INTRODUCTION TO HUMAN EMBRYOLOGY
Embryology deals with the study of the human embryo during the first 8 weeks of development. It 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 3-9) when there is extensive growth and differentiation and when the embryo acquires a more human form (see separate chapter). Development is a long complex process which transforms a single cell (fertilized ovum) into a complex multicellular organism. The development of the embryo and foetus is the prenatal period of development.
[A timetable of prenatal development is illustrated in Moore, 1989]
Embryonic development begins at fertilization (day 1) followed by cleavage of the embryo and its
implantation in the uterus (week 1). This is followed by the differentiation of the primary germ layers (week 3) and culminates in primary organogenesis when the tissues and major organ rudiments of the embryonic body are formed (weeks 4-8). The embryonic period may be temporally considered in 4 phases:-
Second Week: Formation of bilaminar embryo, deep implantation and establishment of uteroplacental circulation.
Third Week: Formation of trilaminar embryo (gastrulation), somites, chorionic villi and neurulation.
Fourth to Eighth Weeks: Formation of the embryonic body, primary organogenesis and placentation.
The physiology of the embryo (foetal membranes and placenta) and some congenital malformations during critical periods of development will also be dealt with in this chapter.
The pre-implantation phase of development begins with fertilization and culminates in the implantation of the blastocyst in the uterine endometrium. Our knowledge of the first week of development has advanced immensely in the past 20 years, since the advent of in vitro fertilization (IVF) and assisted reproductive technologies (ART). [See our atlas of Early Human Development for ART – Sathananthan et al 1993;1996 for illustrations.]
Briefly, the spermatozoon is a highly specialized torpedo-shaped cell (~ 30mm long) for motility and, of course, for fertilization. It is composed of a head, midpiece (body) and a tail (flagellum). The head carries the nucleus with the paternal chromosomes and is capped by a modified lysosome (acrosome) covered by the cell membrane. The midpiece consists of the base of the axoneme originating from the sperm-neck, where is located the functional paternal centriole hidden in a “black box”. The midpiece has a spiral of mitochondria around the axoneme which provides ATP for sperm motility and nine dense fibres surrounding the axoneme. The axoneme consists of 9 peripheral doublets and a central doublet of microtubules (9+2 organization) that extends into the tail, characteristic of cilia and flagella and is the motile component of the cell. The tail has a ring of 7 dense fibres and a fibrous sheath around the axoneme.
The mature egg or oocyte is a large unspecialized cell (~120mm in diameter) compared to the spermatozoon. It has no nucleus but has a maturation spindle arrested at metaphase II of meiosis. The spindle is barrel-shaped and aligned at right angles to the oocyte surface, where the first polar body is located within the PVS. The egg vestments consist of a gelatinous zona pellucida (shell) surrounded by several layers of follicle cells that make up the cumulus oophorus. The cumulus is expanded and gelatinous in mature oocytes. The egg cell has most of the components of somatic cells but lacks rough endoplasmic reticulism. It has cortical granules beneath the cell membrane, which play a role in fertilization.
[See our atlases Sathananthan et al, 1993; 1996 published by the department for details of gamete structure and function, and CD-ROM, Sathananthan & Edwards, 1995]
b) Gamete transport
Mature sperm (several millions) are ejaculated into the vagina after intercourse, enter the uterus through the cervix and swim up to the far end of the oviduct (ampulla) in about 5 minutes, where fertilization occurs. Of the millions ejaculated, only a few hundred reach the ampulla. These are the most motile in the ejaculate. Sperm capacitation (a physiological process) occurs somewhere in the reproductive tract, where sperm acquire the ability to penetrate the vestments of the oocyte. In vitro, it takes place during the sperm preparation procedure (washing and layering) for ART. Once at the surface of the egg, it has to penetrate the cumulus oophorus (follicle cells around the egg) and the gelatinous shell (zona pellucida) before gamete fusion occurs. Penetration is aided by sperm motility and the acrosome reaction (AR), which is the morphological expression of capacitation. During the AR the surface membranes of the acrosome vesiculate releasing 2 important enzymes – hyaluronidase and acrosin (protease), which enable sperm to penetrate the cumulus and zona, respectively, and reach the perivitelline space (PVS) surrounding the egg.
The mature egg (oocyte) is ovulated around day 14 of the natural menstrual cycle [See Moore, (1989); Larsen, (1998)], released from the ovary into the peritoneal cavity and immediately sucked up through the infundibulum of the Fallopian tube (oviduct) by its ciliary action into the ampulla. Here fertilization occurs and development begins. Oocytes remain viable for up to 12 hours after ovulation, while sperm can survive up to 24 hours in vivo. Sperm, however can be kept alive in vitro up to 2-3 days and still retain their fertilizing capacity, while eggs too can survive for about 20-24 hours, in vitro.
This involves the spontaneous fusion of two germ cells – sperm and egg – to form a zygote. The zygote or fertilized egg is the first cell of the new baby. Fertilization essentially restores the diploid number (2n = 46) of chromosomes of somatic (body) cells bringing the father’s and mother’s genomes together.
The mature sperm cell is formed in the testis (male gonad) by a process of meiotic maturation called spermatogenesis, while the mature oocyte is formed likewise in the ovary (female gonad) during oogenesis. Each gamete has a haploid number of chromosomes (n = 23), having undergone meiosis (reduction division) during gametogenesis (formation of gametes). The zygote has the diploid number of chromosomes (2n = 46). Biparental inheritance of chromosomes leads to species variation, since there is independent assortment of paternal and maternal chromosomes among the germ cells during meiosis. Sex is also determined at fertilization.
d) Mechanics of fertilization
The major events that take place simultaneously or soon after fertilization are summarized as follows:
(i) Sperm – oocyte fusion: the midsegment of the sperm membrane fuses with that of the egg followed by sperm incorporation into the egg to form a male pronucleus (MPN).
(ii) Soon after gamete fusion there is a sperm-induced calcium wave in human oocytes.
(iii) The egg completes its second maturation division, abstricting the second polar body and a female pronucleus (FPN) is formed within the ooplasm. The egg has now completed meiosis and the FPN is haploid.
(iv) Sperm fusion triggers the cortical reaction which involves the exocytosis or release of cortical granules into the PVS.
(v) Contents of released cortical granules interact with the zona – zona reaction - which chemically hardens the zona to prevent polyspermy. Only one sperm needs to fertilize the egg.
(vi) A sperm aster is formed and the male and female pronuclei migrate to the centre of the egg, where they associate but do not fuse. Apart from the father’s chromosomes, the sperm introduces the centriole, which is the active division centre within the embryo during mitosis (cleavage).
Thus fertilization is a complex process, which essentially involves activation of the egg to develop further into an embryo and foetus.
[For diagrams and illustrations see atlases, Sathnanthan et al, (1993; 1996)]
CLEAVAGE: REPEATED MITOSES (CELL DIVISION)
Soon after fertilization, the activated egg (ovum) cleaves as it travels down the oviduct to enter the
uterus. Cleavage involves repeated mitotic divisions with little or no growth. The ovum or zygote divides into 2, 4, 8, 16, 32 cells and becomes a morula and blastocyst, when divisions become irregular. The functional sperm centriole (centrosome) introduced at fertilization replicates and first forms a sperm aster, which then splits to form a bipolar spindle about 20 hours after fertilization. The male and female pronuclei, formed ~12 hours after fertilization, dissociate their envelopes and the paternal and maternal chromosomes come together and organize themselves on this spindle, a stage called “syngamy”. This establishes the embryonic genome, which is activated later on between the 4-8 cell stage in the human. The first cell division occurs around 24 hours after fertilization and a 2-cell embryo is formed. The embryo then divides into 4, 8, 16, 32 cells called blastomeres and forms a morula (mulberry), which develops a fluid filled cavity to become a blastocyst. After the 8-cell stage, the embryo becomes compact (compaction) when cells increase contact with one another and develop an inside-outside polarity, which later gives rise to a mass of cells within the blastocyst - the inner cell mass that becomes the embryo proper (embryoblast) and an outer layer of cells which forms an epithelium and gives rise to placental membranes (trophoblast).This is the first sign of cell differentiation (cells becoming different) in the human embryo. The morula enters the uterus on day 3-4 of development.
BLASTOCYST HATCHING AND SUPERFICIAL IMPLANTATION (DAY 6)
The blastocyst expands as hydrostatic pressure of fluid increases within its cavity and it hatches out of the zona (day 5) through a hole believed to be digested by enzymes, and squeezes its way out. It now closely adheres to the uterine endometrium and trophoblast cells proliferate and form a syncytiotrophoblast (common cytoplasm with several nuclei) which invades the uterine epithelium. The endometrial stroma responds by becoming secretory (decidual reaction), while uterine glands enlarge and the uterus becomes vascularized and edematous, due to the influence of progesterone and oestrogen secreted by the corpus luteum of the ovary. After implantation the trophoblast produces human chorionic gonadotrophin (hCG) which can be detected in the mother’s urine during the first 2 months of pregnancy (pregnancy test). Later the placenta secretes progesterone, as well. If there is no pregnancy the corpus luteum degenerates after about 13 days. The normal site of implantation is the endometrial lining around the cavity of the uterus (body of the uterus). Ectopic pregnancies result when the embryo implants outside this region, e.g., in the tube or cervix – this will be dealt with in another chapter.
[For diagrams of cleavage-stage embryos see atlas- Sathananthan et al (1993); Moore (1989); Langman (1982)]
CLINICAL APPLICATIONS: TREATMENT OF INFERTILITY
The greatest advances in assisted reproduction stem from the recent development of IVF and ART technologies which has led to a better understanding of human development in the first week. This was a grey area before the advent of these techniques. Consequently, we have now an indepth understanding of the events of fertilization, cleavage and abnormalities, thereof. Apart from IVF, the single technique that has revolutionized the treatment of infertility is intracytoplasmic sperm injection (ICSI). This successful technique of assisted fertilization violates most of the norms of fertilization since the sperm is injected directly into the oocyte, bypassing its vestments and pre-empting natural sperm-egg fusion. We have to wait and see the long term effects of this technology. This will be dealt with at length in another chapter on subfertility.
We have now a better understanding of hormonal interactions and monitoring in the stimulated cycle; embryo development in vitro including co-culture methods; chromosomal aberrations in gametes and embryos; uterine receptivity to implantation and, of course, the techniques 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 without ART [see Atlas, Sathananthan et al, 1996]
THE SECOND WEEK: Bilaminar embryo: Deep implantation
Most things happen “in twos” during this week.
(i) Embryoblast splits into two layers (beginning of gastrulation)
(ii) Two new cavities are formed: amniotic and chorionic
(iii) Trophoblast differentiates into cyto and syncytio-trophoblast
a) Bilaminar Embryo
The embryo is discoidal and spherical and consists of two layers: epiblast (above) and hypoblast (below) formed by delamination (splitting) of a single layer. The hypoblast (day 7) becomes the endoderm and the epiblast will later differentiate into ectoderm and mesoderm (week 3). The endoderm proliferates and lines the blastocoele cavity (Heuser’s membrane), now called the primitive yolk sac below the embryonic disc (day 8). There is no yolk in the human – yet a yolk sac appears as in yolky embryos, e.g. chick. However, the yolk sac is vital for formation of blood and germ cells.
b) Amnion and chorion and their cavities.
These two membranes are foetal (extraembryonic) membranes while the yolk sac is another. The conceptus at this stage can be regarded as being composed of 3 balloons – amniotic sac above and yolk sac below, pressed against the embryonic disc. The chorionic sac is the largest balloon on the outside (trophoblast), covering the 2 sacs and the embryonic disc.
The Amnion: The amnion appears on day 8 on the roof of the embryonic disc. Fluid accumulates and a thin epithelium is formed from the cytotrophoblast. The epiblast forms the floor of the amniotic cavity. This is the beginning of the water bag. The amniotic cavity enlarges and by week 8 encloses the entire embryo.
The Chorion: The chorion is formed after the appearance of the extraembryonic mesoderm (days 10/11). These cells arise from the epiblast and migrate to form two layers – the inner lining the Heuser’s membrane (secondary yolk sac) and the other the inner surface of the cytotrophoblast which becomes the chorion. The cavity formed is the extraembryonic coelom or chorionic cavity (days 12/13). The chorionic cavity enlarges and the embryonic disc with its dorsal amniotic sac and ventral yolk sac is suspended in the chorionic cavity by a connecting stalk of mesoderm.
[For diagrams see Moore (1989); Langman (1982); Larsen (1998)]
c) Cyto- and syncytiotrophoblasts (deep implantation)
The trophoblast has already differentiated into a cellular (cytotrophoblast-CT) and syncytium
(syncytiotrophoblast-ST) at the embryonic pole of the blastocyst. The ST is invasive and helps the embryo implant in the endometrium. Deeper implantation (days 7-9) involves extensive growth of the conceptus and its complete implantation within the endometrium, leaving a scar or blood clot (closing plug). Hydrolytic enzymes secreted by the trophoblast digest the extracellular matrix between the endometrial cells and processes of the trophoblast penetrate deep into the endometrium. The ST grows all around the conceptus and lacunae or cavities appear within it (day 9). Soon maternal blood capillaries anastomose and invade the lacunae establishing a primitive uteroplacental circulation. Extensions of the CT extend and grow into the overlying ST establishing the primary chorionic villi (beginnings of the placenta).
d) Clinical Applications
Spontaneous Abortions: These are quite common during early development. An abortion is defined as a termination of pregnancy before 20 weeks of gestation. Almost all abortions that occur in the first 3 weeks are spontaneous (not induced). It is estimated that 50% of all such abortions are caused by chromosomal abnormalities. It is also reported that 30-50% of zygotes never become blastocysts and implant.
Hydatiform Mole: Few of the pregnancies result in hydatiform moles in which the embryo is entirely missing and only a placenta is present. Primary chorionic villi are present without embryonic vessels – detectable by ultrasound. Complete moles have 46 chromosomes, all of paternal origin because zygotes have two male pronuclei. Partial moles are formed from triploid dispermic zygotes. Both types of moles tend to abort spontaneously or are surgically removed. Persistent molar tissue may result in trophoblastic disease.
THE THIRD WEEK: Trilaminar embryo – formation of germ layers.
The most dramatic event in early development is the formation of the 3 primary germ layers: ectoderm, endoderm and mesoderm – referred to as gastrulation in animal terms. It is from these 3 germ layers that all the tissues and organs of the human body are formed. Basically, ectoderm forms the skin and nervous system (outside), endoderm the gut and associated glands including the respiratory system (inside), while mesoderm forms all the other organ systems (in between).
[For details of derivatives of germ layers see Moore (1989)]
Primitive streak: Mesoderm formation
The embryonic disc now becomes pear-shaped and develops a linear primitive streak, dorsally in the epiblast, which is a heap of cells that proliferate and migrate to the centre line. It appears first at the caudal end at the beginning of week 3 and grows towards the centre of the disc. The primitive streak (day 16) signals the formation and separation of mesoderm from the ectoderm overlying it. It may be likened to a permanently closed keyhole, where cells migrate inwards, sideways and forwards to form the mesoderm sandwiched between the ectoderm and the endoderm (hypoblast) which was formed in week 2. This is best appreciated in transverse sections of the disc. [Excellent 3-D images of the process, as well as sections, are presented in Langman (1981); Moore (1989); Larsen (1993). The process of mesoderm formation is called “immigration”, while endoderm formation is referred to as “delamination”. Both processes result by differentiation and movement of cells to take up their definitive positions in the embryo, of course regulated by the genes on the chromosomes of maternal and paternal origin. The genes control the process of development before and after birth. Thus cell differentiation and cell movements are integral processes of development – it is important to put the right cell in the right place at the right time to ensure normal development. Any disturbances in this co-ordinated interaction of cells will eventuate in abnormal development. Both genetic and environmental factors are involved in many developmental processes. Morphogenesis (the development of complex from simple structure) is regulated by cascades of gene expression. At first the maternal genes are switched on, which then activate the zygotic genes. Important events such as gastrulation are controlled by such genes. The embryo is now trilaminar and after all mesoderm cells have migrated along the primitive streak it regresses to the caudal region of the embryo and degenerates. If it persists a tumor or teratoma is formed.
Neurulation and formation of the notochord
Neurulation involves the formation of the neural plate and tube (primitive nervous system). This is preceded by formation of a hollow rod of cells beneath the ectoderm as the primitive streak recedes. The mesoderm cells that migrate through the primitive knot (cranial end of streak) become the notochordal process. This is later transformed into a solid rod – the notochord, the embryonic axial skeleton - which is replaced by the vertebral column. On day 17 the notochordal mesoderm induces the overlying ectoderm to form the neural plate. This is a good example of chemical induction where one tissue induces the formation of another.
Neural tube formation
At first the neural plate is oval but later elongates over the underlying notochord along the whole axis
of the embryo. The plate invaginates towards the notochord to form a neural groove, which deepens progressively to form a tube by fusion of the lateral neural folds. This nerve tube is hollow and is lined by pseudostratified columnar epithelium. The cells of the neural crest separate from the tube to develop into the spinal and autonomic ganglia and pigment cells, later. The nerve tube is formed on days 19-21, and its closure begins in the middle of the embryo and progresses towards cranial and caudal ends by the end of week 4. The anterior end swells to become the brain and the rest forms the spinal cord. The neural tube is open cranially and caudally, forming the neuropores, which close in week 4.
Development of somites
Somites are blocks of paraxial mesoderm which appear in pairs on either side of the notochord. The first somite appears on day 20 behind the base of the future skull. This is the first sign of segmentation in the embryo. Subsequent somites form behind the first progressively till 42-44 pairs are formed by week 4/5. The number of somites are used to determine the age of the embryo. The somites give rise to most of the axial skeleton, associated musculature and dermis of the skin. The intraembryonic coelom (body cavity) is formed in mesoderm lateral to the somites.
Formation of heart and blood vessels
The rudiments of the heart and blood vessels are also laid down in week 3. Blood vessels are formed in the mesoderm of the yolk sac and chorion as spaces within mesenchyme cells (blood islands). They are soon lined with endothelium, and unite with other vessels to form a primitive cardiovascular system. The heart is formed at the end of week 3 in much the same way as enlarged blood vessels in the cranial region. Paired 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. The heart begins to beat and the vascular system is the first to become functional.
[A diagram of the primitive vascular system is shown in Moore, 1989]
The fourth foetal membrane appears in week 3 as a diverticulum of the yolk sac (caudal wall). It remains small and is also involved in angiogenesis (formation of blood vessels) and is later associated with the development of the urinary bladder.
The primary chorionic villi have a core of connective tissue and eventually develop blood capillaries and become secondary and tertiary villi and cover the entire surface of the chorion (see placenta). Their vessels also connect up with vessels inside the embryo. Nutrients and other substances are exchanged between maternal and foetal circulations.
Disturbances in neurulation cause some abnormalities of the brain and spinal cord. Failure of
closure of the neural tube in the caudal region (caudal neuropore) results in spina bifida. These defects also involve the tissues overlying the spinal cord (meninges, vertebral arches, dorsal musculature and skin). The most frequent site of failure of neurulation is the cranial neuropore resulting in anencephaly.
WEEKS 4 – 8:CRITICAL PERIODS OF DEVELOPMENT: Formation of all major organ rudiments and the embryonic body
The last 5 weeks of embryonic development are very critical since all the main organ systems are laid down, both externally and internally, and the embryo takes shape and finally takes the characteristic human form by week 8. It is a critical period since major developmental disturbances that occur now could result in major congenital malformations in each system of the human body. During this period the embryo is susceptible to teratogens (agents that induce malformations).
[A schematic illustration of critical periods for some organs is presented in Moore (1989)]
Formation of the embryonic body: Folding and Flexion
Up to the end of week 3 the embryo was a flat disc. In week 4, the embryo grows very rapidly and becomes progressively cylindrical and takes a characteristic C-shaped form, common to vertebrate embryos. This is caused by folding of the embryo, cranially, caudally and laterally, at the same time positioning and demarcating some of the organs within the embryo.
The effects of folding can best be seen in longitudinal and transverse sections of embryos [see Langman (1981); Moore (1989); Larsen (1993)]. Head and tail flexion occurs ventralwards during week 4, raising and demarcating the embryonic body from the disc. The developing brain grows cranially, tucking the heart and future mouth cavity ventrally. Part of the yolk sac is incorporated into the embryo as the foregut. Soon the tail fold at the caudal end projects over the cloacal region and incorporates part of the yolk sac as the hindgut. After folding, the connecting stalk and yolk stalk remain attached to the ventral surface of the embryo as the umbilical cord. Simultaneously the lateral folds establish the almost cylindrical form of the rest of the embryo – best visualized in transverse sections. As the lateral folds grow medially, the roof of the yolk sac is incorporated into the embryo as the midgut, and the yolk sac is reduced to a narrow yolk stalk. The somites differentiate into sclerotome, myotome and dermatome components internally and are prominent externally.
The C – shaped embryo, thus formed, undergoes further flexion and growth in weeks 5 and 6. It develops fore and hind limb buds and a heart prominence in the chest region. The eye develops on the sides of the forebrain region and 4 branchial arches appear on the sides of the foregut region (pharynx). The brain has divided into fore, mid and hind brain compartments and is demarcated from the rest of the spinal cord. The somites (33-35 pairs) become more prominent on either side of the spinal cord – evidence of segmental development. The head has grown much larger than the body in week 6 due to the growth of the brain. The forelimb develops digital rays – future digits. The external auditory meatus (ear) appears where the first branchial groove is located. The peripheral nervous system begins to form, integrating the developing nervous system. Neural crest cells migrate from the neural tube and aggregate to form ganglia of the sympathetic nervous system and the sensory spinal ganglia.
During the weeks 7 and 8 limbs have formed, the fingers of the hand have separated and the feet are webbed, which then separate into toes. The tail stub disappears altogether. The head is more round and erect but is disproportionally large. The abdomen is flatter but the intestines have herniated in the proximal region of the umbilical cord. Eyelids have formed and are usually open. The auricles of the external ear have formed. The head has enlarged immensely and the embryo is now unquestionably human in appearance.
Determination of Embryonic Age and Measurement
Two criteria are used to determine day 1 in the natural cycle to estimate age:- time of fertilization or onset of last menstrual cycle. The latter becomes complicated for those who have discontinued oral contraception. The day of fertilization is the most reliable 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. In vitro, estimation is easier, since fertilization occurs 2 – 3 hours after insemination and pronuclei are formed 12 – 16 hours after insemination [see atlases, Sathananthan et al, 1993;1996] Embryonic age is determined from external features and measurement of length, usually after abortions. Their greatest length , crown – rump length (sitting height) or crown – heel length are often used. More accurate measurements can be made in utero by ultrasound at weeks 4 – 5, when external structures can be visualized. [See Moore, 1989, for measurements and tables]
Internal differentiation: Organogenesis
The 3 germ layers formed from the inner cell mass in week 3 give rise to all the tissues and organs of the human body. Cells of each germ layer proliferate, migrate, reaggregate and differentiate into various tissues that form the organs (organogenesis)
Ectoderm This outermost layer forms the epidermis of the skin and glands, the central nervous system (brain and spinal cord), peripheral nervous system (nerves and ganglia), and the sensory epithelia (eye, ear and nose).
Mesoderm This intermediate layer forms the majority of tissues and organs: connective, skeletal, muscular tissues, cardiovascular, urinary and reproductive systems, body cavities and linings and the spleen.
Endoderm This innermost layer forms the epithelium of the gastro-intestinal and respiratory systems and associated glands (liver and pancreas), epithelial lining of urethra and parts of the ear, thyroid, parathyroid and tonsils.
Critical periods for most organ systems generally range from 3 – 8 weeks of development, when the rudiments are laid down. Teratogens kill the embryo in the first two weeks or damage some cells
allowing the embryo to recover [see Moore, 1989]
PHYSIOLOGY OF THE EMBRYO: Placentation
The placenta is an organ derived from the trophoblast of the blastocyst and is composed of some foetal
membranes. These membranes are mostly extraembryonic and consist of the amnion, chorion, yolk sac and allantois, which combine to various degrees to form the placenta and umbilical cord. Humans are placentals and the foetal placenta is allanto-chorionic , with the villous chorion forming the major part of the organ. It is essentially involved in the physiology of the embryo and has many functions:- protection, nutrition, respiration, excretion and also has an endocrine function producing hormones. The term conceptus refers to the embryo plus the foetal membranes.
The placenta has both foetal and maternal components. The maternal portion is the gravid (pregnant) endometrium, which is cast off at birth (parturition) – hence termed decidua. The decidua are named acccording to their relation to the implantation site:- a) Decidua basalis – underlying the conceptus forming the maternal component; b) Decidua capsularis – the superficial wall overlying the conceptus; c) Decidua parietalis – the remaining uterine mucosa or wall. As the conceptus grows, the capsularis bulges into the uterine cavity and fuses with the parietalis, obliterating its cavity. The capsularis degenerates and disappears by about week 22. The basalis forms the maternal placenta and is usually discoidal in shape.
Development of the placenta
We have already dealt with the origins of the foetal membranes in weeks 2 and 3. It is by week 4 that the essential parts of the placenta are established and become functional. By weeks 20 – 22 it is fully formed [see Moore, 1989, for diagrams]. It is the chorionic villi, embedded in the decidua basalis, that are involved in foeto-maternal exchanges, later on.
Until about week 8, the chorionic villi (CV) cover the entire surface of the chorionic sac. Eventually the villi over the decidua capsularis degenerate due to reduced blood supply, forming the smooth chorion. Those CV associated with the decidua basalis persist, multiply and branch profusely to form the villous chorion, the foetal placenta. The foetal component is thus composed of the chorion and its CV that are bathed in the maternal blood – hence called a haemo-chorial placenta. The villous chorion, composed of tertiary villi, is anchored to the maternal decidua basalis by anchoring villi. The full term placenta has a very complicated structure composed of branched CV with foetal blood vessels embedded in the decidua basalis. The CV are bathed with maternal blood filling the intervillous spaces, flowing from the endometrial spiral arteries, in spurts.
[See Moore, (1989); Langman, (1981)]
Chorionic villi : Villous chorion
Primary CV were formed on day 14 when a primitive utero-placental circulation became functional. These had a core of cytotrophoblast (CT) surrounded by syncytiotrophoblast (ST). The core is eventually invaded by extraembryonic mesoderm, which becomes mesenchyme or connective tissue (secondary villi) and later by foetal capillaries formed in week 3 (tertiary villi). These have both CT and ST covering the villus. The full-term CV has an epithelium of ST with little or no CT and several foetal capillaries. The whole theme in this development is to make the barrier between the foetal and maternal blood as thin as possible, to increase the efficiency of foeto-maternal exchanges. Thus we have arrived at the most efficient placenta in mammalian evolution and the barrier is called the placental membrane.[An excellent diagram showing the embryonic and villous circulatory system is shown in Moore (1989)].
The placental membrane: Villus.
This membrane consists of 4 layers that separate the foetal from the maternal blood:- foetal capillary
endothelium, foetal connective tissue, CT and ST. The CT is scanty and the maternal blood bathes the villus. By full term, the foetal capillaries are brought very close to the maternal blood, there being little or no intervening connective tissue.
The placenta has three main functions – metabolism, transfer and endocrine. Many substances can permeate the placental membrane. These include gases (O2 and CO2) by diffusion; nutrients (glucose, fatty acids, amino acids, water, electrolytes, vitamins): foetal waste products (CO2, urea, uric acid); some hormones and antibodies (IgG) and harmful substances such as drugs, poisons, teratogens, alcohol, tobacco, cocaine, viruses (rubella) and the syphilis bacterium. Most bacteria and heparin cannot cross the membrane. HIV could be transmitted across the placenta during childbirth or by breastfeeding. The placenta also produces steroid hormones (progesterone and estrogen), human chorionic gonadotrophin (hCG) and other protein hormones and prostaglandins. Foetal hCG is excreted in the mother’s 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 which connects them to the
placenta. It is gradually formed after week 4, replacing the connecting stalk and attains a cord-like form by week 20. It is derived from 3 foetal membranes, amnion, yolk sac and allantois, and the cord takes shape during extensive growth of the amniotic cavity [best visualized in diagrams-see Moore, 1989]. It is composed of gelatinous mesenchyme called Wharton’s jelly and usually contains 2 arteries and 1 vein. The cord usually connects up near the centre of the discoidal placenta. Knotting or looping of the cord could be dangerous to the foetus, especially if it is around the neck. It may impede circulation and cause death.
THE FOETAL MEMBRANES: Physiology of the embryo.
There are 4 foetal membranes:- chorion, amnion, yolk sac and allantois. They are extraembryonic in origin and are composed of either ectoderm or endoderm combined with mesoderm. They play an important role in the functioning of the embryo and contribute to the formation of the placenta, as well.
a) Amnion and Amniotic Fluid.
The amnion or waterbag forms a fluid-filled sac around the embryo and its chief functions are protection, providing an ancient watery environment, and preventing dessication. All vertebrate embryos develop in a watery environment and the human is no exception. Amniotic fluid is derived from the maternal blood and later the foetus excretes urine into the fluid. The fluid is also swallowed by the foetus and absorbed in the gut. The embryo floats freely in the fluid, permitting growth and free movement. It cushions the embryo and acts as a shock absorber. Further it prevents adherence of membranes and limbs and maintains a constant body temperature.
b) Yolk Sac
Though the human egg has no yolk, it forms a yolk sac, like the chicken embryo. It develops in week 2 as a sac and is later reduced to a pear-shaped vestige (week 5). Its functions are:- (i) transfer of nutrients in weeks 2 and 3 when the utero-placental circulation is established; (ii) Blood islets form in the mesoderm of the yolk sac wall in week 3 (hemopoietic activity); (iii) The roof of the yolk sac forms the primitive gut (week 4); (iv) Primordial germ cells appear in the wall of the yolk sac, which later migrate to gonads to become germ cells.
c) The Allantois
Appears in week 3 as a small diverticulum in the hindgut region of the embryo and grows into the connecting stalk. It is non-functional as an embryonic bladder in human embryos but is involved in the formation of blood during the first 2 months and the allantoic blood vessels become the umbilical vessels. The allantois also forms the urachus connected to the bladder which becomes the median umbilical ligament after birth.
d) The Chorion
This is the most important foetal membrane, since it forms the foetal placenta – villous chorion, which we have already dealt with. Above all it is the outermost membrane covering both the embryo and other foetal membranes. Hence it also has an overall protective function apart from its placental function. Foetal membranes are extra-embryonic and are involved in the functioning of the embryo and foetus.
TWINS AND MULTIPLE PREGNANCIES
Multiple pregnancies will be dealt with in another chapter. However, with respect to foetal membranes, twins may share the amnion or chorion and even the placenta, depending on whether they are dizygotic or monozygotic. If dizygotic blastocysts (originating from 2 zygotes) implant close together, they may share the same placenta. Monozygotic twins (originating from 1 zygote by division of the inner cell mass) usually have a single chorion and a common placenta. Monozygotic twins 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 conjoined (Siamese) or may be parasitic, depending on complete or incomplete division of the embryonic disc. Monozygotic twins are rarely delivered alive since their umbilical cords are entangled when circulation stops and foetuses die. Twins are usually smaller than a single foetus, since crowding interferes with growth and nutrition. [See Moore, 1989, for diagrams and photographs]
Langman J. Medical embryology (Fourth Edition) 1981; Williams & Wilkins, Baltimore. pp 384.
(Concise text with colour diagrams and photographs.)
Larsen WJ. Human embryology 1993; Churchill Livingstone, New York. pp 479
(Advanced text with colour illustrations and clinical applications)
Moore KL. Before we are born: basic embryology and birth defects (Third Edition) 1989; Saunders, Philadelphia. pp 306.
(Simple text with colour illustrations and photographs)
O’Rahilly RA. Colour atlas of human embryology 1975; Saunders, Philadelphia.
(Comprehensive set of 35 mm slides – the real thing: whole embryos and sections.)
Sathananthan AH, Ng SC, Bongso A, Trounson A, Ratnam SS. Visual atlas of early human development for assisted reproduction technology 1993. National University, Singapore. pp 209
(Pre-implantation development: illustrations and microphotographs.)
Sathananthan AH (ed.). Visual atlas of human sperm structure and function for assisted reproduction technology 1996; National University, Singapore. pp279.
(Microscopical images of gametes, fertilization and zygotes, some in colour.)
Sathananthan AH, Edwards RG. From sperm binding to syngamy – computer enhanced images of human fertilization (CD-ROM) 1995; Human Reproduction Update 1:1