Continued to post Connective tissue-

Based on different composition of cells and extracellular matrix, following types of connective tissue are present:

(A) Loose connective tissue- Areolar connective tissue, Adipose tissue, Reticular connective tissue

(B) Dense connective tissue- Dense regular connective tissue, Dense irregular connective tissue, Elastic connective tissue

(C) Cartilage- Hyaline cartilage, Fibrocartilage, Elastic cartilage

(D) Bone tissue

(E) Liquid connective tissue- Blood tissue, Lymph

Let’s discuss all these types one by one.

 (A) Loose connective tissue– fibers of this tissue are loosely arranged between cells in ground substance. It has following types:

• Areolar connective tissue- it is the most widely distributed C.T. in the body. It consists of all 3 types of fibers, i.e. collagen, elastic and reticular fibers arranged randomly in C.T. It has fibroblasts, macrophages, mast cells, plasma cells, adipocytes, and WBCs in ground substance made of hyaluronic acid. It is present all over the body under the epithelia so called as Packing material of the body (wraps and cushions organs). Present in deep layers of the skin. Provides strength, support and elasticity.

• Adipose tissue- it has adipocytes (fat storing cells), so adipose tissue is specialized for storage of triglycerides (fats), as large, centrally located droplet. Adipocytes store fat as single large droplet which pushes nucleus and cytoplasm to the periphery of the cell.

Adipose tissue

This tissue has 2 types- White adipose tissue, and Brown adipose tissue. Brown adipose tissue has very rich supply of blood, is darker in color, and has pigmented mitochondria in comparison to white adipose tissue. 

Present in yellow bone marrow, behind eyeball sockets, in padding around joints. . It helps in maintaining energy reserve, reduces heat loss through skin.

White adipose tissue is found in adults while brown is found in infants and fetus. Obese person has white adipose tissue. Brown tissue in newborns help to maintain proper body temperature by generating heat so it acts as an insulator.

• Reticular connective tissue– it has fine network of reticular fibers. It makes stroma (supporting framework) of organs like liver, spleen, red bone marrow, around blood vessels etc. Also acts as a filter for old RBCs in spleen and for microbes in lymph nodes.

(B) Dense connective tissue– it has more fibers which are densely packed and have fewer cells than loose connective tissue. It has following types:

• Dense regular connective tissue– it has white shinny ECM and collagen fibers in the form of bundles arranged regularly in rows with fibroblasts. It forms tendons (attach muscle to bone ), ligaments (attach bone to bone) providing strong attachment between various structures and strength in one direction. Ligaments are very elastic and has great strength, while tendons has only low flexibility with great strength.

• Dense irregular connective tissue– it has collagen fibers arranged irregularly in tissue with few fibroblasts. It forms sheets in various organs like fasciae of skin, fibrous pericardium of heart, periosteum of bone, perichondrium of cartilage, heart valves. It provides strength in many directions.

• Elastic connective tissue- it has yellowish appearance. Made up of elastic fibers with fibroblasts between them. It allows stretching of various structures like walls of arteries, suspensory ligaments of penis, lung tissue, bronchial tubes etc.

(C) Cartilage- it is made up of dense network of collagen and elastic fibers found in ground substance chondroitin sulphate and glucosamine. So the solid ECM of cartilage is made up of sugars and proteins. Cartilage can resist more stress than dense C.T.  due to more collagen fibers in it and it can regain its original shape after deformation due to chondroitin sulphate. Cartilage don’t have blood supply of its own as it secrets blood cell growth prevention factor called as antiangiogenesis factor.

Cells of mature cartilage are called as Chondrocytes. These cells are present in spaces called lacunae, as single cell or in groups. Perichondrium  surrounds most cartilage, which is the source of new cartilage cells.  Perichondrium also provides cartilage blood supply and nerves.

Since cartilage has no blood supply, injury of cartilage heals very slowly.

Cartilage forms the embryonic skeleton and is the precursor of bone. It is replaced by bone during development. After birth, cartilage is present as growth plates in bone for bone elongation.

Cartilage has following types:

Hyaline cartilage

• Hyaline cartilage– Most abundant cartilage in body. It appears as bluish-white, glassy and shiny substance in the body. Chondrocytes present in collagen fibres, which are not clearly visible. This cartilage is present at ends of long bones (smoothens bone surface at joints), anterior end of ribs, nose, parts of larynx, trachea, bronchi, embryonic and fetal skeleton. It provides smooth surface, flexibility and support for the movement of joints. It is weakest type of cartilage and can be fractured.

• Fibrocartilage- It is the strongest type of cartilage.It has chondrocytes in thick bundles of collagen fibers which are clearly visible. Perichondrium is absent. It is present in pubic symphysis (hip bone joint), intervertbral discs, cartilage pads of knee. It provides strength, support, join structures together and help in compression shock absorption.

• Elastic cartilage- It has chondrocytes within network of elastic fibers. Perichondrium is present. It makes part of external ear, eustachian tube, epiglottis. It provides flexibility and strength to structure and also maintains its shape.

(D) Bone Tissue- Bone tissue is also called as Osseous tissue, and as the name suggests found in bone. It forms a framework to support the body and main organs of the body. It works with skeletal muscles to generate movement. It is a strong and non-flexible tissue. Bones store calcium and phosphorus, red bone marrow (produces blood cells) and yellow bone marrow (stores triglycerides). Bone tissue also has periosteum (can be called as outer covering of bone), and endosteum (membrane within bone that stores yellow bone marrow).

Bone tissue has 2 types: Compact bone and Spongy bone.

Osteocyte

Compact Bone:  Basic unit of compact bone is Osteon or Haversian canal. Osteon consists of 4 parts :  (1) Lamellae- are concentric rings of ECM which stores calcium and phosphorus, responsible for compactness of bone, (2) Lacunae- small spaces between lamellae which contains osteocytes (mature bone cells), (3) Canaliculi- these are the minute canals projecting our from lacunae which contains processes of osteocytes. It provides passage way to osteocyte for nutrients intake and waste disposal. (4) Central canal– also called as Haversian canal, contains blood vessels and nerves.

Spongy Bone: Spongy bone lacks osteons. It contains lamellae, lacunae, canaliculi and osteocyte in thin columns called as trabeculae. Red bone marrow fills the space between trabeculae.

(E) Liquid connective tissue– Blood tissue, Lymph- discussed in more detail in other post.

Blood has a fluid (liquid) ECM called plasma, in which red blood cells (RBCs), white blood cells (WBCs) and platelets are suspended. The plasma contains proteins, salts and hormones. Blood flows and transports gases, digested food, hormones and waste materials to different parts of the body.

Lymph is the extracellular fluid that flows in lymphatic vessels. It helps in transportation of WBCs throughout the body. Also it provides immunity against disease-causing agents.

NOTE —  All these types of connective tissue arise primarily from mesenchyme (type of Embryonic C.T. , present in embryo). Other embryonic C.T. is mucous C.T., which forms umbilical cord of fetus.