Anatomy: Cells and Tissues

• Anatomy studies cells and tissues.
• Cells, tissues, organs, systems, and organisms represent a hierarchy of organization.

Cells

• Smallest living subunits of multicellular organisms, such as humans.
• Conduct all chemical activities to sustain life.
• Serve as the building blocks of all living things.
• Bathed in interstitial fluid, a dilute saltwater solution from blood.
• Grouped by structure and function to form more complex tissues.
• Differ widely in size, shape, and function.
• Organized into three main regions: plasma membrane, cytoplasm, and nucleus.

Plasma Membrane

• Surrounds each cell, separating it from the external environment.
• It acts as a barrier for cell contents.
• It exhibits semi-permeability due to its double phospholipid layer.
• Hydrophilic heads like water whereas hydrophobic tails fear it.
• Proteins act as receptors and channels for transport.
• Cholesterol keeps the membrane fluid and stable.
• Glycoproteins function as receptors, for cell-to-cell interactions.

Cytoplasmic Organelles

• Cytoplasm is the gel-like substance largely composing the inside of the cell; it lies outside the nucleus and inside the plasma membrane.
• Cytosol is the fluid containing dissolved nutrients in water that suspends organelles, metabolic machinery, and inclusions.
• Mitochondria, the "powerhouses" of the cell, continuously change in shape, possesses a double membrane and carry out cellular respiration, providing ATP for cellular energy.
• Ribosomes, made of protein and RNA, are protein synthesis sites and are found free in the cytoplasm or attached to the rough endoplasmic reticulum.
• Endoplasmic Reticulum (ER) exists in two forms: rough ER (studded with ribosomes, where cellular membrane materials are built) and smooth ER ( functions in cholesterol synthesis and breakdown, fat metabolism, and detoxification of drugs).
• The Golgi apparatus modifies and packages proteins, producing various packages like secretory vesicles (containing proteins for export) or cell membrane components.
• Lysosomes contain enzymes that digest non-usable materials, such as old organelles, bacteria, and viruses.
• Peroxisomes are membranous sacs with enzymes that catalyze a variety of biochemical reactions, including the detoxification of materials.
• Centrosomes, specialized structures near the nucleus, organize microtubule formation and contain two centrioles that distribute chromosomes during mitosis.
• Microfilaments are rods of actin protein forming meshworks or bundles, giving structural support, maintaining cell shape, and enabling movement, and aggregating into myofibrils in muscle cells.
• Microtubules are long tubes of tubulin molecules involved in cell movement and organelle movement within cells.
• Microvilli are finger-like projections that increase surface area for absorption.
• Cilia and flagella are not in all cells; cilia move materials across the cell surface (short and many), while flagella propel the cell (long and few).

Nucleus

• The nucleus is the cell's control center containing genetic material (DNA).
• The nuclear membrane acts as a barrier and is made of a selectively permeable, double phospholipid membrane with nuclear pores that allow for material exchange.
• Nucleoli are the sites of ribosome production whereas ribosomes then migrate to the cytoplasm through nuclear pores.
• Chromatin condenses to form chromosomes when the cell divides.

Body Tissues

• Cells specialize in structure and function create tissues.
• Epithelium is for covering, connective tissue gives support, nervous tissue gives control, and muscle tissue allows for movement.

Epithelial Tissues

• Lacking capillaries and receiving oxygen and nutrients from underlying connective tissue.
• Found in body coverings and linings.
• Functions include protection, absorption, filtration, and secretion.
• Cells fit closely together and are avascular, depending on diffusion for nutrients.
• Classified by number of cell layers (simple or stratified) and shape of cells (squamous, cuboidal, or columnar), and cell modification (ciliated, keratinized, etc).
• Simple epithelium consists of a single layer of identical cells, and comes in 3 forms, squamous, cuboidal, and columnar.
• Simple squamous consists of a single layer of flat cells and commonly forms membranes where filtration and exchange happen, as in the alveoli of the lungs.
• Simple cuboidal consists of a single layer of cube-like cells and is common in glands and kidney tubules.
• Simple columnar consists of a single layer of tall cells that include goblet cells and often line the digestive tract.
• Pseudostratified columnar is a single layer, but cells vary in height and may have cilia.
• Stratified epithelium consists of multiple cell layers.
• Stratified squamous, has a protective role consisting of flattened surface cells and deeper columnar cells.
• Keratinized stratified squamous epithelium is found on dry surfaces like skin, hair, and nails.
• Non-keratinized stratified squamous epithelium is found on wet surfaces subject to wear and tear.
• Transitional epithelium is characterized by its ability to stretch.
• Glandular epithelium is responsible for secretion, is either unicellular (single cell) or multicellular (group of cells).
• Exocrine glands secrete into ducts, while endocrine glands secrete hormones directly into the blood or interstitial fluid.

Connective Tissue

• Found throughout the body, it is the most abundant and widely distributed tissue type.
• It binds body tissues together, supports the body, and provides protection.
• These tissues vary in blood supply; some are well vascularized, while others have poor blood supply.
• The extracellular matrix surrounds living cells.
• Types include fibroblasts, macrophages, mast cells, plasma cells, and fat cells.
• Collagenous fibers are thick, strong, and slightly elastic.
• Elastic fibers contain elastin, and stretch and return to their original shapes and lengths.
• Reticular fibers are thin collagenous fibers that form a supporting network.
• Connective tissue includes bone, hyaline cartilage, fibrocartilage, elastic cartilage, dense connective tissue, loose connective tissue, and blood.
• Connective tissue types performs different functions, bones protect and support the body and blood transports.

Muscle Tissue

• Function is to produce movement by contracting or shortening.
• Smooth muscle is visceral, involuntary, and spindle shaped.
• Skeletal muscle is controlled and striated (marked with stripes).
• Cardiac muscle is found only in the heart, it is involuntary, and striated.

Nerve Tissue

• Includes neurons and nerve-supporting cells that insulate, support, and protect neurons.
• Nerve tissues function to receive and send impulses and exhibit irritability and conductivity.