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. They 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.

Specification

Germ cells become eggs or sperm. There are two ways these cells are made 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 it will 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 help start this process.

Scientists think that the signal method might be the older way. The germ plasm method might have come later in some animals. One big difference is when these cells are decided: germ plasm happens very early, while signals happen a bit later.

Migration

Main article: Primordial germ cell migration

Primordial germ cells are cells that will become eggs or sperm. They 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 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 grow into eggs or sperm. This process changes for each animal, but it usually has similar steps. Both making eggs (oogenesis) and sperm (spermatogenesis) use meiosis, change shape, and need fertilization to live.

Even though they share some steps, there are important differences. Making sperm makes four same cells, but making eggs makes one egg and two smaller cells called polar bodies. Also, egg development can stop for a long time, but sperm development happens fast without stopping.

Meiosis
Oogenesis
Spermatogenesis

Oogenesis

When animals grow, 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 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. This helps them share important materials, making sure all the sperm work properly.

Diseases

A germ cell tumor is a rare type of cancer. These tumors can happen in people of any age. They are often found in the gonads, which include 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 move to the right place and can cause a tumor to grow. The exact reason for this is still unknown. These tumors can be non-cancerous or cancerous.

Induced differentiation

Scientists are trying to find ways to turn some cells into germ cells. Germ cells can become eggs or sperm. This might help people who have trouble having children.

Researchers at Kyoto University made early germ cells from cells of embryos and skin. They used special timing and a protein called Bmp4 to help the cells change. These early germ cells can be used to make sperm and eggs.

But using human cells is still very hard. The early germ cells made in labs often do not work like natural ones. There is still much to learn before this can be used by many people. A study also showed that growing human cells with special cells from pigs can help make germ cells.