Cells and Tissues

Cells and Tissues Unit 2

Overview of the Cellular Basis of Life • Cells are primarily made of 4 elements: • Carbon, oxygen, hydrogen, and nitrogen • Water (H2O is the most abundant substance • Also contain small amounts of other elements: • Calcium for blood clotting • Iron for hemoglobin, which carries oxygen in blood • Iodine makes thyroid hormone that controls metabolism • Metals (calcium, sodium, potassium, etc.) can carry electrical charge  electrolytes that are essential for nerve impulses

Cell Functions • In multicellular organisms, cells can be specialized to perform specific tasks • However, all cells are able to perform the following functions: • Metabolism • Reproduction • Irritability • Mobility • Grow • Digestion • Excretion

Different Cell Shapes • Cubelike • Tilelike • Disk-shaped • Round spheres • Branching • Cylindrical

Interstitial fluid • Surrounds all cells • Contains salt and other materials  similar to sea water

Structure and function • A cell’s structure greatly impacts its function • Neurons (nerve cells) have long processes for receiving and transmitting messages and are covered with an extensive plasma membrane. • Fat cells are large and spherical due to a large lipid droplet in their cytoplasm. • Squamous epithelial cells are flat and fit together like tiles in order to protect the body. • Skeletal muscle cells are elongated and filled with contractile filaments to help facilitate mobility.

Anatomy of a cell

From cells to tissues • Cells come together to form tissues, joined at cell junctions • Types of junctions: • Tight junctions – impermeable, bind cells into leak-proof sheets. Adjacent plasma membranes fuse together tightly like a zipper. • Desmosomes – anchoring junctions that prevent cells from being pulled apart. Button-like thickenings of adjacent plasma membranes, which are connected by fine protein filaments. • Gap junctions – allow communication, chemical molecules can pass from one cell to another, neighboring cells are connected by connexons, hollow cylinders composed of protein.

Types of tissues • Epithelial – lining, covering, glandular • Connective – connects body parts • Muscle – able to contract (shorten) • Nervous – conduct electrochemical impulses from one part of the body to another  irritability and conductivity

Epithelial tissue • Cells fit closely together to form continuous sheets  Neighboring cells are bound together at cell junctions • Membranes always have one free (unattached) edge = apical surface  exposed to exterior OR cavity of internal organ • Lower surface rests on basement membrane, a structureless material secreted by the cells • Don’t have a blood supply of their own  rely on diffusion from capillaries

Epithelial tissue • Named for cells at the free surface, not those at basement membrane • Simple epithelium = one layer of cells • Stratified epithelium = more than one cell layer • Squamous cells = flattened • Cuboidal = cube-shaped • Columnar = shaped like columns

Simple squamous epithelium • Single layer of thin squamous cells resting on a basement membrane • Fit closely together, like floor tiles • Usually found where filtration occurs or where substances are exchanged through rapid diffusion • Example: Air sacs of lungs and walls of capillaries

Simple cuboidal epithelium • Single layer of cuboidal cells resting on a basement membrane • Common in glands and their ducts • Forms walls of kidney tubules and covers surface of ovaries

Simple columnar epithelium • Single layer of tall cells that fit closely together • Line the entire length of the digestive tract from stomach to anus

Pseudostratified columnar epithelium • All of the cells rest on a basement membrane, however, some appear shorter than others • False impression of stratification • Lines the respiratory tract

Stratified squamous epithelium • Consists of several layers of squamous cells • Most common stratified epithelium in the body • Outer portion of skin, esophagus

Stratified cuboidal and stratified columnar epithelia • Usually only two layers of cells that are cuboidal or columnar in shape • Fairly rare in the body, found mainly in the ducts of large glands

Transitional epithelium • Highly modified, stratified squamous epithelium that forms the lining of only a few organs • Urinary bladder, ureters, part of urethra  all part of urinary system

Glandular epithelium • Glands make and secrete a particular product • Endocrine glands – “ductless” glands  secrete directly into blood stream • Exocrine glands – secrete through ducts to the epithelial surface

Connective tissue • Connects body parts • Most abundant and widely distributed of tissues • Most have a good blood supply, but there are some exceptions, such as tendons, ligaments, and cartilages • Extracellular matrix – nonliving substance found outside of the cells

Bone • Osseous tissue  bone cells sitting in cavities called lacunaeand surrounded by layers of a very hard matrix that contains calcium salts and collagen fibers • Very hard  protects and supports body

Cartilage • Less hard and more flexible than bone • Hyaline cartilage = abundant collagen fibers hidden by a rubbery matrix; forms larynx, attaches ribs to sternum, and covers ends of bones where they form joints • Fibrocartilage = cushionlike discs between vertebrae of spinal column • Elastic cartilage = found where a structure with elasticity is desired, such as the external ear

Dense connective tissue • Collagen fibers = main matrix element • Fibroblasts = fiber-forming cells located between collagen fibers • Forms strong, ropelike structures such as tendons (attach skeletal muscles to bones) and ligaments (connect bones to bones)

Loose connective tissue • Softer, more cells, fewer fibers • Areolar tissue – cushions and protects body organs, helps hold them together and in place, acts as a sponge and soaks up excess fluid • Adipose tissue – “fat,” forms subcutaneous tissue beneath the skin, insulates body and protects some organs, acts as fuel if needed • Reticular connective tissue – delicate network of interwoven fibers that helps support many free blood cells in lymphoid organs

Blood • Blood is considered a connective tissue because it consists of blood cells surrounded by a nonliving, fluid matrix called plasma • The “fibers” are soluble protein molecules visible during clotting • Carries nutrients, wastes, respiratory gases, and many other substances

Muscle tissue • Specialized to contract, or shorten • Three types: • Skeletal • Cardiac • Smooth

Skeletal muscle • Packaged by connective tissue sheets into organs called skeletal muscles • These muscles can be controlled voluntarily • When they contract, they pull on bones or skin • Cells are long, cylindrical, and multinucleate • Obvious striations (stripes)

Cardiac muscle • Found only in the heart • Has striations like skeletal muscle, but the cells only have one nucleus • Branching cells fit tightly together like clasped fingers at junctions called intercalated disks • Contain gap junctions that allow ions to pass freely from cell to cell, resulting in rapid conduction of the electrical impulses • Involuntary control

Smooth muscle • No striations • Cells have a single nucleus and are spindle shaped • Found in the walls of hollow organs, such as the stomach, bladder, uterus, and blood vessels • Smooth muscle contractions help propel substances through an organ • Involuntary contractions

Nervous tissue • Neurons – cells that receive and send electrochemical signals from one part of the body to another • Cytoplasm is drawn out into long extensions, which allows a single neuron to conduct an impulse over long distances

Cells and Tissues