| by Kathy Holohan
Cardinal O'Hara H.S., Springfield ,PA
Notes to the Teacher:
1. Since it is difficult to find text material that is both detailed yet not overwhelming for the high school student, this module includes an overview of development . There is also a brief description of cleft lip and palate, neural tube malformations, and septal defects. Included with this information are the approximate rates of incidence and possible means of treatment and detection.
2. The vocabulary and review activities can be used as assignments or as aids for discussion and student involvement.
3 The game, Fetal Pursuit, may be used as a unit extension, a quiz on the unit, or just for fun.
4. Because the unit has many parts, the time required will vary with the parts the teacher chooses to use. If the whole unit were to be used, the time required would be approximately 4 to 5 class periods of 50 minutes each.
5. It should be noted that the discussion of this topic may open up emotional issues among the students especially among those who have been affected by the developmental disorders described here. For some people, fear is dispelled by understanding and so encouraging the student to do more outside research is helpful. This module does not contain exhaustive information on the topic. In any case it is important to be sensitive to these concerns and emphasize that many times the severity of conditions vary; that many cases are treatable and can be detected by prenatal screening.
** the two cell layers of the cell mass separate, creating a cavity which will become the amniotic cavity. Cells
form the cell mass will form a lining, developing the first of four extra-embryonic membranes, the amnion or
amniotic sac, which provides a fluid filled space that acts as a protective cushion, permits symmetrical, growth , helps the embryo control body temperature, prevents adherence of the embryo to the amnion and protects against infection. The cell mass has become flattened with two cell layers--the ectoderm and the endoderm. It is then called a blastodisc.
**during gastrulation a third layer is added in between the other two and this is called the mesoderm. Each layer is destined to form part or all of the organs that will develop during gestation.
** the yolk sac is the second extraembryonic membrane to develop. It forms from the endoderm and mesoderm. It will begin making blood until the liver, spleen and then bone marrow take over the job. An outpocketing of the endoderm of the yolk sac along with some mesoderm forms the 3rd extraembryonic membrane--the allantois a part of which gives rise to the urinary bladder. Mesoderm associated with the allantois combines with the outermost layer of cells around the blastula to form the chorion which will develop the chorionic villi essential to the formation of the placenta.
** the placenta begins to form (placentation). It is made up of maternal tissue and embryonic tissue. The placenta will be the location of exchange of nutrients and wastes between mother and child.
** the embryonic disk becomes more tubular because of differences in the growth rate in different locations.
** the neural groove forms and this will be the site of development of the spinal cord. One end of the neural
groove bends and this will develop into the brain. Pairs of cell clumps (somites) gather on either side of the
groove. These will be the site of formation of muscles and vertebrae.
** the heart takes shape. It is a tube-like structure with one chamber. Forming blood vessels begin to link up.
** at this point the embryo is between 1.5 and 2.0 mm--about the thickness of 2 dimes.
** during this week things really start to happen quickly!!
** in the heart, differences in wall thickness allow the atria to be distinguished from the ventricles. By day 25, the heart is beating, circulating blood between the embryo, the yolk sac and the umbilical cord. The umbilical cord is formed by the gathering of the yolk stalk, which connects the embryo to the yolk sac and the body stalk. The body stalk is formed from the chorion and the allantois.
** at the center the neural groove sinks and the sides grow up to begin the formation of the neural tube . Eventually the sides of the tube will meet and close. It is at this stage that neural tube disorders occur. The outcome of the failure to close is determined by the location at which this occurs. (See anencephaly and spina bifida) A region of cells at the top of the neural tube will form the neural crest. These cells will later migrate and contribute to the formation of several structures.
** limbs arise during this week in the form of limb buds.
** folds of the head and face become distinct and spots form marking the beginning of the development of the eyes and ears.
**the thyroid, pancreas,liver, kidneys and lungs are identifiable.
**the embryo's sex cells migrate to a location near the developing kidney to form a genital ridge.
**by the end of the 4th week, the embryo is curled into an arc about 4mm across--about the size of a small pea.
**arms and legs get longer and acquire bends where knees and elbows will form
**tiny paddles develop at the ends of the limbs--these will become hands and feet.
**eyes and ears are distinguishable but the eyes are still very far apart on the face.
**the brain continues to develop. Cranial nerves form. The brain curves more due to differential growth rates.
**the embryo grows a tail that will later disappear
**cartilage begins to form in the head, alongside and underneath the brain.
**the development of the embryo kidney continues.
**atria and ventricles begin to divide. The septa between the two atria has a gap in it called the foramen ovale.
This does not close until birth and serves to close off circulation to the lungs until that time. There is a flap to revent the backflow of blood from the left atrium to the right atrium. The atrio-ventricular septa develop at
this time. (See Septal Disorders).
**muscle organization increases
**cartilage begins to form in limb buds; the hands and feet get rays of cartilage that will develop into fingers and toes.
**by the end of the week, some ossification begins.
**the liver grows rapidly creating a bulge near the heart. It takes over the production of blood cells from the yolk sac
**cells in the male begin to produce testosterone which directs the formation of male reproductive ducts and external genitalia. (If no testosterone is produced, the female structures will develop. Much discussion is going on about the significance of this.)
**by the end of this week, the embryo is 15 mm long, about the size of a one- peanut peanut shell.
**spinal nerves begin to form
**the neural crest cells migrate to form such structures as specialized ganglia, glial cells, cranial and spinal meninges , and spinal nerves that will innervate the limb buds.
**by the end of the 8th week, the face develops an upper lip and nostrils. It is during this time, as the skin folds around the closing parts of the arches to form the top lip and nostrils, the problem of cleft lip can occur.
**the neck lengthens and makes the jaw stick out a bit
**the ears that were at the level of the neck move to the side of the head and are surrounded by a skin flap
**eyes are still far apart on the face but have moved forward from their original position and are pigmented.
**internal organs are well defined
**sex organs form around germ cells in the genital ridge
**muscles in the neck and body contract spontaneously causing movement although the infant is too small for
the mother to detect this motion.
**vertebrae enlarge. The only part of the notochord left forms the center part of each intervertebral disc.
**basic heart structure is complete
**cartilage around the brain enlarges. There is more bending of the tissues in the brain which has a dominating cerebral cortex.
**the pleural cavity is completely separate from the pericardial cavity and the diaphragm is in place, separating the thoracic and abdominal cavities.
**formation of the adult form of the kidney begins
**the 8th week marks the end of the embryonic period--the time in which most errors in development occur.
By the end of this week, the embryo is about 30 mm long--about the size of a two peanut shell.
WEEKS # 9 & 10
**the embryo is now called a fetus.
**the head makes up half of the crown to rump length.
**the neck grows longer and straightens out a little
**eyelids grow over the eyes closing them until the 6th month
**hands and feet are well developed and fingernails and toenails start to form.
**while some ossification began earlier, it is during the 9th week that bone formation begins in earnest.
**the mandible forms from part of the mandibular arch
**frontal and parietal bones form on the top of the skull
**the maxilla, an important facial bone, begins to form. Ossification centers begin to form the palatine
processes which will fuse with the maxilla to form the roof of the mouth. It is at this time that cleft palate
**the diaphragm is completed
**external genitalia begin to develop. Up to this time it was not possible to distinguish the male fetus from the
female fetus externally--both had a creased bump called the phallus. During this week in boys a burst of
testosterone causes the crease to disappear and the phallus to remain. In the weeks that follow, nothing dramatic happens to the female fetus but in the next few weeks the crease stays and the phallus retracts.
**intestines move into the coelom.
**the second pharyngeal arch forms the hyoid bone which supports the tongue; arches 3,4, and 6 will form
cartilage making up the larynx.
WEEKS 11 & 12
**during week 11 the fetus opens its mouth and will suck a finger if it happens to get in there.
**by week 12, the fetus begins to practice swallowing by taking in amniotic fluid.
**legs are folded; arms have almost reached their final length
**the size of the amniotic sac doubles
**cerebral hemispheres grow over the midbrain. The cerebellum forms.
**ossification of the cranium is well underway.
**the gall bladder and pancreas are complete.
**the spleen begins blood cell formation
**the mature fetal form of male or female genitalia is in place
**urine formation begins and urine is discharged into amniotic fluid
**fetal wastes are transferred across the placenta
**by the end of this month, the total length of the fetus is about 87mm.
OVERVIEW OF SOME DEVELOPMENTAL DISORDERS
Single defects in development with prenatal onset fall into 3 categories:
malformation--usually arises from a localized error in morphology eg. cleft lip and/or palate, cardial septal defects, and defects in neural tube closure.
deformation--an alteration of the shape and or structure of a part that has differentiated normally. Many times this is caused by some issue of fetal position or available room for the fetus to grow.
disruption--refers to a structural defect from the destruction of a previously normal part. There are two basic mechanisms: entanglement--followed by the tearing away of a normally developed structure eg. partial or
complete amputation of a limb by an amniotic cord; interruption of blood supply which involves necrosis of a
normal part because of something that cuts off circulation.
The type of disorders covered in this overview are all due to malformations.
Cleft Lip and Cleft Palate
Cleft lip without cleft palate occurs in about 1 out of 1000 Caucasian births. The frequency is lower in Afro-American groups (1/2500) and higher in Oriental births (1.7/1000 births). The occurence of cleft lip with
with cleft palate is highest in Native American populations (3.6/1000 births). Isolated cleft palate occurs in about
about 1/2000 Caucasian births and 2/2500 Afro-American births.
Cleft lip is a vertical, usually off-center, split in the upper lip. It varies from just a notch in the lip to a split that extends to the nose. In some cases the upper gum is also cleft and the nose is crooked. Another variation is the presence of 2 notches, one on each side of the mouth. There is considerable molding of skin that occurs around the final formation of the nose and mouth. Remnants of that process are seen in the flaring skin around the nostrils and the little groove between the nose and the upper lip. When this does not occur completely or correctly, cleft lip results. This occurs around the 8th week of development.
Cleft palate, which may or may not be present with cleft lip, occurs when the palatine processes of the maxilla which form the hard palate in the roof of the mouth do not fuse properly. This creates a gap in the roof of the mouth that runs along the midline from behind the teeth to the nasal cavity. Fusion usually occurs between weeks 8 and 10 of development.
Feeding is difficult for babies with cleft palate. In most cases they must bottle feed because of the inability to create enough sucking pressure to breast feed. Cleft lip is treated with plastic surgery usually around the age of three months. The lip can be restored to almost normal appearance and there are rarely any speech defects resulting. Cleft palate is usually surgically repaired around age one year. More than one operation may be required and some speech therapy may also be needed. The inheritance of both conditions is complicated.
Neural Tube Disorders
Neural tube disorders are caused by the failure of the neural tube to close properly during the 4th week of development. The type of disorder occurring depends to a great extent on the location that remains open. Each of the conditions varies in severity from individual to individual depending on the extent of error and therefore causes different amounts of problems.
If the neural tube does not close properly at the cranial end, anencephaly results. While there are different variations of this condition, most commonly an infant with anencephaly has no cranial vault (top of the skull) and so no cerebral hemispheres develop. While many infants with anencephaly are stillborn, those who survive until birth usually only live a few hours. Anencephaly occurs in about 5 in 1000 pregnancies. In about one third of the instances, the pregnancy does not continue to term.
If the failure to close occurs farther down the neural tube, spina bifida occurs. This is due to the failure of the vertebrae to develop completely leaving a portion of the spinal cord exposed. The severity of the condition depends on how much nerve tissue is exposed. The incidence of spina bifida is about 1 case per thousand babies born and thought to increase with increased maternal age. There are four basic types of spina bifida:
occulta--in which only the two arches of the vertebrae fail to close, so there may be only a dimple or tuft of hair at the location on the back. It may go unnoticed at first. Some symptoms associated with this condition
are leg weakness and urinary incontinence. It can be diagnosed by an X-ray.
meningocoele--in which the spinal cord is intact but the tissues around it (the meninges) produces a bulging sac covered by skin at the site of the problem. This may be repaired by surgery in the first few days after birth.
myelocoele--a severe form of spina bifida in which the baby is born with a raw swelling over the spine consisting of malformed spinal cord . The section of spinal cord may or may not be contained in a membranous sac. Hydrocephalus, the build-up of cerebrospinal fluid in the brain, may also be present and cause brain damage. Treatment of hydrocephalus usually involves putting in a shunt or tube to drain off excess fluid. Other
associated abnormalities are: cerebral palsy, epilepsy, mental retardation, bladder paralysis and paralysis of
encephalocoele--is the most rare form of spina bifida. In this type the level of protrusion is at the skull , resulting in severe brain damage.
The frequency of neural tube disorders is thought to be reduced if women supplement their diet with folic acid (one of the B vitamins). The presence of neural tube defects can be ascertained prenatally with some degree of accuracy by doing an alpha-feto-protein test. In this test the amount of alpha-feto-protein that has escaped through the placenta into the mother's blood is measured. This protein is only produced by the developing fetus and the amount normally found in the mother's blood is dependant on the age of the fetus. So, this test is accompanied by an ultrasound to determine gestational age in order to evaluate the results correctly. It is most accurately done between the 16th and 18th weeks of gestation.
Septal defects occur when the separation between the atria or ventricles is not complete. The error usually occurs somewhere between the 5th and 8th week of development. (The exception to this is the closure of the foramen ovale which usually does not occur until birth.) There are basically four kinds of congenital heart formations.
ventricular septal defects-- are the most common type making up 30% of the cases with an incidence of 200 babies in 100 000 births. This is caused by incomplete development of the interventricular septum.
Teterology of Fallot--occurs in about 50 out of 100 000 births. It includes narrowing of the pulmonary blood vessels, incomplete interventricular septum, improperly placed aorta, and an enlarged right ventricle.
Atrio-ventricular septal defect--occurs when the valves between the atria and ventricles are not complete.
These conditions are usually diagnosed at birth with a stethoscope and
are further investigated by chest X-rays and ECG.
Small holes usually have little to no effect and often grow closed as the
child ages. Infants with larger holes in the atrial or ventricular
septa are usually treated with drugs first to help control symptoms to
make sure that the problem does not correct itself. If it does not,
surgery can be done to repair the opening.
|placentation||amniotic sac||yolk sac|
|neural groove||neural tube||neural crest|
|palatine process of maxilla||foramen ovale||atrio-ventricular valves|
|cleft lip||cleft palate||spina bifida|
|amniocentesis||chorionic villus sampling||embryogenesis|
1. Write the vocabulary words on index cards and give one to each student. Have the student write the definition on the card and then give them about five minutes to find the two or three other people having words that are in the same category. For instance--the four extraembryonic membranes: yolk sac, allantois, chorion and amnion. Once the students have arranged themselves in groups, each student reports on his vocabulary word and then the group tells how the terms are related.
2. Divide the students into pairs and have them read the dialog taking the roles of Micro and Slider in: Micro and Slider Visit the Hospital.
MICRO AND SLIDER VISIT THE HOSPITAL
Micro and Slider went to the hospital to visit Micro's sister who had and appendectomy. After they left her room, Slider wanted to look at the babies in the nursery before they left the hospital.
Slider: Oh, Micro, thanks so much for stopping
by the nursery window on our way out of the hospital. I just love
to look at all those new babies--it kind of fills me with hope.
Micro: Well, yes it does, Slider. And, in a place like this where some folks spend the end of their lives, it's good to see the beginning of life's cycle as well.
Slider: Remember our old biology class in high school, Micro? I remember how amazed I was that something as miraculous as these babies could come from one fertilized cell.
Micro: Of course I remember our biology class, Slider. So many things in that class were pertinent to our everyday lives--a sort of "The Way Things Work" for living creatures. I especially enjoyed the part of genetics when we were learning about how we inherited traits from our parents.
Slider:You know, Micro, this having babies is serious stuff. Did you realize that by the time a woman knowns she's pregnant a lot of the development of her baby has already occurred. By 4 weeks in development, the embryo which is only the sizeof a pea already has the beginnings of the nervous system, arms and legs, face with spots for eyes and ears, and many internal organs including a beating heart.
Micro: That's just amazing. It's so incredible that so much could be happening in that tiny creature.
Slider: Oh,no. Look over there, Micro. That baby in the middle has a split in his upper lip that goes all the way to his nose. If I remember correctly our Biology teacher told us that was called cleft lip.
Micro: What causes that, Slider?
Slider: Well, I think it's due to an error in development. While it doesn't usually have one specific cause, it occurs because development does not happen in the right order or without all the steps. In the case of cleft lip, the skin that should fold over and help to form the outline of the nose and mouth does not do so correctly. You can see evidence of correct molding on your face--there--where your nostrils flare and at that groove under your nose. In a child with cleft lip, that does not happen which leaves a vertical slit that can be only a notch or go all the way up to the nose. The gums may even be affected. And that error occurs at only 8 or 9 weeks of development!
Micro: Will that baby be OK?
Slider: Oh, yes. They will do surgery and he will be just fine. Some babies born with cleft lip also have cleft palate. Cleft palate occurs when the palatine processes that fuse to form the hard palate do not close. It may also occur alone. Cleft palate is also repairable by surgery.
Micro:You know, now that you're mentioning all this stuff, I think I remember another one of those errors in development. It had something to do with the spine.
Slider: Yeah, that's called spina bifida. I believe it has something to do with the improper closing of the tube that forms the spinal cord and occurs around the 4th week of development.
Micro: Are there many other kinds of errors in development, Slider?
Slider: Well, there are several different kinds but even the most common are still pretty rare. Cleft lip, for example only occurs in about one birth per thousand.
Micro: I can see now why it is important for prenatal care, especially in the first three months. I've heard that many errors of development can be detected prenatally. Well, I guess we'd better go Slider. They're going to close the curtain soon. Let's stop for some dinner on the way home !
FETAL PURSUIT--a game to review the first 12 weeks of development
MATERIALS: 1 gameboard for 2-4 players, 1 playing piece for each player , 1 pair of dice and a score sheet for each person.
1.Each player should have a playing piece, either red, blue, green or yellow and a score sheet.
2. Play begins with each player's piece on the matching color square at the corner of the game board.
3. The first player rolls the dice and moves the appropriate number of spaces on the game board.
4. The person to the right of the player then selects a question from the correct color category . (Blue--month1,
green--month 2, yellow--month 3 and red--risks.) He/she reads the question to the player and if it is answered
correctly, the player may take another turn. Play then moves to the next player. Correct answers are recorded
in the appropriate space on the score sheet.
5. In order to win, a player must have answered at least one question correctly in each category and then move to the middle of the board by the appropritate pathway. Rolling the exact number is necessary. Once in fetal pursuit, the player must answer one more question correctly in the category of his choice.
1. Putting the game board on a piece of cardboard or foam core board will make it more durable for reuse.
2. Playing pieces may be made by putting colored dot stickers on pennies--one of each color: green, blue, yellow
3. The score sheets may be collected and used as quiz grades. Teachers may also offer extra points for questions answered correctly over 5 required to win.
4. Category questions may be placed in folders or can be cut apart and put on colored index cards.
5. A score sheet may be made for the students to indicate the number of questions they have answered correctly in each color category: blue--month 1, green-month 2, yellow-month 3, red-risks.
Month 1 Month 2
|What is the name of the process during which cells divide?
|What forms the framework for bone?
|What is the name of the process in which the blastula
embedded in the uterus? implantation
|What organ takes over the job of blood cell formation
from the yolk
|Is the placenta made up of tissue from the mother,the
embryo , or
|What is the name given to the gap that exists between
the 2 atria?
|At the end of cleavage, a solid ball of cells is formed?
What is this
|When does the foramen ovale close?
|During what time are identical twins usually formed? 1st week||By what week in development are most of the major internal
well defined? week 8
|What happens if the blastula begins to divide but the
incomplete? conjoined twins
|What is the name of the structure at which reproductive
to develop? genital ridge
|What is another name for the cell mass that develops
|What is the name of the process in which cartilage is
|What are the names of the 3 tissue layers that form in
pre-embryo? endoderm, ectoderm, mesoderm
|Why does brain tissue begin to curve at one end?
different growth rates in different areas
|Tor F All organ systems are generated from
one or more of the first
3 tissue layers. T
|T or F In the second month, a human embryo
has a tail at one point.
|What attaches the embryo to the placenta? umbilical cord||Do ears develop on the head or the neck?
|What extraembryonic membrane is involved in respiration
excretion and will contribute to forming the bladder? allantois
| T or F The embryo is 7x larger at
the end of month 2 than it was at the end of month 1.
|What does the pre-embryo have to do to implant itself in the uterine wall? breakdown some uterine lining cells||What structure forms during month 2 that will separate the abdominal and the thoracic cavities? diaphragm|
|What is the name of the structure that develops in the 3rd week that will form the spinal cord? neural groove||What is the name applied to the developing baby up to 8 weeks of development? embryo|
|What are the names of the paired clumps of cells that occur on either side of the neural groove & contribute to muscle formation? somites||What is the name of the hormone that induces the development of the male reproductive tract? testosterone|
|T or F Blood does not circulate to the embryonic lungs? T||Do neural crest cells contribute to the formation of
the spinal cord?
Month 3 Risks
|What is the name applied to the developing baby after the second month of development? fetus||What is the name given to the category of developmental errors that arises from a localized error in structure? malformation|
|What happens to the eyes during month 3? eyelids form and close the eyes until month 6||What kind of developmental errors result from a change in shape or structure of a normally differentiated part? deformation|
|Pharyngeal arches would mark the formation of what structures in fishes? gills||What developmental error involves the improper molding of skin around the nose and mouth? cleft lip|
|What bones fuse to form the hard palate? the palatine processes of the maxilla||What is the name given to the category of developmental errors which includes spina bifida & anencephaly? neural tube disorders|
|What reproductive structures develop first, male or female? seems to be the male||T or F Cleft lip may or may not be present with cleft palate. T|
|What part of the brain is the largest by the 3rd month of development? cerebrum||What causes cleft palate? failure of palatine processes to fuse|
|What process occurs in larger scale in the 3rd month than it had previously? ossification||What determines the affected area in neural tube disorders? the place where the neural tube failed to close|
|T or F Because the fetus receives nourishment by way of the umbilical cord, it doesn't learn to swallow until after birth. F||T or F Cleft lip and cleft palate both occur between the 8th and 10th week of development. T|
|T or F The size of the fetus in the 3rd month is 3x that of the size in the 2nd month. T||T or F By the 8th week the heart is basically complete. T|
|T or F By the third month the ears have migrated from the neck to the sides of the head. T||T or F The foramen ovale closes by the end of the 3rd month of development. F|
|T or F By the end of month 3, the sex of the child is evident. T||Name two kinds of septal defects. atrioventricular
|What hand and feet structures form during the 3rd month? fingernails and toenails||Name two ways in which septal defects can be detected
stethoscope, chest X-ray, ECG
|What brain structure forms during month 3? cerebellum||T or F Most septal defects will correct themselves. If not they can be treated surgically. T.|
|What product is the fetus able to make by the end of
the 3rd month?
|T or F In the cases of each of the errors in development mentioned in this unit, severity varies from case to case. T|
|T or F The first pharyngeal arch in the human fetus forms the jaw bone. (mandible) T||What is the gap between the left and right atrium called? foramen ovale|
Martini, Frederic, Fundamentals of Anatomy and Physiology, 3rd edition, Prentice Hall Publishing, 1995
Moore, Keith and Persaud, T.V.N., The Developing Human--Clinically Oriented Embryology, W.B. Saunders Publishing, 1993
Vaughn, Christopher, How Life Begins: The Science
in the Womb, Time Books, Random House Publishing, 1996.
Principles and Practices of Medical Genetics,
edited by Alan H Emery and David L.Remoin, Churchill Livingstone Publishers,
http://www.mic.ki.se/other.htm Kurinski Institute
orphan@NORD-rbd.com NORD--The National Organization for Rare Diseases