Kara: Is an egg multicellular?

In biology, the word "egg," can refer to two different things. One is the fertilized cell or zygote that now contains all of the genetic information and can grow and develop into a new organism. This is always a single cell. The other use of the word egg is to describe something like a chicken egg.

Although many online sources will say that a chicken egg is one cell, this is not accurate in terms of how we typically define a cell.

One issue with thinking about an entire egg as being one cell is that there are multiple membranes within the egg. There are membranes around the yolk (the vitelline membranes) as well as an inner and outer membrane that surround the egg white or albumen.

Photo credit: https://www.researchgate.net/figure/Schematic-diagram-of-egg-anatomy_fig1_335619708

Another issue is that the part of a fertilized cell that will grow into a chicken is located on the yolk, and it's technically multicellular. It is called the geminal disk and it's visible as a small white spot (about 4 mm) on the yellow yolk. In addition to containing the cell that can develop into an embryo, the geminal disk also has granulosa cells which help produce the hormones needed for development.

But apart from those cells, the rest of the chicken egg structure really doesn't have any cells. The yellow yolk is primarily fat and the white part of the egg is primarily water and protein. The shell is a mineral layer of calcium carbonate.

Most eggs in the grocery store have not been fertilized and would never develop into chickens. But they still have a geminal disk (The geminal disk is also called a blastoderm in a fertilized egg and a blastodisk in an unfertilized egg).

So the best answer to this question is "multicellular." There is that one important cell that can develop into an embryo (if fertilized) and a few helper cells in the geminal disk that can produce hormones to help the process along. But the rest of the egg? All of that yolk and egg white? That's cell-free.

Amy: What is the largest single-celled organism we have discovered?

Here are three of my favorite large single-celled organisms:

Stentor coeruleus is an 2 mm long, single-celled pond organism with a distinct trumpet shape. The name stentor is a reference to its trumpet shape, while coeruleus describes the blue-green pigment specific to the species. It was discovered in 1744. Stentor is most famous for its amazing regenerative abilities. If a Stentor cell is cut in half, each half regenerates a half-sized cell with normal anatomy. Even if a single cell is cut into multiple small fragments, each fragment will generate into a normal-looking cell. Almost any piece of Stentor can regenerate as long as it contains part of the macronucleus and a small portion of the original cell membrane.

Valonia ventricosa, also known as sea pearl, sea grape or sailor's eyeballs, is a single-celled organisms and can grow to the size of a golf ball. It is a multinucleated cell, meaning it has more than one nucleus within the same cytoplasm. These squishy balls can be found in tropical oceans and reproduce through an asexual process, so if you accidentally pop one you’ll end up with a lot of ‘daughters.’ This ability to clone themselves means they can be invasive.

Caulerpa taxifolia is a green algae that can grow up to 24 inches tall! Like a terrestrial plant it has stolons (a stem-like part that runs along the ground), leaf-like fronds, and root-like structures that attach to the ground. But all of these structures are just one cell that can be up to twelve inches long! Like sea pearls, caulerpa taxifolia is considered a single cell because it has one cytoplasm, but within that cytoplasm are numerous nuclei. It lives in the ocean and prefers warm waters. It is also called killer algae because it can be highly invasive and is toxic to some fish