Tissues And Other Levels Of Organization

Companion Cell

Step 1: Phloem is made of four components – Sieve tube elements, Companion cells, Phloem fibres, and Phloem parenchyma. Step 2: Sieve tubes are elongated living cells with perforated end walls (sieve plates). Importantly, they lose their nucleus at maturity, which means they cannot independently carry out metabolic activities. Step 3: Companion cells are long rectangular living cells that are closely associated with sieve tube elements. They retain their nucleus and are metabolically very active. Step 4: Companion cells are believed to control the activities of the associated enucleate sieve t

Because being arranged in multiple layers, upper layers can be worn away while lower layers regenerate and replace them.

Step 1: Simple epithelium has cells in a single layer and is found where absorption and filtration are primary functions. Step 2: Stratified (compound) epithelium has cells arranged in multiple layers. This structural arrangement is specifically suited for protection. Step 3: In areas of heavy wear and tear like skin and the inner lining of the cheeks, the outermost cells get continuously abraded and worn away. The multiple layers allow this loss without exposing underlying tissue, as lower layers continuously divide to replace the lost cells. Step 4: Option A describes ciliated epithelium fun

Mast cells

Step 1: Areolar tissue is the most widely distributed connective tissue in the body. It has several types of cells embedded in its matrix. Step 2: Fibroblasts are cells in areolar tissue that produce the matrix fibres – yellow (elastin) fibres and white (collagen) fibres. Step 3: Macrophages (also called histiocytes) are phagocytic cells in areolar tissue that engulf bacteria and foreign micropathogens, providing defense. Step 4: Mast cells are specialised cells in areolar tissue that secrete heparin. Heparin is an anticoagulant that helps in blood clotting regulation. This is the correct answ

It is the central canal around which concentric rings (lamellae) of bone matrix are arranged, carrying blood vessels and nerves to nourish the bone cells.

Step 1: Compact bone has a highly organised structure. The matrix is arranged in concentric rings called lamellae, and these rings surround a central canal. Step 2: This central canal in compact bone is called the Haversian canal. Osteocytes (bone cells) are located on the lamellae and send out branched processes that connect adjacent cells. Step 3: The Haversian canal runs longitudinally through compact bone and carries blood vessels and nerves, thus providing nourishment to the deeply embedded osteocytes. Step 4: Option A describes the medullary cavity of long bones which contains bone marro