All right, welcome to chapter 28, the last chapter of the semester. I hope you guys enjoyed every single chapter and you learn a lot. Today we'll be learning about embryology and the human development chapter 28. Make sure you answer all the questions that are popping up. And if you have any questions of your own, please reach out to me. See you later. Development is a remarkable process fundamentally characterized by cell differentiation. This is a process by which initially similar cells become specialized adopting unique structures and functions to form the diverse array of cells types we have encountered throughout our study of the human body. from muscle cells to neurons to epithelial cells. These cells do not just appear in their specialized form. They have to undergo a process of commitment and specialization which is differentiation. We can broadly divide human development into two major periods. Prenatal development, the time from conception, which can also be called fertilization to childbirth, also known as parturation, and post-natal development, which occurs after birth and continues until maturity. For the majority of this lecture, we will be focusing on prenatal development. Within this period, we further distinguish three key stages. The pre-mbbriionic period lasting appropriately the first two weeks from fertilization to implantation in the uterine wall. The embryionic period spanning from implantation to the end of the eighth week of pregnancy, an crucial time for the formation of major organs and the fetal period from the ninth week of pregnancy until birth characterized by growth and maturation of these specialized organs. Once a baby is born, we enter postnatal development, a lifelong process that extends from birth to maturity, a somewhat loosely defined endpoint that generally occurs around 15 to 20 years of age, though individual timelines are vary. Let's begin at the very beginning. fertilization. This is the pivotal moment when two hloid gamuts, the sperm from the male and the oite from the female unite to form a single deploy called a single deploy cell called a zygote. The primary function of these hloid gamtins is to deliver the chromosomes that carry the genetic information. The sperm contributes the paternal chromosomes while the oside provides the maternal chromosomes. However, the egg contributes more than just genetic material. It is it also contains essential organels such as the mitochondria to provide energy for a developing embryo as well as various nutrients and other cytoplasmic components that will support these early stages of embionic development. If we remember from our um biology knowledge, mitochondria has their own DNA. That means that the DNA that we have in our mitochondria comes from our mothers and their DNA comes from their mothers and their mother's mothers and their mother's mothers. So mitochondria DNA is a maternal lineage. And this is why when I joke with my dad about my genetic information, I always tell him that I have more genetic material for my mom than him. Fertilization typically occurs in an specific region of the uterine tube called the ampula. Recall that after ovulation, the oite is released from the ovary and travels through the fimry and the infund divulum making its way into the ampula which is located approximately the distal two/3 of the uterine tube away from where it joins the uterus. This amp ampula ampula provides the ideal environment for the sperm and oite to meet and fuse. on the other side of this equation, we have the incredible journey of the sperm to reach the egg. During intercourse, approximately 200 million, that's with an M, million sperm cells are deposited into the vaginal canal. These sperm then undergo a process within the female reproductive tract called capacitation. Think of capacitation as a functional maturation of the sperm. While they may look mature, they need this final activation within the female environment to become fully capable for fertilization. One key aspect of capacitation is that it increases the motility of the sperm, giving them a more directed and vigorous swimming pattern, essentially enabling them to actively seek out the egg. Of these initial 200 million sperm, only about 10,000 sperm successfully navigate the challenging journey through the vaginal canal, the cervix, the uterus, and finally reach the uterine tubes. If you use the math, that is less than 1% of the total sperms. Exactly more like 0.5%. That is incredible. No. Then of these 10,000 sperms, remarkably less than 100 will actually come into contact with the egg. Ultimately under normal circumstances only one single sperm will successfully fertilize the egg. This entire journey from depos deposition in the vagina to fertilization in the ampula can take anywhere from 30 minutes to two hours. highlighting the incredible race against time and the numerous obstacles the sperm must overcome. This image beautifully illustrates a secondary oite surrounded by numerous sperms emphasizing the competition with only one destination with one goal to penetrate and initiate the development of a new individual. While the meeting of a sperm and egg might seem straightforward, there are several challenges that make fertilization a rather selective process. Firstly, the oite once it is ovulated from the ovary is still surrounded by a layer of cells called the corona radiata which we discuss in our female reproductive system lecture. This cellular barrier needs to be penetrated by the sperm. Secondly, the oite itself is in a state of suspended animation. Its metabolism has been arrested and it is not yet actively develop developing. Finally, the oi has only progressed to metaphase or meiosis 2. Remember that meiosis is the cell division that produ that produces the chromosome number. The chromosomes have lined up at the metaphase plate M for middle but the sister chromatids have not yet separated. Meiosis 2 will only be completed after fertilization happens. So how does a single sperm overcome these obstacles? Typically it is the action of enzymes released by the sperm that allows penetration of the corona radiata. Many sperm release an enzyme called hyoluides. This enzyme helps to break down the extracellular matrix holding the cells of the corona radiata together. allowing them to work their way through this outer layer. Eventually, the first sperm to make direct contact with the oytes plasma membrane will have its own membrane fuse with the oite membrane. Once this fusion occurs, the sperm nucleus and the cytoplasm enters the oplasm which is the cytoplasm of the oite. This moment of a sperm entry triggers a cascade of events within the oite leading to the oite activation. A crucial consequence of this activation is that the oside membrane becomes impenetrable to any other sperm. Various mechanisms are put in place to prevent polypermy the fertilization by more than one sperm which will result in an abnormal number of chromosomes. Following activation, the Oside will finally complete meiosis 2, resulting in the formation of a mature oven and the extrusion of a second polar body, a small nonfunctional cell containing the extra set of chromosomes. With meiosis 2 now completed in the oite we have two distinct hloid nuclei within the opplasm. The hloid nucleus from the sperm is referred to as the male pr-ucleus carrying the paternal genetic material. The hloid nucleus of the mature oon is called the female pr--ucleus containing the maternal genetic material. These two pron-uclei will then migrate towards each other and eventually fuse their membranes. The fusion of the male and the female pronuclei marks the completion of fertilization and results in the formation of a single deploy zygote. The very first cell of the new individual now containing a complete set of chromosomes half from the mother and half from the father. Prenatal development is a relatively long process spanning approximately 9 months which is conventionally divided into three roughly equal threemonth periods known as trimesters. The entire 9-month period from fertilization to birth is also referred to as the gestation period. Each trimester is characterized by a specific developmental milestones. During the first trimester, the major organs and organ systems begin to form through the process of organogenesis. The second trimester is marked by further development and refinement of these organs and organ systems as well as a period of significant and rapid growth of the fetus. In the third trimester, most of the organ systems become functional preparing the fetus for independent life outside the wound and there is contin growth and accumulation of fat tissue. The first trimester spanning from week one to week 13 of pregnancy is arguably the most critical and also the most vulnerable period of prenatal life. It is estimated that only about 40% of conception survive beyond this initial trimester. This highlights the complexity and potential for early developmental issues. There are four main and interconnected events that occur during this crucial time. First, we have cleavage which is a rapid series of mitoic cell divisions of the zygote without any significant increase in overall size. This process leads to the formation of a multisellular structure called a blastoyst. Next, the blastosis undergoes implantation embedding itself into the nutrientrich endometrial lining of the uterus. Following implantation is placentation, the formation of the placenta, the vital organ that will mediate the exchange of nutrients, gases and waste between the mother and the developing embryo and fetus. Finally and encompassing much of the first trimester is embryogenesis. The complex and highly orchestrated process of the development of the embryo itself, including the formation of the basic body plan and the initial development of all major organ systems. The first major event of the first trimester is cleaves and blastosis formation. Cleaves is a unique type of cell division characterized by rapid motic division without self-growth. This means that the saigo divides rapidly but the resulting cells become progressively smaller and smaller rather than growing between division. This is a very rapid process largely because it escapes a typical growth phase of the cell cycle. So after the first cleavage, the two resulting cells called blastores are each about half the size of the original zygote. With a second division, you now have four blastores, each about one quarter the size of the zygote and so on. These early cells resulting from cleavage are collectively known as blastores. After several rounds of cleavage around day three or four, the blossoms form a solid ball of cells called a morula. Imagine a small mulberry like this one. That's where the name comes from. Morula. As cleavage continues, the morula undergoes a process of cavitation. where fluid accumulates within it transforming it into a hollow ball of cells called a blastosis. The blastosis has two main components. The inner cell mass and the trophoblast. There is also a blastoal. The open region filled with fluid between the inner cell mass and the throphoblast. The inner cell mass is a cluster of cells located at one pole of the blastoyst. These cells are plur potent meaning they have the potential to develop into all the tissues of the embryo itself. The throphoblast, the outer layer of cells surrounding the blasto seal is responsible for providing nutrients to the developing embryo and plays a crucial role in implantation attaching the blastois to the uterine lining. This timeline illustrates the progression of this process. We start with fertilization and over the next few days cleavage happens leading to the formation of the morula and then the blastoyst. Around day six or seven we reach the point where the blastocyst is ready for implantation. It is important to note that implantation must happen in the uterine wall. If implantation happens in any part of the fallopian tube, this is called an ectopic pregnancy. Any ectopic pregnancy needs to be terminated because the fetus will not be able to survive and it will cause tremendous pain to the mother. Once formed, the blastosis needs to establish a connection with the maternal tissue to receive nourishment and continue developing. This process is called implantation. As the blastois contacts the endometrial lining of the uterus, the cells of the throbblast begins to proliferate rapidly and invade the endometrium. The cytoroolast and the sincitool blast. The outer layer is the sinclast is a unique structure. It is a multi-ucleated mass of cytoplasm meaning they lack distinct cell boundaries. This sincer blast actively erodess the maternal tissues allowing the blastosis to borrow into the endometrium. The sensitial part of the name empa emp emp emphasizes that it is essentially one large continuous mass of cytoplasm with many nuclei. and the trophoblast indicates its origin from the throbblast cells. Simultaneously, the inner cell mass also undergoes reorganization forming a flattened disc like structure called the blastois or bilaminar disc. The blasto disc is composed of two distinct layers. The hypoblast, a layer of a small cubial cells facing the blastoal and the epiblast, a layer of columnar cells located above the hypoblast facing an newly formed cavity called the amniotic cavity. The amniotic cavity begins to form between the epibblast cells and the throphoblast. The blastois with its two layers of epiblast and hypoblast is a crucial intermediate structure. it will eventually transform into the embryionic disc and this transformation occurs through a process called gastrolation. Gastrillation is one of the most fundamental events in embryogenesis as it establishes the three primary germ layers which are the foundational tissues form from which all the tissues and organs of the body will arise. These three germ layers are ectoerm, mesoderm and endoderm. Gastrolation involves a dramatic reenarrangement of the cells. Cells from the epiblast, the blue layer begins to migrate towards the center of the blasto forming a thickened line called the primitive streak. These epibblast cells then invaginate meaning they fall inward and migrate between the existing epibblast and hypoblast layers. The cells that migrate inward will become the mesoderm forming a new middle layer. The cells that remain on the original surface of the epibblast become the ectoerm. the outermost layer. Finally, the endoderm is formed by cells from the hypoblast that are displaced by the migration of the epiblast cells. So essentially the epiblast is the source of both the ectoerm and the mesoderm while the hypoblast contributes to the endoderm. This process of gastrolation is a crucial turning point establishing the body's basic blueprint and setting the stage for organogenesis. As a developmental biology myself, this is an amazing topic to discuss because it literally happens to every single animal in this planet we call Earth. Gastrolation can be seen from sea urchins to fish to flies to humans. It is amazing. Gastrolation establishes the three primary germ layers ectoerm, mesoderm and endoderm. These layers are not just transient structures. They are the fundamental building blocks that will give rise to all the tissues and organ systems of the body. Each germ layer has a specific fate meaning it will differentiate into particular cell types and contribute to the formation of a specific organs. It is crucial that you become familiar with which germ layers contribute to which organ systems. This table provides a valuable overview of these developmental origins and you should use it as a guide to understand how the body is constructed from these three primary layers. In addition to forming the embryo itself, the germ layers also contribute to the formation of the extra embionic membranes. The term extra embionic is key here. It means outside the embryo. These are membranes that develop outside the embryo but are essential for its survival and development. There are four main extra embriionic membranes. The amnon, the alanto, the yolk sack and the coron. The amnon is a membrane derivive from both ectoerm and mesoderm. It forms a fluid fil sack called the amniotic cavity that surrounds the embryo and later the fetus. This amniotic fluid provides a protective cushion shielding the developing organism from mechanical shocks and preventing dehydration. The alantoys also derived from ectoerm and messoderm is a small outpocketing of the early embryos yok sac. While it has a limited role in humans it contributes to the development of the urinary bladder. The yolk sack formed from endoderm and messoderm is an early site of blood cell formation. It is important in the early stages before the bone marrow takes over this function. Finally, the coron composed of mesoderm and throphoblast is the outermost membrane. It forms the blood vessels that will eventually connect the embryo or fetuses to the maternal tissues via the placenta establishing the crucial link of nutrient and and gas exchange. The process of implantation is amazing. When I was a first year graduate student at NYU doing my PhD, uh I worked for a professor uh whose lab was in charge of finding out why so many women have um miscarriages. Now miscarriages usually happen during the process of implantation. If the fetus cannot be implanted properly, it will be miscarriage. is only one of the many ways that you can get a miscarriage. Well, he found out that when in normal pregnancy, a gene supposed to be turned off. This gene, let's say, is some kind of um a chemical um sensor that calls for um T- cells. If you remember from the blood lecture, these cells are part of the immune system. They're in charge of killing um invaders. Now, you may be asking, hold on, invaders. This is an embryo. But remember, embryos are half yourself and half the father's DNA. So, these cells actually think that an embryo is an invader. So in normal pregnancies these um chemicals supposed to be turned off but in this woman that my professor was studying he found out that this gene was not turned off. So in other words the tea cells were able to go into the embryo and kill it. So all of these little little very intricate steps that we have discussed so far even though they sound super mundane and super uh trivial. Everybody knows about trans implantation during the first semester it actually is very very necessary to understand all the processes to understand what happens when something goes wrong. Don't worry about it. These last few seconds were not part of your exam. One of the most important extra embriionic structures is the placenta. It is a unique organ that develops from both maternal and embryionic tissue serving as the interface between the mother and the developing offspring. Placentation is a process of placenta formation. It begins early in development. The coron the outermost embionic membrane develops fingerlike projections called corionic vi vi. These corionic vi invade and extend into the endometrium lining of the uterus. specifically into the sinclast. Recall that the sincoblast is the outer layer of the throphoblast that eroded the maternal tissue to facilitate implantation. This vi established the intricate network of blood vessels that will allow for the exchange of substances between the maternal and fetal circulations. As the fetus continues to grow and develop, it becomes increasingly re reliant on the placenta for its needs. The connection between the fetus and the placenta is established and maintained by the umbilical cord. This core serves as a lifeline transporting nutrients and oxygen to the fetus and carrying away carbon dioxide and other waste products. As we discussed in a previous lecture, the umbilical arteries carry deoxxygenated blood and fetal waste products from the fetus to the placenta where these substances can be transferred to the maternal circulation for elimination. Conversely, the umbilical veins carry oxygenated nutrientrich blood from the placenta to the fetus, providing the essential substances the fetal needs for growth and development. Because the fetal lungs are not functional during prenatal development, the fetus obtains its oxygen from the mother through this placental exchange. The final major event of the first trimester is embryogenesis. the process by which the embryo itself takes shape and separates from the surrounding extra embriionic membranes. This process occurs relatively soon after gastrolation typically be becoming evident by the fourth week of development. A key future of embryogenesis is the development of the head and tail folds. You can see in these images how the initial flat embryionic disc begins to fall, creating a three-dimensional structure. The embryo essentially curls up on itself, separating from the yolk sack and the amnon. This folding process is crucial for establishing the basic body plan and positioning the developing organs correctly. You can see here the yolk sack amnion and the developing embryo proper taking shape. Around 12 weeks post fertilization the developing organs begin to take on their more recognizable forms. This process is called organogenesis. As the name suggests, organo for organs, genesis for origin or creation. It refers to the formation of the organ systems. The development of most organ systems began relatively early within the first month of gestation. However, the in intergmentaryary system or outside skin, endocrine system and reproductive systems initiate their development slightly later during the second month. It is fascinating to note that many organ systems share similar patterns of organization and utilize comparable developmental mechanisms during their formation. This is largely due to the fact that many of these organ systems begin to develop at the same time during those critical first two months of gestation. These share mechanisms involve intricate processes like cell migration, cellto cell interactions, and program cell death. All precisely coordinated in a space and time. While we do not have time to dive into the specifics of each of these complex processes in this course, I encourage you to explore them further in your future studies of developmental biology. As we move into the second trimester, several significant developmental milestones occur. By this stage, the fetus is completely enclosed within the amnon bathed in amniotic fluid. A notable feature of the second trimester is a rapid growth of the fetus, often at a faster rate than the growth of the placenta. This period is characterized by significant refinement of the organ systems that were initially established in the first trimester. In the third trimester, the final phase of prenatal development, the fetal organ systems continue to mature and develop their functionality. They become increasingly capable of supporting independent life outside the womb, preparing the fetus for birth. I want to take a moment to discuss the remarkable changes that occur within the female reproductive system, particularly in the uterus during prenatal development. The uterus undergoes dramatic transformations to accommodate the growing fetus. One of the most striking changes is a significant increase in size. The uterus will expand considerably in length, increasing from its nonpregant size of approximately 7 cm to around three ci to around 30 cm by the end of the ninth monestation period. You can appreciate this drama dramatic size increase in these images showing the uterus and its normal size compared to its greatly expanded size at term. The uterus does not just increase in length. It also expands in volume to accommodate the growing fetus and amniotic fluid. By the end of the 9 months, the uterus will contain nearly 5 lers of fluid. The combined weight of the uterus and its contents can reach approximately 22 pounds, which is right below 10 kilograms. This substantial increase in size and volume exerts significant pressure on the surrounding maternal abdominal organs causing them to be displaced from their normal position. As you can see in these images, the abdominal cavity is significantly altered to make room for the expanding uterus. Normally the abdominal cavity houses the gastrointestinal tract, the organs of the digestive system including the small and large intestines, the stomach and the liver. However, as the uterus enlarges, these organs are often compressed and pushed superiorly towards the upper abdomen. Observe how the stomach, small intestine, transverse colon, and liver are all shifted significantly upwards to accommodate the growing uterus. At the same time, if you can notice here, the bladder is pushed down. This shows this this is the reason why so many pregnant women towards the third trimester have so much uh need of urination because the fetus is pushing down in the bladder. All right, so that's the end of chapter 28, the embryology and human development chapter. Uh please make sure you reach out if you have any questions or if any of the topics was not very well understood or explained or if you want to just talk more about embryology and development. All right, see you later.