Structures & Functions of Animal Cells and Tissues

Inspiration from Nature & the Rule of “Structure Dictates Function”

• Living structures are routinely copied in engineering and design (e.g., aircraft wings mimic bird wings; slide references to “British Airways / The shape” and “Convening Lap Assin Gaang” illustrate this idea).
• Guiding principle: form is always tied to job; understanding morphology lets us predict performance.
• Core question posed (Slide 5): “Are the structure and functions of the components that make up an organism’s body related to each other?” → every subsequent topic answers yes.

Why Cells Matter – The Basic Unit of Life

• All organisms are composed of at least one cell; hence the cell is the basic, smallest, functional unit of life.
• Cells in multicellular animals are
  • Diverse in shape and size.
  • Highly specialized for distinct tasks (e.g., gas exchange vs. contraction).

Hierarchy of Biological Organization (from smallest to largest)

• $\textbf{Chemical Level}$
  • Atoms & biomolecules (carbohydrates, proteins, lipids, nucleic acids).
• $\textbf{Organelle Level}$
  • Membrane-bound sub-cellular units (mitochondria, nucleus, Golgi apparatus, ER).
• $\textbf{Cell Level}$ – skin cells, neurons, muscle fibers, blood cells.
• $\textbf{Tissue Level}$ – groups of similar cells performing a common function. Four animal types: epithelial, connective, muscular, nervous.
• $\textbf{Organ Level}$ – two or more tissues integrated (heart, lungs, brain, skin).
• $\textbf{Organ-System Level}$ – coordinated organs (circulatory, respiratory, integumentary, etc.).
• $\textbf{Organism Level}$ – a living individual maintaining homeostasis.
• $\textbf{Population}$ – same-species organisms in one area.
• $\textbf{Community}$ – different populations co-inhabiting an area.
• $\textbf{Ecosystem}$ – communities interacting with abiotic factors (soil, water, sunlight).
• $\textbf{Biosphere}$ – the entire layer of Earth where life exists.

Animal Tissues – Overview

• $4$ fundamental tissue categories: epithelial, connective, muscular, nervous.
• Each exhibits a unique structural hallmark that matches its function.

Epithelial Tissue

• General traits
  • Continuous sheets of tightly packed cells; little extracellular matrix.
  • Lines cavities & surfaces; forms glands.
  • Polarity: apical (free) surface vs. basal surface attached to basement membrane.
  • Avascular yet innervated; rapid regeneration.
• Classification keys
  1. Number of layers
  • $\textit{Simple}$ – single layer.
  • $\textit{Stratified}$ – multiple layers.
  • $\textit{Pseudostratified}$ – single layer with nuclei at varying heights giving false stratification.
  1. Cell shape
  • $\textit{Squamous}$ – flat.
  • $\textit{Cuboidal}$ – cube-like.
  • $\textit{Columnar}$ – tall/rectangular.

Major Sub-types, Locations & Functions

• $\textbf{Simple Squamous}$
  • Air sacs (alveoli), capillary walls.
  • Diffusion, filtration, secretion.
• $\textbf{Simple Cuboidal}$
  • Kidney tubules, glands, ovary surface.
  • Absorption, secretion.
• $\textbf{Simple Columnar}$ (often with goblet cells)
  • GI tract lining, body cavities.
  • Absorption, mucus secretion.
• $\textbf{Pseudostratified Columnar}$ (ciliated)
  • Respiratory tract lining.
  • Secretion, propulsion of mucus.
• $\textbf{Stratified Squamous}$
  • Epidermis, mouth, esophagus, vagina.
  • Protection against abrasion.
• $\textbf{Stratified Cuboidal}$
  • Sweat, salivary, mammary gland ducts.
  • Protection, secretion.
• $\textbf{Stratified Columnar}$
  • Male urethra,
some glandular ducts.
  • Protection, secretion.

Connective Tissue

• Unifying features: cells + abundant extracellular matrix (ECM = ground substance + fibers).
• Functions: binding/support, protection, insulation, transport, energy storage.

ECM Components

• Ground substance – interstitial fluid, proteoglycans, cell-adhesion proteins.
• Fibers
  • Collagen (strength).
  • Elastic (stretch & recoil).
  • Reticular (fine branching networks).

Major Classes

1. $\textbf{Bone (Osseous) Tissue}$
   • Osteocytes in lacunae; rigid matrix of $Ca^{2+}$ salts + collagen.
   • Protects (skull, ribs), supports, mineral reservoir.
2. $\textbf{Cartilage}$ – chondrocytes in lacunae; avascular & flexible.
   • $\textit{Hyaline}$: ends of long bones, nose, fetal skeleton.
   • $\textit{Fibrocartilage}$: intervertebral discs, pubic symphysis, menisci; highly compressible.
   • $\textit{Elastic}$: epiglottis, external ear; great elasticity.
3. $\textbf{Dense Connective (Fibrous)}$
   • Predominantly collagen; few cells (fibroblasts).
   • Tendons (muscle→bone) & ligaments (bone→bone).
4. $\textbf{Loose Connective}$
   • Softer ECM, more cells.
   • $\textit{Areolar}$: universal packing material; anchors skin & organs.
   • $\textit{Adipose}$: adipocytes store triglycerides; insulation, protection, energy reserve.
   • $\textit{Reticular}$: fine reticular fiber network; scaffolding for lymphoid organs (spleen, lymph nodes, marrow).
5. $\textbf{Blood}$ – fluid connective tissue.
   • Plasma = liquid matrix; soluble fiber proteins visible only during clotting.
   • Cellular elements:
     • $\textit{Erythrocytes}$ – biconcave, anucleate; $Hb$ transports $O<em>2$ & $CO</em>2$.
     • $\textit{Leukocytes}$ – immune defense.
       • $\textit{Granulocytes}$ (neutrophils, eosinophils, basophils).
       • $\textit{Agranulocytes}$ (lymphocytes, monocytes).
     • $\textit{Thrombocytes (Platelets)}$ – clot formation.

Muscular Tissue

• Composed of elongated muscle fibers (cells) specialized for contraction.
• Cellular anatomy
  • Sarcolemma – plasma membrane.
  • Sarcoplasm – cytoplasm rich in glycogen & $Ca^{2+}$-laden sarcoplasmic reticulum.
  • Myofibrils – contractile threads showing alternating light (isotropic) & dark (anisotropic) bands ⇒ striations.
• Basic physiological properties: contractility, excitability, extensibility, elasticity.

Three Muscle Types

Nervous Tissue

• Specialized for irritability & conductivity – rapid transmission of electrochemical impulses.

Principal Cell Types

1. Neurons – structural/functional unit.
   • Dendrites (input), soma, axon (output).
   • Classified by number of processes (unipolar, bipolar, multipolar).
2. Neuroglia (Glial Cells) – support, protect, insulate neurons.
   • $\textbf{Astrocytes}$ (CNS) – star-shaped; regulate extracellular milieu; most abundant.
   • $\textbf{Microglial Cells}$ (CNS) – transform into macrophages; phagocytize debris.
   • $\textbf{Ependymal Cells}$ (CNS) – ciliated lining of brain ventricles & spinal canal; circulate cerebrospinal fluid.
   • $\textbf{Oligodendrocytes}$ (CNS) – form myelin sheath around CNS axons.
   • $\textbf{Satellite Cells}$ (PNS) – surround neuron cell bodies in ganglia.
   • $\textbf{Schwann Cells}$ (PNS) – myelinate PNS axons; aid regeneration.

Integrative Perspective

• Epithelial: tight cell sheets ⇒ ideal for lining & protection.
• Connective: abundant ECM ⇒ binding, support, metabolic exchange.
• Muscle: contractile proteins + excitability ⇒ movement.
• Nervous: excitable membranes + synapses ⇒ communication & coordination.
• Theme: architecture underlies capability; disturbances in structure (injury, mutation) impair function (e.g., collagen defects → weak connective tissues).

Practice & Application (Slide 70–79 Activities)

• Sample identification quiz
  1. “Group of organs that coordinate to perform a specific function” → $\textbf{Organ System}$.
  2. “Butterflies of the same species living in the same area” → $\textbf{Population}$.
  3. “Atoms and molecules interacting” → $\textbf{Chemical Level}$.
• Tissue-recognition quiz
  1. Extracellular matrix with ground substance & fibers → $\textbf{Connective Tissue}$.
  2. Classified by cell shape & arrangement → $\textbf{Epithelial Tissue}$.
  3. Cells conducting electrochemical signals → $\textbf{Nervous Tissue}$.
• “Draw Me!” laboratory assignment: students illustrate each epithelial, connective, muscle, and nervous tissue type across three consecutive days; graded on creativity ($20$ pts), color quality ($15$ pts), and neatness ($15$ pts) for a total of $50$ pts.

Key Take-Home Messages

• $11$ nested levels organize biological complexity from molecules to biosphere.
• Animal bodies rely on $4$ primary tissue types, each with distinctive morphology matching its job.
• Mastery of tissue histology enables understanding of organ function, pathology, and bio-inspired design.