ANIMAL TISSUES AND CELLS
The animal cells are grouped together to form animal tissues. These tissues vary in their
structure, function, and origin. The animal tissues are divided into epithelial, connective,
muscular and nervous tissues.
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1. Epithelial tissue covers body surfaces and lines body cavities. Epithelial tissue is made of
closely-packed cells arranged in flat sheets. Epithelia form the surface of the skin, line
the various cavities and tubes of the body, and cover the internal organs. Epithelia that
form the interface between the internal and external environments. Skin as well as the
lining of the mouth and nasal cavity.
These are derived from ectoderm. Inner lining of the GI tract, lungs, urinary bladder,
exocrine glands, vagina and more. These are derived from endoderm.
The apical surface of these epithelial cells is exposed to the "external environment", the
lumen of the organ or the air.
Mesothelia. These are derived from mesoderm.
o pleura — the outer covering of the lungs and the inner lining of the thoracic (chest)
cavity.
o peritoneum — the outer covering of all the abdominal organs and the inner lining of
the abdominal cavity.
o pericardium — the outer lining of the heart.
Endothelia. These are derived from mesoderm. The inner lining of the heart, all blood and
lymphatic vessels.
The basolateral surface of all epithelia is exposed to the internal environment extracellular fluid (ECF). The entire sheet of epithelial cells is attached to a layer of
extracellular matrix that is called the basement membrane or, better (because it is not a
membrane in the biological sense), the basal lamina.
The function of epithelia always reflects the fact that they are boundaries between
masses of cells and a cavity or space.
Three types of epithelium occur:
 Squamous epithelium is flattened cells that allows the movement of materials
and nutrients through the cells, that is, passive diffusion.
 Cuboidal epithelium is cube-shaped cells located in the pancreas and kidney
tubules, they help in absorption of nutrients as well as secretion of hormones,
sweat, wax, digestive enzymes and even milk
 Columnar epithelium consists of elongated cells. The function of the columnar
epithelial cells is secretion and absorption of nutrients. The columnar epithelial
cells in the intestine contain microvilli that helps in increasing the surface are for
absorption.
2. Connective tissue serves many purposes in the body. The cells of connective tissue are
embedded in a great amount of extracellular material. This matrix is secreted by the
cells. It consists of protein fibers embedded in an amorphous mixture of proteinpolysaccharide ("proteoglycan") molecules. Functions include binding, supporting,
protecting, forming blood, storing fats and filling space.
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Bone is the strongest connective tissue with little ground substance, hard matrix
of calcium and phosphorous and specialized bone cells called osteocytes. It
provides protection to internal organs and supports the body.
Dense connective tissue is often called fibrous connective tissue and
include Tendons and Ligaments. Tendons connect muscle to bone with a The
matrix is principally Type I collagen, and the fibers are all oriented parallel to
each other. Tendons are strong but not elastic. Ligaments attach one bone to
another and contain both collagen and also the protein elastin. Elastin permits
ligaments to be stretched.
Loose connective tissue is found between many organs where it acts both to
absorb shock and bind tissues together. It allows water, salts, and various
nutrients to diffuse through to adjacent or imbedded cells and tissues. Loose
connective tissue includes adipose tissue, areolar tissue and reticular tissue.
 Adipose tissue consists mostly of fat storage cells called adipocytes.
 Areolar tissue shows little specialization. It contains all the cell types and
fibers and is distributed in a random, web-like fashion. It fills the spaces
between muscle fibers, surrounds blood and lymph vessels, and
supports organs in the abdominal cavity.
 Reticular tissue is a mesh-like, supportive framework for soft organs such
as lymphatic tissue, the spleen, and the liver. Reticular cells produce the
reticular fibers that form the network onto which other cells attach.
Blood is a connective tissue of cells separated by a liquid (plasma) matrix. Two
types of cells occur. Red blood cells (erythrocytes) carry oxygen. White blood
cells (leukocytes) function in the immune system. Plasma transports dissolved
glucose, wastes, carbon dioxide and hormones, as well as regulating the water
balance for the blood cells. Platelets are cell fragments that function in blood
clotting
3. Muscle tissue facilitates movement of the animal by contraction of individual muscle
cells (referred to as muscle fibers).
 Skeletal (striated) muscle fibers have alternating bands perpendicular to the long
axis of the cell. These cells function in conjunction with the skeletal system for
voluntary muscle movements.
 Smooth muscle fibers function in involuntary movements and/or autonomic
responses (such as breathing, secretion, ejaculation, birth, and certain reflexes).
These fibers are components of structures in the digestive system, reproductive
tract, and blood vessels.
 Cardiac muscle fibers are a type of striated muscle found only in the heart. The
cell has a bifurcated (or forked) shape, usually with the nucleus near the center
of the cell. The cells are usually connected to each other by intercalated disks.
The functions of the cells within the heart occur as part of the autonomic
nervous system. This system controls organs, like the heart, that work
involuntarily, which means without active control from the brain.
4. Nervous tissue is composed of nerve cells called neurons and glial cells. Neurons are
specialized for the conduction of nerve impulses; a typical neuron consists of a cell
body which contains the nucleus; a number of short fibers — dendrites — extending
from the cell body and a single long fiber, the axon. The nerve impulse is conducted
along the axon. The tips of axons meet other neurons at junctions called synapses,
muscles (called neuromuscular junctions) and glands.
Glia
Glial cells surround neurons. Once thought to be simply support for neurons (glia = glue), they turn
out to serve several important functions. There are three types:
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Schwann cells. These produce the myelin sheath that surrounds many axons in the
peripheral nervous system.
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Oligodendrocytes. These produce the myelin sheath that surrounds many axons in the
central nervous system (brain and spinal cord).
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Astrocytes. These, often star-shaped cells are clustered around synapses and the nodes of
Ranvier where they perform a variety of functions such as:
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modulating the activity of neurons
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supplying neurons with materials (e.g. glucose and lactate) as well as some signaling
molecules
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regulating the flow of blood to their region of the brain. It is primarily the metabolic
activity of astrocytes that is being measured in brain imaging by positron-emission
tomography (PET) and functional magnetic resonance imaging (fMRI).
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pruning away (by phagocytosis) weak synapses