Cellular Organization and Tissue
Classification
Cells exhibit a high degree of
organization. However, in multicellular organisms, they do not function in
isolation. Instead, they work collectively in groups of similar cells known as
tissues. A tissue is a collection of similar cells and their intercellular
matrix that share a common embryonic origin and collaborate to perform
specialized functions. The scientific study of tissues is known as Histology.
The human body contains four primary
types of tissues, classified based on their function and structure: epithelial,
connective, muscular, and nervous tissues.
Epithelial
Tissue
Epithelial tissue covers the body's
surfaces, lines its cavities and ducts, and forms glands. It is further divided
into:
- Covering and Lining Epithelium
- Glandular Epithelium
Covering
and Lining Epithelium
This type of epithelium forms the
outer surface of the body and the external surface of some internal organs. It
also lines body cavities, the respiratory and gastrointestinal tracts, blood
vessels, and ducts. In coordination with nervous tissue, it contributes to
sensory organs and serves as the source of gametes (egg and sperm).
Classification of Covering and
Lining Epithelium:
- Based on Layer Arrangement:
- Simple Epithelium:
A single layer, specialized for absorption and minimal wear and tear.
- Stratified Epithelium: Multiple layers found in areas experiencing high
wear and tear.
- Pseudostratified Epithelium: Appears multilayered but is actually a single layer.
- Based on Cell Shape:
- Squamous:
Flat and scale-like.
- Cuboidal:
Cube-shaped.
- Columnar:
Tall and cylindrical.
- Transitional:
Adaptable shape, found in distensible areas.
Simple
Epithelium
- Simple Squamous Epithelium: Facilitates diffusion and filtration; found in air
sacs of lungs, kidneys, blood vessels, and lymph vessels.
- Simple Cuboidal Epithelium: Covers ovaries, lens of the eye, retina, and kidney
tubules.
- Simple Columnar Epithelium: Lines the gastrointestinal tract, gallbladder, and
excretory ducts of glands; functions in secretion, absorption, protection,
and lubrication.
Stratified
Epithelium
- Stratified Squamous Epithelium: Exists in two forms—keratinized (skin) and
non-keratinized (mouth, tongue, vagina).
- Stratified Cuboidal Epithelium: Found in sweat gland ducts, eye conjunctiva, and male
urogenital system.
- Stratified Columnar Epithelium: Occurs in mammary gland ducts and the anus; functions
in protection and secretion.
Transitional
Epithelium
Specialized for stretching; found in
the urinary bladder, ureters, and urethra.
Pseudostratified
Epithelium
Lines excretory ducts, epididymis,
parts of the male urethra, and auditory tubes; functions in protection and
secretion.
Glandular
Epithelium
Specialized in secretion, glands can
be:
- Exocrine:
Secrete into ducts (e.g., sweat and salivary glands).
- Endocrine:
Secrete into the bloodstream (e.g., pituitary, thyroid, and adrenal
glands).
Connective
Tissue
Connective tissue is classified into
embryonic and adult types.
- Embryonic Connective Tissue:
- Mesenchyme
- Mucous Connective Tissue
- Adult Connective Tissue:
- Connective Tissue Proper: Includes loose (areolar), adipose, dense
(collagenous), elastic, and reticular connective tissues.
- Cartilage:
Includes hyaline, fibrocartilage, and elastic cartilage, providing
structural support.
- Osseous Tissue (Bone): Composed of osteocytes embedded in a mineral-rich
matrix, forming the skeletal system.
- Vascular Tissue (Blood): A fluid connective tissue containing plasma,
erythrocytes, leukocytes, and thrombocytes, playing a role in transport
and defense.
Muscle
Tissue
Muscle tissue consists of
specialized cells responsible for motion, posture, and heat production. It is
classified into:
- Skeletal Muscle:
Attached to bones, voluntary, striated, and multinucleated.
- Cardiac Muscle:
Located in the heart, involuntary, striated, and uninucleated.
- Smooth Muscle:
Found in hollow internal structures, involuntary, and non-striated.
Nervous
Tissue
Nervous tissue consists of:
- Neurons (Nerve Cells):
Convert stimuli into nerve impulses and consist of cell bodies, axons, and
dendrites.
- Neuroglia (Supporting Cells): Provide protection and nourishment to neurons.
Membranes
Membranes are thin, flexible layers
of epithelial and connective tissues. They are classified into three types:
- Mucous Membranes:
Line cavities that open to the exterior and secrete mucus.
- Serous Membranes:
Cover organs within body cavities that do not open to the exterior,
secreting lubricating fluid.
- Synovial Membranes:
Found in freely movable joints, secreting synovial fluid for lubrication.
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