An introduction to animal structure

Although there is much debate as to how much cell specialization can be affected by external conditions, scientists do agree that there are three main factors which can influence cell specialization: 1. The contents of the cytoplasm 2. Environmental conditions 3. Neighbouring cells

Mitosis occurs in such a way as to ensure that each daughter cell receives an identical set of chromosomes. However, the contents of the cytoplasm may differ in each daughter cell. Why?

Cell specialization can also be affected by the environmental conditions it experiences as it develops. Such conditions include temperature and the presence/absence of certain nutrients. This helps explain why cells with identical genes will develop differently. Why? What other conditions might affect development?

Nearby cells in an organism’s body have one of the biggest influences on a cells specialization. When cells are close to one another, the substances produced by one cell can sometimes diffuse into the other cell, affecting how the DNA of the second cell is expressed. How do you think this happens?

Although it is thought that there is somewhere between 75 to 100 trillion cells in the average human body, those cells combine to form only a few different kinds of tissues. Animals have four main tissues: Epithelial Muscle Nervous Connective

- Line the surfaces of the body, both as a body covering and in between internal organs. - Made of cells with strong connections between adjoining cell membranes, so they form a barrier. Some types: Skin epithelial: made of thin, flat cells that form sheets and act as a semi-permeable barrier between the inside and outside of the body. Columnar epithelial: -made of columns of cells that line the small intestine, the stomach, and glands - May secrete mucus, have finger like projections called cilia, and absorb materials

- Designed to change their shape - Act by shortening or lengthening Some types: Skeletal muscle: - made of cells that line up in the same direction, making the tissue look striped, or striated. - Attaches to bone, making it possible for the body to move. - Is found in limbs, like arms and legs, and places where the body needs support, such as around the lower abdomen and back. Smooth muscle: - Made of cells that are tapered at both ends and do not have a striated appearance

- is found in blood vessels and the walls of internal organs like the esophagus and stomach - Contracts more slowly than skeletal muscle, but its action can be sustained for a long time Cardiac muscle: - Made of cells whose nuclei sometimes appear to be between cells - Are branched and unevenly striated - Contracts as a unit - Found only in the heart

- Made of cells called neurons, which have finger-like projections to receive and transfer signals - Coordinates body actions Some types: - Are varied in their actions: - some relay signals from the brain and spinal cord to muscles and glands - others direct information from their environment and trigger the body’s responses

- strengthens, supports, protects, binds, or connects cells and tissues - Consists of cells in an extracellular matrix that can range from a liquid (in blood) to elastic materials that can stretch (in ligaments), to mineral deposits (in bone) Some types: Bone: - Made of cells surrounded by calcium- hardened tissue through which blood vessels run - Needed for movement, support, and protection Fat: - Made of large, tightly packed cells

- Found under the skin and around organs - Needed for energy storage, padding, and insulation Blood: - Includes red blood cells, white blood cells, and platelets within a straw-coloured liquid matrix called plasma - Transports nutrients and oxygen - Clots when the skin is cut - Attacks invaders such as bacteria and viruses

As we have already learned, plants have cells called meristematic cells that are unspecialized and can continue dividing. Animals too have unspecialized cells that can continue dividing, called stem cells. Stem Cell: An unspecialized cell that can produce various specialized cells. Although some animals do have the ability to use their stem cells indefinitely to rebuilds certain organs, humans do not. This is because human organs are formed in the embryo. Early in their development, human embryos have a special kind of stem cell that can become any cell in the body, however shortly after development they no longer possess this ability and become a different, but similar, stem cell

Early in development: Totipotent - very versatile, can become any cell in the body Later in development: Pluripotent - Not as versatile; can only become certain cells in the body Embryonic stem cells: An unspecialized cell that is found in very young embryos – less than a week – and can become any one of an organisms body cells

Up until now, we have learned about cells, cell specialization, and tissues. As we have also already learned, a collection of tissues working together are called organs. Anytime a group of organs works together, the process is called a system (biological). Systems is the study of how all the organs in an organism function together and for what purpose. Biologists have many tools at their disposal that they can use to study and understand human systems. Such technologies are: 1. X ray 2. CT or CAT scan 3. Ultrasound 4. MRI Scan

The main functions in the human body needed to sustain life are accomplished by 11 organ systems working together. Circulatory system: -Transports blood, nutrients, gases, and wastes Digestive system: -Takes food and breaks it down -Absorbs nutrients -Removes solid waste from the body Respiratory system: -Controls breathing -Exchanges gases in the lungs Excretory system: - Removes liquid waste from the body

Immune system: -Defends the body against infections Muscular system: -Works with the bones to move parts of the body Endocrine system: - Manufactures and releases hormones that act, along with the nervous system, to keep various body systems in balance Reproductive system: - Includes reproductive organs for producing offspring Integumentary system: -Includes skin, hair, and nails - Creates a waterproof barrier around the body

Nervous system: -Detects changes in the environment and signals these changes to the body, which then responds Skeletal system: - Supports, protects, and works with muscles to move the parts of the body