comprising the bulk of the body fat. Cf. brown adipose t.
adrenogenic tissue, fetal zone of adrenal cortex.
analogous tissue, accidental tissue that is similar to one found normally in other parts of the body.
areolar tissue, areolar connective tissue, a type of connective tissue made up largely of interlacing fibers. Called also cribriform t. and loose connective t.
basement tissue, the substance of a basement membrane.
bony tissue, osseous t.
brown adipose tissue, brown fat.
bursa-equivalent tissue, bursal equivalent tissue, a hypothesized lymphoid tissue in nonavian vertebrates including human beings, equivalent to the bursa of Fabricius in birds: the site of B lymphocyte maturation. It now appears that B lymphocyte maturation occurs primarily in the bone marrow.
cancellous tissue, the loose spongy tissue of the interior and articular ends of bone.
cartilaginous tissue, the substance of the cartilages.
cavernous tissue, erectile t.
cellular tissue, loose connective tissue with large interspaces.
chondroid tissue, an embryonic form of cartilage composed of vesicular cells provided with elastic capsules and having collagenous fibers in its interstitial substance. Called also fibrohyaline t. and pseudocartilage.
chordal tissue, the tissue of the notochord.
chromaffin tissue, tissue composed largely of chromaffin cells, well supplied with nerves and vessels; it occurs in the adrenal medulla and also forms the paraganglia of the body.
cicatricial tissue, the dense fibrous tissue forming a scar or cicatrix and derived directly from granulation tissue; called also scar t.
compact tissue, the hard external portion of a bone.
connective tissue, the tissue that binds together and is the support of the various structures of the body. It is made up of fibroblasts, fibroglia, collagen fibrils, and elastic fibers, is derived from the mesoderm, and in a broad sense includes the collagenous, elastic, mucous, reticular, osseous, and cartilaginous tissue. Some authorities also include the blood. Cf. fibroblast. Connective tissue is classified according to concentration of fibers as loose (areolar) and dense, the latter having more abundant fibers than the former.
cribriform tissue, areolar t.
dartoic tissue, dartoid tissue, tissue that resembles the tunica dartos, such as in a tumor with muscular elements; see leiomyoma cutis.
dense connective tissue, see connective t.
elastic tissue, elastic tissue, yellow, connective tissue made up of elastic fibers, frequently massed into sheets.
endothelial tissue, endothelium.
episcleral tissue, the loose connective tissue over the sclera, between it and the conjunctiva.
epithelial tissue, epithelium.
epivaginal connective tissue, connective tissue surrounding the sheath of the optic nerve.
erectile tissue, tissue containing large venous spaces with which arteries communicate directly, as in the penis and clitoris. Another type formed of dilated venules occurs in the nasal mucosa. The smooth muscle of the nipples constitutes another erectile organ.
extracellular tissue, the total of tissues and body fluids outside the cells, including the plasma volume and all plasma components, the extracellular fluid volume and its components, plus the intercellular and extracellular tissue solids, most notably the collagen, cartilage, bone, elastin, and other connective tissues of the body framework and viscera.
extraperitoneal tissue, fascia extraperitonealis.
fatty tissue, adipose t.
fibrohyaline tissue, chondroid t.
fibrous tissue, the ordinary connective tissue of the body, made up largely of yellow or white fibers.
fibrous tissue, white, that which is composed almost wholly of collagenous fibers.
Gamgee Tissue, trademark for a surgical dressing consisting of a thick layer of absorbent cotton between two layers of absorbent gauze.
gelatiginous tissue, that which yields gelatin on boiling with water.
gelatinous tissue, mucous t.
glandular tissue, an aggregation of epithelial cells that elaborate secretions.
granulation tissue, the newly formed vascular tissue normally produced in the healing of wounds of soft tissue and ultimately forming the cicatrix; it consists of small, translucent, red, nodular masses or granulations that have a velvety appearance.

gut-associated lymphoid tissue, lymphoid tissue associated with the gut (primordial digestive tube), including the tonsils, Peyer patches, lamina propria of the gastrointestinal tract, and appendix.
hematopoietic tissue, see under system.
heterologous tissue, tissue unlike any other that is normal to the organism.
heterotopic tissue, choristoma.
homologous tissue, tissue identical with another tissue in structural type.
hyperplastic tissue, tissue affected by hyperplasia.in dentistry, an overgrowth of tissue about the maxilla or mandible that is excessively movable, or more readily displaced than is normal.
indifferent tissue, undifferentiated embryonic tissue.
interstitial tissue, stroma.
junctional tissue, the portion of the conducting system of the heart forming a bridge between the atrium and ventricle of the heart, comprising the atrioventricular node and the bundle of His.
Kuhnt intermediary tissue, glial tissue surrounding the optic nerve and separating it from the retina.
lardaceous tissue, tissue having the appearance of lard as a result of a degenerative process.
loose connective tissue, areolar t.
lymphadenoid tissue, tissue resembling that of the lymph nodes, found in the spleen, bone marrow, tonsils, and other organs.
lymphatic tissue, lymphoid tissue, a latticework of reticular tissue the interspaces of which contain lymphocytes; lymphoid tissue may be diffuse, or densely aggregated as in lymph nodules and nodes. See also under system.
mesenchymal tissue, mesenchyme.
metanephrogenic tissue, the nephrogenic tissue of the metanephros; it gives rise to the nephrons of the permanent kidney.
mucosa-associated lymphoid tissue, a type of specialized lymphoid tissue found in association with certain types of epithelia; it usually has prominent B-cell follicles and sometimes has zones of T cells.
mucous tissue, a jellylike mucoid connective tissue, such as occurs in the umbilical cord.
muscle tissue, muscular tissue, tissue specialized for contraction, which produces movement of the body and its parts; it consists of muscle fibers, muscle cells, connective tissue, and extracellular material. Called also flesh. See also muscle.
myeloid tissue, medulla ossium rubra.
nephrogenic tissue, see under cord.
nerve tissue, nervous tissue, the specialized tissue making up the central and peripheral nervous systems; it consists of neurons with their processes, other specialized or supporting cells such as the neuroglia, and extracellular material.
nodal tissue, tissue made up of nerve and muscle fibers, such as that composing the sinoatrial node of the heart.
osseous tissue, bone that has become ossified, whether normal or of a pathologic change to soft tissue. Called also bony t.
osteogenic tissue, that part of the periosteum adjacent to bone and concerned in the formation of osseous tissue; any tissue capable of generating bone.
osteoid tissue, uncalcified bone tissue.
parenchymatous tissue, parenchyma.
protochondral tissue, centers of chondrification.
reticular tissue, reticulated tissue, connective tissue consisting of reticular cells and fibers.
rubber tissue, rubber in sheets for use in surgery.
scar tissue, cicatricial t.
sclerous tissues, a category that includes cartilaginous, fibrous, and osseous tissues.
shock tissue, that tissue in the animal body which bears the brunt of the antigen-antibody reaction in anaphylaxis.
skeletal tissue, the bony, ligamentous, fibrous, and cartilaginous tissue forming the skeleton and its attachments.
splenic tissue, pulpa splenica.
subcutaneous tissue, tela subcutanea.
subcutaneous tissue of abdomen, tela subcutanea abdominis.
subcutaneous fatty tissue, panniculus adiposus.
subcutaneous tissue of penis, tela subcutanea penis.
subcutaneous tissue of perineum, tela subcutanea perinei.
sustentacular tissue, a non-nervous structure of the retina composed of its Müller fibers.
symplastic tissue, symplasm.
target tissue, tissue, either in vivo or in vitro, against which humoral or cell-mediated immunity is directed.the tissue that responds specifically to a given hormone.
tuberculous granulation tissue, the tissue that forms the characteristic tubercle in tuberculosis, composed of epithelioid cells in concentric masses, lymphocytes, and often Langhans giant cells.
vesicular supporting tissue, chondroid t.

Tissue is a group of cells that have similar structure and that function together as a unit. A nonliving material, called the intercellular matrix, fills the spaces between the cells. This may be abundant in some tissues and minimal in others. The intercellular matrix may contain special substances such as salts and fibers that are unique to a specific tissue and gives that tissue distinctive characteristics. There are four main tissue types in the body: epithelial, connective, muscle, and nervous. Each is designed for specific functions. Use the hyperlinks below to branch into a tissue type and learn more about the topic.
Epithelial tissues are widespread throughout the body. They form the covering of all body surfaces, line body cavities and hollow organs, and are the major tissue in glands. They perform a variety of functions that include protection, secretion, absorption, excretion, filtration, diffusion, and sensory reception.

The cells in epithelial tissue are tightly packed together with very little intercellular matrix. Because the tissues form coverings and linings, the cells have one free surface that is not in contact with other cells. Opposite the free surface, the cells are attached to underlying connective tissue by a non-cellular basement membrane. This membrane is a mixture of carbohydrates and proteins secreted by the epithelial and connective tissue cells.

Epithelial cells may be squamous, cuboidal, or columnar in shape and may be arranged in single or multiple layers.

Simple cuboidal epithelium is found in glandular tissue and in the kidney tubules. Simple columnar epithelium lines the stomach and intestines. Pseudostratified columnar epithelium lines portions of the respiratory tract and some of the tubes of the male reproductive tract. Transitional epithelium can be distended or stretched. Glandular epithelium is specialized to produce and secrete substances.

Connective tissues bind structures together, form a framework and support for organs and the body as a whole, store fat, transport substances, protect against disease, and help repair tissue damage. They occur throughout the body. Connective tissues are characterized by an abundance of intercellular matrix with relatively few cells. Connective tissue cells are able to reproduce but not as rapidly as epithelial cells. Most connective tissues have a good blood supply but some do not.

Numerous cell types are found in connective tissue. Three of the most common are the fibroblast, macrophage, and mast cell. The types of connective tissue include loose connective tissue, adipose tissue, dense fibrous connective tissue, elastic connective tissue, cartilage, osseous tissue (bone), and blood.

Muscle tissue is composed of cells that have the special ability to shorten or contract in order to produce movement of the body parts. The tissue is highly cellular and is well supplied with blood vessels. The cells are long and slender so they are sometimes called muscle fibers, and these are usually arranged in bundles or layers that are surrounded by connective tissue. Actin and myosin are contractile proteins in muscle tissue.

Muscle tissue can be categorized into skeletal muscle tissue, smooth muscle tissue, and cardiac muscle tissue.

Skeletal muscle fibers are cylindrical, multinucleated, striated, and under voluntary control. Smooth muscle cells are spindle shaped, have a single, centrally located nucleus, and lack striations. They are called involuntary muscles. Cardiac muscle has branching fibers, one nucleus per cell, striations, and intercalated disks. Its contraction is not under voluntary control.

Nervous tissue is found in the brain, spinal cord, and nerves. It is responsible for coordinating and controlling many body activities. It stimulates muscle contraction, creates an awareness of the environment, and plays a major role in emotions, memory, and reasoning. To do all these things, cells in nervous tissue need to be able to communicate with each other by way of electrical nerve impulses.

The cells in nervous tissue that generate and conduct impulses are called neurons or nerve cells. These cells have three principal parts: the dendrites, the cell body, and one axon. The main part of the cell, the part that carries on the general functions, is the cell body. Dendrites are extensions, or processes, of the cytoplasm that carry impulses to the cell body. An extension or process called an axon carries impulses away from the cell body.

Nervous tissue also includes cells that do not transmit impulses, but instead support the activities of the neurons. These are the glial cells (neuroglial cells), together termed the neuroglia. Supporting, or glia, cells bind neurons together and insulate the neurons. Some are phagocytic and protect against bacterial invasion, while others provide nutrients by binding blood vessels to the neurons.


Body membranes are thin sheets of tissue that cover the body, line body cavities, and cover organs within the cavities in hollow organs. They can be categorized into epithelial and connective tissue membrane.

Epithelial Membranes

Epithelial membranes consist of epithelial tissue and the connective tissue to which it is attached. The two main types of epithelial membranes are the mucous membranes and serous membranes

Mucous Membranes
Mucous membranes are epithelial membranes that consist of epithelial tissue that is attached to an underlying loose connective tissue. These membranes, sometimes called mucosae, line the body cavities that open to the outside. The entire digestive tract is lined with mucous membranes. Other examples include the respiratory, excretory, and reproductive tracts.

Serous Membranes
Serous membranes line body cavities that do not open directly to the outside, and they cover the organs located in those cavities. Serous membranes are covered by a thin layer of serous fluid that is secreted by the epithelium. Serous fluid lubricates the membrane and reduces friction and abrasion when organs in the thoracic or abdominopelvic cavity move against each other or the cavity wall. Serous membranes have special names given according to their location. For example, the serous membrane that lines the thoracic cavity and covers the lungs is called pleura.

Connective Tissue Membranes

Connective tissue membranes contain only connective tissue. Synovial membranes and meninges belong to this category.

Synovial Membranes
Synovial membranes are connective tissue membranes that line the cavities of the freely movable joints such as the shoulder, elbow, and knee. Like serous membranes, they line cavities that do not open to the outside. Unlike serous membranes, they do not have a layer of epithelium. Synovial membranes secrete synovial fluid into the joint cavity, and this lubricates the cartilage on the ends of the bones so that they can move freely and without friction.

The connective tissue covering on the brain and spinal cord, within the dorsal cavity, are called meninges. They provide protection for these vital structures.