An egg is a self-contained biological package designed to nourish and protect a developing embryo. In everyday language, “egg” usually refers to the chicken egg you buy at the grocery store, but biologically, eggs are reproductive structures produced by birds, reptiles, fish, amphibians, and even mammals. The chicken egg is one of the most nutritionally dense single foods available, packing about 6 grams of protein and a wide range of vitamins into a compact, edible shell.
How an Egg Works Biologically
The eggs laid by birds and reptiles are called amniotic eggs, and they’re essentially portable life-support systems. Inside the shell, a series of fluid-filled membranes each handle a specific job for the developing embryo. The amnion surrounds the embryo in stable fluid, acting like a personal cushion. The yolk sac delivers food, shrinking as the embryo grows. The allantois handles gas exchange (letting the embryo “breathe”) and collects waste. And wrapping around everything is the chorion, which serves as the outermost living barrier before the shell itself.
The shell prevents the contents from drying out while still being porous enough to let air through. A small air pocket forms between the inner and outer shell membranes after the egg is laid, giving the embryo an extra buffer against environmental shifts. This layered design is what allowed vertebrates to reproduce on dry land millions of years ago, freeing them from needing water to keep their eggs moist.
Anatomy of a Chicken Egg
A chicken egg has more structural complexity than most people realize. From the outside in, here are the key parts:
- Shell: Made almost entirely of calcium carbonate (the same mineral in limestone), the shell is rigid but breathable. A hen’s body deposits about 0.33 grams of calcite per hour during shell formation, pulling roughly 2 grams of calcium from her blood supply each day. About 40% of that calcium comes from her bones, since shell formation happens mostly at night when she isn’t eating.
- Shell membranes: Two thin, transparent layers sit just inside the shell. They’re made partly of keratin, the same protein in human hair, and they serve as an efficient barrier against bacteria.
- Air cell: This pocket of air forms at the wide end of the egg as the contents cool and contract after laying. It grows larger as the egg ages, which is why older hard-boiled eggs are easier to peel (and why that flat crater appears at one end).
- Albumen (egg white): Four alternating layers of thick and thin white contain about 40 different proteins, along with niacin, riboflavin, magnesium, potassium, and sodium. The albumen also acts as a chemical defense system: very little bacteria can survive in it, thanks to proteins that actively inhibit or kill microbes.
- Chalazae: These are the opaque, rope-like strands you sometimes notice when you crack an egg. They anchor the yolk to the shell membrane, keeping it centered and suspended.
- Yolk: The yellow center contains most of the egg’s fat, along with the majority of its vitamins and minerals, including iron, vitamin A, vitamin D, phosphorus, and calcium. The yolk’s fat content also makes it a natural emulsifier, which is why egg yolks are the key ingredient in mayonnaise and hollandaise sauce.
Nutritional Profile
A large chicken egg (about 50 grams) delivers roughly 6 to 7 grams of protein, 4.5 to 5.5 grams of fat, and around 200 milligrams of cholesterol. Protein is split between the white and yolk, with the white contributing slightly more than half. The yolk, however, carries nearly all of the egg’s vitamins and minerals.
Eggs are one of the few foods that naturally contain vitamin D. They’re also rich in B vitamins, particularly riboflavin (B2) and pantothenic acid (B5). One nutrient that sets eggs apart is choline, a compound essential for brain function and cell membrane integrity. Eggs are a primary source of choline in Western diets, and most people don’t get enough of it from other foods.
The yolk also concentrates lutein, a pigment that accumulates in the retina and brain. Research suggests that both choline and lutein play important roles in brain development during pregnancy and infancy, and that adequate lutein intake in middle and later adulthood may help protect against age-related cognitive decline. The fat in the yolk actually helps your body absorb these pigments more efficiently than you would from plant sources like spinach.
What About Cholesterol?
Eggs contain about 400 milligrams of cholesterol per 100 grams, which is relatively high compared to most foods. For years, dietary guidelines recommended strict limits on cholesterol intake. Current evidence, however, shows that for most people, dietary cholesterol has a smaller effect on blood cholesterol than once believed. Saturated and trans fats in the overall diet have a larger impact. That said, individual responses vary, and people with certain metabolic conditions may be more sensitive to dietary cholesterol.
Egg Grading and Sizing
If you’ve ever wondered what the letters and size labels on an egg carton mean, they come from USDA grading standards. Eggs are graded by inspecting them against a bright light (a process called candling) to evaluate the shell, air cell, white, and yolk without breaking them open.
- Grade AA: The freshest quality. The air cell is less than 1/8 inch deep, the white is firm, and the yolk is barely visible when the egg is held up to light.
- Grade A: Slightly less firm white and a marginally larger air cell (up to 3/16 inch). This is what you’ll find in most grocery stores.
- Grade B: The white may be thin and watery, the yolk can appear dark or flattened, and the shell may have minor staining. These eggs are typically used in processed egg products rather than sold in cartons.
Egg sizes are determined by total weight per dozen, not by individual egg measurements. A dozen large eggs must weigh at least 24 ounces. Extra large eggs come in at a minimum of 27 ounces per dozen, and jumbo eggs at 30 ounces. On the smaller end, medium eggs weigh at least 21 ounces per dozen, small eggs 18 ounces, and peewee eggs 15 ounces. Most recipes assume large eggs unless stated otherwise.
Safe Handling and Storage
In the United States, commercially sold eggs are washed and sanitized before packaging, which removes the natural protective coating on the shell. This is why American eggs need refrigeration, while eggs in many other countries (where washing isn’t standard) can be stored at room temperature. Keep your eggs at 40°F or below.
For cooking safety, both the yolk and white should be firm. Dishes that combine eggs with other ingredients, like casseroles or quiches, should reach an internal temperature of 160°F. If you’re reheating a cooked egg dish, bring it to 165°F. The air cell inside the egg grows as the egg ages and moisture escapes through the porous shell, so a simple freshness test is to place an egg in water: fresh eggs sink and lie flat, while older eggs tilt upward or float as the air cell expands.
Beyond the Chicken Egg
While chicken eggs dominate global consumption, many other eggs are eaten around the world. Duck eggs are larger, richer in fat, and have a slightly gamier flavor. Quail eggs are tiny (about one-fifth the size of a chicken egg) and are common in Japanese and Southeast Asian cuisine. Goose eggs can be nearly three times the size of a chicken egg. Fish eggs, known as roe or caviar, are a completely different category but share the same fundamental biological purpose: delivering nutrients to a developing organism.
Even among chicken eggs, shell color varies. White-feathered breeds with white earlobes typically lay white eggs, while red or brown-feathered breeds with red earlobes lay brown eggs. The color difference is purely cosmetic. Nutritional content is determined by what the hen eats, not by shell pigment. Hens fed diets rich in omega-3 fatty acids or given access to pasture with insects and greens produce eggs with higher levels of those nutrients.