lec2 anatomy

Anatomy Dr.Israa H. Mohsen

Lecture 2

TISSUES
Although the human body contains trillions of cells, all of those cells can be categorized as belonging to one of four distinct groups of tissue. Tissues are simply groups of similar cells that perform a common function. The four categories of tissue are epithelial, connective, nervous, and muscular.
Immediately after an egg and sperm unite to form a single cell, the cells begin to divide rapidly. At first, all the cells are identical.
Soon, the cells organize into three layers: the ectoderm (outer layer), the mesoderm (middle layer), and the endoderm (inner layer). The cells of each layer continue to divide, becoming increasingly distinct from the cells of the other layers. Eventually each layer gives rise to the different types of tissue, a process called differentiation.



Epithelial Tissue
epithelial tissue is a continuous sheet of tightly packed cells; it covers the body’s surface, lines the body cavities and many of the organs, and forms certain glands.
The key functions of this tissue involve protection, absorption, filtration, and secretion.
In a sense, the epithelium is a surface tissue: its top surface is usually exposed to the environment—such as occurs with the skin or the inside of the mouth—or to an internal body cavity; its bottom surface adheres to underlying connective tissue by means of a basement membrane. Epithelial tissue is too thin to contain blood vessels; therefore, it depends on the connective tissue beneath to supply its needs for oxygen and nutrients. Epithelial tissue is classified by the shape of the cells as well as by the number of layers.
Cell Shape
Epithelial cells may assume one of three basic shapes: squamous, cuboidal, or columnar

Glandular Epithelium
There’s another type of epithelium: glandular epithelium. A gland is a collection of epithelial cells that specializes in secretion of a particular substance.
? Exocrine glands secrete their products (such as tears, sweat, or gastric juices) into ducts. The ducts then empty onto a body surface or inside a body cavity. For example, sweat glands secrete sweat, which flows through ducts and onto the skin’s surface.
? Endocrine glands are often called ductless glands. These glands secrete their products, called hormones, directly into the blood. For example, the adrenal glands secrete epinephrine and norepinephrine into the bloodstream. FACT
Goblet cells are modified cells containing secretory vesicles that produce large quantities of mucus
Connective Tissue
The most widespread, and the most varied, of all the tissues is connective tissue. Existing in a variety of forms—ranging from tough cords to elastic sheets to fluid—connective tissue performs a variety of tasks INCLUDING connect the body together and to support, bind, or protect organs.
The key component of connective tissue—called extracellular matrix—Extracellular matrix is the framework into which the cells of tissue are embedded. The matrix consists of varying kinds and amounts of protein fibers and fluid; it’s the variation in composition that gives tissue its characteristics. For example, the matrix of blood is fluid; it contains many cells but no fibers. In contrast, the matrix of bone contains few cells and many fibers, making it hard and brittle. The matrix may also be gel-like, flexible, tough, or even fragile.
The fibers found in connective tissue may be one of three types:
? Collagenous fibers: These are strong and flexible but resist stretching; these are the most abundant fibers.
? Reticular fibers: These occur in networks and support small structures such as capillaries and nerve fibers.
? Elastic fibers: Made of a protein called elastin, these fibers can stretch and recoil like a rubber band.
Types of Connective Tissue

Type Location Function
1-Loose fibrous connective
• Areolar Beneath the epithelia; between muscles; surrounding blood vessels and nerves Connects tissues and organs together (such as skin to muscles)
• Adipose Beneath the skin, breast, heart’s surface; surrounding kidneys and eyes Provides protective cushion, insulation; stores energy
• Reticular Spleen; lymph nodes; bone marrow Provides a supportive framework
2-Dense fibrous connective Tendons; ligaments; fascia; dermis of the skin Provides durable support
3-Cartilage
• Hyaline Ends of bones in joints; connecting point between ribs and sternum; rings in trachea and bronchi; larynx; fetal skeleton Eases joint movement; firm but flexible support
• Elastic External ear Provides flexible support
• Fibrocartilage Intervertebral discs; knee joint; pelvis Resists compression and absorbs shock
4-Bone Skeleton Provides support, protection; serves as calcium reservoir
5- Blood Inside blood vessels throughout the body Transports oxygen, nutrients, hormones, wastes from one part of the body to another

Nervous Tissue
Nervous tissue has a high degree of excitability and conductivity—more so than other tissues. It’s these characteristics that allow it to communicate rapidly with other parts of the body. Found in the brain, spinal cord, and nerves, nerve tissue consists of two types of cells:
? Neurons, the units that conduct nervous impulses
? Neuroglia, which protect and assist neurons.


Muscle Tissue
Muscle tissue consists of elongated cells that contract in response to stimulation. The body contains three types of muscle tissue: skeletal, cardiac, and smooth.
Skeletal Muscle
consists of long, thin cells called muscle fibers. Skeletal muscle may also be called striated muscle (because its light and dark bands give it a striped, or striated, appearance) or voluntary muscle (because we can move it voluntarily).
Most skeletal muscle is attached to bone. This is the muscle that makes body movements possible. It is also the muscle responsible for breathing, speech, control of urination, and facial expression.
Cardiac Muscle
found only in the heart. While cardiac muscle also appears striated, it is uniquely different from skeletal muscle. For one thing, cardiac muscle cells are shorter than those of skeletal muscle.
In addition, the cells are joined together with junctions called intercalated discs.
These junctions allow electrical impulses to spread
rapidly from cell to cell; this rapid transmission permits almost simultaneous stimulation and contraction. Finally, cardiac muscle is involuntary muscle: its contraction is not under voluntary control.
Smooth Muscle
consists of long, spindle-shaped cells—lacks the striped pattern of striated muscle. smooth muscle is not under voluntary control. This muscle lines the walls of many organs, including those of the digestive, respiratory, and urinary tracts. Smooth muscle controls the diameter of blood vessels, making it important in controlling blood pressure and flow.

Membranes
Thin sheets of tissue, called membranes, fulfill many crucial functions in the body. In general, membranes line body cavities, cover body surfaces, and separate organs (or parts of organs) from each other. Some membranes secrete lubricating fluids to reduce friction during movement, such as when the heart beats or a joint bends. The two categories of membranes are epithelial membranes and connective tissue membranes.
Epithelial Membranes
The body contains three types of epithelial membranes: mucous membranes, cutaneous membranes, and serous membranes.
1.Mucous membranes
Mucous membranes line body surfaces that open directly to the body’s exterior, such as the respiratory, digestive, urinary, and reproductive tracts. True to the name, mucous membranes secrete mucus, a watery secretion that coats and protects the cells of the membrane. Mucus also acts as a lubricant to help propel food through the digestive tract; in the respiratory tract, it traps dust and bacteria.
2 Cutaneous membrane
Known as the skin, this is the body’s largest membrane. It consists of a layer of epithelium resting on a layer of connective tissue.
3Serous membranes
serous membranes line some of the closed body cavities and also cover many of the organs in those cavities.The serous membrane that lines the body cavities is actually one continuous sheet: part of the membrane (called the parietal membrane) lines the wall of the cavity; it then folds back and covers the organs. The part of the membrane that covers the organs is called the visceral membrane. There are three serous membranes:
• The , or pleural membrane, surrounds each lung and lines the thoracic cavity.
• The , or pericardial membrane, surrounds the heart.
• The , or peritoneal membrane, lines the abdominal cavity and covers the abdominal organs.
Serous membranes secrete serous fluid, which helps prevent friction as the heart beats and the lungs expand.



Connective Tissue Membranes
Some joints are lined by membranes made of connective tissue. For example, synovial membranes line the spaces between bones, where they secrete synovial fluid