In all connective tissue except blood (a fluid connective tissue), the cells secrete fibers of structural proteins called collagen or elastin.

Connective Tissues and Cell Secretion

Connective tissues are the most abundant and diverse tissues in the human body. They are responsible for providing support, structure, and strength to various organs and tissues. Connective tissues consist of cells and an extracellular matrix that is composed of fibers and ground substance.

Types of Connective Tissues

1. Bone: Bone is a type of specialized connective tissue that forms the skeletal system. It provides structural support and protection to internal organs. The cells in bone tissue are called osteoblasts, which secrete collagen fibers and mineral salts, forming a hard and rigid matrix.

2. Cartilage: Cartilage is a semi-rigid connective tissue found in various parts of the body, such as the joints, nose, and ears. The cells in cartilage, called chondroblasts, secrete collagen and elastin fibers, giving the tissue its flexibility and strength.

3. Areolar Connective Tissue: Areolar connective tissue is a loose and widely distributed connective tissue. It is found throughout the body, surrounding and supporting organs and structures. The cells in areolar connective tissue secrete collagen and elastin fibers, providing strength and elasticity to the tissue.

4. Fluid Connective Tissue: Fluid connective tissue includes blood and lymph. Unlike other connective tissues, the cells in fluid connective tissue do not secrete collagen or elastin fibers. Instead, they are responsible for other functions such as carrying oxygen, nutrients, and waste products.

Explanation of Correct Answer

The correct answer is option 'D' - Fluid connective tissue. In fluid connective tissues like blood and lymph, the cells do not secrete fibers of collagen or elastin. Instead, these cells have specialized functions such as carrying oxygen (in red blood cells), fighting infections (in white blood cells), and transporting nutrients and waste products (in plasma).

While blood does contain fibrinogen, a soluble protein that plays a crucial role in blood clotting, it is not secreted by blood cells. Fibrinogen is produced by the liver and released into the bloodstream.

Therefore, in all connective tissues except fluid connective tissue, the cells do secrete fibers of collagen or elastin proteins, providing strength, support, and elasticity to the tissues.