Tissues

Overview of Tissues

• Tissues: group of cells with similar structure and function plus extracellular substance (matrix).
• Histology: study of tissues.
• Main tissue types in animals: epithelial, connective, muscular, nervous.
• Humans: about $3.7\times 10^{13}$ cells, $\approx 200$ cell types.
• Key example equation: $E=mc^2$

Epithelial Tissues

• Characteristics: cells close together, very little extracellular matrix; free surface; basal surface attaches to basement membrane; avascular; high regeneration.
• Basal/basement membrane: anchors epithelium to underlying tissues; nutrients diffuse across basement membrane from capillaries in connective tissue.
• Classification by layers: Simple (1 layer); Pseudostratified (appears multilayered, actually 1); Stratified (more than 1); Transitional (stretches).
• Classification by shape: Squamous, Cuboidal, Columnar.
• Common types:
  • Simple squamous
  • Simple cuboidal
  • Simple columnar
  • Pseudostratified columnar
  • Stratified squamous (keratinized vs non-keratinized)
  • Stratified cuboidal
  • Stratified columnar
  • Transitional
• Functions: protection, barrier, diffusion/filtration, secretion, absorption.
• Goblet cells: mucus secretion (often in simple columnar and pseudostratified).
• Key features to remember: free surface, basement membrane, little extracellular matrix between cells.

Glands

• Glands secrete substances onto surfaces, into cavities, or into blood.
• Exocrine glands: have ducts (e.g., sweat, oil glands).
• Endocrine glands: no ducts; secrete into bloodstream (e.g., thyroid, pituitary).
• Exocrine gland types (classic):
  • Simple vs Compound (branches of ducts)
  • Tubular vs Alveolar (acinar)
• Unicellular gland example: goblet cells in epithelium.

Connective Tissues

• Characteristics: cells are relatively far apart; abundant extracellular matrix (ECM): ground substance + protein fibers + fluid.
• Functions: enclose and separate, connect, support and movement, storage (calcium, fat), cushion/insulate, transport (blood), protect (immune cells).
• ECM components vary by tissue type.
• Protein fibers: Collagen (rope-like, strong), Reticular (support networks), Elastic (recoil).
• Common connective tissue types:
  • Loose (areolar) connective tissue
  • Dense connective tissue (dense regular and dense irregular)
  • Adipose tissue
  • Cartilage (Hyaline, Fibrocartilage, Elastic)
  • Bone
  • Blood (liquid connective tissue)

Cartilage

• Chondrocytes in lacunae; matrix with collagen; resists compression; provides support and flexibility.
• Types:
  • Hyaline cartilage: covers bone ends; reduces friction; embryonic skeleton; location: joints, ribs, trachea.
  • Fibrocartilage: strong, resists compression; intervertebral disks, pubic symphysis.
  • Elastic cartilage: flexible; external ear, epiglottis.

Bone

• Hard connective tissue: compact and spongy bone.
• Structure: osteocytes in lacunae; lamellae organized in osteons (Haversian systems) in compact bone.
• Functions: strength, support, protection, attachment for muscles; calcium storage.

Blood

• Liquid connective tissue: cells suspended in plasma.
• Components: erythrocytes, leukocytes, platelets.
• Functions: transport nutrients, gases, wastes, hormones.

Muscle Tissues

• Three types:
  • Skeletal muscle: striated, voluntary, multiple peripheral nuclei.
  • Cardiac muscle: striated, involuntary, single central nucleus, intercalated discs.
  • Smooth muscle: non-striated, involuntary, single central nucleus; found in walls of hollow organs.
• Key structures:
  • Sarcomere (unit of striated muscle)
  • Myofibrils, muscle fibers, intercalated discs (cardiac)

Nervous Tissue

• Composed of neurons and glial cells.
• Location: brain, spinal cord, peripheral nerves.
• Functions: control and coordinate body movements and activities.
• Neuron structure: dendrites, cell body, axon; synapses with other cells; myelin (Schwann cells in PNS) speeds signaling.

Inflammation and Tissue Repair

• Inflammation: response to injury; mediators cause vasodilation, increased vascular permeability, redness, heat, swelling, pain.
• Process: recruit white blood cells (e.g., neutrophils) to phagocytize debris/bacteria.
• Repair options:
  • Regeneration: replacement with the same cell type (no scar).
  • Replacement: scar formation with different tissue type.
• Important: chemical mediators signal defenses and vascular changes.

Specialization and Modifications of Epithelial Cells

• Apical modifications: Cilia, Flagella, Microvilli, Pseudopodia.
• Basal modifications: Basal infoldings (active transport sites), Hemidesmosomes (attach epithelial cells to basement membrane).
• Lateral modifications (cell-to-cell): Tight junctions, Adhering (Adherens) junctions, Desmosomes, Gap junctions.
• Junctional complex at the apex of lateral surfaces includes tight junctions and adherens junctions; desmosomes provide strong adhesion; gap junctions allow intercellular communication.
• Desmosomes vs Hemidesmosomes: Desmosomes join cells to other cells; Hemidesmosomes join cells to the basement membrane.

Cilia, Microvilli, and Flagella

• Cilia: Motile cilia beat in coordinated waves (e.g., trachea, Fallopian tubes); Non-motile primary cilia act as sensors.
• Structure: axoneme with 9+2 arrangement for motile cilia; 9+0 for primary cilia; basal body anchors cilium.
• Microvilli: increase surface area (brush border) for absorption/secretion; core of actin filaments; common in intestinal and kidney epithelia.
• Flagella: tail-like structures (e.g., sperm) with a 9+2 axoneme; propulsion.
• Basal infoldings and hemidesmosomes support transport and adhesion, respectively.

Quick reference terms

• Free surface: exposed to body cavity or lumen.
• Basement membrane: structural anchor between epithelium and connective tissue.
• Goblet cell: mucus-secreting unicellular gland.
• 9+2 vs 9+0 axoneme: structure of cilia (motile vs primary).
• 3 principal connective tissue fibers: $\text{Collagen},\; \text{Reticular},\; \text{Elastic}.$