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Sunday, July 11, 2021

Histology of Dense irregular connective tissue

Dense connective tissue is characterized by large, robust collagen fibres with less number of connective tissue cells.

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Fibres are so numerous that the key identifying trait of this tissue is the absence of open spaces between cells or fibres. Since the protein fibres are the dominant component of these tissues, the types of fibres and their orientation within these tissues is the basis for the naming scheme. Dense CTs contain either collagen or elastic protein fibres; therefore, there are dense collagenous CTs and dense elastic types. The collagenous types are far more abundant and are called fibrous or ‘white’ CT. Elastic fibres, on the other hand, appear yellow in unstained tissues and are commonly referred to as ‘yellow’ CT (e.g. the yellow ligaments of the spine). Fibroblasts are the only cells visible and are arranged in rows between the fibres. Their function is to create the collagen fibres of the tissue.

The main roles of dense CT are to transmit forces over a distance and to connect different organs/muscles. Collagen fibres are disposed along the direction of mechanical loads present in that specific tissue. The capacity of dense CT to resist mechanical stress is directly related to the structural organization of the ECM and above all, the collagen fibres.

The dense CT is subclassified as follows:

•Dense, irregular CT has irregularly arranged collagen fibres and usually comprises the dermis and fasciae. In the last few years, it has been demonstrated that the irregular appearance of deep fasciae may be due to its multilayered structure, but in actuality each layer presents its own regularity (see Chapter 3). Consequently, the deep fasciae could be classified as dense regular CT.

•Dense, regular CT is a white, flexible tissue that contains tightly packed bundles of collagen fibres. All of these fibres run in one direction and are arranged parallel to the direction of forces exerted on the particular body part where the tissue is located. This arrangement is typical of tendons and ligaments, but according to recent studies (Benetazzo et al 2011) the deep fasciae could also be classified in this group. Purslow (2010) demonstrated that the epimysium and perimysium have a very specific organization that also may classify them as dense, regular CT, and Huijing and Baan (2003) demonstrated this tissue’s role in force transmission (see Chapter 3). Finally, a specific discussion of the endomysium is necessary as it is not clear whether this has a structure similar to a microtendon (Purslow 2010) or if it is better classified as loose CT (Testut 1905). In the following chapters, the characteristics and functions of the deep fasciae, epimysium and perimysium will be described in greater detail.

Histology of Dense irregular connective tissue

1. connective tissue cells are less

2. fibers are more, irregular and loosely arranged

Example : submucosa of GIT

Histological features of Loose connective tissue

Loose connective tissue also known as areolar tissue is the most widespread CT of the body.

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It is characterized by an abundance of ground substance, relatively few fibres and connective tissue cells .

The main cellular elements are fibroblasts and a smaller amount of adipocytes. Fat cells are a normal constituent of loose CT, but when they are abundant and organized into large lobules for storage purposes the tissue is better classified as adipose tissue. The adipocytes present in loose CT are generally isolated cells or small aggregations that do not function as storage depots, and their principle function is to facilitate gliding and to act as interstitial filler. The adipocytes of the loose CT usually do not increase in volume when individuals gain weight. Collagen is the principal fibre of loose CT and is arrayed in all directions to form a loose network in the intercellular material. Many elastic fibres are also present.

The loose CT has a viscous, gel-like consistency and its consistency may fluctuate in different parts of the body due to variations in temperature or pH. This CT allows gliding between the various muscles and organs and permits the diffusion of oxygen/nutrients from small vessels to the cells and the diffusion of metabolites back to the vessels. It is the initial site where antigens, bacteria and other agents that have breached an epithelial surface can be destroyed. It also forms a mesh-like tissue with a fluid matrix that supports the epithelia, such as the skin and other membranes. This CT fills the spaces between various organs and thus holds them in place while cushioning and protecting them; it also surrounds and supports the blood vessels.

A particular type of loose CT is the reticular tissue that contains only reticular fibres made of type III collagen. The reticular cells have a stellate shape and long processes that make contact with neighbouring cells, and the subsequent tissue supports a number of bodily structures, such as the liver, spleen, bone marrow and lymphatic organs.