Animal Tissue PDF

ANIMAL TISSUES THE HAVERSIAN SYSTEM BONE TISSUE This tissue is dynamic and elastic. It can change its structure in response to organic and mechanical stimuli. It is formed by organic and inorganic components. The organic components are the typical cells of bone tissue and the ex...

ANIMAL TISSUES THE HAVERSIAN SYSTEM BONE TISSUE This tissue is dynamic and elastic. It can change its structure in response to organic and mechanical stimuli. It is formed by organic and inorganic components. The organic components are the typical cells of bone tissue and the extracellular matrix (amorphous substance and type I collagen). As for the inorganic components, they are minerals such as calcium and magnesium phosphates and Na, Mn, K citrates that are located in the extracellular matrix. The organic component of extracellular matrix corresponds to 35% of the dry weight of the bone and it provides strength and elasticity, while the mineralized inorganic component corresponds to 65% of the dry weight and provides bone tissue with consistency and hardness. This tissue often changes its structure and function due to age, diet and the general conditions of the individual. CELLS OF BONE TISSUE OSTEOPROGENITOR CELLS - responsible for the development of new bone tissue. OSTEOBLASTS - osteocyte precursors; guarantee organic matrix (osteoid) production as well as inorganic matrix deposition. They produce type 1 collagen, osteocalcin, osteopontin and bone sialoprotein. OSTEOCYTES - After bone development, when osteoblasts are trapped inside the lacunae within the matrix they produced, they become osteocytes. Their shape is irregular, their nuclei are elongated and clearly visible and their cytoplasm displays various extensions. They are located in bone lacunae from which multiple microscopic canaliculi branch off. Through these caniculi, osteocytes cytoplasmic extensions connect among themselves with communicating junctions and capillaries of bone canals. In this way, they allow metabolic exchanges among osteocytes and between osteocytes and blood. Osteocytes maintain the extracellular matrix of bone. OSTEOCLASTS – do not come from osteoblasts - they are derived from the fusion of numerous monocyte progenitors (up to 30) and their function is to destroy (resorb) and remodel bone tissue. They are large cells and their diameter can exceed 100 µm. Moreover, they have multiple nuclei. Osteoclasts are highly polarized too: if they become activated, their cytoplasmic face near the bone is characterized by highly movable ripples. Osteoclasts adhere to the bone surface, giving rise to a microenvironment that is separated by the surrounding (sealing zone). This environment is acidified through the subsequent activation of enzymes, whose derivation can be both lysosomal and non-lysosomal (in the first case, proteinase and phosphatase, in the second case metalloproteinase). This process causes the erosion of bone matrix and develops a resorption bay known as Howship’s lacuna. Loose connective tissue This tissue is characterized by the presence of numerous different cells and by a scarcely dense amorphous substance lacking in fibers. Reticular connective tissue This tissue is packed with reticular fibers composed of type III collagen. These fibers surround the single muscle fibers and the peripheral nerve fibers, in order to isolate them from each other. They associate to the basal lamina, underneath epithelia, surround the adipocytes and compose the thin meshwork that constitutes the connective stroma of lymphatic organs and large glands, both exocrine and endocrine. In embryonic life, during the transformation from mesenchymal to connective tissue, reticular fibers appear first. Subsequently collagen fibers substitute most of them. Skin of human palm, transverse section. Collagen reticular fibers, stained black by metallic silver, that contribute to formation of the basement membrane. Bielschowsky X100 Elastic connective tissue Elastic connective tissue is composed of non-birefringent fibers, different from the ones composing collagen connective tissue. These fibers are formed by elastin, amorphous substance and fibrillin. The structure of connective tissue can be organized in parallel fibers (e.g. the nuchal ligament of ruminants, human spine ligaments and tendons) or in fibers, in larger or smaller amounts, scattered in collagen (epiglottis, external ear, urinary bladder). Arterial walls do not present fibers and elastin is composed of fenestrated Human skin. The elastic fibers, stained black-purple (arrows), are wavy lamellae called elastic because they were fixed in resting state. In order to have a better contrast, the membranes. sample was stained also with H&E. Weigert/H&E X100 Dense connective tissue This tissue is marked by multiple fibers made of type I collagen. They are disposed in bundles (that can be even very thick), immersed in amorphous substance and oriented towards various directions. They can be parallel (tendons), crossed (cornea) or interwoven fibers (dermis). There are fewer cells, compared to loose connective tissue. Dense and loose connective tissues do not have precise borders between them when they Human eyelid. Dense connective tissue of the eyelid (tarsal plate) made up of are in contact. interwoven fibers, stained light blue-gray (arrows). Mallory-Azan X40 ADIPOSE TISSUE The cells composing adipose tissue are called adipocytes and their function is to accumulate fats into cytoplasmic vacuoles. There are two types of adipose tissue: white (unilocular) and brown (multilocular) White adipose tissue Its adipocytes contain a single large drop of lipid (the reason why it is called unilocular). This drop moves and flattens the nucleus in an eccentric position. Since it is not surrounded by a membrane, the lipid drop represents a cellular inclusion. Functions of white adipose tissue include, besides energy storage, insulation, cushioning and protection of some structures, such as in the orbits and retroperitoneal space. Brown adipose tissue The cytoplasm of adipocytes in brown adipose tissue contains multiple lipid droplets (that is why it is described as multilocular). Its nucleus is central, round and the cytoplasm is clearly visible. The function of brown fat is to accumulate fat reserves which will later be depleted, mainly in thermal energy. Hibernating animals have large amounts of brown adipose tissue. In humans, it is abundant in newborns and protects them from low temperatures. The amount of brown adipose tissue gradually decreases as the body grows and can be found around kidneys, in interscapular, axillary and pubic regions. Both types of tissue derive from a common progenitor cell, called preadipocyte, which is able to differentiate into white or brown fat. Adipose tissue of a marmot. You can see white (unilocular) adipose tissue, identifiable by the scant cytoplasm and the peripherally flattened nucleus of the adipocytes, along with adipocytes with a central roundish nucleus and an obvious cytoplams where there are several lipid droplets. These cells make the brown or multilocular adipose tissue. In mammals, this type of adipose tissue is normally present in the fetus, in the first months of life and in those animals that hibernate and must have a good reservoir of heat for a long period. In the other adult mammals, the brown adipose tissue is scarce. H&E X200 CARTILAGE Cartilage is a specialized connective tissue, formed by chondroblasts and chondrocytes. Such cells are surrounded by a jelly extracellular matrix with fibers. Cartilage is the only connective tissue lacking in blood vessels: for this reason, it gets its nourishment through the permeability of the extracellular matrix. Generally, cartilage is surrounded by the perichondrium, except in joints, where it is in direct contact with the synovial fluid. Chondroblasts are located in specific lacunae inside the extracellular matrix. where they can divide and create small cell groups known as isogenous groups, (surrounded by extracellular matrix.) Depending on the extracellular matrix characteristics and functions, cartilage can be divided into: Hyaline cartilage Fibrocartilage Elastic cartilage HYALINE CARTILAGE It is the most common cartilage in mammals. Its chondrocytes present a visible nucleus with one or more nucleoli. They are located in lacunae or depressions of the extracellular matrix. When hyaline cartilage is mature, chondrocytes tend to cluster in islets, the isogenous groups. They are more numerous in the deep zone of cartilage and tend to diminish in peripheral areas (intermediate zone and superficial zone). Isogenous groups are generally absent in immature hyaline cartilage. This cartilage is avascular and generally surrounded by the perichondrium. The matrix of hyaline cartilage lacks in fibers and is formed by an amorphous substance packed with proteoglycans, whose concentration is higher in the matrix surrounding isogenous groups (territorial matrix). The remaining matrix is characterized by fewer proteoglycans (interterritorial matrix). In addition to its supportive function, hyaline cartilage must provide flexibility to the skeleton. Elastic cartilage It has very little amount of extracellular substance, which lacks in amorphous component; The abundance of elastic fibers, which are interconnected especially in the deep zone, in order to create a network that surrounds the lacunae containing chondrocytes. The lack of isogenous groups, formed by few cells. Elastic cartilage is avascular and surrounded by perichondrium and it provides organs with higher elasticity (epiglottis, auricle, auditory or Eustachian tube). Blood It is a typical red fluid that circulates in a closed system of channels: the blood vessels. It derives from the mesenchyma and is composed of a corpuscular part (erythrocytes, leukocytes and platelets) and a liquid part (plasma). Its main function is to transport oxygen to the cells inside the tissues, other than transporting hormones. Erythrocytes These are small cells rich in hemoglobin, the substance from which they get their red color. Their main function is to bind oxygen inside the lungs and transport it to the various tissues. During maturation in the bone marrow, they lose DNA-dependent properties and the nucleus regresses until it disappears. In fish, reptile and bird erythrocytes, the nucleus is still present. The reduced size of red blood cells allows them to cross the most peripheral capillaries, while their typical biconcave shape makes sure that, size being equal, they have a greater exchange surface. Platelets Platelets are created inside the bone marrow, because of the budding of small parts of large cell cytoplasm, called megakaryocytes. Therefore, they are small, round or elongated corpuscles without a nucleus and microscopically visible only at high magnification, because their size is reduced compared to other blood elements, including erythrocytes. Platelets take part in the coagulation process, which is essential to reduce haemorrhages. Leukocytes Their nuclei are highly visible. When in the blood, they have a round shape, while in the connective tissue they take on an ameboid aspect. There are five types of leukocytes, classified depending on the presence and amount of cytoplasmic granules (called granular or agranular leukocytes) and on the shape of the nuclei (monomorphic nuclear leukocytes and polymorphonuclear leukocytes). Granular leukocytes are classified depending on the staining characteristics of the cytoplasmic granules: basophilic, eosinophilic and neutrophilic. Agranular leukocytes are divided, according to their size and shape of their nucleus, in lymphocytes and monocytes. Neutrophils These are the most numerous leukocytes, around the 50-70% of the total leukocytes. With a light microscope, they can be distinguished because of the shape of their nucleus, which presents three or more lobes, connected by thin chromatin bridges. In addition, bilobed nuclei have been detected, but they are very rare (at least in the blood). The number of lobes depends on the age of the cell: the more lobes it presents, the older the cell is. From these data, the level of regenerative capacity of the hemopoietic tissue may be easily deduced. In approximately 3% of these cells, a small, nuclear, drumstick-shaped appendage can be seen. This appendage is called Barr body (inactive X chromosome), an indicator of the female gender. Despite being clearly visible, the neutrophilic granules are generally poorly stained, hence their name. Neutrophils have phagocytic capabilities and contribute to protect the organism against bacterial infections. To do so, they migrate towards the endothelial cells and penetrate the connective tissue where they may carry out their function at their best. During bacterial infections, these leukocytes are produced in larger amounts. Eosinophils They are easy to recognize because of their specific granules, intensely stained with eosin and acidic staining methods in general. These cells always present a bilobed nucleus, whose lobes are connected by a thin isthmus. Generally, they represent 2-4% of blood circulating leukocytes, but they increase remarkably in case of allergy, parasite attack, inflammation and neoplastic disease. Basophils Basophils appear in many specific kinds of inflammatory reactions, particularly those that cause allergic symptoms. Basophils contain anticoagulant heparin, which prevents blood from clotting too quickly. They also contain the vasodilator histamine, which promotes blood flow to tissues. They can be found in unusually high numbers at sites of ectoparasite infection (e.g., ticks). Lymphocytes A lymphocyte is a type of white blood cell that is part of the immune system. There are two main types of lymphocytes: B cells and T cells. The B cells produce antibodies that are used to attack invading bacteria, viruses, and toxins. The T cells destroy the body's own cells that have themselves been taken over by viruses or become cancerous. Monocytes Monocytes represent 3-8% of blood circulating leukocytes. They are large, their nucleus is evident and its lobes are more numerous depending on the age, so that their look from that of a kidney to a horseshoe. Although the cytoplasm is rich in primary lysosomes, it does not present granules, which might be visible with a light microscope. They pass through the blood and migrate towards the connective tissue where they turn into either macrophages or dendritic cells. https://www.researchgate.net/figure/Photomicrograph-of-a-Human-Blood-Sample-37_fig1_356904462 THE NEURON Each neuron consists of: (1) a cell body, called soma, which includes the nucleus and cytoplasm (perikaryon) from which one or more cytoplasmic processes leave. (2) Such processes are suitable to receive impulses and they are known as dendrites. (3) The other neuronal component is the only cytoplasmic protrusion that transmits impulses: the axon (also known as neurite). The nucleus is located inside the soma, together with the cytoplasmic organelles and several ribosomes associated with the rough endoplasmic reticulum to form clusters. Such clusters are known as tigroid substance (Nissl bodies) and can be highlighted as basophilic granules with a light microscope. Dendrites are thin and highly branched. They can also present protrusions called spines. Generally, axons are much longer than dendrites. They arises from the soma, in the axon hillock, and can repeatedly branch out. Each branch ends with a button-shaped swelling: the synaptic ending or bouton. Synaptic endings can connect with both neurons and effector organs. The neuroglial cells In contrast to neurons, these cells generally maintain their proliferative capacity. Their aim is to support and cooperate during the neuronal functions. In the CNS, there are oligodendrocytes, astrocytes, microglia and ependymal cells, while in the PNS there are the Schwann cells and satellite cells. Oligodendrocytes and Schwann cells form the myelin in the CNS and PNS, respectively. The myelin is a lining formed by the wrapped oligodendrocyte/Schwann cell membrane, surrounding a section of the axon. Multiple following sections of the lining cover the whole axon with myelin. Astrocytes Astrocytes can be found in the CNS, where they represent the most important physical support for neurons and they contribute to creating the blood-brain barrier. They have a star-like shape and their extensions end with pedicels. Microglia cells These cells derive from the mesoderm and have a main phagocytic function. This is why they are considered the effectors of the immune protection of the CNS. Ependymal cells They line the brain ventricles, the central canal and the choroid plexuses. They contribute to the production of the cerebrospinal fluid. Ganglia (singular: ganglion) A ganglion is a group of neuron cell bodies in the periphery (a.k.a. the peripheral nervous system). Ganglia can be categorized, for the most part, as either (a) sensory ganglia or (b) autonomic ganglia, referring to their primary functions. Autonomic ganglia are clusters of neuron cell bodies that transmit sensory signals from the periphery to the integration centers in the CNS (central nervous system) https://histology.siu.edu/ssb/motoneur.htm https://www.ouhsc.edu/histology/Glass%20slides/3_09.jpg