Human Anatomy and Physiology - I. V. Gaivoronsky 2011

The Human Body as a Single Entity. Fundamentals of Cytology and Histology
Tissues

Cells in the body cannot exist in isolation; together with the Extracellular matrix, they form tissues.

A tissue is an integration of cells and extracellular matrix specialized to perform specific Functions. In many cases, the cells that make up a tissue share a common origin and Structure. The extracellular matrix is the collective product of cellular activity, and its content, composition, and physicochemical properties serve as a hallmark of each tissue. Cells are the primary component of a tissue, though in some instances the extracellular matrix may play a more prominent role, providing, for example, the mechanical strength of bone or Cartilage.

Class="center">Table 3.2 MAIN TYPES OF Tissues

There are four main morphofunctional groups of tissues: epithelial, connective, muscular, and nervous. Each group comprises several subtypes, the primary ones being summarized in Table 3.2.

Epithelial Tissues

Epithelial tissues (epithelia) primarily perform boundary, or lining, and secretory functions. Situated at the interface between body tissues and the external environment, they fulfill a protective, or barrier, role. They also mediate the Metabolic exchange between the Organism and the external environment.

Epithelia cover the body surface and hollow Organs, forming an integral part of the mucous membranes lining the digestive tract, respiratory tract, Urogenital System, and others. Epithelial tissues also form numerous glands that secrete various products.

The main morphological features of epithelium are as follows:

1) a boundary position between internal and external environmental tissues;

2) arrangement of cells into tightly adherent sheets;

3) positioning of cells in one or multiple layers on a basement membrane (the basement membrane is a specialized structural formation between the epithelium and the underlying loose Connective Tissue);

4) a minimal amount of intercellular substance;

5) the absence of Blood Vessels, meaning that nourishment occurs through diffusion from underlying tissues;

6) a high capacity for regeneration—recovery following injury.

Epithelial tissues perform numerous functions in The Human Body:

1) boundary and barrier function—The primary function of the epithelium, which involves separating the body's internal and external environments;

2) protective function—preventing the damaging effects of mechanical, physical (Temperature, radiation), chemical, and microbial factors through both mechanical strength and the secretion of a protective mucus layer, The formation of cornified scales, and The production of antimicrobial substances;

3) transport function—The transfer of various nutrients across the epithelium into internal environments, or the movement of mucus containing dust particles and other debris along its surface;

4) absorption function—epithelia actively absorb various substances, which is most prominently expressed in the intestines and renal tubules;

5) secretory function—the epithelium forms the mucous membranes of hollow organs that secrete various fluids, and also serves as the primary tissue component of major glands;

6) excretory function—participation in eliminating final metabolic products from the body (via urine, sweat, and Bile) as well as various compounds, such as medications;

7) sensory (receptor) function—while performing its boundary role, the epithelium utilizes specialized structures to perceive mechanical, chemical, and Other types of signals originating from both the external and internal environments.

Based on their function, epithelia are categorized into glandular, lining, and sensory. Glandular Epithelium forms the mucous membranes of Internal Organs and major glands; lining epithelium forms various surfaces and linings, such as being a component of the Skin; sensory epithelium is a constituent of the Sense Organs.

Depending on the shape of the cells that form epithelial tissues, squamous, cuboidal, columnar, and cylindrical epithelia are distinguished.

According to the number of layers, epithelia are classified into simple and stratified. If all cells rest on the basement membrane, the epithelium is simple. In turn, simple epithelia can be single-layered (simple) or pseudostratified. Pseudostratified epithelium differs from Stratified Epithelium in that in pseudostratified epithelium every cell rests on the basement membrane, whereas in stratified epithelium only the basal layer contacts the basement membrane, while subsequent layers contact solely adjacent epithelial cells (Fig. 3.2). Depending on the presence or absence of a keratin layer, stratified squamous epithelium is subdivided into keratinized or non-keratinized. General Information on the localization of epithelia of various shapes in the human body is presented in Table 3.3.

Fig. 3.2. Main types of epithelium:

a — simple columnar; b — simple cuboidal; c — simple squamous (mesothelium); d — pseudostratified; e — stratified squamous non-keratinized; f — Stratified squamous keratinized; g — stratified transitional (organ empty); h — stratified transitional (organ full)

Table 3.3 Localization of epithelium in the human body

Connective Tissues

Connective tissues are widespread throughout the human body. They primarily perform mechanical and binding functions by connecting various structures together, form the internal environment of the body, and participate in maintaining its Homeostasis.

They are characterized by a pronounced predominance of extracellular matrix over cells.

Connective tissues perform numerous functions in the human body:

1) trophic — providing other tissues with nutrients;

2) transport — carrying nutrients, gases, and metabolic products;

3) regulatory — influencing the functions of other tissues via Hormones and BIOLOGICALLY ACTIVE SUBSTANCES;

4) protective — providing mechanical defense along with specific and non-specific immune responses;

5) respiratory — connective tissues participate in gas exchange processes occurring in tissues and organs;

6) skeletal/supportive — connective tissue forms the passive part of The Musculoskeletal System (bones and cartilage), forms the stroma of most internal organs establishing their internal framework, and forms the external framework of organs (capsules).

Connective tissues include: Proper Connective Tissue, which comprises loose connective tissue and Dense connective tissue; skeletal connective tissues, namely cartilage and bone; and Connective tissue with special properties, a group that includes adipose tissue, blood, lymph, and hematopoietic tissues.

Proper connective tissue. It contains reticular, Collagen, and elastic fibers. Loose connective tissue (Fig. 3.3) is characterized by a relatively low content of reticular fibers in the extracellular matrix, which form delicate, stretchable three-dimensional networks. It covers the exterior of Muscles and A number of internal organs. Collagen fibers exhibit high mechanical strength and form The basis of dense Fibrous connective tissue (tendons, ligaments, and fasciae). Elastic fibers are less mechanically durable, yet capable of stretching and returning to their original length and thickness once the tensile force ceases. Dense connective tissue is distinguished by a high content of fibers—predominantly collagenous—forming thick bundles that occupy the bulk of the tissue volume.

Fig. 3.3. Proper connective tissue:

a — loose; b — dense

Fig. 3.4. Types of cartilage tissue:

a — hyaline cartilage; b — elastic cartilage; c — fibrocartilage

Skeletal connective tissues. This group includes cartilage and Bone Tissues. Cartilage Tissues, in turn, are subdivided into hyaline, elastic, and fibrocartilage (Fig. 3.4).

Hyaline cartilage is the most common type of cartilage tissue in the body. It forms the fetal Skeleton, the anterior ends of the Ribs, the nasal cartilages, most of the laryngeal, tracheal, and major bronchial cartilages, and covers articular surfaces. The tissue gets its name from its Glass-like appearance (from the Greek hyalos meaning glass) and has a bluish tint.

Elastic cartilage is characterized by flexibility and The ability to undergo reversible deformation. It forms the cartilage of the auricle, the external acoustic meatus, the auditory tube, and the epiglottis. This cartilage has a yellowish color and, unlike hyaline cartilage, contains not only cells (chondrocytes) but also collagen and elastic fibers.

Fibrocartilage possesses significant mechanical strength. It forms the intervertebral discs and the Pubic Symphysis. The extracellular matrix of this cartilage contains dense fibers, which impart its special strength.

Bone tissues form the skeleton, which protects internal organs from damage, makes up the Locomotor System (movement), and serves as a mineral depot in the body. Bone tissue is formed by bone cells and a calcified (impregnated with minerals, predominantly calcium) extracellular matrix (Fig. 3.5). The following bone cells are distinguished: osteoblasts, osteocytes, and osteoclasts.

Fig. 3.5. Bone tissue

Osteoblasts are young, actively dividing bone cells that secrete unmineralized extracellular matrix and ensure its calcification.

Osteocytes are the main cell type of mature bone tissue. They are formed from osteoblasts and maintain the constant COMPOSITION OF THE bone matrix (extracellular matrix).

Osteoclasts are giant multinucleated cells responsible for bone resorption. Their number increases in old age and in a number of diseases, leading to Osteoporosis (thinning) of the bone tissue.

Bundles of collagen fibers are located in the extracellular matrix of bone tissue. Depending on the degree of their Organization, two Types of bone tissue are distinguished: woven-fibrous and lamellar.

Woven-fibrous bone tissue is characterized by an unorganized, chaotic arrangement of collagen fibers in the bone matrix, has low mechanical strength, and typically forms when osteoblasts produce extracellular matrix at a high rate. Fetal bones consist of this type of tissue, which is replaced by lamellar bone tissue as the fetus grows and matures. Its mineralized extracellular matrix consists of specialized bone lamellae containing highly organized, parallel-arranged collagen fibers.

Adipose tissue. It is a specialized variety of connective tissue in which the bulk is occupied by fat cells—adipocytes. In humans, Two Types of adipose tissue are distinguished: white and brown.

White adipose tissue forms superficial (subcutaneous adipose tissue) and deep accumulations (the omentum, adipose tissue around internal organs such as the Kidneys and eyeball). Connective tissue septa divide white adipose tissue into compartments (lobules). Brown Adipose tissue is found in humans in only a few locations: between the scapulae, in the axillae, and around the major Blood vessels of the neck; it is abundant in fetuses and newborns. The main functional difference of brown tissue is its tendency toward high oxidative activity under certain conditions, leading to the release of a large amount of heat accompanied by a sharp increase in blood flow within its vessels. Apparently, due to this, this type of adipose tissue is particularly well developed in newborns, whose thermoregulation function is imperfect.

Adipose tissue performs an energy function in the human body, serving as a reserve energy source when oxidative processes are activated, especially during periods of fasting. Its supporting and protective functions stem from its ability to cushion shocks and impacts, as it is located beneath the skin or around internal organs. The thermoregulatory function is related to the fact that this tissue is a good thermal insulator and prevents excessive heat loss from the body; under certain conditions, adipose tissue undergoes oxidation, which ensures heat production. In addition, it performs a storage function for Fat-soluble Vitamins and a number of hormones.

BLOOD AND LYMPH. They consist of a fluid portion and formed elements. The fluid portion of blood (plasma) is a specialized liquid extracellular matrix containing nutrients, hormones, dissolved gases, and cell metabolic products. Blood Plasma contains formed elements such as erythrocytes, leukocytes, and thrombocytes. The formed elements of lymph are lymphocytes, and its fluid portion is represented by interstitial (tissue) fluid, which is compositionally similar to blood plasma.

Hematopoietic tissues. Such tissues are located in the Red Bone Marrow (myeloid tissue), Thymus, Lymph Nodes, Spleen, Tonsils, and lymphoid nodules of the gastrointestinal mucosa (lymphoid tissue).

Muscle Tissues

Muscle tissues perform a contractile function in the body, which is carried out due to special Organelles called myofibrils. Muscle tissues exist in the form of smooth and striated (skeletal and cardiac) musculature (Fig. 3.6).

Smooth muscle tissue. It is found in the walls of internal organs, blood vessels, and Lymphatic vessels, as well as within certain glands. It consists of cells called smooth myocytes.

Fig. 3.6. Types of muscle tissue:

a — smooth muscle tissue; b — striated muscle tissue; c — Cardiac muscle tissue

Striated muscle tissue. It forms the basis of skeletal muscles and certain muscles within internal organs (muscles moving the eyeball; Muscles of the oral cavity walls, Tongue, pharynx, larynx, and upper third of the Esophagus). It consists of striated muscle fibers, which exhibit cross-striation due to the ordered arrangement of protein filaments: Actin and Myosin. The peculiarity of these muscle fibers is that they are multinucleated, formed As a result of the fusion of many cells (myoblasts). Skeletal Muscle contraction occurs voluntarily at a person's will. The structural principles of striated muscle tissue will be discussed in more detail in Chapter 6.

A special form of muscle tissue is cardiac striated musculature, which has a cellular structure (cardiomyocytes). The contractions of smooth muscles and cardiac muscle are not subject to conscious control. These muscles are involuntary.

Nervous tissue

Nervous tissue plays an integrating role in the organism, as its activity unifies the functions of numerous organs and individual body parts into a single, cohesive system. Nervous tissue comprises the nervous tissue proper, represented by Nerve Cells, and neuroglia, represented by glial cells.

Each nerve cell consists of a cell body with a nucleus, specialized inclusions, several short, tree-like branching processes known as dendrites, and a single (usually long) axon extending from The Cell body. Nerve cells are capable of perceiving stimuli from the external or internal environment, transforming the energy of the stimulus into a Nerve Impulse, conducting, analyzing, and integrating these impulses. Nerve impulses travel via dendrites toward the nerve cell body, and via the axon away from the body toward the next nerve cell or effector organ.

Neuroglia surrounds nerve cells (neurocytes), performing delimiting, supportive, trophic, and protective functions. Neuroglial cells also vary significantly in shape, size, and their relationships with Neurons.

The structural principles of nervous tissue will be discussed in greater detail in Chapter 14.

Tissue Regeneration

Tissue regeneration is the process that ensures tissue renewal during normal physiological activity (physiological regeneration) or recovery following injury (reparative regeneration). Reparative regeneration relies on the same mechanisms as physiological regeneration, though the processes occur more intensely. In complete regeneration, tissue is fully restored at the expense of its own resident cells. Incomplete regeneration results in a failure to fully restore tissue mass and volume, or the tissue is repaired through the proliferation of connective tissue, leading to scar formation (sclerosis).

Regeneration occurs at both the cellular (cellular regeneration) and subcellular (intracellular regeneration) levels. Cellular regeneration is accomplished through mitotic Cell Division. Intracellular regeneration ensures the continuous renewal of Cellular Structural Components under physiological conditions or following injury.

Cellular regeneration must be distinguished from cellular hypertrophy. Cellular hypertrophy is an increase in cell volume and functional activity accompanied by a simultaneous accumulation of intracellular structures. It results from enhanced intracellular regeneration under conditions where anabolic processes predominate. Hypertrophy typically leads to the greatest volumetric increase in those components that ensure adaptation of a given cell type to altered conditions (for example, Hypertrophy of the contractile and energy-generating apparatus in cardiac myocytes during increased physical exertion). Cellular atrophy is characterized by the reverse manifestations.

Tissue hypertrophy is observed either as a result of the hypertrophy of its individual cells while their number remains constant, or as a result of hyperplasia—an increase in cell number due to enhanced cell proliferation—or through a combination of these processes.