TBL1 - Embryology Flashcards
(39 cards)
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)