Human Body: Cells and Tissues

Introduction to Cells

• Cells form the foundation of the human body
• In simple organisms like amoebas, the entire organism is one cell
• The human body has grouped single cells that form complex systems
• Billions of cells make up the human body
• Protoplasm supports internal cell structures, a viscous liquid matrix.
• Protoplasm, a colorless substance, exists in small amounts in each cell
• Protoplasm facilitates intracellular transport of nutrients, signaling molecules, ATP, and proteins.
• Cells that perform similar jobs form tissues
• Organs are formed by groups of tissues performing interrelated jobs.
• Homeostasis is the stability of the body's normal internal environment
• Disruption of homeostasis can significantly impact cell to the whole organism.

Structure of the Cell

• The human body has two classes of cells: sex cells and somatic cells
• Sex cells (germ or reproductive cells) exist in sperm and oocytes
• Somatic cells, meaning "body", include all non-sex cells
• The chapter focuses on somatic cells
• Cell membranes, nucleus and cytoplasm are the three basic parts of a cell
• Cell membranes enclose the cell, nucleus, organelles and cytoplasm
• The nucleus contains genetic material and controls cell activities
• The Cytoplasm fills out the cell and maintains cell shape
• Organelles carry out functions required by the cell
• Cell membrane, also called plasma membrane, controls substance movement in/out of cells
• Extracellular space is the outside of the cell
• Intracellular space is the inside of the cell
• The membrane consists of molecules arranged to alter structure based on the cell's needs
• Atoms make up molecules and are comprised of smaller particles
• Protons are positively charged, electrons are negatively charged, and neutrons are uncharged
• The cell membrane facilitates communication between intracellular and extracellular spaces and aids in movement
• The cell membrane:
  • Gives the cell form
  • Conducts much of the cell's biologic activities
  • Detects external signals and transmits them inward
  • Adheres to other cells is tissue formation
• Cell membranes are semipermeable and allow certain elements to pass through while blocking others
• Selective permeability: only some substances can enter or leave the cell
• Lipids and proteins primarily make up cell membranes
• A double layer of phospholipid molecules usually exists
• The phosphate portion forms the outer surface
• The fatty acid portion forms the inner surface
• This double layer is also called a bilayer
• Proteins periodically interrupt the lipid bilayer and serve as transporters, signal receptors, doorways (channels), or ion channels
• Cell membrane is selectively permeable, it allows some, but not all, substances to pass through to maintain homeostasis
• Separation of extracellular from intracellular areas helps maintains homeostasis
• Enzymes, sugar molecules, and electrolytes pass freely into and out of the cells
• Electrolytes act as ionic conductors when dissolved in solvents such as water
• Diffusion, osmosis, facilitated diffusion, and active transport help materials pass through the cell wall
• Diffusion: particles or solutes move from high to low concentration areas
• The permeability of the membrane and the concentration gradient dictates the degree of diffusion
• The concentration gradient is the difference in concentrations of a substance on either side of a membrane
  • Small molecules diffuse more easily than large molecules
  • Watery solutions diffuse faster than viscous solutions
• Osmosis: solvent movement, such as water, from low to high solute concentration through a selectively permeable membrane
• Osmosis occurs when the membrane is permeable to the solvent but not the solute
• Movement continues until solute concentrations equalize on both sides
• Osmotic pressure measures the tendency of water to move by osmosis
• Crenation: cell shrinkage from too much water moving out
• Lysis: cell swelling & bursting from too much water moving in
• Facilitated diffusion: carrier molecule moves substances into/out of cells from high to low concentration areas
• Energy is not required in facilitated diffusion, but the number of molecules transported is directly proportional to the concentration
• Active transport: the movement of a substance against a concentration or gradient
• Active transport requires energy and a carrier mechanism
• Active transport movement opposes normal diffusion
• This Mechanism may exchange one substance for another
• Endocytosis: uptake of material by the cell membrane
• The cell membrane surrounds and engulfs material within it
• Phagocytosis: a form of endocytosis involving solid particles, meaning "cell eating"
  • Occurs when infection-fighting white blood cells consume bacteria and foreign particles
  • In Certain diseases, cells lose the ability to phagocytize, leading to life-threatening infections.
• Pinocytosis: endocytosis of liquids, or "cell drinking"
• Exocytosis: the release of secretions accumulated in vesicles:
• Secretions accumulate within vesicles, which move to the cell membrane
• Vesicles bond/fuse to the membrane, and the content is eliminated
• Secretion of digestive enzymes by the pancreas, and mucus secretion by the salivary glands are examples of exocytosis

Nucleus

• Nucleus is the largest, most visible structure inside a cell and controls cellular operations
• Nucleus contains DNA, which controls cell activities
• A cell without a nucleus cannot repair itself, and will disintegrate within 3 to 4 months
• The nucleus holds the genetic instructions to synthesize proteins that determine cell structure and function
• Instructions are stored in the chromosomes
• Most cells contain a single nucleus although skeletal muscle cells contain numerous nuclei
• Mature red blood cells contain no nuclei
• The nucleus is round and enclosed in a double nuclear envelope with inner and outer lipid membranes
• The nuclear envelope has a protein lining, allowing certain molecules to exit
• The nucleoplasm is a fluid inside the nucleus that suspends structures like the nucleolus and chromatin
• Nucleolus: a "mini nucleus" of RNA and protein molecules, has no surrounding membrane and forms ribosomes that migrate to the cytoplasm
• Chromatin: loosely coiled DNA and protein fibers that condenses, DNA controls protein synthesis, and chromatin fibers coil tightly to form chromosomes during division

Cytoplasm

• Cytoplasm is the substance containing all cellular contents between the cell membrane and the nucleus
• Serves as a matrix substance where chemical reactions occur
• Cytoplasm makes up most of each cell's volume
• Its a gel-like material suspending the cell's organelles
• Organelles: internal structures carrying out necessary processes for the human body to function
• Centrioles: important in the formation of the spindle apparatus
• Spindle fibers and microtubules radiate from the centrioles
  • Essential in cell division, forms the spindle-shaped structure needed for movement of DNA strands
• Cilia and flagella – structures that extend from certain cell surfaces
  • Cilia - Hair-like projections on a cell, often several exist on one cell
    • Cilia are capable of motion, and move materials over the cell surface
    • In the trachea and bronchi, cilia continuously move a layer of mucus from the lower portions of the lungs to the throat to trap particles of dirt, microorganisms, and other foreign matter that are then swallowed or expectorated
    • Cigarette smoking paralyzes cilia motion, resulting in an accumulation of foreign substances in the lungs
  • Flagella - tail-like microtubule structures
    • Flagella Propels the cell
    • Sperm has a flagellum that propels it to fertilize an ovum
• Ribosomes: organelles containing RNA and protein
  • Interact with RNA from other cell parts to join amino acids to form proteins
  • It takes place in the endoplasmic reticulum
• Endoplasmic reticulum: a series of membranes where specific proteins and fats (lipids) are manufactured
• Golgi apparatus: a set of membranes within a cell's protoplasm that are associated with the formation of various carbohydrate and complex protein molecules
• Lysosomes: membrane-bound vesicles with enzymes that function as an intracellular digestive system
• Peroxisomes: similar to lysosomes, found in liver and neutralize toxins such as alcohol
• Mitochondria: small, rod-like organelles, the metabolic center of the cell
  • Use oxidative metabolism to produce adenosine triphosphate (ATP)
  • ATP is the primary molecule used by cells to store and transfer energy
• Nucleus contains DNA and RNA
  • DNA in the chromosomes
  • RNA in the nucleoli
    • Nucleoli are spherical intranuclear structures
    • The nucleus is surrounded by the nuclear envelope
    • The nucleus is embedded in the cytoplasm
• Cytosol is the fluid portion of cytoplasm, containing water, glucose, amino acids, fatty acids, ions, lipids, proteins, ATP, and waste products
• Cytosol is the site of cell chemical reactions

Cell Division

• Cell remodeling: ongoing cell renewal process due to cell division and cells dying/being replaced
• Cell division occurs via mitosis and meiosis
• Meiosis: specialized cell division to produce mature sperm and ova
• Normal cells contain 46 chromosomes (diploid number), with one set from each parent
• Sperm and ova contain half (23 chromosomes, called haploid number)
• When sperm and egg unite, the final organism has 46 chromosomes (normal number)
• Mitosis: division of the nucleus
• Cytokinesis is the division of cytoplasm
• All cells except egg and sperm cells divide by mitosis
• Division must be precise to duplicate DNA accurately
• Cell division occurs in four stages:
  • Stage One (prophase) – two new centriole pairs move to opposite ends of the cell.
  • Stage Two (metaphase) – the chromosomes align in the middle (equatorial plane) between the centrioles, with spindle fibers attached.
  • Stage Three (anaphase) – centromere sections of each chromosome are pulled apart to become individual chromosomes and move toward opposite ends
    • Cytoplasmic division begins during this phase
  • Stage Four (telophase) – chromosomes arrive at each pole and new nuclear membranes form

Types of Tissue

• Tissue: groups of similar cells working together, classified into epithelial, connective, muscle, and nerve
• Epithelial tissues cover external body surfaces, line organs, and make up glands. Enabling secretion and absorption
• Most epithelial tissues have a basement membrane, a noncellular layer securing the overlying tissues
• Epithelial tissue is classified by the number of cell layers and the cell shape
• Cell shapes include squamous, cuboidal, and columnar
  • Squamous epithelium = flat sheets
  • Cuboidal epithelium: rows of square-shaped cells
  • Columnar epithelium rows of tall, thin cells
• Simple epithelium consists of a single layer of cells, all touching the basement membrane
• Stratified epithelium consists of more than one layer of cells, only one layer touching the basement membrane
• Pseudostratified epithelium consists of a single layer of varying heights, with all cells attached to the basement membrane
• Transitional epithelium consists of layers of stratified cells that change shape from cuboidal to squamous, like when the organ is stretched
• Connective tissues bind other tissue types together
• Connective tissue proper includes many types of cells with extracellular fibers in ground substance
• Dense connective tissue contains collagenous fibers and appears white
• Loose connective tissue:
  • Adipose tissue - lies beneath the skin, between muscles, around the kidneys, behind the eyes, and heart's surface.
  • Areolar tissue binds skin to organs and fills in spaces between muscles
  • Reticular tissue creates frameworks inside organs (spleen and liver)
• Supporting connective tissues have less diverse cells and more densely packaged fibers
  • Supports soft tissues and carries weight.
  • The two types of supporting connective tissue are cartilage and bone
• Cartilage is rigid connective tissue with a gelatinous matrix and fibers
  • Supports, frames, and attaches to underlying tissues/bones
• Bone: the most rigid type of connective tissue, with high mineral content.
  • Establishes the framework of the body
  • Attaches to muscles, which protect and support body structures
• Fluid connective tissues have distinct cell populations suspended in a water matrix and contain dissolved proteins
  • Blood contains formed elements (red/white cells, platelets) in blood plasma liquid matrix
    • Blood cells are formed in red bone marrow
  • Lymph forms when interstitial fluid enters lymphatic vessels

Muscle Tissue

• Muscle tissue overlies the skeleton and is enclosed by connective tissue
• Muscles are classified structurally and functionally
• Structurally, muscle tissue is striated or nonstriated (smooth)
• Striated = microscopic bands or striations
• Nonstriated = lacking striations; smooth muscle is nonstriated
• Functionally, muscle is voluntary or involuntary
• Voluntary muscle is consciously controlled
• Involuntary muscle operates without conscious control
• The three muscle types are skeletal, cardiac, and smooth muscle
  • Skeletal muscle is striated voluntary muscle and is used volitionally
• Cardiac muscle is striated involuntary muscle (myocardium)
  • The heart consists of cardiac muscle
  • Cardiac muscle contains less connective tissue than skeletal muscle
  • It relies on pacemaker cells or nodes for regular contraction
  • Each cardiac tissue has automaticity: the ability to generate an impulse by hypoxia or electrolyte imbalance
• Smooth muscle is nonstriated involuntary muscle
• Can regenerate, composes hollow organ walls (intestines and stomach)
• No conscious control over the function of smooth muscle

Nervous Tissues

• Nervous tissues specialize in electrical impulse conduction
• Contains neurons and neuroglia (glial cells.)
• Found in the brain, spinal cord, cranial nerves, and peripheral nerves
• Peripheral nerves all nerves extending from the brain and spinal cord.
• Neurons are the main conducting cells of nerve tissue
  • Dendrites – projections receiving electrical impulses from axons, move them towards the cell body
  • Axons – projections conducting impulses away from the cell body
  • Each neuron has one axon but can have several dendrites

Organs and Body Regions

• Various tissues work together in organs to perform tasks, such as pumping blood
• The skin (integument) contains all four tissue types and is the largest organ
  • Preserves heat, prevents fluid loss, and protects against infection
• Other organs include the liver, spleen, digestive/reproductive organs, and sensory organs
• Organ Systems: groups of organs working together to maintain homeostasis
  • Organ systems include skeletal, muscular, respiratory, circulatory, lymphatic, nervous, integumentary, digestive, endocrine, urinary, and genital
  • Combined organ systems form an organism or individual living thing
  • Anatomy and physiology of each system are described in the following chapters
• The body is divided into regions
  • The Appendicular region includes extremities and their associated girdles
    • Girdles: bony structures that attach limbs to the body
    • The upper extremity divides into the arm, forearm, wrist, hand, and attaches at the shoulder girdle
      • Axilla: region inferior to the shoulder (armpit)
      • Antecubital: the bend of the elbow
    • The lower extremity divides into the thigh, leg, ankle, and foot, attached by the hip/pelvic girdle
      • Popliteal fossa: the knee space
  • Axial region – the head, neck, and trunk - Scalp is the skin layer of the head, excluding the face and ears, normally covered with hair - Consists of skin, subcutaneous tissue, muscle, loose connective tissue, and periosteum - Highly vascular tends to bleeds freely - Meninges: membrane layers inside the skull covering the brain containing dura mater, arachnoid, and pia mater - Periosteum: membrane covering the outside the skull and all bones - Skull: houses the brain and is composed of the cranium and facial bones - Sutures - Skull bones connected at special joints - Fontanelles: soften/expand during birth, link sutures - Tissue through fontanelles is layers of scalp and thick-overlying membranes - Brain may not be felt through fontanelle - These sutures should solidify and close in by age 2 - The neck contains anterior, carotid, and posterior triangles - Anterior triangle: bound by the sternocleidomastoid muscle, anterior midline of the neck, and inferior border of the mandible. - Carotid triangle: lies within the anterior triangle and contains the carotid artery and internal jugular vein. - Posterior triangle: runs from the posterior portion of the sternocleidomastoid muscle to the posterior midline of the neck and the base of the skull - Contains lymph nodes, the brachial plexus, spinal accessory nerve, and a subclavian artery portion - Trunk: subdivides into the thorax, abdomen, and pelvis

Body Cavities

• Hollow areas within the body containing organs and systems
• The skull and vertebral column hold the brain and spinal cord
  • Cranial cavity: the skull's hollow portion, is continuous with the spinal cavity
  • Spinal cavity: spinal column or vertebral canal, that extends through vertebrae
  • The muscular diaphragm divides the trunk into thoracic and abdominal cavities
• Serous membrane: thick connective tissue lining trunk cavities
  • Parietal part of serous membrane: lines the cavity wall
  • Visceral part of serous membrane covers internal organs
  • Serous membranes secrete fluid to fill the space between membranes
  • The fluid and membranes protect internal organs from friction
  • Fluid volume depends on the cavity
• Thoracic cavity (thorax): the cavity between the neck base and the diaphragm, formed by the rib cage
  • Organs belonging to the cardiovascular/respiratory systems i.e. heart and lungs
    • Lungs are surrounded by pleura or serous membranes
    • Pleural space is a space between the visceral and parietal pleura that rarely contains anything
  • Mediastinum: space between the lungs containing the heart, trachea, mainstem bronchi, part of the esophagus, and large blood vessels - The pericardium surrounds the heart, containing a set of serous membranes - Pericardial sac is the space between serous membranes that rarely contains fluid and surrounds the heart
• Abdominal cavity: cavity extending from the diaphragm to the pelvic bones, lined with the peritoneum serous membrane
  • Mesentery: membranous double fold of tissue in the abdomen attaches organs to the wall, supplies blood to the organs
  • The abdomen divides into quadrants by crossing the umbilicus with imaginary perpendicular lines (the umbilicus is the navel)
  • Retroperitoneal space: the posterior area to the parietal peritoneum
  • Retroperitoneal organs term given to the kidneys, pancreas, duodenum, and abdominal cavity's major blood vessels
• Pelvic cavity: lower portion of the abdominal area containing urinary/reproductive organs and the gastrointestinal system, supported by the pelvic girdle

Cellular Injury

• Cellular injury results from hypoxia (lack of O2), ischemia (lack of blood), chemical or infectious injury, immunologic/inflammatory injury, and physical damage
• Manifestation depends on how many and which cells are damaged
• Hypoxia is a common cause of cell injury
  • Can result from decreased O2 in the air, loss of hemoglobin, fewer red blood cells, respiratory/cardiovascular disease, or loss of cytochromes
• Hypoxic cells for more than a few seconds produce mediators that may damage local or distant body areas
  • Mediators cause more cell damage, which leads to more hypoxia, resulting in tissue damage
• Chemical Injury: injure and destroy cells through poisons, lead, CO, ethanol, cyanide (blocks oxygen metabolism in mitochondria), pesticides (prevents proper nerve impulse transmission)
• Infectious Injury caused by bacteria, fungi, or viruses that may cause injury directly or release toxins
  • Viruses often initiate inflammatory responses that damage cells
• Immunologic and Inflammatory Injury: the immune system attacks and removes foreign organisms (bacteria/viruses)
  • Inflammation occurs without bacterial invasion for protection
  • Infection is the invasion of microorganisms causing injury, that leads to the inflammatory response
  • Injury direct contact with cellular/chemical immune/inflammatory components such as in Phagocytes, histamine, antibodies, and lymphokines
• Cell membrane alteration causes water inflow, and potassium outflow, that can rupture the nuclear, organelle, and cell membranes, leading to cell death
• Genetic, nutritional, or physical factors/agents can cause cell injury and death. i.e, genetic disorders
  • Chromosomal disorders (Down syndrome) or meiosis mutation causes abnormal development
• Imbalances in nutrition can destroy cells and the overall organism - Obesity, malnutrition, vitamin or mineral surplus or deficiency
• Excessive heat or cold exposure, and radiation may also lead to cellular injuries