Download Development of Extraembryonic Membranes and Bilaminar Embryo

Development of Extraembryonic Membranes and Bilaminar Embryo, 11/5/2010
- Embryo is term used for 1-8 weeks of development, afterwards called fetus (actual fetus on projector)
-At the end of 1st 8 weeks, all of the organs have been established, then head into the fetal period where
there is growth and maturation physiologic and anatomic maturation occurs of tissues
-18 weeks old=size of the hand; 22 weeks are about the same size but these fetuses could be viable,
depending on lung maturation; size doesn’t really matter, could be v. small and still survive.
-Maturation of the lungs/respiratory function enables baby to survive.
-Umbilical cord connects out and attaches to the fetal side of the placenta (in a depression); on the
outside is the midterm placenta and other tissue that extends from placenta and completes the chorion
sac (outer extraembryonic membrane), part of this becomes the placenta which is the interface btw the
mother and fetus.
-Amnion (part of the chorion must be removed to show this)—membrane that surrounds the
embryo/fetus and contains amnionic fluid (little one develops in this waterbath derived from maternal
tissue fluid, diffuses through the chorion and amnion to be within the amnionic cavity; particles from
amnionic fluid can also back diffuse back into tissue fluids of endometrium).
-When fetus enlarges, there is added amnionic fluid in the form of fetal urine (not like our urine
bc kidneys are not concentrating; placenta is used to get rid of fetal waste, diffusing across the
placental membrane into maternal blood to be eliminated by maternal kidneys; fetal kidneys
are producing fluid that is excreted into amnionic fluid and adds to this). Concurrently with this,
embryo begins to swallow around this time, it swallows amnionic fluid which is taken down into
stomach and absorbs in proximal small intestine—practice for life outside; this is a secondary
balance within the amnionic fluid component, partially derived from maternal tissue fluid. Fetus
begins to add to the fluid and then take away from it.
-If fetus cannot take away fluid (cannot swallow)- POLYHYDRAMINOUS, fluid builds up.
OLIGHYDRAMINOUS- not enough fluid (fetal kidneys do not develop, cannot provide its own
fluid).
-Amnionic fluid- in contact with the surface of the fetus so that it helps with fetal cells
exfoliating; exfoliated cells can be found within the amnionic fluid. Syringe into amnionic cavity
this is done to examine DNA and see genetics. Also can examine fetal maturity this way by
looking at the chemicals (especially lung fetal maturity).
-Amnionic fluid has no nutritive value. Function is to keeps little one hydrated; it is a body
temperature waterbath. Before skin maturations, little one loses water through the skin, so it is
one with it’s surroundings- fluid moves back and forth. Amnionic fluid= chemical environment
for fetus (secreted by the embryo/fetus) to develop in; serves as a cushion, especially for the
developing skeleton. Embryo and early fetus is gelatinous (all cellular) and cannot withstand
pressure. Chorion grows rapidly, amnion and fetus grow within this. Amnionic cavity filled with
it’s fluid provides a buffer for the little one therefore uterine contractions do not hurt fetus.
Amnioniic fluid also serves as a hydrallic wedge, when little one wants to get out. Urinary
contractions force down on amnion, which pressurizes on fetus, also moving inferiorly into the
cervical canal (lower part of uterus which holds little one in place, preventing it from escaping).
Cervix has softened in 3rd trimester, cervical canal gets dilated by pressure produced by uterine
contractions. Cervix must be widely dilated before membranes rupture and amnionic fluid is
released; otherwise the head will produce the wedge with dilates the cervix; molding of head by
passage through cervical canal and into vagina. Amnionic membrane is rather fragile (can be
durable though, depends on exact dimensions); chorion is tougher.
- placental portion of chorion- divided into lobules-Cotelydons (first 2 “leaves” in botony), each is a
functional unit of the placenta.
Mature placenta- maternal aspect is deep within, away from amnionic cavity containing fetus.
Cotelydons (pic on right) have increased in number at this point. As placenta develops, it explands
adding more tissue to it, developing into more peripheral cotelydons. See umbilical cord and vessels
radiating out. Around egde is amnion and chorion. Middle thin tissue is amnion.
Looking at ID twins (monozygotic), embryo divided very early into 2 separate units; can develop within
one chorion but each has their own amnionic cavity.
Fraternal twins (dizygotic twins) developing from separate oocytes being fertilized have own separate
chorion and own separate amnion, but developing within uterus at same time. Sometimes twins share
circulation back and forth (in humans, dangerous in animals btw male and female animals bc male
hormones will negatively affect female body); one placenta may be larger than the other. Placenta is
about size of paper plate and about 2 inches thick.
- MRI of mother (who was having pain, about mid pregnancy; though she may have had appendicitis),
do not do CTs or Xrays on pregnant woman (MRI’s are safe, but don’t use unless you have to). See little
one in normal position, head down; it can move around. Can see deformation of body wall or feel baby
moving on mom who is moderately pregnant, would not see this early on. Amnionic fluid appears white,
fetus has grown enough to fill chorion. Mid pregnancy and on mom goes to the bathroom a lot because
baby pushes on bladder. Can see umbilical chord coming out of the little one going into the placenta.
Anterior wall is thicker than the posterior wall in uterus because this is where placenta is. Usual place for
implantation is rather high on the posterior wall within the uterine cavity. Little one fills space within
amnionic cavity and uterus; under other circumstances that area would be filled with intestinal tract
this is squished over and moved to side. Mature fetuses (just before birth) occupy entire space of
uterine cavity; there is some fluid there.
Menses= beginning of cycle, endometrium from previous cycle is lost; lasts 3-4 days. Follicles within
ovary develop during this time and releases estrogen which cause the endometrium (lining of uterus) to
proliferate/regenerate. At mid cycle, one follicle in humans usually ruptures (ovulates) releaseing a
2ndary oocyte and other materials; 2ndary oocyte is picked up by uterine tube to travels to uterine
cavity. If it is fertilized, development will begin within distal portion of uterine tube. Zygote will transit
down uterine tube (1 week),
These developing cells are surrounded by zona pellucida (acellular substance; produced partially by
oocyte and partially by cells surrounding it known as corona radiata As cells divide by mitosis this
increases # but cells become smaller units, so DNA is replicating (can’t enlarge bc of ZP surrounds and
confines it.). Functions of ZP:
1)
2)
somewhat controls/monitors the env. that oocyte is developing in;
aids sperm in fertilizing oocyte
3) helps hold early cells together
4) prevents implantation until conceptus is old enough to implant properly and then it
disappers.
Cells are organizing themselves. Cavity develops within the developing mitotic cells= blastocele (blast=
formative structure; this particular one is cystic in nature) blastocyst= the whole thing, space within is
blastocele (“cele”= body cavity). As it enters uterine cavity, ZP dissolves. Conceptus now comes in
contact with the lining of the uterine cavity and begins to burrow in/implant.
** Important: 2 weeks after ovulation the ovarian structure that remains (corpus luteum,) degenerates
and it’s fxn ends. This is what has been producing progesterone which maintains endometrium.
Estrogens produced by developing follicle causes regeneration of the endometrium. Progesterone in
maternal system ripens endometrium so it is ready for implantation and maintains it. CL under normal
circumstances has effects of producing progesterone only last for 14 days after ovulation. Loss of
progesterone (14 days after ovulation), and the endometrium begins to degenerate, which shifts into
menstrual period and gets ready for next ovarian cycle. Uniform/constant time period occurs between
the time of ovulation and the onset of the next menstrual period. Variable time is btw onset of
menstruation and ovulation. Little one puts out hormones/substances as moving through uterine tube
and implanting to notify the mother that it is present and mother’s body needs to prepare for the little
one to have a place to live (early pregnancy factors) to maintain CL, maintain progesterone and maintain
endometrium..
Conceptus= products of fertilization; includes embryo proper (becomes new individual) and the
extraembryonic tissue; there may be intraembryonic or extraembryonic portions; BVs may be intra or
extraembryonic
Mesoderm= can be intra or extraembryonic
Corona radiata are follicular cells, ovulated with the oocyte, make oveulated mass easier for the uterine
tube be able to handle and transport more easily; surrounded by ZP and inside is oocyte. Initially oocyte
is right up against ZP. Sperm that make it through the cervical canal into the uterine tubes (traveled 8-10
inches), they approach the ovulated mass which generates chemical signaling into fluid in uterine tube
for the sperm to come to it (also peristaltic activity moves oocyte downward);
Sperm that reach sperm release acrosomal enzymes from their head, to break down CR. This allows
other sperm to be able to make contact with ZP, which has chemical receptors on it allowing sperm to
dock which then release more enzymes creating a path through ZP. 1 Sperm gets in; fuses with oocytic
membrane that is in contact with ZP; membranes fuse, nuclear material and mitochondria move into the
cytoplasm of oocyte.
This triggers a cortical reaction, enzymes in storage vesicles around outside periphery of ooctye are
released causing a chemicals change within ZP so that further sperm cannot digest there way through,
oocyte then retracts a bit from the ZP (a gap is created btw ZP and outercell membrane of ooctyte), this
is good bc it takes time for chemical changes to take place, so that sperm cannot get in to reach the
oocyte even if it has pushed through ZP.
In some cases 2-3 sperms can come in contact with oocyte producing a multiple fertilization
(“ployspernia”), these conceptuses are typically not viable and die very quickly.
**Recap: ZP is docking site for sperms helping them in fertilization process, impedes sperm to a degree
because they need to digest their way through, undergoes a chemical change to prevent further sperm
from moving in. Excess nuclear material is discarded in form of polar bodies ZP then surrounds cells
undergoing mitosis, keeping them corralled; at day 5-6 conceptus moves into uterine cavity, it hatches
from ZP which then breaks down allowing conceptus to move out and swell and enlarge, blastocyst
hatches, blastocele forms and gets bigger (there is an outercell mass and an innercell mass, these have
begin to differenciate, no longer multipotential); at day 6-7 it begins to make contact with
endometrium, begins implantation process.
Morula= “mulberry”; loose connection of cells before the cavity develops, cells have microvilli
extending from surfaces; are joined together with jxnal complexes so that they can share info back and
forth. Early on can take cells from blastocyst (in animals), mix them up and let them reaggregate and
they go on and continue proper development; once there is an outer and inner cell mass these cells
have partially differentiated. Inner cell mass becomes embryo proper, outer cell mass becomes
supportive tissues for developing embryo.
Endometrium- mucosal lining of uterus; consists of epithelium, stroma (connective tissue), maternal BVs
(endometrium is a very glandular and vascular tissue).
Myometrium- muscular wall of uterus
-Implantation of blastocyst: implanatation= growing into another structure; in this case it is movements
of conceptus from uterine cavity into stroma of edometrium; humans have interstitial implantation,
conceptus invades and migrates into stroma of endometrium (implantation occurs in interstitial,
intercellular space of endometrium, losing contact with uterine cavity), while breaking through epithelial
lining (becoming invasive); different animals have different types of implantation, some not as intimate
as in humans (sometimes conceptus remains in the uterine cavity). There are pros and cons to each.
What’s happening on the outside (establishes what implants into maternal body)-
Blastocyst (stage at which inner cell and outer cell mass exist) looks for an attachment point on
endometrium. Implantation is not a one way street: Endometrium releases substances that
attract conceptus, and conceptus releases substances that move it towards endometrium
-
(handshake relationship, reciprocal; if coordinated appropriately then a successful implantation
will take place); birthcontrol interferes with this!
Decidua= altered endometrial stroma (to fall away); during implantation, as conceptus invades,
stroma is stimulated to change its nature (this is normal); at birth most of decidua comes off.
3 types/portions of decidua:
-
-
1. Decidua Basalis= btw conceptus and myometrium, 2. Decidua Capsularis- btw conceptus and
uterine cavity; makes a capsule over conceptus at implantation site (temporary layer), as
conceptus enlarges blood supply to Decidua capilaris is compromised and perfusion drops,
tissue degenerates 3. Decidua Parietalis- everything else, wall of uterine cavity.
Eventually all of endometrium undergoes decidual reaction
Eventually there is a drop of perfusion and the decidua capsilaris degenerates, everything is well
anchored into deciduas basalis by then, so not in danger of dislodging.
BLASTOCYST-- Group of cells on inside= inner cell mass (embryoblast will give rise to embryo proper),
on outside= outercell mass (trophoblast)
Embryoblast is divided into an epiblast (gives rise to embryo proper…ectoderm, mesoderm ( inside
embryo), endoderm) and a hypoblast (temporary, will give rise to extraembryonic mesoderm= this
mesoderm is outside embryo!)
Trophoblast (= “to feed or support”) differenciates into 2 different types of tissue:
1) Cytotrophoblast- will be discrete cells, individualized, these tend to be reserve cells
meaning that they will develop into something else and will be reduced in number as
development proceeds, their cytoplasm will grow into syncytiotrophoblast and merge to
give rise to more syncytiotrophoblast). (Syncytium= “multinucleated cytoplasmic mass” ).
Cytotrophoblast comes in contact with maternal tissue but not maternal blood.
2) Synctitiotrophoblast (within trophoblast, within placenta)- differentiated, specialized tissue,
this gives rise to hormones and make contact will maternal blood. Cytotophoblast cells
continue to migrate through syncyctiotrophoblast and migrates into desidua; increases
amount of synctytiotrophoblasts, as cytotrophoblast cell membranes break down and they
fuse to form syncyctiotrophoblast. Synctyctiotrophoblast comes in contact with maternal
tissue and blood. As implantation continues, there is more and more synctyciotrophoblast
Lacuna= (“space”) membrane bound spaces within the syncytiotrophoblast; will become the intervillous
space within the placenta; these membrane bound spaces will ultimately contain the maternal blood,
when it comes in contact. Membrane bound spaces will coalesce to form larger spaces.
See fig 4.15 in powerpoint
Maternal Blood reaching Lacunae:
When trophoblast comes in contact with the decidua (or with the endometrium), the
cytotrphoblastic cells begin to proliferate, these will break down and join together to form
syncytiotrophoblast located on outside of outercell mass in contact with maternal tissue.
Syncytiotrophoblast grows into endometrium and decidual reaction occurs. Syncytiotrophoblast
comes in contact with small maternal BVs (hopefully, so that pressure is low and it doesn’t cause
hemmorage) within endometrium ; eats its way into maternal vessels, which releases maternal
blood into lacunae.
Chorionic villus has several stages of development, chorionic villi hang into the blood-filled intervillus
space; in mature state they contain fetal BVs
Primary chorionic villi- cyto and synctitiotrophoblast
Secondary chorionic villi- (developmental stage)syncytiotrophoblast, cytotrophoblast, and
extraembryonic mesoderm that’s grown in from hypoblastic cells; no BVs
Tertiary chorionic villi- has core extraembryonic mesoderm and embryonic fetal BVs derived
from this extraembryonic mesodermal core which is surrounded by the 2 trophoblastic layers
** Overall Process, review: Blastula is freed from ZP, comes in contact with maternal tissue
(endometrial wall); wants a blood supply; cytotrophoblast proliferate and develop syncytiotrophoblast
which invades endometrium; syncytiotrophoblast develops on outside of outer mass where it meets
endometrium. Hypoblastic cells proliferate and migrate down on inside of cytotrophoblast (on inside of
outercell mass), forming a second lining now, giving rise to extraembryonic mesoderm which develops
btw inner cell mass and 1st layer of hypoblastic cells (hypoblast is temporary layer and eventually
becomes extraembryonic mesoderm or a structural lining). As implantation continues, cytotrophoblasts
proliferate and grow into synctitiotrophoblast (there are no discrete cellular boundaries here) enabling
this to grow further into endometrium. Concurrently, extraembryonic mesoderm also follows the
cytotophoblasts responsible for syncytiotrophoblast growth, forming extraembryonic mesodermal cores
and extraembryonic BVs develop within extraembryonic mesoderm giving rise to vessels on the
placenta. Finally, extraembryonic BVs develop within the extraembryonic mesoderm, which will link up
with circulatory system developing within the embryo proper. Once we have invaded all the way in,
endometrial epithelium closes over.
Cytotrophoblastic shell- before this develops, endometrium has changed into deciduas, which is unable
to recognize foreign substances, therefore it does not attack the developing embryo. Extraembryonic
mesoderm gives rise to BVs on the placenta, this will go all the way around conceptus.
Hypoblast eventually becomes extraembryonic mesoderm
Thin membrane btw maternal and fetal blood; fetal BVs come in direct contact from basal lamina with
basal lamina of syncytiotrophoblast, and eventually there is a barrier btw maternal and fetal blood, and
this barrier is not very selective. Nutrition is picked up from diffusion of particles via maternal blood,
nourishing the embryoblast