Overview of Tissues- Chapter 4
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Tissue: a group of cells that usually have a common embryonic origin and function together to carry out
specialized activities.
o Similar cells are put together into tissues. Various tissues are put together to make organs and
other body parts.
There are four main types of tissues in our body: epithelial, connective, muscle, and nervous tissue. (We
will learn a little about muscle tissue and nervous tissue but will learn much more about those in future
units. So we will focus more on epithelial and connective tissue right now).
Cell Junctions
 Cells are held together with various cell junctions: contact points between the cell membranes of
tissue cells.
 The five types are:
o Tight junctions
o Adherens junctions
o Desmosomes
o Hemidesmosomes
o Gap junctions
Epithelial Tissue
 Also called epithelium.
 Definition: cells arranged in continuous sheets, in either single or multiple layers.
 There are four sides of an epithelial cell: apical surface, basal surface, and lateral surfaces.
 Epithelial cells attach to the basement membrane, which is deep to the cells.
o Basement membrane: thin extracellular layer that commonly consists of two layers, the
basal lamina and the reticular lamina.
o The basal lamina is closer to the cells (superficial) and the reticular lamina is closer to the
connective tissue that is underneath (deep) to the epithelium.
 There are two main types of epithelial tissue: covering and lining epithelium and glandular
epithelium.
Covering and Lining Epithelium
 There are two characteristics that classify covering and lining epithelium:
o Arrangement of cells in layers
 Simple, stratified, pseudostratified
o Cell shapes
 Squamous, cuboidal, columnar, transitional
 By putting these two characteristics together, there are 8 unique types of epithelium in
our body:
o Simple squamous epithelium
o Simple cuboidal epithelium
o Simple columnar epithelium (nonciliated and ciliated)
o Pseudostratified columnar epithelium (nonciliated and ciliated)
o Stratified squamous epithelium
o Stratified cuboidal epithelium
o Stratified columnar epithelium
o Transitional epithelium
 Each of these unique types of epithelium has special functions, characteristics, and
locations in the body. These functions and locations are based on what the arrangement
and cell shapes are of each type.
Glandular Epithelium
 The main function of glandular epithelium is secretion. Therefore, it will be found
wherever molecules are secreted.
 Glandular epithelium forms glands: a single cell or a group of cells that secrete
substances into ducts (tubes), onto a surface, or into the blood.
 Glands are found scattered throughout covering and lining epithelium, so you won’t see a
large “chunk” or section of glandular epithelium, like you do with covering and lining
epithelium.
 There are two main types of glands:
o Endocrine glands- the secretions enter the interstitial fluid and then diffuse
directly into the bloodstream without flowing through a duct. These secretions are
mainly hormones.
o Exocrine glands- the secretions empty into the ducts of the gland and then empty
onto the surface of a covering and lining epithelium such as skin or inside a
hollow organ.
 Exocrine glands can be functionally classified into 3 types:
 Merocrine glands
 Apocrine glands
 Holocrine glands
o The difference between the types is how the secretions are
released.
 Exocrine glands can be structurally classified by their shape and structure.
 They can be unicellular or multicellular.
 Multicellular exocrine glands are categorized by two
characteristics: whether their ducts are branched or unbranched and
the shape of their secretory parts.
 Ducts: can be simple or compound.
 Shape: can be tubular or acinar.
 If you put all these characteristics together, there are 8 types of
multicellular exocrine glands:
o Simple tubular
o Simple branched tubular
o Simple coiled tubular
o Simple acinar
o Simple branched acinar
o Compound tubular
o Compound acinar
o Compound tubuloacinar
Connective Tissue
 Connective tissue is the most abundant type of tissue in our body- you pretty much find it
everywhere!
 The variety of functions of connective tissue include:
o It binds together, supports, and strengthens other body tissues.
o It protects and insulates internal organs.
o It compartmentalizes structures such as skeletal muscles.
o It serves as the major transport system within the body (through blood and lymph).
o It is the primary location of stored energy reserves (in adipose or fat tissue).
o It is the main source of immune responses.
 Connective tissue is made of two main components: extracellular matrix and various cells.
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Extracellular matrix:
o Definition: the material located between the widely spaced cells.
o The extracellular matrix is secreted by the cells in the connective tissue and determines
the tissue’s qualities.
o It consists of protein fibers and ground substance.
 Protein fibers are various filaments made of different proteins.
 The ground substance is the material between the cells and the fibers.
o So: if you put all this together, there are 3 things in connective tissue: ground substance
(the “filler” material), various protein fibers, and cells.
 Ground substance: may be fluid, semifluid, gelatinous (like jell-o), or calcified
(solid, like bone). The function is to support cells, bind them together, store water,
and provide a medium (material) through which substances are exchanged
between the blood and cells. It also plays an active role in how tissues develop,
migrate (move), proliferate (divide), change shape, and in how they carry out their
metabolic functions. Ground substance itself contains water and an assortment of
large organic molecules (proteins, carbohydrates, etc.).
 Fibers: The function of the protein fibers is to strengthen and support connective
tissue. There are 3 types:
 Collagen fibers
 Elastic fibers
 Reticular fibers
Cells- there are 6 types of cells in connective tissue:
o Fibroblasts (the suffix “blast” indicates it is an immature cell that can do cell division)
o Adipocytes (the suffix “cyte” indicates it is a mature that does not generally divide)
o Mast cells
o White blood cells
o Macrophages
o Plasma cells
Classification of Connective Tissue
 Embryonic connective tissue: only found in embryos and fetuses, as they are developing.
o Mesenchyme
 Mature connective tissue: found in newborns and older ages. There are 5 types and 11
subtypes.
o Loose Connective Tissue: fibers are loosely arranged between cells.
 Areolar connective tissue
 Adipose connective tissue
 Reticular connective tissue
o Dense Connective Tissue: contains more numerous, thicker, and denser fibers but
fewer cells.
 Dense regular connective tissue
 Dense irregular connective tissue
 Elastic connective tissue
o Cartilage: is a dense network of collagen fibers or elastic fibers embedded in a
gel-type ground substance.
 Hyaline cartilage
 Fibrocartilage (or fibrous cartilage)
 Elastic cartilage
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Extra info: the mature cells of cartilage are called chondrocytes. They are found
in open spaces called lacunae in the extracellular matrix. These lacunae and
chondrocytes can be easily seen in cartilage tissue.
o Bone Tissue- has a calcified (hard) ground substance
o Liquid Connective Tissue- has liquid extracellular matrix with suspended cells
 Blood tissue
 Lymph
Muscular Tissue
 Consists of elongated cells called muscle fibers that can use ATP (energy molecule) to generate
force. Because of this, the functions are to produce body movements, maintain posture, generate
heat, and provide protection.
 There are 3 types of muscular tissue:
o Skeletal muscle tissue: attached to bones. Characteristics are striations (“lines” or
alternating light and dark bands within the fibers) and it is voluntarily moved.
o Cardiac muscle tissue: forms the wall of the heart. Characteristics are striations and is
involuntarily moved.
o Smooth muscle tissue: located in the walls of hollow internal structures such as blood
vessels, airways to the lungs, and parts of the gastrointestinal tract (stomach, intestines,
etc.). Characteristics are nonstriated (no striations) and involuntarily moved.
Nervous Tissue
 Nerve cells have the ability to respond to and send electrical signals called action potentials.
 There are two main types: neurons (the main cells that send/receive the messages) and neuroglia
(used for support for neurons but do not send/receive messages).
 The structure of neurons is very unique and they contain dendrites, a cell body, and an axon.
Membranes
 Membranes are flat sheets of pliable tissue that cover or line a part of the body.
 There are two main types of membranes: epithelial membranes and synovial membranes.
o Epithelial membranes: are made from an epithelial tissue layer and a connective tissue layer.
 Three types of epithelial membranes:
 Mucous membrane: lines a body cavity that opens directly to the exterior.
 Serous membrane: lines a body cavity that does not open directly to the exterior
and covers the organs that lie within the cavity.
o Two types- parietal layer: lines the cavity wall; visceral layer: covers the
organs.
o Many of the visceral layers have their own unique names, based on where
they are found.
 Pleura- lines the thoracic cavity and covers the lungs.
 Pericardium- lines the heart cavity and covers the heart.
 Peritoneum- lines the abdominal cavity and covers the abdominal
organs.
 Cutaneous membrane: also known as skin, covers the entire surface of the body.
o Synovial membranes: function is to line joints, is made from connective tissue only (no epithelial
tissue). They line structures that do not open to the exterior.