Tissues, Organs, and Functions 2/3

Body tissues come in a limited number of basic types which specialize into numerous variations in terms of both specific functions and locations within the body.

Physiology

The study of the functioning of living things, especially including above the level of individual biochemical pathways or, for multicellular organisms, including above the level of the functioning of individual cells.

The science of physiology tends to be somewhat rooted in the chemistry of living things, that is, biochemistry. For multicellular organisms such as ourselves, this can include both what is going on inside of cells and what is going on outside of cells, and otherwise is seen in the functioning of larger, multicellular structures of the body.

The actual study of human physiology tends to place substantial emphasis particularly on what is going on outside of cells, which is versus the emphasis of the science of cell biology, which considers instead particularly what is going on inside of cells. Nonetheless, in the study, for example, of muscles, substantial emphasis is placed on the physiology of what is going on inside of cells, which happens to be where muscle contraction actually takes place.

Anatomy

The study of the morphology of living things, particularly the more macroscopic aspects of living things.

Anatomy tends to consider those qualities of a body that can be discerned without aid, that is, by use of the standard, unaided human senses alone, particularly sight. That statement comes with the caveat, however: The study of anatomy, at least historically, has been from the perspective of dead organisms when considering those aspects that are found below the skin. The study of the anatomy of living organisms, if not done invasively (that is, via especially surgery), instead can require substantial augmentation of our senses such as via the use of X-rays, MRI machines, endoscopes, etc. Human anatomy nonetheless is strongly rooted in the dissection along with visual inspection of cadavers.

The study of microscopic anatomy, which inherently requires an augmentation of our sight via magnification, instead is called histology.

Development

Cell division and cellular differentiation that results in conversion of an organism from immature or juvenile form to mature form.

Development in most animals begins at the point of zygote formation from the fusion of egg and sperm. It then proceeds through embryonic development and what is known as organogenesis. It continues even after birth, or hatching in the case of most non-mammalian animals, and even following the attainment of sexual maturity and/or adulthood.

In addition to the replication of cells, along with their physiological differentiation (i.e., cellular differentiation), development also involves the death of cells including in terms of cell-mediated suicide called apoptosis, which is how the body gets rid of unneeded, unnecessary, or even dangerous cells.

The above video has a nice introduction but, alas, it gets its definition of gastrula wrong.

The above video provides a quick introduction to early embryonic development. Note, though, the less than wonderful sound quality.

The above video provides a very human perspective of what it means to be a male, during development.

Tissue

Multiple cells possessing the same origin, displaying similar functions, and often similarly located within an organism.

The concept of tissue actually has multiple subtle meanings. The most basic of these are the primary tissues, which in animals come in either two, or more commonly, in three types. A second, and more general meaning, is as a description of living material, that is, the living tissue that makes up some generally macroscopic aspect of an organism. The third, and here the intended meaning, is that of subsets of organism bodies that consist of the same type of cells.

In animal tissues by this latter meaning come in four basic types: connective tissue, epithelial tissue, muscle tissue, and nervous tissue. Organs in turn consist of combinations of multiple basic tissue types. The study of tissues generally is done on a microscopic level and thus represents a microscopic aspect of anatomy, that is, which instead is called histology.

The above video is rather well done, providing a great overview of the four basic animal tissue types.

Primary tissues

Two or typically three layers of tissue found in early animal embryos that are the product of early cellular differentiation and specialization, and which are formed in the course of a developmental process known as gastrulation.

The primary tissues, also known as primary germ layers, consist of what are known as endoderm, mesoderm, and ectoderm. Following gastrulation, the early embryo possesses both an interior surface and an exterior surface. The interior surface lines a primitive cavity (not a body cavity, however) that forms into the alimentary canal, and this cavity is lined with endodermal tissue.

The exterior of the embryo is covered with tissue that will become our skin, and this consists of ectodermal tissue, which also gives rise to our nervous system (i.e., nervous tissue). The mesoderm forms between these two layers and gives rise to both connective tissue and muscle tissue.

The above video provides a very fast introduction to what primary tissues, here "embryonic germ tissue layers", are all about. The sound quality is not great, however, and this video also is presented above on this same page.

Epithelial tissue

Groups of cells in animals specialized in covering or otherwise encasing other cell types.

Epithelial tissue overlies basement membranes, receiving blood from the further underlying vasculature (capillaries), and generally possesses large numbers of between-cell junctions (such as tight junctions, gap junctions, desmosomes, etc.).

Epithelial tissue also serves as the lining of blood vessels themselves, there described as endothelium. Epithelial tissues serve as the absorptive surfaces within the gastrointestinal tract, and serve as the lining of the gastrointestinal tract generally, as for example the intestinal epithelium. Epithelial tissue also lines the lungs and, as groups of epithelial cells, make up endocrine glands.

Epithelial tissue comes in three types. These are called simple (single layer of cells), stratified (multiple layered), and an in-between type known as pseudostratified epithelium. Individual epithelial cells can be described as columnar epithelial cells (column-like), cuboidal epithelial cells (cube-like), and squamous epithelial cells (flattened).

In terms of primary germ layers, epithelial tissue comes from all three primary tissues, with the endoderm giving rise to the lining of the gastrointestinal tract, the ectoderm giving rise to the body's outer surface, and the mesoderm giving rise to internal epithelium, such as that covering organs as well as lining body cavities (e.g., such as the abdominal cavity).

The name "epithele" itself is derived from Greek meaning upon ("epi") and nipple ("thele"). Epithelial tissue is also one of four basic tissue types found in animals, the others being muscle tissue, nervous tissue, and connective tissue.

Nice, cute introduction to how to classify the different types of epithelial tissue.

The above video is pretty heavy duty in terms of its presentation of what epithelial tissue is all about.

Muscle tissue

Groups of cells in animals specialized for contraction.

Muscle tissue comes in three basic types, what are known as skeletal muscle, smooth muscle, and cardiac muscle. Each of these consist of contractile tissue, meaning that they are cells that can forcefully shorten themselves, but they do so in subtlety different ways. Basically these different ways are (1) substantially but at the expense of tiring, (2) slow and steady but without tiring, and (3) endlessly repetitively, respectively for skeletal muscle, smooth muscle, and cardiac muscle.

The basic unit of muscle is the muscle cell which contains the proteins actin and myosin. These proteins, coupled together, are able to push and pull against the other, resulting collectively in a shortening of the muscle cell. Muscle tissue is derived from the mesodermal primary tissue, that is, the stuff in the middle between the outside of an animal and that of the lining of the gastrointestinal tract (a.k.a., alimentary canal). Muscle tissue is also one of four basic tissue types found in animals, the others being epithelial tissue, nervous tissue, and connective tissue.

This video provides a nice, basic introduction to the three types of muscle tissue.

Nervous tissue

Groups of animal cells specialized in the conduction of information via impulses of membrane depolarization.

Nervous tissue is derived from ectodermal primary tissue, that is, as found on the outside of animal gastrula. Nerve impulses themselves are carried along what are known as nerve cells, which can be thought of as carrying information from one location to another. This is information that is "written" in binary bode with zeros represented by a lack of nerve impulse whereas ones are represented by the presence of a nerve impulse.

Additional nervous tissue cells are involved in support of the nerve cells, with the supporting nervous tissue cells called glial cells. Generally nervous tissue can be differentiated into that which makes up the brain or spinal cord (central nervous system or CNS) and that which is peripheral to the brain and spinal cord (peripheral nervous system or PNS).

Short and sweet, the above video provides a very quick introduction to what neurons, that is, nerve cells, are all about, at least anatomically.

Connective tissue

Groups of cells and often associated fibers in animals that are notable especially in terms of their association with a specialized extracellular matrix.

Connective tissue consists of a substantial diversity of tissue types including blood, bone, cartilage, and adipose. Connective tissue is involved in holding cells together. In soft tissues such as the spleen or bone marrow, connective tissue forms a reticular "skeleton" known as stroma. Connective tissue is also involved in connecting together other tissues as in the case of tendons or ligaments. Connective tissue also contributes to the outer margins of various discrete body parts such as organs, bone, and even muscle.

The extracellular matrix of connective tissue is referred to as its ground substance. When that ground substance consists of fibers, there are three basic types: collagen fibers (consisting of collagen), elastic fibers (consisting in part of elastin), and reticulating fibers (which also consist of collagen protein). Fibrous connective tissue also can be distinguished into loose connective tissue, dense connective tissue, elastic connective tissue, and reticular connective tissue.

The cells that secrete the ground substance are called fibroblasts, and connective tissue, like muscle tissue, is derived from the mesodermal primary tissue. Basically, then, connective tissue consists of mesodermal primary tissue cells called fibroblasts plus extracellular matrix, while extracellular matrix typically will consist of less fibrous ground substance plus fibers.

The above video provides a fairly good introduction to what connective tissue is all about, though it is a bit on the long side.

Dense connective tissue

Groups of similar types of animal cells that are associated with secretion of large amounts of collagen protein.

Dense connective tissue is especially strong, forming for example ligaments and tendons. Dense connective tissue is also found in association with skin as well as with scar tissue. Cartilage, in terms of its collagen density, can be viewed as representing a specialized form of dense connective tissue.

In association with the role of dense connective tissue in providing strong connections. Dense connective tissue therefore is typically relatively lacking in vascularization, that is, blood vessels, since those would represent relatively collagen-less weak points. As a consequence of the resulting relative lack of nutrients that otherwise would be supplied by blood, dense connective tissue can take both a long time to strengthen and a long time to heal if damaged.

For example with regard to strengthening, while it is relatively easy to build muscle via exercise programs, connective tissue will tend to lag in terms of its growth, resulting in an increased potential for, for example, tendon injuries or ligament injuries (often pushing yourself really hard early on in an exercise program is the worst thing you can do since it is so likely to give rise to injury). This is an important justification for starting out slowly when working on increasing the amount of exercise in your life.

A whole video discussing dense connective tissue (regular as well as fibrous). Pretty cool, if a bit redundant…

Elastic connective tissue

Groups of similar types of animal cells that are associated with secretion of collagen protein and which are able to sustain fairly substantial amounts of reversible deformation.

Aspects of the body, such as organs, that must reversibly withstand stretching or other means of deformation but which otherwise are relatively soft – and certainly not consisting primarily of skeletal muscle – will tend to be held together with elastic connective tissue. Elasticity is not just an ability to deform but also an ability to resist, to a degree, that deformation, returning to the original conformation once the deforming force is removed.

Lungs, for example, stretch as they fill with air, and then return to the original conformation (shape) as air is released. Indeed, part of the process of breathing includes exhalation that is driven in part by preceding deformation of elastic connective tissue. So too the pressure of our blood (blood pressure) is sustained between heartbeats to a large extent by the stretching of elastic connective tissue found in blood vessel walls in combination with the application of pressure against the blood as elastic fibers seek to return to their original shape.

These processes of course are much like what can be observed with a rubber band, which can be stretched and which then seeks to return to its original, unstretched shape.

Loose connective tissue

Groups of similar types of animal cells that are associated with secretion of collagen protein and which are associated especially with a substantial amount of flexibility.

Loose connective tissue provides flexible coverings to various somewhat discrete body parts such as muscles, organs, and also blood vessels and nerves. It notably possesses relatively little collagen fiber, though collagen fibers nonetheless are still present, just not as abundantly as compared with dense connective tissue. Loose connective tissue also possesses relatively little elastin fiber (compared to elastic connective tissue).

The fibers found in loose connective tissue are not aligned but instead can provide their strength in multiple directions. The result is strength and flexibility but neither exceptional strength nor exceptional flexibility/deformability. Loose connective tissue can be distinguished into areolar connective tissue and reticular connective tissue. Included among loose connective tissue, among other types, is adipose tissue.

Reticular connective tissue

Groups of similar types of animal cells that are associated with secretion of collagen protein and which provide an internal structural support to various soft organs.

While loose connective tissue consists of loosely associated, multidirectional fibers, they are found in more or less two-dimensional arrangements whereas reticular connective tissue is found instead in three-dimensional arrangements. Thus, while loose as well as elastic connective tissues can be viewed as holding together structures from the outside, essentially serving as a wrap around those structures, reticular connective tissue holds together what often are the same structures from within the tissues making up those structures.

This provides a resistance to tearing that also maintains the internal structure of organs as more than just a "soup" of associated cells. The liver, for example, is held together somewhat by reticular connective tissue, as too is the spleen.

Adipose tissue

Groups of cell types employed by the body to store fat.

The fat cells themselves are called adipocytes. Adipose tissue is employed for long-term storage of excess energy by the body as well as for cushioning organs and insulation against the cold. Adipose tissue is a form of connective tissue, and thus has a mesodermal origin, but is not an example of fibrous connective tissue. Indeed, adipose tissue possesses few extracellular protein fibers or indeed ground substance.

The utility of storing energy as fat is a consequence of the compactness and relatively low mass of stored fat which is the result of fat, gram-for-gram, containing somewhat more Calories than protein or carbohydrate (9 Calories per gram versus 4). In addition, fat molecules, unlike protein and carbohydrate, are not highly hydrated (i.e., there is little associated water) and thus as hydrated tissue (as is all tissue in our body) possesses even less mass.