Tissues: The Tissue Level of Organisation

Learning Outcomes

Identify the four primary tissue types and subdivisions.
Describe the structure of primary tissue types & divisions.
Understand the function of primary tissue types.

Tissues

A group of cells similar in function and structure.
Four primary tissue types:
- Epithelial: covers/lines body surfaces, hollow organs, cavities etc.
- Connective: protects and supports the body and its organs.
- Muscle: cells specialised for contraction & force generation.
- Nervous: detects changes in a variety of conditions inside & outside the body and responds via generating electrical signals.

Tissue Histology

Histology is the microscopic study of tissues and organs through sectioning, staining, and examining those sections under a microscope.
Allows visualisation of tissue structure and characteristic changes.
Most commonly used stain is Haematoxylin and Eosin (H&E) staining.
- Haematoxylin stains acidic structures (nucleic acids).
- Eosin stains basic structures (proteins/cytoplasm).

Epithelium

Several special characteristics:
1. Cells have polarity
  - Apical (upper, free) and basal (lower, attached) surfaces
  - Apical surfaces may have microvilli (absorption) or cilia (movement)
  - Non-cellular basal lamina of glycoprotein and collagen fibres lies adjacent to basal surface
2. Closely packed cells and special contacts
  - Continuous sheets joined by tight junctions and desmosomes
3. Supported by connective tissue reticular lamina
  - Deep to the basal lamina
  - NB: reticular lamina + basal lamina = basement membrane
4. Avascular, but innervated
5. High rate of regeneration
Two forms:
- Covering and lining epithelium (external and internal surfaces)
  - Endothelium (lymphatic vessels, blood vessels, heart)
  - Mesothelium (serous membranes in the ventral body cavity)
- Glandular epithelium
  - Cells or groups of cells producing secretions
  - Endocrine (secrete directly into blood supply, eg: hormones)
  - Exocrine (secrete via a duct, eg: liver secreting bile; pancreas secreting digestive enzymes; mucous, sweat, oil, salivary glands)

Epithelial membranes

Cutaneous membrane (dry)
- Skin
Mucous membranes (moist)
- which line body cavities that open to the exterior of body (line hollow organs)
- eg: oral cavity, anal cavity, vagina
Serous Membranes (moist)
- Serosa—membranes in a closed ventral body cavity
- Parietal serosa line internal body walls
- Visceral serosa cover internal organs
- eg: surrounding lungs, heart, synovial joint cavities

Epithelium Classification

Simple epithelium
- Single layer of cells that functions in diffusion, osmosis, filtration, secretion, or absorption.
Pseudostratified epithelium
- Appears to have multiple layers of cells because the cell nuclei lie at different levels and not all cells reach the apical surface; it is actually a simple epithelium because all its cells rest on the basement membrane.
- Note: Cells can also have apical surface specialisations: ie microvilli or cilia
Stratified epithelium
- Consists of two or more layers of cells that protect underlying tissues in locations where there is considerable wear and tear.

Epithelium Classification

Squamous
- Squamous cells are thin/flat, which allows for the rapid passage of substances through them.
Cuboidal
- As tall as they are wide and are shaped like cubes or hexagons. They may have microvilli at their apical surface and function in either secretion or absorption.
Columnar
- Taller than they are wide, like columns, and protect underlying tissues. They are often specialised for secretion and absorption.
- Note: Cells can also have apical surface specialisations: ie microvilli or cilia
Transitional cells: change shape, from squamous to cuboidal and back

Connective tissue

Most abundant and widely distributed tissue
Major Functions:
- Binding and support
- Protection
- Insulation
- Transportation (blood)
Characteristics:
- Cells separated by nonliving Extracellular Matrix (ECM)
- ECM consists of ground substance and fibres, allowing for weight bearing, tension etc
- Varying ratio of ground substance: fibres; cells in connective tissue (some very strong, some very delicate)
- Varying degrees of vascularity (blood supply)
- Mesenchyme (embryonic tissue) as their common tissue of origin

Connective tissue

Structural elements:
1. Ground Substance (space-filler between cells)
  - Interstitial fluid
    - Dissolved substances can diffuse from capillaries to cells (molecular ‘sieve’)
    - Fibres in interstitial fluid can interfere with this solute diffusion
  - Cell adhesion proteins
    - Act as ‘glue’ allowing connective tissue cells to attach to
  - Proteoglycans
    - Protein core + large polysaccharides (glycosaminoglycans or GAGs)
    - Varying volumes of water trapped and this affects viscosity
2. Fibres:
  - Collagen fibres (white fibres)
    - Strongest and most abundant
    - Provides high tensile strength
  - Elastic fibres (yellow fibres)
    - Networks of long, thin elastin fibres
    - Allows stretch and recoil (skin, lungs)
  - Reticular fibres
    - Short, fine, highly branched collagenous fibres
    - Allow more stretch than larger collagen fibres
3. Cells:
  - Cell populations found as:
    - Immature (-blast) form (mitotically active and secretory)
    - Mature (-cyte) form (maintain ‘health’ of tissue)
    - Fibroblastð fibrocyte (connective tissue proper)
    - Chondroblastð chondrocyte (cartilage)
    - Osteoblastð osteocyte (bone (osseous) tissue)
  - Other cells found in connective tissue include:
    - Adipocytes (fat cells)
    - Leucocytes (white blood cells)
    - Mast cells (release histamine, heparin, enzymes if tissue damaged)
    - Macrophages (‘big eater’) engulf foreign materials, dead/damaged cells etc

Connective tissue Classification

Common origin (mesenchyme)
- Connective tissue proper
  - Loose
    - Areolar
    - Adipose
    - Reticular
  - Dense
    - Regular
    - Irregular
    - Elastic
- Connective tissue specialised
  - Supporting
    - Cartilage
      - Hyaline
      - Fibrocartilage
      - Elastic
    - Bone
      - Compact
      - Spongy
  - Fluid
    - Blood
    - Lymph

Loose Connective tissue

Areolar connective tissue
- Description
  - Areolar connective tissue is one of the most widely distributed connective tissues; consists of fibres (collagen, elastic, reticular) arranged randomly and several kinds of cells (fibroblasts, macrophages, plasma cells, adipocytes, mast cells, and a few white blood cells) embedded in semifluid ground substance (hyaluronic acid, chondroitin sulphate, dermatan sulphate, and keratan sulphate).
- Location
  - In and around nearly every body structure (thus, called 'packing material' of the body): in subcutaneous layer deep to skin; papillary (superficial) region of dermis of skin; lamina propria of mucous membranes; around blood vessels, nerves, and body organs.
- Function
  - Strength, elasticity, support.
Adipose tissue
- Description
  - Adipose tissue has cells derived from fibroblasts (called adipocytes) that are specialised for storage of triglycerides (fats) as a large, centrally located droplet. Cell fills up with a single, large triglyceride droplet, and cytoplasm and nucleus are pushed to periphery of cell. With weight gain, amount of adipose tissue increases and new blood vessels form. Thus, an obese person has many more blood vessels than does a lean person, a situation that can cause high blood pressure, since the heart has to work harder. Most adipose tissue in adults is white adipose tissue (just described). Brown adipose tissue (BAT) is darker due to very rich blood supply and numerous pigmented mitochondria that participate in aerobic cellular respiration. BAT is widespread in the foetus and infant; adults have only small amounts.
- Location
  - Wherever areolar connective tissue is located: subcutaneous layer deep to skin, around heart and kidneys, yellow bone marrow, padding around joints and behind eyeball in eye socket.
- Function
  - Reduces heat loss through skin; serves as an energy reserve; supports and protects organs. In newborns, BAT generates heat to maintain proper body temperature.
Reticular connective tissue
- Description
  - Reticular connective tissue is a fine interlacing network of reticular fibres (thin form of collagen fibre) and reticular cells.
- Location
  - Stroma (supporting framework) of liver, spleen, lymph nodes; red bone marrow; reticular lamina of basement membrane; around blood vessels and muscles.
- Function
  - Forms stroma of organs; binds smooth muscle tissue cells; filters and removes worn-out blood cells in spleen and microbes in lymph nodes.

Dense Connective Tissue

Dense regular connective tissue
- Description
  - Dense regular connective tissue forms shiny white extracellular matrix; mainly collagen fibres regularly arranged in bundles with fibroblasts in rows between them. Collagen fibres (protein structures secreted by fibroblasts) are not living, so damaged tendons and ligaments heal slowly.
- Location
  - Forms tendons (attach muscle to bone), most ligaments (attach bone to bone), and aponeuroses (sheetlike tendons that attach muscle to muscle or muscle to bone).
- Function
  - Provides strong attachment between various structures. Tissue structure withstands pulling (tension) along long axis of fibres.
Dense irregular connective tissue
- Description
  - Dense irregular connective tissue is made up of collagen fibres; usually irregularly arranged with a few fibroblasts.
- Location
  - Often occurs in sheets, such as fasciae (tissue beneath skin and around muscles and other organs), reticular (deeper) region of dermis of skin, fibrous pericardium of heart, periosteum of bone, perichondrium of cartilage, joint capsules, membrane capsules around various organs (kidneys, liver, testes, lymph nodes); also in heart valves.
- Function
  - Provides tensile (pulling) strength in many directions.
Elastic connective tissue
- Description
  - Elastic connective tissue contains predominantly elastic fibres with fibroblasts between them; unstained tissue is yellowish.
- Location
  - Lung tissue, walls of elastic arteries, trachea, bronchial tubes, true vocal cords, suspensory ligaments of penis, some ligaments between vertebrae.
- Function
  - Allows stretching of various organs; is strong and can recoil to original shape after being stretched. Elasticity is important to normal functioning of lung tissue (recoils in exhaling) and elastic arteries (recoil between heartbeats to help maintain blood flow).

Cartilage Connective Tissue

Hyaline cartilage
- Description
  - Hyaline cartilage (hyalinos = glassy) contains a resilient gel as ground substance and appears in the body as a bluish-white, shiny substance (can stain pink or purple when prepared for microscopic examination; fine collagen fibres are not visible with ordinary staining techniques); prominent chondrocytes are found in lacunae surrounded by perichondrium (exceptions: articular cartilage in joints and cartilage of epiphyseal plates, where bones lengthen during growth).
- Location
  - Most abundant cartilage in body; at ends of long bones, anterior ends of ribs, nose, parts of larynx, trachea, bronchi, bronchial tubes, embryonic and foetal skeleton.
- Function
  - Provides smooth surfaces for movement at joints, flexibility, and support; weakest type of cartilage and can be fractured.
Fibrocartilage
- Description
  - Fibrocartilage has chondrocytes among clearly visible thick bundles of collagen fibres within extracellular matrix; lacks perichondrium.
- Location
  - Pubic symphysis (where hip bones join anteriorly), intervertebral discs, menisci (cartilage pads) of knee, portions of tendons that insert into cartilage.
- Function
  - Support and joining structures together. Strength and rigidity make it the strongest type of cartilage.
Elastic cartilage
- Description
  - Elastic cartilage has chondrocytes in threadlike network of elastic fibres within extracellular matrix; perichondrium present.
- Location
  - Lid on top of larynx (epiglottis), part of external ear (auricle), auditory (eustachian) tubes.
- Function
  - Provides strength and elasticity; maintains shape of certain structures.

Bone Connective Tissue

Description
- Compact bone tissue consists of osteons (haversian systems) that contain lamellae, lacunae, osteocytes, canaliculi, and central (haversian) canals. By contrast, spongy bone tissue (see figure 6.3) consists of thin columns called trabeculae; spaces between trabeculae are filled with red bone marrow.
Location
- Both compact and spongy bone tissue make up the various parts of bones of the body.
Function
- Support, protection, storage; houses blood-forming tissue; serves as levers that act with muscle tissue to enable movement.

Liquid Connective tissue

Blood
- Description
  - Blood plasma and formed elements: red blood cells (erythrocytes), white blood cells (leukocytes), platelets (thrombocytes).
- Location
  - Within blood vessels (arteries, arterioles, capillaries, venules, veins), within chambers of heart.
- Function
  - Red blood cells: transport oxygen and some carbon dioxide; white blood cells: carry on phagocytosis and mediate allergic reactions and immune system responses; platelets: essential for blood clotting.
Lymph
- Description
  - Lymph is the extracellular fluid that flows in lymphatic vessels. It is a liquid connective tissue that consists of several types of cells in a clear liquid extracellular matrix that is similar to blood plasma but with much less protein. The composition of lymph varies from one part of the body to another. For example, lymph leaving lymph nodes includes many lymphocytes, a type of white blood cell, in contrast to lymph from the small intestine, which has a high content of newly absorbed dietary lipids.

Muscle tissue

Skeletal muscle tissue
- Description
  - Skeletal muscle tissue consists of long, cylindrical, striated fibres (striations are alternating light and dark bands within fibres that are visible under a light microscope). Skeletal muscle fibres vary greatly in length, from a few centimetres in short muscles to 30-40 cm (about 12-16 in.) in the longest muscles. A muscle fibre is a roughly cylindrical, multinucleated cell with nuclei at the periphery. Skeletal muscle is considered voluntary because it can be made to contract or relax by conscious control.
- Location
  - Usually attached to bones by tendons.
- Function
  - Motion, posture, heat production, protection.
Cardiac muscle tissue
- Description
  - Cardiac muscle tissue consists of branched, striated fibres with usually only one centrally located nucleus (occasionally two). Attach end to end by transverse thickenings of plasma membrane called intercalated discs (in-TER-ka-lät-ed; intercalate = to insert between), which contain desmosomes and gap junctions. Desmosomes strengthen tissue and hold fibres together during vigorous contractions. Gap junctions provide route for quick conduction of electrical signals (muscle action potentials) throughout heart. Involuntary (not conscious) control.
- Location
  - Heart wall.
- Function
  - Pumps blood to all parts of body.
Smooth muscle tissue
- Description
  - Smooth muscle tissue consists of fibres usually involuntary, nonstriated (lack striations, hence the term smooth). Smooth muscle fibre is a small spindle-shaped cell thickest in middle, tapering at each end, and containing a single, centrally located nucleus. Gap junctions connect many individual fibres in some smooth muscle tissue (for example, in wall of intestines). Can produce powerful contractions as many muscle fibres contract in unison. Where gap junctions are absent, such as iris of eye, smooth muscle fibres contract individually, like skeletal muscle fibres.
- Location
  - Iris of eyes; walls of hollow internal structures such as blood vessels, airways to lungs, stomach, intestines, gall bladder, urinary bladder, and uterus.
- Function
  - Motion (constriction of blood vessels and airways, propulsion of foods through gastrointestinal tract, contraction of urinary bladder and gall bladder).

Nervous tissue

Description
- Nervous tissue consists of (1) Neurons (nerve cells), which consist of cell body and processes extending from cell body (one to multiple dendrites and a single axon); and (2) neuroglia, which do not generate or conduct nerve impulses but have other important supporting functions.
Location
- Nervous system.
Function
- Exhibits sensitivity to various types of stimuli; converts stimuli into nerve impulses (action potentials); conducts nerve impulses to other neurons, muscle fibres, or glands.

Lecture summary

The 4 primary cell types
The subclassifications of each primary tissue type
The special features/characteristics of each tissue type
Their function and location within the body