Types of Tissues

• The term tissue is used to describe a group of cells found together in the body.

The Four Types of Tissues

• 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.

• 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.

Embryonic Origin of Tissues

• The first embryonic cells generated have the ability to differentiate into any type of cell in the body and, as such, are called totipotent, meaning each has the capacity to divide, differentiate, and develop into a new organism.

• Each germ layer is identified by its relative position: ectoderm (ecto-= “outer”), mesoderm (meso- = “middle”), and endoderm (endo- = “inner”).

• Tissue Membranes: A 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 Membranes

• The connective tissue membrane is formed solely from connective tissue.

  • These membranes encapsulate organs, such as the kidneys, and line our movable joints.

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

Epithelial Membranes

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

• The mucous membrane is also a composite of connective and epithelial tissues.

• The underlying connective tissue, called the lamina propria (literally “own layer”), helps support the fragile epithelial layer.

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

• The skin is an epithelial membrane also called the cutaneous membrane.

Epithelial Tissue

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

• The epithelial cells exhibit polarity with differences in structure and function between the exposed or apical facing surface of the cell and the basal surface close to the underlying body structures.

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

• The basal lamina attaches to a reticular lamina, which is secreted by the underlying connective tissue, forming a basement membrane that helps hold it all together.

Cell to Cell Junctions

• At one end of the spectrum is the tight junction, which separates the cells into apical and basal compartments.

• An anchoring junction includes several types of cell junctions that help stabilize epithelial tissues.

• A 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 Epithelium

• The cells in simple squamous epithelium have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell.

• The endothelium is the epithelial tissue that lines vessels of the lymphatic and cardiovascular

• system, and it is made up of a single layer of squamous cells.

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

• In simple cuboidal epithelium, the nucleus of the box-like cells appears round and is generally located near the center of the cell.

• In 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.

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

Stratified Epithelium

• 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.

• Another kind of stratified epithelium is 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”).

Methods and Types of Secretion

• Merocrine secretion is the most common type of exocrine secretion.

• Apocrine secretion accumulates near the apical portion of the cell.

  • That portion of the cell and its secretory contents pinch off from the cell and are released.

• The process of holocrine secretion involves the rupture and destruction of the entire gland cell.

• The serous gland produces watery, blood-plasma-like secretions rich in enzymes such as alpha amylase, whereas the mucous gland releases watery to viscous products rich in the glycoprotein mucin.

Connective Tissue Supports and Protects

• 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 matrix.

• The major component of the matrix is a ground substance often crisscrossed by protein fibers.

Embryonic Connective Tissue

• The first connective tissue to develop in the embryo is mesenchyme, the stem cell line from which all connective tissues are later derived.

• A second type of embryonic connective tissue forms in the umbilical cord, called mucous connective tissue or Wharton’s jelly.

Classification of Connective Tissues

• 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.

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

Cell Types

• The most abundant cell in connective tissue proper is the fibroblast.

• A 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.

• The mesenchymal cell is a multipotent adult stem cell.

Connective Tissue Fibers and Ground Substance

• 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.

Loose Connective Tissue

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

• Areolar tissue shows little specialization. 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.

  • It derives its name from the Latin reticulus, which means “little net.”

Cartilage

• Embedded within the cartilage matrix are chondrocytes, or cartilage cells, and the space they occupy are called lacunae (singular = lacuna).

• 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

• Bone is the hardest connective tissue.

  • It provides protection to internal organs and supports the body.

• Bone’s rigid extracellular matrix contains mostly collagen fibers embedded in a mineralized ground substance containing hydroxyapatite, a form of calcium phosphate.

Muscle Tissue and Motion

Comparison of Structure and Properties of Muscle Tissue Types

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

• The 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.

Nervous Tissue Mediates Perception and Response

• Two main classes of cells make up nervous tissue: the neuron and neuroglia.

  • 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.

• A long “tail,” the axon, extends from the neuron body and can be wrapped in an insulating layer known as myelin, which is formed by accessory cells.

• 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.

Tissue Injury and Repair

• 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.

• Upon tissue injury, damaged cells release inflammatory chemical signals that evoke local vasodilation, the widening of the blood vessels.

• Increased blood flow results in apparent redness and heat. In response to injury, mast cells present in tissue degranulate, releasing the potent vasodilator histamine.

• 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.

• A primary union describes the healing of a wound where the edges are close together.

  • When there is a gaping wound, it takes longer to refill the area with cells and collagen.

• The process called secondary union occurs as the edges of the wound are pulled together by what is called wound contraction.

Tissue and Aging

• According to poet Ralph Waldo Emerson, “The surest poison is time.”

  • In fact, biology confirms that many functions of the body decline with age.

• The skin and other tissues become thinner and drier, reducing their elasticity, contributing to wrinkles and high blood pressure.

• Many tissues, including those in muscles, lose mass through a process called atrophy.

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

  • Telomeres, regions of the chromosomes necessary for cell division, shorten each time cells divide.