Cells in the human body
The human body comprises more than 200 types of cells, and every one of these cell types arises from the zygote, the single cell that forms when an egg is fertilized by a sperm. Within a few days, that single cell divides over and over again until it forms a blastocyst, a hollow ball of 150 to 200 cells that give rise to every single cell type a human body needs to survive, including the umbilical cord and the placenta that nourishes the developing fetus.
Basic cell biology
Each cell type has its own size and structure appropriate for its job. Skin cells, for example, are small and compact, while nerve cells that enable you to wiggle your toes have long, branching nerve fibers called axons that conduct electrical impulses.
Cells with similar functionality form tissues, and tissues organize to form organs. Each cell has its own job within the tissue in which it is found, and all of the cells in a tissue and organ work together to make sure the organ functions properly.
Regardless of their size or structure, all human cells start with these things in common:
- A nucleus that contains DNA, the genetic library for the entire body. Different cells read and carry out different instructions from the DNA, depending on what those cells are designed to do. Your DNA determines virtually everything about your body, from the color of your eyes to your blood type and even how susceptible you are to certain diseases. Some diseases and conditions, such as color blindness, also are passed down through DNA.
- Cytoplasm – the liquid outside the nucleus. The cytoplasm contains various components that make the materials that the cell needs to do its job.
- The cell membrane – the surface of the cell, a complex structure that sends
and receives signals from other cells and lets material in and out of the cell. Cells have to be able to communicate to work together in tissues and organs.
Most cells divide. Shortly before division, the DNA replicates and then the cell divides into twodaughtercells. Each has a complete copy of the original cell’s DNA, cytoplasm and cell membrane.
About stem cells
Stem cells are the foundation of development in plants, animals and humans. In humans, there are many different types of stem cells that come from different places in the body or are formed at different times in our lives. These include embryonic stem cells that exist only at the earliest stages of development and various types of tissue-specific (or adult) stem cells that appear during fetal development and remain in our bodies throughout life.
Stem cells are defined by two characteristics:
- They can make copies of themselves, or self-renew
- They can differentiate, or develop, into more specialized cells
Beyond these two things, though, stem cells differ a great deal in their behaviors and capabilities.
Embryonic stem cells are pluripotent, meaning they can generate all of the body’s cell types but cannot generate support structures like the placenta and umbilical cord.
Other cells are multipotent, meaning they can generate a few different cell types, generally in a specific tissue or organ.
As the body develops and ages, the number and type of stem cells changes. Totipotent cells are no longer present after dividing into the cells that generate the placenta and umbilical cord. Pluripotent cells give rise to the specialized cells that make up the body’s organs and tissues. The stem cells that stay in your body throughout your life are tissue-specific, and there is evidence that these cells change as you age, too – your skin stem cells at age 20 won’t be exactly the same as your skin stem cells at age 80.
Learn more about different types of stem cells here.