Connective Tissue

Connective tissues are very important in all organs. Connective tissues have an involvement with all other tissues but can be a structural entity in its own right ( e.g. Cartilage). Connective tissue is unique as the main building blocks are not cells but extracellular matrix (ECM) which is produced by the cells. Different proteins suspended in the matrix of proteoglycans which are a large collection of Glycosaminoglycans (GAGs) give different connective tissues their properties. For example, Hyaline cartilage contains a specialised glycoprotein called chondroitin and parallel rows of collagen that give it its strength and means it does not stretch. Connective tissue can be loose or or dense. Loose connective tissue has scattered cells within the ECM (e.g. adipose tissue) whereas dense connective tissue has less ground substances and a higher proportion of cells (e.g. Tendons). Connective tissues are derived from mesenchymal stem cells (read more about Development of tissues here)

Types of Connective tissue

Loose connective tissue - Typically found beneath the skin and between organs. They generally give support and insulation but can serve as a food storage (adipose tissue). The cells they contain are fibroblasts, macrophages, mast cells and fat cells.

Dense connective tissue - Most commonly found as tendons providing flexibility and strong connections between muscles. The cells they contain are fibroblasts.

Cartilage - Found in most joints, make tracheal rings and is the predecessor to bone in developing animals. Provide flexible support, shock absorbtion and the reduction of friction in joints. Contain chondrocytes. The types of cartilage include: hyaline, fibrous and elastic.

Bone - Makes up the skeletal system in animals. Bone is a calcified structure which protects organs and provides rigid support for muscle attachment. Main cell type is chondrocytes.

Blood - Found in the circulatory system. Functions as a highway of the immune system as well as the method of carrying nutrients, oxygen and removing waste products such as C02. Made up of erythrocytes and leukocytes.

[1]

Loose Connective TissueBoneCartilage (Hyaline)Dense Connective Tissue (Tendon)Blood (erythrocytes)How do these cell types make up the tissue

Fibroblasts are the cells that produce the ECM. GAG production starts in the cytoplasm of these cells. The production of GAGs is a complicated process involving uridine diphosphate (UDP) derived activated sugars and a large number of modifications within the Golgi apparatus. GAG synthesis is driven by the combined action of many tissue specific enzymes within fibroblasts [2]. On top of this Fibroblast produce collagen an extremely important protein in the strength of all connective tissues and the most common protein found in the human body. There are 28 different types of collagen each with different functions.

Chondrocytes are found in cartilage. They perform a slightly different function in each type of cartilage. The main function of chondrocytes is the production of type II, IX, and XI collagens and proteoglycans. The regulation of the ECM is controlled by chondrocytes releasing pro-inflammatory cytokines and growth factors which have anabolic and catabolic effects. Theses molecules play a major role in the degradation and synthesis of matrix macromolecules. This is of particular importance in growth plates. The size of a human is predetermined, the growth plates show how much a human will grow in time. Figure 1 shows the growth plate of a rat. Resting chondrocytes are transformed into bone causing the length of the animals bone to grow. Animals only have a certain number of resting cells so their bones can only grow to a certain length.

Bone contains 2 types of specialised chondrocytes : Osteoblasts and osteoclasts. The former creates the calcified mesh of bone and the latter degrades it. This process of synthesis and degradation can happy very quickly as the bones "mesh" interior changes structure as mechanical loading changes. Osteoclasts and osteoblasts can communicate with each other through direct cell-cell contact, cytokine or cell-bone matrix. The production of bone is highly enzymatically and hormonally controlled. An example of this is the activity of ephs and ephrins. These are types of transmembrane ligand that have been found to perform different functions in the cell. ephb4 produced by osteoblasts can inhibit osteoclast formation and ephrin2 can stimulate osteoblast stimulation [4]

How do cells regulate the ECM and therefor regulate the tissue?

First of all why does the ECM need to be regulated and degraded? The ECM needs to degraded for a number of reasons. First of all for normal tissue turnover, keeping the tissue healthy which can be fast or slow e.g. ligaments in your teeth can take only 4 days to fully breakdown and be rebuilt. The remodelling of the ECM for growth and development as seen in the growth plates in figure 1. The ECM needs to be degraded in order to repair damaged tissue around wounds to make space to reform tissue. Finally the ECM needs to be degraded in order to allow fine tuning of ECM synthesis.

Cells need to be able to communicate with one another in order to regulate the tissue. This is done differently in each type of connective tissue. For example in hyaline cartilage the cells are isolated from one another. They communicate with each other through cell to matrix interactions. Cells release different types of matrix proteases which breakdown different ground substances in the ECM. These are as follows: Matrix metalloproteinases (MMPs), the Plasmisn family , Aggrecanases, Elastases, Cathepsin and Glycosidases. MMPs degrade collagens, gelatin, fibronectin, laminin, elastin and aggrecan. Plasmin family degrades fibronectin, laminin and type IV collagen. Aggrecanases degrade aggrecan, versican and brevican. Elastases degrade elastin. Cathepsin degrades collagen in bone resorption. Finally glycosidases remove glycosylations. Obviously if left unregulated these proteinases would have a devastating effect on tissues so there is a large amount of control at multiple levels. From transcriptional to post-translational regulation. At the transcriptional level the expression of proteinases is tightly controlled so that they are only produced in specific cells at specific times. Post-transcriptionally there are many ways these ECM proteinases are regulated. One of these is by distributing them to specific extracellular locations. MMPs and plasmin exist as precursors that are inactive until they are processed into mature proteinases. In MMPs (the most important ECM degradation molecule) this is done using a pro-domain at the amino terminus which when present masks the catalytic Zn-binding motif. The pro-domain is removed by other proteinases which can be other MMPs. This means that MMPs can activate each other and all become activated at a rapid rate. [5]

How do cells with the matrix?

Cells sense changes in the extracellular matrix using a series of different receptors that act as mechanical links between the cell and the ECM. These receptors send mechanical information down the actin filaments of the cells cytoskeleton. Focal adhesions are protein containing structures that link the cell cytoskeleton to the ECM through intergrins. (find more on intergrins here). Mechanical changes detected by the cell can cause conformational changes in the cell, for example opening gates in the cell membrane to allow proteins and GAGs through. [6]

It was recently found that the degradation of large fibrillar proteins such as collagen and fibrin are tension-sensitive. Enzyme activity is inhibited when the proteins are under tensile force. This means that mechanical loading can have an effect on the shape of the ECM through the biosynthetic pathway giving another way in which cells control the ECM. [6]

References and Extra Reading

[1] Mason, K. A., Losos, J. B. and Singer, S. R. 2014. Biology Tenth Edition. New York: McGraw-Hill Education.

[2] Casale J, Crane JS. Biochemistry, Glycosaminoglycans. [Updated 2021 May 9]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK544295/

[3] Kemer, S., Karademır, F., Aydemir, G., Kucukodaci, Z., Pirgon, O., Genc, F. and Aydinoz, S. 2015. Effects of Inhaled Corticosteroids on the Growth Plates of Infant Rats. *Fetal and pediatric pathology* 34,  doi: 10.3109/15513815.2015.1042606

[4]Chen, X., Wang, Z., Duan, N., Zhu, G., Schwarz, E. M. and Xie, C. 2018. Osteoblast-osteoclast interactions. *Connective tissue research* 59(2), pp. 99-107. doi: 10.1080/03008207.2017.1290085

[5]Lu, P., Takai, K., Weaver, V. M. and Werb, Z. 2011. Extracellular matrix degradation and remodeling in development and disease. *Cold Spring Harbor perspectives in biology* 3(12), p. a005058. doi: 10.1101/cshperspect.a005058

[6]Muntz, I., Fenu, M., van Osch, G. J. V. M. and Koenderink, G. H. 2022. The role of cell–matrix interactions in connective tissue mechanics. *Physical Biology* 19(2), p. 021001. doi: 10.1088/1478-3975/ac42b8

Chiquet, M., Renedo, A. S., Huber, F. and Flück, M. 2003. How do fibroblasts translate mechanical signals into changes in extracellular matrix production? *Matrix Biology* 22(1), pp. 73-80. doi: https://doi.org/10.1016/S0945-053X(03)00004-0

[Narayanan, A. S., Page, R. C. and Swanson, J. 1989. Collagen synthesis by human fibroblasts. Regulation by transforming growth factor-beta in the presence of other inflammatory mediators. *The Biochemical journal* 260(2), pp. 463-469. doi: 10.1042/bj2600463

Akkiraju, H. and Nohe, A. 2015. Role of Chondrocytes in Cartilage Formation, Progression of Osteoarthritis and Cartilage Regeneration. *Journal of developmental biology* 3(4), pp. 177-192. doi: 10.3390/jdb3040177

Alberts, B. 2017. Molecular Biology of the Cell. CRC Press.