Germ cell

A germ cell is a special kind of cell that helps living things have babies. These cells develop into eggs or sperm, which are needed for sexual reproduction. In many animals, these cells start in a place called the primitive streak and move through the embryo to reach the gonads, where they change and become mature eggs or sperm.

Scientists still discuss exactly where these cells come from in some animals, but they know they are very important for continuing life. Unlike animals, plants do not have germ cells set aside early in their growth. Instead, these cells can develop from regular body cells in adult plants, like in the parts that make flowers. This shows how different living things have their own ways of creating new life.

Introduction

Multicellular living things are made of two main types of cells: germ and somatic cells. Germ cells are special because they can make eggs or sperm and are the only cells that can go through a process called meiosis as well as mitosis. Somatic cells are all the other cells that build the body, and they only divide by mitosis.

Germ cells start forming very early in an embryo. In many animals, they begin during a step called cleavage, or in birds and mammals during a step called gastrulation. These cells then move to the area where the reproductive organs develop. In humans, the process that leads to making eggs or sperm starts about 6 weeks after a baby is conceived. Under certain lab conditions in vitro, germ cells can change and act like embryonic stem cells. Both of these cell types can develop into many different types of cells in a lab, but only embryonic stem cells have shown this ability inside a living body.

Specification

There are two ways that germ cells, which become eggs or sperm, are formed in an embryo. In some animals, these cells get special instructions from parts of the egg called germ plasm. These parts have important materials that tell the cell what to become.

In mammals like mice and humans, germ cells are formed when signals from nearby cells tell a few early embryo cells to become primordial germ cells. These early cells can become any type of cell in the body. Special signals, like those from a protein called BMP, help start this process.

Scientists think that the signal method might be the older way, and the germ plasm method might have developed later in some animals. One big difference is when these cells are decided: germ plasm happens very early, while signals happen a bit later. Because cells formed early don’t divide much, they might be less likely to change and make mistakes.

Migration

Main article: Primordial germ cell migration

Primordial germ cells, which are cells that will become eggs or sperm, divide many times as they move through the body to reach the developing gonads, or reproductive organs.

In animals like fruit flies, these cells move from the back of the embryo into the gut and then into the body wall where they continue moving until they reach the gonads.

In vertebrates such as frogs, these cells begin in a specific part of the early embryo and move through the gut to reach the gonads. In mammals, including humans, these cells start from a layer of cells and travel through the body to the gonads over several weeks.

In reptiles and birds, these cells travel through the bloodstream to reach the gonads.

Gametogenesis

Gametogenesis is the process where cells develop into eggs or sperm. This process is different for each animal, but it generally follows similar steps. Both the development of eggs (oogenesis) and sperm (spermatogenesis) include meiosis, changes in shape, and the need for fertilization to survive.

Even though they share some steps, there are important differences. Making sperm results in four identical cells, while making eggs produces just one egg along with two smaller cells called polar bodies. Also, egg development can stop at various points for a long time, while sperm development happens quickly without stopping.

Meiosis
Oogenesis
Spermatogenesis

Oogenesis

When animals develop, special cells called germ cells move to form parts called ovaries. These cells, called oogonia, multiply a lot but many disappear, leaving about 50,000. They then become primary oocytes. In humans, these cells start a process called meiosis before birth but stop until later. During this time, they make a protective layer and get ready for fertilization.

As the female grows, hormones cause one of these cells to leave the ovary each month in a process called ovulation. This cell continues its development and, if met with a sperm, finishes meiosis to become an egg ready for starting a new life.

Egg growth

Eggs need to grow much bigger than regular cells. They do this by pausing part of their development while they grow, and some animals make extra copies of important instructions. Other cells nearby also help feed the growing egg with nutrients.

Mutation and DNA repair

Studies in mice show that the cells that become eggs have fewer changes than other body cells. These egg cells also have ways to fix mistakes in their instructions during a long waiting period.

Spermatogenesis

Mammalian spermatogenesis is how most animals make their sperm. In boys, this process starts at puberty in the seminiferous tubules inside the testicles and continues throughout their lives.

Spermatogonia are the early, immature germ cells. They keep dividing and some of them become primary spermatocytes. These cells go through a process called meiosis, which creates four spermatids. These spermatids then change and become sperm, growing special parts like the acrosome and flagellum.

During this process, the developing germ cells stay connected through special bridges, which help them share important materials. This ensures that all the sperm have what they need to work properly.

Diseases

A germ cell tumor is a rare type of cancer that can affect people of any age. As of 2018, these tumors made up 3% of all cancers in children and teenagers aged 0 to 19 years.

These tumors are usually found in the gonads, which are the ovary and testis, but they can also appear in the abdomen, pelvis, mediastinum, or brain. Sometimes, the cells that should become eggs or sperm do not reach their right place and can cause a tumor to grow there instead. The exact reason for this is still not known. These tumors can be non-cancerous or cancerous.

Induced differentiation

Scientists are working on ways to turn certain cells into germ cells, which are cells that can become eggs or sperm. This could help people who have trouble having children and might even let couples who are not biologically related have children together.

Researchers at Kyoto University have made early-stage germ cells from cells taken from embryos and from the skin. They use special timing and a protein called Bmp4 to help these cells change. These early germ cells can then be used to make sperm and eggs.

However, using human cells is still very difficult. The early germ cells made in labs often do not work as well as natural ones, and they do not always develop correctly. There is still much more to learn before this method can be used widely. Another study showed that growing human embryonic stem cells with special cells from pigs can also lead to the formation of germ cells.