Meiosis enables the production of cells with half the genetic material of the parent cell. This in turn enables the joining together of two haploid cells to form a zygote that has the full number of chromosomes again e.g. diploid in humans.
Gametes are these cells that are haploid are are used as part of sexual reproduction in mammals to enable more genetically diverse offspring, compared with asexual reproduction. Gametes are egg and sperm cells. Their development through meiosis involves multiple stages, dispersed throughout an individual’s lifetime as we will see for eggs which finish the first part of development even prior to the birth of the individual who may use them later.
The precursor (primordial) cells of eggs are oogonia (oogonium, sg.) and those of sperm are spermatogonia (spermatogonium, sg.). These grow and divide (mitosis) before undergoing meiosis I, maturing into oocytes and spermatocytes. These cells are termed primary, while the resulting cells after meiosis I are termed secondary.
So primary spermatocyes undergo the first stage of meiosis, producing secondary spermatocytes. Primary oocytes divide unevenly, producing one large offspring cell which is the secondary oocyte and a smaller cell which doesn’t go any further in the process, called a first polar body (yes, there will be a second polar body during meiosis II).
Meiosis II sees the secondary oocytes and spermatocytes divide again and produce spermatids and ootids. For sperm, this gives 4 cells, but with another second polar body (actually 3 this division, not shown; so 4 overall counting the first polar body) produced alongside the first discarded polar body, there is only one egg per meiosis process.
Finally, mature gametes spermatozoa and ova (ovum, sg.) arise following the differentiation of the spermatids and ootids. Spermatozoa have developed flagella while ova have a thicker membrane – at this point fertilisation has occurred.
The process of making sperm, spermatogenesis, kickstarts at puberty. By this time, its counterpart oogenesis is already at the primary oocyte level. Ovulation is the step towards making the secondary oocytes. In the absence of fertilisation, oocytes stop here. Upon fertilisation, meiosis carries on to develop the ootid and ovum. This is now a zygote.
We’ve already treaded onto fertilisation territory, by describing the final developmental stage of eggs from secondary oocytes into ootids and ova. However, this is once fertilisation has occurred. Let’s step back and rewind in more detail the steps from the first contact of the two gametes to the moment their membranes fuse together to make one complete diploid cell – the zygote.
The sperm head contains enzymes that can break down the outer layer of the egg. It’s called hyalunoridase and the release of the sperm nuclear content initiates the further developmental responses by the egg including the release of the second polar body and preparing for division (cleavage).
The tip of the sperm head is called acrosome. This is where the enzyme starts to break down the egg layer once it has approached the surface. Once the first sperm has broken down the egg coat and entered, the process is closed off to the remaining sperm cells.
Early embryo development
What follows next is a series of zygote cleavages into a small number of cells through to a mass known as a morula, before partitioning away within the membrane and leaving a cavity at the stage of blastocyst.
At this stage implantation takes place where the embryo attaches to the lining of the uterus (endometrium) and the fetus starts to develop.
The zona pellucida is a glycoprotein layer surrounding the embryo membrane. In the late blastocyst, the epiblast is the source of the actual embryo that is about the grow, while the trophectoderm is the tissue that will provide nutrients to it and become a big part of the placenta.
The primitive endoderm is one of three (the other two being the mesoderm and ectoderm) primitive layers of differentiating tissue that provide the source for many different subsequent tissues and organs in the developing embryo. The endoderm provides the lining cells in many systems including digestive, respiratory, auditory and urinary.