SUGAR MILLS HIGH SCHOOL

Animal tissues can be grouped into four basic types: connective, muscle, nervous, and epithelial. Multiple tissue types comprise organs and body structures. While all animals can generally be considered to contain the four tissue types, the manifestation of these tissues can differ depending on the type of organism. For example, the origin of the cells comprising a particular tissue type may differ developmentally for different classifications of animals.

The epithelium in all animals is derived from the ectoderm and endoderm with a small contribution from the mesoderm, forming the endothelium, a specialized type of epithelium that comprises the vasculature. By contrast, a true epithelial tissue is present only in a single layer of cells held together via occluding junctions called tight junctions, to create a selectively permeable barrier. This tissue covers all organismal surfaces that come in contact with the external environment such as the skin, the airways, and the digestive tract. It serves functions of protection, secretion, and absorption, and is separated from other tissues below by a basal lamina.

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning. Organs are then formed by the functional grouping together of multiple tissues.

The study of tissue is known as histology or, in connection with disease, histopathology. The classical tools for studying tissues are the paraffin block in which tissue is embedded and then sectioned, the histological stain, and the optical microscope. In the last couple of decades, developments in electron microscopy, immunofluorescence, and the use of frozen tissue sections have enhanced the detail that can be observed in tissues. With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of clinical diagnosis and prognosis.

Organs are the next level of organization in the body. An organ is a structure that contains at least two different types of tissue functioning together for a common purpose. There are many different organs in the body: the liver, kidneys, heart, even your skin is an organ. In fact, the skin is the largest organ in the human body and provides us with an excellent example for explanation purposes. The skin is composed of three layers: the epidermis, dermis and subcutaneous layer. The epidermis is the outermost layer of skin. It consists of epithelial tissue in which the cells are tightly packed together providing a barrier between the inside of the body and the outside world. Below the epidermis lies a layer of connective tissue called the dermis. In addition to providing support for the skin, the dermis has many other purposes. The dermis contains blood vessels that nourish skin cells. It contains nerve tissue that provides feeling in the skin. And it contains muscle tissue that is responsible for giving you 'goosebumps' when you get cold or frightened. The subcutaneous layer is beneath the dermis and consists mainly of a type of connective tissue called adipose tissue. Adipose tissue is more commonly known as fat and it helps cushion the skin and provide protection from cold temperatures.

	Epidermis
	Dermis
	Subcutaneous layer

Organ Systems
Organ systems are composed of two or more different organs that work together to provide a common function. There are 10 major organ systems in the human body, they are the:

• Skeletal System:

Image courtesy of A. McGann	Major Role: The main role of the skeletal system is to provide support for the body, to protect delicate internal organs and to provide attachment sites for the organs.
	Major Organs: Bones, cartilage, tendons and ligaments.

• Muscular System:

Image courtesy of G. Huang	Major Role: The main role of the muscular system is to provide movement. Muscles work in pairs to move limbs and provide the organism with mobility. Muscles also control the movement of materials through some organs, such as the stomach and intestine, and the heart and circulatory system.
	Major Organs: Skeletal muscles and smooth muscles throughout the body.

• Circulatory System:

Image courtesy of G. Huang	Major Role: The main role of the circulatory system is to transport nutrients, gases (such as oxygen and CO2), hormones and wastes through the body.
	Major Organs: Heart, blood vessels and blood.

• Nervous System:

Image courtesy of G. Huang	Major Role: The main role of the nervous system is to relay electrical signals through the body. The nervous system directs behaviour and movement and, along with the endocrine system, controls physiological processes such as digestion, circulation, etc.
	Major Organs: Brain, spinal cord and peripheral nerves.

• Respiratory System:

Image courtesy of A. McGann	Major Role: The main role of the respiratory system is to provide gas exchange between the blood and the environment. Primarily, oxygen is absorbed from the atmosphere into the body and carbon dioxide is expelled from the body.
	Major Organs: Nose, trachea and lungs.

• Digestive System:

Image courtesy of A. McGann	Major Role: The main role of the digestive system is to breakdown and absorb nutrients that are necessary for growth and maintenance.
	Major Organs: Mouth, esophagus, stomach, small and large intestines.

• Excretory System:

Image courtesy of G. Huang	Major Role: The main role of the excretory system is to filter out cellular wastes, toxins and excess water or nutrients from the circulatory system.
	Major Organs: Kidneys, ureters, bladder and urethra.

• Endocrine System:

Image courtesy of G. Huang	Major Role: The main role of the endocrine system is to relay chemical messages through the body. In conjunction with the nervous system, these chemical messages help control physiological processes such as nutrient absorption, growth, etc.
	Major Organs: Many glands exist in the body that secrete endocrine hormones. Among these are the hypothalamus, pituitary, thyroid, pancreas and adrenal glands.

• Reproductive System:

Female: Male: Images courtesy of G. Huang	Major Role: The main role of the reproductive system is to manufacture cells that allow reproduction. In the male, sperm are created to inseminate egg cells produced in the female.
	Major Organs: Female (top): ovaries, oviducts, uterus, vagina and mammary glands. Male (bottom): testes, seminal vesicles

Cells are the fundamental units of life. All living organisms are composed of and depend on cells to function normally. Did you know that there are more bacterial cells in the body than human cells? Learn more interesting facts about cells!