Overview
This lecture introduces histology (the study of tissues), focusing on the four main tissue types and emphasizing epithelial tissues—their structure, classification, and function.
Four Types of Tissues
- The four tissue types in the human body are epithelial, connective, muscle, and nervous tissue.
- Epithelial tissue covers body surfaces, lines cavities, forms glands, and covers organs.
- Connective tissue supports, protects, and connects other tissues and organs (e.g., bone, fat, blood).
- Muscle tissue is excitable and contracts to produce movement; includes skeletal, smooth, and cardiac types.
- Nervous tissue is excitable and transmits electrical signals (mostly in brain and spinal cord).
Tissue Origins: Embryonic Germ Layers
- All tissues and organs originate from three germ layers: ectoderm (outer), mesoderm (middle), and endoderm (inner).
- Ectoderm forms the epidermis (skin) and nervous system.
- Mesoderm forms all connective and muscle tissues, and serous membranes.
- Endoderm forms internal organ linings like respiratory and digestive systems.
- Epithelial tissues can arise from all three germ layers; other types usually come from one.
Tissue Membranes
- Four types of membranes: serous (internal cavities), mucous (open to exterior), cutaneous (skin), synovial (joints).
- Serous and mucous membranes are epithelial; synovial is connective; cutaneous is both.
- Membranes serve protective, lubricating, and lining roles throughout the body.
Epithelial Tissue Structure and Features
- Epithelial tissues are sheets of tightly packed cells with little extracellular matrix.
- They show polarity: apical (top) and basal (bottom) surfaces.
- Always sit atop a basement membrane made from both epithelial and connective tissues.
- Epithelial tissues are avascular (lack blood vessels) and highly mitotic (divide quickly).
Cell Junctions in Epithelia
- Tight junctions seal cells together near the apical surface to prevent leakage.
- Gap junctions allow direct communication between cells via protein channels.
- Desmosomes act as strong "buttons" for tissues under stress.
- Hemidesmosomes anchor epithelial cells to the basement membrane.
Classification of Epithelial Tissues
- Cell shapes: squamous (flat), cuboidal (cube-shaped), columnar (tall).
- Number of layers: simple (one layer) vs. stratified (multiple layers).
- Simple types: simple squamous (e.g., lung alveoli, blood vessels), simple cuboidal (e.g., kidney tubules), simple columnar (e.g., intestine lining), pseudostratified columnar (e.g., respiratory tract, with cilia and goblet cells).
- Stratified types: stratified squamous (skin, mouth), transitional (bladder, ureters—cells change shape as organ stretches).
Glands and Modes of Secretion
- Glands are mostly epithelial and classified as exocrine (secrete out) or endocrine (secrete into body/hormones).
- Exocrine glands have ducts and secrete products (e.g., sweat, saliva, breast milk) onto surfaces.
- Modes of exocrine secretion:
- Merocrine: product released by exocytosis (cell stays intact).
- Apocrine: apical portion of cell pinched off (e.g., breast milk).
- Holocrine: entire cell ruptures (e.g., sebaceous glands).
- Glands produce serous (watery) or mucous (thick) secretions; some, like salivary glands, can do both.
Key Terms & Definitions
- Histology — study of tissues.
- Epithelial tissue — tissue covering surfaces and lining cavities.
- Connective tissue — tissue supporting and connecting other tissues.
- Muscle tissue — tissue that contracts for movement.
- Nervous tissue — tissue that transmits electrical signals.
- Germ layer — embryonic layers (ectoderm, mesoderm, endoderm) forming all tissues.
- Basement membrane — thin layer between epithelial and connective tissue.
- Avascular — lacking blood vessels.
- Simple epithelium — single cell layer.
- Stratified epithelium — multiple cell layers.
- Cilia — hair-like structures moving substances along surfaces.
- Microvilli — projections increasing surface area for absorption.
- Goblet cell — specialized cell producing mucus.
- Exocrine gland — gland that releases secretions via ducts to body surfaces.
Action Items / Next Steps
- Review lab manual and tables outlining tissue types, locations, and features.
- Watch assigned animations on tissue structure and membrane types.
- Prepare questions for upcoming lab or lecture sessions on epithelial tissues.
- Complete Prelab 2 related to histology.