Histology: The Four Basic Tissue Types

Epithelial Tissue

Classification

Criterion	Types
Cell layers	Simple (1), Stratified (2+), Pseudostratified (appears layered)
Cell shape	Squamous (flat), Cuboidal (cube), Columnar (tall), Transitional (variable)

Simple epithelia:

Type	Location	Function
Simple squamous	Endothelium (vessels), mesothelium (serosa), lung alveoli, glomerulus	Diffusion, filtration
Simple cuboidal	Kidney tubules, thyroid follicles, pancreatic ducts	Secretion, absorption
Simple columnar	Stomach, intestine (ciliated in bronchi, fallopian tubes)	Secretion, absorption, propulsion
Pseudostratified columnar	Trachea, bronchi (ciliated with goblet cells)	Mucus secretion, clearance

Stratified epithelia:

Type	Location	Function
Keratinized squamous	Epidermis (skin)	Protection
Non-keratinized squamous	Oral cavity, esophagus, vagina, cornea	Protection (moist)
Stratified cuboidal	Sweat gland ducts, esophageal glands	Protection, secretion
Stratified columnar	Large duct of salivary glands, conjunctiva	Protection, secretion
Transitional	Urinary tract (bladder, ureters, urethra)	Stretch

Special Features

Feature	Description
Basement membrane	Basal lamina (type IV collagen, laminin) + reticular lamina
Tight junctions (zonulae occludentes)	Apical seal (prevents paracellular leakage)
Adherens junctions (zonulae adherentes)	Lateral adhesion (cadherins)
Desmosomes (maculae adherentes)	Spot adhesion (intermediate filaments)
Gap junctions	Intercellular communication (connexins)
Hemidesmosomes	Basal adhesion (to basement membrane)
Microvilli	Brush border (absorption - intestine, kidney)
Cilia	Motile (respiratory tract), sensory (inner ear)
Stereocilia	Long microvilli (epididymis, inner ear hair cells)

Glandular Epithelium

Type	Description	Example
Exocrine	Duct to surface	Sweat, salivary, pancreas
Endocrine	No duct (to blood)	Thyroid, pituitary
Merocrine	Vesicle exocytosis	Sweat, salivary
Apocrine	Apical cytoplasm pinched	Mammary, axillary sweat
Holocrine	Whole cell released	Sebaceous

Connective Tissue

Components

Component	Description
Cells	Varied (see below)
Fibers	Collagen (types I-IV), elastic, reticular
Ground substance	Proteoglycans, GAGs, glycoproteins

Classification

Type	Cells	Fibers	Matrix	Location
Areolar (loose)	Fibroblast, macrophage, mast cell	Collagen (sparse), elastic	Gel-like	Subcutaneous, around vessels
Adipose	Adipocytes	Reticular	—	Fat depots
Reticular	Fibroblast, lymphocytes	Reticular (type III)	—	Lymph nodes, spleen
Dense regular	Fibroblasts	Collagen (parallel)	Dense	Tendons, ligaments
Dense irregular	Fibroblasts	Collagen (random)	Dense	Dermis, capsule
Elastic	Fibroblasts, smooth muscle	Elastic (dominant)	—	Aorta, lung
Cartilage (hyaline)	Chondrocytes	Type II collagen	Proteoglycan (aggrecan)	Joints, nose, trachea
Cartilage (fibrocartilage)	Chondrocytes	Type I + II collagen	Dense	Intervertebral discs, menisci
Cartilage (elastic)	Chondrocytes	Elastic + type II	—	Ear, epiglottis
Bone	Osteocytes, osteoblasts, osteoclasts	Type I collagen	Hydroxyapatite (CaPO₄)	Skeleton
Blood	RBC, WBC, platelets	—	Plasma	Blood vessels

Connective Tissue Cells

Cell	Origin	Function
Fibroblast	Mesenchyme	Collagen, elastin, GAG production
Chondroblast / chondrocyte	Mesenchyme	Cartilage matrix
Osteoblast / osteocyte	Mesenchyme	Bone formation
Osteoclast	Monocyte-macrophage	Bone resorption
Adipocyte	Mesenchyme	Fat storage
Macrophage	Monocyte	Phagocytosis
Mast cell	Bone marrow	Histamine, heparin
Plasma cell	B lymphocyte	Antibody production

Extracellular Matrix

Component	Subtypes	Function
Collagen	Type I (bone, tendon), II (cartilage), III (reticular), IV (basement membrane)	Tensile strength
Elastin	Elastic fibers	Elastic recoil
Proteoglycans	Aggrecan, versican, decorin, perlecan	Hydration, compression resistance
GAGs	Hyaluronan, chondroitin sulfate, heparan sulfate	ECM hydration
Glycoproteins	Fibronectin, laminin, tenascin	Cell attachment

Muscle Tissue

Feature	Skeletal	Cardiac	Smooth
Striation	Yes	Yes	No
Nuclei	Multiple (peripheral)	1-2 (central)	1 (central)
Branching	No	Yes	No
Gap junctions	No	Yes (intercalated discs)	Yes
Control	Voluntary	Involuntary	Involuntary
Location	Attached to bones	Heart	Blood vessels, viscera
T-tubules	A-I junction	Z-line	Caveolae (no T-tubules)
Sarcoplasmic reticulum	Extensive	Moderate	Variable
Regeneration	Limited (satellite cells)	None (cell death → scar)	Good (can hyperplastic)

Skeletal muscle anatomy:

• Epimysium → Perimysium → Endomysium
• Myofiber → Myofibril → Sarcomere (Z-disc, A-band, I-band, H-zone, M-line)
• Thick filament (myosin II), thin filament (actin, tropomyosin, troponin)

Nervous Tissue

Cell Type	Location	Function
Neuron	CNS, PNS	Signal transmission
Astrocyte	CNS	Blood-brain barrier, support, neurotransmitter cycling
Oligodendrocyte	CNS	Myelination (multiple axons)
Schwann cell	PNS	Myelination (1 axon), regeneration
Microglia	CNS	Immune surveillance (resident macrophage)
Ependymal cell	CNS (ventricles)	CSF production (choroid plexus)

Neurons

Criterion	Types
Function	Sensory (afferent), Motor (efferent), Interneuron
Polarity	Unipolar (sensory), Bipolar (retina), Multipolar (motor)
Neurotransmitter	Glutamate (excitatory), GABA (inhibitory), Dopamine, ACh, Serotonin, etc.

Structure: Soma (cell body) → Dendrites (input) → Axon hillock → Axon (myelinated/unmyelinated) → Axon terminals (synaptic boutons) → Synapse

Synapse types:

Type	Description
Chemical	Neurotransmitter release (most common)
Electrical	Gap junctions (fast, bidirectional)
Neuromuscular junction	Motor neuron → skeletal muscle (ACh, nicotinic)

Neuroglia Ratios

Region	Neurons	Glia	Glia:Neuron Ratio
Cerebral cortex	~16 billion	~60 billion	~3.75:1
Cerebellum	~69 billion	~16 billion	~0.23:1
Total CNS	~86 billion	~85 billion	~1:1

Tissues by Primary Germ Layer

Germ Layer	Tissues
Ectoderm	Epithelium (skin, cornea), neural tissue (neurons, glia), adrenal medulla
Mesoderm	Connective tissue (all types), muscle (all types), endothelium, mesothelium, kidney, gonads
Endoderm	Epithelium (GI tract, respiratory tract, liver, pancreas, thyroid, bladder)