Epithelial and Connective Tissues & The Integumentary System

Epithelial Tissue

Epithelial tissue is a fundamental tissue type that covers all body surfaces and organs, both internally and externally, and forms glands. It plays crucial roles in absorption, filtration, protection, secretion, and sensory reception.

• Key Characteristics:

  • Cells are tightly packed, forming continuous layers and sheets.

  • Apical surface: The free or exposed surface of the cell.

  • Basal surface: The surface attached to the basement membrane, opposite the apical surface.

  • Avascular: Epithelial tissue lacks a direct blood supply.

• Naming Criteria:

  • Number of layers:

    • Simple: One layer of cells.

    • Stratified: More than one layer.

    • Pseudostratified: Appears layered due to cell nuclei at different heights, but all cells touch the basement membrane.

  • Shape of cells:

    • Squamous: Flat, thin, scale-like cells.

    • Cuboidal: Cube-shaped cells, similar in height and width.

    • Columnar: Taller than wide, column-like cells.

Example: The lining of the alveoli in the lungs is composed of simple squamous epithelium, facilitating gas exchange.

Types of Epithelium

• Simple Squamous Epithelium:

  • Single layer of flat cells.

  • Locations: Kidney glomeruli, alveoli of lungs, lining of heart, blood vessels (endothelium), lymphatic vessels, serous membranes.

  • Function: Diffusion and filtration.

• Simple Cuboidal Epithelium:

  • Single layer of cube-shaped cells.

  • Locations: Kidney tubules, liver cells, small ducts and glands.

  • Function: Secretion and absorption.

• Simple Columnar Epithelium:

  • Single layer of column-shaped cells.

  • Locations: Lining of digestive tract, gallbladder, ducts of large glands, small bronchi, uterine tubes.

  • Specializations: Microvilli (increase absorption), cilia (move substances), goblet cells (secrete mucus).

• Stratified Squamous Epithelium:

  • Multiple layers of flat cells; deeper layers may appear cuboidal.

  • Keratinized: Surface layers are dead and filled with keratin (e.g., epidermis).

  • Non-keratinized: Surface layers are alive (e.g., esophagus, mouth, vagina).

  • Function: Protection against abrasion.

• Pseudostratified Columnar Epithelium:

  • Single layer of columnar cells of varying heights, giving a stratified appearance.

  • Locations: Respiratory tract, male reproductive ducts, ducts of large glands.

  • Specializations: Cilia, goblet cells.

• Transitional Epithelium:

  • Stratified tissue with large, rounded cells in the apical layers.

  • Locations: Urinary bladder, ureters, urethra.

  • Function: Allows stretching and recoiling.

Connective Tissue

Connective tissue is the most abundant tissue type in the body, serving to bind, support, protect, and fill spaces. It contains fewer cells than epithelial tissue, and the cells are separated by an extracellular matrix composed of fibers and ground substance.

• Matrix: The space between cells, containing collagen, elastic, and reticular fibers.

• Main cell types: Fibroblasts, mast cells, macrophages, adipocytes, chondrocytes, osteocytes.

Major Classifications of Connective Tissue

• Areolar (Loose) Connective Tissue:

  • Contains collagen and elastic fibers, loosely arranged.

  • Cells: Fibroblasts, mast cells, macrophages.

  • Locations: Papillary layer of dermis, around capillaries, between organs.

  • Function: Cushions organs, provides support but allows movement.

• Dense Connective Tissue:

  • Dense Regular: Parallel bundles of collagen fibers; strength in one direction (e.g., tendons, ligaments).

  • Dense Irregular: Random arrangement of collagen fibers; strength in all directions (e.g., reticular layer of dermis, fibrous capsules).

• Reticular Connective Tissue:

  • Matrix consists of reticular fibers; main cell is reticular cell.

  • Locations: Lymph nodes, spleen, bone marrow.

  • Function: Forms supportive framework for soft organs.

• Adipose Tissue:

  • Composed of adipocytes that store lipids (fat).

  • Function: Padding, insulation, energy storage.

  • Locations: Subcutaneous layer, around kidneys, heart, breasts, abdominal cavity, joints.

• Cartilage:

  • Avascular; cells are chondrocytes in lacunae.

  • Types:

    • Hyaline Cartilage: Most common; found at ends of long bones, nose, larynx, trachea.

    • Elastic Cartilage: More elastic fibers; found in epiglottis, pinna of ear.

    • Fibrocartilage: Good shock absorber; found in intervertebral discs, menisci, pubic symphysis.

• Compact Bone:

  • Calcified collagen fiber matrix; cells are osteocytes in lacunae.

  • Composed of osteons; found in bones of the skeleton.

  • Function: Support, protection, mineral storage.

• Blood:

  • Watery matrix called plasma; no fibers.

  • Cells: Red blood cells (erythrocytes), white blood cells (leukocytes), platelets.

  • Function: Transport of gases, nutrients, wastes; immune defense.

  • Location: Within blood vessels.

The Integumentary System

The integumentary system consists of the skin and its accessory organs, including sweat glands, sebaceous glands, hair, sensory receptors, and nails. The hypodermis (subcutaneous layer) is not technically part of the skin but separates it from deeper tissues and stores energy in its adipose tissue.

• Main Functions:

  • Acts as a two-way barrier.

  • Regulates body temperature.

  • Provides sensory input.

  • Excretes waste products.

  • Synthesizes vitamin D.

Layers of the Skin

• Epidermis:

  • Superficial layer; composed of keratinized stratified squamous epithelium.

  • Avascular; has 5 layers in thick skin, 4 layers in thin skin.

  • Thick skin: Found on soles of feet and palms of hands; contains stratum lucidum.

  • Thin skin: Covers the rest of the body; lacks stratum lucidum.

  • Main cell type: Keratinocytes (produce keratin).

• Dermis:

  • Deeper layer; highly vascular.

  • Contains glands, hair follicles, sensory receptors.

  • Has two layers: papillary (areolar connective tissue) and reticular (dense irregular connective tissue).

• Hypodermis (Subcutaneous Layer):

  • Deep to the dermis; composed of adipose tissue.

  • Functions in insulation and energy storage.

Accessory Structures

• Sweat Glands:

  • Merocrine (Eccrine): Most common; secrete watery sweat for temperature regulation.

  • Apocrine: Larger; secrete thicker substance responsible for body odor; found in axilla and genital regions.

• Sebaceous Glands:

  • Secrete oily sebum; lubricates skin and hair.

  • Secrete directly into hair follicles.

• Hair:

  • Present on most body surfaces except soles, palms, sides of phalanges, and parts of external genitalia.

  • Produced at the base of the follicle; cells become keratinized as they move toward the surface.

  • Arrector pili muscle: Smooth muscle attached to hair follicle; contraction causes hair to stand up (goosebumps).

• Nails:

  • Composed of dead, keratinized cells.

  • Main parts: nail plate, lateral nail fold, lunula, cuticle, nail root, free edge.

Table: Types of Epithelial Tissue

Table: Types of Connective Tissue

Additional info:

• Keratin is a tough, fibrous protein that provides protection and waterproofing to the skin.

• Collagen fibers provide tensile strength, while elastic fibers allow tissues to stretch and recoil.

• Stratum basale is the deepest layer of the epidermis where new keratinocytes are produced.

• Merocrine (eccrine) sweat glands are important for thermoregulation.