Tissue Types and Histology — Epithelium, Connective Tissue, Muscle and Nerve

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Histology is a core topic in INI CET — understanding the microscopic structure of tissues and organs is fundamental to anatomy and pathology. The four primary tissue types are epithelium, connective tissue, muscle, and nervous tissue. Focus on identifying tissue types from photomicrographs and understanding the structure-function relationships.

High-Yield Facts for INI CET:

• Simple squamous epithelium (single layer flat cells): allows diffusion; found in alveoli, glomeruli, endothelium
• Stratified squamous epithelium (multiple layers): protection; found in skin, oesophagus, vagina
• Loose areolar connective tissue: the “packing material” of the body — fills spaces between organs
• Skeletal muscle: striated, multinucleated, voluntary control; cardiac muscle: striated, single nucleus, intercalated discs; smooth muscle: non-striated, involuntary

⚡ Exam tip: Identify tissues by their structural features — number of layers, cell shape, presence/absence of striations, and the type of connective tissue they rest on. Always look at the basement membrane side (basal surface) vs free surface (apical surface).

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Epithelial Tissue:

Classification of Epithelium

Epithelium is classified by:

1. Number of layers: Simple (single layer) vs Stratified (multiple layers) vs Pseudostratified (single layer but nuclei at different levels — appears multilayered)
2. Cell shape (apical surface): Squamous (flat), Cuboidal (cube-shaped), Columnar (tall rectangular)

Simple Epithelia:

• Simple squamous: Single layer of flat cells; nuclei are central and flattened; allows rapid diffusion

  • Endothelium (blood vessels, heart): thin, smooth lining for blood flow
  • Mesothelium (peritoneum, pleura, pericardium): serous membranes
  • Alveoli of lungs: gas exchange surface
  • Glomerular capsule (Bowman’s capsule): filtration barrier

• Simple cuboidal: Single layer of cube-shaped cells with central, round nuclei

  • Thyroid follicles: hormone storage
  • Kidney tubules (proximal and distal convoluted tubules): absorption and secretion
  • Surface of ovary, lining of bronchioles
  • Choroid plexus (CSF production)

• Simple columnar: Single layer of tall rectangular cells; nucleus located basally

  • With cilia: Respiratory tract (trachea, bronchi) — ciliated columnar; goblet cells present for mucus
  • Without cilia (microvilli): GI tract (stomach, intestines) — absorptive cells with striated border (microvilli); goblet cells scattered
  • Gallbladder

Stratified Epithelia:

• Stratified squamous (keratinised): Surface cells are dead, keratinised; found in skin (epidermis)
  • Layers: stratum basale (stem cells), stratum spinosum, stratum granulosum, stratum lucidum (palms/soles), stratum corneum (keratinised)
• Stratified squamous (non-keratinised): Moist surfaces — mouth, oesophagus, vagina, cornea
• Stratified cuboidal: Rare — sweat gland ducts, large salivary ducts
• Stratified columnar: Rare — large ducts of glands, conjunctiva

Pseudostratified Columnar Epithelium:

• Single layer but nuclei at multiple levels (appears stratified)
• Usually ciliated with goblet cells
• Respiratory tract: trachea, bronchi — called “respiratory epithelium”
• Male reproductive tract: epididymis, vas deferens

Transitional Epithelium (Urothelium):

• Found in urinary tract — renal calyces, ureter, bladder, proximal urethra
• Dome-shaped surface cells (umbrella cells) that can flatten when stretched
• Multiple layers; cells vary in shape depending on distension
• Barrier against urine

Epithelial Specialisations:

• Microvilli: Finger-like projections; increase surface area; found in small intestine (striated border), proximal convoluted tubule (brush border)
• Stereocilia: Long microvilli; in epididymis and inner ear (cochlea)
• Cilia: Hair-like projections with 9+2 axoneme; found in respiratory tract, fallopian tubes
• Desmosomes: Cell-cell adhesion; spot welds; connect to intermediate filaments (keratin); numerous in cardiac muscle and stratified squamous epithelium

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Connective Tissue:

Components of Connective Tissue

1. Cells: Fibroblasts (make fibers), adipocytes (fat cells), macrophages (phagocytosis), plasma cells (antibody secretion), mast cells (histamine, heparin), leukocytes
2. Fibers:
   • Collagen fibers: Thick, eosinophilic; Type I collagen (bone, tendon, skin); Type II (cartilage); Type III (reticular fibers in organs); most abundant protein in body
   • Elastic fibers: Thin, branching; contain elastin; in lungs, arteries, skin
   • Reticular fibers: Thin, branching network; Type III collagen; in liver, spleen, lymph nodes (forming the stroma)
3. Ground substance: Gel-like matrix — proteoglycans, glycoproteins (fibronectin, laminin); gel-like consistency; permits diffusion of nutrients

Loose Connective Tissue (Areolar)

• Most widely distributed CT; “packing material” between structures
• All three fiber types present loosely arranged
• Contains fibroblasts, macrophages, mast cells
• Under skin (subcutaneous), between muscles, surrounding blood vessels and nerves

Dense Connective Tissue

• Dense regular: Parallel bundles of thick collagen fibers; tendons (muscle to bone), ligaments (bone to bone), aponeuroses; poor blood supply → slow healing
• Dense irregular: Thick collagen fibers in random arrangement; dermis of skin, submucosa of GI tract, sclera of eye; more blood supply than regular

Specialised Connective Tissue

Adipose Tissue:

• White fat: Unilocular (single large lipid droplet), nucleus pushed to periphery; energy storage, insulation, cushioning; found subcutaneously and around organs; leptin secreted by adipocytes (regulates appetite)
• Brown fat: Multilocular (multiple small lipid droplets), central nucleus; abundant in infants (between shoulder blades); thermogenesis (via uncoupling protein-1, UCP1); adults have minimal brown fat

Cartilage:

• Hyaline cartilage: Most common; glassy appearance; collagen II fibers not visible
  • Fetal skeleton (most eventually replaced by bone), articular surfaces of bones, costal cartilages, nose, larynx, trachea rings
• Elastic cartilage: Contains elastic fibers; more flexible; external ear (pinna), epiglottis
• Fibrocartilage: Thick bundles of collagen I; intervertebral discs (nucleus pulposus + annulus fibrosus), pubic symphysis, menisci of knee; avascular → poor healing

Bone (Osseous Tissue):

• Compact bone: Dense outer layer; Haversian systems (osteons) — central canal + concentric lamellae + lacunae (osteocytes in spaces) + canaliculi
• Spongy bone: Trabecular network; red marrow between trabeculae; found in epiphyses of long bones
• Bone matrix: Type I collagen + hydroxyapatite (calcium phosphate crystals)
• Cells: Osteoblasts (bone forming), Osteocytes (mature bone cells), Osteoclasts (bone resorption — derived from monocytes)

Blood:

• Connective tissue with liquid matrix (plasma)
• Formed elements: RBCs (biconcave discs, no nucleus), WBCs (neutrophils, eosinophils, basophils, monocytes, lymphocytes), platelets (cell fragments from megakaryocytes)

Muscle Tissue

Skeletal Muscle:

• Multinucleated fibers; long cylindrical cells; striated (alternating A and I bands)
• Voluntary (somatic nervous system control)
• Sarcomere structure: Z lines, A bands, I bands, H zone, M line; sliding filament theory (actin slides over myosin)
• Satellite cells: Stem cells for regeneration

Cardiac Muscle:

• Single nucleus; striated; branching cells; intercalated discs (gap junctions, desmosomes, fascia adherens)
• Involuntary (autorhythmic — pacemaker cells)
• Found only in heart (myocardium)

Smooth Muscle:

• Spindle-shaped cells; single nucleus; non-striated (no sarcomeres)
• Involuntary; arranged in sheets or bundles
• Found: Walls of hollow organs (intestine, blood vessels, uterus, bladder), iris, ciliary body

Nervous Tissue

• Neurons: Cell body (soma) + dendrites (receive signals) + axon (transmit signals); unipolar, bipolar, multipolar; synapse (chemical transmission via neurotransmitters)
• Neuroglia: CNS — astrocytes (blood-brain barrier), oligodendrocytes (myelin production), microglia (macrophages), ependymal cells; PNS — Schwann cells (myelin), satellite cells
• Myelinated axons: Saltatory conduction (faster) in CNS (oligodendrocytes) and PNS (Schwann cells); Nodes of Ranvier between myelin internodes

Synapse and Neurotransmitters:

• Presynaptic terminal → synaptic vesicle release → neurotransmitter → postsynaptic receptor → response
• Acetylcholine: Neuromuscular junction, parasympathetic
• Noradrenaline (norepinephrine): Sympathetic
• Dopamine, serotonin, GABA, glutamate: CNS