MESENCHYME: TYPES, FUNCTIONS AND DISEASES - BIOLOGY - 2024

The mesenchyme is loose connective tissue that has significant amounts of extracellular matrix, is viscous and rich in various proteins, such as collagen. Embryologically, it comes from the mesoderm and by cellular differentiation processes it gives rise to a large number of tissues in organisms.

These tissues include connective tissue, smooth muscle, organs and structures related to the circulatory and lymphatic systems, among others. Mesenchyme is a medium for the exchange of substances for the body, provides the necessary structural support and protects the body.

In addition, it is responsible for the accumulation of reserve substances, such as fat. The cell types derived from this tissue are fibroblasts, mesothelium, endothelium, adipocytes, myoblasts, chondroblasts, and osteoblasts.

General characteristics

The term mesenchyme refers to a mesodermal tissue that helps maintain the shape of organs. Cells in these tissues do not have connections and are freely arranged in the medium, separated by abundant extracellular matrix.

The extracellular matrix is ​​secreted by fibroblasts and is mainly composed of various proteins, proteoglycans, glycosaminoglycans, and hyaluronic acid.

It is considered a zone of integration in the tissues, occupying the "empty" intercellular space. The matrix allows cells to compress and stretch.

The main component of "soft" tissues is collagen, a protein molecule whose structure is a fiber. Collagen gives two important properties to tissues: flexibility and resistance.

The properties of mesenchymal tissue are totally opposite to that of epithelial tissue, characterized by presenting tightly knit cells with little extracellular matrix. All the organs of an individual are composed of an epithelium and a mesenchyme.

In the literature, it is common for the terms "mesenchymal tissue" and "connective tissues" to be used interchangeably.

Mesenchymal cells

Mesenchymal cells are small in size, their shape is generally elongated or stellate, and they have a heterochromatic nucleus.

These are responsible for giving rise to the cell types that make up the connective tissue: fibroblasts, adipose cells, mast cells, pericytes and histiocytes.

- Fibroblasts are characterized by being spindle-shaped and presenting flat nuclei. These are responsible for generating all the components of the extracellular matrix. When fibroblasts can contract, they are called myofibroblasts.

- Adipocytes are large cells that store lipids as a reserve substance in organisms. Likewise, they can be reservoirs for certain hormones and inflammatory mediators.

- Mast cells, also called mast cells, are related to the immune response of the individual. When a foreign body is detected, these cellular agents secrete inflammatory substances (such as histamine) and other factors that are responsible for attracting cells related to the immune response.

- Pericytes, or Rouget cells, are elongated cells associated with blood vessels and endothelial cells. They have the ability to contract and can differentiate into smooth muscle and endothelial cells.

Mesenchyme in invertebrates

In some groups of invertebrates -such as porifers, cnidarians and some acellomed- the term “mesenchyme” refers to a poorly organized gelatinous tissue with varied cell types. It is generally located between the epidermis and the epithelial lining of the digestive tract.

In the aquatic invertebrates belonging to the Phylum Porifera, the mesenchyme is called mesohilo.

Similarly, in the Phylum Cnidaria the mesenchyme is derived entirely from the ectoderm. Therefore, in this lineage of organisms the type of mesenchyme is ectomesodermal.

Finally, in acellomized animals with three embryonic leaves (ectoderm, endoderm and mesoderm), the term "parenchyma" is often used to refer to the intermediate layer. Other terms used in invertebrate zoology to describe the mesenchyme are: collenchyma and mesoglea.

Types and functions

Thanks to the presence of stem cells, the mesenchyme has the ability to form the following tissues:

Connective or connective tissue

Connective tissue can be loose or dense. The first group has support functions and forms the filling of the organs. The second type contains more collagen in its composition, is less flexible and is located in tendons, ligaments and around the bones.

Woven bone

Bones are tubular structures responsible for supporting the body. There are three cell types related to bones: osteoblasts, osteocytes, and osteoclast.

Its structures are rigid and strong, thanks to which the extracellular components undergo a calcification process, which gives rise to the bone matrix.

Bone tissue can be spongy or compact. The former is found in short bones and in the endings of long bones, while compact tissue is found in long, flat bones and in some regions of the short bones.

Adipose tissue

Adipose tissue is what is collectively called "fat." It is made up of specialized cells with large amounts of cytoplasm inside, whose job is to store lipids.

There is a particular type of fat called brown fats, which are involved in the thermoregulation of small mammals and infants in humans.

Cartilaginous tissue

Cartilage is a strong and sufficiently dense structure, but retains resilient properties. It is composed mainly of collagen.

The cells that make up mature cartilage are chondrocytes, present in low numbers and surrounded by abundant extracellular matrix.

Depending on the composition of said matrix, the cartilage can be divided into hyaline, elastic and fibrocartilage.

Muscle tissue

Muscle tissue is divided into three types: skeletal, cardiac, and smooth. Skeletal muscle is voluntary and is composed of myofibrils, which are multinucleated.

Myofibrils are made up of myofilaments: actin and myosin, the contractile proteins responsible for movement.

The heart muscle is similar to skeletal in structure, but it is involuntary. The fibers of the heart muscle are organized in a syncytium (a multinucleated cytoplasm) and not in myofibrils. This muscle type has a high number of mitochondria and myoglobin.

Smooth muscle is also involuntary and is part of the gastrointestinal tract and urinary system. The cells of this tissue are spindle-shaped and have a central nucleus.

Hematopoietic tissue

Hematopoietic tissue is composed of blood plasma, which has nutrient transport and gas exchange functions.

It is responsible for the production of blood cells such as erythrocytes, granulocytes, monocytes, lymphocytes, platelets, among others.

It is located mainly in the bone marrow, and secondarily in the thymus, spleen, and lymph nodes.

Diseases

Tumors

The tumors of the mesenchymal tissue are: angioma, cavernoma, hemangiopericytoma, lipoma, chondroma, chordoma and histiocytoma.

Agioma

Angiomas are benign tumors caused by the abnormal growth of blood vessels (veins, arteries, or capillaries). They usually affect infants and are ball or ball shaped. They can be located in the region of the face such as eyes, nose and mouth, or also in the anal area.

Angiomas are not able to migrate to other tissues of the individual and do not form malignant tumors. This pathology is thought to be hereditary.

Cavernoma

Cavernoma or cavernous angioma is a malformation related to vascular structures. This lesion is characterized by taking the shape of a blackberry made up of capillaries, reaching sizes of up to 5 centimeters.

Hemangiopericytoma

Hemangiopericytoma is a tumor that originates from Zimmerman's pericytes, usually in the retroperitoneal space and in the lower extremities.

It is a rare lesion that presents as a progressive and abnormal cell growth that does not present pain, and may or may not compress other structures.

Chondroma

Chondromas are benign tumors that occur in the bones, often in the hands. They are the product of uncontrolled cell proliferation in mature hyaline cartilage, in the metaphyseal regions of the endochondral ossification bones.

The frequency with which chondromas occur is quite high. In addition, they can occur singly or together.

Chordoma

Like chondromas, chordomas are bone tumors, although the latter are malignant. They occur frequently in the spinal column or in the support area of ​​the skull (in the upper portion of the spine).

It is more common in men than in women and usually appears between 50 and 70 years of age, although it also appears earlier in life.

Due to its location, it is a difficult lesion to treat, since it can affect other vital structures such as the carotid artery and part of the brain tissue. It can be treated through surgery, radiation therapy, and chemotherapy.

Lipoma

Lipomas are benign tumors and are quite common in mesenchymal tissue. In 20% of cases, they occur on the head and neck, and predominantly affect males between 40 and 60 years of age. They are classified as conventional, infiltrating or deep.

Histiocytoma

Histiocytomas are tumors formed in soft tissues and can be benign or malignant.

Malignant fibrous histiocytoma can occur in all parts of the body, in soft parts or in the bone, although it is more common in the bones of the extremities (femur, tibia, humerus) and abdomen.

The growth of the lesion is accelerated and can migrate to other areas of the body, such as the lungs. Its frequency is high in older adults.

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