3. First Week
• Continuous process
• Human development begins at fertilization
a zygote (a single totipotent cell )
• The zygote, contains chromosomes and genes that are
derived from the mother and father.
• The unicellular zygote multi-cellular human being
through;
– Cell division
– Cell migration
– Cell growth
– Apoptosis
– Cell rearrangement
– Cell differentiation
4. • Definition:
• Embryology means the study of embryos
• The term generally refers to prenatal development of embryos and
fetuses.
• Developmental Anatomy
• Is the field of embryology concerned with the changes that cells, tissues,
organs, and the body as a whole undergo from a germ cell of each
parent to the resulting adult.
• Teratology
• Study of abnormal embryonic and fetal development.
• Branch of embryology concerned with congenital anomalies or birth
defects and their causes.
5. a. Prenatal period
b. Postnatal period
• Prenatal period: Period before birth
• 38 weeks from conception to birth (average)
• Gynecologic timing has been from LNMP ,
• Therefore, refers to 40 weeks “gestational” age
• Date of conception has been difficult to time
• LNMP is on average two weeks before ovulation
• Gestational (menstrual) age is widely used in
clinical practice because the onset of the LNMP
is usually easy to establish.
6. • The first 38 weeks of human development
• Pre-embryonic period:
• First 2 weeks of development
• Zygote becomes a spherical, multicellular
structure.
• Embryonic period:
• Includes the 3rd through 8th week of development.
• All major organs formed during this period.
• Known as the period of organogenesis.
• The developing human from 3rd – 8th week is called
Embryo
7. • The Fetal Period
• Period that begins at week nine and ends at
birth.
• The fetus continues to grow
• Its organs increase in complexity
• The developing human from 9th week to birth is
called fetus or offspring.
8.
9. • Divided into three trimesters
• First trimester: fertilization to 12weeks.
• Second trimester: 13 – 24 weeks.
• Third trimester: 25 – 36 weeks.
10. First trimester:
• Is most critical stage of development.
• Period of organogenesis: all of the major organ-systems
begin to form.
• Is also the period when the developing organism is most
vulnerable to the effects of drugs, radiation and
microbes.
11. Second trimester:
• Is stage of complete development of organ systems.
• By the end of this stage, the fetus assumes distinctively
human features.
Third trimester:
• Represents a period of rapid fetal growth.
• Weight of the fetus doubles.
• Most of the organ systems become fully functional.
• Final touch is made on many organs.
12. Significance of Embryology
Bridges the gap between prenatal development and
obstetrics, perinatal medicine, pediatrics, and clinical
anatomy.
Develops knowledge concerning the beginnings of
human life and the changes occurring during prenatal
development.
Helps to understand the causes of variations in human
structure.
Illuminates gross anatomy and explains how normal
and abnormal relations develop.
14. Embryological Terminology
• Oocyte (ovum)
• The female germ or sex cell produced in the ovaries.
• When mature, the oocyte is called a secondary oocyte
or mature oocyte.
• Sperm or Spermatozoon
• The sperm, or spermatozoon, refers to the male germ
cell produced in the testes.
• Zygote
• Cell results from the union of an oocyte & a sperm
during fertilization.
• A zygote is the beginning of a new human being (i.e.,
an embryo).
15. • Conceptus
• The embryo & its associated membranes
• Primordium
• Is the beginning or first discernible indication
of an organ or structure.
Embryological Terminology
16. Fetus
After the embryonic period (8th weeks), the developing
human is called a fetus.
During the fetal period (9th week to birth),
differentiation & growth of the tissues & organs
formed during the embryonic period occur.
The rate of body growth is remarkable, esp. during the
3rd & 4th months, & weight gain is phenomenal during
the terminal months.
Abortion
A premature stoppage of development & expulsion of a
conceptus from the uterus or expulsion of an embryo or
fetus before it is viable.
Embryological Terminology
17. Types of abortion
Threatened abortion
• (Bleeding with the possibility of
abortion) is a complication in about
25% of clinically apparent pregnancies.
An accidental abortion occurs because of
an accident.
17
18. Spontaneous abortion
Is one that occurs naturally & is most common
during the 3rd week after fertilization.
About 15% of recognized pregnancies end in
spontaneous abortion, usually during the first 12
weeks.
Habitual abortion
Is the spontaneous expulsion of a dead or
nonviable embryo or fetus in three or more
consecutive pregnancies.
Types of abortion
19. Types of abortion
Induced abortion
Is a birth that is induced before 20 weeks (i.e.,
before the fetus is viable).
Refers to the expulsion of an embryo or fetus that
is brought on intentionally by drugs or mechanical
means.
A complete abortion
Is one in which all the products of conception are
expelled from the uterus.
Legally induced abortions (elective, justifiable, or
therapeutic abortions) usually induced:
19
20. Criminal abortion
Is one that is produced illegally.
Miscarriage
Is the spontaneous abortion of a fetus &
its membranes before the middle of the
second trimester (about 135 days).
An abortus
Is the products of an abortion (i.e., the
embryo/fetus & its membranes).
Types of abortion
21. Trimester
A period of three calendar months during a pregnancy.
The 9-month period of gestation is commonly divided into
three trimesters.
The most critical stages of development occur during the first
trimester (12 weeks) when embryonic & early fetal
development is occurring.
Congenital Anomalies or Birth Defects
Abnormalities of development that are present at birth
Postnatal period
The changes occurring after birth are more or less familiar to
most people.
Embryological Terminology
32. Gametogenesis
• Is the process of formation and development of
specialized generative cells called gametes.
• Spermatogenesis in males
• Oogenesis in females.
• The sperm and oocyte, the male and female
gametes, are highly specialized sex cells that will be
produced.
• Begins with meiosis.
• Chromosome number is reduced by half
35. Spermiogenesis
• Transformation of spermatids into spermatozoa.
• The entire process of spermatogenesis, which includes
spermiogenesis, takes approximately 2 months (72
days).
• From a single spermatocyte, four new sperm are formed.
• No cell division occurs during this process
• Spermatids contain 23 chromosomes
• All sperm have 22 autosomes and either an X
chromosome, or a Y chromosome.
36. Spermatids
– small size (7–8 µm in
diameter)
– haploid nuclei with highly
condensed chromatin
– position near the lumen of
the seminiferous tubules
37. Process of spermiogenesis
• Formation of the acrosome
• Condensation and elongation of the
nucleus
• Development of the flagellum (tail)
• Shedding of much of the cytoplasm
43. Oogenesis
• The sequence of events by which oogonia are transformed into
mature oocytes.
• Begins before birth and is completed after puberty.
• Oogonia are diploid cells.
• In females, the sex cell produced is called the secondary oocyte.
• This cell will have 22 autosomes and one X chromosome.
44. Oogenesis cont’d
• Contains;
• Prenatal maturation phases
• Formation and proliferation of Oogonia
• Oogonia enlarge to form primary oocytes
• Formation of follicular cells
• Formation of zona pellucida
• Beginning of the first meiotic division, but
INCOMPLETE!
48
45. Oogenesis cont’d
• Contains;
• Post-natal maturation phases
• Ovulation
• Completion of First meiotic division
• Division of unequal cytoplasm
• The cell is arrested at Metaphase II
49
52. Capacitation reaction
Elimination of certain molecules from the surface of
the sperm cells
In the female genital ducts
Involves removing the glycoprotein coat and seminal
plasma proteins from the head of the sperm.
Increases motility of the spermatozoa
Prepares for acrosomal reaction
60. Fertilization Cont’d
71
The usual site of fertilization is the
ampulla of the uterine tube, its longest
and widest part.
If the oocyte is not fertilized here, it slowly
passes along the tube to the uterus, where
it degenerates and is resorbed.
Although fertilization may occur in other
parts of the tube, it does not occur in the
uterus.
Chemical signals secreted by the oocyte
and surrounding follicular cells, guide the
capacitated sperms to the oocyte.
61. Phases of Fertilization
1. Passage of a sperm through the corona radiata
2. Penetration of the zona pellucida
3. Fusion of plasma membranes of the oocyte and sperm
4. Completion of the second meiotic division of oocyte and
formation of female pronucleus
5. Formation of the male pronucleus
6. As the pronuclei fuse into a single diploid aggregation of
chromosomes, the ootid becomes a zygote
64. Early pregnancy factor
76
An immunosuppressant
protein;
Secreted by the
trophoblastic cells.
Appears within 24 to 48
hours after fertilization.
Forms the basis of a
pregnancy test during the
first 10 days of
development.
65. Results of Fertilization
1. The oocyte is metabolically activated
2. Completion of the 2nd meiotic division of the oocyte that was
arrested at the metaphase stage giving rise to the definitive
oocyte and secondary polar body
3. Restoration of the diploid state of the chromosome (46) in the
zygote
4. Determination of the genetic sex of the offspring
5. Produce genetically unique individual
6. Initiation of the cleavage
68. Cleavage
• Repeated mitotic divisions of the zygote, resulting in a rapid
increase in the number of cells.
• Smaller blastomeres are formed with subsequent cleavage
divisions.
• After the nine-cell stage, the blastomeres change their shape
and tightly align themselves against each other to form a
compact ball of cells – compaction.
• When there are 12 to 32 blastomeres, the developing human
is called a morula.
69. Morula
• Internal cells of the morula (inner
cell mass) are surrounded by a
layer of cells that form the outer
cell layer
• forms approximately 3 days after
fertilization and enters the uterus.
70.
71. Early Blastocyst
• Occurs when the morula enters the uterus by the 4th day after
fertilization
• Fuid passes from the uterine cavity to the morula through the
zona pellucida
• Now the morula is called blastocyst, fluid-filled space inside the
morula is called blastocystic cavity
72. Late Blastocyst
• As fluid increases in the blastocystic cavity, it separates the cells into
two parts:
• Trophoblast
• thin, outer cell layer
• gives rise to the embryonic part of the placenta
• Embryoblast
• group of centrally located cells
• inner cell mass
• gives rise to the embryo
74. Late Blastocyst
• Embryoblast projects into the blastocystic cavity and the
trophoblast forms the wall of the blastocyst.
• Free blastocyst floats in the uterine secretions for
approximately 2 days.
• Zona pellucida gradually degenerates and disappears.
• Blastocyst increase rapidly in size.
• Early embryo derives nourishment from secretions of the
uterine glands.
75. Implantation
• Approximately 6 days after fertilization, the blastocyst attaches to the endometrial
epithelium, usually adjacent to the embryonic pole
• Trophoblast starts to proliferate rapidly and gradually differentiates into two layers:
• An inner layer of cytotrophoblast
• An outer layer of syncytiotrophoblast consisting of a
multinucleated protoplasmic mass in which no cell
boundaries can be observed
77. Implantation
• At approximately 6 days, syncytiotrophoblast invades the
endometrial epithelium and underlying connective tissue.
• At approximately 7 days, a layer of cells, the hypoblast (primary
endoderm), appears on the surface of the embryoblast facing the
blastocystic cavity.
• By the end of the first week, the blastocyst is superficially
implanted in the endometrium.
78.
79. Ectopic pregnancy
• An ectopic pregnancy occurs when a fertilized egg implants
and grows outside the main cavity of the uterus.
116
81. Second Week of Development
Formation of Bilaminar Germ Disc
82. • Major events during second week;
• Completion of implantation of blastocyst
• Formation of bilaminar embryonic disc
• Formation of Extraembryonic structures;
• The amniotic cavity
• Amnion
• Umbilical vesicle (yolk sac)
• Connecting stalk
• Chorionic sac
Second Week of Development
83. COMPLETION OFIMPLANTATION AND CONTINUATION OFEMBRYONIC
DEVELOPMENT
• Implantation of the blastocyst is completed.
• It occurs during a restricted time period 6 to 10 days after ovulation.
• Trophoblast contacts the endometrium and differentiates into;
• Cytotrophoblast
• Synctiotrophoblast- invades endometrial CT
• Endometrial wall undergoes apoptosis
• Implantation involve synchronization between the
invading blastocyst and a receptive endometrium.
84. Endometrium receptive in the presence of the following
substances
The microvilli of endometrial cells
(pinopodes),
Cell adhesion molecules,
Cytokines,
Prostaglandins,
Homeobox genes,
Growth factors,
Matrix metalloproteins
121
85. COMPLETTE DIFFERENTIATION TROPHOBLAST
The TROPHOBLAST
differentiation:
An inner layer
Mononucleated cells
= Cytotrophoblast
An outer layer
Multinucleated
= Syncytiotrophoblast
86. Decidual reaction
Changes in the endometrium.
Results from the adaptation of these tissues in
preparation for implantation.
The connective tissue cells with glycogen and lipids.
Degeneration of the cells adjacent to the penetrating
syncytiotrophoblast.
- decidual cells
The syncytiotrophoblast engulfs these degenerating cells.
123
87. A small space appears in the
embryoblast.
◦ primordium of the amniotic cavity
Soon amniogenic cells form the
amnion.
◦ encloses the amniotic cavity
Morphologic changes occur in
the embryoblast
◦ Flat bilaminar plate of cells
embryonic disc 12
4
88. Formation of Embryonic Disc
• Cells of inner cell mass also differentiate
into two layers.
– Hypoblast
– Epiblast
• A small cavity appears within
the epiblast.
• This cavity enlarges to become
the amniotic cavity.
• Epiblast cells adjacent to the
cytotrophoblast are called amnioblasts.
89. DEVELOPMENT OF EXTRAEMBRYONIC MESODERM
Cells arising from primitive yolk sac.
Form a layer of loose connective tissue.
Around yolk sac & amnion.
Small cavities appears in the extra
embryonic mesoderm.
Un split part of the extra-embryonic
mesoderm appears.
This mesoderm forms a structure called
the connecting stalk.
92. Day 9
• Blastocyst is more deeply
embedded
• The penetration defect is
closed
• Vacuoles appear in the
syncytium.
• Establishment of the lacunar
stage.
• Hypoblast cells form a thin
membrane.
(exocoelomic (Heuser’s)
membrane)
• Primitive yolk sac
(exocoelomic cavity forms &
is lined by hypoblast &
exocoelomic membrane.
93. Days11 & 12
• Establishment of primordial
uteroplacental circulation
• Lacunar spaces in the syncytium that form
an intercommunicating network.
• Cells of syncytiotrophoblast
• Penetrate deeper into the stroma
• Erode the endothelial lining of the
maternal capillaries.
• These capillaries, which are
congested and dilated, are
known as sinusoids.
• Maternal blood enters the
lacunar system.
94. DEVELOPMENT OF THE CHORIONIC SAC
The end of the second week is characterized by the appearance of primary
chorionic villi.
Proliferation of cytotrophoblastic cells produces cellular extensions that grow
into the syncytiotrophoblast.
The growth of these extensions is thought to be induced by the underlying
extraembryonic somatic mesoderm.
The cellular projections form primary chorionic villi, the first stage in the
development of the chorionic villi of the placenta.
The extraembryonic somatic mesoderm and the two
layers of trophoblast form the chorion.
131
95. DEVELOPMENT OF THE CHORIONIC SAC
• Transvaginal ultrasonography
(endovaginal sonography) is used
for measuring the chorionic
(gestational) sac diameter.
• This measurement is valuable for
evaluating early embryonic
development and pregnancy
outcome.
132
99. Day 14
• At 14th day, embryo is in
the form of flat bilaminar
disc.
• In a circular area, cubical
hypoblastic cells become
columnar
– PRECHORDAL PLATE
101. Development of Third Week
• Beginning of the EMBRYONIC PERIOD (3-8 weeks)
• Period of organogenesis through:
– Cell division
– Cell migration
– Programmed cell death
– Differentiation
– Growth
– Rearrangement
102. Formation of Germ Layers and Early Tissue and Organ
Differentiation: Third Week
• Characterized by;
– Appearance of primitive streak
– Development of three germ layers
– Development of notochord
– Formation of neural tube
– Development of intra-embryonic mesoderm
– Development of somites
– Early development of cardiovascular system
– Development of chorionic villi
139
103. Gastrulation
Bilaminar Trilaminar embryonic germ disc
The embryo is called Gastrula at this stage
Indicated by the formation of the primitive streak
The process of morphogenesis
104. Formation of Germ Layers and Early Tissue and Organ
Differentiation: Third Week
105. FORMATION OF GERM LAYERS AND EARLY TISSUE AND
ORGAN DIFFERENTIATION: THIRD WEEK
From Epiblast cells
Migrate to the midline and then
inward and laterally between
the epiblast and hypoblast
106. Function of primitive streak
Enables identification of:
Craniocaudal axis
Cranial from caudal end
Dorsal from ventral surface
Right from left side
Initiates formation of notochord
145
111. Neural crest
• By cells detaching from the edges of the neural folds.
• Give rise to:
– Peripheral nervous system and their ganglia
– Chromaffin cells of the adrenal medulla
– Schwann cells
– Mesenchyme of the pharyngeal arches as
ectomesenchyme
– Odontoblasts
– Meninges (Arachnoid mater and pia mater)
– Parafollicular cells of the thyroid gland
112. Differentiation of the intraembryonic mesoderm
• Regionally differentiates into:
– Paraxial mesoderm
– Intermediate mesoderm
– Lateral mesoderm
113. Cylindrical column lateral to the notochord
It differentiates and condenses into somitomers
Form somites
Beginning from day 20, three somites appear
each (days 20 – 30)
115. Somites
Give a total of 42 – 44 pairs by the end of 5th week.
Identified as:
4 pairs of occipital somites
8 pairs of cervical somites
12 pairs of thoracic somites
5 pairs of lumbar somites
5 pairs of sacral somites
8-10 pairs of coccygeal somites
116. Somites
Each somite differentiates into two parts:
The ventromedial part is sclerotome
Medially surrounding the notochord & neural tube
Its cells form the vertebrae and ribs
The dorsolateral part is the dermomyotome
Cells from myotome
give rise to most of the skeletal muscles of the body
Cells from dermatome form the dermis of the skin
118. Lateral plate mesoderm
Cavities appear in the
lateral mesoderm and
coalesce giving horseshoe
shaped intraembryonic
coelomic cavity
Develops into:
Pericardial cavity
Pleural cavity
Peritoneal cavity
119. Lateral mesoderm
• Divided by the intraembryonic coelomic cavity into:
• Somatic (parietal) mesoderm layer
– Continuous with the extraembryonic mesoderm covering
amnion
– Give rise to the body wall or somatopleure with overlying
ectoderm
• Splanchnic (visceral) mesoderm layer
– Continuous with the extraembryonic mesoderm covering the
umbilical vesicle
– Gives rise to the embryonic gut wall or splanchnopleure with
the endoderm.
120. Development of blood vessels
Blood vessel formation (Vasculogenesis &
angiogenesis)
Begins in the extraembryonic mesoderm of:
Yolk sac (Umbilical vesicle)
Connecting stalk
Chorion
In the intraembryonic sites appears later
121. Development of blood vessels
Vasculogenesis – formation of new vascular
channels by assembly of individual cell precursors.
Angiogenesis – formation of new vessels by
budding and branching from pre-existing vessels.
173
122. Development of blood
Appears first in the extraembryonic sites of the yolk
sac and allantois.
In embryo appears by the 5th week, first in the liver
and then in spleen, bone marrow and lymph
node.
123. Development of the heart
Develops by fusion of a bilateral endocardial heart
tubes in the cardiogenic area (mesoderm located
cranial to the prechordal plate)
Is the first organ system to reach to a functional state
by day 21 (22).
125. Development of the chorionic villi
Has three stages:
1. Primary chorionic villi – core of
cytotrophoblast covered by
syncytiotrophoblast
1. Secondary chorionic villi – when
extraembryonic mesoderm grows into
the core of primary villi
2. Tertiary chorionic villi – when blood
vessels (with blood cells and capillaries)
appear in the connective tissue core
126. Types of Tertiary chorionic villi
1. Anchoring (stem) villi – connected to the decidua
basalis (run from chorionic plate to decidual plate)
1. Floating (branch) villi
Branches of the stem villi floating in the maternal blood in the
intervillous space
As placental membrane are engaged in exchange of materials
between maternal and embryonic blood by the end of the 3rd
week
127.
128. Development of tissues & organs
Body forms take place
Occurs in 3 phases
Growth
Development
Differentiation
The body form
folding of the embryo
Trilaminar to Cylindrical Embryo
Development of the 4th – 8th Weeks
133. Development at the beginning of 4th Week
• Major changes occur in
the body form.
• Embryo is almost
straight.
• Has 4 – 12 Somites and
produce surface
elevations.
• Neural tube shows
rostral and caudal
134. Development at the 24 days
• The 1st and 2nd pharyngeal arches
are distinctly visible.
• The embryo is slightly curved
because of the head and tail folds.
• Heart is prominent and has begun to
pump blood.
135. Development at the 26 days
• Three pairs of pharyngeal arches are
visible.
• Rostral neuropore is closed.
• Forebrain makes prominent elevation.
• The embryo is curved to become C-
shaped.
• Upper limb buds appear.
• Otic and lens placodes appear.
136. Development at the end of 4th week
• The 4th pharyngeal
arches appear.
• Lower limb buds appear.
• A long tail-like caudal
eminence is visible.
• Caudal neuropore is
closed.
137. Development at the 5th week
• Changes in the body form are
minor.
• The head grows because of the
growth of the brain.
• Face contacts the heart
prominence because of the
head fold.
138. Development at the 6th week
• The embryo shows reflex response to touch.
• The embryo shows spontaneous movement, such as twitching
of the trunk and limbs.
• There is rapid regional
differentiation of upper limb.
• Elbow & hand plates develop.
139. Development at the 6th week
The external acoustic
meatus develops.
Eyes are obvious.
Umbilical intestinal
herniation takes place.
140. Development at the 7th week
• More differentiation
occurs in the limbs.
• Ossification of upper limb
bones has begun.
141. Development at the 8th week
• Further development of the limbs takes place showing
purposeful movement.
• Tail has become
smaller.
• Scalp vascular plexus.
142. Development at the end of the 8th week
Tail disappears
Head is large and makes almost half of the embryo
Neck is established
Sex is different in appearance of
the external genitalia, but not
distinctive enough.
The embryo appears distinctly
human looking.
Embryo is about 27-31mm long
143. 1. Measuring the length of embryo
2. By counting the number of somites
144. Greatest length (GL)
Is the length from most cranial to the caudal ends of the embryo
Is suitable for embryos of 3rd and early 4th weeks as they are straight
at this period
Crown-rump length (CRL)
Is the sitting height
In greatly flexed embryo, is neck-rump length
Suitable for older embryos
Crown-heel length (CHL)
Is the standing height
Sometimes suitable for embryos of 8th weeks
Measuring the length of the embryo
146. Derivatives of the Ectoderm: Neuroectoderm
Neural tube
CNS
Retina
Pineal body
Posterior pituitary
Neural crest
Peripheral nervous system and
their ganglia
Chromaffin cells of the adrenal
medulla
Schwann cells
Mesenchyme of pharyngeal
arches as ectomesenchyme
Odontoblasts
Meninges
Parafollicular cells of the thyroid
gland
Cells of the trancoconal cushions
of the heart
147. Derivatives of the Ectoderm
• Surface ectoderm
– Epidermis, hairs, nails and cutaneous & mammary glands
– Epithelia of the cornea and conjunctica, lacrimal glands &
nasolacrimal ducts
– Epithelia of nasal, paranasal sinus, lips, cheeks, gums &
palate
– Salivary glands
– Epithelia of lower anal canal & terminal male urethra
– Epithelia of external acoustic meatus
– Anterior pituitary gland
– Enamel
– Inner ear
– Lens
148. Derivatives of the Mesoderm
In the head region
Skull
Muscle
Connective tissue of
head
Dentine
Paraxial mesoderm
Muscles of trunk
Skeleton, except skull
Dermis
Connective tissues
Intermediate mesoderm
Urogenital system
Lateral mesoderm
Connective tissue & muscles
of viscera and limbs
Serous membrane of pleura,
pericardium & peritoneum
Cardiovascular and
lymphatic system & blood
cells
Spleen
Adrenal cortex
149. Derivatives of the Endoderm
Epithelium of:
Gastrointestinal tract, liver, pancreas
Respiratory tract
Urinary bladder, most of urethra & urachus
Tympanic cavity, tympanic antrum, pharyngotympanic
tube and tonsils
Thymus, thyroid and parathyroid glands
150.
151. The Fetal Period
• The period from the beginning of the 9th week to birth
• Further differentiation and development of organ
systems.
• Generalized slow down of growth of the head
• The length of the fetus is calculated by
– Crown-rump length (CRL)
– Crown-heel length (CHL)
154. 9th-12th weeks
Urine formation & urination into amniotic fluid occurs
During the 12th week
External genitalia is different in the two sexes
Erythropoiesis has decreased in the liver and has
begun in spleen
Upper limbs have almost reached their final
155. 13th-16th weeks
Growth is rapid
Scalp hair pattern has
developed
During the 14th week
Slow eye movements occur
13-week fetus
156. 13th-17th weeks
– The proportion of the head is relatively smaller
compared with that of the 12th week
– Lower limbs have lengthened
– Ovaries are differentiated and contain primary
follicles
– Eyes and external ears are close to their definitive
position
157. 13th-17th weeks
– Lower limbs reach their final
relative proportional length.
– Quickening (fetal movement)
commonly felt by the mother
– Skin is covered with vernix
caseosa:
158. 17th-20th weeks
By 18 weeks, the uterus is formed and canalization
of the vagina has begun.
During the 19th week
Testes and ovaries have begun to descend, but are still
found on the posterior abdominal wall
Lanugo hairs (fine wool-like hairs)
Cover the body completely
159. 21st -25th weeks
• During the 21st -25th weeks
– Growth slows down
– Pronounced weight gain occurs
– Skin is usually wrinkled and more translucent and
reddish
• During the 24th week
– Type II pneumocytes have begun to secret surfactant
–Fingernails are present
160. 26th-29th weeks
• During the 26th-29th weeks
– Lungs are capable of functioning
– CNS has matured to a stage of controlling respiration
and body temperature
– If born, the fetus often survives with intensive care
• During the 26th weeks
– Eyes reopen
– Lanugo hairs and head hairs are well developed
161. 30th week
Pupillary light reflex can be elicited
During the 34th week
Skin is pink and smooth
Upper and lower limbs appear fat
Fetus of 32 weeks and older usually survives
162. 35th- 38th weeks
Nervous system is sufficiently mature to
carry out some integrative function
Growth slows down
163. Developments at full term
• Amount of white fat is about 16% of body weight
• Skin is bluish-pink
• Chest is prominent and breasts protrude slightly in both
sexes
• Testes have descended and are usually in the scrotum
• Head is still one of the largest region of the fetal body
164. The Neonate
Measures approximately 36cm in crown rump length
(CRL) or 50cm in crown heel length (CHL)
Weighs 3000-3400gm.
However, if weighs
500-1000gm is immature, but may survive
1500-2500gm is premature and survive, but faces
difficulties
165. Intrauterine growth retardation (IUGR)
Weigh 2500gm or less
The skin is wrinkled because of lack of subcutaneous
fatty tissue
Cause may be:
Placental insufficiency
Multiple gestation
Malnourishment of the mother
Smoking of the mother
Hormonal effect (maternal or fetal)
Cardiovascular malformations or other congenital
malformations
166. Prolonged pregnancy & postmaturity
syndromes
Thin and dry parchment-like skin
No lanugo hairs
Vernix caseosa will be reduced or absent
Long nails
Overweight
167. The status of fetus could be assessed by
• Ultrasonic measurement
• Chorionic villus sampling (CVS)
– Could be performed as early as the 9th week
– Provide information, such as
• Chromosomal abnormalities
• Sex-linked disorders
• Identification of the sex
168. The status of fetus could be assessed by
Diagnostic amniocentesis
By sampling amniotic fluid
Done during 15th – 18th weeks
Used for;
Identification of the sex of the fetus
Any chromosomal abnormalities
170. The Placenta
The placenta
organ that facilitates nutrient & gas exchange between
the maternal & fetal blood streams.
Fetal membranes include:
Chorion
Amnion
Umbilical vesicle
Allantois
The placenta and fetal membranes
Separate the fetus from the endometrium.
Shortly after birth, are expelled from the uterus
171. The Placenta
Primary site of nutrient and gas exchange between the
mother and embryo/fetus.
Fetomaternal organ that has two components:
Fetal part – Develops from the chorionic sac – Villous
chorion (Trophoblast & extraembryonic mesoderm)
Maternal part – Derived from the endometrium
(decidua basalis)
173. The Decidua
After implantation, the uterine endometrium
is called the decidua.
Decidua refers to the gravid endometrium
Has three parts
Decidua basalis – part deep to the conceptus
Forms the maternal part of the placenta
Decidua capsularis – superficial part overlying the
conceptus.
Decidua parietalis – all the remaining parts of the
decidua.
175. Full-Term Placenta
• Discoid in shape
• 15 to 25 cm in diameter, 3 cm thick, about 500 – 600 gm.
• At birth, it is torn from the uterine wall and is expelled as the
afterbirth.
• It presents two surfaces: maternal and fetal
– Its maternal surface presents 15–20 lobes/ cotyledons.
– Its fetal surface presents a smooth shining surface.
177. Full-Term Placenta
Fetal surface:
This side is smooth and shiny.
It is covered by amnion.
The umbilical cord is
attached close to the center
of the placenta.
The umbilical vessels
radiate from the umbilical
cord.
They branch on the fetal
surface to form chorionic
vessels.
179. Functions of the Placenta
• The placenta has three
main functions:
– Metabolic functions
• Synthesis of glycogen,
cholesterol, and fatty
acids
– Transport of gases
and nutrients
• Simple diffusion
• Facilitated diffusion
• Active transport
• Pinocytosis
• Endocrine functions
– Human chorionic
gonadotropin [hCG]
– Progesterone
– Estrogen
– Human chorionic
somatomammotropin
– Human chorionic thyrotropin
– Human chorionic corticotropin
182. Amniotic Fluid
Clear, watery fluid which fills amniotic cavity
It is derived from:
Amniotic cells by filtration or secretion
Fetal urine when kidneys start functioning (500 ml in
late pregnancy)
Secretion of lung cells
Secretion by placenta
183. Amniotic Fluid
Amount
30ml at 10 weeks
450 ml at 20 weeks
800 to 1000 ml at 37 weeks
The volume of amniotic fluid is replaced every 3 hrs
Fetus drinks about 400ml of amniotic fluid per day
235
184. Amniotic Fluid
Functions
Permits symmetric external growth of the embryo and fetus
Cushions the embryo and fetus against injuries
Prevents adherence of the embryo to the amnion
Allows free fetal movements
Helps control the embryo's body temperature by maintaining
a relatively constant temperature
185. Multiple Pregnancy
The nurturing of two conceptuses at the same time is
termed twinning.
Twins that originate from two zygotes are dizygotic (DZ)
twins or fraternal twins, whereas twins that originate from
one zygote are monozygotic (MZ) twins or identical twins.
The fetal membranes and placentas vary according to the
origin of the twins.
In the case of MZ twins, the type of placenta and
membranes formed depends on when the twinning process
occurs.
Approximately two thirds of twins are DZ.
186. Dizygotic Twins
2/3rd of twins are dizygotic
formed from fertilization of two oocytes
DZ twins develop from two zygotes
May be of the same sex or different sexes.
They are no more alike genetically than brothers or sisters
born at different times.
DZ twins always have two amnions and two chorions, but the
chorions and placentas may be fused.
DZ twinning shows a hereditary tendency.
Recurrence in families is approximately three times that of
the general population.
187.
188. Monozygotic Twins
formed from the fertilization of one oocyte
develop from one zygote
results from splitting of zygote at various stages of development
MZ twins are of the same sex
genetically identical, and very similar in physical appearance.
Splitting occurs at 2 cell stage
MZ twinning usually begins in the blastocyst stage,
Two embryos, each in its own amniotic sac, develop within the
same chorionic sac and share a common placenta (a
monochorionic-diamniotic twin placenta).
In rare case spliting occurs at bilaminar disc stage
189. Monozygotic Twins
The outcome of the twinning process depends on when the
division occurs.
If division occurs within the first 72 hours after fertilization, the inner
cell mass (morula) has yet to form and the outer layer of blastocyst
has not yet committed to become chorion. Two embryos, two
amnions, and two chorions develop, and a monozygotic, diamnionic,
dichorionic twin pregnancy evolves. Two distinct placentas or a
single fused placenta may develop.
If division occurs between the fourth and eighth day, the inner cell
mass has formed and cells destined to become chorion have
already differentiated, but those of the amnion have not. From this
division, two embryos develop, each in a separate amnionic sac
covered by a common chorion. This division gives rise to a
monozygotic, diamnionic, monochorionic twin pregnancy.
190. Monozygotic Twins
The outcome of the twinning process depends on when the
division occurs.
If occurs by about 8 days after fertilization, division results in
two embryos within a common amnionic sac
monozygotic, monoamnionic, monochorionic twin pregnancy.
If division is initiated after the embryonic disk has formed,
cleavage is incomplete and conjoined twins result
194. Human Birth Defects
Structural, behavioural, functional and metabolic disorders
present at birth
Teratology is the science that study birth defects
May be Minor or Major
195. Minor Congenital Anomalies
Occur in about 15% of the newborns
Are not detrimental to the health of the individual
In some cases, are associated with major defects and
could serve as a clue for diagnosing more serious
underlying defects
90% of infants with multiple minor anomalies have one
or more associated major anomalies
196. Major Congenital Anomalies
2-3% in the newborn infants
10-15% in early embryos, but decrease later
because of spontaneous abortion during the first 6-8
weeks
About 1/3rd of all zygotes formed will never reach the
stage of blastocyst and get implanted because of
lethal chromosomal abnormalities or poorly developed
endometrium
197. Major Congenital Anomalies
The incidence of congenital anomalies in the major
organs 3% all together and is:
1% for the Brain
0.8% for the Heart
0.4% for the Kidneys
0.2% for the Limbs
0.6% for all the other organs
0.7% of the newborns have multiple major
anomalies
198. Major Congenital Anomalies
Additional anomalies can be detected after birth raising
the incidence of major congenital anomalies to about:
6% in 2-year-olds
8% in 5-year-olds
Approximately 2% additional anomalies are detected
later (e.g., during surgery, dissection, or autopsy)
199. Causes of congenital anomalies
1. Genetic factors – accounts for 13-15%
2. Environmental factors – accounts for 7-10%
3. Combination of genetic and environmental factors –
account for 20-25%
Unknown factors – account for 50-60%
201. Genetic Factors
Could be:
1. Numerical chromosomal abnormalities
Involving changes in the number of chromosomes
Occur due to
failure of meiotic division to occur or
abnormal meiotic division during gametogenesis
2. Structural chromosomal abnormalities
Involving changes in the structure of chromosomes
Deletion, Translocation…
3. Mutations
202. Numerical chromosomal abnormalities
May be:
1. Aneuploidy
Involves a specific chromosome
Caused by failure of separation of the chromosomal
pair of the two chromatids of a chromosome during
cell division as nondisjunction
Occur as hypodiploidy or hyperdiploidy
2. Polyploidy
Involves the whole set of chromosome
chromosome number is increased in a multiple of haploid
(23) set of chromosomes
As triploidy & tetraploidy
203.
204.
205. Hypodiploidy (Monosomy)
When one of the paired chromosomes is missing
leading to monosomy
Could occur in the sex chromosomes or any of the
autosomal chromosomes
206. Monosomy in the sex chromosomes
Cause death in about 99% of the cases
In about 1% of the cases and when the present single
chromosome is the X chromosome, the embryos
survive and show Turner’s syndrome
207. Turner’s syndrome
45, X0 chromosomes
Female phenotype
Gonadal dysgenesis and no ovary
Webbed neck
Lymphedema of the extrimities
Skeletal deformation, broad chest
and short stature
Mental retardation
209. Monosomy in the autosomal chromosomes
The embryo die leading to spontaneous abortion,
and are not seen in the population
210. Hyperdiploidy
Presence of extra chromosome(s) leading to trisomy,
tetrasomy, etc. in a specific chromosome(s)
Most commonly involves trisomy of either the sex or
autosomal chromosomes
211. Trisomy in the sex chromosomes
Is common, but not usually detected until at the
adolescence age
Could be:
1. 47, XXX
• Is normal appearing and usually fertile female, but
in about 15-25% may show mild mental retardation
2. 47, XYY
Is normal appearing male, but aggressive behaviour
Usually tall
212. Trisomy in the sex chromosomes
3. 47, XXY
Is male and shows Klinefelter’s
syndrome with:
− Sterile with testicular atrophy and
hyalinization of the seminiferous
tubules and gynecomastia
− Long lower limbs
− Lower intelligence
213. Trisomy in the autosomal chromosomes
Those seen include:
1. Trisomy of chromosome 21
About 75% will die and get aborted, 20% stillborn
and remaining will cause the Down syndrome
Increases with increasing maternal age
Trisomy 21 occurs once in:
1100 births in mothers aged 25 years
350 births in mothers aged 35 years
25 births in mothers aged 45 years
217. Trisomy in the autosomal chromosomes
Those seen include:
2. Trisomy of chromosomes 13 (Patau syndrome) &
18 (Edwards syndrome)
Are less commonly seen because of presence of several
malformations and usually die before reaching the age of
6th months.
218. Polyploidy
Multiplication of the haploid chromosomes in sets and could
be:
1. Triploidy with 3n (69 chromosomes)
May be caused by dispermy or failure of the
separation of the 2nd polar body from the oocyte
results in spontaneous abortion of the conceptus
or brief survival of live-born infant after birth
2. Tetraploidy with 4n (92 chromosomes)
Abort very early and not seen
219. Mosaicism
Presence of two cell lines with two or more different
genotypes, one normal and the other(s) defective
Could affect either autosomes or the sex chromosomes
Usually causes less serious anomalies than those of
monosomy or trisomy for the defect is not fully
expressed
Usually caused by nondisjunction during early
cleavage of the zygote
220. Structural chromosomal abnormalities
Mainly by breakage and reconstitution of
chromosomes during meiotic division in abnormal
combination as:
− Inversion
− Translocation
− Deletion
Abnormalities caused depend on the fate of the
broken piece, e.g.,
– Translocation of chromosome number 21 is the cause for
3-4% of the Down syndrome
221. Structural chromosomal abnormalities
Deletion in the short arm of
chromosome number 5 cause
Cri du chat syndrome with:
weak cat-like cry
Microcephaly
Mental retardation
Congenital heart malformation
222. Gene mutation
Permanent change in the sequence of genomic DNA, causing
loss or change in the function of a gene as single gene
mutation
Occur as:
Dominantly inheritable congenital anomalies
Cause birth defects in a single (hetrozygous) dose
Recessively inheritable mutations
Cause birth defect when occur in a double
(homozygous) dose and rarely in a single
(hetrozygous) dose
223. Dominantly inheritable congenital anomalies
Achondroplasia
Homozygous mutant genes are fatal
before or shortly after birth.
Hetrozygous cause:
Short stature and limbs
Normal length of the trunk
Relatively large head
Depressed nasal bridge
224. Dominantly inheritable congenital anomalies
Polydactyly (extradigits)
By an autosomal dominant mutations in a variety of single
genes
225. Environmental factors (Teratogens)
Agents that cause or raise the incidence of congenital
anomalies
Determined by the following three factors:
1. The developmental period
First two weeks (predifferentiation or pregerm layer)
Embryonic period
Fetal period
2. Dose of the teratogen
3. Genotype of the embryo
226. Effect of teratogens during the first 2 weeks
This is a period of predifferentiation or pregerm layer
Teratogens within this period interfere with cleavage,
implantation and formation of the extraembryonic
structures and could cause:
a. Early death and spontaneous abortion, or
b. Damage only to some of the cells. Remaining undamaged
cells compensate for those loss and develop normally
causing no congenital anomalies.
227. Effect of teratogens during the embryonic period
This is organogenetic period
Disrupt organogenesis and cause birth defects, each
tissue or organ showing its own critical period of
susceptibility
228. Effect of teratogens during the fetal period
This is a period of growth and maturation of organ
systems
Teratogens cause only functional defects, such as mental
retardation, and/ or minor congenital anomalies
229. Environmental factors (Teratogens)
Dose of the teratogens
The severity of the birth defect is directly related to the
dose of the teratogen
Genotype of the embryo
All genotypes are not equally susceptible to a specific
teratogen causing congenital anomalies.
230. Types of teratogens causing human birth
defects
1. Infectious agents
• Metabolic (Diseases)
2. Drugs and chemicals
• Alcohol, Heroin, Narcotics, Nicotine
3. Radiations
231. Metabolic Teratogens
Rubella
cardiovascular defects, deafness, blindness,
slow growth of fetus
Syphilis
deafness, mental retardation, skin & bone
lesions, meningitis
Toxoplasmosis
microcephaly, hydrocephaly, cerebral
calcification, mental retardation
Diabetes
cardiac and skeletal malformations, central
nervous system anomalies; increased risk of
stillbirth
Herpes Simplex skin lesions, encephalitis
Mumps spontaneous abortion
232. Chemical Teratogens
Alcohol
growth & mental retardation, microcephaly,
facial and trunk malformations
Chemotherapy major anomalies throughout body
Diethylstilbestrol cervical and uterine abnormalities
Lithium hearing anomalies
Mercury
mental retardation, cerebral atrophy,
spasticity, blindness
Streptomycin hearing loss, auditory nerve damage
Tetracycline staining of tooth enamel and bones
Thalidomine limb defects, cardiovascular anomalies
233. Alcohol & Nicotine
The most common defect of addictive substances, including
nicotine, is low birth weight
Infants born to addicted women will also be addicted.
Fetal Alcohol Syndrome
Growth deficiencies
Skeletal and facial deformities
Organ deformities: heart defects; genital malformations;
kidney and urinary defects.
Central nervous system handicaps: small brain; mental
retardation learning disabilities; hyperactivity, poor
coordination.
