Tissues, Organ Systems & Homeostasis

Primary Germ Layers (Embryonic Origins)

Vertebrate embryos differentiate into three primary germ layers, each giving rise to specific adult tissues.
- Ectoderm (outer layer)
- Develops into the nervous system (brain, spinal cord, peripheral nerves).
- Forms the epidermis of skin and its derivatives (hair, nails, sweat- and sebaceous-glands).
- Mesoderm (middle layer)
- Generates muscular tissue (skeletal, cardiac, smooth) and most connective tissues (bone, cartilage, blood, adipose).
- Forms kidneys, gonads, serous membranes, and much of the cardiovascular system.
- Endoderm (inner layer)
- Lines the digestive and respiratory tracts.
- Gives rise to accessory organs (liver, pancreas, portions of lungs, thyroid, parathyroid, thymus).

Four Fundamental Tissue Types (Adult)

Epithelial tissue – covers and lines; forms glands.
Connective tissue – binds, supports, transports; most abundant.
Muscle tissue – contracts to produce movement.
Nervous tissue – detects stimuli and conducts impulses.

Epithelial Tissue

General Characteristics

Continuous sheets of closely packed cells, minimal extracellular matrix (ECM).
One free (apical) surface exposed to body fluid/exterior; opposite (basal) surface anchored to a basement membrane (non-cellular, adhesive layer of proteins + polysaccharides that attaches epithelium to underlying CT).
Avascular yet capable of rapid regeneration via mitosis.

Classification Scheme

By cell shape
- Squamous – flat, scale-like.
- Cuboidal – as tall as wide.
- Columnar – taller than wide.
By layers
- Simple – single cell layer → efficient exchange (diffusion, absorption, filtration).
- Stratified – ≥2 layers → protection from abrasion, chemical stress.
- Pseudostratified – single layer that appears multi-layered because nuclei are at different heights; often ciliated.
- Transitional – stratified epithelium capable of stretching (ureter, urinary bladder).

Representative Examples & Functions

Simple squamous
- Location: alveoli of lungs, lining of blood/lymph vessels (endothelium).
- Function: diffusion, osmosis, secretion of serous fluid.
Stratified squamous (keratinized & non-keratinized)
- Keratinized: epidermis – waterproof & abrasion-resistant.
- Non-keratinized: mouth, esophagus, vagina – moist protection.
Simple columnar
- Non-ciliated: lining of small intestine (microvilli form brush border for absorption).
- Ciliated: upper respiratory tract, uterine (fallopian) tubes (cilia propel mucus/oocyte).
Pseudostratified columnar ciliated
- Trachea & bronchi; mucociliary escalator.
Transitional
- Bladder; dome-shaped superficial cells flatten when organ distends.

Glandular Epithelium

Endocrine glands – ductless, secrete hormones → interstitial fluid → blood.
- Thyroid: releases $T<em>3$ and $T</em>4$ ↑ basal metabolic rate (BMR).
- Pituitary: diverse hormones (TSH, FSH, ADH, etc.).
Exocrine glands – secrete into ducts that open onto surfaces.
- Sweat glands: watery solution of $\text{Na}^+$ , $\text{Cl}^-$ for thermoregulation.
- Salivary glands: mucus + amylase initiate starch digestion.

Connective Tissue (CT)

Defining Features

Most abundant, widely distributed tissue type.
Vascularity varies: bone highly vascular, cartilage avascular, dense CT limited.
CT = Cells + Matrix
- Cells
- Fibroblasts (produce fibers), macrophages (phagocytes), adipocytes, chondrocytes, osteocytes, mast cells, plasma cells.
- Matrix
- Ground substance: water, GAGs, proteoglycans.
- Fibers: collagen (strength), elastic (stretch), reticular (supportive network).

Major CT Sub-types & Examples

Loose (areolar) CT – loosely arranged fibers, many cell types; packaging material around organs; vascular.
Dense regular CT – parallel collagen bundles; tendons/ligaments; poor blood supply → slow healing.
Cartilage (hyaline, elastic, fibrocartilage) – resilient, avascular; precursor of bone, articulating surfaces, tracheal rings.
Bone
- Compact: concentric lamellae (osteons) µ $\text{Ca}^{2+}$ , $\text{PO}_4^{3-}$ salts, osteocytes in lacunae.
- Spongy: trabeculae + red marrow (hematopoiesis).
Blood – liquid matrix (plasma) + formed elements (RBCs, WBCs, platelets); transport + immunity.
Adipose – adipocytes store triglycerides; insulation, energy reserve, cushions organs (e.g., around heart, subcutaneous layer).

Concept Check

Question: Which of the following is NOT a connective tissue? • Options: A) Blood B) Squamous epithelium C) Bone D) Hyaline cartilage.
Answer: B) Squamous epithelium (it is epithelial).

Muscle Tissue

General Attributes

Composed of elongated muscle fibers containing contractile proteins actin & myosin.
Contraction explained by Sliding Filament Model: myosin heads form cross-bridges → pull actin toward sarcomere center → $\text{sarcomere length} \downarrow$ → muscle shortens; ATP & $\text{Ca}^{2+}$ required.

Three Muscle Types

Skeletal
- Striated, voluntary, multinucleated; attached to bones via tendons → body movements; neurogenic (requires motor neuron’s acetylcholine).
Cardiac
- Striated, involuntary, single nucleus, branched cells interconnected by intercalated discs (gap junctions + desmosomes); myocardium pumps blood.
Smooth
- Non-striated, involuntary; walls of hollow organs (GI tract, blood vessels, uterus); sustained contractions, peristalsis.

Nervous Tissue

Neurons

Functional unit; specialized for electrical excitability.
- Cell body (soma) – nucleus & organelles.
- Dendrites – multiple, branched, receive input.
- Axon – single, conducts action potentials to effectors/other neurons.

Neuroglia (Support Cells)

Central Nervous System (CNS)
- Astrocytes – maintain blood–brain barrier, regulate neurotransmitters, form scar tissue.
- Oligodendrocytes – myelinate CNS axons.
- Microglia – phagocytic, remove debris/pathogens.
- Ependymal cells – line ventricles/central canal; produce & circulate cerebrospinal fluid.
Peripheral Nervous System (PNS)
- Schwann cells – myelinate PNS axons; aid regeneration.
- Satellite cells – regulate microenvironment of ganglionic neurons.

Tissue Membranes

Epithelial membranes
1. Mucous (mucosa) – line body cavities that open to exterior; moist (e.g., GI tract).
2. Serous – line closed ventral body cavities; visceral & parietal layers secrete serous fluid (pleura, pericardium, peritoneum).
3. Cutaneous – skin (epidermis + dermis).
Connective tissue membrane
1. Synovial – line joint cavities; produce lubricating synovial fluid.

Organs and Organ Systems

Organ = ≥2 tissue types integrated to perform specific function.
- Example: Heart – cardiac muscle (pump), epithelial (endocardium), CT (valves, blood vessels), nervous tissue (pacemaker) → pumps blood.
Organ system = group of organs with coordinated, broad function.
- Example: Cardiovascular system – heart + blood vessels → transport nutrients, wastes, hormones, heat.

Homeostasis

Negative Feedback Loop (General Model)

Stimulus → Sensor (receptor) detects change → Integrator (control center) compares to set-point → Effector carries out corrective response → Response reverses stimulus → loop stops.
Mathematical abstraction: $\text{Error} = \text{Set point} - \text{Current value}$ , controller acts to minimize $\text{Error}$ (similar to engineering PID control).

Thermoregulation Example

Control center: hypothalamus.
When core temperature ↓
- Cutaneous blood vessels constrict.
- Skeletal muscles shiver (increase heat via ATP hydrolysis).
- Thyroid releases more $T_3$ → ↑ metabolic heat.
When core temperature ↑
- Blood vessels dilate; warm blood radiates heat.
- Sweat glands activate; evaporation absorbs heat $(\Delta H_{vap} \approx 2.43 \text{ kJ g}^{-1})$ .
Feedback inhibition: normalized temperature suppresses hypothalamic output.

Glucose Homeostasis

Rise after eating → pancreatic beta cells secrete insulin → body cells & adipose uptake glucose; liver converts glucose → glycogen.
Fall during fasting/exercise → pancreatic alpha cells release glucagon → liver glycogenolysis releases glucose.
Disorders: diabetes mellitus (insulin deficiency/resistance) = homeostatic failure.

Integumentary System (Skin)

Functions

Protection: physical barrier vs. trauma, pathogens, UV.
Water balance: prevents dehydration.
Thermoregulation: sweat glands, vasomotor changes.
Sensation: tactile, thermal, pain receptors.
Metabolic: synthesizes vitamin D precursor when epidermal 7-dehydrocholesterol absorbs UVB.

Layers & Cells

Epidermis (keratinized stratified squamous)
- Keratinocytes: produce keratin → waterproof, toughness.
- Melanocytes: synthesize melanin → UV protection; variations create skin color.
- Continuous mitosis in basal layer; superficial cells slough off.
Dermis (1–2 mm dense irregular CT)
- Fibroblasts: secrete collagen & elastin (↓ with age → wrinkles).
- Dermal papillae: fingerlike projections interlock with epidermis; house capillaries & Meissner’s corpuscles; create fingerprints.
- Nerves, blood vessels, hair follicles, sebaceous & sweat glands.
Hypodermis (subcutaneous)
- Adipose + areolar CT; thermal insulation, shock absorption, major energy reserve.

Accessory Structures & Receptors (see schematic)

Hair shaft/root, sebaceous glands (sebum), arrector pili muscles, eccrine/apocrine sweat glands, free nerve endings, Pacinian & Meissner corpuscles.

Common Skin Issues

Skin cancers (most common cancers)
- Basal cell carcinoma – arises from stratum basale; rarely metastasizes.
- Squamous cell carcinoma – keratinocytes of stratum spinosum; moderate metastasis risk.
- Melanoma – melanocytes; highly metastatic; ABCDE rule (Asymmetry, Border, Color, Diameter > $6$ mm, Evolving).
Acne – bacterial infection (often Propionibacterium acnes) in obstructed sebaceous glands; inflammation, pustules.
MRSA infections – Staphylococcus aureus resistant to methicillin; spreads via skin breaks; community acquired (CA-MRSA) increasing.
Psoriasis – chronic autoimmune; T-cells trigger rapid keratinocyte proliferation; plaques of red skin with silvery scales.
Sunburn (acute solar dermatitis)
- UV damages epidermal DNA → inflammatory cascade.
- Vasodilation → redness & warmth; damaged cells release cytokines perceived as burning pain.
- WBCs remove damaged cells → itching, peeling; cumulative UV = photoaging & cancer risk.

Sample Review Activity (Matching)

Alveoli of lung → A. Epithelial (simple squamous)
Epidermis of skin → A. Epithelial (keratinized stratified squamous)
Myocardium of heart → B. Muscular (cardiac)
Pain sensor in dermis → D. Nervous (free nerve ending)
Diaphysis of long bone → C. Connective (compact bone)
Tendon → C. Connective (dense regular CT)
Muscular layer of digestive tract → B. Muscular (smooth)

Nervous System Pathways (Integration with Homeostasis)

Sensory (afferent) division brings signals from skin, muscle spindles, organs to CNS.
Motor (efferent) division
- Somatic: voluntary control of skeletal muscle.
- Autonomic: involuntary regulation of smooth muscle, cardiac muscle, glands; subdivided into sympathetic ("fight-or-flight") & parasympathetic ("rest-and-digest").

Conceptual Connectivity & Real-World Relevance

Tissue structure ↔ function (e.g., thin simple squamous ideal for gas exchange; dense collagen rich tendon resists tension).
Disorders highlight significance: diabetes (connective adipose & endocrine), cardiac ischemia (muscle, connective, nervous interactions), burns (integumentary & fluid homeostasis).
Ethical/medical: MRSA antibiotic stewardship, UV exposure guidelines, tanning culture vs. melanoma risk.

Key Numerical / Chemical References

$T<em>3$ , $T</em>4$ – thyroid hormones regulating \text{BMR (kcal·day}^{-1}).
Heat of vaporization of water $\Delta H_{vap} \approx 2.43 \text{ kJ g}^{-1}$ crucial for sweat cooling.
Normal fasting glucose \approx 70–110\,\text{mg·dL}^{-1}; insulin/glucagon maintain range.