4-EMBRYOLOGICAL_DEVELOPMENT_OF_BODY_TISSUES,_ORGANS_AND_SYSTEMS.[1].pptx

EMBRYOLOGICAL
DEVELOPMENT OF BODY
TISSUES, ORGANS AND
SYSTEMS.

Objectives
By the end of this section, you should be able
to:
1. Define terms used in embryology.
2. Describe human fertilization and
implantation.
3. Describe the developing human

Write down the definition of the following
terms
1.Fertilization
2.Implantation
3.Cleavage
4.Morphogenesis
5.Differentiation

terms…
Fertilization
• Fertilization is the process whereby two sex cells (gametes) fuse together to create
a new individual with a genome derived from both parents.
• Fertilization, union of a paternal sperm nucleus with a maternal egg
nucleus to form the primary nucleus of an embryo
• Human fertilization is the union of a human egg and sperm, occurring in
the ampulla of the fallopian tube.
• Fertilization can be defined as the union of two haploid gametes, the
spermatozoa and the oocyte, hereto referred to as egg, to restore the diploid
state

Implantation
• The process by
which the zygote
(fertilized ovum)
adheres to the
uterine lining and
becomes embedded
in the endometrial
layer of the uterus
and starts to grow
into a new form.

terms…
Cleavage
• Cleavage is a process
by which the
multicellular
organisms undergo a
series of mitotic
divisions to become a
blastomere following
fertilization.

Morphogenesis
• This is the process by which
competent cells responds to
inducers- paracrine factors
through signal transduction
pathways to interact with
other cells in the integration
of organism, genetic, and
cellular processes of embryo
development

terms…
Differentiation
• Differentiation- is the development of specialized cell
type.
• In order for this to happen the cell must undergo overt
changes in cellular biochemistry and function which are
preceded by a process- resulting in the commitment of
the cell to a certain fate.

What factors do you think may affect implantation process?

Human Fertilization and implantation
• This is the fusion of a spermatozoon and an ovum to form a single cell, the zygote.
• When semen is deposited in the vagina, the spermatozoa travel through the cervix
and body of the uterus into the fallopian tubes where they will meet the ovum and
fertilize it.
• Only one spermatozoon of the many millions that are deposited in the female
genital tract with a single ejaculation is required to fertilize an ovum.
• Many sperms surround and make an attempt to penetrate the protective shell- like
barrier that surround the ovum.
• Fertilization usually takes place in the lateral third of the fallopian tube.
• When the motile spermatozoon reaches the zona pellucida of the ovum, the
sperm head starts to penetrate this layer

Sperm attempting to gain entry into the ovum

Human Fertilization and implantation …
• Once the head of the spermatozoon has pierced the zona pellucid, the movement of the tail ceases.
• This time the entire spermatozoon, including the tail, is engulfed by the cytoplasm of the ovum and drawn
inward.
• Following the penetration of the ovum by one spermatozoon, the permeability of the zona pellucid changes
and become hard and impenetrable.
• Therefore even though the other competing spermatozoa become attached to the zona, they are unable to gain
entrance to the ovum.
• During this process, the ovum and the remaining 22+X chromosomes reconstitute themselves into a nucleus
and form the female pronucleus.

• The nucleus of the head of the spermatozoon becomes swollen to form the male pronucleus, and
then the body and tail disintegrate and disappear.
• The two pronuclei now appear.
• Meanwhile each pronucleus loses its nuclear membrane combining their chromatin into a
complete single set of 23 pair of chromosomes
• Thereafter the chromosomes become organized on the spindle and the 23 paternal and 23 maternal
chromosomes split longitudinally at the centromere.
• The process initiates normal mitotic division to take place, facilitating the first cleavage division.
• Fertilization thus results in the re-combination of the male and female chromosomes and the
restoration of the full diploid number.
• The resulting combination is called a zygote.

• Prior to fertilization, each ovum contains a complete set of human genome, including a
single X but no Y chromosome.
• Likewise, each spermatozoon contains a complete set of autosomes and a single sex
chromosome, either X or Y.
• The resulting zygote is similar to the majority of somatic cells because it contains two
copies of the genome in a diploid set of chromosomes.
• One set of chromosomes come from the nucleus of the ovum and the second set from the
nucleus of the sperm.
• If the spermatozoon contributes a Y chromosome then the zygote will develop into a
male.
• Unlike the X chromosome, the Y chromosome contains very little genetic information.
• However, it does contain a gene SRY, which will switch on androgen production at a later
stage, leading to the development of a male body type.
• In contrast, the mitochondrial genetic information of the zygote comes entirely from the
mother via the ovum.

• The zygote spends the next few days travelling along the fallopian
tube on its way to the uterus.
• During this time it divides several times to form a ball of cells known
as a morula.
• The cell divisions continue and form a cavity in between.
• This stage transit to another stage called a blastocyst in which further
cell division take place to produce and form yet other smaller cells.
• By the fifth day, it reaches the uterus following fertilization.

• The zona pellucid which is a glycoprotein disintegrate to allow the development of
trophectoderm cells which form the extra-embryonic structures such as the
placenta which come into contact with the luminal epithelial cells of the
endometrium.
• It then attaches to the uterine lining and subsequently burrows into the endometrial
cell layer where it anchors itself.
• This process is called implantation.
• In most successful pregnancies, the conceptus implants 8 to 10 days after
ovulation (Wilcox et al, 1999).
• The inner cell mass forms the embryo, while the outer layers form the membranes
and placenta.
• Together, the embryo and its membranes are referred to as a conceptus or ‘the
products of conception’.
• Implantation of the blastocyst is completed during the 2nd week of embryonic
development.

The developing human
• Human prenatal development is divided into an
embryonic period and a fetal period.
• The embryonic period begins with fertilization and
ends eight weeks later.
• Following fertilization, the early development of the
zygote is all the same in all mammals.
• It is after a period of about four weeks that there is a
significant change from the mammalian structure to
that of a miniature human being.
• This is evident through the use of an ultrasound
machine.

Embryonic period
• The embryonic period in human begin at fertilization 12-24 hours after
ovulation.
• This is expected to occur between the 2nd and 3rd week of gestation.
• The period normally continues till the end of the 10th week of
gestation, all this occurs in the fallopian tube.

The first week.
• The zygote undergoes cleavage which is a rapid succession of mitotic divisions
produce a large number of smaller cells called blastomeres.
• The cell size becomes smaller with each division.
• At about 40 to 50 hours following fertilization the four- cell stage occurs.
• The large numbers of new cells are located internal to the zona pellucida.
• They collectively have the appearance of a mulberry and are known as the
“morula”.
• As the morula is slowly being propelled through the uterine tube by the peristaltic
waves with the aid of the contraction of the muscle in its wall cell division will
continue taking place.

The first week…
• However, the beating of the cilia of the columnar cells of the mucous membrane of the tube,
provide a fluid vehicle which helps in the transport of the morula along its lumen.
• The tube also provides some nourishment for the actively dividing cells.
• The morula enters the uterine cavity when the cell is about twelve- to sixteen- cell stage, and this
is achieved at about 5 to 8 days after ovulation.
• Glandular secretion from the uterine cavity passes through the zona pellucid and diffuses between
the cells of the morula.
• As soon as the morula reaches the uterus a cavity is formed.
• At this stage it is referred to as a blastocyst which consists of three structures:
• 1. The inner cell mass, or embryoblast, which will form the embryo and some extra-embryonic
tissues.
• 2. A blastocyst cavity which is a fluid- filled.
• 3. The trophoblast, which is a thin outer layer of cells

The first week…
• The trophoblast encloses the inner cell mass and blastocyst cavity and later forms extra-
embryonic structures and embryonic part of the placenta.
• The zona pellucida is shed after four to five days following fertilization, and the trophoblast
adjacent to the inner cell mass attaches to the endometrial epithelium.
• The trophoblast adjacent to the embryonic pole differentiates into two layers, and these are:
• a) Syncytiotrophoblast -which is an outer layer.
• b). Cytotrophoblast. - is an inner. Layer
• The syncytiotrophoblast burrows the endometrial epithelium and underlying connective
tissue.
• The cuboidal layer of hypoblast forms on the deep surface of the inner cell mass.
• Implantation of the blastocyst in the endometrium of the uterus is completed by the end of the
first week.

The second week
• The second week is characterized by a more complex process of:
• 1. Formation of the germ layer.
• 2. Early tissue and organ differentiation.
• This is a period of rapid proliferation and differentiation of the trophoblast.
• The trophoblast completes its implantation in the endometrium, resulting into
various endometrial changes known as decidual reaction.
• The primary yolk sac forms with the extra-embryonic mesoderm developing from
the endoderm of the yolk sac.
• The extra-embryonic coelom develops from spaces in the extra-embryonic
mesoderm, which later becomes the chorionic cavity.

The second week…
• The primary yolk sac disappears and a secondary yolk sac develops.
• Other changes which occur include:
• 1. Amniotic cavity which arises from the cytotrophoblast and the inner
cells mass or embryoblast.
• 2. A bilaminar embryonic disc an inner cell mass which consist of
epiblast, related to the amniotic cavity, and hypoblast.
• 3. The prechordal plate develops as a localized thickening of the hypoblast
(primary endoderm); it outlines the future cranial region of the embryo
and the future site of the mouth.
• 4. The prechordal plate is an important organizer of the head region.

The third week
• The third week is characterized by the differentiation and formation of organs through a process known as
organogenesis
• Organ genetic period
• During this period, the bilaminar embryonic disc is converted into a trilaminar embryonic disc during
gastrulation.
• A primitive streak appears as a result the migration of epiblastic cells to the median plane of the embryonic
disc.
• Invagination of epiblastic cells from the primitive streak form mesenchymal cells.
• These cells migrate ventrally, laterally, and cranially between the epiblast and hypoblast to initiate
communication and development.
• The primitive streak begins to produce mesenchymal cells, the epiblast layer is known as the embryonic
ectoderm.
• Some cells of epiblast displace the hypoblast and form the embryonic endoderm.

The third week…
• Mesenchymal cells organize themselves into a third gem layer, which is the intra-
embryonic-mesoderm.
• Cells of the intra-embryonic mesoderm migrate to the edges of the embryonic disc, where
they join the extra-embryonic mesoderm covering the amnion and yolk sac.
• By the end of the third week, mesoderm exists between the ectoderm and endoderm
everywhere except at the oropharyngeal membrane.
• The notochord, occupies the median plane at the cloacal membrane.
• The notochordal plate forms the notochord, the primordial axis of the embryo which
again forms the axial skeleton.
• The neural plate forms the neural tube called neuralation.
• The neural folds fuse to form neural tube.

The third week…
• The neuro-ectodermal cells migrate dorso-lateral to form neural crest
from which it will divide into two cell masses which will subsequently
develop into the sensory ganglia of the cranial and spinal nerves.
• Other neural crest cells migrate from neural tube and help to form
various structures, such as the retina.
• The mesoderm which is on each side of the notochord thickens to form
longitudinal columns of paraxial mesoderm.
• Division of these paraxial columns into pairs of somites which begins
cranially is complete by the end of the third week.

The third week…
• Remember, the number of somites present is an indication of the age of the embryo.
• The coelom (body cavity) develops from isolated spaces or vesicles in the lateral
mesoderm and cardiogenic mesoderm.
• The coelom vesicles infold forming a cavity which subsequently will form body cavities
such as the peritoneum.
• The blood vessels first appear in the wall of the yolk sac and in the chorion known as
allantois.
• They rapidly develop within the embryo starting as blood islands within the mesenchymal
spaces.
• The spaces soon become lined with endothelium derived from the mesenchymal cells.
• These primordial vessels unite with other vessels to form a primordial cardio-vascular
system.

The third week…
• Towards the end of the third week, the heart is almost completed, represented by a paired
endothelial heart tubes that are joined to the blood vessels in the embryo and in the extra-embryonic
membranes (yolk sac, umbilical cord, and chorionic sac).
• By the end of the third week, the endothelial heart tubes have fused to form a tubular heart that is
joined to vessels in the embryo, yolk sac, chorion, and connecting stalk to form a primordial
cardiovascular system.
• The primordial blood cells thus ‘hemocytoblast’- are formed from the endothelial cells of blood
vessels in the walls of the yolk sac and allantois.
• It is believed that fetal and adult erythrocytes are formed from hematopoietic precursors.
• By the third week, formation of chorionic villi is completed.
• The exchange of oxygen and nutrients and other substances between the maternal and embryonic
circulation occur as a result of the rapid development and increases of the surface area of the chorion.

Week 4-5.
• The embryo produces a chemical which stops the woman’s menstrual period.
• Other structural development which occur include; neurogenesis which initiate brain
activities, a miniature heart which begins its activity.
• The limb buds for the arms and legs appear which will grow later.
• This is the period of organogenesis.
• The head is the first part to appear and is proportionally a large mass representing half of
the embryo.
• The brain develops into five areas.
• Tissues start to form and develop into the vertebra column and associated bones.
• Cardiac activity with blood flow is evident at around 6th week.

Week 6-8.
• This period is characterized by rapid myogenesis, neurogenesis, organogenesis and growth.
• The embryo’s eyes and facial features begin to form and movement of the embryo is possible.
• Essential organs including the hair start to form and by the end of 8th week, the embryonic period
comes to an end and fetal period begins.
• Unfortunately this is also a period in which some embryos do not survive.
• Miscarriages do occur to about 25% of pregnancies.
• The common causes of miscarriage being chromosomal abnormalities, maternal age, infections,
chemical agents, and radiation.
• However, some abortions occurring during embryonic period may be induced due to a variety of
reasons which include; interruption of education, work, desire to delay or end childbirth or
unstable relationships.
• Abortion may be performed in cases of either rape or incest which has resulted to pregnancy.

Fetal period
• It is during this period that the foundations of all major organs and systems
of the body have been made from the three germ layers.
• At the beginning of the fourth week, folding in the median and horizontal
planes converts the flat trilaminar embryonic disc into a C- shaped,
cylindrical embryo.
• There is a continuous sequence of events that results into the formation of
the head, tail, and lateral folds with a constriction between the embryo and the
yolk sac.
• The endodermal-lined yolk sac is incorporated into the embryo during
folding and forms the primordial gut (foregut, midgut and hindgut).

Fetal period…
• The head region folds ventrally, incorporating part of the endodermal
layer into the developing embryonic heads as the foregut.
• The folding of the head region results in the formation of
oropharyngeal membrane and heart.
• The two structures are then placed in ventral position.
• The developing brain becomes the most cranial part of the embryo.
• The part of the endodermal germ layer is incorporated into the caudal
end of the embryo as the hindgut, which subsequently expand to form
the cloaca.

Fetal period…
• Again the tail region folds resulting into the formation of the cloacal membrane,
allantois, and connects the stalk to the ventral surface of the embryo.
• The midgut is formed as a result of folding of the embryo in the horizontal plane
which incorporates part of the endodermal gem layer.
• The yolk sac remains attached to the midgut by a narrow stalk.
• The lateral and ventral body walls are formed during the horizontal folding plane.
• As the amnion spread progressively, it envelops the connecting stalk, yolk stalk
and allantois again forming an epithelial covering of the umbilical cord.
• The end of the embryonic period is marked by the three germ layers which
differentiate into various cells, tissues and organs thereby, setting the beginning of
all the organ systems.

Fetal period…
• The formation of the brain, heart, liver, somites, limbs, ears, nose, and eyes is generally affected by
external appearance of the embryo.
• As these structures develop, the appearance of the embryo changes so that it has unquestionably
human characteristics by the end of eighth week.
• The essential external and internal structures are formed during the fourth to eighth week.
• However, this is the most critical period of development, and any disturbances during this period
may give rise to major congenital anomalies of the embryo.
• It is important to note that at this particular time, reasonable estimates of the age of the embryos
can be determined from:
• 1. The day of the last menstrual period (LMP).naegles formular 1-8-2022 edd-
• 2. The estimated time of fertilization or conception.
• 3. Ultrasound measurements of the chorionic sac and embryo.
• 4. Examination of the external characteristics of the embryo.

Fetal period…
• The fetal period begins after fertilization and ends at birth.
• It is characterized by rapid body growth and differentiation of tissues and organ systems.
• An obvious change in the fetal period is the relatively slowing of the head growth as compared with the rest
of the body.
• The beginning of the twentieth week is marked by appearance of lanugo and head hair.
• The fetal skin is coated with vernix caseosa.
• The eyelids are closed during most of the fetal period, but begin to reopen at about 26 weeks.
• At this time, the fetus is usually capable of extra-uterine existence, mainly because of the maturity of its
respiratory system.
• The fetus appears reddish and wizened at 30 weeks because of the thinness of its skin and the relative absence
of subcutaneous fat.
• At 6 to 8 weeks, subcutaneous fat usually develops rapidly giving the fetus a smooth, plump appearance.
• The remaining period is devoted mainly to building up of tissues and preparing systems involved in the

Fetal period…
• Changes occurring during the fetal period are not as dramatic as those appearing in the embryonic period, but they are very
important.
• At this time fetus is less vulnerable to the teratogenic effects of drugs, viruses, and radiation.
• Remember that these agents may interfere with growth and normal functional development, especially of the brain and
eyes.
• The status of the fetus can be assessed using various techniques which are readily available in diagnosing certain diseases
and developmental anomalies before birth.
• It is now possible to determine whether or not a fetus has a particular disease or congenital anomaly by using various
diagnostic techniques, by using amniocentesis and ultrasonography.
• Diagnosis can be made early enough during pregnancy to allow termination of the pregnancy if need be in cases of serious
anomalies which are incompatible with life.
• As you know, various treatments can be given in selected cases of fetal anomaly (Soothill, 1996), for example, the
administration of drugs to correct cardiac arrhythmia or thyroid disorders.
• Surgical correction of congenital anomalies in utero is also possible, such as; ureterostomies on fetuses that have ureters
that do not open into the bladder (Harrison, 1991).

Ninth week
• The fingers and toes have completely separated webbed feet-polysydct, the taste buds are starting to develop,
Baby has tooth buds, can swallow and stick out his or her tongue.
• Whole body except tongue is sensitive to touch.
• Cartilage now calcifying to become bone.
• If it is a male, the testicles are starting to produce the testosterone hormone.
• The brain is growing rapidly and producing more than 250,000 nerve cells a minute.
• The heart is almost completely developed and very much resembles that of a newborn baby.
• An opening the atrium of the heart and the presence of a bypass valve divert much of the blood away from the
lungs, as the child's blood is oxygenated through the placenta.
• The eyelids have fused shut and will not open again until around week 27.
• The wrists and ankles have formed and the fingers and toes are clearly visible.
• By this week the placenta has developed enough to support most of the critical job of producing hormones.

Tenth week
• The fetus starts moving spontaneously.
• The face is beginning to look like a baby's face.
• The pancreas is functioning and producing insulin.
• Fingernails and toenails appear.
• The baby can suck his thumb, and get hiccups.
• From this week you may well be able to hear the baby's heart beat
through a doppler monitor.
• You will notice that the rate is up to 160 a minute(110-160bpm),
double that of a normal adult. 80-100bpm

Tenth week …
• The baby now has a chin and a nose and a facial profile.
• Vocal chords are complete, and the baby can and does sometimes cry
silently.
• The brain is fully formed, and the baby can also feel pain.
• The fetus may even suck his thumb.
• The eyelids now cover the eyes, and will remain shut until the seventh
month to protect the delicate optical nerve fibers.
• The hair is on the head and the fingers and toes have developed soft
nails.
• The kidneys are developed and begin to secrete urine.

Eleventh week
• The fetus has grown to about 3 inches (8cm) long.
• Weight approximately 25 grams.
• Bone is beginning to replace cartilage and the ribs are appearing.
• The nose and chin are well defined.
• Movements can be measured.
• The fetus can open and close its mouth.
• The external genitalia are almost defined; it may be possible to
determine the sex of the fetus.

Twelfth –fifteenth week
• The fetus grows rapidly during this period nearly doubling its length, while the head remain relatively large
about a third of the body’s size.
• The forehead is high and prominent with a broad face.
• The ears are located on both sides of the jaw.
• The eyes have lids covering them and it will take some time before they open, usually at the seventh month.
• The first rudiments of fine hair, known as lanugo, appear over the forehead and eyebrow region and start to
grow.
• The nails appear on the tips of the fingers.
• The midgut has been withdrawn into the abdomen from the extra- embryonic coelom in the umbilical cord.
• The kidney start to function in secreting urine and the buccopharyngeal membrane rupture to facilitate the
fetus to swallow amniotic fluid.
• The external genitalia are now developed and sex of the fetus is evident.
• The fetus measures about 56mm in length and weighs about 14 gm.

Sixteenth to nineteenth week
• There is continuous growth in size and the fetus now looks definitely
human with individual differences which can be recognized.
• Although the head is still large, the face is broad and the eyes are well
separated with fused eyelids.
• The mandible and the chin are developed and the ears now on the
same higher side of the head.
• The fetus measures about 112mm in length and weighs about 105 gm.

Twentieth to twenty third weeks
• The entire body is covered by lanugo and sebaceous glands which secretes
sebum that mixes with the epithelium to form a greasy, cheesy substance
known as vernix caseosa, which covers the entire skin from then till birth.
• Hair appears on the head.
• Fetal movement can be experienced by the mother and is known as
quickening.
• Fetal heart is evident and can be detected by use of a fetal stethoscope
placed on the mother’s abdomen.
• The size of the fetus at this time is 160 mm in length and weighs about
310gm.

Twenty fourth to twenty seventh weeks
• The fetus skin is wrinkled and red due to inadequate
subcutaneous fat, and the red myohaemoglobin of the muscle
shows through the translucent skin.
• At this particular time, there is significant development, and
the fetus looks more like a child with open eyelids.
• The fetus measures approximately 200mm in length and
weighs about 640gm.

Twenty eighth to thirty first weeks
• There is progressive deposition of subcutaneous fat which is going to
fill the fetus crevices so as to make it smooth.
• The head is covered with long hair and it has increased its length to
240mm and weighs 1080gm.
• The fetus is viable and if born at this time and given good care, will
survive.
• It has all the attributes of a baby and will move and cry normally like
any baby.

Thirty second to thirty fifth weeks
• There is progressive deposition of subcutaneous fat and the fetus looks
a bit wrinkled and scraggy.
• The fetus’s skin is covered by a thick layer of vernix caseosa which
protects it from liquor amnii.
• Both the finger nails and the toe nails have developed and reached
their ends.
• The fetus measures about 280mm and weighs about 1670gm.
• If born at this time the fetus has good chance of survival.

Thirty sixth to thirty ninth weeks
• At this time there is further deposition of subcutaneous fat making the
end of wrinkled appearance.
• The look of a child is more pronounced with fully developed external
genitalia.
• For the male, the testis has descended into the scrotum; while in the
female both labia minora and majora are fully formed.
• The fetus measures about 310mm in length and weighs about 2400gm.
• If the fetus happen to be born at this time, it has all the possibilities of
survival.

Fortieth week
• At this time, the fetus has achieved full development.
• There is the disappearance of lanugo hair.
• The finger nails and toe nails have grown beyond their terminal ends.
• The bones are fully formed and especially those of the skull are firm.
• The head circumference is larger than any other part of the body.
• This is an important factor during childbirth.
• The fetus is fully grown to a baby to be born.
• It measures about 350mm in length and weighs about 3300gm.
• This weight may vary from one baby to another and genetic predisposition play an
important part.
• However, with different babies, the weight is between 2500gm to 5,000gm

Placenta Formation
In addition to the embryo and fetus, the fetal membranes and the major part of
the placenta originate from the zygote.
The placenta consists of two parts
1. A large fetal part derived from the villous chorion.
2. A smaller maternal part developed from the decidua basalis
• The two parts are held together by stem chorionic villi that attach to the
cytotrophoblastic shell surrounding the chorionic sac, which attaches the sac to the
decidua basalis
• The placenta begins to develop upon implantation of the blastocyst into the
maternal endometrium.
• The outer layer of the blastocyst becomes the trophoblast, which forms the outer
layer of the placenta.

Placenta Formation…
• This outer layer is divided into two further layers: the underlying
cytotrophoblast layer and the overlying syncytiotrophoblast layer.
• The syncytiotrophoblast is a multinucleated continuous cell layer that
covers the surface of the placenta.
• It forms as a result of differentiation and fusion of the underlying
cytotrophoblast cells, a process that continues throughout placental
development.
• The syncytiotrophoblast (otherwise known as syncytium), thereby
contributes to the barrier function of the placenta.
• The placenta grows throughout pregnancy.
• Development of the maternal blood supply to the placenta is complete by
the end of the first trimester of pregnancy (approximately 12–13 weeks).

The functions of the placenta
• The functions of the placenta include the following:
1. Metabolism – such as the synthesis of glycogen, cholesterol and
fatty acids
2. Respiration- gas exchange (oxygen, carbon dioxide and carbon
monoxide)
3. Transfer of nutrients such as vitamins, hormones, and
antibodies
4. Elimination of waste products
5. Endocrine secretion (e.g. hCG) for maintenance of pregnancy

The functions of the placenta…
• All these functions are essential for maintaining pregnancy and promoting normal fetal
development.
• The fetal circulation is separated from the maternal circulation by a thin layer of extra-
fetal tissues known as placental membrane.
• The membrane is permeable and allows water, oxygen, nutritive substances, hormones,
and noxious agents to pass from the mother to the embryo or fetus.
• Waste products pass through the placental membrane from the fetus to the mother.
• You also need to note that the fetal membranes and placenta(s) in multiple pregnancies
vary considerably, as a result of the embryos and the time when division of embryonic
cells occurs.
• The common type of twins is dizygotic twins.
• They have two amnions, two chorions, and two placentas that may or may not be fused.

The functions of the placenta…
• Monozygotic twins, the less common type, represent about a third of all twins;
they develop from one zygote.
• The monozygotic twins have one chorion, two amnions, and one placenta.
• Twins with one amnion, one chorion, and one placenta, are always monozygotic
and their umbilical cords are often entangled.
• Other types of multiple births (triplets, etc.) may be derived from one or more
zygotes.
• The yolk sac and allantois are primary structures; and their presence is essential
to normal embryonic development.
• They are early sites of embryonic and fetal development.
• Primordial germ cells also originate in the wall of the yolk sac.

The amniotic fluid
• The amnion forms a sac for amniotic fluid and
provides a covering for the umbilical cord.
• The amniotic fluid has three main functions:
1. To provide a protective buffer for the embryo or
fetus
2. To allow room for fetal movements
3. To assist in the regulation of fetal body temperature.

Human birth defects
• Defects:
•Body cavities.
• Cardiovascular
• Mesenteries.
• Muscular system.
•Diaphragm. .
•Eye and ears.
•The pharyngeal (bronchial) apparatus. .
•Limbs.
•The respiratory system. .
•Nervous system.
•Urogenital. .
• Skin etc.

Human birth defects
• Congenital abnormality is a structural abnormality of any type that is present at birth.
• It may be macroscopic or microscopic, on the surface or within the body.
• There are four clinically significant types of abnormalities:
• 1. Malformation.
• 2. Disruption.
• 3. Deformation.
• 4. Dysplasia.
• Scientific evidence shows that some of the congenital abnormalities may be induced by genetic factors or by
environmental factors.
• They cause derangements during prenatal development.
• However, most of the congenital abnormalities show family patterns that are expected of multi-factorial
inheritance and are determined by a combination of genetic and environmental factors.

Human birth defects…
• About 3% of all live born infants have an obvious major abnormality.
• Additional abnormalities are detected after birth; thus, the incidence is about 6% in 2-year olds and
8% in 5- year olds.
• Other abnormalities (about 2%) are detected later in life (e.g. during surgery or autopsy).
• Congenital abnormalities may be single or multiple and of minor or major clinical significance.
• Single minor abnormalities are present in about 14% of newborns.
• These abnormalities are of no serious medical consequences, but they alert the clinician to the
possible presence of an associated major abnormality.
• Ninety percent of infants with multiple minor abnormalities have one or more associated major
abnormalities.
• Of the 3% of infants born with a major congenital abnormality, 0.7% has multiple major
abnormalities.

Human birth defects…
• It is worth noting that majority of abnormalities are more common in early embryos (up to 15%) than they are
in newborn infants (up to 3%).
• Most severely malformed embryos are usually spontaneously aborted during the first 6 to 8 weeks.
• Some congenital abnormalities are caused by genetic factors (chromosome abnormalities and mutant genes).
• A few congenital abnormalities are caused by environment factors (infectious agent, environmental
chemicals, and drugs); however, most common abnormalities result from a complex interaction between genetic
and environmental factors.
• The causes of most congenital abnormalities remain unknown.
• During the first 2 weeks of development, teratogenic agents usually kill the embryo or have no effect, rather
than cause congenital abnormalities.
• During the organogenetic period, teratogenic agents disrupt development and may cause major congenital
abnormalities.
• During the fetal period, teratogens may produce morphological and functional abnormalities, particularly of
the brain and eyes.

Summary
• Fertilization usually takes place in the lateral third of the fallopian tube.
• Fertilization results in the combination of the male and female chromosomes and the
resulting combination are called a zygote which differentiates to form a morula then a
blastocyst.
• Implantation occurs 8 to 10 days after ovulation.
• The embryonic period ends eight weeks after which it is referred to as a fetus.
• Fetus develops week by week.
• The placenta begins to develop from the outer layer of the blastocyst upon implantation
into the maternal endometrium.
• Congenital abnormality which is structural abnormality of any type that is present at
birth.
• The four clinically significant types of abnormalities as; malformation, disruption,
deformation and Dysplasia.

Review questions
1. Describe the process of fertilization.
2. Outline four methods of estimating the age of an embryo.
3. State three changes that occur during the following stages of
human development;
a.Second week
b.Ninth week
c.Twenty fourth week
•

1. The outer covering of the cell is:
• a) A cytoplasm
• b) Cell layer
• c) Protoplasm
• d) Cell membrane
2. The following cell structure is involved in protein synthesis?
• a) Nucleus
• b) Golgi Apparatus
• c) Smooth endoplasmic reticulum
• d) Rough endoplasmic reticulum
3. Cellular activities are conducted by:
• a) Mitochondria
• b) Nucleus
• c) Adenosine Tri-Phosphate (ATP)
• d) Nucleolus

4. Members of paired chromosomes are called;
• a) Chromatids
• b) Chromatin
• c) Autosomes
• d) Karyotype
5. The full complement of chromosomes is known as;
• a) Autosomes
• b) Diploid
• c) Haploid
• d) Karyotype
6. Genes are carried on;
• a) Chromosomes
• b) Nucleus
• c) Nucleolus
• d) Chromatids

7. When new strands of DNA are synthesized with its base inserted by mistake into a gene, the result is,
• a). RNA
• b). gene destruction
• c). mutation
• d). Adaptation
• 8. Tissues are made up of:
• a) Many types of cells
• b) Epithelium
• c) Cells
• d) Organelles
•

9. Functions of the body are conducted by:
• a) Cells
• b) Organs
• c) Body system
• d) Tissues
• 10. Interaction of cell network is done by:
• a) Inductive interaction
• b) Instructive permissive interaction
• c) epithelia-mesenchymal interaction
• d) Paracrine factor

SECTION TWO: SHORT ANSWER
QUESTIONS
1. State five functions of the placenta (5mks)
2. Outline three roles of amniotic fluid (3mks)
3. Explain four ways in which age of an embryo can be estimated
(4mks)
4. Describe the process of meiosis (5mks)
5. Explain the process of normal inheritance (5mks)
6. Outline four clinical types of abnormalities (4mks)

SECTION THREE: LONG ANSWER
QUESTION
Describe the process of human development under the following:
a. Fertilization
b. Implantation
c. Cleavage
d. Morphogenesis
e. Differentiation (20 marks)

REFERENCES
• Balinsky BI. 1970. An Introduction to Embryology.1.st Edition. New York. W B.
Saunders Company.
• Gilbert SF. 2003. Developmental Biology. Seventh Edition; Sinauer Associates, Inc;
Sunderland, Massachusetts, 181-221.
• Moore K L. and Persaud T.V.N. 1993. The Developing Human, Clinically Oriented
Embryology. 6th Edition, W.B. Saunders Company. pp 18-45.
• Snell RS. 1975. Clinical Embryology for Medical Students.2nd Edition, Printed in USA. P
25-69.
• Williams PL; Wendell- Smith CP; and Treadgold S. 1969. Basic Human Embryology
for medical students. 2nd edition. Printed in USA. Pg 13, 25 -69.
•

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