Animal Histology Cells and Tissues Tissues How do

Animal Histology: Cells and Tissues

Tissues • How do we define tissue? • Tissues are groups of specialized cells that work together for a particular function. • There are four types of tissue. • Epithelial (covering) • Connective (support) • Muscle (movement) • Nervous (control)

Epithelial Tissues • Epithelial tissues cover body surfaces. – Outer layer of skin and the lining of organs – Also found in glandular tissue – Play roles in absorption, filtration, secretion, and protection against foreign substances

Special Characteristics of Epithelium • Usually fit closely together in continuous sheets • Exception: glandular epithelium • Avascular (Have no blood supply of their own) • How do they get nutrients? • If well nourished, epithelial cells regenerate themselves easily.

Special Characteristics of Epithelium • Have one free (unattached) surface or edge called the apical surface – Exposed to body’s exterior or to the cavity of an internal organ • Lower surface of an epithelium rests on a basement membrane – Structureless material secreted by both the epithelial cells and the connective tissue cells

Classification of Epithelium • Each epithelium is given 2 names • First: indicates relative number of cell layers – Simple (one layer of cells) – Stratified (more than one cell layer)

Classification of Epithelium • Second: describes the shape of cells – Squamous (cells flattened like scales) – Cuboidal (cube -shaped) – Columnar (shaped like columns)

Simple Squamous Epithelium • Simple squamous (SS) tissue is composed of flat, scale-like cells that usually forms membranes • It lines the walls of blood vessels, pulmonary alveoli (shown here), and the lining of the heart, lung, and peritoneal cavities.

Simple Cuboidal Epithelium • This tissue is composed of a single layer of cube-like cells. • It lines the walls of kidney tubules, covers the surface of ovaries, and is common in glands and their ducts.

Simple Columnar Epithelium • This tissue is composed of a single layer of tall cells. • It often includes mucus-producing goblet cells. • It often lines the digestive tract.

Pseudostratified Columnar Epithelium • Although this tissue appears stratified, it is actually composed of a single layer of cells of different types. • Although their nuclei are found at different levels, each cell adjoins the basal membrane (BM). • This tissue lines the larger respiratory passageways. • It is often ciliated (arrows).

Pseudostratified Columnar Epithelium • Main functions: absorption and secretion • Ciliated variety lines respiratory tract – Mucus produced by goblet cells traps dust and other debris – Cilia propel mucus upward and away from the lungs

Stratified Squamous Epithelium • The term “stratified” refers to the layered arrangement of cells. • The outer layers of cells appear flat, but the inner cells vary in shape from cuboidal to columnar. • Stratified squamous epithelium serves as a barrier to the outside environment in locations such as the skin, mouth, and esophagus.

Stratified Cuboidal Epithelium • Stratified cuboidal epithelium (SC) is found in the ducts of sweat glands and surrounds Graafian follicles of ovaries (shown below).

Transitional Epithelium • Shape of cells depends upon the amount of stretching • It lines organs of the urinary system.

Ciliated Epithelium • Some epithelial membranes are made up of cells with cilia, tiny projections that beat in unison to move mucus along the surface. • Ciliated epithelia in the trachea, for example, sweep debris out of the respiratory tract.

Glandular Epithelium • A gland is defined as one or more cells responsible for secreting a particular product • Two major gland types – Endocrine gland • Ductless since secretions diffuse into blood vessels • All secretions are hormones • Examples include pituitary, pancreas, ovaries, testes – Exocrine gland • Secretions empty through ducts to the epithelial surface • Include sweat and oil glands

Glandular Epithelium

Connective Tissue • Connective tissue differs from other tissues in that it contains large amounts of extracellular matrix. • Extracellular matrix is nonliving material that surrounds living cells • It is found everywhere and includes the most abundant and widely distributed tissue

Connective Tissue • Connective tissues function to • bind other tissues together • provide support • provide nourishment • store wastes • repair damaged tissues • These tissues are generally well vascularized • Exceptions: tendons, ligaments, cartilage • The exceptions are avascular

Extracellular Matrix Two main elements: 1. Ground substance • Mostly water along with glycoproteins and large polysaccharide molecules • May be liquid, gel-like, or rock-hard 2. Fibers • Collagen fibers (high strength) • Elastic fibers (stretch and recoil) • Reticular fibers (fine fibers, internal ‘skeleton’)

Types of Connective Tissue • • • Bone Various types of cartilage Adipose tissue Dense and loose connective tissue Blood

Bone (Osseous Tissue) • Composed of • Bone cells in lacunae (cavities) • Hard matrix of calcium salts • Large numbers of collagen fibers • Used to protect and support the body

Hyaline Cartilage • Most common type of cartilage • Composed of collagen fibers hidden by a rubbery matrix with a glassy blue-white appearance • Found in the larynx, attaches ribs to sternum, & at the end of many bones • Entire fetal skeleton before birth

Elastic & Fibrocartilage Elastic • Provides elasticity • Supports the external ear Fibrocartilage • Highly compressible • Forms cushion-like discs between vertebrate

Dense Connective Tissue • Dense connective tissue contains a large number of fibers with only a few cells. • Fibers shown here all running parallel to each other, and no cells are present. • Tendons (muscle to bone) and ligaments (bone to bone) are composed of dense connective tissue.

Loose Connective Tissue • Loose connective tissue has few fibers, a number of cell types, and a large amount of matrix. • It functions to bind epithelia to underlying tissues. • Includes areolar, adipose, & reticular connective

Areolar Tissue • Most widely distributed connective tissue • Soft, pliable “cobwebby” tissue that cushions and protects the body’s organs it wraps • Holds internal organs together and in their proper positions • Under microscope: matrix appears as empty space, reservoir of water and salts

Edema • When a body region is inflamed, the areolar tissue in the area soaks up the excess fluid like a sponge, and the area swells and becomes puffy.

Adipose Tissue • Adipose cells are bundled together by connective tissue. • Each cell appears as a clear space, representing the site of the large drop of lipid (fat) before it dissolved during preparation of the microscope slide. • The nuclei appear as small disks on the periphery of cells. • Functions to insulate the body, protect organs, and fuel storage

Reticular Connective Tissue • Consists of a delicate network of interwoven reticular fibers • Forms the stroma (internal framework) which can support free blood cells in lymphoid organs (lymph nodes, spleen, & bone marrow)

Blood (Vascular Tissue) • Consists of blood cells surrounded by nonliving, fluid matrix called blood plasma • ‘Fibers’ only visible during blood clotting • Functions as a transport medium for materials

Muscle Tissue • Muscle is a contractile tissue. • There are three types of muscle: – Skeletal – Cardiac – Smooth • Main function is to produce movement.

Skeletal Muscle • • Under voluntary control Contracts to pull on bones or skin Produces gross body movements or facial expressions Characteristics of skeletal muscle cells – Striated (stripe-like pattern) – Multinucleate (more than one nucleus) – Long, cylindrical

Cardiac Muscle • • Under involuntary control Found only in the heart Function is to pump blood Characteristics of cardiac muscle cells – Cells are attached to other cardiac muscle cells at intercalated disks – Striated – One nucleus/cell

Smooth Muscle • Under involuntary muscle • Found in walls of hollow organs such as stomach, uterus, and blood vessels • Characteristics of smooth muscle cells – No visible striations – One nucleus/cell – Spindle-shaped cells

Nervous Tissue • Nervous tissue, which occurs throughout the body, receives and transmits stimuli. • It converts a stimulus, whether chemical or physical in nature, into an electrical impulse that is conducted by neurons. • Nervous tissue also consists of glia, which are the various types of supporting cells in the nervous system.

Cerebellum • The surface of the cerebellum is highly fissured. • Outer layer is the molecular layer, which contains some glial cells but consists mostly of neuronal processes (dendrites and axons). • Darkly stained area is the granular layer and is composed mostly of nerve cell bodies of tiny granule cells (a neuron type) along with some larger Golgi cells (another neuron type). • Between the molecular and granular layers lies the Purkinje layer.

How Histology Can Be Used… (One example) Immunohistochemistry is used to reveal Brd. U (brown), a thymidine analog that is incorporated in cells undergoing S phase. Cells stained brown were caught in the act of DNA synthesis. These cells are found in the hippocampus, one of the only brain regions where new neurons are formed.

Immunofluorescence labeling shows new neurons in the hippocampus. Green is Brd. U (the marker that labels cells undergoing S phase), red is a Neu. N (a neuron-specific marker), and blue is GFAP (a glial marker). Cells co-labeled green and red are new neurons.