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: $E=mc^2$

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 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.

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)

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.

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.

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)

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: 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.

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: 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.

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: $\text{Collagen},\; \text{Reticular},\; \text{Elastic}.$