Body Tissues and Membranes

A tissue is composed of similarly specialized cells that perform a common function in the body.


Epithelial Tissue

Epithelial tissues cover the body surface, line most cavities, and form glands.

Functions include protection, absorption, secretion, and filtration.

They may be classified according to cell shape (i.e., squamous, cuboidal, or columnar) and whether or not they form layers: unstratified (simple), stratified, or pseudostratified.

Special Characteristics of Epithelium

Composed of closely packed cells with little extracellular material between. Cells secrete a thin, underlying sheet of material (primarily glycoproteins) called the basal lamina or basement membrane. The basal lamina acts both as a filter that determines which molecules can diffuse from the deeper connective tissue and as a scaffold.

Adjacent epithelial cells are bound together by specialized contacts such as desmosomes and tight junctions.

Exhibits polarity by having an apical surface (free) and a basal surface (attached).

Supported by the underlying connective tissue.

Innervated (has nerves) but avascular (blood supply is in supporting connective tissue).

Has a high regeneration capacity.

Absorptive cells may have microvilli on their apical surface to increase the absorptive surface area.

Some epithelial cells are ciliated; the cilia of respiratory epithelium moves mucus over the surface of the cells, ciliated cells in the female reproductive tract move oocytes into and down the uterine tubes.


Squamous Epithelium

Simple Squamous Epithelium

Stratified Squamous Epithelium

Cuboidal Epithelium

Columnar Epithelium

Simple Columnar Epithelium

Pseudostratified Columnar Epithelium

Transitional Epithelium


Connective Tissue

Connective tissues bind structures together, provide support and protection, fill spaces, and store fat. The cells of connective tissues are separated by a nonliving, noncellular matrix which often contains fibers.

Fibers of the connective tissue provide support.

Collagen fibers are extremely strong and provide high tensile strength to the connective tissue. Their strength and ability to withstand twisting makes collagen fibers excellent choices for construction of "cables" that attach bone to bone (ligaments) and muscle to bone (tendons).

Elastic fibers contain elastin, which allows them to be stretched and to recoil. They're great for keeping your skin taut (watch the wrinkles appear when the elastin goes way), allowing blood vessels to stretch when blood is pumped through, and allowing your ear to pop back into shape after your mom uses it to pick you up off the floor when you misbehave at the grocery store, but they aren't really very strong, certainly not compared to collagen.

Reticular fibers are fine, collagenous fibers that form networks.

Each major class of connective tissue has a fundamental cell type that exists in immature and mature forms.

Connective tissues include loose connective tissue, fibrous connective tissue, cartilage, bone, and blood.

Loose (Areolar) Connective Tissue

Areolar connective tissue serves to bind body parts together while allowing them to move freely over one another, wraps small blood vessels and nerves, surrounds glands, and forms the subcutaneous tissue.


Adipose (fat) tissue is a richly vascularized tissue that functions in nutrient storage, protection, and insulation.

Fibrous (Dense) Connective Tissue

Dense regular connective tissue contains closely packed bundles of collagen fibers running in the same direction and makes up tendons and ligaments.

Dense Regular


Dense irregular connective tissue contains thick bundles of collagen fibers arranged in an irregular fashion, and is found in the dermis and fibrous capsules of joints and some organs.

Dense Irregular


Reticular Connective Tissue

Reticular connective tissue forms the internal framework of the lymph nodes, the spleen, and the bone marrow. Reticular fibers work a bit like spider webs, allowing leukocytes to attach and sample lymph or blood as it flows through lymphoid organs.


Cartilage lacks nerve fibers and is avascular.

Hyaline cartilage is the most abundant cartilage providing firm support with some pliability.



Elastic cartilage is found where strength and exceptional stretchability are needed, such as the external ear and epiglottis; the extracellular matrix contains elastin fibers for flexibility.

Fibrocartilage is found where strong support and the ability to withstand heavy pressure are required, form intervertebral discs and meniscii; the extracellular matrix contains dense collagen fibers.


Bone (osseous tissue) has an exceptional ability to support and protect body structures due to its hardness, which is determined by the additional collagen fibers and calcium salts found in the extracellular matrix.

Two forms, compact and spongy (more later).


A type of connective tissue in which the extracellular matrix is liquid and contains soluble proteins that aren't made by the cells of the tissue (exception: some leukocytes secrete gamma globulins, or antibodies).

Muscular Tissue

Muscular tissue is composed of fibers (cells) that contract. Skeletal muscle is under voluntary control and functions to move body parts. Both smooth and cardiac muscle are under involuntary control. Smooth muscle is found in blood vessels and visceral organs, and cardiac muscle is found in the heart.







Nervous Tissue

Nervous tissue is composed of conducting cells called neurons and supporting cells called neuroglia.

Neurons initiate and/or conduct nerve impulses.

Neuroglia of the CNS

Astrocytes - provide nutrients to neurons, recycle some neurotransmitters, control the chemical environment around neurons, and secrete glia-derived growth factor, which supports the growth and health of neurons.

Microglia - phagocytes, related to macrophages, clear debris in the CNS.

Oligodendroglia - form myelin sheaths around the axons of neurons in the CNS.

Ependymal cells - filter plasma to make cerebrospinal fluid (CSF).

Neuroglia of the PNS

Schwann cells - form the myelin sheath around axons of neurons in the PNS, gaps between Schwann cells are nodes of Ranvier

Extracellular Junctions


Composed of epithelial cells

Exocrine - secrete through ducts onto surfaces

Endocrine - ductless, secrete hormones into body fluids

Goblet cells - unicellular glands, secrete mucin onto mucous membranes


Mucous membranes line the interior walls of tubes that open to the outside of the body.

Serous membranes cover organs and line body cavities.

Synovial membranes line freely movable joint cavities.

Meninges cover the brain and spinal cord.

The cutaneous membrane (or skin) covers the body surface.