Tissue Types and the Extracellular Matrix (ECM)

Comprehensive practice flashcards covering structural levels of organisation, epithelial and muscle histology, nervous system structure, connective tissue classifications, extracellular matrix components, mechanotransduction, and relevant clinical pathologies.

What are the six structural and functional levels of organization in the human body?

1. Chemical level (atoms combine to form molecules)
2. Cell level (molecules form organelles that make up cells)
3. Tissue level (similar cells and surrounding materials make up tissues)
4. Organ level (different tissues combine to form organs)
5. Organ system level (organs make up an organ system)
6. Organism level (organ systems make up an organism)

What two primary components constitute a tissue, and what comprises the extracellular matrix (ECM)?

A tissue is composed of cells and extracellular matrix (ECM). The ECM consists of protein fibres (e.g., collagen) and ground substance.

What are the four primary tissue types found in the human body and their basic roles?

1. Epithelial tissue: covers body surfaces, lines internal cavities and vessels, and forms barriers and glands.
2. Connective tissue: protects and supports other tissues and organs (e.g., fat, blood, cartilage, bone).
3. Muscle tissue: cells that contract to generate force.
4. Nervous tissue: cells that generate electrical signals in response to the environment.

How do the dominant functional tissue (parenchyma) and connective tissue components correspond in the skin, brain, heart, liver, lung, and urinary bladder?

• Skin: Epithelial parenchyma; dermis connective tissue.
• Brain: Nervous parenchyma; meninges, blood vessel connective tissue, and ECM.
• Heart: Muscle parenchyma; fibrous skeleton and supporting stroma.
• Liver: Epithelial glandular parenchyma; connective tissue capsule and septa.
• Lung: Epithelial parenchyma; interstitial connective tissue.
• Urinary bladder: Transitional epithelium parenchyma; lamina propria and connective tissue layers.

How do cardiac, skeletal, and smooth muscle compare regarding location, microscopic structure, control, and power?

• Cardiac: Located in the heart; striated; involuntary control; high power.
• Skeletal: Attached to bones; striated; voluntary control; high power.
• Smooth: Located in the gut and blood vessels; non-striated; involuntary control; low power.

Which specific muscle tissue type is depicted in this histological section?

Skeletal muscle tissue (characterized by striations and peripheral nuclei).

Which specific muscle tissue type is depicted in this histological section?

Smooth muscle tissue (characterized by non-striated, spindle-shaped cells with central nuclei).

Which specific muscle tissue type is depicted in this histological section?

Cardiac muscle tissue (characterized by branching, striated fibres and intercalated discs).

What are the anatomical divisions of the central nervous system (CNS) and peripheral nervous system (PNS)?

• Central Nervous System (CNS): Brain and Spinal Cord.
• Peripheral Nervous System (PNS): Cranial Nerves and Spinal Nerves.

What histological structure of the central nervous system is shown in this cross-section?

The spinal cord (showing central butterfly-shaped grey matter surrounded by white matter).

What are the two major cell classes of the nervous system and their primary functions?

1. Neurones: The functional units of the nervous system; transmit electrical signals, interact via chemical signals, receive sensory inputs, and activate motor contraction.
2. Glial cells: Support cells that protect, provide nutrients, and offer immune functions to neurones.

What are the defining general characteristics of epithelial tissue?

• Highly cellular cohesive sheets bound by cell junctions and adhesions to a basement membrane
• Avascular (lacks blood vessels)
• High capacity for regeneration
• Polarity (distinct apical, lateral, and basal surfaces)
• Covers body surfaces, lines cavities, and forms glands

What are the main functions performed by epithelial tissues?

• Protecting underlying structures from abrasion and mechanical damage
• Permitting the selective passage of substances (absorption/filtration)
• Acting as a barrier to prevent unwanted substances from moving through
• Secreting substances onto surfaces or into the body/blood
• Providing sensation via a large sensory nerve supply

What defines the three distinct polarized surface domains of an epithelial cell?

1. Apical surface: Free surface not attached to other cells, often lining the lumen of ducts or cavities.
2. Lateral surface: Attached to adjacent epithelial cells via junctions.
3. Basal surface: Attached to the basement membrane, which anchors the epithelium to underlying connective tissue.

What are the differences between microvilli, stereocilia, and cilia on epithelial apical surfaces?

• Microvilli: Non-motile cytoplasmic protrusions forming a 'brush border' that increase surface area for absorption.
• Stereocilia: Long, non-motile protrusions with limited distribution (epididymis, vas deferens, and inner ear sensory hair cells).
• Cilia: Motile hair-like protrusions (several hundred per cell) that beat in coordination to propel substances over the epithelium (found in respiratory epithelium and fallopian tubes).

What apical specialisation is shown in this scanning electron micrograph of an inner ear hair cell?

Stereocilia.

What apical surface specialisations are shown in this scanning electron micrograph?

Cilia.

What are the major types and functions of epithelial cell junctions?

• Occluding/Tight junctions: Seal cells together to prevent paracellular leaking.
• Anchoring junctions: Mechanically attach cells to neighbours or ECM (zona adherens and desmosomes on lateral domains; hemidesmosomes on basal domains anchoring to the basement membrane).
• Communicating/Gap junctions: Allow the passage of electrical and chemical signals directly between cells.

On what criteria are epithelia classified and named?

1. Number of cell layers: Simple (single layer), Stratified (two or more layers), or Pseudostratified (single layer of varying cell shapes with nuclei at different heights).
2. Cell shape at the free surface: Squamous (flat), Cuboidal (cube-shaped), Columnar (tall, cylindrical), or Transitional (changes shape with stretching).
3. Surface specialisations: Ciliated, keratinized, microvilli, etc.

What epithelial tissue type is pictured here lining the pulmonary alveoli and capillaries?

Simple squamous epithelium.

What are the structure, primary functions, and typical locations of simple squamous epithelium?

• Structure: Single layer of flat, often hexagonal cells with flattened nuclei.
• Function: Diffusion, filtration, some secretion and absorption; provides little protection against friction.
• Locations: Lining of blood vessels, heart endocardium, and alveoli of the lungs.

What epithelial tissue type is shown lining the tubules in this histological cross-section?

Simple cuboidal epithelium.

What are the structure, primary functions, and locations of simple cuboidal epithelium?

• Structure: Single layer of cube-shaped cells; some have microvilli or cilia.
• Function: Secretion, absorption, diffusion, and movement of mucus-embedded particles out of terminal bronchioles by ciliated cells.
• Locations: Kidney tubules, glands, and the lining of terminal bronchioles of the lungs.

What type of epithelium is shown in this micrograph, consisting of a single layer of tall cells with oval nuclei?

Simple columnar epithelium.

What are the structure, functions, and locations of simple columnar epithelium?

• Structure: Single layer of tall, narrow cells; may possess microvilli or cilia.
• Function: Movement of substances, absorption, and secretion; provides more protection than flatter epithelia.
• Locations: Stomach, intestines, glands, ducts, and bronchioles of the lungs.

What type of stratified epithelium is identified in this section of skin?

Keratinized stratified squamous epithelium.

How do keratinized and nonkeratinized stratified squamous epithelia differ in structure, characteristics, and anatomical distribution?

• Nonkeratinized (moist): Outer layers retain nuclei and are coated with fluid; provides protection against abrasion without drying out. Found in the mouth, throat, larynx, esophagus, anus, and vagina.
• Keratinized: Deep layers are living cells, but outer layers are dead, filled with keratin protein, creating a dry, durable, moisture-resistant barrier. Found in the skin, gums, and hard palate.

Where are stratified cuboidal and stratified columnar epithelia found, and what are their functions?

• Stratified cuboidal: Rare; functions in secretion, absorption, and protection. Located in sweat gland ducts, salivary gland ducts, and ovarian follicular cells.
• Stratified columnar: Multiple layers with tall cells on top of cuboidal cells; functions in protection and secretion (no absorption). Located in mammary gland ducts, larynx, and parts of the male urethra.

What type of respiratory epithelium is illustrated and photographed in this diagram, featuring cilia and goblet cells?

Pseudostratified columnar epithelium.

What are the structural features, functions, and locations of pseudostratified columnar epithelium?

• Structure: Single layer of cells all touching the basement membrane, but because cells vary in height and nuclei sit at different levels, it appears multi-layered; almost always ciliated and contains goblet cells.
• Function: Synthesizes, secretes, and moves mucus and trapped foreign particles out of air passages.
• Locations: Lining of the nasal cavity, pharynx, trachea, and bronchi of the lungs.

What unique epithelium depicted here shifts from $5-6$ layers to $2-3$ layers upon stretching?

Transitional epithelium.

What are the unique properties and anatomical locations of transitional epithelium?

• Properties: Accommodates large volume fluctuations by shifting cells over one another; unstretched tissue appears cuboidal and has $5-6$ layers, whereas stretched tissue appears flattened/squamous and thins to $2-3$ layers.
• Locations: Urinary bladder, ureters, renal pelvis of the kidney, and superior part of the urethra.

In a 55-year-old chronic smoker presenting with a productive cough and frequent chest infections, what epithelial function has been compromised?

Ciliary motility / mucociliary clearance (smoking reduces ciliary beating, impairing the upward clearance of mucus and trapped bacteria).

In a patient presenting with chronic diarrhoea, weight loss, and small intestine mucosal changes (e.g., Coeliac disease), what epithelial function is most affected?

Absorption (blunting or loss of microvilli dramatically reduces surface area for nutrient uptake).

Why is the esophagus lined by stratified squamous epithelium in the context of gastro-oesophageal reflux disease (GORD)?

Stratified squamous epithelium provides multi-layered physical protection against abrasion and chemical erosion from swallowed food and regurgitated gastric contents.

What functional suffixes designate cells that create, maintain, or break down the extracellular matrix in connective tissue?

• '-blasts' create the ECM (e.g., osteoblasts in bone, chondroblasts in cartilage, fibroblasts in CT proper).
• '-cytes' maintain the ECM (e.g., osteocytes, chondrocytes).
• '-clasts' break down the ECM (e.g., osteoclasts, chondroclasts).

Besides resident blast/cyte/clast cells, what wandering or specialized cell types inhabit connective tissue?

Adipose cells (adipocytes), mast cells (mastocytes), macrophages (phagocytes), white blood cells (leukocytes), lymphocytes, and undifferentiated mesenchymal stem cells.

What are the primary constituents of ground substance in the extracellular matrix?

Tissue fluid (water), cell adhesion proteins (connective tissue 'glue' like fibronectin, osteonectin, and chondronectin), and proteoglycans.

What molecular complex of the ECM ground substance is depicted in this structural diagram?

A proteoglycan aggregate (composed of a hyaluronic acid central filament, protein cores, and attached glycosaminoglycans such as chondroitin sulfate and keratan sulfate).

What is the structural composition of a proteoglycan and why is it functionally significant in weight-bearing joints?

A proteoglycan consists of a central protein core to which glycosaminoglycans (GAGs) are covalently attached. This arrangement confers high viscosity and low compressibility, enabling connective tissues to absorb compressive forces.

What are five important glycosaminoglycans (GAGs) found in the ECM ground substance?

1. Hyaluronic acid
2. Chondroitin sulfate
3. Heparan sulfate
4. Dermatan sulfate
5. Keratan sulfate

What biomedical process is illustrated by the white organ structure on the right, and what is its primary advantage?

Organ de-cellularisation (leaving an ECM scaffold). Recellularising this scaffold with a patient's own cells eliminates adverse immune responses compared to traditional donor transplants.

What are the structural features and biochemical assembly steps of collagen fibres?

Fibroblasts assemble three amino acid chains into a triple helix (requiring vitamin C). In the ECM, procollagen is modified into tropocollagen molecules ($\sim 300\,\text{nm}$), which align into fibrils ($\sim 1\,\mu\text{m}$), and then bundle into thick collagen fibres ($\sim 10\,\mu\text{m}$). Collagen represents approximately $6\%$ of total body weight.

What are the distribution differences among Type I, Type II, and Type III collagen?

• Type I: The most abundant type; forms thick, strong fibres in tendons, ligaments, and bone.
• Type II: Found primarily in cartilage.
• Type III: Forms delicate, highly branched reticular fibres.

What are the characteristics and locations of elastic fibres and reticular fibres?

• Elastic fibres: Long, thin, branching fibres containing elastin secreted by fibroblasts; provide elasticity and recoil in lungs and blood vessel walls.
• Reticular fibres: Short, fine ($0.5 - 2\,\text{mm}$ reported slide diameter) Type III collagen fibres forming mesh-like networks that allow 'give'; abundant in liver, spleen, lymph nodes, and basement membranes.

What are the two embryonic connective tissues, and how do they differ?

Both arise from embryonic mesoderm:

• Mesenchyme: Irregularly shaped cells in semifluid ground substance containing reticular fibres; found under embryonic skin and along developing bones.
• Mucous connective tissue (Wharton's Jelly): Scarce fibroblasts suspended in viscous jelly-like ground substance with collagen fibres; found exclusively in the umbilical cord.

What type of embryonic connective tissue found in the umbilical cord is shown in this histological section?

Mucous connective tissue (Wharton's Jelly).

At what gestational age has most mesenchymal tissue differentiated into specialized mature connective tissue?

By $8\,\text{weeks}$ of development.

What are the three main classes of mature connective tissue and their subtypes?

1. Connective tissue proper:
   • Loose (Areolar, Reticular, Adipose)
   • Dense (Dense regular collagenous, dense regular elastic, dense irregular collagenous, dense irregular elastic)
2. Supporting connective tissue:
   • Cartilage (Hyaline, Fibrocartilage, Elastic)
   • Bone (Spongy, Compact)
3. Fluid connective tissue:
   • Blood (Red cells, white cells, platelets)
   • Haemopoietic (Red marrow, yellow marrow)

What subtype of dense connective tissue found in tendons and ligaments is shown in this micrograph?

Dense regular collagenous connective tissue.

What subtype of dense connective tissue found in the aorta wall is shown in this micrograph?

Dense irregular elastic connective tissue.

What is mechanotransduction in the context of the extracellular matrix?

The cellular process whereby mechanical signals from the ECM (such as stiffness or tension) are transmitted across transmembrane integrins in focal adhesions to the cytoskeleton and nucleus, triggering cellular responses such as differentiation, proliferation, or migration.

What three mechanosensitive mechanisms do cells utilize to regulate ECM properties?

1. Stiffening their environment by generating forces through transmembrane receptors.
2. Remodelling their environment via deposition and crosslinking of ECM components.
3. Degrading the ECM in response to altered tension using secreted enzymes (e.g., matrix metalloproteinases, MMPs) via a 'use-it-or-lose-it' mechanism.

How does collagen fibril diameter differ between tendons and the cornea, and why?

• Tendon: Fibril diameter is approximately $200\,\text{nm}$ to maximize tensile strength under unidirectional strain.
• Cornea: Fibril diameter is tightly regulated to approximately $30\,\text{nm}$ to ensure optical transparency.

What is the biochemical cause and clinical presentation of scurvy?

Scurvy is caused by vitamin C (ascorbic acid) deficiency, which prevents proper proline and lysine hydroxylation required to stabilize the collagen triple helix. Symptoms include bleeding gums, loosened teeth, capillary fragility, and petechial hemorrhages in skin, mucous membranes, and eyes.

What ocular clinical manifestation of Marfan's syndrome is shown in this image?

Lens subluxation (ectopia lentis).

What is the genetic and structural etiology of Marfan's syndrome, and what are its cardinal clinical signs?

Marfan's syndrome results from abnormal production of Fibrillin-1, leading to weak elastic fibres and tissue overgrowth. Cardinal signs include tall stature, long limbs, scoliosis, arm span exceeding height, high-arched palate with crowded incisors, lens subluxation, mitral valve prolapse, and aortic dilation/aneurysms.

What gross pathological condition characterized by architectural obliteration and honeycomb scarring is shown in this section of a lung?

End-stage pulmonary fibrosis.

What is tissue fibrosis, and how does it compromise organ function?

Fibrosis is the excessive deposition of fibrous connective tissue in an organ. Confluent scarring destroys normal parenchymal architecture and stiffens tissues, severely restricting fluid flow and gas exchange (as seen in idiopathic pulmonary fibrosis).