Embryology Flashcards
Prenatal period
The period of gestation is prior to birth
Perinatal period
22 weeks gestation to 28 days after birth
Postnatal period
after birth
neonatal period
up to 1 month after birth
infancy
first postnatal year
childhood
12 months to 12-13 years
Puberty
10-15 years (girls); 12-17 years (boys)
Adolescence
3-4 years post puberty
adulthood
from 18-25 years to..
What is the estimated pregnancy length for clinicians, and patients?
Clinicians: 40 weeks, patients: 9 months
What are the 3 stages of embryonic development from a clinicians point of view, and an embryologists?
Clinicians: first, second and third trimester. Embyologist: preimplantation, embryonic, fetal stage
What happens during week one of the prenatal period?
Preimpantation stage: zygote, morula, blastocyst
What happens during week 2 of the prenatal period?
inner cell mass forms bilaminar embryo
What happens during week 3 during the prenatal period?
Bilaminar embryo becomes trilaminar embryo
What happens during week 3-8 during the prenatal period?
Embryonic period- organogenesis
What happens during week 9 to term during the prenatal period?
Fetal period- grown and differentation
During what time in development is it considered the “all or nothing” period?
The first 2 weeks
What stage of development is most susceptible to teratogenic agents?
The embryonic stage, also termed the “critical period”
What stage is most likely to produce minor structural defects or functional abnormalities?
The fetal period
Gestational age (GA)
The age of the embryo/fetus from the presumed first day of the last menstrual period- clinical notion
Fertilization age
the age of the embryo/fetus from the fertilization day- not used as much in clinic
Whats the relationship between fertilization age and gestational age?
GA is approx. 2 weeks longer than fertilization age because the oocye is not fertilized until about 2 weeks after last menstrual period (around ovulation time)
Naegele’s rule
First day of the LMP subtract 3 months then add one year and one week , so LMP- 3 months + 1 year + 1 week
What percentage of women deliver on their due date? What percentage deliver within 13 days of their due date?
5% due date, 60-70% within 13 days
Abortion
Expulsion/removal of the embryo/fetus from uterus prior to age of viability (~
Miscarriage; what are most caused by?
spontaneous expulsion from uterus of embryo/fetus relatively early (usually <20 weeks); most caused by chromosomal abnormalities
Premature birth; most common cause?
Infants born between stage of viability and 37 weeks –>majority caused by maternal complications of the pregnancy
Term birth
Infants born after at least 37 weeks of gestation
Describe the process of gametogenesis from primordial germ cells up until gonocytes
Primordial germ cells are identified during the 4th week in the yolk sac. Week 4-6 they migrate to the future genital ridge are. PGCS continue to multiply by mitosis during their migration. PGCS stimulate the surrounding epithelium to create the somatic support cells, which causes swelling, and this becomes the primitive gonad. PGCs are now called gonocytes.
What are the somatic support cells for gonocytes in males and females?
ovarian follice (females), sertoli cells (males)
Gamete maturation is called what in males? in females?
Males, spermatogenesis
Females, oogenesis
What are the two basic processes of gametogenesis that are the same in males and females? What is different between them?
several mitotic division, two meiotic divisions; the timing of the maturation of germ cells is different in the male and female
Gonocytes have how many chromosomes?
23 pairs, total of 46 (diploid). One chromosome of each pair is obtained from maternal gamete and the other from the paternal gamete.
Mature sperm and oocytes have how many chromomoses? Why? When does this happen?
23 (haploid), during gametogeneis the chromosome # is reduced by half, this happens during meiosis
Oogonia divide by what? By the 5th month of pregnancy the number of germ cells in the ovary reaches what?
Mitosis, they reach their max (7mil) most degenerate
What happens to the surviving oogonias in oogenesis? What are they enclosed in?
They enter prophase of meiosis 1; enclosed in follicle
By puberty, about how many oogonias remain in females?
about 400,000
What cell stage are the surviving oocytes arrested in?
Prophase of meoisis 1
Describe what happens to the primary oocyte during oogenesis
How many follicles resume maturation each month?
As a follicle matures, the primary oocyte increses in size and shortly before ovulation, completes the first meoitic division to give rise to a secondary oocyte and the first polar body.
5-12 resume maturation each month, one becomes the dormant one, the other degerates
How is cytoplasm distributed amongst the secondary oocyte during oogenesis?
The secondary oocyte receives almost all the cytoplasm and the first polar body receives very little. (polar body is small, nonfunctional cell)
What happens to the secondary oocyte at ovulation? What stage does it progress to?
It enters second meiotic division; progresses only to metaphase, when division is arrested
Secondary meiotic division is completed only if what?
fertilization occurs
Spermatogonias inside embyronic male gonads remain what from 6th week until puberty?
Dormant
Spermatogenesis takes place ____from puberty until death (___-___ million sperm/day)
continuosly; 200-300 million
What happens to sertoli cells at puberty, and why?
Under the influence of testosterone, Sertoli cells differentiate into a system of seminferous tubles. The dormant PGCs resume development, divide several times by mitosis, and then differntiate into spermatonia
The secondary oocyte is surrounded by zona pellucida, which is what?
a thin layer of transparent gelatinous layer of protein and polysaccharides
Corona radiata
several layers of ovarian follice
What does the acrosome reaction accomplish?
What is the result of cleavage of the zygote?
Following fertilization, the zygote undergoes a series of mitotic cell divisions (cleavage) as it migrates toward the uterus. The process of cleavage divides the zygotes cytoplasm into increasingly smaller cells known as blastomeres (blastula).
After the 3rd cleavage, the blastomeres tightly align via tight junctions via a process known as compaction.
About 3 days after fertilization (12-32 cell stage) the embryo is termed a morula, consisting of a solid ball of cells with an inner and outer mass. The inner mass will become the embryo, the outer mass wil become the placenta. The morula enters the uterus by day 4
What does the morula form?
Fluid from the uterus penetrates the zona pellucia into the morula to form a hollow cavity, transforming the morula into a blastocyst
What is the difference between the trophoblast and embryoblast? What does each come from?
The trophoblast is the thin outer cell mass. It will make human corionic gonadotropin (hCG) which is the basis for pregnancy testing.
The embryoblast is the inner cell mass, which is a group of centrally located blastomeres.
Explain the steps of implantation
Normally, the blastocyst implants along the anterior or posterior wall of the body of the uterus.
In the area over the embryoblast, the trophobolast differentiates into 2 layers, the cytotrophoblast (inner layer of mononucleated cells) and the synctiotrophoblast (outer layer of multinucleated zone that invades the uterine endometrium to implant blastocyst).
Cytotrophoblast
inner layer of monoucleated cells with mitotic capacity
synctiotrophoblast
outermultinucleated zone that invades the uterine endometrium to implant the blastocyst
Where and how do amnion and amnionic activity develop?
The embryoblast differentiates into 2 cellular layers: dorsal epiblast and ventral hypoblast.
Within the epiblast, clefts develop to form the amniotic cavity and amnion
Where and how do the chorion and chorionic cavity develop?
Hypoblast cells migrate and line the inner surface of the cytotrophoblast, forming the exocoelomic membrane (Heuser’s membrane), which surrounds a space known as the primitive yolk sac.
The extraembryonic mesoderm (from the primary yolk sac) proliferates to fill the spaces between the cytotrophoblast and either the amnion or the primitive yolk sac.
Cavities form within the extraembryonic mesoderm to form the chorionic cavity
What layers form the chorionic membrane (chorionic plate)
the extraembryonic mesoderm
the cytotrophoblast
the synctiotrophoblast
What are the primary chorionic villi and how do they develop?
cytotrophoblasts proliferate to form extensions (primary villi) taht grow into synctiotrophoblast. These primary villi grow out into the lacunae, carrying with them a covering of synctiotrophoblast
Describe the development of the primary and secondary yolk sacs
The hypoblast cells migrate out along the surface primitive yolk sac (exocoelomic coelom) to form a smaller sac (definitive or secondary yolk sac/umbilical vesicle)
What does the connecting stalk form?
the umbilical cord
Why are most abnormal blastocysts not detected?
What are hydatidiform moles? How are they formed?
What is placenta previa?
The blastocyst may implant close to the internal os (opening) of the cervix. In this case, the placenta may cover the internal os, resulting in painless vaginal bleeding in the later stages of pregnancy.
What is an ectopic pregnancy? Where do 95% occur? What are signs of this?
Give a summary of steps of week 2 in embryonic development
Embryoblast become bilaminar disc (epiblast and hypoblast)
Amnionic cavity and primary yolk sac (primary umbilical vesicle) form
Extraembryonic mesoderm develops
chorionic cavity develops as spaces within extraembryonic mesoderm appear
definitive yolk sac (secondary umbilical vesicle) forms as the primary yolk sac regresses
What is the primitive steak? what 3 things does it consist of?
Primitive groove: longitudinal midline groove on the embyonic disk which elongates to occupy half the length of the embry
Primitive node: Elevated area at the cephalic “head” end of the primitive groove
Primitive pit: Depression in the middle of the primitive node
Cells from which layer of the bilaminar embryonic disk migrate through the primitive streak and pit? What do they do once they arrive?
Cells of the epiblast migrate toward the primitive streak, upon arrival at the primitive groove, they detach from the epiblast and slip beneath it via the process of invagination
What happens to the epiblast cells once they have migrated through the primitive groove?
Some displace the hypoblast, creating endoderm (which will form the lining of the future gut tube and gut derivatives)
Others come to lie between epiblast and endoderm to form intra embryonic mesoderm
After the endoderm and mesoderm layers are formed, cells remaining in the epiblast become ectoderm
What do cells form that move through the primitive pit?
At the same time that cells migrate laterally through the primitive groove, they also move cranially through the primitive pit to form:
the pre or pro-chordal plate (located caudal to the buccopharyngeal membrane and cranial to the primitive pit; serves as an important organizer of the head region
The notochord ( solid cord of cells that serves as the basis for the axial skeleton
Explain what happens when the intra-embryonic mesoderm spreads out from the primitive streak.
What happens to the length of the primtive streak?
What happens to the primtive streak when gastrulation is complete?
As the intra-embryonic mesoderm spreads out from the primitive streak, the whole embryo increases in size and the primitive streak becomes relatively smaller.
The primitive streak regresses caudally, becoming gradually shorter.
When gastrulation is complete, the primtiive streak disappears
Explain how the notochord forms
Cells destined to form the notochord migrate through the primitive pit cranially until they reach the pre or pro-chordal plate, where they form the hollow notochordal process
The notochordal process then detaches from the endoderm as a solid cylinder known as the notochord
What is chordoma?
Tumor arises from remnants of notochord
intracranially or sacrally; more common in adult men in their 50’s
What is neurolation?
Formation of neural tube. As the notochord develops, it induces the overlying ectoderm to thicken and from the neural plate.
What happens to the neural plate early in the 3rd week?
The neural plate invaginates to form the median neural groove, flanked by neural folds on either side
What happens to the neural folds later in the 3rd week?
The neural folds fuse to form the neural tube (CNS precursor), which separates from the surface ectoderm as the folds fuse
As the primitive streak regresses, what happens to mesodermal cells?
mesodermal cells that migrated laterally through the streak condense into rod or sheet-like sturctes on either side of the notochord
What is the paraxial mesoderm?
Thick plate of mesoderm located on each side of midline, organized into segments known as somites beginning on day 20
What is the intermediate mesoderm, what is it important in?
Longitudinal dorsal ridge of mesoderm between paraxial and lateral mesoderm
forms urogenital ridge, which is involved in formation of kidneys and gonads (ovaries and testes)
What are the red, blue, and yellow strucutres?
ectoderm in blue, mesoderm in red, endoderm in yellow
What is the lateral mesoderm?
Part highlighted in yellow,
continuous with extraembryonic mesoderm
Large spaces develop within this layer to form intra-embryonic coelom–which will form pleural (around lung), pericardial (around heart) and peritoneal (abdominal) cavities in adult
What organ does the splanchnic mesoderm, of the lateral mesoderm, go on to form?
heart and great vessels
What does the septum transversum of the lateral mesoderm go on to form?
It forms the intial partition between the thoracic and abdominal cavities and will form the central tendon of the diaphragm
blue sac is amnionic sac
Describe what happens during the transverse folding of the embryo during week 4
Ectoderm covers the entire surface of the 3D embryo except for the future umbilical region where the yolk sac and connecting stalk emerge
Fusion of the lateral edges of the yolk sac endoderm finishes creating the gut tube
What is gastroschisis? What does it occur from?
Occurs if lateral or transverse folds dont completely fuse.
Its an abdominal wall defect, characterized by an opening in the abdominal wall of the fetus. Intestines and other abdominal organs herniate (protrude) through and openining in the abdominal wall and spill out into the amniotic fluid around the fetus
How are the foregut and hindgut created during the differentiation of the endoderm? What do each go on to form?
When folding occurs, the cranial and caudal portions of the endoderm are converted into blind ended tubes: the foregut and hindgut.
The foregut goes on the create esophagus and stomach, and several digestive accessory digestive organs.
The hindgut will form things like the distal colon and the rectum
When does the allantois form, from what layer, and what will it later become as an adult?
During week 3, it arises as a ventral endodermal outgrowth from part of the hindgut that forms the urinary bladder
extends into the connecting stalk
With what does the midgut communicate?
The midgut lies between the fore and hindgut and initally is open to the yolk sac.
Connection between the midgut and yolk sac narrow to form the yolk stalk (vitelline duct)
Briefly describe the steps in the process of neurulation.
The notochord induces the ectoderm above it to thicken, creating the neural plate. The neural plate then migrates toward the notochord. The neural plate pinches off from the ectoderm, creating neural tube above the notochord, and between the two mesodermal layers
What are the 3 types of intraembryonic mesoderm and how are they related to one another anatomically?
Paraxial (somites) located on each side of the midline
Intermediate mesoderm
Lateral mesoderm
Paraxials are on each side of the midline, and are called somites
Intermediate mesoderm is below the paraxial (somites), and its smaller
Lateral is split by the intraembryonic coelom into 2 layers: somatic and splanchnic. Somatic forms parietal serous lining of cavities, and splanchnic forms the visceral serous coverings of the organs
What type of mesoderm forms the kidneys?
The intermediate mesoderm
What germ layer forms the allantois? What does it form in the adult?
The germ layer is the endoderm, and it forms the median umbilical ligament
With what does the midgut communicate?
The midgut communicates with the yok sac, which narrows to form the yok stalk (also known vitelline duct)
