Embryology Flashcards
Explain the events that lead up to fertilization
Gametogenesis
- oogenesis
- spermatogenesis
ovulation-mature oocyte released
final maturation of sperm
-capacitation
(hyperactivation)
-acrosomal reaction
3 phase process of fertilization
During the embryonic period, all organ systems are established and all major structures go through initial development, True or False?
TRUE
- some even become functional (heart)
- period where the basic form and shape of what is developing is created
During the embryonic period, all organ systems are established and all major structures go through initial development, True or False?
TRUE
- some even become functional (heart)
- period where the basic form and shape of what is developing is created
What is the fetal period and when does it occur?
The period during which differentiation and growth of organs & tissues occurs, from the 9th week (57day) to Term, or the 38th week (measured from fertilization)
-form portions of lungs
growing the organs and tissues you laid down in week 1-8 so that the fetus can thrive and survive by term
Week 1 starts with a fertilized cell, and lots of cell division occurs very quickly, via what process?
Mitosis
Mitosis begins with one cell with _N and ends with _ cells of _N
1 cell, 2N -> 2 cells of 2N
In what order are the stages of Mitosis?
1 Cell 2N Preprophase 4N Prophase Metaphase Anaphase Telephase Cytokinesis 2N
N represents
haploid numbers of chromosomes (23 chromosomes)
ploidy = # of copies of chromosomes present in a cell nucleus
N = # of copies of each unique double stranded DNA molecule
Normal cells duplicate themselves via Mitosis, then meiosis occurs in germ cells to produce haploid offspring. Describe the process in terms of ploidy and N.
(normal cell division where a diploid cell (2N) replicates its DNA (becoming 4N) and undergoes a single division to yield 2 diploid daughter cells. The diploid germ cell replicates its DNA to become 4N but undergoes 2 successive (qualitatively different) nuclear and cell divisions to yield 4 x 1N or 4 haploid offspring.
Somatic cells and Primordial germ cells (PGCs) both start as _ploid _N cells, they both replicate their DNA to become _N.
Diploid 2N, 4N
How are the male and female mitotic and meiotic processes different?
The mitotic processes are the same for both females and males.
The meiotic male and female processes are each different from each other. The female meiotic cell divisions are dramatically unequal
Oocyte (1:3) 1N
Spermatids (4) 1N
What are the stages of Meiosis?
Meiosis I Preprophase 4N Prophase Metaphase Anaphase Telephase Cytokinesis 2N Meiosis II Prophase 2N Metaphase Anaphase Telephase Cytokinesis 1N
Oocyte (1:3) 1N
Spermatids (4) 1N
Describe Meiosis I vs MeII?
Meiosis I Preprophase 4N Prophase Metaphase Anaphase Telephase Cytokinesis 2N Meiosis II Prophase 2N Metaphase Anaphase Telephase
Germ cells are also known as _____
Primordial sex cells (PGCs)
How do germ/primordial sex cells originate?
As specific cells lining the yolk sac (umbilical vesicle)
-from the endodermal cells of the umbilical vesicle near the origin of the allantois
-large spherical cells 1st recognizable 24 days after fertilization.
(2N)
Where do germ/primordial sex cells migrate to?
to occupy gonads and mature into gametes (mature sex cells)
When do PGCs migrate to the gonadal region and how?
PGCs originate in the lining of the yolk sac and migrate along the dorsal mesentery of the hindgut to the gonadal ridge during the 5th week of embryonic development.
What happens during week 6 with the PGCs ?
They enter the underlying mesenchyme and are incorporated into the gonadal cords. At this time the gonads of the 2 sexes are identical in appearance and are thus = indifferent gonads.
Male and female differentiation begins during the _th week for the males and at approximately the _th week in females the ovaries are distinguishable histologically.
7th week for males
10th week for females
What is gametogenesis (gamete formation)?
the process of formation and development of specialized generative cells,gametes(oocytes/sperms) from precursor cells. This development, involving the chromosomesandcytoplasmof the gametes, prepares these sex cells for fertilization. During gametogenesis, the chromosome number is reduced by half and the shape of the cells is altered.
During gametogenesis, the chromosome number is reduced by half and the shape of the cells is not altered, True or False?
FALSE, the chromosome number is reduced by half but the shape of the cells is altered as well during gamete formation
male vs female gametes
sperm; oocytes
Sperm
male gamete: 23X or 23Y, motile
Acrosome, head, neck, tail, mitochondrial sheath
Oocyte
female gamete: 23X, immotile
-do not move self, depend on smooth muscle movement and pressure changes in fallopian or uterine tubes
Zona pellucida, Corona radiata
zona pellucida
thick glycoprotein cell layer, EC membrane on top of oocyte itself
corona radiata
follicular cells, follicular layer around zona pellucida
Spermatogenesis is the process by which spermatagonia become mature sperm (spermatazoa), True or false?
When does this process begin?
TRUE, spermatogenesis begins at puberty
1 precursor becomes 4 gametes
What is oogenesis? When does it begin?
the process by which Oogonia become > mature oocytes
- Begins before birth, is halted at METAPHASE stage of MEIOSIS II, continues at puberty
- -primary oocyte (diploid still….starts going into Me 1 but halts)
- ONLY finishes oogenesis (meiosis II) IF FERTILIZATION occurs
How is the secondary oocyte transferred to the fallopian tube?
via the action of the fimbriae
What happens with the secondary oocyte during ovulation?
~3 hours before ovulation, 2° oocyte enters meiosis II and arrests in metaphase
Will complete meiosis II upon fertilization
Smooth m. contractions help to expel 2° oocyte
Surrounding the oocyte is the zona pellucida & corona radiata (all ovulated together)
Beginning at puberty, cyclic FSH (follicle stimulating hormone) + LH (luteinizing hormone) released from the anterior pituitary gland
FSH (triggers follicular growth) + LH surge (triggers ovulation)
The mature Oocyte is released from the ovary during ovulation
Ampulla of the uterine tube is the normal site of fertilization
When are FSH and LH released first, and from which portion of the brain?
Beginning at puberty, cyclic FSH (follicle stimulating hormone) + LH (luteinizing hormone) released from the anterior pituitary gland
What is the role of FSH in ovulation?
Triggers follicular growth
What does the LH surge trigger?
Ovulation
What is the normal site of fertilization?
The AMPULLA of the uterine tube (fallopian tube)
Which stage in the final maturation of sperm allows the acrosome to react with the corona radiata & zona pellucida (ZP) to release enzymes?
Capacitation, the first stage of the final maturation of sperm, during which surface proteins are removed from the acrosome
What is the acrosome reaction?
Occurs upon contact with the ZP
Results in release of enzymes from acrosome, allowing sperm to penetrate ZP
What is capacitation?
(takes place within uterine wall)
Begins after ejaculation is initiated by substances secreted in the uterus and uterine tubes. Capacitation is required to mature the sperm allowing removal of the glycoprotein coat and seminal proteins from the surface of the sperm acrosome to allow the acrosomal reaction to occur.
hyperactivation of the sperm (increased activity) happens between this and the acrosomal reaction
What is the acrosome reaction?
Occurs upon contact with the ZP
Results in release of enzymes from acrosome, allowing sperm to penetrate ZP
occurs when the capacitated sperm acrosome binds to the zona pellucida. The acrosome begins to perforate in the presence of several enzymes and substances, breaking down the membranes producing apertures. Several enzymes are released to facilitate fertilization.
What are the three phases of fertilization?
Phase 1: Penetration of the corona radiata
Phase 2: Penetration of the zona pellucida
Phase 3: Fusion of the sperm and oocyte cell membranes
Phase 1 fertilization
Sperm penetrates the corona radiata
Phase 2 fertilization
Sperm then penetrates the zona pellucida (ZP) > ZP = a specialized extracellular matrix surrounding the developing oocyte (egg) within each follicle within the ovary.
At this point, the sperm that penetrates the ZP contacts the oocyte cell membrane enabling a reaction where the ZP becomes impermeable to other sperm
Phase 3 fertilization
The cell membranes of the oocyte & sperm fuse and break down
The pronuclei of the oocyte & sperm fuse, forming zygote > pronuclei = the nucleus of a sperm or an egg cell during the process of fertilization, after the sperm enters the egg, but before the genetic material of the sperm and egg fuse
If there was any chromosomal abnormalities during the gametogenesis period for either the sperm or oocyte and those respective structures go through fertilization this could lead to what?
spontaneous abortions
What results from fertilization?
Haploid pronuclei (of sperm and oocyte) fuse, forming diploid zygote
Restoration of diploid number of chromosome
Determination of genetic sex
Initiation of cleavage
What week starts with a fertilized cell? What is this called?
Week 1, Zygote
Zygote Cleavage
~30 hours after fertilization, zygote undergoes cleavage (mitosis) to make early embryonic cells called blastomeres
ZP hasn’t broken down providing framework/protection
Cell divisions result in successively smaller blastomeres that remain confined within the space available within the zona pellucida = compaction
Compaction begins
@ ~ 8 cell stage
blastomeres are
early embryonic cells created when the zygote undergoes mitotic division (cleavage) about 30 hours after fertilization
Compaction
begins at 8 cell stage
Cell divisions result in successively smaller blastomeres that remain confined within the space available within the zona pellucida
When does the zygote become a morula?
3-4 days following fertilization as the embryo enters the uterus, the zygote is in the 12-32 cell stage and is renamed a morula, as it now resembles a mulberry. (IN WEEK ONE don’t need to know exact day)
Morula Stage
During this stage, the blastomeres begin to organize into an inner cell mass (which will lead to the emrbyo proper) and an outer cell mass (which is the primary source of the fetal component of the placenta.)
When and how does the morula become a blastocyst?
By four days of development, the morula begins to absorb fluid. A large cavity called theblastocyst cavity (synonym = blastocoel)forms within the morula. It is now called a blastocyst.
The embryoblast cells (which was the inner cell mass) then form a compact mass at one side of this cavity, and the trophoblast (which was the outer cell mass) organizes into a thin, single-layered epithelium. The side of the blastocyst containing the inner cell mass is called theembryonic poleof the blastocyst, and the opposite side is called the abembryonic pole.
embryonic poleof the blastocyst
side of the blastocyst containing the inner cell mass
abembryonic pole
the opposite side of the inner cell mass of the blastocyst
Attachment & Differentiation of the Trophoblast
Trophoblast cells attach to endometrium (posterior or anterior wall of body of uterus)
Upon attachment, trophoblast differentiates into (around day 7):
Cytotrophoblast
Syncytiotrophoblast
Cytotrophoblast
differentiation of trophoblast creates this division of cells that line the wall of the blastocyst
Syncytiotrophoblast
Cells that are directly embedded in the endometrium
Ectopic pregnancy
Occasionally, a blastocyst implants in the abdomen, on the surface of the ovary, within the oviduct, or at an abnormal site in the uterus. Theseectopic pregnanciesoften threaten the life of the mother because blood vessels that form at the abnormal site are apt to rupture as a result of growth of the embryo and placenta.
Typically, ectopic pregnancy is revealed by symptoms of abdominal pain and/or vaginal bleeding.
Drugs or surgical intervention is usually required to interrupt the pregnancy.
Where do ectopic pregnancies occur usually?
Within the uterine tubes *most common
Within the abdomen (ex: surface of the ovary, rectouterine pouch, mesentery)
Abnormal site within the uterus (internal os, cervix)
Summary of Week 1
What is the developmental process of the embryo during week 1?
The fertilization & formation of zygote
The zygote undergoing cleavage to get to the morula stage and continuation into the blastocyst stage
We also saw the attachment to the endometrium & formation of cytotrophoblast & syncytiotrophoblast.
Describe the developmental processes of the embryo during the second week.
During this time we will see the trophoblast differentiating into 2 layers, the embryoblast forming 2 layers, the extraembryonic mesoderm splitting into 2 layers, and 2 cavities forming. These membranes and cavities serve in the maintenance of the developing embryo. (“Week of 2s”)
Overview:
-Implantation completed-fully implanted by week 2
-Trophoblast differentiates into 2 layers
- Embryoblast forms 2 layers
- then 2 cavities form
- then 2 yolk sacs form
- Extraembryonic mesoderm splits in to 2 layers
- Uteroplacental circulation established
At the beginning of Week 2, where and how is the embryo implanted?
The embryo is superficially implanted in the endometrium (attached Day 6)
-not fully invested in tissue, just superficially implanted
In week 2, which two layers does the embryoblast differentiate into?
- Epiblast
- columnar type cells
- rather tall
- will form 3 primary germ layers in week 3* - Hypoblast
- inferior to epiblast
- cuboidal type cells
- will form part of lining of one cavity
- will start to be origin of how we form ___
While the embyroblast is differentiating in week 2, what are the other 3 events that are occurring simultaneously?
- Amniotic cavity forming
- Implantation continuing
- Syncytiotrophoblast continuing to grow
How are epiblast cells different from hypoblast cells?
epi: columnar and tall, form 3 primary germ layers (week 3)
hypo: cuboidal, form lining of cavity, inferior to epiblast
Primitive endoderm is another name for which layer of the differentiated embryoblast? Once differentiated, the two-layer embryoblast is called what?
Hypoblast
Bilaminar embryonic disc
At what point does the embryoblast become the bilaminar embryonic disc?
By day 8 (week 2), when the embryoblast has differentiated into 2 distinct layers
How is the Amniotic cavity formed? When?
Fluid begins to collect between cells of the epiblast and overlying trophoblast (~day 9)
-Amniotic cavity
A layer of epiblast cells expands toward the embryonic pole and differentiates into a thin membrane separating the new cavity from the cytotrophoblast. This membrane is the lining of the amnion. Although the amniotic cavity is at first smaller than the blastocyst cavity, it expands steadily. By the eighth week, the amnion encloses the entire embryo.
In the second week, while the amniotic cavity is forming, four other events simultaneously occur, which are what?
Implantation completed (note: coagulation plug)
Hypoblast cells migrating
Trophoblastic lacunae form in syncytiotrophoblast
Development of primary yolk sac
What is the coagulation plug? What does its formation mark in terms of implantation?
light tissue, mark that implantation completed entire embryo embedded in uterine wall
How is the primary yolk sac formed?
First wave of hypoblast cells migrate to line blastocyst cavity; forming Heuser’s membrane (exocoelomic membrane)
-Once lined, cavity is called primary yolk sac
Heuser’s membrane
exocoelomic membrane formed after the first wave of hypoblast cells migrates to line the blastocyst cavity
-membrane of primary yolk sac (primary umbilical vesicle)
Why does the embryo need the primary yolk sac?
As the embryo is developing during the early days it is reliant on the primary yolk sac for nutrient transfer and for the development of the definitive or secondary yolk sac.
While the primary yolk sac is forming, what two other events are occurring simultaneously?
Formation of extraembryonic mesoderm
Trophoblastic lacunae anastomose with maternal capillaries
What is the large cavity called that develops in extraembryonic mesoderm?
Chorionic cavity
Formation of Chorionic Cavity
the second cavity to form, formed by splitting of the extraembryonic mesoderm into two layers, then cavity continues to expand
-separates the embryo with its attached amnion and yolk sac from the outer wall of the blastocyst, now called the chorion.
Formation of the extraembryonic mesoderm
Forms as primary yolk sac is forming
-filled blastocyst cavity with loosely arranged cells; which will then expand between the amnion and cytotrophoblast, eventually forming around the entire embryo proper and later going on to become some portions of the placenta as well as the umbilical cord (connecting stalk)
What are the two layers of extraembryonic mesoderm after splitting during formation of the chorionic cavity?
The extraembryonic mesoderm splits into two layers:
1) Extraembryonic splanchnic mesoderm: coating the outside of Heuser’s membrane
2) Extraembryonic somatic mesoderm: lining the inside of the cytotrophoblast. The space between the two layers is the chorionic cavity.
How is the secondary (definitive) yolk sac formed?
A second wave of hypoblast cells migrate pushing the primary yolk sac & breaking it up/causing out pocketing, gets pinched away, forming the secondary yolk sac
What happens to the primary yolk sac as the second yolk sac forms?
As the definitive yolk sac develops on day 13, the primary yolk sac breaks up and is reduced to a collection of vesicles at the abembryonic end of the chorionic cavity.
Where can primordial germ cells first be identified in humans?
primordial germ cellscan first be identified in humans in the wall of the yolk sac
What is the connecting stalk? What does it develop into?
By the end of the second week, the definitive (2a) yolk sac loses contact with the remnants of the primary yolk sac, and the bilaminar embryonic disc with its dorsal amnion and ventral yolk sac is suspended in the chorionic cavity by a thick connecting stalk.
Connecting stalk will develop into the umbilical cord with development of blood vessels
From which embryonic structure does the umbilical cord form?
Connecting stalk (with development of blood vessels)
How is the uteroplacental circulation established?
-Trophoblastic lacunae open within syncytiotrophoblast & maternal capillaries expand and anastomose with these lacunae
- Cytotrophoblast proliferates locally & forms extensions that grow into blood-filled lacunae
- -Outgrowth is called a primary villus
Although this system begins to form on day nine it will not be fully formed and specialized until the placenta is established.
Describe a summary of Week 2 events
- Implantation completed
- Embryoblast develops into bilaminar disc
- Amniotic & chorionic cavities form
- Extraembryonic mesoderm forms & splits
- Primary & secondary yolk sacs form
- Uteroplacental circulation established
How is the uteroplacental circulation established?
-Trophoblastic lacunae open within syncytiotrophoblast & maternal capillaries expand and anastomose with these lacunae
- Cytotrophoblast proliferates locally & forms extensions that grow into blood-filled lacunae
- -Outgrowth is called a primary villus
Although this system begins to form on day nine it will not be fully formed and specialized until the placenta is established.
Describe a summary of Week 2 events
- Implantation completed
- Embryoblast develops into bilaminar disc
- Amniotic & chorionic cavities form
- Extraembryonic mesoderm forms & splits
- Primary & secondary yolk sacs form
- Uteroplacental circulation established
Describe a summary of Week 3 events
Week 3: become TRIlaminar disc, form THREE primary germ layers
- Formation of primitive streak & node
- Gastrulation occurs (begin process to become trilaminar disc)
- Establishment of body axes
- Formation of notochord (mesoderm)
- Growth of the embryo
- Further development of uteroplacental circulation
- 1a villi -> 2a villi -> 3a villi
True or False: ectoderm and endoderm originate from the epiblast, but mesoderm has a different origin
FALSE
All layers formed from epiblast*
Gastrulation
Formative process by which the 3 embryonic germ layers arise
Ectoderm*
Mesoderm*
Endoderm*
(takes bilaminar disc and transforms it into trilaminar disc)
All layers formed from epiblast* (ALL from SAME original source)
-starts with formation of primitive streak (runs along midline on dorsal aspect of epiblast)
True or False: In gastrulation, epiblast cell migration causes both the primitive streak and the primitive pit to move forward (cranially)
FALSE
The primitive pit remains in the same place, but the primitive streak moves forward
What two parts make up the primitive streak?
- Primitive groove (indentation into epiblast)
- Primitive node
- Contains: primitive pit (in middle, opening where cells begin to jump in)
- expanded end looks like its moving cranially
During gastrulation, the epiblast cells that do not migrate during invagination form the _______
ECTODERM
How do invaginating epiblast cells form endoderm?
displace/replace hypoblast cells (cells lining yolk sac)
Hypoblast cells are fairly thin and not robust; the migrating epiblast cells which displace/replace the hypoblast cells are much more robust and substantial; the same goes for the epiblast cells which remain in the epiblast and do not migrate, they will also become more robust to form the ectoderm
Describe the formation of the notochord in week 3
- Notochordal process -Formation begins with cranial midline extension from the primitive node of a hollow tube
- Will grow proximally towards oropharyngeal membrane - Notochordal plate
- Ventral floor fuses with the underlying endoderm and 2 layers break down, leaving behind the flattened plate
-At primitive pit, yolk sac transiently communicates with amniotic cavity through opening called neurenteric canal
- Notochord- Solid rod
- notochordal plate then detaches from the endoderm, into the mesoderm containing space between the endoderm and ectoderm, forming a solid rod
Notochordal process
ollow tube from which the notochord is formed by the cranial extension from its primitive node
Notochordal plate
flattened version of notochordal process (2nd step), left behind after ventral floor fuses with underlying endoderm and 2 layers break down
Notochordal process
Hollow tube from which the notochord is formed by the cranial extension from its primitive node
Notochordal plate
Flattened version of notochordal process (2nd step), left behind after ventral floor fuses with underlying endoderm and 2 layers break down
At the beginning of week 3, primary villi characterize which embryonic cell?
trophoblast
During week 3, how is uteroplacental circulation further developed
Mesoderm associated with the cytotrophoblast penetrates the core of the primary stem villi, thus transforming them intosecondary chorionic stem villi
- By the end of the third week, this villous mesoderm has given rise to blood vessels that connect with the vessels forming in the embryo proper, thus establishing a working uteroplacental circulation
- These villi containing differentiated blood vessels are called tertiary chorionic stem villi
Secondary Villus
Week 3, formation of uteroplacental circulation
Mesodermal cells penetrate core of primary villi
Tertiary Villus
Week 3, formation of uteroplacental circulation
Mesodermal cells differentiate into blood cells and small blood vessels
Secondary Villus
fingerlike extension grows
Week 3, formation of uteroplacental circulation
(extraembryonic) Mesodermal cells penetrate core of primary villi (cytotrophoblast, syncytiotrophoblast, trophoblastic lacuna)
Tertiary Villus
Week 3, formation of uteroplacental circulation
Mesodermal cells differentiate into blood cells and small blood vessels (villous capillary)
-see distinct oxygenated and deoxygenated blood move with nutrients, connecting embryo and mother
Vasculogenesis
Blood vessels arise from blood islands
(mesoderm cells) > Hemangioblasts > tube formation
Neurulation
Process by which the neural plate forms the neural tube
Location: Occurs in the ectoderm via notochordal induction
When: starts in week 3, Completed by end of 4th week
Fast, 4 main events
What are the 4 main events of Neurulation?
- Formation of neural plate in the ectoderm (1st step in development of future CNS)
- Shaping of neural plate (form tube)
- Invagination of neural plate
- Closure of neural groove
Describe the neural plate
Formation is induced by the notochord which causes the ectodermal cells along the midline axis to thicken to form this plate.
1st event in development of future CNS, in ectoderm
Neural folds
laterally elevated edges
Neural groove
depressed midregion of neural plate which will become the hollow portion (neural tube) once the neural folds move together and fuse
How is the neural tube formed? Is it mesoderm or ectoderm?
Neural folds move together & eventually fuse creating hollow canal within; tube will invaginate/move inward/deep to become embedded in mesoderm and then covered by surface ectoderm (will become epidermis)
- ectoderm (neuroectodermal cells)
- Begins in cervical region (5th somite) (high up in neck)
-Progresses both cranially & caudally (like double zipper on both ends where its going to start to close up)
True or False: Until fusion is completed, the cranial and caudal neuropores allow the neural tube to communicate with the amniotic cavity
TRUE
Once the neural tube is formed, how is it closed?
- Starts closing in the middle, then progresses both cranially and caudally
- The cranial neuropore closes BEFORE the caudal neuropore
-once closed, neurulation is over
What event(s) marks the end of neurulation during week 4?
closure of the (cranial, then caudal) neuropores
neuroectoderm
thickened midline portion of ectoderm forms neural plate, cells in this plate (thickened in middle/along midline) known as neuroectoderm because they will become the actual tissue of the spinal cord and brain
What transition do the neural crest cells have to undergo before entering the underlying mesoderm tissue and migrating to various places in the body?
epithelial-to-mesenchymal
Neural crest cell development
As neural folds fuse (to create neural tube), cells at lateral border (crest) of neuroectoderm dissociate (neural crest cell)
Undergo epithelial-to-mesenchymal transition
-Enters the underlying mesoderm (above neural tube)
(breaks tight connections to migrate to various places in body where they will help form various structures)
-migrate from both cranial & trunk regions of neural tube
Mesoderm differentiation
Mesoderm near midline proliferate to form thickened paraxial mesoderm
More laterally, stays thin, known as lateral plate mesoderm
In between paraxial and lateral plate mesoderm is the intermediate mesoderm
paraxial mesoderm
closest to midline/notochord (form pairs - think para - of somites around neural tube)
- organizes into segments (somites), formed cranially to caudally (beginning in occipital region)
- gives rise to most of axial skeleton, skeletal muscles, and dermis of back
Intraembryonic cavities split intraembryonic lateral plate mesoderm into:
Intraembryonic Somatic or parietal layer (lines the amnion)
intraembryonic Splanchnic or visceral layer (lines yolk sac)
Intraembryonic cavities split lateral plate mesoderm into:
Intraembryonic Somatic or parietal layer (lines the amnion)
intraembryonic Splanchnic or visceral layer (lines yolk sac)
Intraembryonic Somatic or parietal layer
Lines amnion
-from lateral plate mesoderm being split by intraembryonic cavities
Intraembryonic Splanchnic or visceral layer
lines yolk sac
-from lateral plate mesoderm being split by intraembryonic cavities
Somite
-in pairs, formed cranially to caudally
the organization of the paraxial mesoderm which occurs at the beginning of week 3
- mesoderm differentiation: come from paraxial mesoderm
- Can be used to determine age of embryo
- forms dermis (underlying epidermis), muscle, and bone
Somite Differentiation
-give rise to muscle (myotome) and dermis (dermatome) and bone (sclerotome) (NOT spinal nerves)
Somites reorganize into 3 subdivisions:
-Cells in ventral & medial walls shift position to surround neural tube and portion of spinal cord; form sclerotome
-Cells between dorsomedial cells & ventrolateral cells form the dermatome
- Cells at dorsomedial & ventrolateral edges of upper region of somite form precursors for muscle cells
- Cells will become mesenchymal & migrate beneath (deep to) dermatome and form the myotome
Sclerotome
Subdivision of somites
Formed by cells in ventral & medial walls, shift themselves to surround neural tube and portions of notochord
-start to form axial skeleton bones (ribs, vertebrae)
Dermatome
Subdivision of somites that stay toward dorsal surface
Formed by cells between dorsomedial muscle cells & ventrolateral muscle cells
makes dermis, dermal layer
Lateral plate mesoderm derivatives
Will mainly form linings of body cavities or developing organs
Parietal mesoderm layer (intraembr. somatic mesoderm) along with overlying ectoderm form lateral body wall folds
Cells of parietal layer surrounding the intraembryonic cavity form serous membranes (i.e., mesothelial membranes)
serous membrane formation
formed by cells of parietal layer surrounding intraembryonic cavity
aka mesothelial membranes
-line the body cavities (peritoneal, pleural, & pericardial cavities) and secrete serous fluid
Visceral (intraembr. splanchnic) mesoderm layer
form walls of the gut tube
(with the overlying endoderm)
-cells of visceral layer form thin serous membrane around each organ
Blood and blood vessel formation
- Blood islands begin to form in MESODERM (both INTRA and EXTRAembryonic surrounding wall of yolk sac (week 3)
- Then later in the lateral plate mesoderm and other regions
Blood islands
beginning of blood and blood vessel formation/ development of heart; form in both intraembryonic and extraembryonic tissues
- start in mesoderm around wall of yolk sac (week 3)
- later in the lateral plate mesoderm and other regions
Endoderm derivatives
Will form the gastrointestinal tract (GI tract)
- Lines the ventral surface of the embryo & forms roof of yolk sac
- for now, focus on the fact that we have a continuous tube from the oral cavity to the anus
Body folding during the embryonic period
Folding simultaneously occurs in both the sagittal plane (i.e., cranial/caudal folds) as well as the horizontal plane (i.e., lateral folds)
Takes the embryo from a flat disc to a cylinder
As a result of folding, the cranial, lateral, and caudal edges of the embryonic disc are brought together along the ventral midline. The endodermal, mesodermal, and ectodermal layers of the embryonic disc fuse to the corresponding layer on the opposite side, thus creating a tubularthree-dimensional body form.
Cranial Folding
Due to growth of brain in embryo
Sagittal folding
Oropharyngeal membrane (ONLY ecto and endoderm) acts as hinge & swings ventrally
Part of yolk sac endoderm is incorporated into embryo as the foregut (proximal portion of GI tract)
Oropharyngeal membrane separates the foregut from the oral cavity (will rupture ~day 26) (Will rupture into opening/oral cavity)
Caudal Folding
Sagittal folding
Cloacal membrane (endo and ectoderm) swings ventrally (Cloacal> Caudal)
Portion of yolk sac endoderm is incorporated as the embryonic hindgut (distal portion of GI tract)
Connecting stalk (future umbilical cord) is now attached to ventral aspect of embryo
Lateral Folding
Close ventral surface
Dorsal part of yolk sac endoderm is incorporated as embryonic midgut
End 4th week, lateral folds fuse, except at region of connecting stalk
The midgut connection to the yolk sac constricts, leaving a narrower connection: the vitelline duct (omphaloenteric duct)
vitelline duct (omphaloenteric duct)
connection between gut tube and secondary yolk sac
How does the embryo within the cavity become enclosed by the amniotic cavity?
Lateral Folding
embryonic midgut
Dorsal part of yolk sac endoderm
Prior to folding, the intraembryonic body cavity forms a ______ shaped space
horse shoe
With folding there is a change in position of this intraembryonic body cavity
Septum Transversum
Mesoderm between the thoracic cavity and yolk sac/abdominopelvic cavity
major component of diaphragm
above heart before folding, brought to ventral surface with folding
The embryonic body cavity is comprised of:
- pericardial cavity
- 2 pericardioperitoneal canals
- peritoneal cavity
Pleuropericardial folds
(lateral extensions of tissue) grow in medially from the lateral walls of the pericardial cavity
-Dividing it into the definitive pericardial (1) and pleural cavities (2)
Pleuroperitoneal membranes
grow ventrally & meet up and fuse with septum transversum, sealing off the pericardioperitoneal canals
(pleural or thoracic cavity above, from abdominal cavity below)
Formation of the Diaphragm
4 embryonic structures:
- Septum transversum
- Pleuroperitoneal membranes
- Mesoderm from body wall (muscular)
- Esophageal mesoderm
The placenta originates from which embryonic structures in week 2 of development?
extraembryonic SOMATIC mesoderm, Cytotrophoblast, and syncytiotrophoblast (3 closest layers to mother/implanted within uterine wall, growing out and looking for the uteroplacental blood supply/circulation, feeding embryo/ future fetus)
primary villus
formed in the quick succession of events in uteroplacental circulation being established in week 2
cytotrophoblast will proliferate locally and form extensions that grow into blood filled lacunae
-outgrowth of cytotrophoblast pushes syncytiotrophoblast and trophoblastic lacunae (now anastomosed with maternal capillaries) layers out as well causing outgrowth
During which weeks does the amniotic cavity grow, expanding and filling with fluid to eventually overtake all other cavities and completely encase the embryo? What happens to the chorionic cavity?
Between weeks 4 and 8
-obliterates the chorionic cavity in the process (very tiny piece of it left)
The formation of which structure during week 3 let’s you determine the tail and head end of the developing embryo? How do you know which is which?
primitive streak
-the primitive node is formed at the head end (future position of oropharyngeal membrane, the opposite end is the tail end (future position of the cloacal membrane)
The formation of which structure during week 3 let’s you determine the tail and head end of the developing embryo? How do you know which is which?
primitive streak (beginning of gastrulation)
-the primitive node is formed at the head end (future position of oropharyngeal membrane, the opposite end is the tail end (future position of the cloacal membrane)
oropharyngeal membrane is formed from what two layers? How does it facilitate cranial folding?
adhered ectoderm and endoderm
helps cranial folding process by acting as a hinge and swinging ventrally
hypoblast becomes endoderm, true or false?
FALSE
epiblast cells that invaginate become displaced and then move hypoblast cells to become endoderm
intraembryonic mesoderm is formed how?
during gastrulation, the invaginating epiblast cells that remain and lie between epiblast and endoderm become the mesoderm
When the notochordal process completely formed, transformations take it through 3 more forms, 1. 2. and 3.
- Hollow tube
- Flattened plate
- Solid rod
The notochord really doesn’t serve any other process than as the induction to form our CNS, true or false?
TRUE
although some of the bone that will become the vertebrae will develop and grow to enclose some of its remnants in the intervertebral spaces, aka inside the intervertebral discs (nucleus pulposus)
The notochord really doesn’t serve any other process than as the induction to form our CNS, true or false? Is it mesoderm or ectoderm in derivative?
TRUE
although some of the bone that will become the vertebrae will develop and grow to enclose some of its remnants in the intervertebral spaces, aka inside the intervertebral discs (nucleus pulposus)
MESODERM
this is not nervous tissue
True or False, Angiogenesis will occur normally without vasculogenesis
FALSE, you need vasculogenesis in order to have angiogenesis
True or False, The notochord does not become the brain and spinal cord, it is not ectoderm, but it acts as the guideline to form the brain and spinal cord
TRUE
True or false, the oropharyngeal membrane will rupture into the oral cavity in week 4, but before this occurs it separates the oral cavity from the hindgut.
FALSE, separates oral cavity from FOREGUT
Oropharyngeal membrane separates the foregut from the oral cavity (will rupture ~day 26) (Will rupture into opening/oral cavity)
How does the umbilical cord end up on the ventral surface of the embryo?
caudal folding
pericardioperitoneal canals
will form pleural cavities, have connection to abdominal cavities as well
Which of the following are not derived from somites (paraxial mesoderm)? A. axial skeleton B. epidermis C. skeletal muscle D. dermis E. they are all derived from somites
B. epidermis (comes from ectoderm - attractoderm = skin)
What is the embryonic body cavity composed of?
- Pericardial cavity (heart)
- 2 pericardioperitoneal canals (future lungs-> pleural cavities)
- Peritoneal cavity (abdomen/tummy/gut)
Pleuropericardial folds
(lateral extensions of tissue) grow in medially from the lateral walls of the pericardial cavity
Dividing it into the definitive pericardial (1) and pleural cavities (2)
Pleuroperitoneal membranes
grow ventrally & fuse with septum transversum, sealing off the pericardioperitoneal canals.
The diaphragm is formed from _______
- Septum transversum
- Pleuroperitoneal membranes
- Mesoderm from body wall (muscular portion)
- Esophageal mesoderm
Which of the following occur during weeks 3-8 of embryonic development?
A. Neurulation
B. Mesoderm differentiation
C. Body folding
D. Formation of body cavities and diaphragm
E. All of the above
E. All of the above
