5 Embryonic And Fetal Periods Flashcards
What happens in gastrulation?
Three germ layers are established
What form the different tissues and organs during organogenesis?
Ectoderm
Endoderm
Mesoderm
What happens at the end of the embryonic period?
Main organ systems are formed and the external body form is recognizable
Cells of the neural plate make up the
neuroectoderm
Notochord induces _____ to form the neural plate
ectoderm
What is neurulation?
Formation of the neural tube
The neural plate expands towards the _______ ________.
Primitive streak
What organizes migration of cells?
The primitive node
Where does gastrulation occur?
@ primitive streak
What happens by the end of the third week?
The lateral edges of the neural plate form the neural folds
And the depressed mid region forms the neural groove
Gradually the neural folds approach each other in the __________ where they fuse
Midline
Where does fusion begin?
Cervical region ( 5th somite)
How does fusion proceed?
Cranially and caudally forming the neural tube
Until fusion is complete the cephalic and caudal ends of the neural tube communicate with the amniotic cavity using the
Anterior ( cranial) and posterior ( caudal) neuropores respectively
When does the anterior neuropore close?
Around day 25
When does the posterior neuropore close?
Day 28
______ _________ portion of the neural tube forms the brain and the _______ ______ portion forms the spinal cord
Broader cephalic
Narrower caudal
In general terms , the organs and structures that _______ _________ _____ ____________ arises from the ectodermal germ
Layer
Maintain contact with the outside world
What germ layer is the central nervous system derived from?
Ectoderm
What germ layer is the peripheral nervous system derived from?
Ectoderm
What germ layer is the sensory epithelium of the ear, nose and eye derived from?
Ectoderm
What germ layer is the epidermis and its derivatives/appendages derived from?
Ectoderm
What germ layer is the subcutaneous glands derived from?
Ectoderm
What germ layer is the mammary glands derived from?
Ectoderm
What germ layer is the pituitary gland derived from?
Ectoderm
What germ layer is the enamel of the teeth derived from?
Ectoderm
Neural crests are formed from …
the edges of neural folds
______ and _______ ( secreted from ectoderm) are necessary for neural crest formation
BMPs and FGFs
Neural crest cells ( neuroectodermal) transform from
epithelial to mesenchymal form
What is required for the formation and migration of neural crest cells?
Interaction with ectoderm
What is connective tissue and bones of the skull derived from?
Neural crest
What is the cranial and spinal nerve ganglia derived from?
Neural crest
What is the septum of the heart derived from?
Neural crest
What is the sympathetic chain and sympathetic and parasympathetic ganglia derived from?
Neural crest
What is the schwann cells derived from?
Neural crest
What is the adrenal medulla derived from?
Neural crest
What are glial cells derived from?
Neural crest
What are melanocytes ( in the skin and other organs) derived from?
Neural crest
What are odontoblasts derived from?
Neural crest
What results in a neural tube defect?
Neural tube fails to close.
If the neural tube fails to close in the cranial region then
Most of the brain fails to form ( anencephaly)
If the neural tube fails to close below the cervical region failed closure results in
Spina bfida
Spina bfida cases commonly occur in …
The lumbosacral region
What is supplement is used to reduce neural tube defects?
Folic acid
In the US what is recommended to women of childbearing age?
400 micrograms of folic acid
Initially the mesodermal germ layer form
A thin sheet of loosely woven tissue on each side of the midline
Notochord induces
mesoderm to divide into three parts
What happens on the 17th day?
Cells close to midline form a thickened plate known as the paraxial mesoderm ( future somites)
What happens on the 19th day?
More laterally mesoderm layer remains thin is known as a lateral plate
What connects the paraxial and lateral plate mesoderm?
Intermediate mesoderm
Cavities develop in the ______ _______ and it’s divided into two layers : ….
Lateral plate
Somatic (parietal) and splanchnic ( visceral) layers
What does the somatic mesoderm form?
Dermis of the skin in the body wall and limbs and bones and connective tissue
What does the splanchnic mesoderm form?
The muscles of the gut
What does the intermediate mesoderm contribute to?
Formation of the urinary and reproductive systems ( gonads)
What arises from the mesoderm?
Blood cells and blood vessels
What happens by the beginning of the 3rd week?
Paraxial mesoderm begins to be organized into segments
By beginning of the 3rd week, Paraxial mesoderm begins to be organized into segments known as …
somitomeres
Where do somitomeres first appear? And how does their formation proceed?
Somitomeres first appear in the cephalic region and their formation proceeds cephalocaudally
Somitomeres further organize into
Somites
What happens by the end of the 5th week?
42-44 somites develop.
4 occupital
8 cervical
12 thoracic
5 lumbar
5 sacral
8-10 coccygeal pairs
Which somites later disappear?
The 1st occipital and the last 5-7 coccygeal somites
Initially each somite has a center around which
Mesoderm cells are arranged
What do the somites form?
Axial skeleton ( vertebrae, skull,sternum, ribs) body wall muscles and dermis of the skin
What are the three parts of the somite?
Sclerotome myotome and dermatome
What acts on the dorso medial side of the neural tube?
BMP4
What secretes SHH and noggin?
The notochord and floor plate( ventrolateral) of the neural tube
What do SHH and noggin cause?
Cause ventral part of somite to form sclerotome and to express PAX1 which controls chondrogenesis and vertebrae formation
BMP4 acts on the dorsal side of the neural tube to secrete _____
WNT
_________ from _____ neural tube activate _____ which separates/ distinguishes the ___________
WNT proteins from dorsal neural tube activate PAX3 which separates/ distinguishes the dermomyotome.
What is another function of the WNT proteins?
Direct the dorsomedial portion of the somite to differentiate into it muscle precursor cells and to express muscle specific gene MYF5
What happens to the mid dorsal portion of the somite?
Its directed to become the dermis byneurotrophin 3 ( NT3) expressed by dorsal neural tube
additional muscle precursor cells formed from ___________________ under the combined influence of activating ________ and ________ protein which together activate ______ expression
Dorsolateral portion of the somite under the combined influence of activating WNT proteins and inhibitory bone morphogenetic protein ( BMP4) protein which together activate MyoD expression
Lateral plate splits into
Parietal/ somatic and visceral/splanchnic which line the intraembryonic cavity and surround the organs respectively
Scelerotome forms
Tendon cartilage and bone
Myotome forms
Segmental muscle component
Dermatome forms
Dermis of the back
Each myotome and dermatome also has its own
Segmental nerve component
Parietal layer of mesoderm with overlying ectoderm form
Lateral body wall folds
Lateral body wall fold together with head ( cephalic) and tail ( caudal) folds ..
close ventral body wall
Parietal layer aka
Somatic layer
Visceral layer aka
Splanchnic layer
Hemangioblasts are directed to form blood cells by ?
Vascular endothelial growth factor ( VEGF)
By the end of the 4th week / at around day 28 what happens?
Parietal mesoderm and overlying ectoderm form ventral and lateral body wall. Also there is now a peritoneal/ serous membrane.
The endoderm layer covers … and forms …
Covers ventral surface of embryo
Forms roof of the yolk sac
What happens between day 17 and 28?
Growth of the brain vesicles and lengthening of the neural tube causes both head and tail regions to move ventrally forming cranial and caudal body folds and the embryo curves into fetal position and lies within the amniotic cavity
Between the 17th and 28th days, growth of the brain vesicles and lengthening of the neural tube causes both the head and tail regions to move ventrally forming cranial and caudal body folds and the embryo curves into the fetal position; the embryo lies within the amniotic cavity. What happens simultaneously?
the sides of the embryo begin to fold ventrally to form two lateral body wall folds
True or false. The umbilical region in the ventral body closes completely.
False. in the umbilical region the ventral body wall does not close completely
Why doesnt the umbilical region close?
To leave a connection between the yolk sac and the gut
at the cranial end what happens to the foregut?
It is separated from the stomodeum (primitive mouth) by the oropharyngeal membrane
When does the membrane break?
During the 4th week
What happens at the caudal end?
the cloacal membrane separates the hindgut from exterior
When does the cloacal membrane break?
7th week
What events establish continuity with the exterior ( amniotic cavity) ?
Cloacal membrane separating hindgut from exterior and it breaking in the 7th week
The midgut remains in connect with the _______
Yolk sac
What germ layer contributes to the formation of the epithelium of the primitive gut?
Endoderm
What germ layer contributes to the formation of the respiratory tract?
Endoderm
What germ layer contributes to the formation of the thyroid and parathyroid?
Endoderm
What germ layer contributes to the formation of the liver, pancreas, tonsils and thymus?
Endoderm
What germ layer contributes to the formation of the urinary bladder, urethra , tympanic cavity and auditory tube?
Endoderm
When does the external appearance change?
End of 8th week
Why does the external appearance change in the 8th week?
Due to increase in head size and formation of the limbs, face, ears, nose and eyes
What happens during the 5th week?
Limbs appear first upper then lower
And finger differentiation happens
What happens during the 2nd month?
Increase in head size face limbs ears nose and eyes
When are most organs and organ systems formed?
During the embryonic period
What is a critical period for development?
Embryonic period
What happens if external insults occur during the embryonic period?
External insults (thalidomide exposure) will induce abnormal development.
What can be noticed around a 6-8 week fetus?
Yolk sac
Umbilical cord
Chorionic plate
Chorionic villi
Amnion
When is the fetal period?
9weeks to birth
What are characteristics of a 9week fetus?
Large head compared to rest of body
Long vitelline duct and yolk sac
Umbilical cord and herniated intestinal loops
What happens during the fetal period?
Tissues and organs mature and grow rapidly
Growth in length
Weight gain
Slowdown of head growth compared to rest of body
Growth in length is conspicuous in
3rd to 5th month
Weight gain is striking during
Last 2 months of gestation
When does the face become more human looking?
3rd month
When do the eyes and ears move to adult position ?
3rd month
When do the upper limbs reach their relative length ( but not lower limbs)?
3rd month
When do the primary ossification centers ( bone formation) present in skull ?
3rd month
When does The external genitalia develop ( and can be seen in the ultrasound)?
3rd month
When are the intestinal loops withdrawn into the abdominal cavity?
3rd month
When is there muscular activity?
3rd month
When does the fetus lengthen rapidly ( length is 15cm and weight is 500g)?
4-5 months
What is the length and weight of the fetus during 4-5 months?
L 15cm W 500 g
When is the fetus lines with fine hair lanugo hair?
4-5 months
When can movements of the fetus be felt by the mother?
4-5 months
Weight increases rapidly especially ______ .
during last 2 1/2 months
What is a characteristic of the fetus during 6th week?
Skin of the fetus is reddish
Why may a 6 month fetus that was born survive?
Lung development is not complete and alveolar collapse may happen
During the 6th week why is the skin of the fetus reddish?
Due to high levels of angiogenesis ( high vascularity)
In the 4th and 5th months the fetus is lined with fine hair-languo hair. Why?
To protect the skin
A ___ month old fetus would able to survive
7
A fetus has well rounded contours as a result of what?
Deposition of subcutaneous fat
Characters of a fetus in the 7th month
Head size is bigger
Umbilical cord is twisted
There are well developed limbs fingers toes , ears and eyes
Typically when is the time of birth?
266 days / 38 weeks after fertilization
True or false. The day of fertilization is easy to determine
False. It is difficult to determine
How does the doctor calculate the date of delivery?
Doctor calculates the date of delivery as 280 days/40 weeks after last
menstrual period
Why can Implantation bleeding misguide the doctor?
Because it can be mistaken as a menstrual period
What can also misguide the doctor?
Irregular menstrual period
What is it called when delivery takes place much earlier than estimated date?
Premature birth
What is it called when birth takes place much later than the estimated date?
Post mature birth
Why must the size of the fetus be determined?
to manage delivery and growth of
fetus
How can the size of the fetus be determined?
by ultrasound by measuring bi-parietal diameter of the head
or femur length
When is the embryonic period?
Week 3 - week 8
What are the three derivatives of the mesoderm?
Paraxial intermediate lateral
In the second week, what does the extraembryonic cavity help us form?
Yolk sac and chorionic cavity
Extraembryonic mesoderm spilts into two :
Splanchnic and somatic
Notochord functions
Form body axes
Expresses alot of genes
Regulates mesoderm dorsalizing
The lateral will split into :
1)One within the endoderm (splanchnic or visceral, will be in association with the developing gut)
2)and one within the amniotic cavity(somatic)
the notochord once formed begins to signal to the ectoderm above it to
differentiate these cells especially in the cephalic region to change into neuroectoderm (to fold)
The primitive node is the
Organizer
(expresses specific genes to rom the notochord)
Somites parts and purposes
1) Sclerotome->bones&vertebra 2) Myotome->muscles
3) Dermatome->skin
During neurulation what happens on the lateral to the neural groove?
Somites (which are part of the mesoderm) start to appear
As the tube begins to close the somites get larger in number so
somites act as a landmark to tell us which stage of development we are
The neural tube begins to close like a
Zipper cephalocaudally
True or false. Closure of the two pores is unimportant.
False. The closure of the 2 pores is extremely important
If anterior pore fails to close
Anencephaly
Which neuropore should close first anterior or posterior?
Anterior should close first
If posterior pore fails to close
Spina bfida
What are the differences between 25 day and 28 day embryo?
1) increase in size
2) Umbilical cord formation
3) Closure of neuropores
4) Otic placode give rise to eyes and
ears
5) limb ridges appear
6) increase in somite number
Sympathetic ganglion
Autonomic part and is derived from neural crest cells
Formation and migration of Neural crest cells happens during
neural tube closure
BMPs and FGFs regulate the
whole process( formation and migration of neural crest cells), why?
Because they play role in ventralizing the mesoderm, so this is part of the mesoderm cells as well at this point in time, they are derived from the developing
neural tube, they have the ability to shift their profile to become neural crest cell and go to that mesoderm layer and innervate the structure that they are gonna develop to
Dorsal root ganglion
A sensory gateway/door that collects info and it send to the brain (Dorsal root ganglion is formed from neural crest cells
Preaortic ganglion
Control vasodilation and vasoconstriction (part of the nervous system)
Enteric ganglia
Going to GI tract, excelling GI tract to inc or dec digestion (controlled by nervous system also)
Neural tube will give rise to
brain and spinal cord
Locations of somites and what they will form
Most ventral somite -> Sclerotome
Dorsal medial & ventral lateral -> muscles Middorsal -> dermatome
This is the ventral lateral part of the somite It requires which Transcription factor?
MYOD
What regulates that expression( of MYOD) ?
WNT from the ectoderm, and lateral plate mesoderm will secrete BMP4 to express MYOD as as well
This is the middorsal part of the somite
It requires which Transcription factor?
PAX3
What regulates expression of PAX3?
NT- 3 coming from dorsal side of N.T
This part will form the sclerotome
What transcription factor is required in order differentiate somites into sclerotome?
PAX1
What induces expression of PAX1?
SHH and Noggin
Where is the SHH and Noggin coming from?
Ventral part of the neural tube and notochord
This layer is attached to the endoderm and incorporated within the developing embryo. Which layer is it?
Visceral mesoderm layer
This is the dorsal medial part
It requires which Transcription factor?
MYF
What regulates expression of MYF?
WNT coming from the dorsal side of N.T in addition to SHH from notochord and ventral part of N.t
Gonads develop from the intermediate mesoderm BUT there is an exception
(Bladder and urethra) they develop from endoderm
What is true during the 9th week?
All systems are in place and completed development but now are maturing and increasing in size
What is not fully incorporated in a developing fetus?
Herniated intestinal loops
Is the intra-embryonic mesoderm inside the baby? And what does it give rise to?
Yes. give rise to Paraxial Intermediate Lateral
When lateral splits it will give rise to
lining of the gut and membranes around body wall
