Embryology is the study of prenatal development to understand how anatomical structures form. The major events of the first week include conception, formation of the zygote, cleavage into blastomeres, formation of the morula and blastocyst. The blastocyst undergoes implantation in the endometrium between days 6-7. This involves apposition, adhesion, penetration and invasion. The trophoblast differentiates and the inner cell mass forms the embryo proper. During the embryonic period from weeks 4-8, the three germ layers form and all major structures and organs develop. By the end of the embryonic period at 8 weeks, the embryo has distinct human form and features.

1. Embryology
PRESENTED BY-
SHEETAL VERMA
M.SC. (N) 1ST YEAR
AIIMS, RISHIKESH

2. Embryology is the study of events
occurring during prenatal period to gain an
understanding of how the anatomical
structures formed.

3. Embryonic period vs. Fetal
period
Embryonic – first 8 weeks
• Development of the three primary germ layers give
rise to all structures
• Basic body plan takes shape
Fetal period – remaining 30
weeks Structures and organs continue to grow
and develop, increasing in complexity

6. Major Events of First Week
 Conception – in lateral third of uterine tube
 Fusion of female and male pronuclei = amphimixis
 Zygote (46 chromosomes) moves toward the uterus
 Blastomeres – daughter cells formed from zygote

7. Morula (means mulberry) – cluster of
12–16 Blastomeres
Blastophere– about 60 cells Inner cavity
is called the blastocoele
Trophoblast – layer separating
blastocoele from external environment

8. Oogenesis

9. Spermatogenesis

11. Fertilization
The process of fertilization implies the union of
mature germ cells, the ovum and
spermatozoa.

12.  Fertilization life span of oocyte ranges
from 12 hours to 24 hours
 Similarly, fertilizable life span of
spermatozoa ranges between 48 and
72 hours. Majority of pregnancies occur
when coitus takes place within 3 days
prior to ovulation

13. Events in sperm-egg
interaction
 There are three types of glycoprotein in zona
pellucida. These are known as ZP1, ZP2 and
ZP3 of which ZP3 is the most abundant. ZP3
is the primary ligand for the sperm and ZP2 is
responsible for Zona Reaction following
sperm penetration to prevent polyspermy.
 Penetration through the zona is rapid and
mediated by Acrosin, a trypsin like proteinase.

15.  Spermatozoa enters perivitelline space at an
angle. Then there is binding between inner
acrosomal membrane of the sperm head and
oolemma (outer membrane of ooplasm). This
induces cortical and zona reactions which
prevent entry of another spermatozoon into the
oocyte, thereby blocking polyspermy.

17. Pronucleus Formation-
Syngamy- Embryonic
Cleavage
Approximately, 3 hours after entry of
sperm head in the oocyte, the
second meiotic division is
completed and the second polar
body is released with a haploid
complement of oocyte
chromosomes

18.  The remaining haploid number of chromosomes
in the oocyte will form the female pronucleus.
The nucleus of sperm head undergoes
decondensation and the male nuclei migrates
towards each other. When they come in close
proximity, the limiting membrane breakdown.
 There is an exchange of chromosome material
between the male and female pronucleus. The
process is known as Syngamy.

19.  A spindle is formed on which chromosomes
become aligned. The stages for first cell division
has now been organised and with first cell
division a zygote is formed.
 Embryonic genomic cell activity starts between 4
and 8 cell stage of cleavage, 2-3 days after
fertilization. Normal embryonic genomic activity
will now control further cell division into Morula
and blastocyst.

20. Preimplantation Preparatory
Changes
Prior to implantation both developing
endometrium and incoming embryo
undergo some preparatory changes.

21. Endometrial preparation for implantation
(endometrial receptivity)
Endometrial preparation for implantation (endometrial
receptivity) is a complex procedure which includes
interaction of several molecules generated by following
changes within the endometrium.
 Endocrine regulation (E2 and progesterone)
 Physiological changes (pinopod formation)
 Biochemical changes (integrin, selectin)
 Immunomodulatory alteration (formation of protective
cytokines interleukin – 3,5,6,10,13 and suppression of
natural killer.
 Endometrial genetic expression (troponin, transformation
growth factor alpha, PDG-a etc.)

22. Morula
 After the zygote formation, typical mitotic division
of the nucleus occurs producing to Blastomeres.
The two cell stage is reached approximately
after 30 hours of fertilisation. Each contains
equal cytoplasmic volume and chromosome
numbers.

23.  The Blastomeres continue to divide by binary
division through 4,8,16 cell stage until a cluster
of cells is formed and is called Morula,
resembling a mulberry.
 As the total volume of the cell mass is not
increased and the zona pellucida remains intact,
the Morula after spending about 3 days in the
uterine tube enters the uterine cavity through the
narrow uterine ostium (1mm) on the 4th day in
the 16-64 cell stage.

25.  The transport is slow process and is controlled
by muscular contraction and movement of the
cilia.
 The central cell of the Morula is known as inner
cell mass which forms the embryo proper and
the peripheral cells are called outer cell mass
which will form protective and nutritive
membranes of the embryo.

27. Blastocyst
 While the Morula remains free in the uterine
cavity on the 4th and 5th day it is covered by a
film of mucus. The fluid passes through the
canaliculi of the zona pellucida which separates
the cells of the Morula and is now termed
blastocyst.

29.  Zona hatching is the next step so that
trophectoderm cells interact with endometrial
cells and implantation occurs. Due to Blastocyst
enlargement the zona pellucida becomes
stretched, thin and gradually disappears. Lysis
of zona and escape of embryo is called zona
hatching. The cells on the outer side of the
Morula become trophectoderm and the inner
cell become inner cell mass by the mediation of
epithelial cadherin(protein).

30.  Trophectoderm differentiates into chorion and the
inner cell mass into the embryo. Completely
undifferentiated cells are called pluripotent
embryonic stem cells (ES cells). ES cells are able to
produce mature somatic cells of any germ layers
(ectoderm, mesoderm and endoderm).

31. Events after fertilization

32. First week development

33. Implantation
Implantation occurs in the endometrium of the
anterior or posterior wall of the body near the
fundus on the 6th day which corresponds, to the
20th day of regular menstrual cycle.

34. IMPLANTATION occurs through four stages:
 Apposition
 Adhesion
 Penetration
 Invasion

35. Changes in the Blastocyst
 Adhesion of trophoblast cells to the
endometrial cells
 The factors responsible for blastocyst
attachment are P.selectin, heparin sulphate,
proteoglycan, integrin trophinin, tasin and
others.

36. Apposition
 Occurs through pinopod formation.
 These pinopod absorbs the endometrial fluid
which is secreted by the endometrial gland
cells. This fluid rich in glycogen and mucin
provides nutrition to the blastocyst initially.
Unless this fluid is absorbed, adhesion phase
cannot occur.
 Adhesion of blastocyst to the endometrium
occurs through the adhesion molecules like
integrin, selectin and cadherin (glycoproteins).

37. Penetration
 Actual penetration occurs through the stromal cells
in between the glands and is facilitated by the
histolytic action of the blastocyst.
 With increasing lysis of the stromal cells, the
blastocyst is burrowed more and more inside the
stratum compactum of the decidua.
 Concurrently, the syncytical cells penetrate deeper
into stroma and erode the endothelium of the
maternal capillaries.

38. Further penetration is stopped probably by the
maternal immunological factor and the original point of
entry is sealed by fibrin clot and later by epithelium.
The process is completed by 10th or 11th day which
corresponds to D 24-25 from LMP.

39.  The process of implantation is controlled by the
Immunomodulatory role of various cytokines,
many local peptides like epidermal growth factor
and prostaglandins. Both the decidua and
embryo synthesize these molecules.

40. TROPHOBLAST
 differentiation into an outer trophectoderm and inner cell
mass.
 The trophectoderm is further differentiated into an inner
mononuclear cellular layer cell called cytotrophoblast or
langhan’s layer and an outer layer of multinucleated
syncyticum called syncytiotrophoblast.
 Placenta and fetal membranes are developed from the
trophoblast. It serves as-
 Invasion
 Nutrition
 Hormones production

41.  Trophoblasts (from Greek trephein: to feed,
and blastos: germinator) are cells forming the
outer layer of a blastocyst, which provide
nutrients to the embryo and develop into a
large part of the placenta.

42. Decidua
The decidua is the endometrium of the pregnant
uterus. It is so named because of it is shed following
delivery.
 Decidual reaction –
the increased structural and secretory activity of the
endometrium that is brought about in response to
progesterone following implantation is known as
decidual reaction.

43. The well-developed decidua differentiates into
three layers-
 Superficial compact layer
 Intermediate spongy layer
 Thin basal layer

44. After the interstitial implantation of the blastocyst into
the compact layer of the decidua, the different
portions of the decidua are renamed as-
 Decidua basalis (contact with the basal layer of the
blastocyst)
 Decidua capsularis (thin superficial layer covering
the blastocyst)
 Decidua Vera (lining the uterine cavity outside the
site of implantation )

46. Functions of decidua
 good nidus for the implantation of the blastocyst
 It supplies nutrition the early stage of the growing
ovum by its rich source of glycogen and fat
 Deeper penetration of the trophoblast is controlled
by local peptides, cytokines and integrins.
 Decidua basalis takes part in the formation of the
placenta and for the nourishment of the developing
embryo.

47. Chorion and chorionic villi
It consists of two embryonic layer- outer
trophoblast and inner primitive mesenchyme
which appears on the 9th day.

48. Formation of chorionic villi-
 Development of primary stem villi by
syncytotrophoblast
 Lacunar space by Mesodermal cell is known as
intervillous space
 Formation of secondary villi formation on 16th
day
 Development of villous capillary system from
the Mesodermal cells on 21st day also known as
21st day

49. Development of inner cell mass
Along with the changes in the trophoblast, on the 8th day,
the embryoblast differentiates into bilaminar germ disc
which consist of dorsal ectoderm layer and ventral
endodermal layer. Two cavities appear one on each
side of the germ disc-
 Amniotic cavity (formed by extension of epiblast)
 Yolk sac (extension of hypoblast)
Formation of trilaminar germ layer
 Proliferation of Ectodermal cells in midline leads to
formation of primitive streak

54. During the embryonic stage which extends from the 4th to
8th week
 individual differentiation of the germ layers and the
formation of the folds of the embryo occur.
 Most of the tissues and organs are developed during this
period.
 The major structures which are developed from the
three germinal layers from which embryo can be
differentiated as human at 8th week

56. Gastrulation is the process of formation of three
germ layers during 3rd week.
Ectodermal layer
 central and peripheral nervous system
 epidermis of the skin with its appendages
 pituitary gland, salivary glands, mucous
lining of the nasal cavity, paranasal sinus,
roof of the mouth etc.

57. Mesodermal layer
bones, cartilages, muscles, kidney,
gonads, spleen, peritoneal cavity
and cardiovascular system.

58. Endodermal layer
Epithelial lining of the GI tract, liver, gall bladder,
pancreas and mucous membrane of urinary
bladder and urethra etc.
 The embryonic endoderm is the first of three
germ layers from which all tissues (bone,
muscle, connective tissue), organs and
structures derive. Structures such as fetal
membranes, umbilical cord and part of the
placenta also develops from these layers.

60. At the time of implantation (6th day) the
embryo known as bilaminar embryo
because the embryo disc arising from the
inner cell mass has two layers of cells-
a) epiblast
b) hypoblast

62.  From the beginning of the third week, the
primitive stalk, arising from the embryonic
disk, is the growth centre for embryo for
about two weeks after which it
disintegrates. During the third week, the
neural tube, coelomic spaces primitive
blood cells develop.

63. During 4th week
The heart starts to beat at the beginning of 4th week.
 During the 4th week longitudinal and transverse folding
of the embryonic disk takes place which converts the
embryo from straight to a curve form.
 By the end of 4th week, the embryo has assumed its
often called salamander look and has the rudiments of
ear, arm, legs, facial and neck structures.

64. During the 5th week
 rapid development of brain results in extensive growth
of the head and makes it much larger in relation to the
rest of the body.
 Development takes place from cephalic to caudal, with
the development of legs almost a week behind
development of the arms.
 The eyes begin to develop with the lens, vesicles, optic
cups and retinal pigments.

65. During 6th week
 Nose, mouth and palate begin to take form
and the eyelids visible.
 Arms and legs undergo extensive
development and by the end of 7th week
arms and legs are formed with clearly
defined fingers and toes.

66. During 7th week
 the neck region is established, the abdomen is less
protuberant and urogenital development begins.
 The external ears are evident
 The embryo by the end of 7th week has distinctive
human characteristics. The end of the 7th week also
marks the end of the embryonic period.
 All essential internal and external structures are formed.
 They undergo further elaboration and growth, including
the replacement of cartilage with the bone cells

67.  The embryonic period is a critical period
during which any teratogen (e.g. drugs, X-rays,
viruses) may either be lethal or cause any
major congenital malformations

68.  By the end of the first trimester ,the intestines
are fully into the abdomen, the external genitalia
have male or female characteristics but neither
are fully formed, the anus has formed and the
facial characteristics of the fetus now look
undeniably human. The fetus at this stage can
swallow and make respiratory movements,
urinate, move specific parts of limbs and open
and shut his or her mouth.

70. MULTIPLE
CHOICE
QUESTIONS

71. Q-1 The part of the sperm containing proteolytic
enzymes to digest the zona pellucida is the:
A. capacitor
B. head
C. corona
D. acrosome

72. Q-2: The testes descend to the scrotum during the fetal
period at:
a) The 3rd month
b) The 6th month
c) The 4th month
d) Just before the labor

73. Q-3 The first week of human development is
characterized by formation of the:
A. inner cell mass
B. hypoblast
C. Trophoblast
D. blastocyst
E. all of the above

74. Q-4 With the light microscope, the zona
pellucida appears as a translucent membrane
surrounding the:
A. primary oocyte
B. zygote
C. Morula
D. very early blastocyst
E. all of the above

75. Q-5 The first two intraembryonic germ layers to
differentiate are the:
A. ectoderm and hypoblast
B. epiblast and hypoblast
C. ectoderm and endoderm
D. ectoderm and mesoderm

76. Q-6 The most important region of the decidua for
the nourishment of the conceptus is the decidua
___________.
A. frondosum
B. capsularis
C. parietalis
D. basalis

77. Q-7 Failure of the brain to grow may result in:
A. Plagiocephaly
B. craniostenosis
C. acrocephaly
D. microcephaly

78. Q-8 Embryonic period is up to:
a) 8 days
b) 4 weeks
c) 8 weeks
d) 12 weeks

79. Q-9 Implantation Occurs
a) On 24th day
b) 4 weeks
c) 3rd day
d) 6th day

80. Q-10 Identify the picture