Reticular tissue functions. Reticular tissue and capillaries are signs of connective tissues. Micrographs of reticular cells
We already wrote about the main terms and general components of ST in a previous article on the characteristics of connective tissue. Let us now characterize the individual connective tissue groups(ST).
Loose ST- this is the main and main tissue when it comes to connective tissue (Fig. 10). Elastic (1), collagen (2) fibers, as well as some cells, are included in its amorphous component. The most basic cell is the fibroblast (Latin fibra - fiber, Greek blastos - sprout or germ). Fibroblast is able to synthesize the constituent elements of the amorphous component and form fibers. That is, the actual function of the cell - fibroblast - is the ability to synthesize intercellular substance. Fibroblasts (3) with a large nucleus (a) in their endoplasm (b) and ectoplasm (c) contain a rather impressive endoplasmic reticulum, in which proteins such as collagen and elastin are synthesized. These proteins are the builders of the corresponding fibers. Another important cell in loose CT is the histiocyte (4). Microorganisms should be afraid of these cells, because getting into the intercellular substance, it phagocytizes them or, simply speaking, eats them. Finally, in color picture I, you can see another important cell of loose CT - this is a mast cell, it stores two biologically active compounds: heparin and histamine. Heparin is a substance that prevents blood from clotting. Histamine is a substance that takes part in various allergic reactions and inflammatory processes. Due to the release of histamine from mast cells, symptoms such as skin redness, hives, itching, blistering, burning, and anaphylactic shock are observed. 
Picture I. Loose connective tissue
Loose ST accompanies all vessels. The aorta is lined with a whole pillow - adventitia, and the smallest capillaries are surrounded by a very thin cobweb of fibers and cells. Vessels are protected, strengthened and, as it were, rely on this type of ST. And this means that loose ST is located wherever there are vessels. It is for this reason that it should be singled out as the main and main connective tissue.
A practical doctor in his daily work very often encounters one manifestation of loose connective tissue - edema. Glycosaminoglycans, which form an amorphous component, are able to retain water in themselves, which they do whenever possible. And this possibility appears in some pathological processes: heart failure, lymph stagnation, kidney disease, inflammation, and so on. In this case, fluid accumulates in the connective tissue, which swells, making the skin swollen. Sometimes swelling under the eyes can be the initial symptom of a disease such as glomerulonephritis, an immune inflammation of the kidney. 
Dense ST contains a very small number of cellular components and an amorphous component of the intercellular substance, most of the dense connective tissue is made up of fibers. There are two forms of dense ST. Dense unformed ST(Fig. 11) has a complete mess of fibers (4). Its fibers intertwine as they please; fibroblasts (5) can be oriented in any direction. This type of ST is involved in the formation of the skin, it is located under the epidermis (1) and a layer of loose ST (2) surrounding the vessels (3), and gives the dermis a certain strength. But in this she can not be compared with the strength dense decorated ST(Fig. 12), which consists of strictly ordered bundles (5), which in turn have a certain direction of collagen (3) and / or elastic (4) fibers. Formed connective tissue is part of the tendons, ligaments, albuginea of the eyeball, fascia, dura mater, aponeuroses and some other anatomical formations. The fibers are wrapped (1) and "layered" (7) with loose CT containing vessels (2) and other elements (6). Due to the parallelism of the tendon fibers, they receive their high strength and rigidity. 
Adipose tissue(Fig. 13) is distributed almost everywhere in the skin, retroperitoneal space, omentum, mesentery. Adipose tissue cells are called lipocytes (1 and picture II). They are very densely spaced, passing between them only such small vessels as capillaries (2), and with them the ubiquitous fibroblasts with individual fibers (3). Lipocytes are almost completely devoid of cytoplasm and are filled with large continuous drops of fat. The nucleus is shifted to the side, despite the fact that it is the regulator of the cell. 
Picture II. Adipose tissue
Adipose tissue is the body's most important source of energy. Indeed, during the breakdown of fat, much more is released than when using carbohydrates and proteins. In addition, a significant amount of water is formed in this case, so adipose tissue simultaneously turns out to be a reserve reservoir of bound water (it is not for nothing that this particular variant of ST is located in the humps of camels, which slowly break down fat when crossing hot deserts). There is one more function. In newborns, a special subspecies was found in the skin - brown adipose tissue. It contains a huge amount of mitochondria and due to this it is the most important source of heat for the baby that was born. 
Reticular tissue, located in the organs of the lymphatic system: in the red bone marrow, lymph nodes, thymus (thymus gland), spleen, consists of multi-pronged cells called reticulocytes. The Latin word reticulum means "net", which fits this fabric perfectly (Fig. 14). Reticulocytes, like fibroblasts, synthesize fibers (1), called reticular (collagen variant). This type of ST provides hematopoiesis, that is, almost all blood cells (2) develop in a kind of hammock, consisting of reticular tissue(picture III). 
Picture III. Reticular tissue
The last subspecies of ST proper - pigment tissue(Fig. 15) is found in almost everything that is intensely colored. Examples are hair, the retina of the eyeball, tanned skin. pigment fabric represented by melanocytes, cells filled with granules of the main animal pigment - melanin (1). They have a stellate shape: from the nucleus located in the center, the cytoplasm diverges in petals (2). 
These cells can give rise to a malignant tumor - melanoma. The disease has recently become much more common than before. In the last decade, the incidence of skin cancer has increased dramatically, it is believed that this is due to a change in the thickness of the ozone layer, which protects our planet with a powerful layer from the deadly effects of ultraviolet radiation. Over the poles, it has decreased by 40-60%, scientists even talk about "ozone holes". And as a result, in people roasting under the sun, melanocytes of birthmarks are the first to respond to the mutagenic effect of ultraviolet rays. Dividing non-stop, they give rise to tumor growth. Unfortunately, melanoma progresses rapidly and usually metastasizes early.
cartilage tissue(Fig. 16) - a tissue that has a very “good-quality”, concentrated amorphous component in its intercellular substance. Glycosamino- and proteoglycans make it dense, elastic, like jelly. This time, both the amorphous and fibrous components of the intercellular substance are synthesized not by fibroblasts, but by young cells of cartilage tissue, which are called chondroblasts (2). Cartilage has no blood vessels. Its nutrition comes from the capillaries of the most superficial layer - the perichondrium (1), where the chondroblasts are actually located. Only after “growing up”, they are covered with a special capsule (5) and pass into the amorphous substance of the cartilage itself (3), after which they are called chondrocytes (4). Moreover, the intercellular substance is so dense that when a chondrocyte divides (6), its daughter cells cannot disperse, and remain together in small cavities (7). 
Cartilage tissue forms three types of cartilage. The first, hyaline cartilage, has very few fibers, and it is found at the junctions of the ribs with the sternum, in the trachea, in the bronchi and larynx, on the articular surfaces of the bones. The second type of cartilage is elastic (picture IV), containing many elastic fibers, it is located in the auricle and larynx. Fibrous cartilage, in which collagen fibers are mainly located, forms the pubic symphysis and intervertebral discs. 
Picture IV. Elastic cartilage
Bone contains three types of cells. Young osteoblasts are similar in function to fibroblasts and chondroblasts. They form the intercellular substance of the bone, located in the most superficial layer rich in blood vessels - the periosteum. Aging, osteoblasts are included in the composition of the bone itself, becoming osteocytes. During the embryonic period, the human body does not have bones as such. The embryo has, as it were, cartilaginous "blanks", models of future bones. But gradually ossification begins, requiring the destruction of cartilage and the formation of real bone tissue. Destroyers here are cells - osteoclasts. They crush cartilage, making room for osteoblasts and their work. By the way, the aging bone is constantly replaced by a new one, and again, it is the osteoclasts that are engaged in the destruction of the old bone.
The intercellular substance of the bone tissue contains a small amount of organic substances (30%), in particular collagen fibers, which are strictly oriented in the compact bone substance (picture V) and disordered in the spongy one. The amorphous component, “realizing” that it is “superfluous at this celebration of life”, is practically absent. Instead, there are various inorganic salts, citrates, hydroxyapatite crystals, more than 30 trace elements. If you ignite a bone in a fire, then all the collagen will burn out; in this case, the shape will be preserved, but it is enough to touch it with a finger, and the bone will crumble. And after a night in a solution of some acid, in which all inorganic salts dissolve, the bone can be cut like butter with a knife, that is, it will lose strength, but on the neck (thanks to the remaining fibers) it will be tied like a pioneer tie. 
Picture V. Bone tissue
Last but not least connective tissue group, is blood. To study it requires a huge amount of information. Therefore, we will not belittle the meaning of blood by the description here, but leave this topic for separate consideration.
Brown tissue adipocytes are smaller in comparison with adipocytes of white adipose tissue cells, polygonal shape. The nucleus is located in the center of the cell, multiple fat droplets of various sizes are characteristic, therefore brown adipose tissue cells are called multilocular adipocytes. A significant volume of the cytoplasm is occupied by numerous mitochondria with developed lamellar cristae. The lobules of brown adipose tissue are separated by very thin layers of loose fibrous connective tissue, but very abundant blood supply. The terminals of sympathetic nerve fibers are immersed in areas of the cytoplasm of adipocytes. The brownish-red color of this type of adipose tissue is associated with a dense network of capillaries in the tissue, as well as a high content of stained oxidative enzymes - cytochromes - in the mitochondria of adipocytes. The main function of brown adipose tissue is thermogenesis, heat production . There are few oxisomes on the cristae of the mitochondria of the adipocytes of this tissue (the location of the ATP-synthetic complex). Mitochondria contain a special protein - UCP (u n c oupling p rotein - uncoupling protein), or thermogenin, due to which, as a result of fat oxidation, energy is not stored in the form of high-energy compounds (ATP), but is dissipated in the form of heat. The oxidative capacity of multi-lobe adipocytes is 20 times higher than that of single-lobe adipocytes. Abundant blood supply ensures rapid removal of the generated heat. With the flow of blood, heat is distributed throughout the body. The main factor causing thermogenesis and mobilization of lipids from brown tissue is the stimulation of the sympathetic nervous system, adrenaline, norepinephrine.
Reticular tissue
Reticular tissue is a specialized connective tissue that is included as a structural basis ( stroma) in the composition of hematopoietic tissues - myeloid and lymphoid. Its elements are reticular cells and reticular fibers form a three-dimensional network in the loops of which blood cells develop. Reticular cells are large, process-like, fibroblast-like cells that form a network. They are characterized by a rounded light nucleus with a large nucleolus, weakly oxyphilic cytoplasm. The processes of the reticular cells are interconnected by gap junctions.
Functions of the reticular tissue:
supporting;
creation of a microenvironment in myeloid tissue: transport of nutrients; secretion of hematopoietins - humoral factors that regulate the division and differentiation of blood cells; adhesive contacts with developing blood cells.
Synthetic: form reticular fibers and the main amorphous substance.
barrier: control of the migration of formed elements into the lumen of blood vessels.
Reticular fibers formed by type III collagen, braid reticular cells, in some areas are covered by the cytoplasm of these cells. The fibers are quite thin (up to 2 μm), have argyrophilia (stained with silver salts) and give the PAS-PAS reaction (Schiff-iodic acid, detects compounds rich in carbohydrate groups), since the reticular microfibrils are covered with a sheath of glycoproteins and proteoglycans.
Base substance– proteoglycans and glycoproteins bind, accumulate and secrete growth factors that affect the processes of hematopoiesis. Structural glycoproteins laminin, fibronectin and hemonectin promote adhesion of hematopoietic cells to the stroma.
In addition to reticular cells, macrophages and dendritic antigen-presenting cells are present in the reticular tissue.
pigment fabric
Pigment tissue is similar in structure to loose fibrous connective tissue, but contains significantly more pigment cells. Pigment tissue forms the iris and choroid of the eye.
Pigment cells are subdivided into melanocytes and melanophores.
Melanocytes- process cells in contact with other cells of this tissue. The cytoplasm contains a developed synthetic apparatus and a large number of melanosomes - granules containing the dark pigment melanin. These cells synthesize melanin.
Melanophores- have a poorly developed synthetic apparatus and a significant number of mature melanin granules. These cells do not synthesize, but only absorb ready-made melanin granules.
Other cells found in pigment tissue: fibroblasts, fibrocytes, macrophages, mast cells, leukocytes.
Functions of the pigment tissue: protection against damaging and mutagenic effects of ultraviolet radiation, absorption of excess light rays.
mucous tissue
Modified loose fibrous connective tissue with a sharp predominance of intercellular substance, in which the fibrous component is poorly developed. The mucous tissue has a gel-like consistency. It lacks blood vessels and nerve fibers. The mucous tissue fills the umbilical cord of the fetus (the so-called B a rton jelly). A similar structure has the vitreous body of the eyeball.
Cells of the mucous tissue are similar to fibroblasts, but contain a lot of glycogen in the cytoplasm. In the intercellular substance, a homogeneous and transparent ground substance sharply predominates. High content hyaluronic acid in the ground substance, creates a significant t at rgor, which prevents compression of the umbilical cord.
The term "" (Greek Mesos - middle, enchyma - filling mass) was proposed by the Hertwig brothers (1881). This is one of the embryonic rudiments (according to some ideas - embryonic tissue), which is a loosened part of the middle germ layer - the mesoderm. Cellular elements of the mesenchyme (more precisely, entomesenchyme) are formed in the process of differentiation of the dermatome, sclerotome, visceral and parietal sheets of the splanchiotome. In addition, there is an ectomesenchyme (neuromesenchyme) that develops from the ganglionic plate.
mesenchyme consists of process cells, network-like connected by their processes. Cells can be released from bonds, move amoeboidly and phagocytize foreign particles. Together with the intercellular fluid, mesenchymal cells make up the internal environment of the embryo. As the embryo develops, cells of a different origin than those from the above embryonic rudiments migrate into the mesenchyme, for example, neuroblastic differon cells, migrating myoblasts of the anlage of skeletal muscles, pigmentocytes, etc. Therefore, from a certain stage of embryo development, the mesenchyme is a mosaic of cells that have arisen from different germ layers and embryonic tissue primordia. However, morphologically, all cells of the mesenchyme differ little from each other, and only very sensitive research methods (immunocytochemical, electron microscopy) reveal cells of different nature in the mesenchyme.
mesenchymal cells show the ability to differentiate early. For example, in the wall of the yolk sac of a 2-week-old human embryo, primary blood cells - hemocytes - are isolated from the mesenchyme, others form the wall of primary vessels, and others are the source of development of reticular tissue - the backbone of hematopoietic organs. As part of the provisional organs, the mesenchyme undergoes tissue specialization very early, being the source of development of connective tissues.
mesenchyme exists only in the embryonic period of human development. After birth, only poorly differentiated (pluripotent) cells remain in the human body as part of loose fibrous connective tissue (adventitial cells), which can differentiate divergently in different directions, but within a certain tissue system.
Reticular tissue. One of the derivatives of the mesenchyme is the reticular tissue, which in the human body retains a mesenchymal-like structure. It is part of the hematopoietic organs (red bone marrow, spleen, lymph nodes) and consists of stellate reticular cells that produce reticular fibers (a type of argyrophilic fibers). Reticular cells are functionally heterogeneous. Some of them are less differentiated and perform a cambial role. Others are capable of phagocytosis and digestion of tissue decay products. Reticular tissue, as the backbone of hematopoietic organs, takes part in hematopoiesis and immunological reactions, acting as a microenvironment for differentiating blood cells.
These tissues are characterized by the predominance of homogeneous cells, with which the name of these types of connective tissue is usually associated.
Morphofunctional characteristics of reticular, pigment, mucous and adipose tissues.
These fabrics include:
1. Reticular tissue- located in the hematopoietic organs (lymph nodes, spleen, bone marrow). Comprises:
a) reticular cells- process cells that are joined to each other by their processes and are associated with reticular fibers;
b) reticular fibers, which are derivatives of reticular cells. In chemical composition, they are close to collagen fibers, but differ from them in smaller thickness, branching and anastomoses. Under an electron microscope, the fibrils of reticular fibers do not always have a clearly defined striation. Fibers and process cells form a loose network, in connection with which this tissue got its name.
Functions: forms the stroma of hematopoietic organs and creates a microenvironment for developing blood cells in them.
2. Adipose tissue are accumulations of fat cells found in many organs. There are two types of adipose tissue:
BUT) White adipose tissue; this tissue is widespread in the human body and is located under the skin, especially in the lower part of the abdominal wall, on the buttocks, thighs, where it forms a subcutaneous fat layer, in the omentum, etc. This fatty tissue is more or less clearly divided by layers of loose fibrous connective tissue into lobules . Fat cells inside the lobules are quite close to each other. The shape of fat cells is spherical, they contain one large drop of neutral fat (triglycerides), which occupies the entire central part of the cell and is surrounded by a thin cytoplasmic rim, in the thickened part of which lies the nucleus. In addition, small amounts of cholesterol, phospholipids, free fatty acids, etc. can be found in the cytoplasm of adipocytes.
Functions: trophic; thermoregulation; endogenous water depot; mechanical protection.
B) brown adipose tissue found in newborns and in some animals on the neck, near the shoulder blades, behind the sternum, along the spine, under the skin and between the muscles. It consists of fat cells densely braided with hemocapillaries. Fat cells of brown adipose tissue have a polygonal shape, 1-2 nuclei are located in the center, and in the cytoplasm in the form of drops there are many small fatty inclusions. . Compared to white adipose tissue cells, significantly more mitochondria are found here. The brown color of fat cells is given by iron-containing pigments of mitochondria - cytochromes.
Function: takes part in the processes of heat production.
3. Mucous tissue occurs only in the embryo, in particular in the umbilical cord of the human fetus. Built from: cells, represented mainly by mucosal cells, and intercellular substance. In it, in the first half of pregnancy, hyaluronic acid is found in large quantities.
Function: protective (mechanical protection).
4. Pigment fabric — it includes connective tissue areas of the skin in the area of the nipples, in the scrotum, near the anus, as well as in the choroid and iris, birthmarks. This tissue contains many pigment cells - melanocytes.
Connective tissues with special properties include reticular, adipose and mucous. They are characterized by the predominance of homogeneous cells, with which the very name of these types of connective tissue is usually associated.
Reticular tissue
Reticular tissue (textus reticularis) is a type of connective tissue, has a network structure and consists of process reticular cells and reticular (argyrophilic) fibers. Most reticular cells are associated with reticular fibers and are joined to each other by processes, forming a three-dimensional network. Reticular tissue forms the stroma of hematopoietic organs and the microenvironment for developing blood cells in them.
Reticular fibers (diameter 0.5-2 microns) are a product of the synthesis of reticular cells. They are found during impregnation with silver salts, therefore they are also called argyrophilic. These fibers are resistant to weak acids and alkalis and are not digested by trypsin.
In the group of argyrophilic fibers, proper reticular and precollagen fibers are distinguished. Actually reticular fibers are definitive, final formations containing type III collagen.
Reticular fibers, compared to collagen fibers, contain a high concentration of sulfur, lipids and carbohydrates. Under an electron microscope, fibrils of reticular fibers do not always have a clearly defined striation with a period of 64-67 nm. In terms of extensibility, these fibers occupy an intermediate position between collagen and elastic.
Precollagen fibers are the initial form of collagen fiber formation during embryogenesis and regeneration.
Adipose tissue
Adipose tissue (textus adiposus) is an accumulation of fat cells found in many organs. There are two types of adipose tissue - white and brown. These terms are conditional and reflect the features of cell staining. White adipose tissue is widely distributed in the human body, while brown adipose tissue is found mainly in newborns and in some animals throughout life.
White adipose tissue in humans is located under the skin, especially in the lower part of the abdominal wall, on the buttocks and thighs, where it forms a subcutaneous fat layer, as well as in the omentum, mesentery and retroperitoneal space.
Adipose tissue is more or less clearly divided by layers of loose fibrous connective tissue into lobules of various sizes and shapes. Fat cells inside the lobules are quite close to each other. In the narrow spaces between them are fibroblasts, lymphoid elements, tissue basophils. Thin collagen fibers are oriented in all directions between fat cells. Blood and lymphatic capillaries, located in layers of loose fibrous connective tissue between fat cells, tightly cover groups of fat cells or lobules of adipose tissue with their loops.
In adipose tissue, active processes of metabolism of fatty acids, carbohydrates and the formation of fat from carbohydrates take place. When fats break down, a large amount of water is released and energy is released. Therefore, adipose tissue plays not only the role of a depot of substrates for the synthesis of high-energy compounds, but also indirectly the role of a depot of water.
During fasting, subcutaneous and perirenal adipose tissue, as well as adipose tissue of the omentum and mesentery, rapidly lose their fat reserves. The lipid droplets inside the cells are crushed, and the fat cells become stellate or spindle-shaped. In the region of the orbit of the eyes, in the skin of the palms and soles, adipose tissue loses only a small amount of lipids even during prolonged fasting. Here, adipose tissue plays a predominantly mechanical rather than an exchange role. In these places, it is divided into small lobules surrounded by connective tissue fibers.
Brown adipose tissue is found in newborns and in some hibernating animals on the neck, near the shoulder blades, behind the sternum, along the spine, under the skin, and between the muscles. It consists of fat cells densely braided with hemocapillaries. These cells take part in the processes of heat production. Brown adipose tissue adipocytes have many small fatty inclusions in the cytoplasm. Compared to white adipose tissue cells, they have significantly more mitochondria. Iron-containing pigments - mitochondrial cytochromes - give brown color to fat cells. The oxidative capacity of brown fat cells is approximately 20 times higher than that of white ones and almost 2 times the oxidative capacity of heart muscle. With a decrease in ambient temperature, the activity of oxidative processes in brown adipose tissue increases. In this case, thermal energy is released, heating the blood in the blood capillaries.
In the regulation of heat transfer, a certain role is played by the sympathetic nervous system and the hormones of the adrenal medulla - adrenaline and norepinephrine, which stimulate the activity of tissue lipase, which breaks down triglycerides into glycerol and fatty acids. This leads to the release of thermal energy that heats the blood flowing in numerous capillaries between lipocytes. During starvation, brown adipose tissue changes less than white.
mucous tissue
Mucous tissue (textus mucosus) is normally found only in the embryo. The classic object for its study is the umbilical cord of the human fetus.
Cellular elements here are represented by a heterogeneous group of cells that differentiate from mesenchymal cells during the embryonic period. Among the cells of the mucous tissue, there are: fibroblasts, myofibroblasts, smooth muscle cells. They differ in the ability to synthesize vimentin, desmin, actin, myosin.
The mucous connective tissue of the umbilical cord (or "Wharton's jelly") synthesizes type IV collagen, characteristic of basement membranes, as well as laminin and heparin sulfate. Between the cells of this tissue in the first half of pregnancy, hyaluronic acid is found in large quantities, which causes the jelly-like consistency of the main substance. Fibroblasts of the gelatinous connective tissue weakly synthesize fibrillar proteins. Loosely arranged collagen fibrils appear in the gelatinous substance only at the later stages of embryonic development.
Some terms from practical medicine:
reticulocyte - a young erythrocyte, with supravital staining of which a basophilic mesh is detected; not to be confused with the reticular cell;
reticuloendotheliocyte is an obsolete term; earlier this concept included both macrophages, and reticular cells, and endotheliocytes of sinusoidal capillaries;
lipoma, wen - a benign tumor that develops from (white) adipose tissue;
hibernoma - a tumor that develops from the remnants of embryonic (brown) adipose tissue
