Animal Tissues, including Epithelial tissue, Connective tissue, muscle tissue, nervous tissue by Miller Harley

 Animal Tissues 

In an animal, individual cells differentiate during development to perform special functions as aggregates called tissues.

A tissue (Fr. tissu, woven) is a group of similar cells specialized for the performance of a common function. The study of tissues is called histology (Gr. histos, tissue 1 logos, discourse).

 Animal tissues are classified as

1.    1. Epithelial tissue

2.    2. Connective tissue

3.    3. muscle tissue

4.     4. nervous tissue

1.   Epithelial Tissue: 

 Many Forms and Functions

Epithelial tissue exists in many structural forms. In general, it  either covers or lines something and typically consists of renewable sheets of cells that have surface specializations adapted for their specific roles. Usually, a basement membrane separates epithelial tissues from underlying, adjacent tissues. Epithelial tissues absorb (e.g., the lining of the small intestine), transport (e.g., kidney tubules), excrete (e.g., sweat and endocrine glands), protect (e.g., the skin), and contain nerve cells for sensory reception (e.g., the taste buds in the tongue). The size, shape, and arrangement of epithelial cells are directly related to these specific functions.

Epithelial tissues are classified on the basis of shape and the number of layers present. Epithelium can be simple, consisting of only one layer of cells, or stratified, consisting of two or more stacked layers. Individual epithelial cells can be flat (squamous epithelium;), cube shaped (cuboidal epithelium;), or column like (columnar epithelium). The cells of pseudo-stratified ciliated columnar epithelium possess cilia and appear stratified or layered, but they are not, hence the prefix pseudo. They look layered because their nuclei are at two or more levels within cells of the tissues and they grow in height as old cells are replaced by new ones.

Types of Epithelium Tissue:

(a)  Simple squamous epithelium consists of a single layer of tightly packed, flattened cells with a disk-shaped central nucleus .  

  Location:   Air sacs of the lungs,  kidney glomeruli, lining of heart, blood vessels, and lymphatic vessels.

  Function:   Allows passage of materials by diffusion and filtration.

(b)   Simple cuboidal epithelium consists of a single layer of tightly packed, cube-shaped cells. Notice the cell layer indicated by the arrow.

  Location:     Kidney tubules, ducts and small glands, and surface of ovary.

   Function:    Secretion and absorption.

(c)  Simple columnar epithelium consists of a single layer of elongated cells. The arrow points to a specialized goblet cell that secretes mucus.

    Location:    Lines digestive tract, gallbladder, and excretory ducts of some glands. 

    Function:    Absorption, enzyme secretion.

(d)   Pseudostratified ciliated columnar epithelium. A tuft of cilia tops each columnar cell, except for goblet cells

  Location:  Lines bronchi, uterine tubes, and some regions of the uterus.

 Function: Propels mucus or reproductive cells by ciliary action.

(e)   Stratified squamous epithelium consists of many layers of cells.

Location:   Lines esophagus, mouth, and vagina. Keratinized variety lines the surface of the skin. 

Function:   Protects underlying tissues in areas subject to abrasion

1.   Connective Tissue: 

Connection and Support Connective tissues support and bind.

Unlike epithelial tissues, connective tissues are distributed throughout an extracellular matrix. This matrix frequently contains fibers that are embedded in a ground substance with a consistency anywhere from liquid to solid. To a large extent, the nature of this extracellular material determines the functional properties of the various connective tissues.

Connective tissues have two general types of fiber arrangement. In loose connective tissue, strong, flexible fibers of the protein collagen are interwoven with fine, elastic, and reticular fibers, giving loose connective tissue its elastic consistency and making it an excellent binding tissue (e.g., binding the skin to underlying muscle tissue). In fibrous connective tissue, the collagen fibers are densely packed and may lie parallel to one another, creating very strong cords, such as tendons (which connect muscles to bones or to other muscles) and ligaments (which connect bones to bones). Adipose tissue is a type of loose connective tissue that consists of large cells that store lipid. Most often, the cells accumulate in large numbers to form what is commonly called fat.

i.        Loose connective tissue contains numerous fibroblasts (arrows) that produce collagenous and elastic fibers. Location: Widely distributed under the epithelia of the human body. Function: Wraps and cushions organs.

ii.        Fibrous connective tissue consists largely of tightly packed collagenous fibers.

Location: Dermis of the skin, sub-mucosa of the digestive tract, and fibrous capsules of organs and joints.

Function: Provides structural strength.

iii.        Blood is a type of connective tissue. It consists of red blood cells, white blood cells, and platelets suspended in an intercellular fluid (plasma) .

Location: Within blood vessels.

 Function: Transports oxygen, carbon dioxide, nutrients, wastes, hormones, minerals, vitamins, and other substances throughout the bodies of animals.

(a)  Adipose tissue cells (adipocytes) contain large fat droplets that push the nuclei close to the plasma membrane. The arrow points to a nucleus.

Location:   Around kidneys, under skin, in bones, within abdomen, and in breasts.       Function:   Provides reserve fuel (lipids), insulates against heat loss, and supports and protects organs.

Cartilage

Cartilage is a hard yet flexible tissue that supports structures such as the outer ear and forms the entire skeleton of animals such as sharks and rays . Cells called chondrocytes lie within spaces called lacunae that are surrounded by a rubbery matrix that chondros-blasts secrete. This matrix, along with the collagen and/or elastin fibers, gives cartilage its strength and elasticity. Bone cells (osteocytes) also lie within lacunae, but the matrix around them is heavily impregnated with calcium phosphate and calcium carbonate, making this kind of tissue hard and ideally suited for its functions of support and protection.

a)    Hyaline cartilage cells are located in lacunae (arrow) surrounded by intercellular material containing fine collagenous fibers. Location: Forms embryonic skeleton; covers ends of long bones; and forms cartilage of nose, trachea, and larynx. Function: Support and reinforcement.

b)     Elastic cartilage contains fine collagenous fibers and many elastic fibers in its intercellular material. Location: External ear, epiglottis. Function: Maintains a structure’s shape while allowing great flexibility.

c)     Fibrocartilage contains many large, collagenous fibers in its intercellular material. The arrow points to a fibroblast. Location: Intervertebral disks, pubic symphysis, and disks of knee joint. Function: Absorbs compression shock.

d)     Bone (osseous) tissue. Bone matrix is deposited in concentric layers around osteonic canals. Location: Bones.  Function: Supports, protects, provides lever system for muscles to act on, stores calcium and fat, and forms blood cells. 

3.   Muscle Tissue:

 Movement Muscle tissue allows movement. The three kinds of muscle tissue are skeletal, smooth, and cardiac. Skeletal muscle is attached to bones and makes body movement possible in vertebrates. The rhythmic contractions of smooth muscle create a churning action (as in the stomach), help propel material through a tubular structure (as in the intestines), and control size changes in hollow organs such as the urinary bladder and uterus. The contractions of cardiac muscle result in the heart beating.

(a)  Skeletal muscle tissue is composed of striated muscle fibers (cells) that are long and cylindrical and contain many peripheral nuclei. Location: In skeletal muscles attached to bones. Function: Voluntary movement, locomotion.

(b)  Smooth muscle tissue is formed of spindle-shaped cells, each containing a single centrally located nucleus (arrow). Cells are arranged closely to form sheets. Smooth muscle tissue is not striated. Location: Mostly in the walls of hollow organs. Function: Moves substances or objects (foodstuffs, urine, a baby) along internal passageways; involuntary control.

 

(c)   Cardiac muscle tissue consists of branched striated cells, each containing a single nucleus and specialized cell junctions called intercalated disks (arrow) that allow ions (action potentials) to move quickly from cell to cell. Location: The walls of the heart. Function: As the walls of the heart contract, cardiac muscle tissue propels blood into the circulation; involuntary control.

4.   Nervous Tissue:

Communication Nervous tissue is composed of several different types of cells: Impulse-conducting cells are called neurons. cells involved with protection, support, and nourishment arecalled neuroglia; and cells that form sheaths and help protect, nourish, and maintain cells of the peripheral nervous system are called peripheral glial cells. 

Neurons

Neurons in nervous tissue transmit electrical signals to other neurons, muscles, or glands. Location: Brain, spinal cord, and nerves. Function: Transmits electrical signals from sensory receptors to the spinal cord or brain, and from the spinal cord or brain to effectors (muscles and glands).