After fertilization, gestation period begins According to development gestation period divided in to, i. germinal period ii. embryonic period iii. fetal period Here the first week of the germinal period is discussed Zygote undergoes cleavage The process of each step of cleavage is explained and shown diagrammatically The significance of cleavage given. Formation of morula after the compactum of blastocytes Pushing of embryoblast towards the animal pole and blastocoel the formation has taken place. Formation of blastocyst completed. A blastocyst is ready for implantation after loosing Zonapellucida. Implantation begins in the first week of development

1. EMBRYOLOGY -- 1
EMBRYOLOGY - 1
FIRST WEEK OF DEVELOPMENT
DR. VIJAYALAXMI PATTANSHETTI
ASST. PROF IN ANATOMY
SIMS&RH, TUMKURU(KARNATAKA)
17/2/18

2. Gestation period:
Gestation period: Is foetal development period from the time of
conception until birth Or retention of embryo in the uterus until fully
developed new born is born. For humans, the full gestation period is
normally 9 plus months.
 The time interval of a gestation is called the gestation period. In human
obstetrics, gestational age refers to the embryonic or foetal age plus two
weeks.
 The word "gestation" comes from the Latin "gestare" meaning
"to carry or to bear”.
The gestation period divided into
(i) germinal period
(II) embryonic period, and
(iii) foetal period 17/2/2018
Vijayalami Pattanshetti
2

3. GESTATIONAL PERIOD cont…
 1). GERMINAL PERIOD(0-3wks):
This begins at fertilization and extends till the third week, i.e
i.e., up to formation of trilaminar germ disc.
 2).EMBRYONIC PERIOD(4-8wks i.e. 5wks):
This period extends from fourth to eighth week, involving cha
changes in shape and external appearance of the ….. …. …..
cylindrical embryo, i.e., tissue and organogenesis.
(0-8wks=2mnts)
 3).FOETAL PERIOD(3rd month to termination of pregnancy):
This extends from third month up to termination of g
pre pregnancy, during which there is a rapid growth of foetus and
and complete development of placenta.
Vijayalaxmi Pattanshetti 17/2
/18
3

4. WEEKLY DEVELOPMENT DURING GESTATIONAL PERIOD
FIRST WEEK
The changes seen during first week are:
a). Cleavage division,
b). Formation of i). Morula
& ii). Blastocyst
c). Implantation of Blastocyst. 17/2/18
Vijaylaxmi Pattanshetti
4

5. a).CLEAVAGE DIVISION

6. a). cleavage
The process of repeated mitotic divisions of the zygote within the zona ..
pellucida, resulting in rapid increase in the number of cells is known as ..
cleavage.
Two types of cleavage depends on the amount of yolk in the egg
(i) Holoblastic or total cleavage: In mammals , the fertilized ovum has a . ..
small amount of *deutoplasm(yolk) and the whole ovum is involved in
the process of cleavage.
(ii) Meroblastic cleavage: In reptiles and birds, the deutoplasm is
abundant and only a part of it is involved in cleavage.
* deutoplasm – the nutritive substance in the cytoplasm, especially
the yolk of an ovum.
In humans cleavage is holoblastic, i.e., it divides the zygote
completely into daughter cells or blastomeres.
Vijaylaxmi Pattanshetti

7. cleavage cont….
 The first cleavage(two cell stage) takes place about 30 hours after fertilization. It
divides zygote longitudinally into two blastomeres (one slightly larger than the other).
 Of two cells the larger cell divides first, so that we have a ‘ 3-cell stage, -
. Followed by the smaller of two cells divides giving rise to
4-cell, a 5-cell stage etc.
 This process of subdivisions of the ovum in to smaller cells is called cleavage
 The cells become smaller with each cleavage division are known as blastomeres.
 Until 8-cell stage they form loosely arranged clump.
 As the cleavage proceeds, blastomeres maximise their contact with each other, forming
a compact ball of cells held together by
a tight junction.

8. Cell stages
2 – cell stage 30 hour
4 – cell stage 40-50hrs
12-cell stage 72 hrs
16 cell stage 96hrs
The pole of egg with highest
concentration of yolk is called
as vegetal pole while the opposite
is referred as animal pole.
with time
Vijaylaxmi Pattanshetti
FORMATION OF MORULA

9. Significance of Cleavage:
(i) It converts a unicellular zygote into a multicellular embryo.
(ii) It maintains the cell size and nucleo-cytoplasmic ratio of the
species.
(iii) Cleavage produces large member of cells or blastomeres
required for the building of offspring’s body.
(iv) During cleavage quick mitotic division of blastomeres
following which there is no growth of blastomeres.
(v) Cleavage brings about the distribution of cytoplasm among
the blastomeres.

10. b).-1. Formation of morula
 The process of compaction, segregates inner cells from outer cells.
 Approximately 3 days after fertilization cells of compacted embryo comes
to have 16-cells called morula( like mulberry fruit).
 It is still surrounded by zona pellucida
 If we see the cut section of morula – it consists of inner cell mass
surrounded by outer cell layer
 The inner cell mass gives rise to embryo proper, therefore called
embryoblast.
 The cells of the outer layer will give rise to a structure called the
trophoblast. These cells help to provide nutrition to the embryo, which
later contributes to the placenta.

11. b). -2. formation of blastocyst
 Blastocyst begins at day 4/5 after fertilization
 About the time the morula enters the uterine cavity, fluid begins to penetrate through
the zona pellucida in to the intercellular spaces of the inner cell mass.
 Now the inner cell mass partially separates from trophoblasts.
 As the quantity of fluid increases, the morula acquires the shape of a cyst
 The cells of the trophoblast become flattened, and the inner cell mass
comes to be attached to the inner side of the trophoblast on one side.
 The morula has now become a blastocyst.
 The cavity of the blastocyst is called the blastocoel.
 The side of the blastocyst to which the inner cell mass is attached is called embryonic
or animal pole, while the opposite side is the abembryonic pole.
 Human blastocyst comprises of 70 – 100 cells

12. FORMATION OF BLASTOCYST
a) b)
c) d)
e) blastocyst
blastocyst
Vijaylaxmi Pattanshetti
F)

13. Embryonic pole
abembryonic pole

14. HATCHING OF BLASTOCYST BEFORE IMPLANTATION

15. C). IMPLANTATION OF BLASTOCYST
DEFINITION:
* It is an process by which an embryo attaches to an uterine wall and
penetrates first the epithelium and then the circulatory system of the mother
to form the placenta.
* This is the process that is limited in both time and space
TIME TAKEN:
* 2-3 days after the fertilised egg enters the uterus.
* Entry is on 18 0r 19 day of the menstrual cycle.
* 5–7 days after fertilization.
* implantation starts at 7th day and completes at 11th day
SITE
* Usually at upper part of posterior wall of uterus near fundus.
VIJAYLAXMI PATTANSHETTI

16. FIXATION OF EMBRYONIC POLE FOR IMPLANTATION
Vijaylaxmi Pattanshetti

17. SNCYTIOTROPHOBLAST ERODE IN TO ENDOMETRIUM
Vijaylaxmi Pattanshetti

18. STAGE OF IMPLANTATION
Vijaylaxmi Pattanshetti

19. Fibrin plug
COMPLETION OF IMPLANTATION WITH FIBRIN PLUG FORMATION
Vijaylaxmi Pattanshetti

20. 2. IMPLANTATION CONT…
 The blastocyst floats in the uterine secretions for 2 days.
 The zona pellucida gradually degenerates and disappears at the end of the
day.
 The endometrium of uterus is changed in to decidua and stroma cells are
distended with accumulation of glycogen and lipid droplets
 The decidua has three layers: 1, Stratum compactum
2. Stratum spongiosum and
3. Stratum Basale
 On the sixth day after fertilization, the blastocyst attaches to the endometrium
adjacent to the inner cell mass.
 As it attaches to endometrium, trophoblast proliferates rapidly
to form two layers
i). inner cytotrophoblast
ii). Outer syncytiotrophoblast(multinucleated protoplasmic mass)

21. 3. IMPLANTATION CONT…
 Finger-like processes of the syncytiotrophoblast extend in to, and erode the
stratum compactum and spongiosum by proteolytic action of trophoblast assisted
by uterine mucosa.
 The implantation is interstitial and takes place on 6th -7th day after fertilization.
 On the 7th day, a flattened layer of cells called hypoblast(primitive endoderm)
appears on the surface of inner cell mass facing the blastocyst cavity.
 The site of implantation is mostly the junction of fundus and posterior wall of body
of uterus.
 Abnormal implantation could be extrauterine called ectopic, when the embryo
implants in the ovary, fallopian tube, abdominal cavity or intrauterine.
 When the implantation is at the lower part of the cavity overlapping the internal os
placenta previa

22. ABNORMAL SITES OF IMPLANTATION
1). ECTOPIC PREGNANCY in which the foetus develops outside the … ……
uterus.
Vijaylaxmi Pattanshetti

23. ABNORMAL SITE OF IMPLANTATION: 2).PLACENTA PREVIA:
a condition in which the placenta partially or wholly blocks the neck of the
uterus, so interfering with normal delivery of a baby.
There are 3 types of placenta previa:
1).partial placenta previa: The internal cervical opening is partially covered by the
placenta.
2).complete placenta previa: The internal cervical opening is completely covered
by the placenta..
3).marginal placenta previa: The placenta is at the edge of the internal cervical
opening.
Vijaylaxmi Pattanshetti

24. Traditionally placenta previa is divided into four types – is divided into four grades
depending on the relationship and distance to the internal cervical os:
I).Low lying placenta: In this type placenta is extended into the lower uterine segment but does
not reach the internal orifice of cervix. In most of cases it often moves upward in the uterus as the
date approaches
II).Marginal placenta: When placenta just reaches the internal orifice of cervix, but does not
cover it then the condition is known as marginal placenta
III).Partial placenta: In this type placenta partially covers the internal orifice of cervix
IV).Complete placenta previa: In this the internal orifice of cervix is completely covered by
placenta
Vijaylaxmi Pattanshetti

25. Implantation has begun in first
week of development…..
Vijaylaxmi Pattanshetti
Asst. Prof in Anatomy
SIMS &RH, TUMKURU