Epithelial and connective tissue
Epithelium forms the body's surfaces and glands, and connective tissue holds everything together, from bone to blood.
Features of epithelium
Epithelial tissue covers surfaces, lines cavities and tubes, and forms glands. Its cells are tightly packed, sit on a basement membrane, have a free (apical) surface, and lack blood vessels, getting nutrients by diffusion from underlying tissue. Epithelium regenerates quickly, which matters because it suffers constant wear.
Classifying epithelium
Epithelium is named by layers and cell shape. Simple means one layer; stratified means several. Squamous cells are flat, cuboidal are cube-shaped, columnar are tall. Simple squamous epithelium lines the air sacs of the lungs and blood vessels, allowing rapid diffusion; stratified squamous forms the skin and lines the mouth, resisting abrasion.
Special epithelia
Simple columnar epithelium lines the stomach and intestines, often with microvilli to increase absorption. Pseudostratified ciliated columnar epithelium lines the airways, where cilia sweep mucus and trapped particles upward. Transitional epithelium lines the bladder and stretches as it fills.
Glands
Glands are epithelial structures that secrete products. Exocrine glands release secretions through ducts onto surfaces: sweat, saliva, digestive enzymes. Endocrine glands are ductless and release hormones directly into the blood. The pancreas is both, making digestive enzymes and insulin.
Connective tissue basics
Connective tissue is the most abundant tissue. It consists of cells scattered in an extracellular matrix made of protein fibres and ground substance. Collagen fibres give strength, elastic fibres allow stretch, and reticular fibres form supporting meshes. Fibroblasts are the cells that build the matrix.
Connective tissue proper
Loose (areolar) connective tissue cushions organs and holds them in place. Adipose tissue stores fat, insulates, and protects. Dense regular connective tissue, with parallel collagen fibres, forms tendons (muscle to bone) and ligaments (bone to bone). Dense irregular tissue forms the deep skin layer and resists pulling in many directions.
Cartilage, bone, and blood
Cartilage is firm but flexible, with no blood vessels, so it heals slowly. Hyaline cartilage covers joint ends; fibrocartilage forms the discs between vertebrae; elastic cartilage shapes the ear. Bone has a mineralised matrix of calcium phosphate. Blood is a fluid connective tissue with plasma as its matrix.
Tissue repair
Damaged tissue heals by regeneration, replacing it with the same tissue, or by fibrosis, filling in with scar tissue made of collagen. Epithelium and bone regenerate well; cardiac muscle and most nervous tissue do not, so damage from a heart attack leaves scar tissue that does not contract.
Key takeaways
Epithelium covers and lines, has no blood vessels, and is named by layers and shape. Exocrine glands use ducts; endocrine glands release hormones into blood. Connective tissue is cells in a matrix of fibres and ground substance. Tendons and ligaments are dense regular tissue. Cartilage, bone, and blood are specialised connective tissues. Some tissues regenerate; others scar.
Flashcard
Covers surfaces, lines cavities, forms glands
Check your understanding
Why does cartilage heal slowly after injury?