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Chapter 4 Cells, tissues, and organs.

Functional anatomy begins with the cell, and an understanding of cell types and cell processes. Depending on their location, cells develop specialized functions. Next, groups of cells form tissues and then organs. These organs become parts of systems that function together to produce a healthy, productive animal.


After completing this chapter, you should be able to:

* Explain the importance of cells and their function to the horse

* Identify the parts and organelles of animal cells

* List and describe the functions of each of the major types of specialized animal cells

* List the cell organelles and the functions of each part

* Describe how specialized cells are organized to form a tissue type

* List and describe the six types of specialized animal tissues and their individual functions

* Describe the difference between meiosis and mitosis

* Describe blood and its function


adenosine triphosphate (ATP)












Golgi body





meiotic cycle











oxidative phosphorylation

plasma membrane


ribonucleic acid (RNA)









All living material is made of cells or the chemical products of cells. Understanding the cell as the fundamental unit of life is the basis for an understanding of living organisms such as the horse.

Modern cellular biology makes six assumptions:

1. All living material is made up of cells or the products of cells.

2. All cells are derived from previously existing cells; most cells arise by cell division, but in sexual organisms they may be formed by the fusion of a sperm and an egg.

3. A cell is the most elementary unit of life.

4. Every cell is bounded by a plasma membrane, an extremely thin skin separating it from the environment and from other cells.

5. All cells have strong biochemical similarities.

6. Most cells are small, about 0.001 cm (0.00004 inches) in length.

The three general functions of most cells include:

* Maintenance

* Synthesis of cell products

* Cell division

These functions require the cell to take in nutrients and excrete waste products. The nutrients are used either as building blocks in synthesizing large molecules, or they are oxidized--burned--producing energy for powering the cell's activities. Synthesis, maintenance, and mechanical and electrical activity all require energy. Cellular respiration is the process in which the chemical bonds of energy-rich molecules such as glucose are converted into energy usable for life processes. Here is the equation for the oxidation of glucose:

[C.sub.6][H.sub.12][O.sub.6] + 6[O.sub.2] [right arrow] 6C[O.sub.2] + 6[H.sub.2]O + Energy

Cellular respiration occurs in gradual steps that result in conversion of the energy stored in glucose to usable chemical energy in the form of adenosine triphosphate (ATP). ATP is known as the universal currency. ATP is constantly used and regenerated (Figure 4-1). Waste products of cellular respiration (carbon dioxide [C[O.sub.2]] and water [[H.sub.2]O]) are released through exhaled air, sweat, and urine.


Components of Cells

A cell is enclosed by a cell membrane. The material known as the cytoplasm lies within the cell membrane and contains several organelles and granules in suspension (Figure 4-2). Major components of the cell include:

* Plasma membrane

* Nucleus

* Ribosomes

* Endoplasmic reticulum

* Golgi body

* Centrioles

* Microfilaments, microtubules, lysosomes, and storage particles

Plasma Membrane. Cells are surrounded by a thin membrane of lipid (fat) and protein. This membrane controls the transport of molecules in and out of the cell, serving as a boundary between the cell and the surrounding tissue. Other membranes also occur in a cell's interior, for example, as part of the endoplasmic reticulum, nucleus, and mitochondria. The exterior portions of the cell surfaces determine cell-to-cell interactions, so they are important in formation and control of tissues. The extracellular material also acts as a glue that holds cells together in tissues.


Nucleus. Most cells have a single nucleus enclosed by a nuclear envelope, or membrane, with pores. Pores provide continuity between the nucleus and the cytoplasm. The nucleus contains one or more discrete structures, known as nucleoli, which are sites of ribosomal ribonucleic acid (RNA) synthesis. Hereditary information is carried in the DNA contained within the chromosomes in the nucleus. In the nucleus, this information is transcribed into the RNA, which serves as a messenger. The messenger then moves outside of the nucleus to the ribosomes, where it guides the synthesis of proteins. Thus, the nucleus directs the activity of the cell.

Ribosomes. Ribosomes are tiny particles within the cell. Made of RNA and protein, they are present in large numbers in most cells and are the site of protein synthesis (the manufacture of large protein molecules from amino acid subunits).

Endoplasmic Reticulum. Within most cells is a complex set of membranous structures. When viewed with an electron microscope, the membranes are either rough--covered with granules or ribosomes--or smooth. Generally, the rough endoplasmic reticulum is highly developed in cells that make large amounts of protein.

Golgi Body. A special type of membrane mixture is often found near the nucleus. This collection of membranes is called the Golgi body. In cells that synthesize and secrete lipids and proteins, the Golgi body is the site of accumulation.

Mitochondria. Mitochondria are composed of an outer membrane and a winding inner membrane. A series of chemical reactions that occur on the inner membrane convert the energy of oxidation into the chemical energy of ATP. In this process, called oxidative phosphorylation, the predominant energy transfer molecule is ATP. Almost all of the energy passes through this molecule before being used in cell function. Cells with high rates of metabolism usually have a large number of mitochondria.

Centrioles. Most cells have two cylindrical bodies, called centrioles, located near the nucleus. The centrioles appear as sets of triple tubules. Centrioles play a part in cell division.

Centromere. This is the region where chromatids join during the phases of cell division.

Other Organelles. The material containing the organelles is called ground substance, or cytoplasm. It contains proteins, small molecules, and a group of entities organized as microfilaments and microtubules. Microfilaments are long, thin, contractile rods that appear to be responsible for the movement of cells, both external and internal.

Microtubules are hollow, cylindrical groupings of tubelike structures that help give the cell shape and form. They are also involved in other cell processes.

Lysosomes are small bodies where large numbers of enzymes are stored. Some cells may also have particularly large liquid-filled areas known as vacuoles. The vacuoles are believed to be involved in digestion or excretion, or both.

Storage particles comprise a diverse group of structures and contain lipid droplets and glycogen granules whose function is the long-term storage of energy.


All organisms, regardless of their complexity, begin as a single cell. By repeated cell growth and mitosis, or division, the organism eventually develops into an adult containing thousands of billions of cells. This process of development is called morphogenesis. Because many different types of cells exist in fully grown animals, morphogenesis involves not only cell growth but differentiation into specialized types of cells (Figure 4-3). This differentiation is controlled by the genes. The information needed to program and guide the growth is contained within the chromosomes. Size, shape, and chemical activity of the cells are governed to some extent by the function of the tissue in which they are found.

Each cell contains the same total genetic information that was present in the fertilized egg. The cells are not identical, because in different types of cells, groups of genes are controlled--switched on and off--by various biochemical processes. Each cell manufactures the proteins and structures needed for it to function. Blood cells make hemoglobin to carry oxygen. Sperm cells make flagella, and so forth. On average, only about 10 percent of the genes of any cell are functional--which genes, in particular, vary with the type of cell. Although morphogenesis has been scientifically described in great detail for a number of organisms, all of the processes involved at the cellular level are still not understood.

Cell Division

The nucleus controls cell division; cell division depends on two events:

1. The replication (copying) of the DNA molecules that make up the basic genetic material of all cells.

2. The orderly separation of the products of this replication.

To survive, each new cell must have the same genetic code as its parent cell. For cells to reproduce, both the division of the nucleus and the division of the cytoplasm are necessary.

Mitotic Cycle

Mitosis is part of a more complex cycle that includes a long phase, called interphase. Due to the syntheses that take place during interphase, each chromosome consists of two sister chromosomes, called chromatids, that are identical in their structural and genetic organization and joined at the centromere. Chromatids become visible when mitosis sets in; the remainder of the mitotic cycle involves their separation into two offspring nuclei (Figure 4-4). Mitosis depends on four major events--coiling, orientation, movement, and uncoiling. The six essential stages of the mitotic cycle are

1. Prophase

2. Prometaphase

3. Metaphase

4. Anaphase

5. Telophase

6. Interphase

Measuring the Chemical Activity of Cells

Metabolism is the sum of all the chemical reactions in the living
cell. These reactions produce useful work and synthesize cell
constituents. Almost all cellular reactions are catalyzed by complex
protein molecules, called enzymes, that speed reaction rates by a
factor of hundreds to millions.

Many structures in the living cell must be periodically replaced.
This process of building new molecules is called anabolism.
Structures that are worn out or no longer needed are broken down
into smaller molecules and either reused or excreted. This process is
called catabolism. Great quantities of energy are required not only
to produce the work needed for pumping the heart, for muscular
contraction, and for nerve conduction but also to provide the chemical
work needed to make the large molecules characteristic of living cells.
Anabolism and catabolism are aspects of overall metabolism. They occur
interdependently and continuously.

In the digestion and metabolism of feed, oxygen is
used and carbon dioxide is given off. The rate of oxygen
consumption indicates the energy expenditure of a horse, or its
metabolic rate. The metabolic rate of a horse at any given time is
highly variable and is influenced by many factors, including
muscular activity, diet, digestion, lactation, pregnancy, time of
day or year, sexual activity, and stress.

To fix a point of reference, a convention has been adopted to serve as
the standard metabolic rate. The ideal standard established is the
metabolism of an animal under the least physiologically demanding
conditions. In the case of humans and livestock, this
minimal-rate-of-energy metabolism is called the basal metabolic rate
(BMR). It is defined as the rate of metabolism of a fasting animal at
rest and under no heat stress.

Meiotic Cycle

Mitosis rarely lasts more than 2 hours, but the meiotic cycle (meiosis) that produces gametes, or sex cells, may take days or weeks, since it involves two successive sequences of cell division and a reduction in the number of chromosomes. Each cell resulting from meiosis has one-half the number of chromosomes contained in the parent cell or other body cells. Also, the chromosomes in the resulting cells are sorted. The stages of the meiotic cycle include:

* Prophase with five substages

* Prometaphase

* Metaphase

* Anaphase

* Interphase

Types of Cells

During morphogenesis, several major types of animal cells form, including absorptive, secretory, nerve, sensory, muscle, and reproductive cells. All must arise during morphogenesis from cells that are less differentiated.

Absorptive Cells. Absorptive cells often occur as continuous sheets on surfaces where material is transported to the cells. For example, the single layer of epithelial cells lining the surface of the small intestine selectively absorbs food molecules from the gut into the bloodstream. These cells have a free surface that faces the digestive tract and a base surface that is in contact with the capillaries. The free surface is covered with many projections called microvilli, which vastly increase the area available for molecular flow (Figure 4-5).


Similar cells are found in the kidney, where a large surface area is needed for the absorption of protein, water, salts, and other materials. The microvilli are an example of a cell structure fitted to the function of the cell. Because an absorptive cell needs maximum area for transport, the shape of the cell surface is altered to achieve the optimum transfer of molecules.


Secretory Cells. Secretory cells produce products that are subsequently deposited in either the bloodstream or a special duct to an organ, where they are used. The pancreas and pituitary are glands that have large numbers of secretory cells. Proteins and other cell products are synthesized throughout the cytoplasm of these cells and transported to the Golgi apparatus, where they are packaged in membrane-bounded vesicles that come to a cell's surface and discharge the secretion outside the cell. Secretory cells in the spleen, lymph nodes, and other sites synthesize antibodies for the recognition and destruction of foreign molecules.

Nerve Cells. A nerve cell consists of a main cell body and a long, thin structure known as an axon (Figure 4-6). The function of nerve cells is to transmit electrical messages from one part of the cell body to another. These cells function like telephone transmission lines. The connections between nerve cells are called synapses. When these structures are combined, they form an electrical network known as the nervous system. The processes that occur at the synapses are both electrical and chemical. The axon is covered with a layer of insulation called myelin. Axons carry electrical signals called nerve impulses.

Sensory Cells. Sensory cells respond to impulses by emitting electrical signals. An example is the rod cell of the eye, in which the central cell body has two long, thin appendages. One appendage has an outer segment consisting of specialized stacked membranes for the reception of light. At the other end is a long, thin connection to a nerve cell that leads to the optic nerve fiber. About half of the material in the outer segment consists of rhodopsin, the pigment used in detecting light. Other sensory receptors include free nerve endings, pacinian corpuscles, ruffini corpuscles, taste buds, hearing receptors, and smell receptors (Figure 4-7).

Muscle Cells. Muscles are of three types--skeletal, (voluntary) cardiac, and smooth or involuntary (Figure 4-8). Contraction of muscle fibers generates a mechanical force. The skeletal muscle is a multinucleate structure with an outer envelope known as the sarcolemma. Skeletal muscle cells are actually a tissue in which the cells have merged. Most of the interior consists of long, thin myofibrils that are actually the contractile elements.




Reproductive Cells. Gametes are formed after completion of the process of meiosis, which halves the number of chromosomes in each cell. Stallion gametes or spermatoza are motile, whereas a mare's gamete or ovum (egg) is larger and stationary. Fertilization occurs when a sperm is fused with an egg. This stage is followed by morphogenesis (Figure 4-9).


Tissues are structured groupings of cells specialized to perform a common function necessary for the survival of the horse--a multicellular animal. The process of tissue formation or histogenesis evolves from the earlier process of cell differentiation.

The fertilized ovum or egg, a single cell, divides to form the blastula, in which tissues are not yet defined (Figure 4-10). As growth continues, the cells of the blastula begin to form three germ layers-- ectoderm, mesoderm, and endoderm--through the process of gastrulation. Cell differentiation during gastrulation begins the process of histogenesis and continues into the formation of organs.

The cells in a tissue look more or less alike and contribute the same type of service. Five general classifications of tissues are:

1. Nerve

2. Epithelial

3. Muscle

4. Connective

5. Fluid



Nerve Tissue

Nerve tissues consist of extraordinarily complex cells called neurons. The neurons respond in a specific way to a variety of stimuli so as to transfer information from one part of the body to another (Figure 4-11).

Epithelial Tissue

Epithelial tissue consists of a layer of cells covering the external surfaces of an animal and lining its internal tubes for digestion, respiration, circulation, reproduction, and excretion. This layer controls what is absorbed into and lost from the organism. The epithelium is composed of continuous sheets of adjacent cells. Outgrowths and ingrowths of epithelium form the sensitive surfaces of sensory organs, glands, hair and nails, and other structures.


Muscle Tissue

The ability to contract and relax and thus provide movement is characteristic of muscle tissue. Muscle tissue is of three types. Smooth muscle is activated by the autonomic nervous system. Skeletal muscle is controlled by the central nervous system and, to a certain extent, by the will (Figure 4-11). Cardiac muscle is characterized by its ability to contract rhythmically.

Connective Tissue

Connective tissues contain large amounts of extracellular material modified into different types. They are varied in structure to permit them to support the entire body and to connect its parts. Connective tissue includes fibrous tissue found in tendons and ligaments; elastic tissue found in ligaments between the vertebrae, arterial walls, and trachea; cartilaginous tissue found in joints and in the development of bone; and adipose tissue that, with its fat deposits, cushions and supports vital organs and stores excess food.

Fluid Tissue

Fluid tissues are the blood and lymph. These tissues function to distribute food and oxygen to other tissues, carry waste products from the tissues to the kidneys and lungs, and carry defensive cells and other substances to destroy disease-producing agents (Figure 4-12).



Groups of specialized tissues performing a specific function are called organs. The stomach is an organ of digestion. The uterus is an organ of reproduction. A group of organs working together is known as a system. For example, the stomach is only one of the organs in the digestive system, and the uterus is only one organ in the reproductive system.

Chapter 5 discusses the systems formed by the organs in the body of the horse.


All living material is made of cells or the chemical products of cells. Most of the cells in the body of the horse carry on the processes of maintenance, synthesis, and cell division. As cells carry on these life processes, they require energy and produce waste products.

All cells contain the same genetic information, but through morphogenesis cells grow and develop into specialized cells. Tissues are structured groupings of cells specialized to perform a common function necessary to the survival of the horse. The five basic tissue types include nerve, epithelial, muscle, connective, and fluid. Tissues combine to form organs, and organs group to form functional body systems.


Success in any career requires knowledge. Test your knowledge of this chapter by answering these questions or solving these problems.

True or False

1. Prophase occurs in both mitosis and meiosis.

2. Axons are a part of absorptive cells.

3. Groups of specialized cells are called organs.

4. An end product of mitosis is sperm cells or an egg.

5. The pancreas and pituitary are glands that have large numbers of secretory cells.

Short Answer

6. List 5 of the 10 major components of a cell.

7. Name three general functions of all cells.

8. Name two gametes.

9. Identify five general types of tissues.

10. What general type of tissue includes fibrous, elastic, cartilaginous, and adipose tissue?

11. Give an example of a fluid tissue.

12. What type of information is carried in DNA?

Critical Thinking/Discussion

13. Describe six assumptions that make the cell the fundamental unit of life.

14. Discuss the concept of morphogenesis.

15. Explain the difference between mitosis and meiosis.

16. Discuss the differences between skeletal, smooth, and cardiac muscle.

17. Describe the two events that cell division depends upon.

18. What is the importance of ATP?


1. From a biological supply company, obtain prepared microscope slides of epithelial tissue, muscle tissue, nerve tissue, connective tissue, and blood. View these through a microscope, and make sketches of each type. As an alternative, search the Internet for sites with photographs of tissue cross sections and histological (tissue) discussions.

2. Draw and label your own diagram of an animal cell. Label all of the components of the cell.

3. Develop a report on the production of ATP in the body. How is it produced, and where is it used? Briefly, outline the biochemistry involved in producing and using ATP.

4. Obtain a blood smear and staining kit. Using your own blood, make and stain some blood smears, and observe these through the microscope. Make drawings of your observations.



Asimov, I. (1954). The chemicals of life. New York: New American Library.

Aspinall, V., & O'Reilly, M. (2004). Introduction to veterinary anatomy and physiology. Oxford, UK: Butterworth-Heinemann.

Frandson, R. D., Fails, A. D., Wilke, W. L. (2003). Anatomy and physiology of farm animals (6th ed.). Philadelphia: Lippincott, Williams & Wilkins.

Hafez, E. S. E. (2000). Reproduction in farm animals (7th ed.). Philadelphia: Lippincott, Williams & Wilkins.

Kahn, C. M. (Ed.). (2005). The Merck veterinary manual (7th ed.). Whitehouse Station, NJ: Merck & Co.

Thomas, L. (1974). The lives of a cell: Notes of a biology watcher. New York: Viking Press.

Equipment and Supplies

Carolina Biological Supply Company, Carolina Science and Math Catalog 66, 2700 York Rd., Burlington, NC 27215-3398 <>

Fisher Science Education, 4500 Turnberry, Hanover Park, IL 60133 <http://www.>

NASCO Agricultural Sciences, 901 Janesville Ave., Fort Atkinson, WI53533-0901 <>

Nebraska Scientific, 3823 Leavenworth St., Omaha, NE 68105-1180 <http://>


Internet sites represent a vast resource of information, but remember that the URLs (uniform resource locator) for World Wide Web sites can change without notice. Using one of the search engines on the Internet such as Yahoo!, Google, or, find more information by searching for these words or phrases:

cellular biology

components of the cell

plasma membrane



endoplasmic reticulum

Golgi apparatus







cell division (mitotic cycle, meiotic cycle)

absorptive cells

secretory cells

nerve cells

sensory cells

muscle cells

reproductive cells


nerve tissue

epithelial tissue

muscle tissue

connective tissue

fluid tissue

Table A-18 in the appendix also provides a listing of some useful Internet sites that can serve as a starting point for further exploration.
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Author:Parker, Rick
Publication:Equine Science, 3rd ed.
Geographic Code:1USA
Date:Jan 1, 2008
Previous Article:Chapter 3 Breeds, types, and classes of horses.
Next Article:Chapter 5 Functional anatomy.

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