Tissues

Definition

• Body tissues are groups of cells that are similar in structure and function.

EPITHELIAL TISSUES

• Epithelial tissues are the tissues that lines or covers the body. It is the lining, covering and the glandular tissue of the body. Primarily, epithelial tissues functions include:
  1. Protection
  2. Absorption
  3. Filtration
  4. Secretion

In general, the epithelium has the following distinct characteristics:

1. Forms continuous sheet by fitting together closely with the exception of glandular tissues.
2. Have one free surface or edge in the membranes called an apical surface.
3. Has a basement structure where the lower surface of the epithelium rests.
4. Have no blood supply.
5. Depends on diffusion from the capillaries in the underlying connective tissue for food and oxygen.
6. Regenerates themselves if well nourish.

The epithelium is classified into simple and stratified epithelium. The classification is based on the cell arrangements.

Simple Epithelium

This type of epithelium is very thin. It functions for absorption, secretion and filtration. Protection is not one of their specialties.

• Simple squamous epithelium – forms the membranes where filtration or exchange of substances by rapid diffusion takes place.
• Simple cuboidal epithelium – this is one layer of cuboidal cells which lies on the basement of the membrane. It forms the walls of the tubules in the kidney and the one that covers the surface of the ovaries in females.
• Simple columnar epithelium – this is one layer of columnar cells (tall cells) that fits closely together. It covers the entire area of the digestive tract from the stomach to the anus. In this type of epithelium, goblet cells are found. Goblet cells are the one which produce lubricating mucus.
• Pseudistratified Columnar Epithelium – this type of epithelium gives the impression that it is stratified. The main function of this epithelium is on absorption and secretion.

Stratified Epithelium

This epithelium consists of two or more layer of cells. These epithelia mainly functions for protection. It is more durable than the simple epithelia making it possible to carry out its function of protection.

• Stratified squamous epithelium – this is the most common stratified epithelium in the body which consists of several layers of cells. Cells located at the outer portion or edge area are squamous cells while those that are located near the basement are either cuboidal or columnar. This type of epithelium is located in sites where a good deal of abuse or friction is occurring in the body. These areas are the esophagus, mouth and the outer portion of the skin.
• Stratified columnar epithelium – in this type of stratified epithelium the cells are columnar. However the basal cells are in various sizes and shapes. This is a rare epithelium in the body where it is just found in the ducts of the large glands.
• Stratified cuboidal epithelium – like the stratified columnar epithelium these cells are also rare in the body and are only found in the ducts of the large glands in the body. Typically, this type of stratified epithelium has at least two layers of cells with the surface cells becoming cuboidal in shape.
• Transitional epithelium – this epithelium forms the lining of few organs such as the urinary bladder, the ureters and part of the urethra. With a transitional epithelium, cells located at the base are either cuboidal or columnar while those at the free surface are varying in their appearance. Thus, it is a highly modified and stratified squamous epithelium. The transitional cells has the ability of stretching by sliding past one another and changing their shape thus allowing stretching of the organ to take place.

Glandular Epitheliu

The main function of glandular epithelium is for secretion. A gland is composed of one or more cells that makes and secretes a particular product or secretion. This secretion contains molecules in a water based fluid. Two glands in the body are developed from the epithelial sheets namely:

• Endocrine glands – these are the ductless glands which secrete hormones that directly diffuse into the blood that intertwines through the glands. Endocrine glands include the thyroid, adrenals and the pituitary.
• Exocrine glands – are the glands that carry out their function of secretion through their ducts to the epithelial surface. Included in this classification are the sweat and oil glands, the liver and the pancreas.

CONNECTIVE TISSUES

• Connective tissues are located everywhere in the body. Based on its name, it is responsible for connecting body parts. The functions of these tissues are the mainly the following:
  1. Protecting
  2. Supporting
  3. Binding together other body tissues

Connective tissues have the following distinct characteristics:

• Connective tissues have variations in their blood supply. Some connective tissues are poor blood supply such as the tendons and ligaments while the other have no blood supply at all (avascular) such as the cartilage. However, most connective tissues have a good blood supply (vascular).
• All structures of a connective tissue heal very slowly.
• It is composed of an extracellular matrix that enables the connective tissue form a soft packing tissue around other organs allowing the tissues to bear weight and to withstand stretching and other abuses such as abrasion that no other tissue could have endured. This makes connective tissue different from other tissues in the body.

Types of connective tissue:

1. Bone – this is the most rigid connective tissue in the body which is also called osseous tissue. It is composed of lacunae which are bone cells that are sitting in cavities. The lacunae are then surrounded by layers of a very hard matrix that is comprised of calcium salts plus the large amount of collagen fibers. The bone has an outstanding ability to protect and support other body organs because of its rocklike hardness.
2. Cartilage – next to the bone, cartilage is less hard but is more flexible. Though found in only few areas of the body the most widespread is the hyaline cartilage which contains a large amount of collagen fibers. The cartilage is the one that forms the supporting structures of the larynx. Aside from that it is the one responsible for attaching the ribs to the breastbone and covers the ends of the bones where they form the joints. The fibrocartilage is the cushion-like discs that are located in between the vertebrae. Another type of cartilage is the elastic cartilage that supports the external ear and is located in areas where elasticity is desired.
3. Dense Connective tissue – this connective tissue forms the tendons and the ligaments. It is also called dense fibrous tissue where its main matrix is the collagen fibers. In the collagen fibers are the fibroblasts that produce the building elements of the fibers. Tendons are responsible for attaching muscles to the bone whilst ligaments are responsible for connecting bones to bone joints. Aside from tendons and ligaments, dense connective tissue makes up the dermis which is the lower layer of the skin.
4. Loose connective tissue – these are softer tissues containing more cells and fewer fiber. Areolar tissue is under this type which is the most widespread tissue of its kind. Adipose tissues also called fat is an areolar tissue where fat cells are predominating. Reticular connective tissues resemble fibroblasts which form the stroma that supports many free blood cells in the lymphoid organs in the body such as the lymph nodes, spleen and the bone marrow.
5. Blood – Blood is considered a connective tissue which is composed of blood cells that are surrounded by a fluid matrix called blood plasma. It is also termed as vascular tissue where the fibers it contains are soluble protein molecules that become visible only when the blood clots.

MUSCLE TISSUE

• Muscle tissues produce movement in the body by their ability to shorten and contract.

Types of Muscle Tissues

Skeletal Muscle Tissue

• Skeletal muscle tissues are muscles that can be consciously or voluntarily controlled. These tissues are attached to the skeleton which is packaged by connective tissue sheets into the skeletal muscles. When the muscles contract, they pull the bones or skin resulting to the gross body movements or changes in the facial expression. Skeletal muscle cells are elongated. The elongated shape of these muscles provides a long axis for contraction and they are often called muscle fibers. Obvious striations in the muscle cells are noted and they are long, cylindrical and multinucleated.

Cardiac Muscle Tissue

• Cardiac muscle tissues are found only in the heart. Unlike the skeletal muscle, cardiac cells are short and unicleated. However, cardiac muscles have striations also like the skeletal muscles. Presence of cardiac muscles allows the heart to pump blood through the vessels and be distributed to the different parts of the body. These muscle tissues function under involuntary control. Hence, a person cannot consciously control them.

Smooth Muscle Tissue

• Another term for smooth muscle tissue is visceral muscle. As the name implies these muscles are smooth, thus, no striations are visible. They are spindle-shaped and have a single nucleus. It is found in the walls of the hollow organs such as the stomach, the bladder, uterus and the blood vessels. Compare to the two muscle types, smooth muscles contract more slowly.

NERVOUS TISSUE

• Nervous tissues are also called the NEURONS in the body. Neurons are very important in the body as they are responsible for conducting and receiving impulses from a certain part of the body to the other. The distinct characteristics of neurons are their ability to function on IRRITABILITY and CONDUCTIVITY. Unlike the other tissues, their cytoplasm is drawn out into long processes, thus making their structure unique. The cytoplasm of the neurons can be drawn out as much as 3 feet long or more.

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