In sexual reproduction offspring receive two sets of genetic instructions, one delivered by a male gamete (sperm in humans, ova in animals) and the other by a female egg cell. This is a result of a special type of cell division called meiosis that halved the set of chromosomes in each gamete precursor cell.
Characteristics of Male Sperm Cells
Sperm cells (also called spermatozoa) are tiny and motile. They have a head covered by a cap (the acrosome) that contains enzymes to help them enter an egg. A tail made of protein fibers contracts on alternative sides, giving the characteristic wavelike movement seen when a sperm cell moves through seminal fluid and the mucus membranes of the female reproductive tract. Each sperm cell has 23 chromosomes and is haploid. When a sperm cell fuses with an egg cell during fertilization, the two chromosomes join to make a diploid, 46-chromosome new organism.
The process by which sperm develops is known as spermatogenesis and occurs continuously in the testes of mammals and other amniotes. Germ cells (spermatogonia) proliferate and differentiate to become spermatocytes, which undergo meiosis to reduce their number of chromosomes from 23 to 12. Each spermatid then matures into a mature sperm cell that constructs its famous tail.
The motility of sperm is driven by energy, which comes from the mitochondria located in the body of the sperm cell. The mitochondria convert sugar to adenosine triphosphate (ATP), which fuels the flagellum’s protein fibers during their active movement. An additional energy source is provided by the concentration of a sugar-containing substance in seminal fluid and the mucus of the female reproductive tract. This sugar triggers a series of chemical changes in the sperm cell, known as capacitation, that prepares it to participate in fertilization.
Characteristics of Female Egg Cells
Female eggs (or ovum) are produced in the ovaries from birth. At least one egg will be released during each menstrual cycle throughout a woman’s fertile lifetime. The female body typically has as many as seven million eggs at birth.
Unlike sperm cells, egg cells do not have a tail-like flagellum to propel them and are thus non-motile. Instead, ovum is typically large and able to fuse with a spermatozoon during fertilization to create a new diploid organism.
What’s more, unlike normal body cells that undergo mitosis during cell division, ovum has a special mechanism to ensure that each daughter cell receives exactly half of the parent cell’s genetic information by pulling them to opposite sides of the cell. This process, called spindle formation, is mediated by the cell’s protein-rich structures called microtubules.
In addition to this, ovum contains a high concentration of mitochondria—a type of cellular organelle responsible for converting oxygen and nutrients into chemical energy. Anyone who paid attention in 8th grade biology will remember that mitochondria are the powerhouses of the cell. Mitochondria are crucial to the function of most body cells, but ovum requires a particularly high amount of them in order to support embryogenesis. These features, coupled with their extreme size, make egg cells an incredible resource for the development of new embryos.
Characteristics of Human Sperm Cells
While most cells in the body are duplicated during our lifetime through a process called mitosis, sex cells (known as sperm or gametes) are reproduced only once and never replaced. Because of this, sperm are smaller than most other cells in the body and contain very little cytoplasm.
A sperm cell is composed of a head, mid-piece and tail section (or flagellum). The body is made up of tightly concentrated mitochondria, which supply energy to the sperm’s long tail that helps it move towards and eventually fertilize an egg cell.
The head contains a nucleus that carries 23 chromosomes. The head is capped by a component known as the acrosome and contains a set of proteins that help sperm recognise and enter an egg. The acrosome contains enzymes that are inactive but, when they come into contact with diffusible molecules on the surface of an egg, these become active and break down the proteins that line the egg’s membrane, allowing the sperm to pass through it.
The sperm’s tail, which is around 80% of its total length, has a flagellum – a structure that consists of a central axoneme surrounded by nine evenly spaced microtubule doublets. This gives the sperm cell its characteristic ‘wave’ motion that is used to propel it towards and eventually fertilize an egg.
Characteristics of Human Egg Cells
Egg cells, or oocytes, are the largest cells produced by most organisms. They are several times larger than a typical body cell and about 10,000 times bigger than sperm cells. They are filled with nutrients to support the growth of an embryo after fertilization and also contain mitochondria, which convert oxygen and other nutrients into chemical energy.
Scientists have only recently begun to understand why egg cells are so big. The answer appears to lie in the way they are formed. Eggs develop through a special process called meiosis, which separates their genetic material into two different sets. After meiosis, one set of genes becomes the nucleus of the egg and the other forms a collection of specialized proteins called cytoplasm.
The cytoplasm of the egg contains specialized secretory vesicles that are distributed evenly throughout the outer region of the egg, known as the cortex. These vesicles release cortical granules when the egg is activated by sperm, which causes it to change its shape and to prevent more than one sperm from fertilizing it.
A team led by Saitou has now figured out how to push human stem cells into the oogonia stage, the point at which they become immature female egg cells. When these egg cells are fertilized by sperm, they will form the diploid (having a full set of 46 chromosomes) zygote that creates an embryo.