CM 7-8 Histology and Tissue organization

What does Histology Study:

study of microscopic tissue organizations and how structure relates to function.

(Cells, Extracellular matrix, how cells are organized, and how tissue interface)

Pathway of Cell to Human Body

Cell → Tissue → Organ → Organ System → Organism

Cell

Smallest living structural and functional unit and specialized unit that perform specific functions

(retinal ganglion cell)

Tissue

Cells+extracellular components organized to perform a common function. Organized communities of cells and their extracellular environment.

Retina (tissue)

Organ

Different tissue types organized into a functional structure

(Eye)

Organ System

Multiple organs working together toward a broader function

Ex: Visual system (eyes, optic nerves, brain)

Organism

All organ systems functioning as an integrated whole

Ex: human body

What makes up tissues?

Cells and Extracellular components

Cell Dominant

Cells are tightly packed with little extracellular material

Ex: epithelial tissue

Matrix Dominant

Cells are dispersed within abundant extracellular matrix

Ex: Connective tissue

What components help define tissue structure and function?

Relative amount and organization of cells and extracellular matrix

Tissues on the Eye

Nervous tissue, epithelial tissue, connective tissue, and muscle tissue

Epithelial tissue

Forms specialized surfaces and barriers. They are packed cells (sheets/layers).

Ex: corneal epithelium, conjunctival epithelium

Nervous tissue

Receives, processes and transmits visual information

Neural cells/processes

Supporting (glial) cells

Ex: retina, optic nerve

Connective tissue

It is matrix dominant. Provides structural and extracellular support. Cells dispersed.

Ex: corneal stroma, sclera

Muscle tissue

Produces movement and controls dimensions. Has elongated contractile cells (bundles/layers)

Ex: Extraocular muscles, iris smooth muscle

What are the major retinal layers organized between?

Extends from the retinal pigment epithelium (RPE) to the ganglion cell and nerve fiber layers

What are the five major neuronal cell types in the retina?

Photoreceptors (rods and cones), horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells

What are the photoreceptors of the retina?

Rods and cones

Cornea

Transparent

Layered

Avascular

Sclera

Dense

Fibrous

Protective

Iris

Pigmented

Vascular

Contractile

Retina

Neural

Layered

Sensory

Epithelial Recognition clues

AVASCULAR

Packed cells→ very little extracellular space

Sheets/layers → cells form a continuous lining

Avascular → supported by connective tissue below

Basement membrane → defines the epithelial- CT interface

Epithelial tissue found where in the eye?

Cornea, conjunctiva, ciliary body, lens epithelium

What does epithelium do?

Protect → acts as barrier against mechanical, microbial, and environment challenges

Absorb→ allows selective uptake of substances from external/intenral environement

Secrete → Releases fluids, proteins, enzymes, hormones and other products

Transport → Regulates fluid and ion movement across thin epithelial barriers

Sense → interfaces with sensory structures to detect stimuli from the environement (Ex. corneal epithelium with sensory nerve ending)

What does epithelium do in the eye?

Protection → corneal epithelium forms barrier against pathogens/environemental exposure/ mechanical injury

Secretion → Lacrimal gland epithelium produces the aqueous component of tears, helping lubricate and protect the ocular surface

Transport → Corneal endothelial cells regulate fluid and ion movement, helping maintain corneal hydration and transparency

Epithelia named?

By layers (simple, stratified, and pseudo stratified) and shapes (squamous, cuboidal, columnar)

Epithelium Polarity: apical (top part)

Interacts with the free surface or lumen → absorption, secretion, movement and sensing

key structures: microvilli, cilia, stereocilia

Epithelium Polarity: lateral (side part)

connects neighboring cells → adhesion, communication and maintenance of the epithlial barrier

key structures: Tight adherens, desmosomes, gap junctions

Epithelium Polarity: Basal (bottom part)

Anchors the epithelium and creates the interface with the underlying conective tissue

Key structures: Hemidesmosomes, basement membranes

What is epithelial polarity

spacial organization of function

Microvilli (Structure, composition, motility, function, location)

Structure: Short, densely packed projections

Composition: Actin filaments

Motility: Non motile

Function: Increase surface area for absorption

Location: Small intestine - proximal renal tubules

Cilia (Structure, composition, motility, function, location)

Structure: Longer, uniform hair-like projections

Composition: Microtubules

Motility: Motile- coordinated beating

Function: Move material across the epithelial surface

Location: Respiratory tract - uterine tubes

Stereocilia (Structure, composition, motility, function, location)

Structure: very long, often irregular projections

Composition: Actin filaments

Motility: non-motile

Function: absorption or mechanosensation

Location: Epididymis - inner ear

Tight Junction

Seal

Limits paracellular movement between adjacent cells

Adherens Junction

Anchor

Links actin cytoskeletons between neighboring cells

Desmosomes

Resist stress

Links intermediate filaments → strong cell-to-cell adhesion

Gap junction

Communicate

Channels allow ions and small molecules to pass cell-to-cell

Hemidesmosome

Basal anchorage

Anchors basal epithelial cells to basement membrane

Normal cornea

Non- keratinized stratified squamous epithelium

Superficial cells retain nuclei

Smooth epithelial surface supports corneal transparency

Conjunctiva

Non- keratinized stratified epithelium

Goblet cells produce mucins

Contributes to ocular surface protection and lubrication

Pathologic Keratinization

Squamous metaplasia and surface keratinized

May occur in severe ocular surface disease

Loss of normal surface specialization can impair ocular surface function

What does Keratinization represent?

An abnormal ocular surface change

Basement Membrane:

Thin, acellular layer of specialized matrix located between the basal surface of epithelium and the underlying connective tissue.

Avascular→ O2, nutrients and metabolites diffuse from connective tissue

Composition of basement membrane:

Type IV collagen (structural network)

Laminin ( cell attachment and organization)

Proteoglycans + glycoproteins

Organization of basement membrane:

Basal laminate (closest to epithelial cells and rich in laminin + type IV collagen)

Reticular Lamina (Connects the basal laminate to underlying connectve tissue)

Why does basement membrane matter?

Anchorage → attaches epithelium to supporting tissue

Barrier → regulates movement between compartments

Nutrition → supports diffusion to avascular epithelium

Repair → provides a scaffold for regeneration

Cell behavior → influences adhesion, migration and differentiation

Specialized ECM (extracellular matrix)

Provides structural support, selective exchange and control of the epithelial - connective tissue interface

Connective Tissue: Cells in Matrix

• cells are dispersed within an abundant extracellular matrix (unlike epithelium) and more extracellular material b/w cells than epithelium

Cells → Fibroblasts (principle resident cells) and other cell populations vary by tissue and function

Fibers→ Collagen, elastic reticular

Ground substances→ hydrated gel containing proteoglycans, GAGs and glycoproteins

What makes up Extracellular Matrix and what is it?

It is a non living scaffold that supports cells and is made up of fibers and ground substance

Loose connective tissue

More ground substance

Loosely arranged fibers

Flexible supports and diffusion (hydrated)

Dense connective tissue

More collagen

Densely packed fibers

Tensile strength

Cartilage

firm, hydrated matric

cells in lacunae

Resilient support

Bone

Type I collagen + mineral

Mineralized matrix

Rigid support and protection (mineralized)

Connective tissues in the eye

Corneal stroma, sclera, choroid, and laminate criborosa

Corneal Stroma

Regularly arranged type I collagen lamellae + Keratocytes

Transparency

structural support

Sclera

Dense, interlacing type collagen with fibroblasts

Strength, protection, globe shape

Choroid

Vascular, pigmented connective tissue with fibroblasts and melanocytes
Vascular support, light absorption

Lamina Cribrosa

Collagen - and elastin-rich connective tissue at the optic nerve head

Structural support for exiting retinal ganglion cell axons

Stevens - Johnson Syndrome

From ocular surface injury to squamous metaplasia

Stevens - Johnson Syndrome: Acute ocular injury

Corneal epithelial defect and conjunctival membrane

Amniotic membrane was placed to support epithelial healing

Stevens - Johnson Syndrome: Conjunctival Growth

Three weeks later: vascularized conjunctival growth on the amniotic membrane with symblepharon

Stevens - Johnson Syndrome: histopathology

Squamous epithelium growing on aceullular amniotic membrane with loss of goblet cells

Muscle Tissue

Elongated cells specialized to generate force through contraction

recognition clue: elongated cells/fibers arranged for force production

Muscle tissue contractile proteins

Actin and myosin

Muscle tissue: Muscle cell

Muscle fiber

Muscle tissue: cytoplasm

sarcoplasm

Muscle tissue: cell membrane

sarcolemma

Skeletal Muscle

Striated; long, cylindrical fibers

Multiple peripheral nuclei

Voluntary

Tongue, muscles of mastication

Cardiac Muscle

Striated; Branched fibers

Uusally 1 central nucleus

Involuntary

Intercalated discs

Smooth muscle

Non- striated; spindle- shaped cells

1 central nucleus

Involuntary

vessel walls, ducts

Muscles that move the eye:

Superior rectus

Medial rectus

Inferior rectus

Lateral rectus

Inferior oblique

Superior oblique

Eye muscle: skeletal muscle

striated muscle fibers generate rapid and precise contractions

Eye muscle: coordinated action

The muscles work together to position both eyes and change the direction of gas

Eye muscle: Visual function

Their coordinated movement supports fixation, tracking and binocular alignment

Eye muscle: Motor control

Extraocular muscles are controlled by cranial nerves III, IV, and VI

Elevation/Depression

Move the eye upward or downward

Abduction/Adduction

Move the eye away from or toward the nose

Intorsion/Extorsion

Rotate the superior pole of the eye toward or away from the nose

Intraocular muscles

Smooth muscles that adjust the optical system

Muscles inside the eye

Ciliary muscle

Spincter pupillae

Dilator pupillae

Smooth muscle

Ciliary muscle

Contraction changes lens shape during accommodation, helping focus on near objects.

Spincter pupillae

Contraction decreases pupil diameter (miosis).

more light entering under bright conditions

Dilator pupillae

Contraction increases pupil diameter (mydriasis).

Smooth muscle

Unlike the extraocular muscles, these muscles work under autonomic control.

What makes Nervous Tissue different:

NEURONS→ Excitable cells specialized for communication.
Cell body + dendrites + axon.

GLIA → Support the neural environment: structural support, protection, homeostasis and insulation.

BACKGROUND → Very little extracellular matrix. The background is largely neuropil→ a dense network of neuronal and glial processes.

Functional logic for nervous tissue

Receive (Dendrites/sensory input) → Process (cell body + neural circuits) → Transmit (axon → target cell)

What should eye find on nervous tissue:

Large neuronal cell bodies

prominent nucleus/nucleolus

many much smaller glial nuclei

Nucleolus

structure inside nucleus that makes ribosomal components

Organization of Nervous Tissue: Cell body-rich regions

Large neuronal somas may be visible

Prominent nucleus and nucleolus

Numerous smaller glial nuclei

Dense network of cellular processes

(Information processing)

Organization of Nervous Tissue: Axon- rich regions

Axons may dominant the field

Neuronal cell bodies may be absent

Fibers may be grouped into bundles

Supporting cells accompany the axons

(Information transmission)

Central nervous tissue

Large neuronal cell bodies may be visible

Numerous small glial nuclei

Dense background of neural processes

Very little extracellular matrix

(neural network)

Peripheral Nervous Tissue

Nerve fibers organized into fascicles

Connective tissue surrounds and supports the bundles

Neuronal cell bodies are usually not visible

Pattern is dominated by organized axons

Peripheral nerve combines nervous tissue with connective tissue

(organized fiber bundles)

Retina - Sensory processing

Photoreceptors - detect light

Bipolar cells - relay visual signals

Ganglion cells - provide the retinal output, their axons form the optic nerve

Receives and proceses visual information → optic nerve transmits it

Cornea - Sensory Innervation

Sensory fibers - enter the cornea and form dense nerve networks

Free nerve endings - detect touch, irritation, temperature and pain

Important distinction - the cornea contains nerves, but the cornea itself is not nervous tissue

Cornea uses sensory nerves for protection

Optic Nerve - Transmission

Axons of the retinal ganglion cells leave the eye together

Carries visual information from the retina toward the brain

Where is corneal innervation

Where nervous tissue meets epithelium

Dense sensory innervation

The cornea is one of the most densely innervated and sensitive tissues in the body

From stroma to epithelium

Stromal nerves branch and form the subbasal nerve plexus near the basal epithelium