Introduction to Human Embryology

8/12/2019 Introduction to Human Embryology

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INTRODUCTION TO HUMAN EMBRYOLOGY

A.HENRY SATHANANTHAN

Monash Institute of Repro u!tion " De#e$op%ent& Monash Uni#ersit'&Me$(ourne

henr'.sathananthan)%e .%onash.e u.au

OB*ECTI+ES O, THIS CHA-TER

To intro u!e 'ou to /

1. The first 8 weeks of human development the embryonic period.

2. The processes of fertilization, cleavage, germ layer formation and primary

organogenesis.

. !ome clinical aspects of these processes.

". #hysiology of the embryo placentation.

$. %ritical periods of development and some congenital malformations.
http://www.sathembryoart.com/index.htm


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&. 'asic te(ts, atlases, slides and %)*+ -s, since embryology is a visualsub ect.

INTRODUCTION

/mbryology deals with the study of the human embryo during the first 8 weeks of

development. 0t is important to know the events that occur in the beginnings of life,

when all the blueprints of the human body are laid down in the embryo. The

embryonic period is followed by the foetal period months * 3 when there is

e(tensive growth and differentiation and when the embryo ac4uires a more human

form see separate chapter3. )evelopment is a long comple( process which

transforms a single cell fertilized ovum3 into a comple( multicellular organism.

The development of the embryo and foetus is the prenatal period of development.

0A ti%eta($e of prenata$ e#e$op%ent is i$$ustrate in Moore& 12324

)evelopment is a comple( process which transforms a single cell into a comple(multicellular organism.

EMBRYONIC -ERIOD

/mbryonic development begins at fertilization day 13 followed by cleavage of the

embryo and its


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implantation in the uterus week 13. This is followed by the differentiation of the

primary germ layers week 3 and culminates in primary organogenesis when the

tissues and ma or organ rudiments of the embryonic body are formed weeks "*83.

The embryonic period may be temporally considered in " phases5*

First Week: 6ertilization, cleavage and onset of implantation pre*implantation phase3.

Second Week: 6ormation of bilaminar embryo, deep implantation and

establishment of uteroplacental circulation.

Third Week: 6ormation of trilaminar embryo gastrulation3, somites, chorionic

villi and neurulation.

Fourth to Eighth Weeks: 6ormation of the embryonic body, primary

organogenesis and placentation.

The physiology of the embryo foetal membranes and placenta3 and some

congenital malformations during critical periods of development will also be dealt

with in this chapter.

THE ,IRST 5EE6 Ga%etes& ,erti$i7ation& C$ea#a8e an I%p$antation


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The pre*implantation phase of development begins with fertilization and

culminates in the implantation of the blastocyst in the uterine endometrium. ur

knowledge of the first week of development has advanced immensely in the past

27 years, since the advent of in vitro fertilization 0 63 and assisted reproductive

technologies 9+T3. 0See our at$as of Ear$' Hu%an De#e$op%ent for ART 9

Sathananthan et a$ 122:;122< for i$$ustrations.4

a) Gamete structure: sperm and egg

'riefly, the spermatozoon is a highly specialized torpedo*shaped cell : 7 mlong3 for motility and, of course, for fertilization. 0t is composed of a head,midpiece body3 and a tail flagellum3. The head carries the nucleus with the

paternal chromosomes and is capped by a modified lysosome acrosome3 covered by the cell membrane. The midpiece consists of the base of the a(onemeoriginating from the sperm*neck, where is located the functional paternal centriolehidden in a ;black bo( densefibres and a fibrous sheath around the a(oneme.

The spermatozoon is a highly specialized cell for motility and fertilization.

The mature egg or oocyte is a large unspecialized cell :127 m in diameter3compared to the spermatozoon. 0t has no nucleus but has a maturation spindlearrested at metaphase 00 of meiosis. The spindle is barrel*shaped and aligned atright angles to the oocyte surface, where the first polar body is located within the# !. The egg vestments consist of a gelatinous zona pellucida shell3 surrounded

by several layers of follicle cells that make up the cumulus oophorus. The cumulus


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is e(panded and gelatinous in mature oocytes. The egg cell has most of thecomponents of somatic cells but lacks rough endoplasmic reticulism. 0t has corticalgranules beneath the cell membrane, which play a role in fertilization.

0See our at$ases Sathananthan et a$& 122:; 122< pu($ishe (' the epart%entfor etai$s of 8a%ete stru!ture an fun!tion& an CD/ROM& Sathananthan "E =ar s& 122>4

The oocyte is a large unspecialized cell compared to the spermatozoon

b) Gamete transport

-ature sperm several millions3 are e aculated into the vagina after intercourse,enter the uterus through the cervi( and swim up to the far end of the oviduct

ampulla3 in about $ minutes, where fertilization occurs. f the millions e aculated,only a few hundred reach the ampulla. These are the most motile in the e aculate.!perm capacitation a physiological process3 occurs somewhere in the reproductivetract, where sperm ac4uire the ability to penetrate the vestments of the oocyte. 0nvitro, it takes place during the sperm preparation procedure washing and layering3for 9+T. nce at the surface of the egg, it has to penetrate the cumulus oophorus

follicle cells around the egg3 and the gelatinous shell zona pellucida3 beforegamete fusion occurs. #enetration is aided by sperm motility and the acrosomereaction 9+3, which is the morphological e(pression of capacitation. )uring the9+ the surface membranes of the acrosome vesiculate releasing 2 importantenzymes hyaluronidase and acrosin protease3, which enable sperm to penetratethe cumulus and zona, respectively, and reach the perivitelline space # !3surrounding the egg.

!perm penetration is aided by sperm motility and the acrosome reaction

The mature egg oocyte3 is ovulated around day 1" of the natural menstrual cycle0See Moore& ?1232@; Larsen& ?1223@4, released from the ovary into the peritonealcavity and immediately sucked up through the infundibulum of the 6allopian tube

oviduct3 by its ciliary action into the ampulla. ?ere fertilization occurs anddevelopment begins. ocytes remain viable for up to 12 hours after ovulation,while sperm can survive up to 2" hours in vivo. !perm, however can be kept alive


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in vitro up to 2* days and still retain their fertilizing capacity, while eggs too cansurvive for about 27*2" hours, in vitro.

6ertilization occurs in the ampulla of the oviduct soon after ovulation

c) Fertilization

This involves the spontaneous fusion of two germ cells sperm and egg to forma zygote. The zygote or fertilized egg is the first cell of the new baby. 6ertilizationessentially restores the diploid number 2n @ "&3 of chromosomes of somatic

body3 cells bringing the fatherAs and motherAs genomes together.

The mature sperm cell is formed in the testis male gonad3 by a process of meioticmaturation called spermatogenesis, while the mature oocyte is formed likewise inthe ovary female gonad3 during oogenesis. /ach gamete has a haploid number ofchromosomes n @ 2 3, having undergone meiosis reduction division3 duringgametogenesis formation of gametes3. The zygote has the diploid number ofchromosomes 2n @ "&3. 'iparental inheritance of chromosomes leads to speciesvariation, since there is independent assortment of paternal and maternalchromosomes among the germ cells during meiosis. !e( is also determined at

fertilization.

6ertilization involves the spontaneous fusion of two germ cells sperm and egg to form a zygote

d) Mechanics of fertilization

The ma or events that take place simultaneously or soon after fertilization aresummarized as follows5

i3 !perm oocyte fusion5 the midsegment of the sperm membranefuses with that of the egg followed by sperm incorporation into theegg to form a male pronucleus -#B3.


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ii3 !oon after gamete fusion there is a sperm*induced calcium wave inhuman oocytes.

iii3 The egg completes its second maturation division, abstricting thesecond polar body and a female pronucleus 6#B3 is formed withinthe ooplasm. The egg has now completed meiosis and the 6#B ishaploid.

iv3 !perm fusion triggers the cortical reaction which involves thee(ocytosis or release of cortical granules into the # !.

v3 %ontents of released cortical granules interact with the zona zonareaction * which chemically hardens the zona to prevent polyspermy.

nly one sperm needs to fertilize the egg.

vi3 9 sperm aster is formed and the male and female pronuclei migrateto the centre of the egg, where they associate but do not fuse. 9partfrom the fatherAs chromosomes, the sperm introduces the centriole,which is the active division centre within the embryo during mitosis

cleavage3.

Thus fertilization is a comple( process, which essentially involves activation of theegg to develop further into an embryo and foetus.

0,or ia8ra%s an i$$ustrations see at$ases& Sathnanthan et a$& ?122:; 122


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centrosome3 introduced at fertilization replicates and first forms a sperm aster,which then splits to form a bipolar spindle about 27 hours after fertilization. Themale and female pronuclei, formed :12 hours after fertilization, dissociate theirenvelopes and the paternal and maternal chromosomes come together and organizethemselves on this spindle, a stage called ;syngamy


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outside this region, e.g., in the tube or cervi( this will be dealt with in anotherchapter.

E,or ia8ra%s of !$ea#a8e/sta8e e%(r'os see at$as/ Sathananthan et a$ ?122:@;Moore ?1232@; Lan8%an ?123 @4

9fter implantation the trophoblast produces human chorionic gonadotrophin h%C3which can be detected in the motherAs urine

CLINICAL A--LICATIONS TREATMENT O, IN,ERTILITY

The greatest advances in assisted reproduction stem from the recent developmentof 0 6 and 9+T technologies which has led to a better understanding of humandevelopment in the first week. This was a grey area before the advent of thesetechni4ues. %onse4uently, we have now an indepth understanding of the events offertilization, cleavage and abnormalities, thereof. 9part from 0 6, the singletechni4ue that has revolutionized the treatment of infertility is intracytoplasmicsperm in ection 0%!03. This successful techni4ue of assisted fertilization violatesmost of the norms of fertilization since the sperm is in ected directly into theoocyte, bypassing its vestments and pre*empting natural sperm*egg fusion. Fe

have to wait and see the long term effects of this technology. This will be dealtwith at length in another chapter on subfertility.

9nother e(citing finding is the paternal inheritance of the centrosome, whichregulates early cleavage in the human embryo 0see at$as& Sathananthan et a$&122


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implantation and, of course, the techni4ues that have evolved in gamete recovery,such as ultrasound, embryo transfer and embryo biopsy after assisted reproduction.The amazing pictures that have been generated of gametes, fertilization, embryos,and the reproductive tract in recent years would not have been possible without9+T 0see At$as& Sathananthan et a$& 122


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The Amnion 5 The amnion appears on day 8 on the roof of the embryonic disc. 6luidaccumulates and a thin epithelium is formed from the cytotrophoblast. The epiblastforms the floor of the amniotic cavity. This is the beginning of the water bag. Theamniotic cavity enlarges and by week 8 encloses the entire embryo.

The %horion5 The chorion is formed after the appearance of the e(traembryonicmesoderm days 17H113. These cells arise from the epiblast and migrate to formtwo layers the inner lining the ?euserAs membrane secondary yolk sac3 and theother the inner surface of the cytotrophoblast which becomes the chorion. Thecavity formed is the e(traembryonic coelom or chorionic cavity days 12H1 3. Thechorionic cavity enlarges and the embryonic disc with its dorsal amniotic sac andventral yolk sac is suspended in the chorionic cavity by a connecting stalk ofmesoderm .

0,or ia8ra%s see Moore ?1232@; Lan8%an ?123 @; Larsen ?1223@4

c) $ to% and s nc tiotrophoblasts &deep implantation)

The trophoblast has already differentiated into a cellular cytotrophoblast*%T3 andsyncytium

syncytiotrophoblast*!T3 at the embryonic pole of the blastocyst. The !T isinvasive and helps the embryo implant in the endometrium. )eeper implantation

days >* 3 involves e(tensive growth of the conceptus and its complete

implantation within the endometrium, leaving a scar or blood clot closing plug3.?ydrolytic enzymes secreted by the trophoblast digest the e(tracellular matri(

between the endometrial cells and processes of the trophoblast penetrate deep intothe endometrium. The !T grows all around the conceptus and lacunae or cavitiesappear within it day 3. !oon maternal blood capillaries anastomose and invadethe lacunae establishing a primitive uteroplacental circulation. /(tensions of the%T e(tend and grow into the overlying !T establishing the primary chorionic villi

beginnings of the placenta3.

9 primitive uteroplacental circulation is established in week 2 not to be confusedwith the placenta

d) $linical !pplications


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Spontaneous Abortions 5 These are 4uite common during early development. 9nabortion is defined as a termination of pregnancy before 27 weeks of gestation.9lmost all abortions that occur in the first weeks are spontaneous not induced3.0t is estimated that $7I of all such abortions are caused by chromosomalabnormalities. 0t is also reported that 7*$7I of zygotes never become blastocystsand implant.

Hydatiform Mole 5 6ew of the pregnancies result in hydatiform moles in which theembryo is entirely missing and only a placenta is present. #rimary chorionic villiare present without embryonic vessels detectable by ultrasound. %omplete moleshave "& chromosomes, all of paternal origin because zygotes have two male

pronuclei. #artial moles are formed from triploid dispermic zygotes. 'oth types ofmoles tend to abort spontaneously or are surgically removed. #ersistent molartissue may result in trophoblastic disease.

THE THIRD 5EE6 Tri$a%inar e%(r'o 9 for%ation of 8er% $a'ers .

The most dramatic event in early development is the formation of the primarygerm layers5 ectoderm, endoderm and mesoderm referred to as gastrulation inanimal terms. 0t is from these germ layers that all the tissues and organs of thehuman body are formed. 'asically, ectoderm forms the skin and nervous system

outside3, endoderm the gut and associated glands including the respiratory system

inside3, while mesoderm forms all the other organ systems in between @.

0,or etai$s of eri#ati#es of 8er% $a'ers see Moore ?1232@4

The most dramatic event in early development is the formation of ectoderm,endoderm and mesoderm, the three germ layers referred to as gastrulation

'rimiti"e streak: Mesoderm formation

The embryonic disc now becomes pear*shaped and develops a linear primitivestreak, dorsally in the epiblast, which is a heap of cells that proliferate and migrateto the centre line. 0t appears first at the caudal end at the beginning of week andgrows towards the centre of the disc. The primitive streak day 1&3 signals the


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formation and separation of mesoderm from the ectoderm overlying it. 0t may belikened to a permanently closed keyhole, where cells migrate inwards, sidewaysand forwards to form the mesoderm sandwiched between the ectoderm and theendoderm hypoblast3 which was formed in week 2. This is best appreciated intransverse sections of the disc. E E !e$$ent :/D i%a8es of the pro!ess& as =e$$ asse!tions& are presente in Lan8%an ?1231@; Moore ?1232@; Larsen ?122:@. The

process of mesoderm formation is called ;immigration the notochordal mesoderminduces the overlying ectoderm to form the neural plate. This is a good e(ample ofchemical induction where one tissue induces the formation of another.


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(eural tube formation

9t first the neural plate is oval but later elongates over the underlying notochordalong the whole a(is

of the embryo. The plate invaginates towards the notochord to form a neuralgroove, which deepens progressively to form a tube by fusion of the lateral neuralfolds. This nerve tube is hollow and is lined by pseudostratified columnarepithelium. The cells of the neural crest separate from the tube to develop into thespinal and autonomic ganglia and pigment cells, later. The nerve tube is formed ondays 1 *21, and its closure begins in the middle of the embryo and progressestowards cranial and caudal ends by the end of week ". The anterior end swells to

become the brain and the rest forms the spinal cord. The neural tube is opencranially and caudally, forming the neuropores, which close in week ".

The neural tube forms the central nervous system brain and spinal cord.

e"elopment of somites

!omites are blocks of para(ial mesoderm which appear in pairs on either side of

the notochord. The first somite appears on day 27 behind the base of the futureskull. This is the first sign of segmentation in the embryo. !ubse4uent somitesform behind the first progressively till "2*"" pairs are formed by week "H$. Thenumber of somites are used to determine the age of the embryo. The somites giverise to most of the a(ial skeleton, associated musculature and dermis of the skin.The intraembryonic coelom body cavity3 is formed in mesoderm lateral to thesomites.

!omites are blocks of mesoderm used to determine embryonic age

Formation of heart and blood "essels


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The rudiments of the heart and blood vessels are also laid down in week . 'loodvessels are formed in the mesoderm of the yolk sac and chorion as spaces withinmesenchyme cells blood islands3. They are soon lined with endothelium, and unitewith other vessels to form a primitive cardiovascular system. The heart is formed atthe end of week in much the same way as enlarged blood vessels in the cranialregion. #aired heart tubes are formed which begin to fuse to form a primitive heart,which connects up with blood vessels in the embryo, chorion and yolk sac. Theheart begins to beat and the vascular system is the first to become functional.

0A ia8ra% of the pri%iti#e #as!u$ar s'ste% is sho=n in Moore& 12324

!llantois

The fourth foetal membrane appears in week as a diverticulum of the yolk sac

caudal wall3. 0t remains small and is also involved in angiogenesis formation of blood vessels3 and is later associated with the development of the urinary bladder.

$horionic "illi

The primary chorionic villi have a core of connective tissue and eventually develop blood capillaries and become secondary and tertiary villi and cover the entiresurface of the chorion see placenta3. Their vessels also connect up with vesselsinside the embryo. Butrients and other substances are e(changed between maternaland foetal circulations.

'lood vessels are formed in the mesoderm of the yolk sac, chorion and allantois.

$ongenital malformations

)isturbances in neurulation cause some abnormalities of the brain and spinal cord.

6ailure of

closure of the neural tube in the caudal region caudal neuropore3 results in spina bifida. These defects also involve the tissues overlying the spinal cord meninges,vertebral arches, dorsal musculature and skin3. The most fre4uent site of failure ofneurulation is the cranial neuropore resulting in anencephaly.


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5EE6S 9 3 CRITICAL -ERIODS O, DE+ELO-MENT ,or%ation of a$$ %a or or8an ru i%ents an the e%(r'oni! (o '

The last $ weeks of embryonic development are very critical since all the mainorgan systems are laid down, both e(ternally and internally, and the embryo takesshape and finally takes the characteristic human form by week 8. 0t is a critical

period since ma or developmental disturbances that occur now could result inma or congenital malformations in each system of the human body. )uring this

period the embryo is susceptible to teratogens agents that induce malformations3.

0A s!he%ati! i$$ustration of !riti!a$ perio s for so%e or8ans is presente inMoore ?1232@4

-a or congenital malformations may occur during the critical periods of development.

Formation of the embr onic bod : Folding and Fle*ion

Jp to the end of week the embryo was a flat disc. 0n week ", the embryo growsvery rapidly and becomes progressively cylindrical and takes a characteristic %*shaped form, common to vertebrate embryos. This is caused by folding of theembryo, cranially, caudally and laterally, at the same time positioning anddemarcating some of the organs within the embryo.

The effects of folding can best be seen in longitudinal and transverse sections ofembryos Esee Dangman 1 813G -oore 1 8 3G Darsen 1 3K. ?ead and tail

fle(ion occurs ventralwards during week ", raising and demarcating the embryonic body from the disc. The developing brain grows cranially, tucking the heart andfuture mouth cavity ventrally. #art of the yolk sac is incorporated into the embryoas the foregut. !oon the tail fold at the caudal end pro ects over the cloacal regionand incorporates part of the yolk sac as the hindgut. 9fter folding, the connectingstalk and yolk stalk remain attached to the ventral surface of the embryo as theumbilical cord. !imultaneously the lateral folds establish the almost cylindrical


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form of the rest of the embryo best visualized in transverse sections. 9s thelateral folds grow medially, the roof of the yolk sac is incorporated into the embryoas the midgut, and the yolk sac is reduced to a narrow yolk stalk. The somitesdifferentiate into sclerotome, myotome and dermatome components internally andare prominent e(ternally.

The embryonic body is formed by fle(ion and folding resulting in a cylindricalembryo.

The % shaped embryo, thus formed, undergoes further fle(ion and growth in

weeks $ and &. 0t develops fore and hind limb buds and a heart prominence in thechest region. The eye develops on the sides of the forebrain region and " branchialarches appear on the sides of the foregut region pharyn(3. The brain has dividedinto fore, mid and hind brain compartments and is demarcated from the rest of thespinal cord. The somites * $ pairs3 become more prominent on either side of thespinal cord evidence of segmental development. The head has grown much larger than the body in week & due to the growth of the brain. The forelimb developsdigital rays future digits. The e(ternal auditory meatus ear3 appears where thefirst branchial groove is located. The peripheral nervous system begins to form,integrating the developing nervous system. Beural crest cells migrate from theneural tube and aggregate to form ganglia of the sympathetic nervous system andthe sensory spinal ganglia.

The embryo is %*shaped like most other vertebrate embryos, when they look similar.

)uring the weeks > and 8 limbs have formed, the fingers of the hand haveseparated and the feet are webbed, which then separate into toes. The tail stubdisappears altogether. The head is more round and erect but is disproportionallylarge. The abdomen is flatter but the intestines have herniated in the pro(imalregion of the umbilical cord. /yelids have formed and are usually open. The


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auricles of the e(ternal ear have formed. The head has enlarged immensely and theembryo is now un4uestionably human in appearance.

etermination of Embr onic !ge and Measurement

Two criteria are used to determine day 1 in the natural cycle to estimate age5* timeof fertilization or onset of last menstrual cycle. The latter becomes complicated forthose who have discontinued oral contraception. The day of fertilization is the mostreliable for estimating age and this is easily determined by the time of ovulation =12 hours, which is the timeframe within which the egg is fertilized. 0n vitro,estimation is easier, since fertilization occurs 2 hours after insemination and

pronuclei are formed 12 1& hours after insemination 0see at$ases& Sathananthanet a$& 122:;122


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%ritical periods for most organ systems generally range from 8 weeks ofdevelopment, when the rudiments are laid down. Teratogens kill the embryo in thefirst two weeks or damage some cells

allowing the embryo to recover 0see Moore& 12324

The germ layers give rise to all the tissues and organs of the human body.

-HYSIOLOGY O, THE EMBRYO -$a!entation

The placenta is an organ derived from the trophoblast of the blastocyst and iscomposed of some foetal

membranes. These membranes are mostly e(traembryonic and consist of theamnion, chorion, yolk sac and allantois, which combine to various degrees to formthe placenta and umbilical cord. ?umans are placentals and the foetal placenta isallanto*chorionic , with the villous chorion forming the ma or part of the organ. 0tis essentially involved in the physiology of the embryo and has many functions5*

protection, nutrition, respiration, e(cretion and also has an endocrine function producing hormones. The term conceptus refers to the embryo plus the foetal

membranes.

The decidua#

The placenta has both foetal and maternal components. The maternal portion is thegravid pregnant3 endometrium, which is cast off at birth parturition3 hencetermed decidua. The decidua are named acccording to their relation to theimplantation site5* a3 )ecidua basalis underlying the conceptus forming thematernal componentG b3 )ecidua capsularis the superficial wall overlying theconceptusG c3 )ecidua parietalis the remaining uterine mucosa or wall. 9s the

conceptus grows, the capsularis bulges into the uterine cavity and fuses with the parietalis, obliterating its cavity. The capsularis degenerates and disappears byabout week 22. The basalis forms the maternal placenta and is usually discoidal inshape.

e"elopment of the placenta


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Fe have already dealt with the origins of the foetal membranes in weeks 2 and . 0tis by week " that the essential parts of the placenta are established and becomefunctional. 'y weeks 27 22 it is fully formed 0see Moore& 1232& for ia8ra%s4 .0t is the chorionic villi, embedded in the decidua basalis, that are involved in foeto*maternal e(changes, later on.

Jntil about week 8, the chorionic villi % 3 cover the entire surface of thechorionic sac. /ventually the villi over the decidua capsularis degenerate dueto reduced blood supply, forming the smooth chorion. Those % associatedwith the decidua basalis persist, multiply and branch profusely to form thevillous chorion, the foetal placenta. The foetal component is thus composedof the chorion and its % that are bathed in the maternal blood hence

called a haemo*chorial placenta. The villous chorion, composed of tertiaryvilli, is anchored to the maternal decidua basalis by anchoring villi. The fullterm placenta has a very complicated structure composed of branched %with foetal blood vessels embedded in the decidua basalis. The % are

bathed with maternal blood filling the intervillous spaces, flowing from theendometrial spiral arteries, in spurts.

0See Moore& ?1232@; Lan8%an& ?1231@4

The chorionic villi are the chief components of the foetal placenta.

$horionic "illi : -illous chorion

#rimary % were formed on day 1" when a primitive utero*placental circulation became functional. These had a core of cytotrophoblast %T3 surrounded bysyncytiotrophoblast !T3. The core is eventually invaded by e(traembryonicmesoderm, which becomes mesenchyme or connective tissue secondary villi3 andlater by foetal capillaries formed in week tertiary villi3. These have both %T and!T covering the villus. The full*term % has an epithelium of !T with little or no%T and several foetal capillaries. The whole theme in this development is to makethe barrier between the foetal and maternal blood as thin as possible, to increasethe efficiency of foeto*maternal e(changes. Thus we have arrived at the mostefficient placenta in mammalian evolution and the barrier is called the placental


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membrane. 0An e !e$$ent ia8ra% sho=in8 the e%(r'oni! an #i$$ous!ir!u$ator' s'ste% is sho=n in Moore ?1232@4 .

The placental membrane: -illus .

This membrane consists of " layers that separate the foetal from the maternal blood5* foetal capillary

endothelium, foetal connective tissue, %T and !T. The %T is scanty and thematernal blood bathes the villus. 'y full term, the foetal capillaries are broughtvery close to the maternal blood, there being little or no intervening connectivetissue.

The placental membrane separates the foetal from the maternal blood.

Foeto%maternal e*changes#

The placenta has three main functions metabolism, transfer and endocrine. -anysubstances can permeate the placental membrane. These include gases 2 and% 23 by diffusionG nutrients glucose, fatty acids, amino acids, water, electrolytes,

vitamins35 foetal waste products % 2, urea, uric acid3G some hormones andantibodies 0gC3 and harmful substances such as drugs, poisons, teratogens,alcohol, tobacco, cocaine, viruses rubella3 and the syphilis bacterium. -ost

bacteria and heparin cannot cross the membrane. ?0 could be transmitted acrossthe placenta during childbirth or by breastfeeding. The placenta also producessteroid hormones progesterone and estrogen3, human chorionic gonadotrophin

h%C3 and other protein hormones and prostaglandins. 6oetal h%C is e(creted inthe motherAs urine and is the basis for the pregnancy test.

THE UMBILICAL CORD

The vascular life*line of the embryo and foetus is the umbilical cord whichconnects them to the


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placenta. 0t is gradually formed after week ", replacing the connecting stalk andattains a cord*like form by week 27. 0t is derived from foetal membranes,amnion, yolk sac and allantois, and the cord takes shape during e(tensive growthof the amniotic cavity 0(est #isua$i7e in ia8ra%s/see Moore& 12324 . 0t iscomposed of gelatinous mesenchyme called FhartonAs elly and usually contains 2arteries and 1 vein. The cord usually connects up near the centre of the discoidal

placenta. Lnotting or looping of the cord could be dangerous to the foetus,especially if it is around the neck. 0t may impede circulation and cause death.

The umbilical cord is the life*line of the embryo and foetus

THE ,OETAL MEMBRANES -h'sio$o8' of the e%(r'o.

There are " foetal membranes5* chorion, amnion, yolk sac and allantois. They aree(traembryonic in origin and are composed of either ectoderm or endodermcombined with mesoderm. They play an important role in the functioning of theembryo and contribute to the formation of the placenta, as well.

a) !mnion and !mniotic Fluid#

The amnion or waterbag forms a fluid*filled sac around the embryo and its chieffunctions are protection, providing an ancient watery environment, and preventingdessication. 9ll vertebrate embryos develop in a watery environment and thehuman is no e(ception. 9mniotic fluid is derived from the maternal blood and later the foetus e(cretes urine into the fluid. The fluid is also swallowed by the foetusand absorbed in the gut. The embryo floats freely in the fluid, permitting growthand free movement. 0t cushions the embryo and acts as a shock absorber. 6urther it

prevents adherence of membranes and limbs and maintains a constant bodytemperature.

b) .olk Sac

Though the human egg has no yolk, it forms a yolk sac, like the chicken embryo. 0tdevelops in week 2 as a sac and is later reduced to a pear*shaped vestige week $3.0ts functions are5* i3 transfer of nutrients in weeks 2 and when the utero*

placental circulation is establishedG ii3 'lood islets form in the mesoderm of theyolk sac wall in week hemopoietic activity3G iii3 The roof of the yolk sac forms


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the primitive gut week "3G iv3 #rimordial germ cells appear in the wall of the yolk sac, which later migrate to gonads to become germ cells.

c) The !llantois

9ppears in week as a small diverticulum in the hindgut region of the embryo andgrows into the connecting stalk. 0t is non*functional as an embryonic bladder inhuman embryos but is involved in the formation of blood during the first 2 monthsand the allantoic blood vessels become the umbilical vessels. The allantois alsoforms the urachus connected to the bladder which becomes the median umbilicalligament after birth.

d) The $horion

This is the most important foetal membrane, since it forms the foetal placenta villous chorion, which we have already dealt with. 9bove all it is the outermostmembrane covering both the embryo and other foetal membranes. ?ence it alsohas an overall protective function apart from its placental function. 6oetalmembranes are e(tra*embryonic and are involved in the functioning of the embryoand foetus.

T5INS AND MULTI-LE -REGNANCIES

-ultiple pregnancies will be dealt with in another chapter. ?owever, with respectto foetal membranes, twins may share the amnion or chorion and even the placenta,depending on whether they are dizygotic or monozygotic. 0f dizygotic blastocysts

originating from 2 zygotes3 implant close together, they may share the same placenta. -onozygotic twins originating from 1 zygote by division of the innercell mass3 usually have a single chorion and a common placenta. -onozygotictwins formed by division of the embryonic disc in week 2 share a single amniotic

sac, a chorionic sac and a placenta. The twins may be separate or con oined!iamese3 or may be parasitic, depending on complete or incomplete division of

the embryonic disc. -onozygotic twins are rarely delivered alive since theirumbilical cords are entangled when circulation stops and foetuses die. Twins areusually smaller than a single foetus, since crowding interferes with growth andnutrition. 0See Moore& 1232& for ia8ra%s an photo8raphs4


24/24

SELECTED RE,ERENCES

Lan8%an *. -edical embryology 6ourth /dition3 1 81G Filliams M Filkins,'altimore. pp 8".

%oncise te(t with colour diagrams and photographs.3

Larsen 5* . ?uman embryology 1 G %hurchill Divingstone, Bew Nork. pp ">

9dvanced te(t with colour illustrations and clinical applications3

Moore 6L . 'efore we are born5 basic embryology and birth defects Third/dition3 1 8 G !aunders, #hiladelphia. pp 7&.

!imple te(t with colour illustrations and photographs3

O Rahi$$' RA . %olour atlas of human embryology 1 >$G !aunders, #hiladelphia.

%omprehensive set of $ mm slides the real thing5 whole embryos and sections.3

Sathananthan AH& N8 SC& Bon8so A& Trounson A& Ratna% SS . isual atlas ofearly human development for assisted reproduction technology 1 . BationalJniversity, !ingapore. pp 27

#re*implantation development5 illustrations and microphotographs.3

Sathananthan AH ed.3. isual atlas of human sperm structure and function forassisted reproduction technology 1 &G Bational Jniversity, !ingapore. pp2> .

-icroscopical images of gametes, fertilization and zygotes, some in colour.3

Sathananthan AH& E =ar s RG. 6rom sperm binding to syngamy computerenhanced images of human fertilization %)*+ -3 1 $G ?uman +eproductionJpdate 151

Documents

Introduction to Human Embryology