Connective tissue

The term connective tissue is applied to a tissue which fills the interstices between more specialized elements; and serves to hold them together and support them.

1. GENERAL CONNECTIVE TISSUE (CONNECTIVE TISSUE PROPER):

a. fibrous connective tissue

- loose connective tissue

- dense connective tissue (regular, irregular)

b. connective tissue possessing special properties

- mucoid

- reticular

- adipose

2. SKELETAL CONNECTIVE TISSUE:

a. cartilage

- hyaline cartilage

- fibrocartilage

- elastic cartilage

b. bone tissue

c. cement of tooth

d. dentin of tooth

GENERAL CONNECTIVE TISSUE (CONNECTIVE TISSUE PROPER)

FIBROUS CONNECTIVE TISSUE

Fibrous connective tissue consists of cells and intercellular substance. Intercellular substance, in its turn, consists of amorphous ground substance and fibres.

CONNECTIVE TISSUE CELLS

Fibroblasts. These are the most numerous cells of connective tissue. They produce intercellular substance. They are not mobile. Their outlines are indistinct.

Histiocytes (macrophages). Free wandering cells, derived from immigration of monocytes from the blood. They are capable of ameboid movement, and have ability to phagocytose unwanted material. The outline of histiocytes is conspicuous.

Pigment cells. Pigment cells are easily distinguished as they contain brown pigment (melanin) in their cytoplasm. Of the many cells that contain pigment in their cytoplasm only a few are actually capable of synthesizing melanin. Such cells are called melanocytes.

Fat cells (adipocytes). Some cells store fat in large amounts and become distended with it. These are called fat cells, adipocytes, or lipocytes. Aggregations of fat cells constitute adipose tissue.

Plasma cells or plasmatocytes. Plasma cell is small and rounded. It can be recognized by the fact that the chromatin in its nucleus forms four or five clumps near the periphery (of the nucleus) thus giving the nucleus a resemblance to a cart-wheel. The cytoplasm is basophilic, the basophilia is seen to be due to the fact that the cytoplasm is filled with rough endoplasmic reticulum, except for a small region near the nucleus where a well developed Golgi complex is located.

Both these features are indicative of the fact that plasma cells are engaged in considerable synthetic activity. They produce antibodies which may be discharged locally; may enter the circulation; or may be stored within the cell itself in the form of inclusions called Russell’s bodies.

INTERCELLULAR SUBSTANCE

There are two main types of intercellular substance: amorphous (nonformed) and fibrous (formed).

Amorphous intercellular substances. The amorphous materials of connective tissue are composed principally of glycosaminoglycans and glycoproteins.

Fibrous intercellular substances.

Collagenous fibres.Collagenic or collagenous fibres are found in all types of connective tissue and consist of the protein collagen. They are extremely tough.

Collagenous fibres vary from one to twelve micrometers in diameter, although several fibres may be collected together to form a bundle of greater size.

Elastic fibres. Elastic fibres are thin. They run singly (not in bundles). Elastic fibres can be stretched and return to their original length when tension is released.

CLASSIFICATION OF FIBROUS CONNECTIVE TISSUE

The major subdivision in the classification of fibrous connective tissues is determined by the concentration of fibres.

Loose (areolar) connective tissue. Loose (areolar) connective tissue contains many cells and few fibres. It is a loosely arranged, connective tissue. Collagenous fibres are most prominent; elastic fibres, which form a continuous branching network, are relatively inconspicuous. The ground substance is relatively fluid-like and occupies many little areas (areolas) in which no structure ordinarily can be seen.

Dense connective tissue. Dense connective tissue contains few cells and many fibres. It is characterized by the close packing of their fibres. Cells are proportionally fewer than in loose connective tissue, and there is less amorphous ground substance. In areas where tensions are exerted in all directions, fibre bundles are interwoven and without regular orientation and the tissue is termed irregularly arranged. In structures subject to tension in one direction, the fibres have an orderly parallel arrangement and the tissues are designated regularly arranged.

CONNECTIVE TISSUE POSSESSING SPECIAL PROPERTIES

Mucoid tissue. The most conspicuous component of mucoid tissue is a jelly like ground substance rich in hyaluronic acid. Scattered through this ground substance there are star-shaped fibroblasts, some delicate collagen and some rounded cells. This kind of tissue is found in the umbilical cord.

Reticular tissue. This type of connective tissue is characterized by the presence of reticular cells. These cells are stellate and have long cytoplasmic extensions which appear to join with those of other cells. They have large, pale nuclei and abundant basophil cytoplasm.

Adipose tissue. Adipose tissue is an aggregation of fat cells, also called adipocytes. Each fat cell contains a large groplet of fat that almost fills it. As a result the cell becomes rounded. When several fat cells are closely packed, they become polygonal because of mutual pressure. The cytoplasm of the cell forms a thin layer just deep to the plasma membrane. The nucleus is pushed against the plasma membrane and is flattened.

SKELETAL CONNECTIVE TISSUE

CARTILAGE

Cartilage consists of cells and intercellular substance. Intercellular substance (matrix), in its turn, consists of amorphous substance and fibres (mainly collagen).

HYALINE CARTILAGE. The word “hyaline” is derived from the Greek hyalos, meaning glass. Hyaline cartilage appears as a translucent, bluish-white mass in the fresh condition. It forms the articular surfaces of bones within joints, the costal cartilages, and the cartilages of the nose, larynx, trachea, and bronchi. In the fetus nearly all the skeleton is first laid down as hyaline cartilage, which is replaced later by bone.

The cells. The cartilage cells, or chondrocytes, occupy small cavities or lacunae within the matrix (intercellular substance). The cells usually are ovoid or spherical, and each contains a large, spherical, centrally placed nucleus with one or more nucleoli. The surface of each cell is irregular and has short processes that extend into depressions within matrix. The cytoplasm is finely granular and moderately basophil, due to the presence of abundant free ribosomes and of a relatively well-developed granular endoplasmic reticulum; in additions, it contains a prominent Golgi complex, large mitochondria, vacuoles, fat droplets, and some glycogen.

In the centre of a mass of cartilage in the adult, the cells may be arranged in groups, each group representing the offspring of a single parent chondrocyte. Such a group of cells within a single lacuna is referred to as a cell nest or isogenous group. Toward the periphery of a mass of cartilage the cells are elliptical and flattened parallel to the surface. In fetal cartilage the cells often are flattened and cell nests are seen rarely.

The matrix (intercellular substance).

1) Collagenous fibres;

2) Amorphous (ground) substance. The ground substance of cartilage is markedly basophil owing to its content of proteoglycans.

ELASTIC CARTILAGE. Elastic cartilage is similar in many ways to hyaline cartilage. The main difference is that the matrix contains both collagen and elastic fibres. This type of cartilage occurs in locations where support with flexibility is required, as in the external ear, auditory tube, epiglottis, and certain cartilages in larynx.

FIBROCARTILAGE. .

1) The cells. These may occur singly or in groups, but commonly are in short rows.

2) Matrix (intercellular substance) consists of collagen fibres and amorphous substance.

Fibrocartilage lacks a perichondrium.

BONE

Bone consists of:

1) bone cells;

2) intercellular substance:

a. amorphous (ground) substance, in which mineral salts (mainly calcium and phosphorus) are deposited;

b. collagen fibres.

Architecture of compact bone.

1) Outer circumpherential (general) lamellae;

2) Osteogenic layer (osteons and interstitial lamellae);

3) Inner circumpherential (general) lamellae.

Questions:

1. Classification of connective tissue.

2. Classification of the general connective tissue (connective tissue proper).

3. Classification of the skeletal connective tissue.

4. Structural features of the loose connective tissue.

5. Structural features of the dense connective tissue.

6. Structural features of different types of connective tissue possessing special properties.

7. Structural features of the hyaline cartilage.

8. Structural features of the fibrocartilage.

9. Structural features of the elastic cartilage.

10. Structural features of the bone tissue.

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