Chapter 4: The Tissue Level of Organization

**Nervous** tissue

is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body.

**Epithelial tissue**

also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, lines internal cavities and passageways, and forms certain glands.

**Connective** tissue

as its name implies, binds the cells and organs of the body together and functions in the protection, support, and integration of all parts of the body.

**Muscle** tissue

is excitable, responding to stimulation and contracting to provide movement.

**totipotent**

meaning each has the capacity to divide, differentiate, and develop into a new organism.

**ectoderm**

(ecto-= “outer”)

**mesoderm**

(meso- = “middle”)

**endoderm**

(endo- = “inner”)

**tissue membrane**

is a thin layer or sheet of cells that covers the outside of the body (for example, skin), the organs (for example, pericardium), internal passageways that lead to the exterior of the body (for example, abdominal mesenteries), and the lining of the moveable joint cavities.

**connective tissue membrane**

is formed solely from connective tissue.

**synovial membrane**

is a type of connective tissue membrane that lines the cavity of a freely movable joint.

**epithelial membrane**

is composed of epithelium attached to a layer of connective tissue, for example, your skin.

**mucous membrane**

is also a composite of connective and epithelial tissues.

**lamina propria**

(literally “own layer”), helps support the fragile epithelial layer.

**serous membrane**

is an epithelial membrane composed of mesodermally derived epithelium called the *mesothelium* that is supported by connective tissue.

**cutaneous membrane**

The skin is an epithelial membrane also called the

**cell junction**

Adjoining cells form a specialized intercellular connection between their cell membranes called a

**basal lamina**

a mixture of glycoproteins and collagen, provides an attachment site for the epithelium, separating it from underlying connective tissue.

**reticular lamina**

which is secreted by the underlying connective tissue, forming a **basement membrane** that helps hold it all together.

**tight junction**

which separates the cells into apical and basal compartments.

**anchoring junction**

includes several types of cell junctions that help stabilize epithelial tissues.

**gap junction**

forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells.

**simple squamous epithelium**

have the appearance of thin scales.

**endothelium**

is the epithelial tissue that lines vessels of the lymphatic and cardiovascular

**mesothelium**

is a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal organs.

**simple cuboidal epithelium**

the nucleus of the box-like cells appears round and is generally located near the center of the cell.

**simple columnar epithelium**

the nucleus of the tall column-like cells tends to be elongated and located in the basal end of the cells.

**Pseudostratified columnar epithelium**

is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells.

**goblet cell**

is a mucous-secreting unicellular “gland” interspersed between the columnar epithelial cells of mucous membranes.

**Stratified squamous epithelium**

is the most common type of stratified epithelium in the human body.

**Stratified cuboidal epithelium** and **stratified columnar epithelium**

can also be found in certain glands and ducts, but are uncommon in the human body.

**transitional epithelium**

so-called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine.

**Glandular Epithelium**

A gland can be classified as an **endocrine gland**, a ductless gland that releases secretions directly into surrounding tissues and fluids (endo- = “inside”), or an **exocrine gland** whose secretions leave through a duct that opens directly, or indirectly, to the external environment (exo- = “outside”).

**Merocrine secretion**

is the most common type of exocrine secretion.

**Apocrine secretion**

accumulates near the apical portion of the cell.

**holocrine secretion**

involves the rupture and destruction of the entire gland ce~~l~~l.

**serous gland**

produces watery, blood-plasma-like secretions rich in enzymes such as alpha amylase

**mucous gland**

releases watery to viscous products rich in the glycoprotein mucin.

**matrix**

Unlike epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a

**ground substance**

major component of matrix

**mesenchyme**

the stem cell line from which all connective tissues are later derived.

**mucous connective tissue**

A second type of embryonic connective tissue forms in the umbilical cord

**Connective tissue**

proper includes **loose connective** tissue and **dense connective** tissue.

**Supportive connective tissue**

bone and cartilage—provide structure and strength to the body and protect soft tissues.

**fluid connective tissue**

in other words, lymph and blood, various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins.

**fibroblast**

The most abundant cell in connective tissue proper

**fibrocyte**

a less active form of fibroblast, is the second most common cell type in connective tissue proper.

**Adipocytes**

are cells that store lipids as droplets that fill most of the cytoplasm.

**mesenchymal cell**

is a multipotent adult stem cell.

**Collagen fiber**

is made from fibrous protein subunits linked together to form a long and straight fiber.

**Elastic fiber**

contains the protein elastin along with lesser amounts of other proteins and glycoproteins

**Reticular fiber**

is also formed from the same protein subunits as collagen fibers; however, these fibers remain narrow and are arrayed in a branching network.

**Adipose tissue**

consists mostly of fat storage cells, with little extracellular matrix.

**Areolar tissue**

It contains all the cell types and fibers previously described and is distributed in a random, web-like fashion.

**Reticular tissue**

is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver.

**lacunae** (singular = lacuna)

Embedded within the cartilage matrix are **chondrocytes**, or cartilage cells, and the space they occupy

**Hyaline cartilage**

the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans.

**Fibrocartilage**

is tough because it has thick bundles of collagen fibers dispersed through its matrix.

**Elastic cartilage**

contains elastic fibers as well as collagen and proteoglycans.

**Bone**

hardest connective tissue

**Skeletal muscle**

is attached to bones and its contraction makes possible locomotion, facial expressions, posture, and other voluntary movements of the body.

**striation**

is due to the regular alternation of the contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective tissues.

**Cardiac muscle**

forms the contractile walls of the heart.

**Smooth muscle**

tissue contraction is responsible for involuntary movements in the internal organs.

***Neurons***

propagate information via electrochemical impulses, called *action potentials*, which are biochemically linked to the release of chemical signals.

***Neuroglia***

play an essential role in supporting neurons and modulating their information propagation.

**myelin**

A long “tail,” the axon, extends from the neuron body and can be wrapped in an insulating layer known as

**Astrocyte** cells

named for their distinctive star shape, are abundant in the central nervous system.

**Oligodendrocyte** cells

produce myelin in the central nervous system (brain and spinal cord) while the **Schwann cell** produces myelin in the peripheral nervous system.

**Inflammation**

is the standard, initial response of the body to injury.

**Necrosis**

or accidental cell death, causes inflammation.

**Apoptosis**

is programmed cell death, a normal step-by-step process that destroys cells no longer needed by the body.

**vasodilation**

Upon tissue injury, damaged cells release inflammatory chemical signals that evoke local

**Clotting**

(coagulation) reduces blood loss from damaged blood vessels and forms a network of fibrin proteins that trap blood cells and bind the edges of the wound together.

**primary union**

describes the healing of a wound where the edges are close together.

**secondary union**

occurs as the edges of the wound are pulled together by what is called **wound contraction**.

**atrophy**

Many tissues, including those in muscles, lose mass through a process

**Aging**

is also apparent at the cellular level because all cells experience changes with aging.