EMBRYOLOGY AND FOETAL DEVELOPMENT Prepared by: Mayuri Gamit M.Sc Nursing (OBG) Nursing tutor

1. EMBRYOLOGY AND FOETAL DEVELOPMENT
Prepared by: Mayuri Gamit
M.Sc Nursing (OBG)
Nursing tutor

2. TERMINOLOGY
• GAMETOGENES: The process involved in the maturation of the two highly specialized
cells, spermatozoon in male and ovum in female before they unite to form zygote, is called
gametogenesis.
• OOGENESIS : The process involved in the development of a mature ovum is called
oogenesis.
• SPERMATOGENESIS: The process involved in the development of spermatids from the
primordial male germ cells and their differentiation into spermatozoa is called
spermatogenesis.

3. • OVULATION: Ovulation is a process whereby a secondary oocyte is released
from the ovary following rupture of a mature Graafian follicle and becomes
available for conception.
• FERTILIZATION: Fertilization is the process of fusion of the spermatozoon
with the mature ovum.
• ZYGOTE: A zygote is the first diploid cell that results from the fusion of male
and female gametes.

4. • MORULA: After the zygote formation, typical mitotic division of the nucleus on the 4th
day in the 16-64 cell stage. (mulberry" appearance).
• 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.
• Zona hatching: Due to blastocyst enlargement the zona pellucida becomes stretched,
thinned and gradually disappears. Lysis of zona and escape of embryo is called zona
hatching.

5. • Implantation: Blastocyst makes the contact with the
endometrium.

6. OOGENESIS
• The process involved in the development of a mature ovum is called oogenesis.
• The primitive germ cells take their origin from the yolk sac at about the end of
3rd week.
• In the female gonads, the germ cells undergo a number of rapid mitotic
divisions and differentiate into oogonia.
• The number of oogonia reaches its maximum at 20th week, numbering about 7
million.

7. • While the majority of the oogonia continue to divide, some enter into the
prophase of the first meiotic division and are called primary oocytes.
• These are surrounded by flat cells and are called primordial follicles and
are present in the cortex of the ovary.
• At birth, there is no more mitotic division and all the oogonia are replaced
by primary oocytes which have finished the prophase of the first meiotic
division and remain in resting phase between prophase and metaphase.

8. • Total number of primary oocytes at birth is estimated to be about 2 million.
• The primary oocytes do not finish the first meiotic division until puberty is
reached.
• At puberty, some 400,000 primary oocytes are left behind, the rest being
atretic.
• Out of these, some 400 are likely to ovulate during the entire reproductive
period.

9. • Maturation of the oocytes: The essence of maturation is reduction of the
number of chromosomes to half. Before the onset of first meiotic division, the
primary oocytes double its DNA by replication, so they contain double the
amount of normal protein content. There are 22 pairs of autosomes which
determine the body characteristics and 1 pair of sex chromosomes, named “XX”.
• The first stage of maturation occurs with full maturation of the ovarian follicle
just prior to ovulation but the final maturation occurs only after fertilization.

10. • The primary oocyte undergoes first meiotic division giving rise to
secondary oocyte and one polar body. The two are of unequal size, the
secondary oocyte contains haploid number of chromosomes (23, X), but
nearly all the cytoplasm and the small polar body also contains half of the
chromosomes (23, X) but with scanty cytoplasm.
• Ovulation occurs soon after the formation of the secondary oocyte.

11. • The secondary oocyte completes the second meiotic division (homotypical) only after fertilization by the sperm in
the Fallopian tube and results in the formation of two unequal daughter cells, each possessing 23 chromosomes
(23, X), the larger one is called the mature ovum and the smaller one is the second polar body containing the
same number of chromosomes.
• The first polar body may also undergo the second meiotic division.
• In the absence of fertilization, the secondary oocyte does not complete the second meiotic division and
degenerates as such.

12. Structure of a mature ovum:
• A fully mature ovum is the largest cell in the body and is about 130 microns
in diameter.
• It consists of cytoplasm and a nucleus with its nucleolus which is eccentric
in position and contains 23 chromosomes (23, X).
• During fertilization, the nucleus is converted into a female pronucleus.
• The ovum is surrounded by a cell membrane called vitelline membrane.
• There is an outer transparent mucoprotein envelope, the zona pellucida.

13. • The zona pellucida is penetrated by tiny channels which are thought to be
important for the transport of the materials from the granulosa cells to the oocyte.
• In between the vitelline membrane and the zona pellucida, there is a narrow space
called perivitelline space which accommodates the polar bodies.
• The human oocyte, after its escape from the follicle, retains a covering of
granulosa cells known as the corona radiata derived from the cumulus oophorus.

16. SPERMATOGENESIS
• The process involved in the development of spermatids from the primordial male
germ cells and their differentiation into spermatozoa is called spermatogenesis.
• Shortly before puberty, the primordial germ cells develop into spermatogonia and
remain in the wall of seminiferous tubules.
• The spermatogonia, in turn, differentiate into primary spermatocytes which
remain in the stage of prophase of the first meiotic division for a long time (about
16 days).

17. • Each spermatocyte contains 22 pairs of autosomes and 1 pair of sex chromosomes, named
“XY”.
• With the completion of the first meiotic division, two secondary spermatocytes are formed
having equal share of cytoplasm and haploid number of chromosomes either 23, X or 23, Y.
• Then immediately follows the second meiotic division (homotypical) with the formation of
four spermatids, each containing haploid number of chromosomes, two with 23, X and two
with 23, Y.

18. • Immediately after their formation, extensive morphological
differentiation of the spermatids occurs without further cell division
to convert them into spermatozoa.
• The process is called spermiogenesis. In man, the time required for
a spermatogonium to develop into a mature spermatozoon is about
61 days.

19. Structure of a mature spermatozoon:
• It has got two parts, a head and a tail. The head consists principally of the
condensed nucleus and acrosomal cap. Acrosome is rich in enzymes.
• The tail is divided into four zones — the neck, the middle piece, the
principal piece and the end piece.
• Sperm life span varies depending on the location and condition.
• Sperm can survive for up to five day inside the female reproductive tract. In
dry surfaces die within minutes.

22. FERTILIZATION
• Fertilization is the process of fusion of the spermatozoon with the mature
ovum.
• Almost always, fertilization occurs in the ampullary part of the uterine tube
• Its objectives are:
(1) To initiate the embryonic development of the egg and
(2) To restore the chromosome number of the species.

23. FERTILIZATION
Human Fertilization is known as conception, is the
fusion of the sperm with the secondary oocyte, to form
the zygote.
normally occur in the ampulla of Uterine Tube
Oocyte, which is 0.15mm in diameter, passes into the
uterine tube
Process takes 24 hours
Following ovulation

24. Oocyte → no power→ with the help of cilia and peristaltic
muscular contraction of uterine tube moves forward
At the same time cervix under the influence of estrogen,
secretes flow of alkaline mucus that attract the
spermatozoa.
In the fertile male at intercourse approximately 300
million sperm are deposited in the posterior fornix of the
vagina

25. Those that react the loose cervical mucus survive and propel
themselves towards the uterine tube →rest destroyed by the acidic
medium of the vagina.
Inside the uterine tube sperm undergo capacitation Influence
of the secretions from the uterine tube →sperm undergo
changes to the plasma membrane →removal of the
glycoprotein coat.
The acrosomal layer of the sperm→ reactive → release
hyaluronidase enzyme→ reaction known as acrosome reaction(
disperses the outer layer of oocyte- zona pellucida)

26. 1 st sperm that reaches zona pellucida
penetrates it
Upon penetration a chemical reaction known as
cortical reaction occurs
this reaction alters the zona pellucida making it
impermeable to other sperm

27. The plasma membrane of the oocyte and sperm
fuse
The oocyte at this stage completes second
mitotic division and becomes mature
Pronucleus has 23chromosomes, referred to
as haploid

28. The tail and mitochondria of sperm
degenerate .. → as it penetrate oocyte
there is formation of male pronucleus
Male(23) pronucleus + Female (23)
pronucleus
New nucleus (diploid cell)
zygote
Fuse (46)

30. APPROXIMATION OF THE GAMETES:
• The ovum, immediately following ovulation is picked up by the tubal fimbriae
which partly envelope the ovary, especially at the time of ovulation.
• The pick up action might be muscular or by a kind of suction or by ciliary
action.
• The ovum is rapidly transported to the ampullary part.
• Fertilizable life span of oocyte ranges from 12 to 24 hours whereas that of
sperm is 48 to 72 hours.

31. • Out of hundreds of millions of sperms deposited in the vagina at
single ejaculation, only thousands capacitated spermatozoa enter the
uterine tube while only 300–500 reach the ovum.
• The tubal transport is facilitated by muscular contraction and
aspiration action of the uterine tube.
• It takes only few minutes for the sperm to reach the Fallopian tube.

32. CONTACT AND FUSION OF THE GAMETES
• Complete dissolution of the cells of the corona radiata occurs
by the chemical action of the hyaluronidase liberated from the
acrosomal cap of the hundreds of sperm present at the site.
• Penetration of the zona pellucida is facilitated by the release of
hyaluronidase from the acrosomal cap. More than one sperm
may penetrate the zona pellucida.

33. • Out of the many sperms, one touches the oolemma. Soon after the sperm fusion,
penetration of other sperm is prevented by zona reaction (hardening) and oolemma
block.
• Completion of the second meiotic division of the oocyte immediately follows, each
containing haploid number of chromosomes (23, X).
• The bigger one is called the female pronucleus and the smaller one is called second
polar body which is pushed to the perivitelline space.

34. • In the human, both the head and tail of the spermatozoon enter
the cytoplasm of the oocyte but the plasma membrane is left
behind on the oocyte surface.
• Head and the neck of the spermatozoon become male
pronucleus containing haploid number of chromosomes (23, X)
or (23, Y).

35. • The male and the female pronuclei unite at the center with restoration of the
diploid number of chromosomes (46) which is constant for the species.
• The zygote, thus formed, contains both the paternal and maternal genetic
materials.
• Sex of the child is determined by the pattern of the sex chromosome supplied by
the spermatozoon. If the spermatozoon contains ‘X’ chromosome, a female
embryo (46, XX) is formed; if it contains a ‘Y’ chromosome, a male embryo (46,
XY) is formed.

38. DEVELOPMENT OF THE ZYGOTE
❑ Fertilized Ovum is known as zygote.
❑ Zygote passes through the fallopian tube and reach the uterus
on 3rd -4 th day of its development

39. Development of Fertilized Ovum (zygote)
1)
Morula
Stage
2)
Blastocyst
3)
Trophoblast
Cell/ Layer

40. 1. MORULA STAGE
• After the zygote formation, typical mitotic division of the nucleus occurs by
producing two blastomeres.
• The two cell stage is reached approximately 30 hours after fertilization.
• Each contains equal cytoplasmic volume and chromosome numbers.
• 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.

41. • The central cell of the morula is known as inner cell mass
which forms the embryo.
• The peripheral cells are called outer cell mass which will
form protective and nutritive membranes of the embryo.

42. • 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 (1 mm) on the 4th day in the 16-64 cell stage.
• The transport is a slow process and is controlled by muscular
contraction and movement of the cilia.

46. 2. 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 ( Secretes) through the zona pellucida which
separates the cells of the morula and is now termed
blastocyst.

48. • Due to blastocyst enlargement the zona pellucida becomes
stretched, thinned 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
(trophoblast) and the inner cells become inner cell mass.
• Trophectoderm differentiates into chorion (placenta) and the
inner cell mass into the embryo.

49. • embryonic stem (ES) cells.
• ES cells are able to produce mature somatic cells of any germ
layer (ectoderm, mesoderm and endoderm).

50. • ECTODERM:
• Develop central and peripheral nervous system
• Epidermis layer
• ENDODERM:
• Dermis
• Yolk Sac Cavity
• Forms : Muscle, Skeletal Connective Tissue, Blood Vessels
• MESODERM :
• Forms Internal Organs: Heart, Blood Vessels, Liver, Pancreas, Bones, Muscles

52. Blastocyst
Inner cell
mass/embryobl
ast
Embryo ,
amnion,
umbilical cord
Fluid filled
cavity
Outer cell
mass/
trophoblast
Placenta,
chorion

53. 3. TROPHOBLAST
• As previously mentioned, the cells of the blastocyst differentiate
into an outer trophectoderm and an inner cell mass.
• Just before implantation, the trophectoderm is further
differentiated into an inner mononuclear cellular layer called
cytotrophoblast or Langhans’ layer and
• an outer layer of multinucleated syncytium called
syncytiotrophoblast.

55. 1. Inner mononucler cellular layer (cytotrophoblast or langhan’s layer)
• Single layer of cell that produce (HCG) hormone, the hormone that is
responsible for the confirmation of pregnancy.
2. Outer multinucleated syncytium (syncytiotrophoblast)
• Finger like projections that invade the decidua, space created is known as
lacunae, maternal blood is filled up in the space.
• Through villi gaseous exchange takes place

56. • The cytotrophoblasts that line the villous stems are the villous
cytotrophoblasts.
• The cytotrophoblast cells that invade the decidua are known
as ‘interstitial extravillous cytotrophoblast’ and those that
invade the lumens of the maternal spiral arteries are known
as ‘intravascular extravillous cytotrophoblast’.

57. • Throughout pregnancy, syncytiotrophoblast is derived from the
cytotrophoblast.
• Placenta and the fetal membranes are developed from the trophoblast.
• It is involved in most of the functions ascribed to the placenta as a whole.
• Thus, it serves at least 3 important functions — invasion, nutrition and
production of hormones for the maintenance of pregnancy.
• Local cytokines (control inflammation) regulate the invasion of the
cytotrophoblasts in the decidua

59. THE DECIDUA
• The decidua is the endometrium of the pregnant uterus.
• 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.

60. • The well developed decidua differentiates into three layers :
1) Superficial compact layer
2) Intermediate spongy layer
3) Thin basal layer

62. (1) Superficial compact layer : consists of compact mass of
decidual cells, gland ducts and dilated capillaries. The greater
part of the surface epithelium.
(2) Intermediate spongy layer : contains dilated uterine glands,
decidual cells and blood vessels. It is through this layer that the
cleavage of placental separation occurs.
(3) Thin basal layer : containing the basal portion of the glands
and is opposed to the uterine muscle.

63. • After the interstitial implantation of the blastocyst into the compact
layer of the decidua.
The different portions of the decidua are renamed as —
(1) Decidua basalis or serotina— the portion of the decidua in contact
with the base of the blastocyst.
(2) Decidua capsularis or reflexa — the thin superficial compact layer
covering the blastocyst.
3. Decidua vera or parietalis — the rest of the decidua lining the uterine
cavity outside the site of implantation.

64. • Its thickness progressively increases to maximum of 5–10 mm
at the end of the second month.
• As the growing ovum bulges towards the uterine cavity, the
space between the decidua capsularis and the decidua vera,
called the decidual space.

65. • Functions of decidua:
(1) It provides a good nidus for the implantation of the blastocyst.
(2) It supplies nutrition to the early stage of the growing ovum by its
rich sources of glycogen and fat.
(3) Deeper penetration of the trophoblast is controlled by local
peptides, cytokines and integrins.
(4) Decidua basalis takes part in the formation of basal plate of the
placenta.

72. 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 a regular menstrual cycle.

74. CHANGES IN THE BLASTOCYST:
• The polar trophoblast cells adjacent to the inner cell mass are
primarily involved in adhesion to the endometrial cells.
• The factors responsible for blastocyst attachment are: P. selectin,
heparin sulfate, proteoglycan, EGF, integrins, trophinin and others.
• The signals for trophoblast multiplication arise from the inner cell
mass.

76. ENDOMETRIUM AT THE IMPLANTATION SITE :
(1) The endometrium is in the secretory phase corresponding to 20–
21 days of cycle.
(2) The microvilli on the surface of the trophectoderm interdigitate
(interlock like the finger) with the decidual cells to form the
junctional complexes.
(3) Endometrial receptivity and molecular signaling during
implantation is induced by progesterone.

77. Four stages Implantation
Apposition Adhesion
Penetration
and Invasion

78. 1. APPOSITION:
• Occurs through pinopod formation.
• Pinopods are long finger like projections (microvilli) from the endometrial
cell surface.
• These pinopods absorb the endometrial fluid which is secreted by the
endometrial gland cells.
• This fluid, rich in glycogen and mucin provides nutrition to the blastocyst
initially.

79. 2. Adhesion
• Unless this fluid is absorbed, adhesion phase cannot occur.
• Adhesion of blastocyst to the endometrium occurs through the
adhesion molecules like selectin and glycoproteins.

80. 3. PENETRATION and INVASION
• Actual penetration and invasion occur 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
burrowed more and more inside the stratum compactum of the
decidua.

81. • 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.

83. • This type of deeper penetration of the human blastocyst is
called interstitial implantationand the blastocyst is covered on
all sides by the endometrium (decidua).
• Occasionally, there may be increased blood flow into the
lacunar spaces.

86. DEVELOPMENT OF CHORION AND CHORIONIC VILLI
• The two fetal membranes (chorion and amnion).
• It consists of two embryonic layers — outer trophoblast and inner primitive
mesenchyme which appears on 9th day.
• At the beginning of the 3rd week, the syncytiotrophoblast produces irregular
finger like projections which are lined internally by the cytotrophoblast.
• These finger like buds are called primary stem villi — surrounded by lacunar
spaces which will later form into intervillous spaces.

87. • After the appearance of the primitive mesenchyme and the development of
the chorion, the primary stem villi are named chorionic villi.
• With the insinuation of the primary mesoderm into the central core of the
villi structures, secondary villi are formed on 16th day.
• Later on mesodermal cells in the villi begin to differentiate into blood cells
and blood vessels, thus forming villous capillary system.
• These vascularized villi are called tertiary villi which are completed on
21st day.
• Later on, this extra embryonic circulatory system establishes connection
with the intraembryonic circulatory system through the body stalk.

89. • The villi overlying the decidua basalis continue to grow and
expand and are called chorion frondosum which subsequently
forms the discoid placenta.

90. DEVELOPMENT OF INNER CELL MASS
• Along with the changes in the trophoblast, on the 8th day, the
embryoblast differentiates into bilaminar germ disc which consists
of dorsal ectodermal layer of tall columnar cells and ventral
endodermal layer of flattened polyhedral cells.
• The bilaminar germ disc is connected with the trophoblast by
mesenchymal condensation, called connecting stalk or body stalk
which later on forms the umbilical cord.

92. • Two cavities appear one on each side of the germ disc.
(1) On 12th postovulatory day, a fluid filled space appears between the
ectodermal layer and the cytotrophoblast which is called amniotic
cavity. Its floor is formed by the ectoderm and the rest of its wall by
primitive mesenchyme.
(2) The yolk sac appear on the ventral aspect of the bilaminar disk and is
lined externally by the primitive mesenchyme and internally by the
migrating endodermal cells from the endodermal layer of the germ disc.

93. • Formation of trilaminar embryonic disk: Fourteen days after fertilization,
proliferation of ectodermal cells in the midline, leads to formation of
primitive streak.
• Cells within the streak spread laterally between the ectoderm and
endoderm as intraembryonic mesoderm.
• This intraembryonic mesoderm becomes continuous with the
extraembryonic mesoderm at the lateral border of the embryonic disk.

95. Extraembryonic coelom
• Extraembryonic mesenchyme, derived from the trophoblast appears
to separate the yolk sac from the blastocyst wall and also the
amniotic cavity from the trophoblast of the chorion.
• Small cystic spaces (lacuna) now appear within the extraembryonic
mesenchyme.
• These spaces gradually enlarge and fuse to form extraembryonic
coelom.

96. • Progressive enlargement of the extraembryonic coelom,
separates the amnion from the inner aspect of the chorion
except at the caudal end of the embryo. There, the
mesenchymal attachment persists to form body stalk.
• Umbilical cord develops from this body stalk.

97. • The embryo can be differentiated as human at 8th week.
• ECTODERMAL LAYER: Central and peripheral nervous system,
epidermis of skin, pituitary gland, organs, salivary glands;
mucous lining of the nasal cavity, paranasal sinus, roof of the
mouth etc.

98. • MESODERMAL LAYER: Bones, cartilage, muscles, cardiovascular system,
kidney, gonads, suprarenals, spleen, most of the genital tract; mesothelial
lining of pericardial, pleural and peritoneal cavity etc.
• ENDODERMAL LAYER: Epithelial lining of the gastrointestinal tract, liver,
gallbladder, pancreas; epithelial lining of respiratory tract and most of the
mucous membrane of urinary bladder and urethra; bulbourethral and
greater vestibular glands etc.

99. Important Events Following Fertilization
• ‘0’ hour — Fertilization (day-15 from LMP)
• 30 hours — 2 cell stage (blastomeres)
• 40–50 hours — 4 cell stage
• 72 hours — 12 cell stage
• 96 hours — 16 cell stage. Morula enters the uterine cavity
• 5th day — Blastocyst
• 4–5th day — Zona pellucida disappears

100. • 5–6th day — Blastocyst attachment to endometrial surface
• 6–7th day — Differentiation of cyto and syncytiotrophoblastic layers
• 10th day — Synthesis of hCG by syncytiotrophoblast
• 9–10th day — Lacunar network forms
• 10–11th day — Trophoblasts invade endometrial sinusoids establishing uteroplacental circulation ,
Interstitial implantation completed with entire decidual coverage
• 13th day — Primary villi
• 16th day — Secondary villi
• 21st day — Tertiary villi
• 21st–22nd day — Fetal heart. Fetoplacental circulation

101. • 5–6th day — Blastocyst attachment to endometrial surface
• 6–7th day — Differentiation of cyto and syncytiotrophoblastic layers
• 10th day — Synthesis of hCG by syncytiotrophoblast
• 9–10th day — Lacunar network forms
• 10–11th day — Trophoblasts invade endometrial sinusoids establishing uteroplacental circulation ,
Interstitial implantation completed with entire decidual coverage
• 13th day — Primary villi
• 16th day — Secondary villi
• 21st day — Tertiary villi
• 21st–22nd day — Fetal heart. Fetoplacental circulation