What is embryogenesis?
Weeks 1-8 of human pregnancy, when organ primordia are established. Sometimes called organogenesis.
The most vulnerable period for a fetus.
What are Carnegie stages?
A system of stages numbered 1-23 used by embryologists to describe the apparent maturity of embryos (based on its external features), rather than chronological age or size.
What is postovulatory age?
Postovulatory age refers to the length of time since the last ovulation before pregnancy; freq. used by clinicians. It is a good indication of embryonic age because the time of ovulation can be determined, and fertilization must occur close to the time of ovulation.
What happens in the first week of embryological development?
During the first week of embryogenesis an oocyte is fertilized in the ampullary region of the uterine tube.
The zygote then undergoes cleavage divisions to form a morula and then a blastocyst.
By the end of the first week, the blastocyst begins implantation into the uterine wall.
What happens during embryological fertilization?
Fertilization is the process in which male (sperm) and female (oocyte) gametes fuse. Results in the formation of a diploid zygote and activates cleavage divisions.
Without fertilization, the oocyte degenerates 24 hours after ovulation.
What happens after a zygote has reached the two-cell stage?
(At ~30hrs) A series of cleavage mitotic divisions, which reduce the size of the cells (now called blastomeres) and increase the number of cells.
After the 3rd cleavage, blastomeres undergo compaction, maximizing contact with each other and forming a compact ball of cells. ~3 days after fertilization, the compacted embryo divides to form a 16-cell morula.
What is the next stage of development after the morula, and how does it occur?
Compaction at the 8-cell stage segregated the outer cells from the inner cells, which constitute the inner cell mass (ICM) in the morula. ICM → tissues in the embryo proper, and the outer cells → the trophoblast.
Trophoblast cells secrete fluid into the morula to create a blastocoel (process of cavitation). At this time the embryo is a blastocyst, whose walls are formed by the flattening of outer cells.
What are embryonic stem cells, and from where do they derive?
ES cells are derived from the ICM of the embryo. These cells are pluripotent (they can give rise to all of the cell types that make up the body), meaning they have potential to treat a variety of diseases such as diabetes, Alzheimer’s disease, and Parkinson’s disease.
ES cells pose ethical issues, however, since these cells must be harvested from a viable embryo. Induced pluripotent stem cells (IPS cells) are alternative stem cells that are generated from adult cells.
What is the difference between totipotency and pluripotency?
Totipotent cells can differentiate into any cell type, including extraembryonic tissues (e.g., placental).
ES cells are pluripotent; they can differentiate into any cell type in the body.
What happens after blastocyst formation?
- Blastocyst hatches from the zona pellucida
- Endometrium “catches” blastocyst on ECM
- L-selectins on trophoblast cells mediate blastocyst-uterus attachment
- Trophoblast secretes proteases to digest uterine ECM → blastocyst buries itself w/in uterine wall
What can be the consequence of abnormal embryological implantation?
Occasionally, implantation occurs near the cervix or outside of the uterus (AKA extrauterine/ectopic pregnancy). Ectopic pregnancies occur in 2% of all pregnancies and account for 9% of all pregnancy-related maternal deaths. In most ectopic pregnancies, the embryo dies during the 2nd month of gestation and may cause severe hemorrhaging in the mother.
What occurs during the second week of embryological development?
The second week of development is the ‘week of twos.’ Think CITY (cavities, ICM, trophoblast, yolk sacs):
- Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast, which contribute to extraembryonic tissues
- ICM differentiates into epiblast and hypoblast to form a bilaminar disc
- Primary yolk sac forms, is displaced by hypoblast cells that give rise to a secondary yolk sac
- Amniotic cavity forms btwn the epiblast and trophoblast; chorionic cavity forms between extraembryonic mesoderm
How does the bilaminar germ disc form?
By day 9, the cells of the ICM have differentiated into two layers that form a bilaminar disc and establish the dorsal-ventral embryonic axis:
- Hypoblast (ventral)
- Epiblast (dorsal)
By the end of the second week, the bilaminar embryonic disc (with a dorsal amnion and ventral yolk sac) is suspended in the chorionic cavity.
What are chorionic villi?
Chorionic stem villi are protrusions that grow into blood-filled lacunae. By the end of the third week, blood vessels develop in the villi that connect with the embryo.
Do the fetal and maternal circulatory systems merge?
Normally, the fetal and maternal circulatory systems never merge, and maternal and fetal blood cells usually do not mix.
Diffusion of soluble substances, however, can occur through the villi. In this manner, the mother provides the fetus with nutrients and O2, and the fetus sends its waste products (mainly CO2 and urea) into the maternal circulation.
How are the different kinds of twins related to embryological development?
Twins are either monozygotic (identical, ~0.25% of live births, below) or dizygotic (fraternal).
- Single embryo splits → identical twins
- Separation occurs before trophoblast forms on day 5 → identical twins w/ 2 separate chorions, 2 separate amnions (dichorionic diamniotic monozygotic, ~1/3)
- Separation comes after chorion forms on day 5 but before amnion forms on day 9 → identical twins w/ 1 shared chorion, 2 separate amnions (monochorionic diamniotic monozygotic, ~2/3)
- Split after day 9 → identical twins w/ 1 shared chorion, 1 shared amnion (mono-mono). Risk of conjoined twins
What are the origins of embryonic tissues?
Epiblast → all tissues in the embryo proper
Hypoblast + trophoblast → extraembryonic tissues
What is gastrulation, and when does it happen?
During the 3rd week of development, bilaminar germ disc → trilaminar embryonic disc w/ 3 germ layers: ectoderm, mesoderm, endoderm
- On epiblast surface, primitive streak forms → defines all major body axes
- Primitive streak → primitive node surrounding primitive pit at cephalic end
- Ingression: epiblast cells migrate thru primitive streak → below, into interior → away as individual cells
- Displacement of hypoblast → endoderm
- Cells btwn epiblast and endoderm → mesoderm
- Cells still in epiblast → ectoderm
Establishment of the body axes occurs before and during gastrulation.
What are the fates of the cells from the 3 germ layers?
Endoderm → "endernal organs": lining of the gut and other internal organs
Mesoderm → "means o' gettin' around": muscle, skeletal system, circulatory system
Ectoderm → "attact-o-derm": skin, brain/nervous system, external tissues (incl. hair)
What is neurulation, and when does it happen?
During the 4th week of development, neural plate → neural tube
- N. plate forms, folds →
- N. crest elevates →
- N. folds converge →
- N. folds brought in contact w/ one another → n. tube closes
What can happen if the neural tube fails to close?
Failure of closure in diff. regions of the n. tube → diff. types of n. tube defects:
- Failure to close the posterior neuropore → spina bifida
- Failure of the neural tube to close in the anterior (cranial) region → anencephaly, a lethal condition in which the forebrain remains in contact with amnionic fluid and degenerates
- Complete failure of closure along the entire neural tube → craniorachischisis
It is estimated that most n. tube defects can be prevented if women take folic acid daily, 3 months prior to conception → throughout pregnancy.
What are neural crest cells, and what is their ultimate fate?
The neural crest are cells arising in the ectoderm at the margins of the neural tube.
These cells migrate to many different locations and differentiate into many cell types within the embryo. Many different systems (neural, skin, teeth, head, face, heart, endocrine, gastrointestinal tract) will have a contribution from the neural crest cells.
When is the embryo most susceptible to birth defects?
During embryogenesis, weeks 3-8 of development. Each organ system may have multiple stages of susceptibility. Overall risk decreases as the fetal period continues, but no stage is safe.
What is Fetal Alcohol Syndrome (FAS)?
The most serious type of FASD. Includes structural defects, growth deficiency and intellectual disability.
- Small head size (microcephaly)
- Indistinct philtrum
- Narrow upper lip
- Low nose bridge
- Flat midface