Chapter Review: Tissues and Cellular Responses

Skeletal vs Cardiac vs Smooth Muscle

• Start with the quiz point: distinguishing muscle types is essential; simply writing “skeletal” without specifying muscle tissue can be interpreted as referring to muscle cells; instructors may award partial credit but you should distinguish tissue from tissue subtype.
• Key terminology:
  • Myoblasts vs Myocytes: myoblasts are precursor cells that fuse to form muscle fibers; myocytes are mature muscle cells. In practice, myoblasts fuse to become muscle fibers;
  • Fibroblasts vs Fibrocytes: blasts are progenitor cells that actively synthesize the connective tissue matrix; cytes are the mature cells.
• Muscle fibers are connected by connective tissue fascia.
• Cardiac muscle (cardiomyocytes):
  • Cardiomyocytes are not multinucleate like skeletal muscle fibers; they are individual cells with their own nuclei.
  • Cardiac muscle has a branching, wavy structure, unlike the parallel alignment of skeletal muscle.
  • Intercalated discs: specialized junctions between cardiomyocytes that anchor cells and form gap junctions, enabling rapid intercellular communication and synchronized contraction.
  • Functional syncytium: the interconnected cardiomyocytes allow the heart to contract as a unit; metabolites and ions pass through the cells to coordinate activity.
• Skeletal muscle:
  • Striated appearance (due to organized myofibrils) and parallel fibers; usually multinucleate; composed of long muscle fibers with multiple nuclei per fiber.
  • Striations are visible; organization supports voluntary movement.
• Smooth muscle:
  • Lacks striations (non-striated); spindle-shaped, mononucleate cells.
  • Autonomic control: does not require conscious brain input for basic rhythmic activity; some smooth muscle can be consciously controlled in rare cases (e.g., breathing can be controlled consciously to some extent).
  • Reflexive activity: maintains automatic tone and rhythmic contractions in organs like vessels and the gut.
• How to differentiate visually (from images or slides):
  • Skeletal muscle: straight parallel fibers with visible striations.
  • Cardiac muscle: branching fibers with intercalated discs; single nuclei per cell is common, though some cells can be binucleate.
  • Smooth muscle: lack striations, spindle-shaped, mononucleate, cells arranged in sheets.
• Practical note on identification:
  • Smooth muscle is trickier to identify in isolation due to lack of striations; look for mononucleate cells and absence of striations.
  • Cardiac muscle shows branching, intercalated discs, and a cellular organization distinct from skeletal muscle.
  • Skeletal muscle shows parallel, striated fibers.

Nervous Tissue

• Nervous tissue is specialized for communication and signaling; it is excitable and transmits electrical and chemical signals.
• Two main cell types:
  • Neurons: propagate electrical signals (action potentials) and neurotransmitters; main communicators of the nervous system.
  • Glial cells (glia): supporting cells that protect, nourish, insulate, and regulate the environment around neurons; they include several subtypes:
  • Astrocytes: regulate ion concentrations and neurotransmitter concentrations around neurons; help prevent overstimulation; support blood-brain barrier and homeostasis.
  • Schwann cells: myelinate axons in the peripheral nervous system (PNS).
  • Oligodendrocytes: myelinate axons in the central nervous system (CNS).
  • Microglial cells: immune cells of the CNS, acting like macrophages to defend against pathogens and remove debris.
• Neuron structure and signal flow:
  • Dendrites: receive signals from other neurons; initial input side.
  • Soma (cell body): integrates input and houses nucleus.
  • Axon: conducts the electrical signal away from the soma; axon ends in a synapse.
  • Synapse: the gap or junction where neurotransmitters are released from the presynaptic neuron and picked up by the dendrites of the postsynaptic neuron; this transmits the signal chemically to the next neuron.
  • Myelin sheath: axons are insulated by a myelin layer, which increases conduction speed and efficiency; in the CNS, myelin is produced by oligodendrocytes; in the PNS, by Schwann cells.
  • Action potential: the electrical signal that travels along the axon; triggered by electrical gradients and ion movements across the neuron membrane; propagation is faster with myelin (insulation).
• Neurotransmission basics:
  • When a neuron fires, it releases neurotransmitters from the axon terminals into the synaptic cleft.
  • Neurotransmitters bind to receptors on the next neuron’s dendrites, initiating a response (electrical signal) in that neuron.
  • The extracellular ionic environment (regulated by astrocytes and other glial cells) is crucial for proper signal propagation.
• Glial cell roles:
  • Astrocytes: regulate extracellular ion concentrations, neurotransmitter clearance, and help maintain homeostasis to prevent overstimulation.
  • Microglia: immune surveillance in the CNS.
  • Schwann cells (PNS) and oligodendrocytes (CNS): produce the myelin sheath around axons; increase conduction speed and efficiency.
• Nervous system organization:
  • Central nervous system (CNS): brain and spinal cord.
  • Peripheral nervous system (PNS): all nerves outside the CNS; Schwann cells myelinate PNS axons.

Four Primary Tissue Types

• Epithelial tissue
• Connective tissue
• Muscle tissue
• Nervous tissue
• Notes:
  • Epithelial and connective tissues are highly diverse; muscle tissue has relatively few primary types with clear distinguishing features; nervous tissue is mainly one tissue type but composed of several cell types.

Tissue Damage and Repair – Inflammation and Healing

• Inflammation as a common tissue damage response:
  • Purpose: cordon off damaged area and promote repair; defend against pathogens; restore tissue integrity.
  • Vasodilation occurs to increase blood flow to the area.
  • Mast cells release histamine, a primary trigger for the inflammatory response; histamine promotes increased vascular permeability and leukocyte recruitment.
  • Consequences: redness, swelling, and pain; increased access of white blood cells to the damaged site.
• Role of histamine and vascular permeability:
  • Histamine increases permeability of vascular epithelium (thin, simple squamous lining) to allow immune cells to extravasate into tissue.
  • The goal is to bring immune cells and nutrients to the site for repair; but this also causes pain and swelling.
• Platelets and clot formation:
  • Platelets accumulate at the wound site and secrete substances to promote attachment to red blood cells and to form a clot.
  • Positive feedback loop: platelets release clotting agents that attract more platelets and red blood cells, enlarging the clot.
  • The clot forms a provisional barrier and scaffold for tissue repair.
• Early tissue repair and fibrous matrix:
  • Connective tissue healing begins with fibroblasts rebuilding the fibrous matrix around the injury.
  • Blood vessels revascularize the damaged area to restore nutrient supply.
• Granulation tissue:
  • A heavily vascularized, temporary tissue that forms as part of the repair process; acts as a bridge to bring nutrients and cells to the wound.
  • Granulation tissue helps remodel the area and supports new tissue formation.
• Scar formation and clot resolution:
  • After repair is near complete, the clot retracts; granulation tissue may be replaced by scar tissue.
  • Visible scars occur when the wound contractures are uneven, leaving raised edges or tissue that protrudes at the surface.
• Tissue repair visualization (described):
  • Damaged vessels and tissue show initial clot formation; fibroblasts rebuild the fibrous matrix; blood vessels extend into the damaged region; elastin and fibrous components form the remodeling matrix; revascularization occurs.
• Atrophy and disuse:
  • Muscle and connective tissue can experience atrophy when not used or during aging; results in reduced mass and strength.
• Aging, telomeres, and senescence:
  • Telomeres: protective caps at the ends of DNA strands; shorten with each DNA replication because DNA polymerase cannot fully replicate the ends of linear chromosomes.
  • Telomere shortening is implicated in cellular aging; once telomeres are depleted, cells may lose the ability to divide effectively, contributing to aging phenotypes.
  • Senescence and cancer: when cells bypass normal growth controls, they can form tumors; telomere dysfunction is linked to genomic instability and cancer risk.
• Tumors and cancer biology:
  • Tumors are aggregates of cancerous cells with distinct architecture; not all tumors are immediately harmful, but they can disrupt normal tissue function.
  • Benign tumors: localized and non-invasive; can still be problematic if they impair organ function or divert blood supply.
  • Cancer progression often involves hypervascularization to supply growing tumor tissue; the body may vascularize tumors to support growth, which is detrimental to normal tissue function.
  • Tumors can become invasive, spreading into neighboring tissues, supporting cancer progression and metastasis if malignant.

Connections, Implications, and Practical Points

• Foundational concepts linked to tissue types and function:
  • Muscle tissue structure aligns with function: skeletal for voluntary movement, cardiac for heart pumping with synchronized contraction, smooth for autonomic, rhythmic control in organs.
  • Nervous tissue architecture (neurons and glial cells) underpins rapid communication and precise control across the body.
  • Inflammation is a protective mechanism but can cause pain and tissue damage if excessive or misdirected; understanding this balance guides medical treatments (e.g., anti-inflammatory strategies).
• Practical implications:
  • Distinguishing muscle types is essential in anatomy, physiology, and clinical diagnosis (e.g., differentiating heart muscle vs skeletal muscle in disease contexts).
  • Myelin and conduction speed: demyelinating diseases (e.g., multiple sclerosis) illustrate the importance of myelin-producing cells for nervous system function.
  • In the healing process, the roles of platelets, fibroblasts, and granulation tissue underscore why clotting disorders and wound healing can be critical clinical topics.
  • Telomere biology informs aging research and cancer risk; interventions affecting telomere maintenance are a focus of biomedicine.
• Ethical, philosophical, and practical considerations:
  • Cancer biology raises questions about screening, early detection, and the ethics of interventions that alter cell proliferation.
  • Treatments targeting inflammation must weigh benefits (infection control, healing) against risks (chronic inflammation, tissue damage).
  • Aging research and telomere biology carry ethical implications about lifespan, quality of life, and access to therapies.

Summary of Key Concepts and Terms (glossary-style)

• Myoblasts vs Myocytes: precursor cells vs mature muscle cells; fusion forms muscle fibers.
• Intercalated discs: specialized cell junctions in cardiac muscle enabling synchronized contraction.
• Syncytium: a network of connected cells functioning as a single unit.
• Sarcomeres and striations: structural units contributing to skeletal muscle’s striped appearance.
• Dendrites: receive signals; Axon: transmits signals; Synapse: gap where neurotransmitters are released.
• Myelin sheath: insulating layer around axons to speed nerve impulse conduction; CNS produced by oligodendrocytes; PNS by Schwann cells.
• Astrocytes: regulate ions and neurotransmitter concentrations; maintain extracellular environment.
• Microglia: CNS immune cells.
• Granulation tissue: temporary, highly vascular tissue formed during healing.
• Atrophy: loss of tissue or muscle mass due to disuse or aging.
• Telomeres: protective ends of chromosomes; shorten with cell division; related to aging and cancer.
• Tumors: abnormal cell growth; benign vs malignant; hypervascularization enables tumor growth.
• Inflammation: vasodilation, increased permeability, leukocyte recruitment, redness, swelling, pain.
• Platelets: initiate clot formation and wound repair; participate in positive feedback loops for hemostasis.

References to Links with Earlier Material

• Revisited fibroblasts/fibrocytes and myoblasts/myocytes discussions from earlier lectures.
• Review of connective tissue fascia as the scaffolding that organizes muscle fibers.
• Concept of neurons and glial cells introduced here aligns with previous topics on neural signaling and homeostasis.