Hard tissue are lightweight yet strong and hard, and serve multiple functions. Mineralized osseous tissue or bone tissue, is of two types – cortical and cancellous and gives it rigidity and a coral-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum, periosteum, nerves, blood vessels and cartilage. Bone is an active tissue composed of different cells. Osteoblasts are involved in the creation and mineralisation of bone; osteocytes and osteoclasts are involved in the reabsorption of bone tissue. The mineralised matrix of bone tissue has an organic component mainly of collagen and an inorganic component of bone mineral
Milky spots Milky spots Milky spots are very small white-coloured areas of lymphoid tissue, found in the peritoneal, pleural and pericardial cavities. These are the three coelomic cavities that form as separate compartments from the intraembryonic coelom. More milky spots are found on the greater omentum in the peritoneal cavity than anywhere else. The milky spots are made up of mesenchymal cells and are covered in a layer of mesothelium. These structures surround the small blood vessels. The enclosing mesothelium contains macrophages, lymphocytes and mast cells. They are also known as secondary lymphoid organs. Most milky spots contain extremely thin-walled lymphatic capillaries.