1.3 embryology - zygote to primitive epithelia Flashcards
embryonic stage of development
fertilization - week 8 (day 56) when all major structures are present
fetal stage of development
day 57 - birth
period of growth and differentiation of tissues and organs formed in embryonic period
fertilization - beginning and end
sperm/oocyte contact to mixing chromosomes
cleavage divisions
mitotic divisions of zygote resulting in collection of blastomeres
blastomere
cell produced by zygotic cleavage. collection forms 12 cell morula
morula enters uterus, develops fluid filled cavity, becomes this
blastocyst
embryoblast
inner cell mass of blastocyst gives rise to embryo
trophoblast
outer cell mass of blastocyst gives rise to placenta
endometrium
luminal layer of uterus, simple columnar epithelium with connective tissue stroma
stroma
connective tissue framework
2 layers of endometrium
functionalis
basalis
decidua
functionalis becomes the decidua during pregnancy
cytotrophoblast
thin, mitotically active layer that develops from the trophoblast after implantation
what happens to the trophoblast after implantation?
proliferates rapidly and becomes the cytotrophoblast and the syncytiotrophoblast
trophoblast turns develops into what 2 layers after implantation?
cytotrophoblast
syncytiotrophoblast
syncytiotrophoblast
outer mass of the trophoblast consisting of a multinucleated mass with no cell boundaries
what is the chorion formed by?
extraembryonic mesoderm, syncytiotrophoblast, and cytotrophoblast
what does the chorion do?
contributes to fetal portion of placenta
ectopic pregnancy
implantation of blastocyst outside of uterus, usually in ampulla of oviduct
bilaminar disc - what does it arise from and when?
arises from changes in the embryoblast in the second week of development
epiblast
thick, dorsal aspect of the bilaminar disc consisting of high columnar cells related to the amniotic cavity
hypoblast
ventral layer of the bilaminar disc located below the epiblast, consisting of small cuboidal cells related to yolk sac formation and extraembryonic mesoderm
what is another name for the hypoblast?
primitive endoderm
does the hypoblast contribute to the tissues of the embryo?
no
yolk sac
ventral structure lined by hypoblast cells, below hypoblast layer
function of the yolk sac?
- origin of primordial germ cells
- early blood cell development
- regulation of nutrients to embryo
how is the amnion formed?
from cells that separate from the epiblast
what does the amnion do?
encloses the amnionic cavity
amnionic cavity
surrounds embryo and fetus with amnionic fluid - critical to health of embryo
primitive streak
thickened band of epiblast cells appearing at the beginning of week 3 in the caudal, dorsal aspect of the bilaminar disc. epiblast cells migrate down through streak to form endoderm and mesoderm during gastrulation
where does the primitive streak first appear?
caudal, dorsal aspect of the bilaminar disc
-aka-
caudal aspect of the epiblast
when does the primitive streak first appear?
beginning of week 3
epiblast cells migrate through the primitive streak to form what 2 layers during gastrulation?
endoderm
mesoderm
gastrulation
period of epiblast proliferation, migration, and differentiation through which the 3 germ layers form
what are the 3 germ layers formed during gastrulation?
ectoderm
mesoderm
endoderm
what do the ectoderm, endoderm, and mesoderm give rise to?
All cells, tissues, and structures of the embryo
brain growth, tooth development, secondary sex characteristics, and ossification of bones are all examples of development that occurs
postnatally
what are the two phases of prenatal development?
embryonic period
fetal period
4 significant developmental processes of the embryonic stage include:
cellular proliferation, migration, differentiation, formation of all major internal and external structures
what is another name for the embryonic period?
the period of organogenesis
what does the “period of organogenesis” refer to?
the embryonic period, in which all the major events of organ formation occur
why is the embryonic period also called the period of organogenesis?
all major events of organ formation occur
time frame of the embryonic period
fertilization to day 56 (week 8)
time frame of fetal period
day 57 (9th week) to birth
what is the fetal period significant for?
growth and maturation of the organ systems developed in the embryonic period
how many chromosomes are typical of a human somatic cell?
46
daughter cells of zygotic cleavage divisions are called
blastomeres
the 12-32 blastomere stage is called the
morula
how many cells in the morula
12-32 cells
the morula turns into the ___ by ____
the morula turns into the blastocyst by absorbing fluid and forming a large central cavity – the blastocyst cavity
what are 3 components of the blastocyst
embryoblast
trophoblast
blastocyst cavity
cells that migrate to the interior of the blastocyst become the
embryoblast (inner cell mass)
cells that migrate to the periphery of the blastocyst become the
trophoblast
why does the zona pellucida degenerate?
to allow for implantation and growth
when does implantation occur?
~day 6
what causes the endometrial layer to to increase in thickness and and provide nourishment for the embryo?
hormonal control
when does the trophoblast begin to proliferate and divide into two layers?
as it contacts the endometrium
teratogen
environmental agents that cause congenital abnormalities (i.e. alcohol, drugs, radiation)
name 3 teratogens
alcohol, drugs, radiation
what is the critical period of susceptibility to teratogens? why?
the embryonic period between weeks 2 and 8. critical period of tissue and organ system development
when in the embyronic period is the embryo apparently not as susceptible to teratogens? why?
first 2 weeks. because death of the embryo (spontaneous abortion) is more likely to occur
when do embryonic cells apparently transition from totipotent to pluripotent?
when the morula becomes the blastocyst
fertilization age vs gestational age
age from fertilization vs age since last normal menstrual period (LNMP)
typical menstrual cycle length
28 days
typical time of fertilization
day 14-15 of menstrual cycle
approximate difference between gestational and fertilization age
14 days (2 weeks)
another name for fertilization age
conception age
which is older, gestational age or fertilization age? by how much?
gestational age is 14 days older
normal gestational age at birth (days)
280 days
normal fertilization age at birth (days)
266 days
how do you calculate estimated due date from date of last menstrual period?
subtract 3 months, add 7 days
-or-
add 9 months and 7 days (280 days)
mom’s last period was Sept 5th, what is the estimated due date (EDD) of her baby?
June 12th
what region of the blastocyst gives rise to only extraembryonic cells?
trophoblast - outer cell mass
what region of blastocyst gives rise to intraembryonic cells?
embryoblast - inner cell mass
what is the function of extraembryonic tissues?
allow embryo to survive in female uterus
implantation induces the trophoblast to do what?
divide into 2 layers: cytotrophoblast and syncytiotrophoblast
of the two layers of the former trophoblast, which is inner and which is outer?
inner - cytotrophoblast
outer - syncytiotrophoblast
of the two layers of the former trophoblast, which produces hCG?
syncytiotrophoblast
the syncytiotrophoblast produces what hormone?
human chorionic gonadotropin, hCG
what hormone can be used to detect pregnancy?
human chorionic gonadotropin, hCG
how many days after implantation can hCG be detected by an early pregnancy detection kit?
6-12 days after implantation
how many days after fertilization can hCG be detected by an early pregnancy detection kit?
12-18 days after fertilization
“ectopic” =
“out of place”
are ectopic pregnancies potentially life threatening? why?
Yes. Internal bleeding.
decidua
the pregnant endometrium
decidua reaction
stromal cells of the decidua become large rounded cells filled with glycogen and lipids
____ penetrates deeper into the stroma of the decidua and erodes the cells and walls of blood vessels
syncytiotrophoblast
what is the role of hCG?
induces the ovary to produce estrogen and progesterone to maintain the pregnancy
hCG is secreted by ___
in order to ___
the uterus-syncytiotrophoblast
induce the ovary to produce estrogen and progesterone to maintain the pregnancy
in what week does the bilaminar disk form?
week 2
which layer of the bilaminar disk encloses the amnonic cavity?
the epiblast
what kind of tissue forms all glands of the body?
epithelial tissue
what kind of tissue covers the body surface and lines cavities?
epithelial tissue
all ____ cells contact a basal lamina
epithelial
what are the two cavities formed in week 2?
amniotic cavity
yolk sac
cells from which part of the bilaminar disc form the amnion that lines the amnionic cavity?
cells from the epiblast
cells from which part of the bilaminar disc form the membrane of the yolk sac?
cells from the hypoblast
early networks that form in the uterine lining to supply blood to the embryo are called
lacunar networks
trophoblastic lacunae and enlarged blood vessels of the uterus meet to form the
primordial uteroplacental circulation
cells of the epiblast surrounding the amniotic cavity are called
amnioblasts
the migration of cells from the hypoblast over the inner surface of the cytotrophoblast form
the primary yolk sac
the secondary (definitive) yolk sac replaces the primary yolk sac through
a second migration of primordial endodermal cells from the hypoblast
2 important functions of the yolk sac
hematopoiesis, primordial germ cells… formation? migration? unclear…
hematopoiesis
blood cell formation
the yolk sac is important during what time period
the first 4 weeks of development, then it regresses and typically vanishes by birth
primordial germ cells are first recognized where
on the wall of the yolk sac prior to migration to gonads
the development of the three layered embryo from the bilaminar disc is called
gastrulation
the primitive streak moves from
caudal to rostral (cranial)
the primitive pit is found on the __ end of the primitive groove
cranial
T/F the primitive streak establishes major body axes
True
4 body axes established by the primitive streak include:
caudal / cranial
medial / lateral
right / left
dorsal / ventral
this structure appears to arise from the cranial end of the primitive streak, inserts into the endoderm, forms a thickening, contributes to the oropharyngeal membrane, forms an important signaling center involved in formation of the forebrain of the central nervous system, and contributes connective tissue to the head region
the prechordal plate
what cells form the primitive endoderm?
hypoblast cells before the primitive streak develops and replaces with the definitive endoderm
summarize the formation of the endoderm during gastrulation
early epiblast cells slip through the basal lamina in the primitive streak and displace the primitive endoderm (hypoblast cells) with the definitive endoderm
summarize the formation of the mesoderm during gastrulation
later epiblast cells migrating through the primitive streak and past the endoderm form the mesoderm
summarize the formation of the ectoderm during gastrulation
cells remaining in the epiblast layer (not migrating through the primitive streak) form the ectoderm
this structure replaces the primitive pit
the notochord
the notochord extends cranially to the
prechordal plate
what is the fate of the hypoblast after gastrulation?
unclear. apparently estraembryonic structures and the cloacal membrane
cloacal membrane
membrane over anus that breaks down early in development
oropharyngeal membrane
membrane over the mouth that breaks down early in development
the notochord is considered a derivative of which germ layer?
the mesoderm
3 layers of the chorion
- extraembryonic somatic mesoderm (and splanchnic mesoderm?) (line amnion & yolk sac)
- cytotrophoblast layer
- syncytiotrophoblast layer
the chorionic cavity divides the extraembryonic mesoderm into the…
-extraembryonic somatic mesoderm (lines trophoblast and amnion)
-extraembryonic splanchnic
mesoderm- lines the yolk sac
the fetal and maternal contributions to the placenta are
chorion (fetal)
decidua (maternal)
can drugs, viruses, or alcohol cross the placental membrane?
Yes.
draw the amnionic cavity, the secondary yolk sac, the chorionic cavity, and the remnant of the primitive yolk sac
draw it
3rd week most significant for
conversion of bilaminar disc to trilaminar disc through gastrulation
what day does the primitive streak appear?
15-16 days post fert
in the bilaminar disc, high columnar cells form the __
epiblast
function of the syncytiotrophoblast
secrete hCG along with the uterus
in week 2 when the yolk sac forms, does the yolk sac contain yolk? what nourishes the embyro?
the yolk sac contains no yolk. the embryo is nourished by the lacunar networks
in week 2, what new layer of cells forms between the yolk sac and the cytotrophoblast?
the extraembryonic mesoderm
cavities form in the extramebryonic mesoderm and form a new space, the…
chorionic cavity
the chorionic cavity divides the extraembryonic mesoderm into these 3 layers…
- extraembryonic somatic mesoderm – lining trophoblast and amnion
- extraembryonic splanchnic mesoderm – lines the yolk sac
- chorion, lines the chorionic cavity
does the extraembryonic splanchnic mesoderm line the primary or secondary (definitive) yolk sac?
the definitive yolk sac – the primary yolk sac is pinched off and regressing
when the primary yolk sac is pinched off it becomes the….
exocoelemic cyst
in what layer of embryonic cells are the lacunar networks formed?
syncytiotrophoblast
when does the decidua reaction begin? how long does it last?
Amusingly, the decidual reaction in humans begins with ovulation. This doesn’t make too much intellectual sense, though, so I’ll forgive you if you want to believe it begins with implantation, as is true in many other animals. A major purpose of decidualization is to prepare the uterine mucosa for a “burrowing” conceptus. The decidualized cells limit the ability of the trophoblast to enzymatically dissolve the mucosa. (You want the conceptus to become encapsulated within the mucosa, but you don’t want it to reach the muscularis layer.) The mucosa remains decidualized for the duration of the pregnancy. (During a normal cycle, the decidualization is relatively insignificant, and any decidualized cells are lost at menstruation.)
