Life Begins at Fertilization with the Embryo's ConceptionLife Begins at FertilizationThe following references illustrate the fact that a new human embryo,the starting point for a human life, comes into existence with theformation of the one-celled zygote:"Development of the embryo begins at Stage 1 when a sperm fertilizes an oocyte and together they form a zygote."[England, Marjorie A. Life Before Birth. 2nd ed. England: Mosby-Wolfe, 1996, p.31]"Human development begins after the union of male and female gametes or germ cells during a process known as fertilization (conception)."Fertilization is a sequence of events that begins with the contact of a sperm (spermatozoon) with a secondary oocyte (ovum) and ends with the fusion of their pronuclei(the haploid nuclei of the sperm and ovum) and the mingling of theirchromosomes to form a new cell. This fertilized ovum, known as a zygote, is a large diploid cell that is the beginning, or primordium, of a human being."[Moore, Keith L. Essentials of Human Embryology. Toronto: B.C. Decker Inc, 1988, p.2]"Embryo: the developing organism from the time of fertilization untilsignificant differentiation has occurred, when the organism becomesknown as a fetus."[Cloning Human Beings. Report and Recommendations of the National Bioethics Advisory Commission. Rockville, MD: GPO, 1997, Appendix-2.]"Embryo: An organism in the earliest stage of development; in a man,from the time of conception to the end of the second month in theuterus."[Dox, Ida G. et al. The Harper Collins Illustrated Medical Dictionary. New York: Harper Perennial, 1993, p. 146]"Embryo:The early developing fertilized egg that is growing into anotherindividual of the species. In man the term 'embryo' is usuallyrestricted to the period of development from fertilization until theend of the eighth week of pregnancy."[Walters, William and Singer, Peter (eds.). Test-Tube Babies. Melbourne: Oxford University Press, 1982, p. 160]"The development of a human being begins with fertilization, a process by which two highly specialized cells, the spermatozoon from the male and the oocyte from the female, unite to give rise to a new organism, the zygote."[Langman, Jan. Medical Embryology. 3rd edition. Baltimore: Williams and Wilkins, 1975, p. 3]"Embryo: The developing individual between the union of the germ cellsand the completion of the organs which characterize its body when itbecomes a separate organism.... At the moment the sperm cell of thehuman male meets the ovum of the female and the union results in afertilized ovum (zygote), a new life has begun.... The term embryocovers the several stages of early development from conception to theninth or tenth week of life."[Considine, Douglas (ed.). Van Nostrand's Scientific Encyclopedia. 5th edition. New York: Van Nostrand Reinhold Company, 1976, p. 943]"I would say that among most scientists, the word 'embryo' includes the time from after fertilization..."[Dr. John Eppig, Senior Staff Scientist, Jackson Laboratory (BarHarbor, Maine) and Member of the NIH Human Embryo Research Panel --Panel Transcript, February 2, 1994, p. 31]"The development of a human begins with fertilization, a process by which the spermatozoon from the male and the oocyte from the female unite to give rise to a new organism, the zygote."[Sadler, T.W. Langman's Medical Embryology. 7th edition. Baltimore: Williams & Wilkins 1995, p. 3]"The question came up of what is an embryo, when does an embryo exist,when does it occur. I think, as you know, that in development, life isa continuum.... But I think one of the useful definitions that has comeout, especially from Germany, has been the stage at which these twonuclei [from sperm and egg] come together and the membranes between thetwo break down."[Jonathan Van Blerkom of University of Colorado, expert witness onhuman embryology before the NIH Human Embryo Research Panel -- PanelTranscript, February 2, 1994, p. 63]"Zygote. This cell, formed by the union of an ovum and a sperm (Gr. zyg tos, yoked together), represents the beginning of a human being. The common expression 'fertilized ovum' refers to the zygote."[Moore, Keith L. and Persaud, T.V.N. Before We Are Born: Essentials of Embryology and Birth Defects. 4th edition. Philadelphia: W.B. Saunders Company, 1993, p. 1]"The chromosomes of the oocyte and sperm are...respectively enclosed within female and male pronuclei. These pronuclei fuse with each other to produce the single, diploid, 2N nucleus of the fertilized zygote. This moment of zygote formation may be taken as the beginning or zero time point of embryonic development."[Larsen, William J. Human Embryology. 2nd edition. New York: Churchill Livingstone, 1997, p. 17]"Although life is a continuous process, fertilization is a criticallandmark because, under ordinary circumstances, a new, geneticallydistinct human organism is thereby formed.... The combination of 23chromosomes present in each pronucleus results in 46 chromosomes in thezygote. Thus the diploid number is restored and the embryonic genome is formed. The embryo now exists as a genetic unity."[O'Rahilly, Ronan and Mller, Fabiola. Human Embryology & Teratology.2nd edition. New York: Wiley-Liss, 1996, pp. 8, 29. This textbook lists"pre-embryo" among "discarded and replaced terms" in modern embryology,describing it as "ill-defined and inaccurate" (p. 12}]"Almost all higher animals start their lives from a single cell, thefertilized ovum (zygote)... The time of fertilization represents thestarting point in the life history, or ontogeny, of the individual."[Carlson, Bruce M. Patten's Foundations of Embryology. 6th edition. New York: McGraw-Hill, 1996, p. 3]"[A]nimal biologists use the term embryoto describe the single cell stage, the two-cell stage, and allsubsequent stages up until a time when recognizable humanlike limbs andfacial features begin to appear between six to eight weeks afterfertilization...."[A] number of specialists working in the field of human reproduction have suggested that we stop using the word embryoto describe the developing entity that exists for the first two weeksafter fertilization. In its place, they proposed the term pre-embryo...."I'll let you in on a secret. The term pre-embryo has been embracedwholeheartedly by IVF practitioners for reasons that are political, notscientific. The new term is used to provide the illusion that there issomething profoundly different between what we nonmedical biologistsstill call a six-day-old embryo and what we and everyone else call asixteen-day-old embryo."Theterm pre-embryo is useful in the political arena -- where decisions aremade about whether to allow early embryo (now called pre-embryo)experimentation -- as well as in the confines of a doctor's office,where it can be used to allay moral concerns that might be expressed byIVF patients. 'Don't worry,' a doctor might say, 'it's only pre-embryosthat we're manipulating or freezing. They won't turn into real humanembryos until after we've put them back into your body.'"[Silver, Lee M. Remaking Eden: Cloning and Beyond in a Brave New World. New York: Avon Books, 1997, p. 39]

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Life Begins at Fertilization with the Embryo's Conception

Lung and Diaphragm Development

Duke LEARNING RESOURCES EB3: Lung and Diaphragm

Click here to launch the Simbryo Lung Development animation (and some really trippy music -you'll understand once the window opens...)

I. Development of the Respiratory Tract

A. Early development

Disruption of the mesoderm, retinoic acid signaling, or TBX4 expression in the endoderm will interfere with this process and can cause defects in lung/trachea development.

Disruption of the formation of the tracheo-esophageal ridges can result in tracheo-esophageal fistulas. This is very often associated with a spectrum of mesodermal defects called the VATER association (Vertebral anomalies, Anal atresia, Tracheoesophageal fistula, Esophageal atresia, and Renal atresia), or, if Cardiac defects and Limb defects are also present, VACTERL.

Tracheoesophageal fistulas occur in about 1/3000 births and most are of the sort where the proximal esophagus ends blindly whereas the distal esophagus communicates with the trachea via a fistula. Complications arise both prenatally and postnatally:

An extreme example is tracheal atresia where the trachea fails to form entirely and the lungs bud directly from the esophagus.

B. Development of the larynx

The process of recanalization can be disrupted resulting in laryngeal atresia (occlusion of the laryngeal lumen, also known as CHAOS, or Congenital High Airway Obstruction Syndrome) or laryngeal web (partial occlusion via a membranous web over the vocal cords). Either of these can be repaired surgically. However, the effects of laryngeal atresia are much more severe: air is trapped in the lungs causing dilation of the lower airways.

C. Development of the trachea

D. Segmental branching and development of the bronchial tree

Branching morphogenesis is MESODERM and RETINOIC ACID-DEPENDENT (along with several other genetic factors such as TBX4 and FGF10, for example). Early disruption of segmental branching can cause the loss, or agenesis, of entire bronchopulmonary segments, lobes, or even an entire lung. Congenital lung cysts arise if the disruption is later in development such that the terminal bronchioles within a small portion of the lung are abnormally dilated. These dilated pockets appear as empty "cysts" in a chest x-ray.

E. Development of the lungs

Because of the fewer number of mature alveoli, the lungs of a newborn are much denser than those of an adult when viewed on a chest x-ray.

F. Surfactant production

Surfactant Protein A plays a role in eliciting uterine contractions by activating as a pro-inflammatory agent on macrophages present in the amniotic fluid. These activated macrophages invade the uterine wall and begin releasing Interleukin-1, which ultimately leads to localized prostaglandin production that stimulates the uterine smooth muscle to contract.

II. Growth of lungs into the body cavity and development of the diaphragm

A. Separation of the pleural and pericardial cavities

B. Separation of the abdominal and thoracic cavities

Closure of the pericardioperitoneal canals is a complex process and disruptions are a frequent cause of congenital diaphragmatic hernias (CDH), in which abdominal contents herniate or protrude into the pleural cavity. The most common site of herniation is at the aortic or esophageal hiatus, but the overall effects are minor since the size of the defect is small. CDH rarely occurs on the right side since the liver is in the way. However, failure of the pericardioperitoneal canal to close on the left can lead to a large defect allowing the intestines to herniate into the left pleural cavity and interfere with development of the left lung, in some cases causing complete agenesis of the left lung.

Questions 1 and 2 refer to the following case: A 35 year-old woman delivers an infant at 40 weeks of gestation (based on the last time of menstruation). While in the neonatal care unit, the infant develops cyanosis and very rapid labored breathing and requires admission to the neonatal intensive care unit. Imaging studies of the thoracic cavity show congestion in the lungs but they appear to be of normal size and there is no apparent abnormality in the diaphragm. The woman reports no family history of lung disease and denies alcohol use, smoking, or taking medications during her pregnancy, and review of the mothers medical records regarding prenatal care and ultrasound imaging is unremarkable.

1. A biopsy of the infant's lung tissue would most likely show:

ANSWER

2.A possible cause of the infant's condition is:

ANSWER

3. The period of lung development in which NO respiratory bronchioles or alveoli have yet formed is known as the:

ANSWER

4. The period of lung development in which surfactant production begins (but is not necessarily sufficient to prevent airway collapse) is known as the:

ANSWER

5. The skeletal muscle of the diaphragm is derived primarily from:

ANSWER

6. The smooth muscle in the wall of the respiratory tract is derived from:

ANSWER

7. Congenital diaphragmatic hernias:

ANSWER

For items 8 10 , select the one lettered option from the following list that is most closely associated with each numbered item below. Options in the list may be used once, more than once, or not at all. a. alveolar stage b. canalicular stage c. terminal sac stage d. pseudoglandular stage 8. stage in lung development at which alveoli have not formed and survival is NOT possible ANSWER

9. premature infants born at this stage have a relatively good prognosis although they will require respiratory support and treatment with exogenous surfactant ANSWER

10. stage in lung development at which there is the most surfactant production ANSWER

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Updated 10/11/11 - Velkey

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Duke Embryology - Lung and Diaphragm

In 2000, the ESHRE Special Interest Group in Embryology (SIG-E) published as a supplement to Human Reproduction the Atlas of Embryology, a long waited reference resource that was extensively used by embryologists in the following many years. More recently in 2012, the same SIG produced an electronic and updated edition of the Atlas. Not only did the new Atlas respond to demands of novelty of contents, but it also met the criteria of accessibility and practicality offered by the PDF format. Now, in 2016, the SIG-E releases a web version of the Atlas of Embryology, accessible from PC, tablets and smartphones. Although not authentically digital native, nevertheless as a generation of embryologists we have at our disposal a multitude of formidable information and communication technologies tools. The development of a web Atlas was therefore inescapable.

Compared with the 2012 publication, the reader will find no novelty in the contents of the web Atlas in its initial version. This does not reflect lack of sensitivity of the web editors for the need of updated information. Rather, it ideally represents the start line for a new development phase. In fact, thanks to the versatility given by its web design, the new Atlas is amenable to future improvement and continued expansion with new sections.

Opportunities are innumerable. Time-lapse microscopy, cryopreservation, micromanipulation, ultrastructure and cytoskeleton are only examples of possible novel contents. The web Atlas therefore has the potential to become a continuously evolving entity. To this end, the contribution of ESHRE members will be crucial, in an era in which creation, sharing, and exchange of information through web-mediated platforms have an increasingly important role in the production of knowledge.

Giovanni Coticchio Co-ordinator SIG Embryology

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Index - Eshre

Between the 3rd month of pregnancy and its end the testes become transferred from the lumbar area (ventro-medial to the mesonephros) into the future scrotum. This transfer is due to a combination of growth processes and hormonal influences (7). The gubernaculum testis also plays a decisive role in this phenomenon.

The gubernaculum testis arises in the course of the 7th week from the lower gubernaculum, after the mesonephros has atrophied. Cranially it has its origin at the testis and inserts in the region of the genital swelling (future scrotum).At the same time, at the inguinal canal along the lower gubernaculum, an evagination of the peritoneum arises, the vaginal process, on which the testes will slide through the inguinal canal.

Fig. 20 The yellow arrow shows the location of the protrusion of the peritoneum and the beginning of the testicular descent into the inguinal canal.

Fig. 21In this diagram, the beginning of the formation of the vaginal process is visible. It enters with the testis into the inguinal canal. Shown in blue is the gubernaculum that becomes increasingly shorter.

The muscle fascia of the transverse muscle is the innermost layer and in the scrotal region, it forms the internal spermatic fascia of the spermatic cord and the scrotum.

The muscle layer of the musculus cremaster is formed from fibers of the oblique internal and transverse muscles.

Externally, the external spermatic fascia is formed from the superficial aponeurosis of the oblique external abdominal muscle.

7

8

910

Fig. 24Detail of the various layers that have formed in the scrotum by the end of the pregnancy.

The region, where the testes pass through the abdominal wall, is called the inguinal canal.

Between the 7th and the 12th week the gubernaculum shortens and pulls the testes, the deferent duct and its vessels downwards. Between the 3rd and 7th month the testes stay in the area of the inguinal canal so they can enter into it. They reach the scrotum at roughly the time of birth under the influence of the androgen hormone.

While in the first year of life the upper part of the vaginal process becomes obliterated, there remains only the peritoneo-vaginal ligament. The lower portion persists as the tunica vaginalis testis, which consists of a parietal and a visceral layer.

The migration anomalies of the testes will be treated in the pathology chapter.

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Descent of the testes - Embryology

Suggested readings from Langman's Medical Embryology (13th. ed.): Ch 15, pp. 225-249 Suggested readings from Langman's Medical Embryology (12th. ed.): Ch 15, pp. 208-231 Suggested readings from Langman's Medical Embryology (11th. ed.): Ch. 14, pp. 209-233

Duke LEARNING RESOURCES EB4: Gut Development Session

Click here to launch the Simbryo GI Development animation (and some really trippy music -you'll understand once the window opens...)

I. Overview

A. Formation of the primitive gut tube

B. Basic subdivisions of the gut tube

FOREGUT

MIDGUT

HINDGUT

C. Definitive subdivisions of the gut tube

D. Cranio-caudal patterning of the gut tube

E. Radial patterning of the gut tube

This occlusion and re-canalization process occurs THROUGHOUT the tube (esophagus to anus) and errors in this process can occur in anywhere along the tube resulting in stenosis (narrowing of the lumen or even outright occlusion) in that region.

F. Mesenteries of the gut tube (refer to the figure on the previous page)

A summary of what is retroperitoneal, intraperitoneal, or secondarily retroperitoneal in the adult:

II. Derivatives of the foregut:

A. Esophagus

Clinical considerations

B. Stomach

Clinical Considerations

C. Liver

D. Pancreas

Errors in the fusion process can result in an annular pancreas that wraps around the duodenum, which can cause obstruction the symptoms of which would be similar to pyloric stenosis except that the vomit may be bilious and there would NOT be a palpable knot in the epigastric region.

E. Proximal or upper duodenum

III. Derviatives of the midgut

A. Distal or lower duodenum

Failure to recanalize the duodenum can result in stenosis (narrowing) or atresia (complete blockage), the symptoms of which would be bilious projectile vomiting an hour or so after feeding.

B. Jejunum, ileum, cecum, appendix, ascending colon, and proximal 2/3 of transverse colon

Failure to obliterate the vitelline duct can result in diverticula (out pouching of the gut tube) called Meckel's diverticula,vitelline cysts or vitelline fistulas (a connection of the small intestine to the skin). These will often be attached at one end to the umbilicus and at the other end to the ileum.

Failure to pull all of the gut contents back into the abdominal cavity or to completely close off the ventral body wall at the umbilicus can result in an oomphalocoele, where the gut contents herniate out of the body wall.

Defects and variations in rotation can cause a variety of aberrant anatomical positions of the viscera that are often asymptomatic, but important to appreciate when trying to diagnose and/or treat gastrointestinal problems (e.g. abnormal positioning of the appendix due to malrotation should be considered when trying to diagnose appendicitis). Malrotation can also cause twisting or volvulus of the gut tube resulting in stenosis and/or ischemia.

III. Derivatives of the hindgut

Failure of the cloacal membrane to break down can result in an imperforate anus.

Failure to generate enough mesoderm during gastrulation can result in anal atresia in which there is insufficient development of the wall (namely the smooth muscle and connective tissue) of the rectoanal canal Failures in the division of the cloaca (usually accompanied by anal atresia) can lead to a variety of aberrant connections of the rectal canal to portions of the urogenital tract.

Failure of neural crest cells to migrate and/or differentiate into neurons in a portion of gut will result in an aganglionic segment (missing submucosal and myenteric ganglia). The main function of these ganglia is to allow local relaxation in the wall of the gut tube, so the aganglionic segment is tonically contracted, leading to obstruction. For a variety of reasons, the distal portions of the colon are most susceptible to this problem, leading to a condition known as Hirschsprung disease or congenital megacolon. The affected individuals often present with a very distended abdomen due to the presence of an aganglionic segment of colon (usually in the sigmoid colon) that causes a blockage and then backup of feces (and massive enlargement) in the descending colon.

Practice Questions

1. Which of the following is NOT derived at least in part from the midgut?

ANSWER

3. During development of the gut:

ANSWER

5. Meckel's diverticula, vitelline cysts, or vitelline fistulas are most commonly found in association with:

ANSWER

6. During development of the gut:

ANSWER

7. The greater omentum is derived from the:

ANSWER

Questions 8 and 9 refer to the following case: A one-week-old male infant is brought in by his parents who report bilious projectile vomiting about 2 hours after each feeding. The child has not gained much weight since birth and the parents comment that the child's diapers are not particularly soiled or when they are changed. On physical exam the child is lethargic and exhibits signs of dehydration. The heart and breathing rates are somewhat elevated, but otherwise the heart and lungs appear normal. On physical exam, the abdomen is unremarkable

8. Which of the following conditions best accounts for the infant's signs and symptoms?

ANSWER

9. The most likely cause of the infant's condition is:

ANSWER

For items 10 12 below, select the one lettered option from the following list that is most closely associated with each numbered item below. Options in the list may be used once, more than once, or not at all. a. ventral mesentery of the liver b. dorsal mesentery of liver / ventral mesentery of stomach c. dorsal mesentery of stomach e. vitelline duct f. allantois

10. urachal cyst ANSWER

11. falciform ligament ANSWER

12. lesser omentum ANSWER

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Updated 10/13/15 - Velkey

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Duke Embryology - Gut Development