Tissues There are 4 primary tissue types in the human body: 1. Epithelial (covering/lining) 2. Connective (support) 3. Muscle (movement) 4. Nervous (control)
Epithelium
Epithelial Tissue Covers the surface of the body, lines the body cavities, forms the external and internal linings of most organs, and constitutes the bulk of most glands. Functions include: 1. Protection 4. Absorption 2. Secretion 5. Filtration 3. Sensory reception
General Features: - Maximum cell-to-cell contact. - Minimum extracellular material. - Cell junctions: Several types of junctional unite (tight junctions, desmosomes and gap junctions). - Supported by basement membrane (basal lamella). - Avascular: There are no blood vessels within the epithelial layer. - Derived from all embryonic germ layers, including endoderm, mesoderm and ectoderm
With the exception of endocrine glands All epithelia have one free surface, called the apical surface, which is exposed at the body surface or at the lumen of the body cavity, duct, tube or vessel. The basal surface of epithelium rests on a basement membrane: a non-living adhesive material secreted by the epithelium and the underlying connective tissue. These cells are often characterized by frequent cell division because they are exposed to wear and tear and injury, necessitating replacement.
How to classify epithelia 1- Number of cell layers present one layer of cells =simple. more than one layer = stratified.
2- Shape of the cells in the apical layer. There are 3 cell shapes: 1.Squamous = flat, scale-like 2.Cuboidal = cube-shaped 3.Columnar = column-shaped
Types of epithelium tissues 1- simple - Simple squamous: lining blood vessels, lung alveoli. - Simple cuboidal: liver, acini of glands, lining ducts. - Simple columnar: stomach, intestine, gall bladder - Simple columnar ciliated: uterus, fallopian tube - Pseudostratified columnar: Respiratory mucosa; trachea
2- Stratified - Stratified squamous keratinized: skin - Stratified squamous non-keratinized: mucus membranes - Stratified cuboidal: lining large ducts of salivary glands - Stratified columnar: male urethra, fornix of conjunctiva - Stratified columnar ciliated: soft palate - Transitional epithelium: urinary bladder
Simple Squamous Epithelium Appearance: Single layer of flattened cells. Functions: Thin = rapid diffusion or transportation Friction Reduction Filtration Secretion Location Alveoli of lungs Kidneys Lining of visceral organs and all blood vessels.
Stratified Squamous Epithelium Microscopic Appearance: Multiple cell layers with cells becoming flatter toward surface. keratinized stratified Dead cells at surface Nonkeratinized stratified squamous No dead cells at surface This is keratinized stratified squamous epithelium from the sole of the foot. It extends the length of the blue line on the left. Notice the multiple layers of clear, dead, enucleated cells.
Location of keratinized version: Epidermis of skin. Locations of non-keratinized version: Lining of oral cavity, tongue, esophagus, vagina, anal cavity
Which of these is keratinized and which is non-keratinized stratified squamous epithelium?
Simple Cuboidal Epithelium Appearance: Single layer of square or round cells. Centrally-placed spherical nuclei. Locations: Liver, Pancreas, Glands, Bronchioles Functions: Absorption Secretion
Simple Cuboidal Epithelium Cross-Section of a kidney tubule
Stratified Cuboidal Epithelium Microscopic Appearance: 2 or more layers of cells. Surface layers are square or round (cuboidal). Locations: Some sweat gland ducts. Ovarian follicle Functions Cells that surround egg PROTECTION Secretion = Sweat and Hormones
Simple Columnar Epithelium Appearance: Single layer of tall, narrow cells. Oval basal nuclei. Locations: lining of stomach, intestines, gall bladder, uterus, and fallopian tubes Functions: Absorption and secretion Small intestine Movement of egg and embryo in uterine tube Fallopian tubes. Secretion of mucus. Large Intestine
Stratified Columnar Epithelium Microscopic Appearance: 2 or more layers of cells. Surface cells tall and narrow with basal nuclei. Locations: Rare. Small portions of anal canal, pharynx, epiglottis, and male urethra. Sometimes seen in large ducts of sweat and salivary glands. Functions: Often seen where 2 other tissue types meet Structural integrity of gland ducts
Pseudostratified Columnar Epithelium Appearance: Mixture of Cells Nuclei are at several levels, cilia, goblet cells Functions: Locations: Trap dust, secrete mucus Respiratory tract from nasal cavity to bronchi.
Appearance: Layered Transitional Epithelium Typically 5-6 cell layers thick when relaxed and 2-3 cell layers thick when stretched. Some cells are binucleate, i.e., they have 2 nuclei. Locations: urinary tract, kidney, urethra Functions: Expand and recoil
Cell polarity or Surface modifications 1- Surface or luminal modifications - Cilia and flagella: especially pseudostratified and columnar epithelia - Microvilli (brush border or striated border): proximal convoluted tubule of the kidney and small intestine. - Stereocilia (long microvillus): in male reproductive tract (epididymis); facilitate absorptive processes
2- Lateral specializations (junctions and folding): - Zonula occludens - Zonula adherens - Desmosomes - Junctional complex: combination of zonula occludens, zonula adherens and desmosomes - Communicating junctions (gap or nexus junctions): contain hundreds of tiny pores (< 2 nm) through connexon complex, permitting passage nutrients).
3- Basal specialization - Folding: ion transport - Hemidesmosome: resemble desmosome but do not occur in pairs.
Glandular epithelium - Generally formed by down growth of surface epithelium into underlying connective tissue, and separated from connective tissue by basal lamina.
Classification of glands: 1- According to presence or absence of ducts: - Exocrine glands (have duct system) - Endocrine glands (ductless) secrete hormones glands.
2- According to Nature of secretion: - Serous secretion: secret watery fluid rich in protein (parotid glands) - Mucous secretion: secret mucus; poor in protein (goblet cells) - Muco-serus secretion: as in mixed salivary glands - Milky secretion: mammary gland - Wax secretion: glands in external ear - Fatty secretion: sebaceous glands - Watery secretion: sweat glands - Cellular secretion: ovary and testis.
3- According to mode of secretion: - Apocrine glands: a small portion of the apical cytoplasm discharged with the secretory products. e.g. Mammary glands and some sweat glands. - Holocrine glands: discharge whole cell; sebaceous glands (sebum). - Merocrine glands: in which secretion occurs by exocytosis; i.e. no cellular changes as parotid glands.
4- According to Number of cells: - Unicellular glands (goblet cells) secrete mucus. - Multicellular glands that can be further classified according to the shape of secretory portion into: -
A- Simple tubular gland: large intestine. B- Simple coiled tubular gland: sweat glands. C- Simple acinar (alveolar) gland: (rounded secretory unit) mucus-secreting glands of penile urethra D- Simple branched tubular gland: stomach. E- Simple branched acinar gland: sebaceous gland F- Simple branched tubulo-alveolar glands: glands of oral cavity. G- Compound tubular gland: liver, kidney H- Compound acinar (alveolar) gland: mammary gland I- Compound tubulo-acinar gland: pancreas