Cell specialization

Cells increase their surface area-to-volume ratio by flattening, by microvilli (finger-like projections of the plasma membrane) and by invaginations (infoldings). Red blood cells are about 8 µm across, biconcave and have no nucleus, so oxygen diffuses only a short way to any haemoglobin, and the cells bend through capillaries. Cells of the proximal convoluted tubule have microvilli facing the filtrate, invaginations facing the blood and many mitochondria, for fast active reabsorption.

Type I pneumocytes are extremely thin and cover most of the alveolar surface, so the diffusion distance for gases is very short. Type II pneumocytes are cuboidal and full of organelles; their lamellar bodies release surfactant by exocytosis. Surfactant lowers the surface tension of the water lining the alveoli, so they do not collapse when we breathe out. The alveolar epithelium is a tissue with more than one cell type, because no single cell could do both jobs.

Both are striated, because myofibrils of actin and myosin are arranged in repeating sarcomeres, and both have many mitochondria. Cardiac muscle cells are short, branched, have one nucleus and are joined by intercalated discs, so the signal to contract spreads quickly through the heart. Striated muscle fibres are long, unbranched and have many nuclei, because they form by fusion of many cells. A fibre has one plasma membrane and normal organelles, but its size and many nuclei make it an unusual cell.

A sperm is small and streamlined: an acrosome with enzymes to digest the zona pellucida, a haploid nucleus, a mid-piece packed with mitochondria and a flagellum. An egg is much larger: its cytoplasm holds nutrients, organelles and mRNA for early development. It is surrounded by the zona pellucida and follicle cells, and its cortical granules harden the zona after one sperm enters, preventing polyspermy.