The document summarizes key stages of embryonic development from the third to eighth week. During this period, the notochord forms and somites appear on each side of the embryo. Organs also begin to form from the three germ layers. The embryonic period is then divided into pre-somite, somite, and post-somite stages based on the presence of somites. Criteria for estimating embryonic age include structures like the primitive streak and number of somites. Folding of the embryo and differentiation of the ectoderm, mesoderm and endoderm are also discussed.

2. KEY POINTS
• Notochord formation
• Development of Somites and Intra embryonic coelom
• Neurulation
• Embryonic folding
• Diagnosis of Pregnancy in first trimester

3. • Period from third week to eighth week is called embryonic period.
• End of the third week ,differentiation of three germ layers and folding of the
embryonic disc to form the species specific shape of embryo
• appearance of mesodermal somites on each side of the midline.
• During the second month, the primordia of organs are formed from the
various germ layers.
• Hence, it is also known as period of organogenesis
INTRODUCTION

4. embryonic period it is divided into the 3 stages
Pre-somite (15th-20th day of development)
Somite (20th–30th day of development)
Post-somite (31st day to 55th day)
CRITERIA USED FOR AGE ESTIMATION OF AN EMBRYO
AT DIFFERENT EMBRYONIC AND FETAL PERIODS
Pre-somite (15th-20th day of development)
The following structures develop during this period
• Primitive streak
• Notochord
• Intraembryonic mesoderm

5. primitive streak
yolk sac
16th day
cloacal membrane
Primitive Streak
The primitive streak gives rise to the
following structures:
1. Intraembryonic mesoderm
2. Septum transversum
3. Notochord
4.Determines the future
craniocaudal axis of the embryo
5. Demarcates the embryo into left
and right halves
Ectodermal cells lying along central
axis near tail end begin to
proliferate & forms elevation in the
amniotic cavity
amniotic cavity

6. primitive streak
primitive node
primitive node
ectoderm
(epiblast)
notochordal process
cloacal
membrane
yolk sac
invagination of
cells - mesodermal
endoderm
(hypoblast)
primitive streak
16th day oropharyngeal membrane
Ingression

7. Formation of Notochord
The notochord is a midline structure that develops in the region between the
primitive streak and the prochordal plate.
It develops from the primitive knot (Hensen’s node) of the primitive streak.
Functions of Notochord
1. It forms the central axis of the developing embryo (embryonic disc).
2. It induces the formation of neural tube from the overlying ectoderm.
3.It provides central column around which vertebral bodies and
intervertebral discs develop.
Remnants of notochord - nucleus pulposus of the intervertebral discs
apical ligament of dens of second cervical vertebra

8. ectoderm
notochordal process
endoderm
mesoderm
allantois
notochordal canaliculus
Development of the notochordal process and notochord

9. ectoderm
canalis neurentericus
extraembryonic
mesoderm
notochordal plate
notochordal plate
endoderm
paraaxial mesoderm
intermediate mesoderm
lateral
mesoderm
allantois

10. ectoderm
endoderm
canalis neurentericus extraembryonic
mesoderm
endoderm
notochord
ectoderm
mesoderm
notochord

11. wall of amniotic
cavity
neural plate
neural plate
neural groove
somite
primitive node
primitive streak
19th day

13. Somite (20th–30th day of development)
• There are 44 pairs of somites that are derived from paraxial mesoderm.
• Once the somites appear, the age is described in terms of the number of
somites present.
• The age of embryo is estimated between 20th to 30th day depending on
the presence of number of somites.
CRITERIA USED FOR AGE ESTIMATION OF AN EMBRYO
AT DIFFERENT EMBRYONIC AND FETAL PERIODS

14. neuroporus anterior
neuroporus posterior
amnion amnion
22th day
23th day
neural plate
heart prominence
somites
somites

15. 28th day
neuroporus anterior
neuroporus
posterior
heart prominence
25th day
amnion
yolk sac
1& 2 branchial
arch
eye placod
otic pit
branchial arches
upper limb bud
heart
prominence
funiculus
umbilicalis

17. Post-somite (31st day to 55th day)
The length and weight of fetus is measured by trans-abdominal ultrasound
• Crown-rump length (CRL)-
Sitting height measured from the vertex of the skull to the
midpoint between the apices of the buttocks
• Crown-heel length (CHL):
Standing height measured from the vertex of the skull to the heel.
CRITERIA USED FOR AGE ESTIMATION OF AN EMBRYO
AT DIFFERENT EMBRYONIC AND FETAL PERIODS

18. The changes during the embryonic period can be studied under the
following headings:
• Folding of embryo
• Differentiation of ectoderm
• Differentiation of mesoderm
• Differentiation of endoderm
EMBRYONIC PERIOD

19. • There is progressive increase in the size of the embryonic disc which results
in formation of a head fold, a tail fold and two lateral folds at the end of
3rd week of development.
• With further enlargement, the edges of embryonic disc become folded on
itself in the median and in the transverse planes.
• The folding in the median plane form ventrally directed head fold and
tail fold
• The folding in the transverse plane forms ventrally directed lateral folds
FOLDING OF EMBRYO

20. Folding of the Embryonic Body

21. • With the formation of the head and tail folds, parts of the yolk sac
become enclosed within the embryo.
• In this way, a tube lined by endoderm is formed in the embryo.
• This is the primitive gut, from which most of the gastrointestinal tract is
derived.
• The part of the gut cranial to this communication and in the head fold is
called the foregut; the part caudal to the communication and in the tail
fold is called the hindgut; while the intervening part is called the midgut.
FOLDING OF EMBRYO
• As a result of these changes, the yolk sac becomes small and inconspicuous,
and is now termed the definitive yolk sac (umbilical vesicle).
• The narrow channel connecting it to the gut is called the vitellointestinal
duct ( vitelline duct; yolk stalk or omphalomesenteric duct).

22. • As a result, the embryo comes to be enclosed all around by ectoderm except
in the region through which the vitello-intestinal duct where there is a
circular aperture which may now be called the umbilical opening.
FOLDING OF EMBRYO
• The folding facilitates growth and expansion of amniotic cavity that comes
to surround the embryo on all sides such that the embryo floats in the
amniotic fluid, which fills the cavity.
• The amnion forms a covering for the umbilical cord.
• The stomodeum is an ectodermal depression at the head end between the
bulging head and pericardial bulge.
• The buccopharyngeal membrane breaks at 4th week and the cloacal
membrane at 7th week.

24. • Connecting stalk- During the formation of
extraembryonic coelom the embryo (along with
the amniotic cavity and yolk sac) remains
attached to the trophoblast only by extra-
embryonic mesoderm into which the coelom
does not extend.
• This extraembryonic mesoderm forms the
connecting stalk.
• Only connecting link between the embryo and
the placenta.
FOLDING OF EMBRYO
• Formation of tail fold moves attachment of
connecting stalk form dorsal end of germ disc to
ventral aspect of embryo and limit it at umbilical
opening.

25. • As the embryo grows, the area of attachment of the connecting stalk to it
becomes relatively smaller.
• Gradually, this attachment is seen only near the caudal end of the
embryonic disc.
• With the formation of the tail fold, the attachment of the connecting stalk
moves to the ventral aspect of the embryo.
• It is now attached in the region of the umbilical opening.
FOLDING OF EMBRYO

26. • At this stage, the amnion has a circular attachment to the margins of the
umbilical opening and forms a wide tube in which the following lie:
Vitellointestinal duct and remnants of the yolk sac.
Mesoderm (extraembryonic) of the connecting stalk. This mesoderm
gets converted into a gelatinous substance called Wharton’s jelly.
Blood vessels that pass from the embryo to placenta.
A small part of the extraembryonic coelom.
• This tube of amnion, and the structures within it, constitutes the umbilical
cord.
FOLDING OF EMBRYO

27. Allantoic Diverticulum
Before the formation of the tail fold, a small tubular
endodermal diverticulum called the allantoic
diverticulum arises from the posterior wall of yolk
sac near the caudal end of the embryonic disc.
• Appears at 16th day
• After the formation of the tail fold, part of this
diverticulum is absorbed into the hindgut (that
forms the urinary bladder).
• It now passes from the ventral side of the hindgut
into the connecting stalk.
FOLDING OF EMBRYO

29. Effect of Head and Tail Folds on Positions of Other Structures
From the cranial to the caudal end, the structures seen in the midline are
1. Septum transversum
2. Developing pericardial cavity and the heart
3. Prochordal plate
4. Neural plate
5. Primitive streak
6. Cloacal membrane
FOLDING OF EMBRYO

30. • Surface Ectoderm - Derivatives of surface ectoderm are:
Buccopharyngeal membrane
Cloacal membrane
Protective covering - Epidermis, hair and nail
Special sense organs - Olfactory pit ,Optic vesicle and lens vesicle , Otic vesicle
Branchial clefts
 Rathke’s pouch
Mammary glands
 Pituitary glands
Others: Parts of mouth, salivary glands, nasal cavity and paranasal air sinuses, Epithelial
lining of cheek, gum, teeth enamel, root of mouth , Sebaceous glands and sweat glands
DIFFERENTIATION OF ECTODERM

31. • Neurectoderm:
the surface ectoderm overlying the developing notochord thickens to
form the neural plate or neurectoderm.
Central part of neural plate shows a depressed area called neural groove.
DIFFERENTIATION OF ECTODERM
There will be two lateral folds known as neural folds. The two folds come
together and fuse in the cervical region.
Later, the fusion of folds that extends cranially and caudally to form the
neural tube.
For sometime the cranial and caudal ends of the neural tube is open and
the open ends are known as anterior and posterior neuropores
respectively.

32. Neurulation
It is the process of formation of neural tube
by conversion of neural plate.
• The neural plate becomes depressed
along the midline with raised margins
(edges) on either side.
• This result in the formation of neural
groove and folds on the dorsal aspect of
the neural plate.
• This groove progressively becomes
deeper.
• The surface ectodermal cells at the
edges of neural plate differentiate into
neural crest cells.
1. Neural plate
2. Neural groove
4. Neural tube
5. Separation of neural
crest
6. Developing of
epidermis
3. Neural crest

33. notochord
amniotic cavity
ectoderm
mesoderm
paraaxial
mesoderm intermediate
mesoderm
amnion neural groove
visceral
mesoderm
intermediate
mesoderm
somite
17th day 19th day
20th day 21th day
endoderm of
yolk sac
parietal mesoderm
yolk sac
endoderm
neural tube

34. • Neural Crest Cells:
At the junction of surface ectoderm with neurectoderm there is formation of specialized
cells called neural crest cells. With the formation of neural tube these cells form bilateral
masses along the dorso-lateral aspect of neural tube deep to surface ectoderm.
Neural crest gives rise to
cranial and sensory nerves and ganglia
Adrenal medulla
Pigment cells
Pharyngeal arch cartilages
Head mesenchyme and connective tissue
Bulbar and conal ridges in heart
Thyroid gland C Cells
DIFFERENTIATION OF ECTODERM

35. • Paraxial mesoderm
The paraxial mesoderm is on either side of the notochord.
It extends from primitive streak caudally to prochordal plate cranially.
The developing otic capsules (membranous labyrinth) divide the paraxial
mesoderm into preotic and postotic parts.
The preotic part is unsegmented where as the postotic part shows 40–44
pairs of segments called somites.
DIFFERENTIATION OF INTRAEMBRYONIC MESODERM

36. The somites appear from day 20 to day 30 in craniocaudal sequence and
are triangular in shape.
Each somite differentiates into three parts:
1. Dermatome: It forms the dermis of the skin.
2. Sclerotome: It helps to form the cartilage and bone components of the
vertebral column and ribs
3. Myotome: It forms skeletal muscles of the trunk and limbs.
DIFFERENTIATION OF INTRAEMBRYONIC MESODERM

37. • Paraxial mesoderm is segmented into cubical masses
• Present either side of midline, little behind the prochordal
plate.
• Formation is in craniocaudal direction on either side of
neural tube
Somitomeres
• Consist of mesodermal cells arranged in concentric whorls
around the center of the unit.
• 1st pair of somites seen in cervical region at about 20th day
further 3pairs / day
• At the end of 5th wk 42-44 pairs
• 4 occipital, 7 cervical, 12 thoracic,5 lumbar ,5 sacral, 8-10
coccygeal
1st occipital & 5-7 coccygeal disappear & remaining somites
form axial skeleton

38. Somite Development
Embryonic Inner Ear
Occipital myotomes
Cervical myotomes
Thoracic myotomes
Lumbar myotomes
Sacral myotomes

39. • Intermediate mesoderm
It connects paraxial mesoderm with lateral plate mesoderm and shows
segmentation in the cervical and upper thoracic regions and remains
unsegmented in the lower thoracic and lumbar regions.
It contributes to the development of kidneys, gonads, and the duct system
of urinary and genital systems.
DIFFERENTIATION OF INTRAEMBRYONIC MESODERM

40. • Lateral Plate Mesoderm:
It is the unsegmented part of intraembryonic mesoderm.
Small cavities appear in the lateral plate mesoderm that later coalesce to
form the horse shoe-shaped intraembryonic coelom
With the formation of intraembryonic coelom the lateral plate mesoderm is
divided into two layers.
the somatopleuric (parietal layer)
the splanchnopleuric(visceral layer)
DIFFERENTIATION OF INTRAEMBRYONIC MESODERM

41. Formation of Coelomic Cavity
• Pericardial sac communicates with intraembryonic coelom to form inverted U-shaped
tubular passage.
• Coelomic cavity forms pericardial, pleural and peritoneal cavities.
Somatopleuric layer of mesoderm forms
– Parietal layers of peritoneal, pericardial and pleural cavities
– Dermis
– Pectoral and pelvic girdles
– Skeletal elements of limbs (muscles develop from migrating myotomes)
Splanchnopleuric layer of mesoderm forms
– Visceral layer of pericardial, peritoneal and pleural cavities
– Musculature and connective tissue of gut, respiratory tract and ear

42. Vasculogenesis and angiogenesis
• Vasculogenesis is formation of a new vessel from
mesenchymal tissue in embryo.
• Angiogenesis is sprouting of vessel into adjacent area
by endothelial budding.
• At beginning of the third week, formation of
vasculature begins in extraembryonic mesoderm of
yolk sac, connecting to stalk and chorion.

43. • The intraembryonic coelom is continuous with extraembryonic coelom
all around the embryonic area except at the cephalic part in relation to
the pericardial cavity part of intraembryonic coelom.
• This part of intraembryonic mesoderm is known as septum
transversum.
DIFFERENTIATION OF INTRAEMBRYONIC MESODERM
The cardiogenic region is a
horseshoe-shaped region of
mesoderm located at the cranial
end of the trilaminar embryonic
disk rostral to the prochordal
plate. This region forms the
future heart.

44. • With the formation of head fold and tail fold the endodermal yolk sac
gets incorporated into the embryo to form the primitive gut that is
subdivided in to foregut, midgut and hindgut from which develops the
gastrointestinal and respiratory tracts
DIFFERENTIATION OF ENDODERM

45. Fourth Week
During fourth week of intrauterine life, the following changes occurs:
• Increase in the somite numbers
• Formation of three brain vesicles from the neural tube
• Formation of head and tail folds
• 21st days and later: Heart starts beating
• 24th day: First pharyngeal arch appears
• 26th day: Three pairs of pharyngeal arches
• 26th–27th day: Rudimentary forelimb bud
• 28th day: Rudimentary hind limb bud
• Formation of prominence of forebrain vesicle
• Otic pits, lens placodes (ectodermal thickening)
Size: 1/6 inch long

46. Fifth Week
During fifth week of intrauterine life, the following changes occur:
• Rapid growth of facial and head prominences
• Rapid growth of a second pharyngeal arch
• Growth of the limb buds
• Formation of alar and basal laminae of the neural tube
• Appearance of olfactory placodes, maxillary and frontonasal process.
• Appearance of gonadal ridges

47. Sixth Week
• Shows spontaneous movements
• Growth of brain vesicles
• Appearance of nasal processes
• Buccopharyngeal membrane ruptures
• Development of appendix, caecum and spleen
• Differentiation of the limb digits begins. Differentiation in lower limb starts later than
upper limb.
• Umbilical herniation is common
• Unfolding
Size: ½ inch long

48. Seventh and Eight Weeks
• Development of face, external ear (auricular hillock) and eye
• Well-defined limbs and digits
• Development of metanephric kidney
• Development of testis and ovaries
• Differentiation of external genitalia
• During the eighth week, coordinated limb movements occur
• Primary ossification centers start appearing
• Caudal tail-like eminence disappears
Size: 1 ¼ inch long

49. • Clinical procedures
amniocentesis
chorionic villus sampling
• Obstetric care
Post term delivery
caesarean section
• Interpretation of antenatal test
Alpha feto protein
Fetal diseases in pregnancy
Fetal growth assessment
Fetal heart rate
IMPORTANCE OF GESTATIONAL AGE
Applied Anatomy of Embryonic Development

50. Symptoms:
• Amenorrhea: Cessation of previously regular menstruation.
• Morning sickness: Nausea with or without vomiting. It usually occurs around 6th week
after last menstrual period.
Per-vaginal Examination by Obstetrician
• Enlarged, globular and soft uterus
• Soft cervix
DIAGNOSIS OF PREGNANCY IN FIRST TRIMESTER
Investigations
• Pregnancy tests: Depend on the presence of human chorionic gonadotropin (hCG) in
maternal urine and serum.
• Ultrasonography: Gestational sac can be detected after 4–5 weeks of amenorrhea.
Fetal heart pulsation can be detected as early as 7 weeks.

51. • This system divides the development of an embryo into various stages
which range from 1 to 23 and cover 1 to 56 days of embryonic
development.
• Used internationally by embryologists and research workers
CARNEGIE SYSTEM OF EMBRYONIC STAGING

52. Sacrococcygeal teratoma
If the cells of the primitive streak remain after the
fourth week, the totipotent (pluripotent) cells of
the primitive streak give rise to a large precoccygeal
tumor called sacrococcygeal teratoma.
Its large size may cause an obstructed labor or even
death of the baby.
most common tumor in newborns, occurring in
1:37,000 pregnancies.
commonly in female babies
The tumor usually becomes malignant during
infancy and therefore must be removed before 6
months of age.

53. This tumor arises from the remnants of notochord.
It is formed either in the cranial region or in the
sacral region.
In the cranial region, it is seen at the base of the
cranium and has a tendency to spread into the
nasopharynx.
commonly occurs in men late in life, viz., over 50
years of age.
About 30% are malignant.
Chordoma

54. Neural Tube Defects

55. Holoprosencephaly
the forebrain is small and the two lateral ventricles fuse into a single cavity
The eyes are closely placed (hypertelorism)
High doses of alcohol in the mother can cause this condition.

56. Caudal dysplasia (sirenomelia)
a collection of syndromes ranging from minor
lesions of lower vertebrae to complete fusion
of the lower limbs.
caused by abnormal gastrulation, in which
the migration of mesoderm is disturbed.
It can be associated with various cranial
anomalies:
1. VATER, which includes vertebral defects,
anal atresia, tracheoesophageal fistula, and
renal defects.
2. VACTERL, which is similar to VATER but also
includes cardiovascular defects and limb
defects.

57. Thalassemia syndromes
heterogeneous group of genetic defects
characterized by the lack or decreased synthesis of either the α-globin chain (α-
thalassemia) or β-globin chain (β-thalassemia) of hemoglobin α2β2.
Hydrops fetalis is the most severe form of αthalassemia and causes severe pallor,
generalized edema, and massive hepatosplenomegaly and invariably leads to intrauterine
fetal death.

58. Allantois cysts
remnants of the extraembryonic portion of the
allantois, are usually found between the fetal
umbilical vessels and can be detected by
ultrasonography.
detected in the proximal part of the umbilical
cord, near its attachment to the anterior
abdominal wall.
The cysts are generally asymptomatic until
childhood or adolescence, when they may
present with infection and inflammation

59. Hirschsprung’s disease
a defect of neural crest development
a condition that affects the large intestine (colon) and causes problems with passing
stool

60. Discussion

61. ectoderm
endoderm
mesoderm

62. References
1. Langman’s Medical Embryology
2.Inderbir Singh’s Human Embryology
3.Text book of Clinical Embryology-Vishram Sing
4.The Developing Human-Keith N.Moore, T.V.N Persaud
5.Human Embryology- Datta A.K
6.Text book of Human Embryology - Yogesh Sontakke

63. Thank you