Lecture 13: Early Human Development Flashcards
What does embryogenesis achieve?
Embryogenesis achieves patterning (cells acquire identity in space and time)
Defines the major axis (anterior/posterior and dorsal/ventral)
Establishes three germ layers (gastrulation), and forms the rudiments of major organs.
What are the three germ layers generated during gastrulation?
The three germ layers are ectoderm (outside), mesoderm (between), and endoderm (inside).
What happens to the egg after it is released from the ovary?
The egg is released into the fallopian tube and the ovary turns into the corpus luteum, which releases progesterone for about 10 days.
What is the role of progesterone in early pregnancy?
Progesterone prepares the uterine lining for the egg.
What occurs after a sperm cell penetrates the zona pellucida?
Once a sperm cell passes the zona pellucida, no more sperm can enter, and the DNA combines into a new cell called a zygote.
What is cleavage in embryonic development?
Cleavage is the rapid cell division of the zygote as it passes through the uterine/fallopian tube, breaking the egg into smaller cells while maintaining the same total size.
What is a morula?
A morula is a hollow ball of cells (16+ cells) formed after about 3-4 days of cleavage.
What is a blastocyst?
A blastocyst is formed from the morula after about 5-6 days, consisting of an inner cell mass (embryoblast) and an outer cell mass (trophoblast).
What happens during implantation?
The blastocyst embeds into the uterine wall, releasing hCG, which tells the corpus luteum to continue releasing progesterone.
What is the difference between a blastocyst and a blastula?
Blastocyst is a stage of the embryo formed in the early development of mammals. In the rest of the animals, it is known as a blastula.
What occurs during fertilization?
During fertilization, a sperm cell enters the oocyte, and Meiosis II is completed.
What layers must a sperm cell penetrate for fertilization to occur?
A sperm cell must penetrate the corona radiata and the zona pellucida to fertilize the oocyte.
What is the significance of the zona pellucida?
Initial gamete recognition, preventing polyspermy, and protecting the embryo during its journey through the oviduct before implantation.
The zona pellucida is not sticky, preventing the blastocyst from sticking in the fallopian tube, which could lead to an ectopic pregnancy.
What occurs after the zona pellucida dissolves?
This is accomplished by a process called blastocyst hatching. A small region of the zona pellucida, usually directly over the inner cell mass in the primate, dissolves, and the blastocyst emerges from the hole.
After the zona pellucida dissolves, the blastocyst implants into the uterine wall at the posterior-superior aspect of the endometrium, known as the decidua.
What is the role of the trophoblast in implantation?
The trophoblast implants in the uterine wall and secretes hCG (human Chorionic Gonadotropin)
It also develops into the chorion, and later the placenta
What are the components of the blastocyst?
Inner cell mass (embryoblast)
Outer cell mass (trophoblast)
Blastocyst cavity (blastocele)
What do the cytotrophoblast and syncytiotrophoblast develop into?
The cytotrophoblast and syncytiotrophoblast give rise to the chorionic villi, which form the fetal placenta.
What is the significance of the primitive streak in embryonic development?
The primitive streak is a thickened structure in the midline of the epiblast that defines the major body axes of the embryo, including cranial and caudal ends.
What happens at the cranial end of the primitive streak?
At the cranial end of the primitive streak, a primitive node with a circular depression known as a primitive pit is formed, leading to the creation of a primitive groove.
What is invagination in embryonic development?
Invagination is the process where epiblast cells migrate towards the primitive streak, detach from the epiblast, and slip into the interior area formed from their absence.
What is the definitive endoderm?
The definitive endoderm is a new proximal cell layer formed when the first cells through the primitive streak invade and displace the hypoblast.
What happens to the hypoblast during embryonic development?
The hypoblast contributes partially to the yolk sac, while the rest is removed through apoptosis.
How is mesoderm formed during gastrulation?
The first set of cells to move down the primitive streak integrate into the hypoblast layer and form endoderm, the first of the three germ layers. The second set of cells to detach and ingress will fill the space between the endoderm and epiblast layer to form the second germ layer, mesoderm
What is the role of the amnion and yolk sac during embryonic development?
The yolk sac is the extraembryonic membrane that surrounds the egg yolk. The yolk sac has a well-developed vascular system that transports nutrients from the egg yolk to the developing embryo while the yolk sac provides nutrition as the placenta is forming.
The amnion is the extraembryonic membrane that surrounds and protects the developing embryo
What is the first indication of gastrulation?
The first indication of gastrulation is the movement of ectodermal cells displacing to become mesoderm.
What happens to the outer cell mass during gastrulation?
The outer cell mass is referred to as the chorion, which consists of the syncytial trophoblast and the cytotrophoblast.
What is embryonic folding?
Embryonic folding involves the embryo with three layers folding its tail and head towards each other.
What are the characteristics of differentiated cells and stem cells?
Differentiated cells have limited divisions, while stem cells can replace themselves and generate differentiated daughter cells.
What defines a stem cell?
A stem cell is defined as a cell that divides to form one daughter that differentiates and one daughter that retains its stem-cell properties.
What are the different types of stem cells based on their developmental potential?
Totipotent (e.g., zygote)
Pluripotent (e.g., blastocyst, ESCs, iPSCs),
Multipotent (specific tissue or organ)
Nullipotent/Unipotent (not capable of giving rise to other cell types).
What is the role of unipotent stem cells in the skin?
Unipotent stem cells in the skin allow for constant renewal of skin cells.
What is the overview of the ex vivo gene therapy strategy for skin cells?
The strategy includes isolating skin cells, using CRISPR/Cas9 to correct mutations, growing gene-corrected skin, and grafting it onto patient wounds.
What are the key stages of embryonic development after fertilization?
After fertilization, the stages are cleavage, blastula formation/implantation, gastrulation (creating three germ layers), and organogenesis.
Why do we need stem cells?
Stem cells are needed to replace differentiated cells for growth, repair, and renewal.
What is the clinical utility of stem cells?
The clinical utility of stem cells includes long-lasting skin grafts, especially when gene-edited.
What occurs during the first trimester of pregnancy?
The first trimester is the most critical stage of development, where the rudiments of all major organ systems appear, and the developing organism is most vulnerable to drugs, radiation, and microbes.
What characterizes the second trimester of pregnancy?
The second trimester is characterized by nearly complete development of organ systems, and by the end, the fetus assumes distinctively human features.
What happens during the third trimester of pregnancy?
The third trimester represents a period of rapid fetal growth, with most organ systems becoming fully functional during the early stages.
What significant events occur during the first week of development?
The first week of development includes fertilization, cleavage of the zygote, blastocyst formation, and implantation.
What happens during fertilization?
During fertilization, the genetic material from a haploid sperm cell and a haploid secondary oocyte merges into a single diploid nucleus.
What is capacitation in sperm cells?
Capacitation is a series of functional changes that sperm undergo in the female reproductive tract, allowing them to fertilize the oocyte.
What changes occur to sperm during capacitation?
During capacitation, cholesterol, glycoproteins, and proteins are removed from the sperm’s plasma membrane, allowing it to fuse with the oocyte’s plasma membrane.
What is the acrosome and its function?
The acrosome is a helmetlike structure covering the head of a sperm that contains enzymes to help penetrate the zona pellucida.
What role does ZP3 play in fertilization?
ZP3 is a glycoprotein in the zona pellucida that acts as a sperm receptor, triggering the acrosomal reaction when bound to specific membrane proteins in the sperm head.
What happens during the acrosomal reaction?
The acrosomal reaction involves the release of enzymes from the acrosome, which digest a path through the zona pellucida, allowing the sperm to reach the oocyte’s plasma membrane.
What events are triggered by the fusion of a sperm cell with a secondary oocyte?
The fusion triggers events that block polyspermy, including the depolarization of the oocyte’s cell membrane and the release of calcium ions.
What is the fast block to polyspermy?
The fast block to polyspermy is the inability of a depolarized oocyte to fuse with another sperm cell, occurring within seconds of fertilization.
What is the slow block to polyspermy?
The slow block to polyspermy involves the release of molecules that inactivate ZP3 and harden the zona pellucida, preventing further sperm entry.
What happens to the oocyte after a sperm cell enters?
The oocyte completes meiosis II, dividing into a larger ovum and a smaller second polar body that disintegrates.
What are the male and female pronuclei?
The male pronucleus develops from the sperm’s nucleus, while the female pronucleus develops from the fertilized ovum’s nucleus.
What is syngamy?
Syngamy is the process where the male and female pronuclei fuse to produce a single diploid nucleus, restoring the diploid number of chromosomes.
What is a zygote?
A zygote is the fertilized ovum that results from the fusion of the haploid pronuclei, containing 46 chromosomes.
What is cleavage in the context of embryonic development?
Cleavage refers to the rapid mitotic cell divisions of the zygote that occur after fertilization.
What are blastomeres?
Blastomeres are the progressively smaller cells produced by cleavage of the zygote.
What is a morula?
A morula is a solid sphere of cells produced by successive cleavages of the zygote, still surrounded by the zona pellucida.
What occurs during blastocyst formation?
During blastocyst formation, the morula enters the uterine cavity, where uterine milk nourishes it, leading to the formation of a blastocyst cavity.
What is the blastocyst?
The blastocyst is the developing mass that forms after the blastocyst cavity is created, consisting of hundreds of cells.
Describe the two distinct cell populations that arise during the formation of the blastocyst.
The two distinct cell populations are the embryoblast, which is the inner cell mass that develops into the embryo, and the trophoblast, which is the outer layer of cells that forms the wall of the blastocyst and will develop into the chorionic sac and placenta.
Explain the role of the trophoblast in the development of the blastocyst.
The trophoblast is the outer superficial layer of cells that forms the spherelike wall of the blastocyst. It ultimately develops into the outer chorionic sac that surrounds the fetus and the fetal portion of the placenta, facilitating the exchange of nutrients and wastes between the mother and fetus.
How does the blastocyst ‘hatch’ from the zona pellucida?
The blastocyst hatches from the zona pellucida by digesting a hole in it using an enzyme and then squeezing through the hole.
Define implantation in the context of blastocyst development.
Implantation is the process by which the blastocyst attaches to the vascular, glandular endometrial lining of the uterus after remaining free in the uterine cavity for about 2 days.
What is the significance of the zona pellucida shedding during blastocyst development?
The shedding of the zona pellucida is necessary to allow the blastocyst to implant into the uterine wall, which is a crucial step for establishing pregnancy.
Explain the function of the embryoblast in the blastocyst.
The embryoblast, or inner cell mass, is located internally within the blastocyst and eventually develops into the embryo.
Describe the secretory phase of the endometrium.
The secretory phase of the endometrium is the stage where the endometrial lining prepares for potential implantation of a fertilized egg, characterized by increased glandular activity and vascularization.
Explain the process of implantation in relation to the blastocyst.
Implantation is the process where the blastocyst loosely attaches to the endometrium about 6 days after fertilization, orienting its inner cell mass toward the endometrium.
How does the blastocyst interact with the endometrium after initial attachment?
After initial attachment, about 7 days post-fertilization, the blastocyst attaches more firmly, leading to enlargement of endometrial glands and increased vascularization of the endometrium.
Define the term ‘decidua’ in the context of pregnancy.
The decidua refers to the modified endometrium after implantation, which surrounds the developing embryo and will separate from the uterus after childbirth.
What happens to the decidua after the fetus is delivered?
After the fetus is delivered, the decidua separates from the endometrium, similar to the process that occurs during normal menstruation.
Identify the decidua basalis and its location.
The decidua basalis is the portion of the endometrium located between the embryo and the stratum basale of the uterus.
Explain the significance of vascularization in the endometrium during implantation.
Vascularization of the endometrium during implantation is significant as it forms new blood vessels that support the developing embryo by providing necessary nutrients and oxygen.
What are the different regions of the decidua named after?
The different regions of the decidua are named based on their positions relative to the site of the implanted blastocyst.
Describe the role of glycogen and lipids in embryonic development.
Glycogen and lipids provide large amounts of energy and nutrients for the developing embryo and fetus.
Explain the function of the decidua capsularis during pregnancy.
The decidua capsularis is the portion of the endometrium that lies between the embryo and the uterine cavity, playing a role in supporting the embryo.
Define the decidua parietalis and its location in the uterus.
The decidua parietalis is the modified endometrium that lines the noninvolved areas of the uterus, surrounding the embryo.
How does the decidua capsularis change as the embryo grows?
As the embryo enlarges, the decidua capsularis bulges into the uterine cavity and eventually fuses with the decidua parietalis, obliterating the uterine cavity.
What happens to the decidua capsularis by about 27 weeks of pregnancy?
By about 27 weeks, the decidua capsularis degenerates and disappears.
Discuss the significance of the maternal part of the placenta.
The maternal part of the placenta, which develops from the decidua, is crucial for nutrient and gas exchange between the mother and the developing fetus.
Describe the development of the trophoblast during the second week of development.
About 8 days after fertilization, the trophoblast develops into two layers: the syncytiotrophoblast, which has no distinct cell boundaries, and the cytotrophoblast, which is composed of distinct cells. These layers become part of the chorion as they grow.
Explain the role of the syncytiotrophoblast during implantation.
The syncytiotrophoblast secretes enzymes that allow the blastocyst to penetrate the uterine lining by digesting and liquefying the endometrial cells, eventually leading to the blastocyst being buried in the endometrium.
Define human chorionic gonadotropin (hCG) and its function in early pregnancy.
Human chorionic gonadotropin (hCG) is a hormone secreted by the trophoblast that has actions similar to LH. It rescues the corpus luteum from degeneration and sustains its secretion of progesterone and estrogens.
How do the layers of the trophoblast contribute to fetal development?
The two layers of the trophoblast, syncytiotrophoblast and cytotrophoblast, become part of the chorion and are essential for implantation and the establishment of the placenta.
What happens to the blastocyst during the second week of development?
During the second week of development, the blastocyst becomes buried in the endometrium and the inner one-third of the myometrium as it undergoes implantation.
Do the layers of the trophoblast have distinct cell boundaries?
The presence of hCG in maternal blood or urine is an indicator of pregnancy and is detected by home pregnancy tests.
Define the bilaminar embryonic disc.
The bilaminar embryonic disc is a flat structure formed by the differentiation of the embryoblast into two layers, the hypoblast and epiblast, around 8 days after fertilization.
How do the hypoblast and epiblast contribute to embryonic development?
A small cavity appears within the epiblast, which eventually enlarges to form the amniotic cavity.
Describe the process of gastrulation in embryonic development.
Gastrulation is the first major event of the third week of development, occurring about 15 days after fertilization. It transforms the bilaminar embryonic disc into a trilaminar disc consisting of ectoderm, mesoderm, and endoderm, which are the primary germ layers that develop into various tissues and organs.
Explain the significance of the primitive streak in gastrulation.
The primitive streak is the first evidence of gastrulation, appearing as a faint groove on the dorsal surface of the epiblast. It establishes the head and tail ends of the embryo, as well as its right and left sides.
Define the term ‘invagination’ in the context of gastrulation.
Invagination is the process during gastrulation where cells of the epiblast move inward below the primitive streak and detach from the epiblast, leading to the formation of the endoderm.
How do the primary germ layers form during gastrulation?
During gastrulation, cells from the epiblast migrate and rearrange. Some cells invaginate to form the endoderm, while others remain between the epiblast and endoderm to form the mesoderm.
What are the three primary germ layers formed during gastrulation?
The three primary germ layers formed during gastrulation are ectoderm, mesoderm, and endoderm.
Describe the role of the primitive node in gastrulation.
The primitive node is a rounded structure formed by a small group of epiblastic cells at the head end of the primitive streak, playing a crucial role in the organization and development of the embryo.
What is the relationship between the epiblast and the hypoblast during gastrulation?
During gastrulation, cells from the epiblast migrate inward and displace the hypoblast to form the endoderm, while other cells form the mesoderm between the epiblast and the newly formed endoderm.
Describe the role of the ectoderm in embryonic development.
The ectoderm develops into the epidermis of the skin and the nervous system.
Explain the significance of the mesoderm in the formation of body structures.
The mesoderm gives rise to muscles, bones, other connective tissues, and the peritoneum.
Define the notochord and its importance in embryonic development.
The notochord is a solid cylinder of cells that plays a crucial role in induction, stimulating the development of adjacent unspecialized tissues into specialized ones.
How does the endoderm contribute to organ formation in the embryo?
The endoderm ultimately becomes the epithelial lining of the gastrointestinal tract, respiratory tract, and several other organs.
Do mesodermal cells migrate during embryonic development? If so, where do they migrate to?
Yes, mesodermal cells from the primitive node migrate toward the head end of the embryo.
Explain the process of induction in embryonic development.
Induction is the process by which one tissue (inducing tissue) stimulates the development of an adjacent unspecialized tissue (responding tissue) into a specialized one.
What is the notochordal process and when does it form during embryonic development?
The notochordal process is a hollow tube of cells that forms in the midline of the embryo about 16 days after fertilization.
Describe the transformation of the notochordal process into the notochord.
By days 22–24, the notochordal process becomes a solid cylinder of cells called the notochord.
How does the notochord influence the development of vertebral bodies?
The notochord induces certain mesodermal cells to develop into the vertebral bodies.
What structures does the mesoderm give rise to besides muscles and bones?
The mesoderm also gives rise to other connective tissues and the peritoneum.
Discuss the relationship between the allantois and the connecting stalk.
The allantois is a small vascularized outpouching of the yolk sac that extends into the connecting stalk during embryonic development.
