Prenatal Development: The Course of Early Development

(whooshing, gleaming) (electronic whirring) (punctuated orchestral music) – Development is a lifespan process that starts with the conception Yes, conception triggers the entire developmental process and is a great example of an individual constraint

As you probably know, timing is a critical factor during the fertilization, and 99% of the sperms will probably fail the fertilization process Timing is critical because there is a small chance in which the ovum will be fertilized The chance are firstly reduced because a woman produce an ovum only every 28 days, and an ovum survives only for about 24 hours Let's look at this animation about the ovulation as the first step to understand the fertilization process – [Female Narrator] Ovulation is a part of the menstrual cycle when the ovary releases a ripe egg, or ovum

Inside the ovary are hundreds of thousands of follicles Each follicle is a hollow ball of cells with an immature egg in the center The typical 28-day menstrual cycle begins on the first day of menstrual bleeding During the first seven days of the cycle, a few follicles begin to grow at the same time These maturing follicles secrete estrogen hormone into the bloodstream to prepare the lining of the uterus for pregnancy

Around day seven, all of the follicles stop growing and begin to degenerate, except for one This dominant follicle continues to grow and nourishes the developing egg inside it Around day 12, the follicle secretes a large amount of estrogen into the bloodstream When the estrogen reaches the hypothalamus and the pituitary gland in the brain, the anterior, or front part of the pituitary gland, releases a huge surge of luteinizing hormone into the bloodstream Around day 14, luteinizing hormone causes the follicle to undergo a sudden growth spurt

Right before ovulation, the egg detaches from the inside of the follicle The bulging follicle releases chemicals, causing one of the two Fallopian tubes to move in closer and surround the follicle The follicle swells until it bursts open, ejecting the egg and fluid from the follicle into the abdominal cavity In response, the fimbriae, tiny projections at the end of the Fallopian tube, sweep across the ovulation site and pick up the egg Microscopic cilia on the fimbriae's surface transport the egg to the entrance of the Fallopian tube

Inside the walls of the Fallopian tube, muscular contractions gently push the egg towards the uterus After ovulation, the egg lives for 12 to 24 hours, so it must be fertilized by a sperm from the male during this time for a woman to become pregnant If it's not fertilized, the egg dissolves away and is shed along with the uterine lining during menstruation – Typical prenatal development begins with the fertilization and ends with birth, lasting between 266 and 280 days, for about 38 to 40 weeks The prenatal development is usually divided into three different periods: the zygotic, the embryonic, and the fetal periods

Learning about these specific changes during these three periods will lead us to a better understanding of how various cells will transform into different parts of our body – [Male Narrator] In this video, then, we'll look at the beginning of development, looking into the ovary at an unfertilized egg It then gets fertilized by sperm, which travel down the Fallopian tube, so here millions of sperm coming along Several of them will hit the egg and try to penetrate it, but one will win, as it were, go into the nucleus, and then there's a reprogramming process where the male and female nuclei have their genes set aside to be turned on and off for early development Here, you see early cleavage stages occurring, and this is one of the early growth phases

As the embryo moves down the Fallopian tube, it's going to form an important stage called the blastocyst here in a few seconds Course, in real life, that takes days About five days At this stage, then, I'd like to draw your attention to the inside of the blastocyst, where there are cells called the inner cell mass, which I'll be abbreviating as ICM Those are the cells that make the entire animal, and the outer cells give rise to the placenta and other supporting tissues

At this stage, the embryo implants into the wall of the uterus This is when a pregnancy is really initiated And now we'll see those blue inner cell mass cells form a disc, and then, as the cells continue to grow, they change their physical positions, their geographical relationship to one another, and you'll see that represented here as this disc gets transformed into an embryo Those lines represent sites where cells are migrating in and out, and here's an important stage, when the three beginning layers of the embryo, the so-called germ layers, are formed, and I'll come back to that in a few minutes As development proceeds, there's more growth and movement of cells, it'll begin to form a neural tube, here it turns, and appendages start to bud out, you see the head forming and the eye, and then eventually we get a small embryo, and some months later, of course, this would be born as a young baby

– Let's start with the first one– the zygotic period This is a period of prenatal development that takes place in the first two weeks after conception It includes the creation of the fertilized egg, called a zygote, cell division, and the attachment of the zygote to the uterine wall The attachment of the zygote to the uterine wall takes place about 11 to 15 days after conception, and this process is called implantation At this stage, the group of cells now called the blastocyst consists of an inner mass of cells that will eventually develop into the embryo and the trophoblast, an outer layer of cells that later will provide nutrition and support for the embryo

Rapid cell division by the zygote continues through all the zygotic period By approximately one week after conception, the differentiation process of these cells will start They be specialized for different tasks What is very interesting here is to realize that timing and constraints are present during this process Know that a female produces an ovum only every 28 days and an ovum survives for only about 24 hours

This is the first constraint in this process On the other hand, a male produces sperm in a quantity of three hundred million, approximately, per day, a total amount that could eventually fertilize almost all women in the US, if you needed a proportion of one to one However, a sperm survive for only about only 48 hours, which is another constraint in this process

So, a first, and probably the only one example of a critical period in development, is that the maximum fertile period is 72 hours out of 28 days But let's move into another period The embryonic period occurs from two to eight weeks after conception During the embryonic period, the rate of cell differentiation intensifies As you could see in this video, the cells start to migrate in and out and three beginning layers of the embryo will start to be formed

The layers are ectoderm, mesoderm, and endoderm An early differentiation process will start as development proceeds, leaving us information of different parts of our human body The mesoderm, for example, is the middle layer, which will become the circulatory system, bones, muscles, the excretory system, and reproductive system The ectoderm is the outermost layer, which will become the nervous system and brain, the sensory receptors, like our ears, nose, and eyes, and the skin parts, like hair and nails, for example Every body part eventually develops from these three layers, so in summary, the endoderm primarily produce internal body parts, the mesoderm primarily produce parts that surround the internal areas, and the ectoderm primarily produce the surface parts

As the inmost three layers form, life support systems for the embryo develop rapidly These life support systems include the amnion, the umbilical cord, both which develop from the fertilized egg, not the mother's body, and the placenta Later, we will notice a clear direction of the development of the central nervous system The directions are two: cephalocaudal, which means cephalic structures will develop first, and the structures close to the end of our spinal cord will develop later At the same time, the development proceeds to be proximodistal, which means that central areas of our body will start developing before the peripheral parts

This is the distal parts Note that both reactions happen at the same time, simultaneously Here is how prenatal structural and functional chains happen during the early periods of our lives – [Male Narrator] Pregnancy and delivery involve the entire female reproductive system, including the ovaries, Fallopian tubes, uterus, and vagina After an egg is released from an ovary and is fertilized, it travels slowly through the Fallopian tube to the uterus

During this time, it begins to divide until it has grown to about one hundred cells The egg, now called a blastocyst, then implants in the tissue lining the wall of the uterus The uterus will then house and protect the developing fetus for 38 weeks, or approximately nine months When the fetus has matured and birth is imminent, the baby begins to go through a series of movements that help it navigate through the birth canal As the uterus begins to contract with the pains of labor, the opening of the uterus, called the cervix, dilates to allow the baby to pass into the vagina

This muscular tube expands to accommodate the baby's head and shoulders, while uterine contractions continue until the baby enters the world in what is, in most cases, a downward-facing position At this point, the baby will be cleaned and checked for good health before being placed on its mother's chest to rest and bond (punctuated orchestral music)

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