Lesson 16: Tissue Anatomy Overview
As explorers of embodiment, yogis will encounter myriad sensations emanating from all the layers of the body/mind. As we dive into the miracle of aliveness and attempt to communicate our experiences with ourselves, our students and the general public, it will be helpful to have a familiar 3 dimensional vision and language of the body. Cells and tissues are fundamental entities that begin our understanding of a living body. They combine to create organs, organ systems, and finally the fully integrated living organism.
The basic unit of life is the cell, which manifests in two basic forms, prokaryotic and eukaryotic. Prokaryotes lack a membrane bound nucleus, can use sources of energy other than oxygen, and, in the form of various bacteria and the recently discovered Archaea, compose the majority of single celled organisms on the planet. Eukaryotes, dependent upon oxygen, have a distinct nucleus as well as other membrane bound, differentiated structures such as mitochondria, lysosomes and vacuoles. They form the basis for all multicellular life forms including plants and animals. In the Quantum Biology section, we will look at a transformationally new model of cell structure and function that moves beyond the Newtonian “cell as a bag with stuff in it” concept. (Although the picture to the left can be useful!)
In multicellular entities, the cells are suspended in a complex fluid/connective tissue matrix somewhat like a sponge, and as we will see in Quantum Biology, the matrix is also suspended inside the cell. It is a continuum. This living matrix is crucial to the health of the cells and the organism and will be the primary focus of our explorations. This matrix performs several necessary functions. It allows the organism to maintain a dynamic 3 dimensional shape by channeling the forces of gravity and water pressure. It provides a support structure for the surrounding blood vessels and nerves which nourish and inform the cells. And it provides a flowing, fluid ground substance that acts as a resevoir to hold electrolytes, nutrients and other chemical messengers, and help transport cellular wastes away.
The human body consists of around 100 trillion eukaryotic cells, (and about 10 times that number of bacterial cells!), divided into about 210 different, specialized cell types, including cells that circulate throughout the body like blood and immune cells, cells that secrete various hormones, and cells that provide a lining to vesels and organs.
The next order of complexity up from the cell is called tissue. A tissue is a collection of cells and associated intercellular materials which are specialized for a particular function. The human body is comprised of four categories of tissue: nervous, muscle, epithelial, and connective. These tissues are used in different proportions and arrangments to create the more complex organs. An organ is a group of tissues designed and organized to perform a particular function or group of functions such as the heart or the liver. Several organs can work together to form organ systems such as the cardiovascular, urinary, or reproductive systems. The living matrix interpenetrates all levels of cells, tissues, organs and organ systems and is the individual embodiment of what Buddhist teacher Thich Nhat Hanh refers to as interbeing, the realization that all of us are enmeshed in a web of interconnected relationship.
Nervous tissue can be considered either central (brain and spinal cord) or peripheral (nerves). It is composed of two major types of cells: nerve cells or neurons and glial cells. Neurons are the cells that carry out the essential function of nervous tissue: communication. They are the primary structural and functional unit of the nervous system and are specialized for sending signals rapidly over long distances to other cells. Neurons have a wide variety of shapes depending on their location and function. Different parts of the neuron are specialized for different tasks. Dendrites, and each cell can have many, receive information for other nerve cells. Axons, only one per cell, send information. Neurons have the capacity to react to various physical and chemical stimuli (irritability) and the ability to transmit the resulting excitation from one locality to another (conductivity).
Glial (glue) cells make up ‘The Other Brain’ as described by R. Douglas Fields in a book of the same name. Totally different from neurons, glia have their own communication network, insulate and support the neurons and even regulate the flow of information between neurons. They also provide nourishment and other aids to neuron function. Glia come in a wide variety of shapes and sizes depending on the particular type and the study of them is one of the more exciting explorations on the frontiers of brain science.
Muscle tissue has specialized, elongated contractile cells that perform their functions by developing a tension along their longitudinal axes. Specialized proteins, actin, myosin and troponin, allow the cells to shorten and lengthen and their ability to contract provides a mechanism for movement of the internal organs and locomotion of the entire organism.
Groups of muscle fibers may form fasciculi (fasciculus = small bundle), and many fasciculi, in turn, are aggregated into units we call muscles. There are 3 major types of muscle tissue: skeletal, cardiac and smooth. Skeletal muscle is attached to bones. Cardiac muscle is found in the heart. Smooth muscle is found in vessels, ducts, skin, and internal organs. Muscle tissue is may be controlled by nerves, hormones, local chemicals, or itself depending on the type and location.
There are various types of epithelial tissue with cells that are often arranged in sheets. This tissue covers all internal and external body surfaces, lying on a basement membrane which serves to anchor the cells. All materials that enter or leave the body do so through an epithelial membrane and thus some fundamental functions of epithelia include protection, absorption and secretion.
Epithelia are classified according to the shape of the cells (squamous, cuboidal,or columnar) and the number of layers (simple, stratified,or pseudostratified). A special type of epithelium, endothelium, makes up the walls of blood and lymph vessels. Another, transitional, is found in the bladder and ureterswhere it can expand and contract. Epithelial tissue is nourished by diffusion from blood vessels situated in the underlying connective tissue as no blood or lymph vessels are found in epithelium.
Epithelial tissue can produce downgrowths into underlying connective tissue called glands and contain cells whose functions are secretion and excretion. There are two main types of glands, exocrine glands and endocrine glands.
Exocrine glands possess ducts which convey the secretory material (mucus, enzymes, etc.) to the surface of the body or cavity lined by the epithelium. Endocrine glands have lost their connection to the epithelial lining from which they were derived and therefore lack ducts. These clumps of cells release their secretions (hormones) directly into the bloodstream, where they are distributed throughout the body.
Classification of glands can be very confusing and is based on branching of the ducts (simple vs compound), shape of the secretory region (tubular, alveolar, tubuloalveolar) and by the type of secretion (mucous, serous or mixed).
In biology, the extracellular matrix (ECM) is any material part of a tissue that is not part of any cell. Extracellular matrix is the defining feature of connective tissue. There are five different types of connective tissue: blood, bone, loose, dense and cartilage. Connective tissue connects, holds and supports other body tissues and cells and consists of three major components: cells, extracellular fibers and extracellular ground substance.
Connective Tissue Cells
Cells which inhabit the connective tissues provide defense as well as produce the supportive structures. Some cells remain in the connective tissue and function in its long-term maintenance. Examples include: fibroblasts
which are responsible for the formation of collagen, elastin and ground substance that comprise the extracellular component of connective tissue: mesenchymal cells (embryonic connective tissue cell commonly called stem cells) and adipose or fat storing cells.
Other wandering cells enter the connective tissue in response to injury or invasion by microorganisms. Examples include mast cells, antibody secreting plasma cells and the trash collecting macrophages. Mast cells are widely distributed in connective tissue and are particularly abundant along small blood vessels. They contain many granules whose contents are responsible for preventing blood clotting and increasing the permeability of capillaries and venules, thus allowing other cells to enter the connective tissue from the blood to fight foreign invaders.
Extracellular fibers, secreted by the fibroblasts, give connective tissue its strength. There are three types: collagen fibers are composed of the protein collagen. They have great tensile strength and are inelastic. Reticular fibers are very thin collagen fibers and form delicate networks around blood vessels, nerves and certain other cells. Elastic fibers are composed of the protein elastin. They stretch easily but return to their original length. They are most abundant in tissues that require flexibility, for example, the ligamentum nuchae on the back of the neck.
Ground substance is the name for the amorphous, gel-like intercellular material in which the cells and fibers of connective tissue are embedded. It is composed of water, proteoglycans (large molecules which help store and regulate the movement of ions and water), other plasma constituents, metabolites and ions, and functions as a medium through which nutrients can diffuse from blood vessels to nourish the cells and the waste products can diffuse back into the blood stream.
Types of Connective Tissue
Blood is a fluid connective tissue in which cells are suspended in a fluid matrix called plasma. Plasma is composed of water, protein and other solutes. Blood cells include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.
Loose Connective Tissue has an abundance of cells and ground substance, but relatively few fibers. It is soft and pliable and serves as a kind of packing material between other tissues and organs. It is found between muscles, allowing one to move freely over the other and supports small blood vessels, lymphatic vessels and nerves. When filled with fat cells, it is known as Adipose Tissue.
Dense or Fibrous Connective Tissue has a greater proportion of fibers, fewer cells and less ground substance than loose connective tissue. There are four types of dense connective tissue; fascia, tendons, ligaments, and aponeuroses. We will take a closer look at fascia in a moment.
Cartilage is a form of connective tissue that is much firmer than dense connective tissue. It consists of a dense network of fibers embedded in a gel-like intercellular material which confers firmness but also permits flexibility. The cells of cartilage are called chrondrocytes. Cartilage has no blood vessels and the cells are entirely dependent on diffusion as the source of their nutrients and oxygen. There are three basic types of cartilage that serve different functional requirements and vary by the type of fiber embedded in the ground substance.
Elastic cartilage has a large number of elastin fibers, allowing it to be very flexible and when deformed, able to immediately return to its normal position. It is found in the auricle of the ear.
Hyaline cartilage, the most widely distributed type of cartilage found in the body, has very little elastin and mostly collagen. This hard, translucent tissue first appears first as the developing bones in the embryo. As growth continues, the cartilage tissue is gradually replaced by bone tissue. At maturity, hyaline cartilage remains at the end of bones where they articulate with one another. Hyaline cartilage also supports the nose, larynx, trachea and bronchi of the respiratory system.
Fibrocartilage is the most abundant cartilage by weight. It is a both tough and elastic tissue present in regions of frequent stress. Containing both elastin and collagen, fibrocartilage is found in the intervertebral discs, the menisci of the knees and the pubic symphasis.
Bone is the most rigid form of connective tissue and is much firmer than cartilage. The hardness of a bone (equal to that of cast iron!) is caused by the presence of calcium phosphate; the small degree of elasticity possessed by bone is caused by the presence of organic collagen fibers. The cells of bone are called osteocytes. Unlike cartilage, bone contains small tubular canals through which the cells are nourished. Bone exists in two forms: compact and spongy.
Fascia, one of the three types of dense connective tissue, is located between the skin and the underlying structure of muscle and bone, and is a seamless web that covers and connects the muscles, organs, and skeletal structures in our body. It consists of three layers: the superficial fascia, the deep fascia and the subserous fascia. Muscle and fascia are united forming the myofascial system, the focus of attention in the body work systems operating under the umbrella of structural integration and including Rolfing and Kinesis Myofascial Integration.
The Superficial Fascia is located directly under the subcutis of the skin. Its functions include the storage of fat and water and it also provides passageways for nerves and blood vessels. In some areas of the body, it also houses a layer of skeletal muscle, allowing for movement of the skin.
The Deep Fascia is beneath the superficial fascia. It aids muscle movements and, like the superficial fascia, provides passageways for nerves and blood vessels. In some areas of the body, it also provides an attachment site for muscles and acts as a cushioning layer between them.
The Subserous Fascia is between the deep fascia and the membranes lining the cavities of the body. There is a potential space between it and the deep fascia which allows for flexibility and movement of the internal organs.
for more information:
http://mywebpages.comcast.net/wnor/ (on line anatomy course with great visuals!)