Specialized Cells for Animals

Organisms’ specialized structures develop based on what they need to survive or adapt to their environments. Similarly, microscopic structures in the body are specialized to carry out activities that other structures cannot.

Microvilli

Microvilli are microscopic projections on the surfaces of certain epithelial cells. They are found in the small intestine (on simple columnar epithelium), the kidney (on simple cuboidal epithelium), egg cells, and some white blood cells. In the small intestine they increase surface area for absorption of nutrients and other substances from the gut into underlying tissues and blood. In the kidney they enable efficient reabsorption of water, electrolytes, and nutrients, helping maintain the body’s internal balance. On the egg cell surface they aid sperm attachment during fertilization. In white blood cells microvilli support migration.

Cilia

Cilia are tiny hair-like projections on the surface of epithelial cells. In the respiratory tract they help keep it free of dust, dirt, and pathogens by moving mucus and particles out of the airways. Cilia are found in the respiratory tract (motility) and in the Fallopian tubes where they move the egg toward the uterus. Some unicellular organisms, such as Paramecium, also have cilia.

Stereocilia

Stereocilia are surface extensions found on the epididymis and in the inner ear. They are mainly responsible for fluid absorption and for detecting sound vibrations. In the epididymis they absorb fluid that propels sperm from the seminiferous tubules; in the inner ear they serve as sensors of vibrations.

Flagella

Flagella are tail-like structures that provide motility to cells. They are found in sperm cells, which use them to travel from the male reproductive tract to the egg in the fallopian tube.

Basal Infoldings and Hemidesmosomes

Basal infoldings and hemidesmosomes are located at the basement membrane of epithelial cells. Hemidesmosomes help the epithelial tissue provide protection and structural support to the underlying cells. Basal infoldings are responsible for increasing surface area and for ion and fluid transport.

Junctions in Epithelia

Tight junctions prevent leakage of substances. Desmosomes connect adjacent cells. Adherens junctions connect adjacent cells. Gap junctions serve as channels of ions, water and other essential substances.

Blood Cells and Sex Cells

Red blood cells are biconcave to aid diffusion of gases between the lungs and body tissues. The lack of nucleus in red blood cells provides more space for hemoglobin, enabling more oxygen transport. They also lack mitochondria and generate energy through anaerobic respiration.

White Blood Cells

White blood cells are responsible for the body’s defenses. They can be granulocytes or agranulocytes.

Egg Cells and Sperm Cells

Egg cells are the largest cells in the human body and are necessary for sexual reproduction. Sperm cells travel from the testes to the fallopian tubes to fertilize the egg.

Specialized Cells in Plants

Plants have specialized cells and structures that support growth, survival, and metabolism.

Trichomes

Trichomes on sundew plants are epidermal outgrowths responsible for preventing insect attacks, shading leaves, and trapping insects.

Root Hairs

Root hairs are tiny hair-like structures that originated from the epidermis of plants and facilitate the absorption of water from the substrate.

Mesophyll Cells

Mesophyll cells are microscopic layers of a leaf, primarily responsible for photosynthesis. They are made up of palisade cells and spongy cells.

Xylem and Phloem

Xylem and phloem tissues consist of specialized cells that transport water, minerals, and food needed by the plants.