Today we will learn about the basic aspects of mammalian development. In the case of mammals, fertilization is internal. It turns out that not only is fertilization internal, but the processes of zygote formation, cleavage, gastrulation, and organogenesis also occur in the female reproductive tract and hence are also internal.
Let us now look at different parts of the female reproductive tract and their role in the developmental processes. Now the ovaries that are shown in the blue boxes are the organs that will produce the oocyte. The oviduct or fallopian tubes are the regions where fertilization occurs.
The uterus is the region where the fertilized egg and the embryo will grow. The endometrium is the lining of the uterus that nourishes that growing embryo. And finally, the cervix and the vagina are the parts of the female reproductive system that will direct the sperm to the oocyte and also enable the embryo to exit from the female reproductive system when it is ready to be born. Most of the embryonic development in mammals occurs in the uterus.
Now the embryo attaches to the uterine wall through a process that's called as implantation, which is shown in the figure to the left. The uterine wall is rich in blood vessels that provide nourishment, and that region is called as the endometrium, which is highlighted in the figure to the right. It should be noted that as we can see in the figure to the right, This layer of the endometrium, which is rich in blood vessels, can be quite thick so that it can provide nourishment to the growing embryo.
If implantation does not occur and the embryo does not attach to the endometrium, then the embryo cannot survive and develop. The thickness of the endometrium is under the regulation of hormones like progesterone. The blood vessels that line the endometrium have to be renewed on a continual basis.
And thus, if an embryo does not get implanted due to the absence of fertilization or due to defects in implantation, then the resulting hormonal changes cause the removal of the blood lining of the endometrium. The removal of the blood lining can be in the form of resorbing, the contents of the womb, and then the subsequent rebuilding of the blood vessels. Or in some animals, it results in the sloughing or shedding of the endometrial contents through the cervix and the vagina.
This is what is observed in humans and this is basically what the menstrual cycle is. Now let us have a look at what happens to the endometrial lining over time. So if we look at the thickness of the endometrium over a period of days, we can see that the endometrial lining is getting thicker and thicker because of more and more blood vessels that are being formed.
However, if there has not been implantation, then the hormonal changes will result in the shedding of that blood lining and that is basically the process of menstruation. where we can see that now there is a decrease in the thickness of the endometrial lining. Once the menstrual cycle is over, the endometrium starts building its wall, and now here we can see in the figure that more blood vessels are being built in that endometrial wall. Now if implantation occurs, the endometrium wall will continue to vascularize, which means that more and more blood vessels will be generated there to provide nourishment to that developing embryo.
Now an interesting fact is that most mammals actually do not menstruate. Menstruation is observed in primates like humans and apes, as well as in other types of mammals like bats, elephant shrews, and the spiny mouse. Thus, we can see that the list is not a really big list.
So if you're curious to know why do humans menstruate instead of reabsorbing the contents of the womb, there is a really well-written article on the BBC that has been posted. So in early mammalian development, the oocyte moves from the ovary to the fembrae to the oviduct. So if we look at the figure, we can see the ovary and we can see an egg or the oocyte being released from the ovary during the process of ovulation. And through the fembrae, it is able to go to the oviduct.
Now fertilization occurs in the ampulla region of the oviduct, which is shown in the blue box. Once fertilization occurs, a microRNA that is made in the sperm is able to initiate S-phase in the zygote. Now the oviduct have cilia that move the embryo to the uterus and during this process the embryo is actually undergoing its development.
So the zygote starts the cleavage step as the embryo moves towards the uterus and that is what is shown here that The embryo undergoes cleavage while moving to the uterus. Thus, to summarize, we learned about the roles of different parts of the female reproductive system in zygote formation and early embryonic development. We also looked at the movement of the oocyte in the female reproductive tract from the ovary to the uterus.