They consist of cellular elements and a large amount of intercellular substance of various chemical composition and structure. They play the role of support, communication, nutrition and protection, as well as a depot of mineral salts of the body. A general idea of the classification of connective tissue species is given in Fig. 8.
Fig. 8. Classification of types of connective tissue
Mesenchyma – embryonic, original connective tissue. It consists of small, loosely located cells connected by processes. Between the cells is a semi-liquid or gelatinous intercellular substance. Mesenchyma fills the space between the germ layers.
Blood and lymph are special types of tissues of mesenchymal origin that make up the internal environment of the body. They have a liquid consistency, consist of intercellular substance (plasma) and shaped elements suspended in it (Fig. 9). (For more information, see Chapter 5, “The system of blood and lymph circulation organs.”)
Fig. 9. Blood:
a – cattle;
b – chicken: 1 – red blood cells; 2.6 – eosinophilic granulocytes;
- 3, 8, 11 —lymphocytes: medium, small, large; 4 – blood platelets;
- 5, 9– neutrophilic granulocytes; segmented (mature), stab (young); 7 – basophilic granulocyte; 10 – monocyte; 12 – erythrocyte nucleus; 13 – non-granular white blood cells; 14 – granular leukocytes; c – red blood cells in a scanning electron microscope (according to V.P. Chumakov, V.N. Pismenskaya)
After slaughter of animals, blood partially remains in the capillaries and therefore is an integral component of meat. Currently, blood is of great importance as an independent raw material for the production of antianemic products; blood fractions use ZO to structure food systems, color products, obtain emulsions, and enrich products with organic iron, which is absorbed by the body 4-6 times faster than other sources.
The bulk of blood proteins is represented by albumin, globulin, fibrinogen and hemoglobin. Blood proteins as food raw materials are more effective than other proteins, can restore plasma proteins and hemoglobin in the body. In this regard, among food proteins, one of the first places belongs to them. The mass fraction of proteins in whole blood depends on the type, age, fatness, conditions of pre-slaughter keeping of animals and on average is: for cattle – 17.41%, for rams – 16.59, for pigs – 2.25%.
Some fractions of plasma proteins have gelling properties. Serum albumin is able to gel plasma. This fact opens up new prospects for the use of blood for food purposes, especially when obtaining structured products.
The use of blood and its fractions for food purposes, including the creation of non-traditional medical and preventive and special products, is relevant for solving the problems of rational use of resources, organizing waste-free technologies at meat processing plants, and improving environmental production conditions.
The endothelium lines the inner surface of all vessels, from the capillaries to the wall of the heart. Endothelial cells can capture tiny particles from the blood and digest them.
Reticular connective tissue has a network-like structure and consists of star-shaped process cells and reticular (argyrophilic) fibers, which, according to the degree of elongation, occupy an intermediate position between collagen and elastic, stained with silver salts. Most of the cells are networked using processes. Distinguish between reticular fibroblast-like cells associated with fibers, phagocytic cells of monocytic origin and low-specialized cells. Reticular tissue forms the stroma of the blood-forming organs and the microenvironment for the developing blood cells in them. It serves as the main tissue of the bone marrow, spleen, and lymph nodes. It occurs in the intestinal mucosa, kidneys, liver and other organs.
Loose fibrous unformed connective tissue is widely represented in the body of adult animals. It penetrates all tissues and organs in the form of layers and membranes, forms a layer under the skin, called subcutaneous tissue. The structure of loose connective tissue includes various types of cells and intercellular substance, represented by two types of fibers – collagenic, or adhesive, and elastic (Fig. 10).
Cell elements include fibroblasts, histiocytes, fat cells (lipocytes), pigment cells, mast cells (tissue basophils), plasmocytes, adventitious, or cambial cells.
Fig. 10. The loose connective tissue of a sheep:
- 1 – collagen fibers; 2 – elastic fibers; 3– fibroblasts;
- 4 – histiocytes
Fibroblasts are the most numerous group of multi-process cells with weakly expressed borders and a pale colored nucleus. They are able to synthesize fibrillar proteins (collagen, elastin).
Histiocytes are actively phagocytic, wandering cells with clearly defined borders and uneven edges.
Their nuclei contain large lumps of chromatin, intensively stained.
Mast cells (tissue basophils) are irregularly oval or rounded, sometimes with wide short processes. Numerous basophilic granules (grains) are located in the cytoplasm. They contain histamine, which helps to dilate blood vessels. Mast cells secrete heparin, which prevents blood coagulation. The number of tissue basophils depends on various physiological conditions of the body. For example, with active digestion, their number in the stomach, intestines and liver increases.
Pigment cells contain the melanin pigment in the cytoplasm. There are many such cells in the choroid of the eye, the skin.
Fat cells (lipocytes) arise from adventitious cells of loose connective tissue, which usually accompany blood vessels. They are located in groups and less often singly. Spherical lipocytes are filled with fat, which occupies most of the cytoplasm, the nucleus is pushed to the membrane. Accumulations of lipocytes form adipose tissue (Fig. 11).
Fibers and cells of loose connective tissue are immersed in a structureless amorphous substance. Collagen fibers consist of the finest protein filaments, or fibrils, have great tensile strength, and when sticking form an adhesive substance. Elastic fibers are thinner than collagen fibers, inferior to them in strength, contain globular protein elastin. Unlike collagen fibers, they do not bundle, but form a network.
Fig. 11. Adipose tissue:
a – in an optical microscope; b – microrelief of fat cells in a scanning electron microscope (according to V.P. Chumakov, V.N. Pismenskaya)
Dense connective tissues have a significant number of bundles of collagen or elastic fibers in the intercellular substance. Distinguish between collagen and elastic dense formed connective tissue (tendons, ligaments, fascia, etc.). Dense, unformed connective tissue forms the basis of the skin (reticular dermis).
Cartilage and bone tissue are skeletal tissue. They perform supporting, protective, mechanical, trophic, electrolytic functions, take part in water-salt, protein and other types of metabolism.
Cartilage tissue is characterized by a dense intercellular substance in which cartilage cells (chondrocytes) are located in groups and one by one. Nutrients diffuse from the surface through the perichondrium, which serves as the cambial and protective element of cartilage. There are three types of cartilage: hyaline, elastic and fibrous (Fig. 12).
Hyaline (vitreous) cartilage is found on the articular surfaces, the tips of the ribs, in the nasal septum, trachea and bronchi. Cartilage cells lie in groups, and closer to the perichondrium – one by one. The intercellular substance consists of amorphous substance and collagen fibers.
Elastic cartilage differs from hyaline cartilage by the presence in the intercellular substance along with collagen elastic fibers that penetrate the intercellular substance in all directions. In the elastic cartilage, calcification does not occur. It gives greater flexibility and elasticity to the auricle, also in the epiglottis, the external auditory canal.
Fig. 12. Cartilage:
a – hyaline cartilage of the rib: 1 – periosteum; 2 – cartilage zone with young cartilage cells; 3 – the main substance; 4 – highly differentiated cartilage cells; 5 – a capsule of cartilage cells; 6 – isogenic groups of cartilage cells; 7 – basophilic layers of the main substance around cartilage cells; b – elastic (mesh) cartilage of the auricle: 1 – perichondrium; 2 – the main substance; 3 – a network of elastic fibers; 4 – cartilage cells;
- 5 – a capsule of cartilage cells; 6 – nuclei of cartilage cells;
- 7 – isogenic group of cartilage cells; in– fibrous: 1 – bundles of chondrin (collagen) fibers; 2 – cartilage cells between bundles
Fibrous cartilage is intermediate between hyaline cartilage, tendons and fascia. The intercellular substance contains ordered bundles of collagen fibers. The result is a striped structure in which bands of hyaline cartilage alternate with bundles of collagen fibers. Fibrous cartilage is found between the vertebrae of the discs, at the junctions from the tendons to the bones.
Bone tissue consists of process cells of osteocytes and intercellular substance, abundantly saturated with calcium phosphate (up to 85%). In the process of aging, the amount of inorganic salts in the bones increases, so the bones of old animals become more fragile and more easily fractured. The intercellular substance of the bone is built from amorphous and fibrous protein matter. Collagen fibers can be randomly located in coarse-fibered bone tissue or in a strictly oriented direction in fine-fibrous (lamellar) bone tissue. In adult animals, coarse fibrous bone tissue is located in the area of overgrown cranial sutures and in the places where tendons are attached to the bones. All other coarse-fiber embryonic bones in adult animals are replaced by lamellar ones.
Lamellar bone tissue is formed by a set of ordered bone plates. The connection between the bone plates is very strong, since collagen fibers from one plate can pass into other neighboring plates. Bone cells and their processes lie in the bone cavities, repeating the contours of the osteocyte. The tubules of the bone cavities are filled with tissue fluid, are connected to each other. A compact and spongy substance is constructed from lamellar bone tissue in most flat and tubular bones of the skeleton (see Chapter 2, “The structure of the bone as an organ”).
Muscle. This tissue performs a motor function, has a complex structure of a protein apparatus capable of producing contraction. It can be of two types: smooth (unstriated) and striated (striated) – skeletal and cardiac.
Smooth muscle tissue develops from mesenchyme, consists of small spindle-shaped cells (myocytes). The rod-shaped core is located in the central part. Each myocyte is surrounded by a membrane. In the cytoplasm of cells in the longitudinal direction there are many thin contractile protein filaments, called myofilaments, which do not give a structural pattern, and appear smooth, not striated. Smooth muscle cells with the help of layers of connective tissue are collected in tightly packed bundles, forming complex systems with a dense network of blood vessels and nerves (Fig. 13). Smooth muscle tissue is found in blood vessels, the walls of hollow internal organs (stomach, intestines, bladder, etc.). It shrinks slowly and involuntarily.
Striated muscle tissueforms the muscles of the skeleton, forming the basis of meat. In morphological terms, meat is a complex tissue complex, which includes muscle tissue together with connective tissue formations, fat, bones, blood and lymph vessels, lymph nodes and nerves. The main and most valuable part of the meat is skeletal muscle. It is built from striated muscle fibers up to 13-15 cm long, with a diameter of 10-150 microns. In each fiber, the outer sheath (sarcolemma), cytoplasm (sarcoplasm), numerous nuclei and contractile protein filaments, myofibrils, are distinguished. The latter have a complex structure, consist of correctly alternating dark and light disks that unequally refract light, which gives the entire fiber a transverse striation (Fig. 14). Dark A-discs are anisotropic,
Fig. 13. The structure diagram of smooth (unstriated) muscle tissue:
- 1 – muscle tissue cell (myocyte); 2– core 3 – bundles of myofilaments;
- 4 – sarcolemma (membrane); 5 – endomysium; 6 – nerve; 7 – blood vessel
In the middle of the A-disk, a light strip N passes, it includes a narrower and darker strip M (mesophragm). Light 1-discs are isotropic and exhibit single refraction. In the isotropic disk of myofibrils there is a thin dense Z-plate (telophragm) dividing it in the middle. The sections of myofibrils enclosed between the Z-plates are called sarcomeres, which serve as structural and functional units of the contractile system. Each sarcomere consists of two halves of a light disk at the ends with Z-plates and a solid dark disk. Myofibrils are composed of the finest protein fibers called myofilaments (Fig. 15).
Fig. 14. Cross-striped muscle fibers of cattle:
a is a longitudinal section; 6 – cross section; c – microrelief of the large lumbar muscle in a scanning electron microscope (according to V.N. Pismenskaya), xZOOO
A-disk myofilaments are thick (10 nm in diameter, 1.5 μm long), and consist of myosin protein. Myofilaments of the 1-disk are thinner (5 nm in diameter, 2 μm long), contain actin protein and tropomyosin. Thin actin filaments are fixed in Z-plates. In the middle of thick myosin myofilaments there is a small thickening, which together creates an M-line (strip). A section of the A-disk occupied by the M-line and adjacent areas, in which only myosin filaments are located, is called the H-strip (bright zone). Actin and myosin myofilaments move relative to each other, due to which the actomyosin complex is formed, which is located in the overlap zone. The degree of entry of thin myofilaments between the thick depends on the degree of contraction of the muscle fiber.
Immediately after the slaughter of the animal, the metabolism characteristic of a living organism ceases. Biochemical processes that occur in meat after slaughter of animals as a result of the activity of tissue enzymes are called autolysis. The cooling temperature and the rate of its change determine the intensity of autolytic processes and affect the nature of changes in protein systems.
Fig. 15. Electronogram of muscle fibers in a transmission (transmission) electron microscope:
a – muscle fiber myofibrils (according to V. N. Pismenskaya ): 1 – strips Z; 2 – thick myosin filaments in a dark A-disk; 3 – thin actin filaments in a light 1-disk; 4 – median M-strip in dark disks; 5 – sarcomeres limited by Z-strips; 6 – sarcosome (mitochondria); b – scheme of the ultrastructure of skeletal muscle fiber
Fig. 16. The nature of crystallization in muscle tissue during freezing
(temperature -50 ° C):
a – cross section (numerous small voids are visible at the place of crystal localization inside the fibers; b – longitudinal section (rows of “distinct” voids) (according to V. N. Pismenskaya )
As a result of the development of enzymatic processes, the physiological apparatus of submicroscopic contraction (the fine structure of the actomyosin complex) is destroyed after a maximum of contractions. Muscle fibers are in varying degrees of contraction and relaxation, super contracted sections (contraction nodes), transverse breaks and longitudinal separation of the fibers occur. The meat becomes tender, its quality improves, but in the stage of complete rigor mortis, the meat is tough. According to the morphological features of the fibers, sarcomeres, super-contracted areas and other signs, it is possible to judge the functional and qualitative differences of individual muscles from different topographic areas of the animal’s body.
The structure of muscle tissue, which determines the quality of meat products manufactured, depends on the type of feeding, conditions of detention, stressful conditions of slaughtered animals, methods of slaughtering livestock, further technological processing of meat (salting, drying, freezing, etc.).
The crystals formed during freezing cause mechanical damage to the structure of muscle fibers, and the general appearance and thickness change. The nature of the distribution of crystals, their number, shape, size, structure and the degree of destruction of the morphological elements of muscle fibers and other parts of meat associated with them depend on the modes and methods of freezing (Fig. 16).
The ambassador helps preserve meat products for a long time, preserving, and in some cases improving their taste. The use of active influences in the process of salting increases the number of transverse microcracks and crevices in muscle fibers, leading to their fragmentation. The yield of fine-grained protein mass, the disappearance or weakening of transverse striation, the swelling of muscle fibers – all this indicates a rapid and uniform distribution of sodium chloride in muscle tissue.
In the process of drying, meat undergoes significant changes. Muscle fibers are reduced in size, one is separated from the other by free spaces. The presence of porosity in dried meat provides a fairly rapid distribution of moisture throughout the volume of the product. After aging in water, the meat approaches in structure to the original fresh form.
Nerve tissue consists of nerve cells (neurons, or neurocytes) and neuroglia. All elements of the nervous tissue make up a single nervous system of the body, which implements the interconnection of all tissues and organs and the connection of the body with the external environment.
Neurons perceive irritations and give response impulses with the help of their processes (Fig. 17). According to the functional value, the processes extending from the body of the neuron are divided into two types. Some perform the function of diverting a nerve impulse from the bodies of neurons and are called axons, or neurites. The axon is always one, it ends with the end device on another neuron or on the tissues of the working organ (muscles, glands). The second type of processes is dendrites,strongly branch, the number, length and nature of their branching is specific for different types of neurons. Dendrites conduct an impulse to the body of the cell. The cytoplasm of neurons is rich in organelles, and the abundance of granular endoplasmic reticulum indicates a high level of synthetic processes, in particular protein synthesis. Lumps of basophilic substance were detected in the body of the neuron and dendrites. In the cytoplasm in the form of a dense network are neurofibrils, which in the processes have a longitudinal orientation. Neurofibrils under an electron microscope correspond to finer filaments of neurofilaments and neurotubules.
The processes of nerve cells coated with membranes are called nerve fibers. They are divided into two groups – myelin and non-myelin. In the center of the myelin fiber lies an axial cylinder, or a process of a nerve cell. Such a fiber has a membrane formed by neuroglia cells – lemmocytes. The axial cylinder is covered with a membrane providing a nerve impulse. Non-myelin nerve fibers have several axial cylinders, and the membrane looks like a homogeneous strand of the cytoplasm of lemmocytes.
The non-myelin nerve fibers are predominantly part of the autonomic (vegetative) nervous system.
Myelin nerve fibers are thicker than myelin-free, found in the central and peripheral nervous system. They have one axial cylinder and two layers of the membrane: a thick myelin (internal) and external (thin, neurilemma), consisting of cytoplasm and nuclei of lemmocytes. Around the axial cylinder, lemmatocytes dressed with a plasmolemma are located in a chain. The axial cylinder is pressed into the cytoplasm of the lemmocyte, dragging along its surface plasmolemma, that is, it is as if suspended on a two-sheeted fold called mesaxone. It is twisted repeatedly around the axial cylinder into a tight spiral, forming a myelin sheath, which is an alternating layer of protein and lipid molecules.
Fig. 17. Scheme of a neuron (according to I. F. Ivanov):
a – the structure of the neuron: 1 – body (pericarion); 2 – core; 3 – dendrites;
4, 6 – neurites; 5, 8 – myelin sheath; 7 – collateral; 9 – interception of the node; 10 – the nucleus of a lemmocyte; 11 – nerve endings; b – types of nerve cells: I – unipolar; II – multipolar;
III – bipolyarnaya: 1 – Neriah; 2 – Dendrite
Neuroglia serves as the backbone in which nerve cells rest and function. According to morphology and function, two types of neuroglia are distinguished: macroglia (gliocytes) and microglia (glial macrophages).
Cells of neuroglia of various shapes with processes, nerve impulses do not conduct. Neuroglia performs supporting, trophic, secretory and protective functions.
In the body, none of the examined tissues in an isolated form is found. For example, in the epithelial tissue there are nerve fibers and nerve endings; in the muscle – nerve elements, blood vessels and connective tissue.
Organs and body. An organ is a formed part of the body that performs a specific function, built from naturally interconnected tissues and has a specific location. In each organ, the stroma is distinguished – the connective tissue skeleton of the organ and the parenchyma – the functional tissue of the organ located in this skeleton (frame). It feeds the blood and lymph vessels and nerves that control the organ.
The place of entry into the organ of blood vessels and nerves and the exit of the excretory ducts in the glandular organ is called the gate of the organs.
The parenchyma reflects the main function of the organ, so in each of them it is specific. For example, in skeletal muscle it is represented by striated muscle fibers, in the liver by hepatocyte epithelial cells. By structure, organs are parenchymal, or compact (liver, testis, etc.), and tubular, or hollow (stomach, intestines, bladder, etc.). Unlike compact organs, tubular organs have a cavity inside and consist of a mucous membrane, muscle and serous, or adventitia. The serous membrane lines the organs of the thoracic and abdominal cavities from the outside; outside their adventitia, the organ is firmly fixed to the fibrous connective tissue surrounding it.
A complex of organs of different structure, location and origin that perform a common vital function in the body are combined into the concept of an apparatus, for example, apparatus of movement, respiration, urination, etc.
The organ system, in contrast to the apparatus, is a complex of morphologically interconnected homogeneous organs that perform a certain function (cardiovascular system, nervous system). Sometimes a system is called organs of the same structure, performing a particular function in the apparatus. So, the bone and muscle systems are parts of the apparatus of movement.
In the body of domestic animals, three groups of systems are distinguished: somatic, visceral and unifying.
By somatic include bone, muscle and skin system (total) cover.
The internal ( visceral ) include digestive, respiratory and genitourinary apparatus. The digestive apparatus provides the body with essential nutrients for life. The breathing apparatus provides oxygen from the atmospheric air to the blood and releases carbon dioxide into the external environment. The urinary system is used to remove harmful metabolic products from the blood into the external environment.
Male and female germ cells are formed in the system of reproductive organs, fertilization and development of the embryo occur.
The unifying ( integral ) include the endocrine, vascular and nervous systems.
The endocrine system provides the interconnection of all organs through special biologically active substances of a protein nature – hormones that are distributed throughout the body by blood and regulate metabolism.
The vascular system is a vicious circle of vessels through which blood and lymph flow. Oxygen, nutrients, etc. come to the organs with blood, and carbon dioxide and metabolic products are carried away from the organs.
The nervous system coordinates the work of all body systems and its relationship with the external environment, due to which the unity of the body and the external environment is realized.
An organism is a complex, unified and integral living system in which everything is in close interconnection and interaction with each other in a genetic, morphological and functional sense.
Therefore, to understand the structural features of the organism of different animals, it is necessary to know not only the structure of organs in relation to their function and stage of development, but also in terms of the integrity of the organism, since any organ is part of the whole organism in interaction with all other organs.