Connective tissue: what it is, characteristics and types

Connective tissue takes many different forms and serves various purposes in the human body.

Recent research estimates that the human body is composed of 30 trillion cell bodies, divided by their capabilities into different functionalities. Eighty percent of all these cells are red Blood cells, as they report the unimaginable figure of 5 million erythrocytes per cubic millimeter of blood of an adult person. They are followed by platelets (4.9%), bone marrow hematopoietic cells (2.5%), lymphocytes (1.6%) and other vascular bodies.

Interestingly, epithelial and organ-forming cells are the minority of the total cell bodies, with the epidermis accounting for only 0.5% of the total. Adipocytes, muscle cells, neurons, hepatocytes and other cellular conglomerates are the immense minority within our organism. Nevertheless, they perform a series of essential functions in the survival of the individual and the species over time.

With these data, we have tried to reflect the immense cellular and tissue diversity of the human body. For all these structures to have a three-dimensional coherence and aggregation, some type of tissue is necessary to act as a bridge and support.. In the following lines, we tell you all about connective tissue.

What is connective tissue?

Connective tissue, also known as connective tissue (CT), is an umbrella term for various types of connective tissue. cellular groupings (in addition to the matrix and its fluids), whose common functionality is to connect, support and help maintain the physical integrity of the various tissues of the body..

Connective tissue is an "umbrella" term, as it encompasses a large number of tissue groups. However, they all have 3 basal characteristics:

• Fundamental substance (SF): a translucent, hydrated material with a gelatinous consistency. In it are suspended the cells and fibers typical of the tissue in question.
• Fibers: of many types, depending on the tissue. Among them, collagen, protein, elastic and microfibrils stand out.
• Cells: almost all are fixed and immobile. They depend on the type of connective tissue consulted.

Types of connective tissue

All connective tissues have these characteristics, but beyond them, the variety prevails over the general. The following are the types of connective tissue, according to their functionality and degree of specificity.

1. Connective tissues proper

The proper connective tissue (or connective tissue proper) is the one with a more general function and a lesser degree of development. It is divided into two variants: lax and dense.

1.1. Lax connective tissue

This is the most common type of connective tissue in vertebrates. It is responsible for holding the organs in place and for attaching the skin to the underlying tissues..

This category includes, for example, mucosal, reticular and mesenchymal connective tissue. Lax connective tissues usually have a mesh-like tissue organization, a fluid matrix and a functionality characterized by the protection of weak organs and structures.

1.2. Dense connective tissue

In contrast to the previous case, dense connective tissue is characterized by a high content of fibers in its matrix, which are usually type I collagen. Among these collagen fibers are fibroblasts, the cells that produce collagen themselves through various metabolic pathways. Due to its solid state and high cohesion, this tissue is ideal for forming part of ligaments and tendons, for example.

In turn, dense connective tissue is subclassified into two variants: regular and irregular.. In the former, collagen fibers are arranged in parallel, while in the latter, collagen is found in disorganized arrangements.

2. Specialized connective tissues

From here, we move away from the "usual" connective tissues, diffuse and mostly undifferentiated, to enter a much more organized and familiar terrain. Let's move on.

2.1. Adipose tissue

It is a tissue of mesenchymal origin, formed by a cellular association in charge of storing lipids: adipocytes.. These cells represent 95% of the total adipose tissue and their function is to store energy in the form of triglycerides.

Within adipose tissue, two types can be differentiated: brown and white. White adipose tissue (unilocular) is the majority in mammals, and is responsible for storing fat in a semi-liquid state in the form of a large droplet.

On the other hand, brown fat stores the lipid content in different droplets (multilocular) and contains a high number of mitochondria in each adipocyte. While the white tissue is responsible for long-term storage of fat, the brown tissue burns it when it is necessary to produce heat.

Bone tissue

Normally, humans have the perception of bone tissue as one that is immovable, strong and permanent over time. While bone tissue is hard (due to the hydroxyapatite crystals that make up its matrix), it also synthesizes and resorbs at variable rates, depending on the needs of the body at any given time.depending on the needs of the body at any given time. 99% of calcium is stored in the bones, and when it is necessary to use it and it is not present in the diet, part of the bone tissue has to be destroyed to recover it.

Depending on its nature, bone tissue can be either dense or spongy.. In both are found various cell bodies, such as osteoclasts, osteoblasts, osteocytes and osteoprogenitor cells. Unfortunately, the more time passes in the life of the individual, the more matrix is broken down and the less is generated. It is no coincidence that up to 80% of the population (especially women) show signs of osteoporosis at 80 years of age.

2.3. Blood tissue

Although it may not look like it, blood is also a specialized type of connective tissue.After all, it brings different organs into contact with each other by means of a series of fibers, matrix (plasma) and cell bodies. Through the blood vessels navigate platelets, white blood cells and erythrocytes, all of them with different and complementary functionalities. Platelets are responsible for clotting the blood in the event of injury and maintaining internal homeostasis, lymphocytes respond to endogenous and exogenous threats, and erythrocytes transport oxygen to each and every tissue.

Although it is a different type of tissue, this group also includes hematopoietic tissue, the precursor of the aforementioned blood cells. This is found mainly in the red bone marrow, the center of synthesis of erythrocytes and white blood cells.

2.4. Cartilaginous tissue

Cartilaginous tissue is a type of elastic, avascular (without blood vessels), specialized connective tissue consisting almost entirely of matrix and scattered chondrocytes..

Cartilage is essential for the movement and locomotion of living beings, as it is found lining the joints, joins the ribs to the sternum, and also serves as a support point and lining for the trachea, bronchi, external ear and nasal septum.

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2.5. Lymphatic tissue

In this section something similar to that mentioned for blood tissue occurs: it is not a solid tissue organization, but it also contains fibers, cells and matrix.so it falls within the definition of "connective tissue", albeit in a highly specialized form.

Lymphatic tissue is the highway of travel (along with blood) for lymphocytes, macrophages and plasma cells. Therefore, its functionality is eminently immunological.

Summary

As you will have seen, connective tissue goes far beyond those diffuse layers that are located holding the skin or protecting the organs and keeping them in place in the abdominal cavity. It also encompasses much more specific concepts, such as fat, blood, lymph, cartilage and even bone.

All these tissues have 3 characteristics in common: they contain fibers, matrix and cells embedded in it. Depending on the dominant cell type and its main function, connective tissue can be analyzed in many different categories and dimensions.