During what stages of embryonic development are embryonic stem cells totipotent?
Up to the eight-cell stage (2 cell, 4 cell, & 8 cell stages), the eight embryonic stem cells can differentiate into all cell types in the embryo and placenta i.e., the cells are totipotent
When are the first four stages of embryonic development achieved?
1) The two-cell stage is reached approximately 30 hours after fertilization 2) The four-cell stage is reached at approximately 40 hours after fertilization 3) The 12 to 16 cell stage is reached at approximately 3 days after fertilization 4) The late morula stage is reached at approximately 4 days after fertilization
What is the zona pellucida and when does it disappear?
1) The zona pellucida is an egg coat that surrounds an egg coat and has an important function in fertilization 2) In humans, at the end of the fourth day after fertilization, the blastocyst performs zona hatching; the zona pellucida degenerates and decomposes, to be replaced by the underlying layer of trophoblastic cell
What is a blastomere?
Once the zygote has reached the two-cell stage, it undergoes a series of mitotic divisions, increasing the numbers of cells. These cells, which become smaller with each cleavage division, are known as blastomeres
What is compaction and when does it occur?
Until the eight-cell stage, blastomeres form a loosely arranged clump. After the third cleavage, however, blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions. This process, compaction, segregates inner cells, which communicate extensively by gap junctions, from outer cells.
What is a morula and what are its components?
Approximately 3 days after fertilization, cells of the compacted embryo divide again to form a 16-cell morula (mulberry). Inner cells of the morula constitute the inner cell mass, and surrounding cells compose the outer cell mass. The inner cell mass gives rise to tissues of the embryo proper, and the outer cell mass forms the trophoblast, which later contributes to the placenta.
List the stages of embryonic development up to and including uterine implantation of a blastocyst
1) Oocyte is formed from ovulation 2) Fertilization, approximately 12 to 24 hours after ovulation 3) Stage of the male and female pronuclei 4) Spindle of the first mitotic division 5) Two-cell stage (approximately 30 hours of age) 6) Morula containing 12 to 16 blastomeres (approximately 3 days of age) 7) Advanced morula stage reaching the uterine lumen (approximately 4 days of age) 8) Early blastocyst stage (approximately 4.5 days of age; the zona pellucida has disappeared) 9) Early phase of implantation (blastocyst approximately 6 days of age). The ovary shows stages of transformation between a primary follicle and a preovulatory follicle as well as a corpus luteum. The uterine endometrium is shown in the progestational stage
What occurs to a zygote once it enters the lumen of the uterus to form a blastocyst?
1) Uterine fluid is used to fill the blastocyst cavity by forming an invagination of the morula to form a blastocyst 2) This forms a separation between an inner cell mass (embryoblast) & outer cell mass (trophoblasts)
What is a blastocyst composed of?
Once zygotic cells have replicated to for roughly a 107-cell human blastocyst, it is composed of an inner cell mass (embryoblasts) and trophoblast cells
What can an embryoblast develop into?
Embryoblasts are pluripotent cells that can differentiate into all cells of the embryo but not the placenta (trophoblast develops into the placenta)
Why do cells from the inner cell mass have clinical potential?
Embryonic stem cells are derived from the inner cell mass. Because these cells are pluripotent and can form virtually any cell or tissue type, they have the potential for curing a variety of diseases including diabetes, Alzheimer's & Parkinson's diseases, anemias, spinal cord injuries, & many others
What is reproductive cloning?
Embryonic Stem (ES) cells may be obtained from embryos after in vitro fertilization (IVF). This approach has the disadvantage that the cells may cause immune rejection, because they would not be genetically identical to their hosts. Another issue is with this approach is based on ethical considerations, as the cells are derived from viable embryos
What is therapeutic cloning or somatic nuclear transfer?
Take nuclei from adult cells and introduce them into enucleated oocytes. These oocytes are stimulated to differentiate into blasocysts, and ES cells are harvested. Because the cells are derived from the host, they are compatible genetically, and because fertilization is not involved, the technique is less controversial
When does a blastocyst penetrate the uterine lining & what part of the blastocyst penetrates the uterine lining?
1) The human blastocyst begins penetrating the uterine mucosa by the sixth day of development 2) Trophoblast cells at the embryonic pole of the blastocyst penetrate the uterine mucosa
What are trophoblasts?
1) Trophoblasts are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta 2) They are formed during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg 3) This layer of trophoblasts is also collectively referred to as "the trophoblast" or, after gastrulation, the trophectoderm, as it is then contiguous with the ectoderm of the embryo
What do embryoblasts and trophoblasts initially form?
1) Embryoblasts form the epiblasts & hypoblasts 2) Trophoblasts form the outside layer of the yolk sac 3) This occurs by day 9 after fertilization
What do embryoblasts and trophoblasts eventually form?
1) Embryoblasts form the endoderm, mesoderm, & ectoderm 2) Trophoblasts form the placenta
How does the amniotic cavity begin to form?
The endometrial stroma and its containing blood vessels are filled with fluid that diffuse into trophoblastic lacunae, allowing the blastocyst to use that fluid to fill the amniotic cavity
What are sinusoids?
1) Cells of the syncytiotrophoblast penetrate deeper into the stroma and erode the endothelial lining of the maternal capillaries. These capillaries, which are congested and dilated, are known as sinusoids 2) The syncytial lacunae become continuous with the sinusoids, and maternal blood enters the lacunar system 3) As trophoblasts continue to erode more and more sinusoids, maternal blood begins to flow through the trophoblastic system, establishing the uteroplacental circulation
How does extra embryonic tissue form?
Between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity are cells derived from yolk sac cells that form a fine, loose connective tissue, the extraembryonic mesoderm. It eventually fills all of the space between the trophoblast externally and the amnion and exocoelomic membrane internally. Soon, large cavities develop in the extraembryonic mesoderm, and when these become confluent, they form a new space known as the extraembryonic cavity, or chorionic cavity. This space surrounds the primitive yolk sac and amniotic cavity, except where germ disc is connected to the trophoblast by the connecting stalk
Describe the primary, secondary, and final yolk sacs
1) Primary yolk sac/primitive yolk sac: it is the vesicle constituted in the second week, its floor is represented by Heuser's membrane and its ceiling by the epiblast. It is also known as the exocoelomic cavity 2) Secondary yolk sac: this first transformation is determined by the modification of its cover, in the connection zone between the hypoblast and the Heuser membrane. We can observe a structure. The two parts detach and the inferior one, which is smaller, forms a cyst destined to be eliminated. The upper one is now covered only by the hypoblast 3) The final yolk sac: during the fourth week of development, during which we can see the shaping of the embryonic areas. A little portion of the sac, in the upper part, constitutes the intestinal tube. On the other side, the distal part forms a little vesicle that is what remains of the yolk sac
Describe the formation of the primitive streak and what occurs at the primitive streak
1) Gastrulation begins with formation of the primitive streak on the surface of the epiblast. Initially, the streak is vaguely defined, but in a 15- to 16-day embryo, it is clearly visible as a narrow groove with slightly bulging regions on either side. The cephalic end of the streak, the primitive node, consists of a slightly elevated area surrounding the small primitive pit 2) Cells of the epiblast migrate toward the primitive streak. Upon arrival in the region of the streak, they become flask-shaped, detach from the epiblast, and slip beneath it. This inward movement is known as invagination
Describe how invagination at the primitive streak forms the three germ layers
Once the cells have invaginated, some displace the hypoblast, creating the embryonic endoderm, and others come to lie between the epiblast and newly created endoderm to form mesoderm. Cells remaining in the epiblast then form ectoderm. Thus, the epiblast, through the process of gastrulation, is the source of all of the germ layers, and cells in these layers will give rise to all of the tissues and organs in the embryo.
Describe the orientation of germ layers when they are formed
The surface of epiblasts facing the amniotic cavity is located in the caudal region (closer to the feet) of the developing embryo and the opposite end is the cranial region (closer to the head). The regions closer to the head are comprised moreso of endoderm and mesoderm as they form first (embryo is formed from the head down)
How is the notochord formed?
1) As the hypoblast is replaced by endoderm cells moving in at the streak, cells of the notochordal plate proliferate and detach from the endoderm 2) They then form a solid cord of cells, the definitive notochord, which underlies the neural tube and serves as the basis for the axial skeleton 3) Because elongation of the notochord is a dynamic process, the cranial end forms first, and caudal regions are added as the primitive streak assumes a more caudal position 4) Note that some cells migrate ahead of the notochord. These mesoderm cells form the prechordal plate that will assist in forebrain induction
When does the primitive streak disappear?
The primitive streak disappears by the end of the 4th week
What are the vitelline vessels and what are their functions?
1) The vitelline veins are veins which drain blood from the yolk sac 2) The vitelline veins give rise to the hepatic veins, inferior portion of Inferior vena cava, portal vein, & superior mesenteric vein 3) The vitelline arteries are the arterial counterpart to the vitelline veins. Like the veins, they play an important role in the vitelline circulation of blood to and from the yolk sac of a fetus. They are a branch of the dorsal aorta. 4) The vitelline artieries give rise to the celiac artery, superior mesenteric artery, and inferior mesenteric artery.
Describe the formation of the umbilical cord
During further development, the amniotic cavity enlarges rapidly at the expense of the chorionic cavity, and the amnion begins to envelop the connecting and yolk sac stalks, crowding them together and giving rise to the primitive umbilical cord 2) Distally, the cord contains the yolk sac stalk and umbilical vessels. More proximally, it contains some intestinal loops and the remnant of the allantoi. The yolk sac, found in the chorionic cavity, is connected to the umbilical cord by its stalk 3) At the end of the third month, the amnion has expanded so that it comes in contact with the chorion, obliterating the chorionic cavity. The yolk sac then usually shrinks and is gradually obliterated
How is the primitive streak related to the formation of teratomas and why do the tumors contain a mixture of tissue types?
Sometimes, remnants of the primitive streak persist in the sacrococcygeal region. These clusters of pluripotent cells proliferate and form tumors known as sacrococcygeal teratomas, which commonly contain tissues derived from all three germ layers. This is the most common tumor in newborns with a frequency of 1 in 37,000. Teratomas may also arise from primordial germ cells that fail to migrate to the gonadal ridge
What does the ectoderm develop into?
Ectoderm forms the spinal cord and tissues that contact the external environment e.g., epidermis of the skin
What does the endoderm develop into?
Endoderm initially forms the lining of the yolk sac and observe endoderm subsequently forms epithelia such as those lining the derivatives of the gut tube
What are mesenchymal stem cells derived from and what can they form?
Mesenchymal stem cells, or MSCs, are multipotent stromal cells that are derived from the mesoderm and can differentiate into a variety of cell types, including: osteoblasts (bone cells), chondrocytes (cartilage cells), and adipocytes (fat cells)
Describe the separation of the mesoderm
1) By approximately the 17th day post fertilization, cells close to the midline of the mesodermal germ layer form a thickened plate of tissue known as paraxial mesoderm 2) More laterally, the mesoderm remains as a thin plate known as the lateral plate
What two layers does the mesoderm develop into?
1) A layer continuous with mesoderm covering the amnion, known as the somatic or parietal mesoderm layer 2) A layer continuous with mesoderm covering the yolk sac, known as the splanchnic or visceral mesoderm layer 3) Together, these layers form the intraembryonic cavity, which is continuous with the extra embryonic cavity on each side of the embryo. Intermediate mesoderm connects paraxial & lateral plate mesoderm 4) Observe the plate splits into parietal and visceral layers, and envision the parietal layer forms the dermis of the skin, and the bones and muscles of the body wall and extremities
What does the thin mesodermal layer give rise to?
1) The thin mesodermal sheet gives rise to paraxial mesoderm (future somites), intermediate mesoderm (future excretory units), and the lateral plate, which is split into parietal and visceral mesoderm layers lining the intraembryonic cavity 2) Envision intermediate mesoderm forms the urinary system and gonads and observe paraxial mesoderm forms block-like somites adjacent to the neural tube. Envision somites form the vertebral column (aka spine) that encloses the neural tube-derived spinal cord.
Describe the structure of mesenchymal cells
Mesenchymal cells create bilateral longitudinal columns composed initially of paraxial mesoderm and intermediate mesoderm
What is the embryo susceptible to during the embryonic period?
During the critical embryonic period (weeks 3-8), the embryo is most susceptible to teratogens e.g., agents such as alcohol and cigarette smoke that cause birth defects
What does the connecting stalk become?
The umbilical cord
When do tissues and organs begin to develop?
Ascertain maturation of developing tissues and organs proceeds during the fetal period that begins at the 9th week and ends at birth, which typically occurs 38 weeks after fertilization