When we first begin to study the human body, it is natural to first look at the larger structures - the organs and organ systems. However, as we try to understand how these structures work, and how they are effected by disease, it is necessary to look at smaller structures, such as cells and tissues. The study of microscopic structures of the body is called histology.
It is said that the smallest living unit of the body is the cell. The cell is contained within a cell membrane (plasmalemma). Within most cells (figure 1), we can see smaller structures called organelles. For example, most human cells contain an organelle called the nucleus. The nucleus contains the cell's genes encoded in molecules of deoxyribonucleic acid (DNA). The genes give the cell instructions on how to develop and behave. Sometimes, for example when a cell is about to divide, the DNA in the nucleus condenses to form chromosomes (figure 2). How many chromosomes are there normally in a human cell? Some special kinds of cells have half the normal number of chromosomes, and some cells have no nucleus or chromosomes at all! Can you think of what types of cells these are? The nucleus is surrounded by a nuclear membrane which communicates with other membranes that run through the cytoplasm.
Most cells contain ribosomes which "read" the instructions on the cell's DNA and make enzymes. Ribosomes are often found attached to an organelle called the endoplasmic reticulum. Endoplasmic reticulum with many ribosomes attached to it is called "rough" endoplasmic reticulum. The endoplasmic reticulum communicates with the Golgi apparatus (or Golgi body) which packages chemicals into vesicles. Cells also contain mitochondria which produce energy for the cell. Other organelles include various vacuoles and vesicles, as well as cilia and flagella. Different cells have different organelles, depending upon what job they have to perform in the body.
Cells which are similar in structure and function are grouped together to form tissues. There are different systems of classifying tissues, such as epithelial tissue (figure 3), connective tissue (figure 4), muscle tissue (figure 5) and nervous tissue (figure 6). There are also sub-classifications of tissues. For example, there are three types of muscle tissue - skeletal muscle, smooth muscle and cardiac muscle.
Different tissues are grouped together to form organs. Examples are the heart and the brain. The internal organs of the chest and abdomen are often called the viscera. Organs which work together form organ systems. For example, together the brain, spinal cord and nerves form the nervous system.
English - Japanese Glossary
cardiac muscle: 心筋 (shinkin); cell: 細胞 (saibou); cell membrane: 細胞膜 (saiboumaku); centriole: 中心小体 (chuushinshoutai); chromosome: 染色体 (senshokutai); cilia: 繊毛 (senmou); connective tissue 結合組織 (ketsugousoshiki); endoplasmic reticulum: 小胞体 (shouhoutai); enzyme: エンチーム(enchiimu); gene: 遺伝子; Golgi body: ゴルジ体 (gorujitai); histology: 組織学 (soshikigaku); mitochondrion: 糸粒体 (shiryuutai); nerve tissue/ nervous tissue: 神経組織 (shinkeisoshuki); nucleus: 細胞核 (saiboukaku); organ: 器官 (kikan); ribosome: リボソーム (ribosoomu);smooth muscle: 平滑筋 (heikatsukin); striated muscle: 横紋筋 (oumonkin); tissue: 組織 (soshiki); vacuole/ vesicle: 空胞 (kuuhou)/ 小胞 (shouhou); viscera:内臓 (naizou)