Cytology, General Histology and Embryology - V. K. Napkhanyuk 2002

Tissues
Connective Tissues
Proper Connective Tissue

Connective Tissues are subdivided into fibrous connective tissues and Connective Tissues with special properties.

The intercellular matrix (substantia intercellularis) of Connective Tissue consists of ground substance (or amorphous ground substance) and fibers.

Ground substance (substantia fundamentalis) is a gelatinous, hydrophilic medium containing carbohydrate compounds known as glycosaminoglycans, such as hyaluronic acid and chondroitin sulfates. These components determine the consistency and Functional Characteristics of the ground substance. In addition to these constituents, the ground substance contains Lipids, Blood albumins and globulins, as well as minerals (sodium, potassium, and calcium salts, etc.).

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Fig. 66. Classification of connective tissue

The amount of ground substance varies across different regions of connective tissue. It is sparse near capillaries, small vessels, and areas containing adipose tissue layers, whereas it is abundant at boundaries with tissues of a different origin, such as epithelium. Ground substance is more abundant in childhood than in adulthood and old age.

The amorphous component of the intercellular matrix is involved in Water METABOLISM, The regulation of ionic composition, Cell-fiber binding, Cell Adhesion, and other processes.

The physicochemical state of the intercellular matrix largely determines the Functional Properties of connective tissue. The denser the intercellular matrix, the more pronounced its mechanical and supporting functions. Conversely, the trophic function is better served by an intercellular matrix of a semifluid consistency, owing to the hydrophilicity of the Proteoglycans within the amorphous component.

Fibers of the intercellular matrix are divided into collagenous and elastic fibers.

Collagen fibers (fibra collagenosa) provide tensile strength to connective tissue.

The Internal Structure of a collagen fiber is determined by the fibrillar protein collagen. A collagen molecule consists of three polypeptide chains wound into a triple helix. Based on Chemical Composition and certain other properties, 12 types of collagen are distinguished.

Type I collagen is primarily characteristic of the connective tissue of the Skin, bones, cornea, sclera, and arterial walls.

Type II collagen is characteristic of hyaline and fibrocartilage, as well as the vitreous body.

Type III collagen is characteristic of fetal dermis, the walls of large Blood Vessels, and reticular fibers.

Type IV collagen is found in basement membranes and the lens capsule.

Type V collagen is located around fibroblasts (the Cells that synthesize it), endothelial cells, and smooth Muscle cells, forming an exoskeleton.

Collagen types VI through XII are insufficiently studied.

Collagen fibers occur in three varieties: true collagen fibers, reticular fibers, and precollagen fibers.

True collagen fibers exhibit high tensile strength and low extensibility. They are composed of Type I collagen, and electron micrographs reveal a transverse banding with a 64 nm periodicity (see Fig. 73).

Reticular fibers (fibrae reticularis) are found in reticular tissue and also form the supporting framework for glandular and muscle cells. They consist of Type III collagen and react with silver salts, which is why they are also termed argyrophilic fibers.

Precollagen fibers are immature collagen fibers composed of the collagen precursor, procollagen.

Elastic fibers (fibra elasticae) impart elasticity to tissues, enabling them to stretch and subsequently return to their original shape and length. Elastic fibers are inferior to collagen fibers in terms of tensile strength.

The primary chemical component of elastic fibers is the globular protein Elastin, which is synthesized by fibroblasts. The most mature elastic fibers contain approximately 90% elastin protein.

In addition to mature elastic fibers, immature elaunin and oxytalan fibers are also distinguished.

Cells of connective tissue

Connective tissue contains several types of cells that are heterogeneous in origin: fibroblasts, macrophages, plasmacytes, tissue basophils (mast cells), adipocytes, pigment cells, reticular cells, adventitial cells, and vascular pericytes (Figs. 67, 69, 70).

Leukocytes that have migrated from the bloodstream are also found in connective tissue.

Fibroblasts and fibroblastocytes (fibroblastocytus) are cells that produce Proteins (tropocollagen, elastin, Fibronectin) and glycosaminoglycans, which form fibers and the ground substance. They develop during Embryogenesis from mesenchymal cells and, after birth, from stem cells.

According to their degree of differentiation and functional activity, fibroblasts are classified into the following types:

— Semiactive stem-cell precursors;

— Underdifferentiated (young) fibroblasts — poorly branched cells with a round or oval Nucleus, a small nucleolus, and basophilic Cytoplasm containing RNA and numerous free Ribosomes. The Cell size does not exceed 20–25 µm;

— Specialized (mature) fibroblasts — actively functioning cells that can reach 40–50 µm in size. They contain light oval nuclei with 1–2 nucleoli. The cytoplasm is slightly basophilic, featuring a well-developed granular Endoplasmic reticulum that contacts the cytolemma in places, and a Golgi apparatus distributed throughout the cell in the form of cisternae and vesicles. Mitochondria and Lysosomes are moderately developed;

— Fibrocytes — definitive forms of fibroblasts, spindle-shaped cells with wing-like processes. They contain a small number of Organelles, vacuoles, lipids, and Glycogen. Collagen synthesis and other metabolic activities in these cells are sharply reduced.

The activity of fibroblasts is associated with The formation of the ground substance and fibers, wound healing, scar tissue development, and the formation of a connective tissue capsule around a foreign body.

In addition to the aforementioned forms, There are also fibroclasts, which participate in the degradation of collagen fibers, and myofibroblasts, which play a role in wound contraction and area reduction.

Macrophages (macrophagocytus) are wandering, actively phagocytic cells rich in organelles for intracellular Digestion of engulfed material and the synthesis of antibacterial and other BIOLOGICALLY ACTIVE SUBSTANCES (pyrogen, interferon, Lysozyme, etc.). Macrophages originate from hematopoietic stem cells. Based on their maturity level in connective tissue, monocytoid, transitional, and mature macrophages are distinguished. Macrophages are characterized by an Abundance of cytoplasmic lysosomes and numerous microvillous projections on their surface.

Tissue basophils, or mast cells (granulocytus basophilus textus), are cells that synthesize, store, and secrete a range of biologically active substances, such as heparin, serotonin, histamine, and dopamine. A characteristic feature of mast cells is the presence of specific granulation in their cytoplasm. These cells act as regulators of local connective tissue Homeostasis and participate in reducing blood clotting, increasing the permeability of the blood-tissue barrier, and mediating inflammation and immunogenesis.

Plasmacytes (plasmocytus) are cells that develop from B lymphocytes and participate in humoral immune responses by producing Antibodies (gamma globulins) against foreign proteins, microorganisms, and their toxins.

The cytoplasm of these cells contains a well-developed granular Endoplasmic reticulum and a Golgi apparatus.

Adipocytes, or fat cells (adipocytus), are cells capable of storing large amounts of reserve fat, which is involved in trophism, energy production, and water metabolism. Adipocytes are located in groups, less frequently singly, typically near blood vessels. When accumulated in large quantities, these cells form adipose tissue.

These cells are spherical in shape. A mature cell generally contains a single large neutral fat droplet occupying the center of the cell, surrounded by a thin cytoplasmic rim with The Nucleus located in its thickened portion.

Pigment cells (cellula pigmentosa) are cells that contain the pigment melanin in their cytoplasm. Pigment cells (melanocytes) have short, variable processes.

Reticular cells (reticulocytus) are the structural analogs of reticular tissue fibroblasts in Hematopoietic Organs, and they produce reticular fibers.

Adventitial cells are poorly differentiated Cells of the fibroblastic Lineage located along blood vessels. They have a flattened or spindle-shaped Morphology with mildly basophilic cytoplasm, an oval nucleus, and poorly developed organelles.

Pericytes (pericytus) are branched cells closely apposed to the exterior of arterioles, capillaries, and venules.

Various types of leukocytes emigrate from blood vessels to perform protective functions within the connective tissue.

Slides for Study

Slide 17. Loose fibrous irregular connective tissue. Whole-mount preparation (Fig. 67).

Low power. Locate the most transparent area of the slide.

High power. Against the Background of the transparent amorphous substance, cells and fibers are clearly visible. Locate fibroblasts, which are characterized by a branched shape, indistinct borders, and a light oval nucleus. Macrophages are distinguished by smaller, darker, round nuclei and darker, vacuolated cytoplasm with sharply defined, irregular borders. Plasmacytes are round with an eccentric nucleus containing condensed Chromatin; an unstained cytoplasmic area (the so-called hof or Golgi zone) is located adjacent to the nucleus.

Each cell must be drawn individually.

Label in the drawing: 1) fibroblast showing: a) processes; b) nucleus; 2) macrophage showing: a) vacuoles; b) nucleus; 3) plasmocyte showing: a) nucleus; b) chromatin; c) Golgi zone (“hof”).

Slide 18. Loose connective tissue. Section of subcutaneous adipose tissue (Fig. 68).

Low magnification. Locate large cells distributed along blood vessels. Their cytoplasm is packed with purple granules.

High magnification. Mast cells are rounded in shape. The nucleus is centrally located and stained orthochromatically blue with Azure. The cytoplasmic granules are stained metachromatically purple.

Fig. 67. Loose irregular connective tissue. Whole-mount preparation. Iron hematoxylin stain. × 600:

1 — fibroblasts; 2 — fibrocyte; 3 — macrophages; 4 — mast cells; 5 — plasmocytes

Draw a mast cell.

Label in the drawing: 1) granules; 2) nucleus.

Fig. 68. Loose connective tissue. Section of subcutaneous tissue. Azure II-eosin stain. × 400:

1 — blood vessels; 2 — mast cells; 3 — endothelial cells; 4 — adventitial cells; 5 — fat cells; 6 — plasma cells; 7 — collagen fibers

Fig. 69. Fibroblast from guinea pig skin wound. Electron micrograph. × 18,000:

1 — fibroblast nucleus; 2 — mitochondria; 3 — endoplasmic reticulum; 4 — collagen fibers (after Ross)

Fig. 70. Macrophage from a Lymph node. Electron micrograph. × 13,000:

1 — cellular microvilli; 2 — lysosomes with a finely granular component; 3 — digestive vacuoles; 4 — mitochondria; 5 — endoplasmic reticulum; 6 — intracellular reticular apparatus (after I. B. Tokin)

Review Questions

1. Connective tissues. General characteristics. Classification.

2. Morphofunctional characteristics of loose connective tissue.

3. Cells of loose connective tissue.

4. Sources of development, structure, and functions of:

a) fibroblasts, fibrocytes, fibroclasts, myofibroblasts;

b) macrophages;

c) tissue basophils;

d) plasma cells;

e) adipocytes;

f) pigment cells;

g) adventitial cells.

5. THE CONCEPT OF the macrophage system.

6. Interrelationship between cells of loose connective tissue and blood.

Situational problems

1. Cytology/cytology/93.html">ELECTRON MICROGRAPHS OF three fibroblasts are provided. One shows a fibroblast with dispersed nuclear chromatin, A large number of free ribosomes and Polysomes in the cytoplasm, and relatively poorly developed membrane-bound organelles; the second shows a fibroblast with compact nuclear chromatin and a small volume of cytoplasm very poor in organelles; the third shows a fibroblast with a well-developed granular endoplasmic reticulum and Golgi apparatus. What types of fibroblasts are represented in the micrographs?

2. Why are plasma cells very rarely found in the subcutaneous connective tissue, whereas they are abundant in the connective tissue of the intestinal mucosa?

3. In histological specimens, a large number of granules are visible next to mast cells. What substances were released from the cells, and what is this process called?

Sample exam questions

1. General morphofunctional characteristics of connective tissues. Classification.

2. Morphofunctional characteristics of loose Fibrous connective tissue.