Tissues

Tissue

A group of cells that are similar in structure and perform a similar (or related) function

Four building block tissues

Epithelial tissue

Connective tissue

Muscle tissue

Nervous tissue

Epithelial tissue function

Covering surfaces (internal & external)

Acts as a boundary or interface between different environments

• Protection (skin)

• Absorption (small intestine)

• Selective permeability

• Filtration (kidneys)

• Excretion (kidney, intestines, skin)

• Secretion (glands, kidney, small int.)

• Sensory reception or perception (skin)

Forms all glands

Connective tissue function

Supporting and connecting other tissues

Muscle tissue function

Movement

Nervous tissue function

Control & coordination

How do cells form tissues?

1. Cell junctions (cells sticking close together) and/or

2. Extracellular matrix (ECM)

Characteristics of epithelium

• Highly cellular - very little to no ECM or space

• Specialised contact between cells - cell junctions

• Polarity - apical (top), lateral (sides) and basal (bottom) domains

• Basal lamina (basement membrane)

• Supporting nutritionally and mechanically by underlying layer of loose connective tissue

• Avascular (no blood vessels)

• Regeneration - can repair itself

• Cell membrane specialisations (microvilli, stereocilia, cilia)

Simple epithelium

One cell layer thick - all cells touch the basal lamina

Stratified epithelium

More than one cell layer thick - not all cells touch the basal lamina

Pseudostratified epithelium

Appears stratified, but is only one cell layer thick - all cells touch the basal lamina but not all cells reach the surface

Squamous cell

Cells are elongated and thin (wide and thin)

Cuboidal cells

Cells are cube shaped (as wide as they are tall)

Columnar cells

Cells are tall and thin

Simple squamous epithelium

Flattened, long nuclei with little cytoplasm

Line the surface where passive transport of gasses or fluids occurs

E.g. alveoli of lungs, glomerulus of kidneys capillaries, external surfaces of many organs (lungs, GIT, heart)

Provide a smooth, frictionless surface

Simple cuboidal epithelium

Spherical nuclei

Many ducts are lined by this

E.g. ducts in kidneys and salivary glands

Simple columnar epithelium

Elongated (sausage shaped) nuclei usually found in a basal position

Found lining parts of GIT, reproductive system and the larger ducts

Stratified squamous epithelium

Many layers of nuclei showing a progressive change in shape

Keratinised (top layers have no nuclei, dead cells) and non-keratinised (nuclei go right to the top

Found in areas that require a protective surface (epidermis of skin, mouth, vagina, anal canal, oesophagus)

Transitional epithelium

Relaxed epithelium - stratified with domed cuboidal cells on surface

Contracted epithelium - flat surface, elongated nuclei

Nuclei can change shape if organ is stretched or not

Found only in the excretory passages of the urinary system (bladder, renal pelvis, ureter)

Endothelium

Epithelium found within blood vessels

Lateral membrane specialisations

Cell junctions

Cell junctions

Create a barrier between cells (tight junctions)

Provide strength and stability (adhering junctions)

Allow for communication (gap junctions)

Tight junctions

Barrier that prevents passage of fluid between cells and maintains apical polarity of membranous proteins

Gap junctions

Allow passage of small molecules and water between cells

Adhering junctions

Proved rigidity

Anchor point for cytoskeleton

Basal surface specialisation

Basement membrane/basal lamina

Basement membrane

Made up of basal lamina (from epithelial cells) and reticular lamina (from connective tissue cells)

Helps epithelial cells to display polarity and shape, acting as a physical scaffolding for epithelial cells

Connects epithelial cells to underlying loose connective tissue

Compartmentalize epithelium from connective tissue and regulated entry & exit (cell migration, filtration)

Apical surface specialisation

Microvilli, cilia, stereocilia

Microvilli

Often called “brush border” under light microscope (fuzzy border)

Way to increase surface area of the cell membrane for absorption

Have a core of microfilaments (actin) that insert into the underlying terminal web

Immotile (doesn't move)

Cilia

Longer than microvilli

Motile - create movement of fluid/material on the apical surface of epithelium (dynein arms)

Core of microtubules in 9+2 pair arrangement (axoneme)

Mainly found in respiratory and reproductive systems

Arise from centriole-like structures (basal bodies)

Glands

• All glands begin as downgrowths of an epithelium

• Endocrine glands break off from epithelium and secrete internally i.e. hormones

• Exocrine glands stay attached to the epithelium/duct and secrete externally(?)

Connective tissue

NOT epithelium, muscle or nervous tissue - everything else in between

Includes skeleton (bone and cartilage,) blood, fat, ligaments and tendons, stroma

Components of connective tissue

Cells: fibroblasts, others (macrophages, plasma cells, adipocytes, WBC, mast cells)

Extracellular matrix (ECM): Ground substance (water and solutes, glycosaminoglycans (GAGs), proteoglycans) and fibres (collagen, reticular, elastin.)

Fibroblasts

• Contains lots of rER

• Active

• Make the connective tissue ECM

• Lots of Golgi

• Produce the ground substance and fibres

Fibrocytes

• inactive

• Little rER & golgi

• Maintain ground substance & fibres

Collagen

• Synthesis requires vitamin C

• Forms a strong triple helix

• 20(+?) types

Endomysium

Loose CT surrounding each individual muscle cell

Perimysium

Dense irregular CT surrounding a group of muscle cells (fascicle)

Epimysiun

Dense irregular CT surrounding the whole muscle proper (all fascicles)

Myofibrils

Densely packed rod-like elements which comprise ~80% of the cell volume (mature skeletal muscle cells)

Contain sarcomeres

Sarcomere

The functional unit within striated muscle cells, contained in myofibrils

Contain myofilaments

Runs between two Z discs -muscles contractile unit

Myofilaments

Thick & thin filaments whose regular arrangement produces the striations - repeating series of dark A bands and light I bands

Actin & myosin

Cardiac muscle

• One (sometimes two) centrally located nucleus

• Striated (organised myofilaments in myofibrils)

• Cells branch so they can contact several other cells

• Cell junctions contain adhering and communicating(gap) junctions (intercalated discs)

• Innervated by autonomic nervous system (and self contracting pacemaker cells)

• 50% of cytoplasm is mitochondria - highly energetic tissue

• Lots of capillaries - high vascular

Thick filaments (A band)

Composed of myosin molecules

Tails contain two interwoven chains

Heads contain two small chains that act as cross bridges during contraction (look like lil golf clubs)

Heads contain binding sites for actin & ATP

Thin filaments

Twisted double stranded of fibrous protein - actin

Tropomyosin and troponin are a regulatory complex bound to actin

Troponin binds to tropomyosin

Tropomyosin covers the myosin binding sites

Ca2+ binds to this complex to expose the binding sites for myosin

Sarcoplasmic reticulum

Network of specialised smooth ER surrounding each myofibril

Function in regulation of Ca2+ levels - stores and releases calcium

T Tubules

Transverse tubules

Invagination of the cell membrane (sarcolemma)

Carries an action potential from outside to inside large muscle cells

Skeletal vs cardiac muscle

Skeletal muscle have lots of SR, cardiac has much less (relies more on extracellular calcium)

Both have T tubules

• Skeletal two SR cisternae and one T tubule in between (triad)

• Cardiac has one SR cistern and one T tubule (diad)

Cardiac has extensive blood supply

Smooth muscle

Consists of many individual, small myofibres with one centrally placed nucleus

Cells are fusiform in shape (tapered at either end)

No striations

Contain only gap junctions that allow ionic transfer to adjacent cells do they can contract as a tissue(syncytium)

Actin anchored to “dense bodies” on sarcolemma and within cytoplasm

Myosin lies close to actin and slides over during contraction|

No fibrils, sarcomeres or T tubules

Nervous tissue

Exists in central nervous system (CNS - Brain and spinal cord) and peripheral nervous system (PNS - nerves, sensory receptors, ganglia)

Cells (neurons) are widely scattered with limited contact at extremities

Cells are characterised by having many cellular processes (cytoplasmic extensions)

Highly specialised contact points (synapses) between cells, enabling signals to be transmitted between cells

Cells of nervous tissue

Neurons

Glial cells - supporting cells

• Oligodendrocytes

• Astrocytes

• Microglial

• Ependymal cells - produce CSF

• Schwann cells (PNS)

• Satellite cells (PNS)

Axon

Long section of the neuron, may be myelinated which allows the action potential to bypass sections of the voltage-gated ion channel (wrapped in myelin sheaths) and jump between nodes of ranvier (no myelin)

Axon hillock

Thick section between axon & soma, where summation occurs

Soma

Cell body of the neuron, contains most of cytoplasm & nucleus

NOT found in peripheral nerves

Axon terminal

Tail of the neuron

Forms the synapses at axon boutons

Neuronal types

Multipolar (motor neurons) - lots of branches of dendrites, soma at the head

Unipolar - one branch of dendrites coming off an axon, soma off the middle

Bipolar neuron - soma in the centre, axon coming off both ends. Dendrites close to external environment (senses- taste, sight etc.)

Synapses

Allow communication

Loose CT

Ordinary/mature CT

Delicate, good for movement

Much ground substance

Few fine fibres

Can have many cells (but doesn't have to)

Dense CT

Regular (1 direction) and irregular (many directions) types

Strong, good for resisting stretch

Many fibres in coarse bundles

Can be collagenous (typical) or elastic (unusual)

Few scattered cells - fibrocytes

Elastic tissue

Special type of dense CT

Found in arteries

Contains elastic tissues called elastic lamina

Maintains pressure when heart is relaxed

Adipose tissue

Highly specialised form of loose CT

Made of adipocytes tightly packed together

Brown (multilocular and found in infants. Produce heat) or white (unilocular)

Energy store

Secrete hormones to regulate metabolism (Leptin)

Adipocyte

Fat cells

Has a large lipid droplet filling the cytoplasm

Nucleus and cytoplasm pushed to edge of the cell

Often found in CT - wherever you find blood vessels

Brown adipose tissue

Lots of mitochondria

Found in infants

Metabolize fat to produce heat

Glial cells

astrocytes

oligodendrocytes CNS

microglia

ependymal cells

Schwann cells PNS

satellite cells

Myelin

Oligendrocytes produce myelin in the CNS
Schwann cells produce myelin in the PNS

A wrapping of the axon by the cell membrane (phospholipid rich)

Allows for saltatory conduction (jumping between nodes)

Oligodendrocytes

Myelinate many different axons at once (or the same axon multiple times) via long cytoplasmic processes that wrap around axons.

Schwann cells (neurolemmocytes)

Schwann cells can only myelinate one part of one axon

Found in the PNS only

Astrocytes

Star cells

have many cellular processes that mechanically and physiologically support neurons, blood vessels, etc. in the CNS; form scar tissue when damage occurs

forms the blood brain barrier

found in CNS

Microglia

small support cells

resident macrophages of the CNS (innate immunological function)

Ependymal cells

line the brains ventricle and the spinal cord’s central canal

help produce & monitor CSF

satellite cells

support cell type found only within ganglia (accumulations of neuronal cell bodies in the PNS) that surround neuronal cell bodies

CNS Spinal cord

Brain & spinal cord are composed of white matter and grey matter

White matter - outside of spinal cord, inside of brain, consists of myelinated axons & glial cells

Grey matter - Inside spinal cord, outside of brain, contains the soma of the neurons - neurons & neuropil

Nerve composition (PNS)

similar to muscle cell structure - groups of myelinated axons form fasicles.

CT layers:

Endoneurium (LCT, found around the individual cell)

Perineurium (DICT, found around each fascicle)

Epineurium (DICT, found surrounding the entire nerve)