Tissues are groups of similar cells, and their products, closely related in
structure (built together) and function (working together).
All animals and plants have cells organized into tissues.
In animals, the four major tissue types are epithelium, connective tissue,
muscle tissue, and nervous tissue.
- Epithelial tissues are tissues that originate
in broad, flat surfaces. Functions include protection, absorption,
and secretion.
- Simple epithelia are one cell layer thick. They may be:
- Squamous, made of flat cells (e.g., lining of capillaries),
- Cuboidal, cells equal in height and width, or
- Columnar, cells much taller than wide, e.g., inner lining of intestine
- Stratified epithelia (many cell layers thick) may be:
- Squamous (surface cells flat, e.g., epidermis of skin)
- Cuboidal (surface cells equal in height and width), or
- Columnar (surface cells tall and skinny).
- Glandular epithelia (highly modified for secretion) may be:
- Exocrine (secretions exit by ducts to targets nearby), or
- Endocrine (secretions carried by bloodstream; targets may be
far away).
- Connective tissues: tissues with much extracellular material (matrix)
- With liquid matrix, capable of solidifying into a clot
- Blood contains:
- Plasma: fluid matrix containing wtaer, ions, proteins, etc.
- Erythrocytes: red blood cells containing hemoglobin
- Leucocytes: white blood cells of many types
(lymphocytes, monocytes, neutrophils, basophils, eosinophils)
- Platelets: cytoplasmic fragments that aid in clotting.
- Lymph, containing leucocytes and plasma only
- With solid matrix, often with fibers of collagen and related proteins.
- Loose (areolar) connectve tissue: simplest, with few fibers
- Regular connective tissue: fibers all arranged in same
direction (as in ligaments and tendons)
- Irregular connective tissue: fibers arranged in all directions
(as in dermis of skin)
- Adipose (fat) tissue: cells filled with fat deposits
- Hemopoietic (blood-forming) tisue: contains precursors
to many connective tissue cells (e.g., bone marrow)
- Cartilage: contains shock-resistant complex of protein
and sugar-like (polysaccharide) molecules
- Bone: strong tissue containing calcium salts
- Muscle tissues, specialized for contraction
- Smooth muscle: involuntary cells with tapering ends but no
cross-banding; smooth, rhythmic contractions; nuclei located centrally;
occurs in digestive organs, reproductive organs, etc.
- Cardiac muscle: involuntary fibers with cross-striations;
cylindrical in shape but branching and coming together repeatedly;
nuclei located centrally; cell boundaries marked by intercalated disks;
rhythmic contractions; occurs in heart only
- Skeletal muscle: voluntary cells with cylindrical shape;
cross-striations caused by alignment of actin and myosin fibers;
many nuclei per fiber; no cell boundaries (each fiber is thus called a
syncytium); rapid, forceful contractions, but fatigues easily;
occurs in muscles; always attaches to connective tissues
- Nervous tissues: contain specialized nerve cells (neurons)
- Neurons: respond to stimuli by conducting impulses
- Neuroglia (several types): hold nervous tissue together
- Plant tissue types (very different from animals):
- Epidermis: arranged in flat surfaces; usually has a protective function
- Meristem: embryonic tissue that continues growing throughout life
- Supporting tissues: hold up the plant and give it shape
- Parenchyma, the simplest and most abundant type of supporting tissue,
made of thin-walled, equidimensional cells (no longer than wide) that
remain living
- Collenchyma, consisting of living cells, somewhat elongated, and
strengthened with strips of thickened cell walls along the edges
- Sclerenchyma, consisting of elongated cells with tapered ends.
These specialized cells die, leaving narrow cavities surrounded by
greatly thickened cell walls. Ropes are made from this material.
- Vascular (conducting) tissues: transport liquids and help in support
- Xylem (wood): generally carries fluid upwards
- Phloem: carries photosynthetic products downwards,
but also sometimes upwards
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