Most of the body is made of connective tissues. While epithelia cover and protect, connective tissues “connect” and store. Connective tissues fill the spaces between other tissues and form organs. In histological slides, we usually see connective tissues between layers of other tissues — for example, between the epithelium and a layer of muscle in the wall of a hollow organ, like the stomach or intestines.
Connective tissues differ significantly from epithelial tissues; they have relatively few cells that are not necessarily connected, and these cells are immersed in a considerable quantity of extracellular matrix. This matrix is a composite of “filler material,” also called ground substance, and varying quantities of fibers. Extracellular matrices can be very diverse, from loosely arranged fibers and a large amount of ground substance in loose connective tissue, to very organized fibers with minimal ground substance in tendons, to fibers covered in crystallized calcium salts in the bone.
Functions of connective tissues
In general, connective tissues:
- form the bulk of organs.
- fill the spaces between other tissues and bind organs together.
- support organs, by forming fascia and sheaths, ligaments, and tendons.
- form support structures in the joints, such as joint capsules, synovial membranes, and tendon sheaths.
- store calcium, fat, immune cells, water, and many bodily chemicals, such as growth factors and hormones.
- insulate (mostly due to stored fat, but also by forming the dermis of the skin).
- transport substances through the body via blood.
Types of connective tissues
Despite their diversity, based on the properties of the extracellular matrix, connective tissues can be grouped into four major types and then further divided into smaller classes.
|“ordinary” connective tissues||“specialized” connective tissues|
|1. connective tissue proper
4. blood and blood-forming tissues
Ground substance is a gel-like material that fills the spaces between connective tissue cells. It is a mixture of glycoproteins, glycoaminoglycans (such as hyaluronic acid), and proteoglycans. Ground substance can be very fluid, as in the vitreous humor in the posterior cavity of the eye, or calcified, as in a bone. The characteristics of ground substance determine the permeability of a connective tissue and its ability to store water.
Fibers in connective tissues
There are three types of fibers in connective tissues:
- collagen fibers
- elastic fibers
- reticular fibers
Collagen fibers, found in tendons, provide tensile strength and ressitance to longitudinal stretching (i.e. they are strong but only in one direction). In contrast, elastic fibers are stretchy in all directions, and help tissues return to their previous shapes after extension (a property called elasticity). Elastic fibers are made of two proteins, elastin and fibrillin. Reticular fibers are very fine collagen fibers that form a net-like structure, the reticulum, which wraps around organs and protects their integrity.
Production and maintenance of connective tissues
Connective tissues are formed by undifferentiated cells (whose names end in “-blast”), that create new ground substance and connective tissue fibers. When these “-blast” cells become surrounded by the ground substance they are producing, they are considered mature and are referred to by names that end in “-cyte.” Thus, a chondroblast is the cell that builds new cartilage within developing fetuses and in the epiphyseal plates of growing bones. Once that chondroblast is surrounded by the cartilage it is itself producing, it is considered mature, and is referred to as a chondrocyte. It is this chondrocyte that maintains the ground substance in post-pubescent adults.
Bone matrix is generated by osteoblasts that are found only on the surface of the bone. Once osteoblasts become trapped within the matrix, they lose the ability to divide and become osteocytes.
Hemocytoblasts produce blood cells in the bone marrow. We will discuss hemocytoblasts, and the various types of blood cells that they produce, in Chapter 10.
Fibroblasts are the most common cells within “ordinary” connective tissue. Fibroblasts produce and maintain ground substance and connective tissue fibers and can migrate through the extracellular matrix to the places which need to be renewed or repaired. The mature fibroblasts located in tissues that don’t actively repair and grow are, in fact, fibrocytes (but we keep the same name). Fibroblasts are spindle shaped, with a single, well-visible nucleus.
Loose connective tissue
Areolar tissue is the most common type of connective tissue. It contains all three types of fibers (collagen, elastic, and reticular), which are distributed randomly and crisscross in all directions, as shown in Figure 1. Areolar tissue wraps around muscles, fills the spaces between muscle fascicles, and surrounds blood vessels and internal organs — especially those in the abdominal cavity. It is usually found just underneath the epithelial layer. Areolar tissue contains fibroblasts and a large number of macrophages, mast cells, and plasma cells.
Reticular tissue forms a mesh-like, supportive framework for soft organs such as the liver, the spleen, and lymphatic tissues. It has a high number of reticular fibers that other cells can use as a scaffold for organ formation (Figure 2).
Adipose tissue consists primarily of adipocytes, or fat cells, which store triglycerides in the form of a fat droplet. There are two types of adipocytes: white and brown. White adipocytes store lipids as a single, large droplet in the middle of the cell, with the nucleus pushed to the periphery. This forms a characteristic shape that is sometimes called a “signet ring”, as shown in Figure 3. Subcutaneous fat is composed of white adipocytes. Brown adipocytes are very rare in humans and practically present only in fetuses and very young babies. They are involved in heat production and help babies adjust to life outside the womb. Brown adipocytes look more like “normal” cells with multiple small fat droplets dispersed in the cytoplasm.
Dense connective tissues
Dense connective tissue contains more collagen fibers than loose connective tissue.
Dense regular connective tissue (Figure 4) has fibers arranged in neatly organized, parallel bundles. It has both collagen and elastic fibers, which provide great tensile strength and elasticity in one direction. This tissue has very few fibroblasts and almost no other cells. Dense regular connective tissue is found in tendons and ligaments.
Dense irregular connective tissue (Figure 5) is composed of haphazardly arranged, densely packed collagen bundles with many purple stained fibroblasts present. Collagen and elastic fibers spread in many directions, providing increased elasticity. Dense irregular connective tissue is found in the dermis layer of the skin.
Cartilage is a very dense and resilient tissue that is mainly composed of collagen fibers within a ground substance made mostly of chondroitin sulfate. It is produced by chondroblasts that become chondrocytes once they are embedded in the matrix. Mature chondrocytes are localized in groups within cartilage. Cartilage has very few cells and is avascular (lacking blood vessels) and aneural (lacking nerves). Nutrition is provided to chondrocytes via diffusion, causing it to heal very slowly.
There are several types of cartilage, based upon the relative amounts of collagen fibers and proteoglycan matrix. Hyaline cartilage (Figure 6) is the most common — and the weakest — and is found in the ribs, nose, larynx, and trachea. It appears glassy in histological slides. In the embryo, bone begins as hyaline cartilage before ossifying into bone.
Fibrocartilage (Figure 7) has many collagen fibers, making it the strongest type of cartilage. It is found in intervertebral discs, joint capsules, ligaments, and the pubic symphysis. Under the microscope, the cartilage fibers appear as whispy lines arranged in an orderly fashion with chondrocytes spaced throughout.
Elastic cartilage (Figure 8) is very springy and yellow, and is found in the epiglottis, the external ear, and the larynx. It consists of fiber bundles that appear dark under the microscope and has many disc-shaped cells interspersed throughout that are white in color.
Bone is a hard, inflexible tissue that is specialized to resist compression. There are two types of bone tissue, compact (or cortical) bone and spongy (or cancellous) bone.
Compact bone (Figure 9) derives its strength from collagen fibers arranged in very defined structures called osteons, cylindrical structures aligned parallel to the long axis of the bone. Each osteon is formed by rings of bone matrix, called lamellae, surrounding a central space, called the Haversian canal, that contains a blood vessel. The collagenous extracellular matrix of bone contains calcium hydroxyapatite, a crystal form of calcium phosphate, and is produced by osteoblasts that are found only on the surface of the bone. Once osteoblasts become trapped within the matrix, they lose the ability to divide and become osteocytes. Osteocytes are located between layers of the matrix (the lamellae) in holes called lacunae, and are connected to neighboring osteocytes by very fine, tentacle-like cell extensions. The canals in which the cell extensions reach toward one another are called canaliculi.
Spongy bone is less dense than compact bone and consists of small plates and spikes of bone called trabeculae that make it look like a spong under the microscope (Figure 10). The spaces between the trabeculae contain red or yellow bone marrow, the primary site of new blood cell production. The canaliculi in spongy bone connects to these marrow-containing spaces to receive their blood supply, rather than a central Haversian canal. Bones will be discussed in further detail in Chapter 8.