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35 Cards in this Set
- Front
- Back
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Describe in general terms the distinct growth phases for each trimester during gestation.
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First trimester: 0-14 weeks; from single cell to baby-looking
Second Trimester: 15-27 weeks; growth in length and organogenesis Third trimester: 28 weeks to term; growth in weight and preparation for birth |
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Which trimester is most critical for normal development?
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The first trimester
- most sensitive to teratogens (eg. drugs and alcohol, high temperatures) - disruption of important events during this time has permanent or lethal consequences, resulting in spontaneous abortion or serious birth defects (eg. NTDs) |
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Fertilization: what occurs? Where?
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sperm meets oocyte, usually occurs in the ampullary portion of the fallopian tube; then cilia sweep the fertilized egg into the uterus for implantation
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Week one is characterized by what process?
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formation of the blastocyst via cellular cleavage
- first cleavage occurs at 24 hours, producing 2 blastomeres - cleavage --> morula (about 30 blastomeres) - compaction of the morula --> blastocyst - the blastocyst then separates into 2 sets of cells |
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What are the two sets of cells that the blastocyst separates into, and what do the become?
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Trophoblast: forms from outer cells which become flat and form tight junctions; gives rise to the placenta/extraembryonic tissues, NOT embryo
Inner Cell Mast: forms from the small collection of internal blastocyst cells; becomes the embryo proper (and some extraembryonic tissues) |
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Describe the process of hatching.
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- The ZP is digested by enzymes secreted by the trophoblast - this is an ACTIVE process that is required for implantation
- occurs 5 days after fertilization; at this point the ICM is a bilaminar disc of epiblast and hypoblast |
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What happens during week 2?
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Embyronic disc formation and implantation
- separating the embryonic region from the extra-embryonic components (placenta) and making intimate contact with maternal tissue |
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Describe the formation of the bilayered disc.
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- the inner cell mass becomes bilayered, comprising the outer epiblast layer and the inner hypoblast layer, surrounded by the trophoblast epithelium
- the trophoblast and hypoblast produce extraembryonic tissues while the epiblast gives rise mostly to the embryo proper |
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Where should the embryo implant?
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the dorsal endometrium normally
- it should first attach to and then invade the endometrium, with the epiblast oriented toward the uterine wall - syncytiotrophoblast cells secrete proteolytic enzymes to allow the blastocyst to enter the endometrial wall |
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During implantation, how does the placenta form?
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the syncytiotrophoblasts generate lacunae (holes) within the endometrium, which fuse with maternal capillaries to fuse maternal and embryonic circulation
- this process continues and elaborates as the placenta forms - then the amniotic cavity forms around and is lined by cells of the epiblast, and the definitive yolk sac forms from the hypoblast |
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What is the function of the yolk sac in mammals?
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a transient site of the earliest formation of blood, which forms from the hypoblast
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What is ectopic pregnancy? How common is it? What are the 7 types?
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Abnormal site of implantation; 2.5% of all pregnancies are ectopic
- ovarian - ampullary - tubal (90% of EPs, requires HUGE blood supply, very high risk) - isthmus - cervix (placenta previa, very dangerous, C-section important) - abdominal - pelvic |
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What happens during week 3?
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Gastrulation: formation of the 3 germ layers (endoderm, mesoderm, ectoderm) via cell migrations
Neurulation: formation of the brain and spinal cord from the ectoderm |
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What do the three germ layers arise from?
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All 3 reside in the epiblast prior to gastrulation
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What general systems do each of the three germ layers give rise to?
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Endoderm; moves in and gives rise to the GI tract and respiratory tract
Mesoderm: follows the endoderm in and gives rise to muscle, bone, blood, urogenital system and dermis of the skin Ectoderm: stays on the surface and gives rise to the brain and spinal cord, as well as the epidermis of the skin |
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What are important "anatomical" features that arise during gastrulation?
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- primitive streak, node and pit
- primative node and streak are essential for specifying the fate of migrating endoderm and mesoderm, inducing the nervous system (neural plate), and defining the rostrocaudal body axis - the shape and position of the node and streak changes during gastrulation - at the end of gastrulation, the germ layer are properly arranged, both rostrocaudally and dorsoventrally |
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What results from persistent primitive streak cells?
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Sacroccygeal teratomas (multi-cell growth) develop at the sacrum/coccyx
- may be external, wholly or partially internal - demand a huge blood supply, so may threaten fetal life en utero and perinatally though they are typically benign |
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What specific tissues develop from each of the three germ layers?
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Ectoderm: epithelium, hair, nails, tooth enamel, CNS and PNS
Endoderm: GI tract epithelium, respiratory tract epithelium, endocrine glands Mesoderm: muscle, skeleton, connective tissue, urogenital tract and germ cells, heart, blood and blood vessels |
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Describe the induction of the neural tube.
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- ectoderm gets divided into 2 territories; neural plate will become the brain and spinal cord
- the lateral ectoderm (cuboidal epithelium) will become the epidermis - the neural plate is induced to form from the ectoderm by signals from the node and the underlying notochord during gastrulation - this induction also establishes the rostrocaudal axis of the future CNS |
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Describe the process of neurulation.
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- after neural induction, the neural plate forms the bilateral neural folds
- the bilateral folds move toward the midline and fuse to form the neural tube - fusion begins at the cervical region, then proceeds both cranially and caudally - cranial and caudal neuropores close at day 25 and 27 respectively - once fusion is complete, the neural tube is covered dorsally by ectodermal epidermis - anterior tube gives rise to the brain, posterior tube gives rise to the spinal cord - the internal neural tube expands and differentiates along its length to form discrete brain regions and the spinal cord |
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What do each of the following differentiate from?
- Forebrain - Midbrain - Hindbrain |
Forebrain: telenecephalon and diencephalon
Midbrain: mesencephalon Hindbrain: myelencephalon and spinal cord |
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What are EMTs?
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EMT = epithelial-mesenchymal transition
ectoderm closes over the neural tube as it forms, and cells at the ectoderm-neural tube border become the neural crest at the neural crest, epithelial cells at the dorsal neural tube become mesenchymal and thus are able to migrate the neural crest gives rise to a large diversity of cells and tissues |
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Failure in neural tube closures range in severity...
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from mild, reparable deficits (eg. spina bifida and other trunk fusion failures) to grave lethal deficits (eg. anencephaly)
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What are 4 risk factors for NTDs?
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- family history
- maternal folic acid deficiency - maternal diabetes/obesity - valproic acid, carbamazepine (anti-convulsants) |
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What happens during week 4?
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Gut tube formation: from the endoderm, via cranial/caudal and bilateral body folding
this generates the abdominal wall, constricts the yolk sac and establishes the embryonic body form Mesoderm development: migration to form muscle, skeleton, dermis, urogenital tract, circulatory system, blood and heart |
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Cephalocaudal folding occurs in the 4th week as well. What are the 5 changes that take place anteriorly?
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- the forebrain folds ventrally and enlarges
- the buccopharyngeal membrane moves posteriorly and ventrally to the oral region - the future heart (cardiogenic field) moves dorsally and posteriorly to the thoracic region - the amniotic cavity becomes a protective sac wrapped the embryo - the foregut forms |
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Cephalocaudal folding occurs in the 4th week as well. What are the 5 changes that take place posteriorly?
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- the cloacal membrane moves ventrally and anteriorly to form the future anus
- the hindgut forms |
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Lateral folding occurs during week 4 - what are the results?
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it results in fused endoderm (eg. gut tube, smooth muscle, skin) ; however the body wall remains open in the region of the yolk sac until week 6
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List the 4 mesodermal subdivisions, from medial to lateral.
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- notochord
- paraxial - intermediate - lateral plate mesoderm |
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What does the axial mesodermal subdivision give rise to?
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axial -> notochord, which contributes some cells to intervertebral discs
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What does the paraxial mesodermal subdivision give rise to?
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Head
Somites - Sclerotome -> skeleton (bone, cartilage) - Myotome -> striated/voluntary muscle - Dermatome -> dorsal dermis of the skin |
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What does the intermediate mesodermal subdivision give rise to?
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kidney and gonads
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What does the lateral plate mesodermal subdivision give rise to?
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Smooth Muscle!
gut smooth muscle, coelom, heart, blood vessels, blood, ventral dermis |
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Describe the formation of somites.
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Beginning at neurulation, somites form in the paraxial mesoderm, in an anterior to posterior progression.
A somite forms sequentially as an epithelial ball, pinching off from the presomatic mesoderm. 44 pairs of somites at the end of somatogenesis. |
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What do the somites give rise to?
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Each somite gives rise, in part, to one vertebra.
Each is partitioned into sclerotome, dermatome, and mytome, giving rise to the skeleton, the dermis, and striated body muscle, respectively. This occurs via transformation of epithelial cells to a mesenchymal form (EMT) to allow migration. |
