Embryonic & Fetal Development Flashcards
Interval of prenatal development
From conception to birth
Interval of infancy and toddlerhood
From birth through 2 years
Interval of early childhood
3 to 5 years
Interval of middle childhood
6 to 11 years
Interval of adolescence
12 to 18 years
What is meant by gestation
Period of time required for full development of a fetus in utero
According to WHO, anyone under the age of ________ is a child
18
Example question :
When we say “28 weeks gestation” , what do we mean by that?
28 weeks in the womb
Or 28 weeks after conception
The term “gestare” means what
To carry or To bear
The first 2 weeks of prenatal development are referred to as the ___________________.
Pre-embryonic stage
When is the pre-embryonic stage ?
First 2 weeks of prenatal development
A developing human during weeks 3-8 is referred to as an ____________.
Embryo
A developing human from the 9th week of gestation until birth is referred to as a ____________.
Fetus
When is a developing human referred to as an embryo ?
Weeks 3-8 gestation
When is a developing human referred to as a fetus?
Week 9 gestation until birth
Embryonic stages of development are characterized by what
- cell division
- migration
- differentiation
True or false
By the end of the embryonic period, all the organ systems are structured in basic form.
True
Describe the function of the organs by the end of the embryonic period?
Either nonfunctional or semi-functional
Describe the stages of pre-embryonic development and
Unfertilized oocyte
Fertilized oocyte
Two-cell stage
Four-cell stage
Eight-cell stage
Morula (16 cells)
Blastocyst (70-100 cells)
After that we have implantation:
- zona pellucida sheds
- blastocyst implants in endometrium of uterus
A morula contains how many cells
16 cells
A blastocyst contains how many cells
+ describe its structure
70-100 cells
It is a sphere of cells surrounding a fluid-filled cavity called blastocoel. The outer layer consists of trophoblast cells that will form the placenta. The inner cell mass (embryoblast) will form the fetus eventually .
When does the process of implantation begin ? And what does it signal ?
Begins at the END of the 1st week.
It signals the END of the pre-embryonic stage of development
What happens during implantation ?
- The blastocyst adheres to the endometrium
THEN - the trophoblast secretes hCG
What is hCG and what is its role?
hCG is Human Chorionic Gonadotropin
It is a hormone that directs the corpus luteum to survive, enlarge, and continue producing progesterone and estrogen to suppress menses (menstruation/period)
When is implantation complete ?
Implantation takes only a few days
It is complete by the middle of the SECOND week when the trophoblast has secreted enough hCG that is now detectable using an at-home urine pregnancy test. (+ve test)
Which hormone is detected in most pregnancy tests ?
hCG
Explain what is going on in this picture ?
When blastocyst implants :
- it digests the uterine mucosa
- the endometrium grows over and surrounds the embryo
- implanted embryo will continue to grow in endometrium
(WARNING: not in the notes but crucial for understanding).
During the second week around implantation, the inner cell mass (embryoblast) divides into the epiblast and hypoblast, forming a two-layered structure called ____________________.
Bilaminar disk
By the beginning of week 3 , what is the shape of the embryo
Oval-shaped disc ( due to the formation of the bilaminar disc)
What process takes place during the 3rd week of embryogenesis ?
Gastrulation
During which days does the inner cell mass divides into an epiblast and hypoblast ?
15-16th day
What occurs during gastrulation ?
The bilaminar disc is converted into 3 layers :
- ectoderm
- mesoderm
- endoderm
(The epiblast divides into these 3 layers)
NOTE: if you are wondering what happened to the hypoblast, it forms the yolk sac
Label
Endoderm layer becomes :
Mesoderm layer becomes:
Ectoderm layer becomes
What does the epiblast and hypoblast give rise to ?
Epiblast gives rise to embryo (ectoderm, mesoderm, endoderm)
Hypoblast gives rise to yolk sac
What process comes after gastrulation
Neurulation
Steps in neurulation
- Formation of a thickened area of cells called the neural plate
(The neural plate forms at the cranial end of the embryo, and grows in a cranial to caudal direction. The cranial or head end of the neural plate indicates the region of the future brain, and the narrower caudal or tail end represents the future region of the spinal cord) - By the end of the 3rd week of development (during 4th week), the lateral edges of the neural plate become elevated and move together to form the neural fold.
- The resulting space created by the folding of the neural plate is called the neural groove.
- The neural folds fuse together, and the neural plate transforms into the neural tube (the precursor to the CNS)
- During the closer of the neural tube, cells on the crest of the neural folds detach, forming a new cell population called the neural crest (these cells contribute to the formation of the PNS)
ONCE THE NEURAL TUBE HAS COMPLETELY FUSED, THE PROCESS OF NEURULATION IS COMPLETE
In the process of neurulation, rudiments of the ___________________ develop from ectoderm
Central nervous system CNS
Rudiments of the central nervous system develop from which layer in the process of neurulation ?
Ectoderm
Specialized neuroectodermal tissue along the length of the embryo thicken to form what structure ?
Neural plate
During the 4th week, what starts to occur in the process of neurulation?
Tissue on either side of neural plate fold upward, creating the NEURAL FOLD.
These two folds converge to form the NEURAL TUBE
The Neural tube lies on top of a rod shaped structure called?
Notochord
Which layer is the notochord derived from?
What does the notochord become?
Mesoderm
Nucleus pulposus of intervertebral discs
What are the block-like structures that are formed on either side of the neural tube?
And what do they become?
Somites (derived from the mesoderm)
They become the axial skeleton, skeletal muscles, and dermis (skin)
During the fourth and fifth weeks, what happens to the neural tube?
The anterior neural tube dilate and subdivides to form vesicles that will become the brain structures
On which day after conception, do we have the thickening of the ectoderm that leads to the development of the neural plate?
18 days after conception
On which day after conception does the neural groove begin to develop?
20 days after conception
On which day after conception does the neural groove close to form the neural tube, which remains open at either end?
22 days after conception
On which days does the anterior and posterior openings ( or neuropores) of the neural tube close
Anterior : 25-26 days
Posterior: 27-28
What supplement is important to the healthy development of the neural tube?
Folate (a B-vitamin)
What happens when there is a deficiency of maternal folate in the first weeks of pregnancy?
Neural tube defects, including:
- Spina bifida
- Anencephaly
What forms the CNS
Neural tube
What forms the PNS
Neural crest
How does the heart begin its development in the embryo?
As a tube like structure that is connected via capillaries to the chorionic villi
True or false
Cells of the primitive tube-shaped heart are capable of electrical conduction and contraction
True
When does the heart begin beating?
When the heart begins beating, does it actually pump embryonic blood right away?
Beginning of the 4th week
It does not actually pump embryonic blood until a week later when the oversized liver has begun producing red blood cells
What is the temporary role of the oversized liver in the embryo?
What organ takes over that role later during fetal development ?
Red blood cell production
Bone marrow
During weeks ______ - ______, the eye pits form, limb buds become apparent, and the basics of the pulmonary system are formed.
Weeks 4 to 5
What happens during the 6th week?
- uncontrolled fetal limb movements begin to occur
- Intestines temporarily loop into the umbilical cord due to the abdomen not being big enough
- Paddle-shaped hands and feet develop
Since the gastrointestinal system develops too fast for the embryonic abdomen to accommodate it, what happens to the embryonic intestines?
The intestines temporary loop into the umbilical cord
How do paddle-shaped hands and feet develop fingers and toes?
By the process of apoptosis (programmed cell death), which causes the tissues between the fingers to disintegrate
What happens during the 7th week?
Facial structure is more complex and includes:
- nostrils
- outer ears
- lenses
What happens during the 8th week?
- Head is as large as the rest of the body + all major brain structures are in place
- External genitalia are apparent, but male and female embryos are indistinguishable (genders are recognized genetically only)
- Bone begins to replace cartilage in the skeleton through the process of ossification
Review this picture
Done
True or false
Fetal circulation is the same as postnatal (after birth) circulation
False
They are different
Explain the fetal circulation
-The oxygenated blood comes from the umbilical VEIN.
-It goes to the liver and splits into 3 branches. One of these branches is called DUCTUS VENOSUS which bypasses the liver to reach inferior vena cava.
-The blood then reaches the inferior vena cava and enters the right atrium.
-In order to bypass the lungs, in the right atrium most of the blood flows to the left atrium directly via an opening called FORAMEN OVALE, which then goes to the left ventricle, aorta, and then to the entire body.
NOTE: some blood does not go through foramen ovale, so instead they go to the right ventricle. After that blood is pumped into the pulmonary artery where it is diverted back into the aorta through DUCTUS ARTERIOSUS so that very little blood passes through lungs while the rest of the oxygenated blood goes to the systemic circulation.
Quick summary
Ductus venosus
Foramen ovale
Ductus arteriosus
What are they?
Ductus venosus: continuation of umbilical vein that bypasses liver and takes oxygenated blood directly to inferior vena cava
Foramen ovale: opening between right and left atrium
Ductus arteriosus: small vessel that connects the pulmonary artery to the descending thoracic aorta to divert more blood into the systemic circulation
In fetal circulation , most of the blood bypasses the lungs and reaches the left ventricle via what two structures?
Foramen ovale
Ductus arteriosus
The fetus uses the placenta to obtain _______ and get rid of ________.
O2
CO2
True or false
The blood traveling to and through the fetal lungs is primarily used for gas exchange .
FALSE
It is primarily used for nourishment and development of lung tissue
Cardiac development is primarily complete by which week
10th week
What are the 3 shunts or bypass channels in the fetal circulation
- Ductus venosus DV
- Foramen ovale FO
- Ductus arteriosus DA
How does deoxygenated blood return from the fetus to the placenta
Lower end of abdominal aorta ➡️ LEFT and RIGHT umbilical arteries ➡️ placenta
The UPPER part of the body receives MORE oxygenated blood compared to the LOWER part .
WHY??
Because of mixing of venous blood in the aorta DISTAL to the connection with the ductus arteriosus DA.
Blood comes into the umbilical vein _____% saturated, and returns back to the umbilical arteries ______% saturated
80%
58%
The blood in the left umbilical vein gradually loses its high oxygen content during its course from the placenta to the fetus.
Why?
Because it mixes with desaturated blood from the body at the DV, FO (left and right atria), and DA.
What causes the blood to move from the right atrium through the foramen ovale into the left atrium ?
Due to resistance in the pulmonary circulation , which shunts most of thuis blood through the foramen ovale into the left atrium .
What are the post nasal changes in fetal circulation
Physiological closure of the 3 shunts and 3 blood vessels occurs IMMEDIATELY after birth
What facilitates the closure of ductus arteriosus ?
The release of bradykinin in lungs after first breath = smooth muscle in their wall contracts
The functional closure of the foramen ovale occurs immediately after birth . What causes this closure ?
The pressure in the right atrium decreases and the pressure in the left atrium increases, thereby pushing the septum primum against septum secundum
Time of anatomical closure of umbilical vessels and DV
2-3 months after birth
Time of anatomical closure of DA
1-3 months after birth
Time of anatomical closure of FO
6 months after birth
What occurs during weeks 9 to 12 of fetal development
- brain expands
- body elongates
- ossification continues
- bone marrow produces RBCs while liver produces bile
- fetus circulates amniotic fluid by swallowing it and urinating
- eyes are well developed but eyelids fused shut
- fingers and toes begin to develop nails
By the end of the 12th week , how much does the fetus measure ?
9 cm (3.5 in)
What occurs during weeks 13 to 16 of fetal development
- sensory organ development
- lips exhibit sucking motion
- ears move up and lie flatter against head
- scalp grows hair
- excretory system are developing : kidneys well formed + meconium accumulates in intestines
What development marks week 13-16
Sensory organ development
What does the meconium consist of
Ingested amniotic fluid
Cellular debris
Mucus
Bile
What occurs during weeks 16 to 20 of fetal development
- limb movements become more powerful (mother may feel this)
- fetus is in fetal position due to restricted space
- sebaceous glands coat the skin with a waxy, protective substance called vernix caseosa
- lanugo (silky hair) covers skin during 17-20 weeks.
What is the fetal position (physiological flexion)
Arms crossed and legs bent at knees
Role of vernix caseosa
Protects and moisturizes the skin + lubrication during childbirth
True or false
Lanugo is shed as the fetus continues to grow
Extremely premature infants sometimes exhibit residual lanugo
True
True
What occurs during weeks 21 to 30 of fetal development
- rapid weight gain (important for stable body temperature after birth)
- bone marrow fully takes over RBC production
- axons of spinal cord begin to be myelinated
- fetus grows eyelashes
- eyelids no longer fused (can be opened and closed)
- lung begins producing surfactant
When does the process of myelination end
Until adolescence
What is surfactant
A substance that reduces surface tension in the lungs and assists proper lung expansion after birth.
Premature newborns have inadequate surfactant production.
What can this cause ?
How is it managed?
Respiratory distress syndrome
Surfactant replacement therapy, supplemental oxygen, or maintenance in continuous positive airway pressure chamber CPAP during their first days or weeks of life.
Measurement of fetus at 30 weeks
28 cm
Almost the same body proportions as a full term newborn but MUCH leaner.
