HEARING AND BALANCE DEVELOPMENT

We use the sense of balance and hearing to position ourselves in space, sense our surrounding environtment and to communicate. Portion of the ear appear very early in development as specialized region (otic palcode) on the embryo surface that sinks into the mesenchyme to form a vesicle (otic vesicle=otocyt)  that form in the inner ear.
This region connects centrally to the nervous system and peripherally through specilized bones to the external ear (auricle). This organisation develop different sources forming the 3 ear parts: inner ear (otic placode, otocyst) middle ear (1st pharingeal pouch and 1st and 2nd arch mesenchyme) and outer ear (1st pharingeal cleft and 6 surface hillocks).
This complex origin, organisation and time course means that abnormal development of any one system can impact upon the development of hearing.

Embriogenic origin overview

External ear:

• Auricle --- Pharingeal arches 1 and 2  (ectoderm and mesoderm)
• External Auditory Meatus  --- Pharingeal arch 1 groove or cleft (ectoderm)
• Tympanic Membrane --- Pharingeal Arch 1 membrane (ectoderm, mesoderm,       endoderm)

Middle ear:

• Middle ear ossicles

#Malleus and incus --- Pharingeal Arch 1 cartilage neural crest (ectoderm)
#Stapes --- Pharingeal Arch 2 cartilage neural crest (ectoderm)

• Middle ear muscles

#Tensor Tympani ---PharingealArch 1 (mesoderm)
#Stapedius  --- Pharingeal arch 2 (mesoderm)

• Middle ear cavity --- Pharingeal Arch 1 pouch (endoderm)

Inner ear:

• Inner ear labyrinth

#Cochlea --- Otic vesicle ---  Otic placode (ectoderm)
#Semicircular canals --- Otic vesicle ---otic placode (ectoderm)
#Saccule and utricle  --- Otic vesicle --- otic placode (ectoderm)

• Cranial nerve VIII

#Auditory component --- Otic vesicle and neural crest (ectoderm)
#Vestibular component  --- Otic vesicle and neural crest (ectoderm)


POSTNATAL CHANGES

There are a number of post natal changes associated with growth of the head that affect the newborn to adult auditory tube and its functions. The auditory tubes (Eustachian, otopharingeal or pharingotympanic) space connects the middle ear cavity to nasopharinx portion of pharinx.
Auditory tube function:

• Ventilation --- pressure equalization in the middle ear
• Clearence --- allow fluid drainage from the middle ear , tube is normally closed  and  opened by muscle

Auditory postnatal :

• Birth --- (neonatal to early childhood)  the tube is initially short (17-18 mm), narrower and runs almost horizontal, The tube is opened by a single muscle, tensor palati muscle.
• Adult --- the tube is longer (twice as long) , wider and runs at aproximately 45 degrees to the horizontal. Tube is opened by two separate muscle, tensor palati and levator palati.

Abnormalities

There are many different abnormalities of hearing development that can result in hearing loss and can be broadly be divided into either conductive or sensorineural loss. These abnormalities can have genetic, environtmental or unknown origins. In addition, abnormalities of the external ear (position and structure) is used as a clinical diagnostic tool for developmental abnormalities in other systems.

1. Inner ear --- common cavity, severe cochlear hypoplasia
2. Middle ear --- rare and  can be part of first arch syndrome, Malleus, Incus, and Stapes Fixation. Cholestoma  is epithelium trapped within skull base in development, erosion of bones (temporal bone, middle ear, mastoid)
3. Outer ear ---several genetics effects and syndromes, environmental effect. Microtia is abnormally small external ear. Prearicular sinus is occurs in 0,25% births, bilateraln (hereditary) 25-50%, unilateral (mainly the left), duct runs inward can extend into the parotid gland, postnatally site for infection.

Congenital deafness

1. Sensory neural --- Cochlear or central auditory pathway

• Hereditary -- Recessive  (severe) and dominant (mild)
• Acquired --- rubella (German measless), maternal infection during 2nd month of pregnancy, vaccination of young girls --- cytomegalovirus ---streptomycin ---antibiotic ---thalidomide

2. Conductive --- Disease of outer and middle ear

• Disease of outer and middle ear
• Can be produced by otitis media with effusion, that is widespread i young children
• Temporary blockage of outer or middle ear

Fetal  alcohol syndrome
Position - lower or uneven height, "railroad track" appearance, curve at top  part of outer ear is under developed, folded over parallel to curve beneath

PEDIATRIC IMPERFORATE ANUS

An imperforate anus is malformation of the rectal are athat may occur in several forms. The rectum may end in  a blind pouch that doesnt connect with the colon. This means that from the outside, what appears to be an anus is evident, but it doesnt go anywhere. Or it may have openings to urethra, bladder, base of the penis or scrotum in boys, or vagina in girls. A condition of narrowing of the anus or absence of the anus may be present. 

Typically, there are two types of imperforate anus:
The "high type" means that no opening is presents on the outside of the childs body and the bowel ends above the muscles at the bottom of the pelvis.
The "low type" means that the bowel ends below the muscles of the pelvis and there is an opening present, but it is  an abnormal position or is covered by a layer of skin or ather type of membrane.

The malformation are caused by abnormal development of the fetus, and many are associated with other birth defects. Imperforate anus is a relatively common congenital malformation that accors in about one out of 5000 infants. It is not the fault of the mother; the cause is unknown.
Anorectal malformations include wide spectrum of defects in the development of the lowest portion of the intestinal and urogenital tracts. Many children with this malformations are said to have an imperforate anus because they have no opening where the anus should be. Although the term may accurately describe a child's outward appearence,  it often belies thr true complexity of the malformation beneath. When malformation of the anus is present, the muscles and nerves associated with the anus often have a similar degree of malformation. The spine and urogenital tract may also involved.


Symptoms may include the absence of anal opening, or one in the wrong area, such as very close to the vaginal opening in females, and no stool within 24 to 48 hours after birth, or stool that comes  out of the vagina, the base of the penis , urethra (where you urinate) or scrotum. Parents also may notice a swollen abdomen.
To diagnose this, the doctor may physically examine the child. He or she also may order a lower abdominal X-ray or enema to see whether fluid inserted into the anus is moving up into the small intestine.


The treatment is surgery. The "low type" may involve opening the thin membrane of skin or other tissue in surgery, repeated stretching of the opening, or surgical reconstruction.
The "high type" is treating by creating a temporary colostomy (re-routing the bowel out the abdominal wall). The baby than has bowel movements into a bag. At about six to 12 months, surgical rebuilding of an anal opening is completed, and any posible fistula is closed,  The colostomy is removed a few weeks later.


The position and nature of these malformations made repair  difficult for early surgeons. The affacted organs are located deep in the pelvis and are not well visualized through abdominal incisions. Traditional surgical dictum did not allow for division of the posterior midline because this division of the muscle was believed, somewhat erroneously, to cause incontinence in the child. Therefore, surgeons approached these  malformation using a combine abdominal, sacral and perineal approach, with limited visibility.  Such approaches have put continence, and surroundings genitourinary structures, at greater risk than simply cutting sphincter limited incisions.  This principle was central to the development of the surgical techniques currently used to repair these malformations.


In 1982, Pena et al reported the results of the use of a posterior sagittal  surgical repair approach. Pena et al  used the traditional approach with sacral incision and made the incisions progresively  larger in an attempt to adequately visualize the anatomy. Eventually, the entire posterior sagittal plene was opened, affording a full view of the complete malformation. This technique, referred to as posterior sagittal anorectoplasty (PSARP) or posterior sagittal anorectovaginourethroplasty (PSARVUP), has led to more complete understanding of the anatomy of these children and of what is required to repair the malformations with optimal results. After reconstructive surgery for the malformation, many children still experience effects in the form of urinary or fecal incontinence. Despite optimal surgical management, no adequate repair for poorly developed muscles or nerves has been developed. Bowel management regiments can provide an excellent quality of life for these children whwn primary continence is not achievable.


The embryogenesis of these malformations remains unclear. The rectum and anus are believed to developed from the dorsal potion of the hindgut or cloacal cavity whwn lateral ingrowth  of the mesenchyme forms the urorectal  septum in the midline. This septum separates the rectum and anal canal dorsally from the bladder and urethra. The cloacal duct is small communication between the 2 portions of the hindgut. Downgrowth of the urorectal septum is believed  to close this duct by 7 weeks gestation. During this time, the ventral urogenital portions acquires an external opening, the dorsal anal membrane opens later. The anus develops by a fusion of the anal tubercles and an external invagination, known as the proctodeum, which deepens toward the rectum but is separated from it by the anal membrane. This separating membrane should disintegrate at 8 weeks gestation.


Interference with anorectal structure development at varying stages leads to various anomalies, ranging from anal stenosis, incomplete rupture of the anal membrane, or anal agenesis to complete failure of the upper portion of the cloaca to descend and failure of the proctodeum to invaginate. Continued communication between the urogenital tract and rectal portions of the cloacal plate  causes rectourethral fistulas or rectovestibular fistulas.
The external anal sphincter, derived from exterior mesoderm, is usually present but has varying degrees formation, ranging from robust muscle (perineal or vestibular fistula) to virtually no muscle (complex long-common-channel cloaca, prostatic or bladder -neck fistula).
Anorectal and urogenital malformations are rarely fatal, although some associated anomalies (cardiac, renal) can be life threatening. Intestinal perforationor post operative septic complications in a newborn with imperforate anus can result in mortality or severe morbidity.


Morbidity generally arises from the following 2 sources:
Malformation-related morbidity and surgery-related morbidity
Malformation-related morbidity relates to associated malformations of rectal motility, anorectal innervation,  and sphincteric musculature. The most common morbidity in this category is constipation. Most children have mild malformations that commonly result in constipation for reasons that remain unclear, If left untreated, chronic  constipation results in rectal dilatation, which worsens the constipation. This become a visious  cycle, which, if untreated, results in fecal impactions and overflow pseudoincontinence, also known as encopresis. The most severe forms of malformation-associated morbidity are fecal and urinary incontinence. Higher malformations, such as long-common-channel cloacae and prostatic or bladder-neck fistulas, are associated with poorer nerve and muscle formation, all of which increase the likelihood of fecal or urinary inconteinence. Malformations that directly involve urinary  sphicteric mechanisms, and spesifically, any malformation in which the rectum and vagina joins the urinary tract at the bladder neck, often results in either urinary incontinence or inability to complete void.


Surgery-related morbidity can include standard complications such as line infections and pnemonia. Wound infections or anastomic breakdowns can occur in any intestinal surgery. Children with imperforate anus are at greater risk for injury to surrounding pelvic organs because these organs  (such as vagina or urethra and seminal vesicles) are located immediately adjacent to the rectum, and may also be involved in the malformation in some unsuspected way. During blind exploration in the pelvis, a dilated ureter can be mistaken for the rectum. Urethras can be opened or transected, and prostates or seminal vesicals can be easily injured. Dissection of these delicate structures can result in ischemia and possible stricture or complete stenosis.


Most children with an anorectal malformation are identified upon routine newborn physical examination. Delayed presentationis often the result of incomplete initial examination.  Newborn anorectal and urogenital examination can be technically challenging and makes many practitioners uncomfortable. Subtle malformation, such as those in some children with perineal fistula that may look normal to the casual glance, may present months or years after birth when the child presents  to a primary care provider  for constipation or urinary tract infection and appears to have a small perineal body upon physical examination. Anorectal malformation in females with a normal-appearing anus who have absent vagina or persistent urogenital sinus may go undiagnosed for years because of examiner reluctance to separate the labia during physical examination, These malformation can be discovered upon evaluation for urinary tract infection or primary ammenorrhea.

LUNG AGENESIS

Pulmonary agenesis is a rare congenital anomaly, the etiology of which is not clearly known. Other systemic comorbidities such as cardiovascular, gastrointestinal, musculoskeletal and urogenital system anomalies can be observed in more than half of the patients.  It is usually diagnosed during childhood. Diagnosis in adulthood  is very rare.


Pulmonary aplasia is a rare congenital pathology in which there is unilateral or bilateral absence of lung tissue. It is distinguished from pulmonary agenesis, although similar, the main difference being that there is a short blind ending bronchus in aplasia.
It is usually unilateral, as bilateral pulmonary aplasia is not viable. It is frequently associated with other congenital abnormalities, mainly cardiovascular, and has been reported to occur with VACTERL syndrome. The acronym VACTERL derives from Vertebratal anomalies (hemivertebrae, congenital scoliosis, caudal regression, spina bifida), Anorectal anomalies (anal atresia), Cardiac anomalies (cleft lip), Tracheo-oesophageal fistula/oesophageal atresia, Renal anomalies (radial ray anomalies), limb anomalies (polydactily, oligodactily). At least three of the above above features (in each category) mis considered necessary for the diagnosis of this condition.


Pulmonary aplasia usually presents with neonatal respiratory distress of variable intensity. In rare cases, it may go unnoticed until later in childhood / adolescence.
Abnormal blood flow in the dorsal aortic arch during the 4th week of gestation had been hypothesized to cause pulmonary agenesis. The contra lateral lung may develop as much as twice more alveoli in response to pulmonary aplasia/agenesis.
On chest x-ray, it can present as an hemithorax white-out or ipsilateral lung volume loss with ipsilateral shift of mediastinal structures and contra lateral lung hyperinflation. A main ipsilateral bronchus is rarely seen, although CT-scan can demonstrate a rudimentary main bronchus.


CT-scan will confirm the absence of lung parenchyma and mediastinal ipsilateral shift. Also, there is an ipsilateral absence of pulmonary artery. It may also show other cardiac congenital malformationsand ipsilateralbronchus remnant.
The main differential diagnosis is pulmonary hypoplasia and complete lung atelectasis.
Pulmonary hypoplasia refers to deficient or incomplete development of parts of the lung. It can develop as a result  of a number of other in-utero anomalies.
The true prevalence of pulmonary hypoplasia is not well known (1,4% of all births according to Knox et.al), but in cases of premature rupture of membranes at 15-28 weeks gestation, the reported prevalence of pulmonary hypoplasia ranges from 9 to 28%.


Pulmonary agenesis means undeveloped pulmonary vessels, bronchi and parenchima. It can be unilateral or bilateral. In unilateral aplasia, the remaining lung contains twice as much alveoli as normal, but has normal bronchi. Although aplasia doesn't have the same structures, it has a rudimentary bronchus. As agenesis and aplasiaare difficult to distinguish from each other clinically and pathologically, they are often use in the  same sense. On the other hand, pulmonary hypoplasia is characterized by immature lung tissue or by lung tissue decreased at varying degrees. 

Pulmonary aplasia (agenesis) thought to result from the negative effects that occur on the 4th week of fetal life. Although its etiology is not fully understood. Its incidence in males and females and the occurence of the anomaly in the right or left lung are about the same. Hypoplasia and aplasia are often observed together with other malformations (diaphragma defects, kidney anomalies, extrapulmonary sequestration, muscle or skeleton system defects). Nearly one-third of the patients have congenital heart diseases. Although the most common one is  the atrial septal defect, vebtricular septal defect, patent ductus arteriosus or aorta coarctation can also be observed.  Aplasic patients are usually asymptomatic. Clinical findings change  with the presence of comorbid anomalies and their severity. Recurrent infections can increase the breathing distress. Although the patients with unilateral lung aplasia (agenesis) are believed to die  usually in the neonatal period, there are patients who live up to the adulthood, some of whom live without any symptoms. In aplasia, secretion not cleared from rudimentary bronchus and/or comorbid bronchiectasis  may partially be responsible for the risk of infection. Chronic respiratory failure and eventually respiratory infections can develop in adults.


Pulmonary hypoplasia is characterized by the presence of both bronchi (albeit rudimentary) and alveoly in an under-developed lobe. Both the size and the weight of the lung is reduced.

There are normally four stages of fetal lung development where the pathological process can occur at any level:
# embryonic: from conception - 5th week
# pseudoglandular: from 5th week - 17 week
# canalicular: from 16th week - 24 week (this stage may be more affected by          oligohydramnions)
# terminal sac or alveolar period: from 24th week to term and beyond

There are several key factors required for the adequate development of the lung. These are:
# sufficient amniotic fluid volumes
# adequate thoracic space-size
# normal breathing movement
# normal fluid within the lung

A deficiency in any of these could lead to pulmonary hypoplasia.
Most cases of pulmonary hypoplasia are secondary to other congenital anomalies or pregnancy complications. Some cases however can occur as primary event.
With secondary causes, it can result from factors directly or indirectly compromising the thoracic space available for lung growth.

RENAL AGENESIS

Congenital abnormalities of the kidney and urinary tract are frequently observed in children and represent a significant cause of morbidity and mortality. These condition are phenottypically variable, often affecting several segments of the urinary tract simultaneously, making clinical classification and diagnosis difficult. Renal agenesis/hypoplasia and dysplasia account for a significant portion of these anomalies, and a genetic contribution to its cause is being increasingly recognized. Nevertheles, overlap between diseases and challenges in clinical diagnois complicate studies attemping to discover new genes underlying this anomaly. Most of the insights in kidney development derive from studies in mouse models or from rare, syndromic forms of human developmental disorders of the kidney and urinary tract. The genes implicated have been shown to regulate the reciprocal induction between the ureteric bud and the metanepric mesenchyme. Strategies to find genes causing renal agenesis/hypoplasia and dysplasia vary depending on the characteristics of the study population available. The approaches range from candidate gene association or resequencing studies to traditional linkage studies, using outbred pedigrees or genetic isolates, to search for structural variation in the genome. Each of these stategies  has advantages and pitfalls and some have led to significant discoveries in human disease. However, renal agenesis/hypoplasia and dysplasia still reperesents a challenge, both for the clinicians who attempt a precise diagnosisand for the geneticist who tries to unravel the genetic basis, and a better classification requires molecular definition to be retrospectively improved. The goals to be feasible with the large multicentric collaborative groups that share the same objectives and resources.


Congenital abnormalities of the kidney and urinary tract are frequently observed in the first year of life, when they collectively represent a significant cause of morbidity and mortality. Data from birth defect registries Metropolitan Atlanta Congenital Defect Program and California Birth Defects Monitoring Program indicate an overall frequency from three to six per 1000 births, and the abnormalities impact life expectancy. Human urinary tract abnormalities are phenotypically variable and may affect several segments simultaneously, often aggregating to form complex phenotypes. Hence, clinical classification and diagnosis may be difficult. As a consequence of the overlap between anatomical defects, many investigators have opted to group renal and urologic malformations under the single label of Congenital Anomalies of the Kidney and Urinary Tract (CAKUT). This broad classification is supported by the fact that a mutation in single gene can have pleiotropic effects on the development of the urogenital tract. For example, mutation in the PAX2 gene cause the renal-coloboma syndrome, but the clinical features of the trait vary significantly between affected individuals, ranging from renal agenesis/hypoplasia to vesicoureteral reflux (VUR) and secondary obstruction. Conversely, mutation in different genes can result in similar renal phenotypes, e.g., EYA1 and PAX2 mutation both can cause the development of hypoplastic kidney. Hence, improved classification of urinary tract malformations may require understanding of primary molecular defects. A broad but clinically useful diagnostic scheme consistof classifying malformations depending on whether  the kidney, the collecting system, or both are affected.  This scheme stems from the fact that the upper tract (glomeruli and tubules) is derived from the metanephric mesenchyme (MM), and the lower urinary tract (collecting duct, renal pelvis, ureter)is derived from the ureteric bud. Even if this is in contrast with more recent data about the reciprocal interaction between the ureteric bud and metanephric mesenchyme, this classification can be clinically useful to partition patients with different types of urinary tract abnormalities. 


Bilateral renal agenesis is a rare and fatal event, usually associated with severe oligohydramnions, which produces a characteristic clinical pattern with facial compression and pulmonary hypoplasia (potter syndrome). An estimate of the incidenceof bilateral agenesisis 0.1/1000 births. Unilateral renal agenesis is more common, although the frequencyis difficult to estimate, as it is usually clinically silent and is commonly detected as a chance observation by autopsy or by prenatal ultrasound.

EMBRIOLOGY

Week 1
For this presentation embriogenic age will be expressed as days or weeks from fertilisation. On this basis human pregnancy lasts about 38 weeks. Obstetricians time pregnancy from the last day of the menstrual period on the assumption that fertilisation take place 2 weeks later. For obstetricians pregnancy last 40 weeks.

Week 2
Implantation
To implant developing embryo must pass through the uterine epithelium. This occurs about 7 days after fertilisation. The trophoblast  produces human chorionic gonadotrophin (hCG) which maintains the corpus luteum of the ovary which in turn produces progestesterone which maintain early pregnancy. hCG can be detected in the maternal blood and forms the basis of the early pregnany test. Detection in urine is less sensitive.

Week 3
Gastrulation
The inner cell mass in the 15-16 day human embryo consist of two layers - an upper epiblast and a lower hypoblast. Gastrulation coverts this bilaminar disc into three layers. An upper ectoderm, a middle mesoderm and a lower endoderm.
Cells from each germ layer have specific fates
Ectoderm

• Epidermis, hair, nails etc
• Brain and nervous system

Mesoderm

• Muscle, cardiovascular system, bones, blood, dermis, gonads, excretory system etc

Endoderm

• Inner lining of digestive tract
• Lining of lungs
• Glands

Week 4
Nervous system development in the human embryo

• At 18 days after conceptions the embryo consistsof 3 layers of cells endoderm, mesoderm and ectoderm. Thickening of the ectoderm leads to the development of the neural plate
• The neural groove begins to develop at 20 days
• At 22 days the neural groove has closed in the middle to form the neural tube. It remains open at either end
• The anterior opening or neuropore closes day 25-26 and the posterior neuropore day 27-28. A 27-day embryo should have a completely closed neural tube. The heart is visible and has already started to beat

Week 5-8
Organogenic Period
32-day human embryo
Organ system development is well underway. This is a critical time for development of the heart, limbs, eyes, upper lip, intestines.
44-day human embryo
At this stage the embryo has completed much of its organogenesis. Still to be completed is the palate and external genitalia and brain development is ongoing.
8-week human fetus
Organogenesis is largely complete, development of the external genitalia is still incomplete. The brain is about to start at 8-week period of massive neuronal cell formation for the cerebral cortex.

Week 9-38
Fetal Period

10-week human fetus
Fetal heart beat can be monitored at this stage. Chorionic villous sampling is usually performed between 8 and 10 weeks. A small piece (villous) of the placenta is removed and cultured in the laboratory. Genetics results are ussually available in 2 weeks.

13-week human fetus
The fetus is surrounded by about 100 ml of amniotic fluid. Amniocentesis can be performed from about 13 to 18 weeks gestation. About 10-20 ml of fluid is removed, the fetal cells are separated and grown in culture and genetic results available in about 2 weeks. An ∝-fetoprotein test can be performed on the maternal blood. This protein is made by the fetus and is in higher concentrations in fetuses with neural tube defects.

16-week human fetus
An ultrasound dating scan can be given at 5-11 weeks to confirm pregnancy, exclude ectopic or molar pregnancies, confirm cardiac pulsation and measure the crown-rump length for dating. An anomaly scan is usually performed at 16-18 weeks to look for congenital malformation. Sex of the fetus can usually be determined at this stage.

24-week human fetus
Head hair appears, the fetus already has a downy hair (lanugo), skin is coated with vernix-a waxy secretion of sebaceous glands.

38-week human fetus
Eyes reopened at about 26 weeks, at about 30 weeks skin becomes thicker and subcutaneous fat appears.

PREGNANCIES

Ectopic Pregnancies

• Implantation in a non-uterine site occurs at a rate of about 0.25-1%
• The ampulla of the uterine tube is the most common ectopic implantation site

In Vitro Fertilisation (IVF)

• The process by which one or more eggs (oocytes) are fertilised outside the body. Fertilisation is either achieved by placing a droplet of washed sperm (∼50.000) onto each egg or if the sperm count is low a single sperm can be injected into each egg (Intra Cytoplasmic Sperm Injection - ICSI)
• Embryos can be placed in to the uterus at the 6-8 cell stage (3-days culture) but some clinics culture the embryos for 5 or 6 days to ensure healthy blastocyst stage embryos are placed into the uterus
• All cells in the early embryo (until about the 8-cell stage) are said to be totipotent. That means each cell is capable of forming a complete human
• So couples using IVF can use genetic screening of their embryos by having a cell removed from their embryos and tested for its genotype. The embryo will still develop normally
• More than 100 disease can be detected including hemophilia A, muscular dystrophy, Tay-Sachs disease, cystic fibrosis and Down syndrome

Stem Cell Research

• Stem cells are unspecialized cells that can renew themselves for long periods through cell division. Under certain experimental conditions, they can be induced to become cells with special functions such as the beating  cells of the heart muscle or the insuline-producing cells of the pancreas
• Human embryonic stem cells are obtained from the inner cell mass-the early embryo
• In the USA it is forbiden to destroy a human embryo to obtain stemm cells. So they are not allowed to remove the inner cell mass in tutipotent cells. There is interest in posibility that at the 8-cell stage the cells are no longer tutipotent but are still pluripotent

Multiple Gestation

• About 1:80 human births is a twin pregnancy
• About 2/3 of twins are fraternal (Dizygotic) - derived from 2 eggs
• About 1/3 of twins are identical twins (monozygotic)
• 35% of mono zygotic twins divide between 2-8 cell stage, get two babies  with two amnion, two chorions and either one fused or two separateplacentas - can not tell difference between these twins and fraternal until genetic testing is done
• 65% of monozygotic twins occur by divisionof the inner cell mass after first week.  At this point two embryos will develop with two amniotic sacs, one chorionic sac and a common placenta
• IVF has greatly increased the number of fraternal (dizygotic) twins

Monozygotic Twins

• Separation at the two blastomere stage can lead to twinning
• Splitting of the inner cell mass is the most common cause of identical twins
• Incomplete separation of the inner cell mass can lead to conjoined twins