Basic Tissues, Cells, and Glands – Lecture Review

The Cell

• Fundamental unit of life; conceptually described as a “bag of fluid” measuring approximately 0.01\text{–}0.05\,\text{mm} in diameter.
• All higher-order tissues, organs, and systems are built from the coordinated activities of trillions of such units.

Cell Membrane

• Semipermeable phospholipid bilayer that encases the cell.
  • Maintains homeostasis by keeping intracellular fluid in and most foreign materials out.
  • Embedded proteins regulate ion passage, receptor signaling, and cell–cell recognition.
  • Clinical significance: damage compromises osmotic balance, leading to cell lysis or apoptosis.

Cytoplasm

• Viscous, protein-rich fluid filling the interior of the membrane.
  • Suspends organelles and provides the medium for metabolic reactions.
  • Site of anaerobic glycolysis and initial protein modification steps.

Nucleus

• Command center controlling heredity and protein synthesis.
  • Generally one per cell—exceptions include skeletal muscle (multinucleated) and RBCs (anucleate).
  • Surrounded by a double nuclear envelope containing nuclear pores for RNA export.

DNA

• Deoxyribonucleic acid arranged in chromosomes during mitosis/meiosis; diffuses through the nucleus as chromatin in interphase.
• Encodes all genetic instructions: \approx 3\times10^{9} base pairs in humans.

RNA

• Ribonucleic acid transcribed from DNA; serves as the working copy.
  • Messenger RNA (mRNA): carries genetic code to ribosomes.
  • Ribosomal RNA (rRNA): part of ribosome structure.
  • Transfer RNA (tRNA): delivers amino acids during translation.
• Usually aggregated in one nucleolus, although multiple nucleoli can exist when protein demand is high.

Organelles

Mitochondria

• “Powerhouses” that generate ATP via oxidative phosphorylation.
  • Inner membrane folds (cristae) expand surface area.
  • Enzyme complexes on cristae regulate metabolic rate; link to basal metabolic rate and thermogenesis.
• Clinical tie-in: mitochondrial myopathies impair energy supply, affecting muscle and nerve tissue first.

Endoplasmic Reticulum (ER)

• Interconnected tubules continuous with the nuclear envelope.
  • Rough ER: studded with ribosomes; synthesizes secretory & membrane proteins under direct mRNA control.
  • Smooth ER: lacks ribosomes; synthesizes lipids, detoxifies drugs, regulates Ca^{2+} (e.g., in muscle sarcoplasmic reticulum).

Golgi Apparatus

• Stack of flattened sacs that modify, sort, and package proteins from ER.
  • Forms secretory vesicles so cargo does not directly mingle with cytoplasm.
  • Malfunction leads to improper glycosylation disorders.

Lysosomes

• Membrane-bound vesicles filled with acid hydrolase enzymes (“lyso” = break apart).
  • Digest bacteria, worn-out organelles, and cellular debris—“scavengers”/“suicide bags.”
  • If membrane ruptures, enzymes autodigest the cell (autolysis).

Cytoskeleton Components

• Microtubules & microfilaments give structural integrity and intracellular transport tracks.
• Centrioles (bundles of microtubules) align chromosomes during mitosis; absent in most plant cells.

Epithelial Tissue

Definition & Origin

• Continuous sheets covering external body surfaces & lining internal cavities/tubes.
• Derived from embryonic ectoderm or endoderm; some glands retain epithelial nature even when buried in connective tissue (e.g., pancreas, liver).

Classification by Cell Layers

• Simple (one layer) vs. Stratified (two or more layers). Basement membrane anchors epithelium to underlying connective tissue.

Simple Squamous Epithelium

• Thin, plate-like cells facilitating diffusion.
  • Endothelium: lines blood & lymph vessels, heart.
  • Mesothelium: lines serous cavities (pleura, pericardium, peritoneum).
• Example: oxygen/CO_{2} exchange across alveolar walls.

Simple Cuboidal Epithelium

• Cube-shaped; unilateral layer thicker than squamous.
  • Found in kidney tubules, small glands, respiratory passages.
  • Ciliated variants move mucus/particles toward pharynx.

Simple Columnar Epithelium

• Taller cells specialized for absorption.
  • Lines stomach & intestines; microvilli (“brush border”) increase surface area.

Pseudostratified Columnar Epithelium

• Appears multilayered because nuclei stagger, yet all cells touch basement membrane.
  • Ciliated versions line trachea & upper respiratory tract; ciliary motion sweeps contaminants.

Stratified Squamous Epithelium

• Most ubiquitous protective covering (skin, oral mucosa, pharynx, esophagus, anal region).
  • Basal cells divide; superficial layers flatten, die, and slough.
  • Keratinized (skin) vs. nonkeratinized (oral cavity) variations.

Pigmentation

• Melanocytes beneath basal layer synthesize melanin, shielding nuclei from UV damage.
  • Quantity & activity, not melanocyte number, dictate skin shade—except in albinism (absence of melanin production).

Glands

Developmental Mechanics

• Down-growth of epithelial basal cells into underlying connective tissue forms a solid cord, later hollowing into a duct.
• Terminal swellings differentiate into secretory end pieces:
  • Acini (berry-like) or tubules (elongated).

Distribution Mechanism

• Exocrine: maintain ducts that deliver product to a surface (e.g., sweat, salivary glands).
• Endocrine: lose ducts; secrete hormones directly into bloodstream (e.g., thyroid, pancreas islets).

Arrangement of Components

• Secretory vs. excretory portions.
  • Simple tubular: straight, unbranched duct (intestinal glands).
  • Compound tubuloalveolar: branched ducts with mixed acinar/tubular end pieces (major salivary glands), resembling a grape cluster.

Types of Secretions (Salivary Example)

• Serous: thin, watery, enzyme-rich (ptyalin/\alpha-amylase).
• Mucous: viscous, glycoprotein-rich, lubricant.
• Seromucous: mixed; proportion varies within a gland (submandibular ≈ \frac{2}{3} serous, \frac{1}{3} mucous).

Embryonic Germ Layers

• Ectoderm (outer): epidermis, central nervous system.
• Endoderm (inner): GI & respiratory epithelia, associated glands.
• Mesoderm (middle, forms later): connective tissues, bone, cartilage, vascular endothelium.

Connective Tissue

Major Categories

1. Connective Tissue Proper
2. Specialized Connective Tissue
   • Cartilage
   • Bone
   • Blood

Connective Tissue Proper — Loose (Areolar)

• Found in superficial/deep fascia; packing & cushioning matrix around organs.
• Components: collagen & elastic fibers, fibroblasts (fiber producers), macrophages (immune surveillance), mast cells (histamine release).

Loose CT with Special Properties

• Mucous CT: embryonic (Wharton’s jelly of umbilical cord) & vocal cords; high fibroblast & collagen content, gelatinous.
• Elastic Tissue: yellow elastic fibers + fibroblasts; in vocal cords & spinal ligamenta flava, allowing recoil.
• Reticular Tissue: network scaffolding of spleen, lymph nodes, liver.

Cartilage (Non-calcified Support Tissue)

• Cells: chondroblasts (produce matrix) → chondrocytes (mature, lacuna-dwelling).
• Matrix: collagen/elastic fibers in firm ground substance.
  1. Hyaline — glassy; fetal skeleton, growth plates, nose, larynx, trachea, bronchi.
  2. Elastic — flexible; external ear, epiglottis, auditory (Eustachian) tube.
  3. Fibrous — dense collagen bundles cushioning joints; intervertebral discs, pubic symphysis, temporomandibular joint (TMJ).

Bone

• ~50\% hydroxyapatite (Ca^{2+} & phosphate) crystals; remainder collagen protein matrix.
• Haversian (osteon) system: concentric lamellae surround a central vascular canal, allowing nutrient diffusion in dense tissue.
• Provides rigid support, locomotion leverage, and mineral reservoir.

Blood

• Specialized liquid connective tissue.
  • Fluid phase: plasma (water, proteins, electrolytes, nutrients, wastes).
  • Cellular phase:
    • Erythrocytes (RBCs): anucleate discs packed with hemoglobin; transport O{2} & CO{2}.
    – Anemia = reduced RBC count/hemoglobin, leading to hypoxia symptoms.
    • Leukocytes (WBCs): immune defense—neutrophils, lymphocytes, monocytes, eosinophils, basophils.
    • Platelets (thrombocytes): cytoplasmic fragments that initiate clotting cascade.

Muscle Tissue

• Excitable tissues converting chemical energy (ATP) to mechanical work.
  1. Skeletal — striated, voluntary, multinucleated; moves skeleton.
  2. Cardiac — striated, involuntary, intercalated discs; pumps blood.
  3. Smooth — non-striated, involuntary; walls of viscera & blood vessels.

Nervous Tissue & System

Overview

• Body’s rapid communication network; integrates sensory input and motor output.
  • Sensory (afferent) pathways: receptors → CNS.
  • Motor (efferent) pathways: CNS → effectors (muscles/glands).

Neurons

• Functional cells composed of:
  • Cell body (soma): houses nucleus & metabolic machinery.
  • Dendrites: branched receivers of signals.
  • Axon: single transmitter (may be >1\,\text{m} long) ending in synaptic terminals.
• Impulse conduction based on \text{Na}^+ influx & \text{K}^+ efflux across axolemma, propagating an electrical action potential.
• Synaptic transmission converts electrical signal to chemical (neurotransmitter) then back to electrical in the next neuron.

Supporting (Neuroglial) Cells (mentioned for completeness)

• Astrocytes, oligodendrocytes/Schwann cells (myelination), microglia, ependymal cells—all derived from ectoderm; maintain optimal neuron environment.

Practical & Clinical Connections

• Tissue choice for grafting depends on vascularity: epithelium heals quickly but poorly vascular; connective tissues support vascular ingrowth.
• Gland malfunction (e.g., salivary hypofunction) leads to xerostomia, impacting digestion & oral health.
• Disorders: cystic fibrosis (defective epithelial Cl^- channel), osteogenesis imperfecta (collagen mutation in bone), leukemia (malignant leukocytes), myasthenia gravis (neuromuscular junction antibodies).