1
Forth stage
Obstetric
Lec-6
.د
ولدان
1/1/2016
Fertilization and implantation
and early development of embryo
Fertilization:
Is the fusion of male and female gametes.
In humans, most cells contain 46 chromosomes (diploid). Meiosis generates mature
(haploid) eggs or sperm containing 23 chromosomes.
A diploid primary spermatocytes undergoes meiosis to produce 4 haploid mature
sperm.
A diploid primary oocytes undergoes meiosis. It produces only one haploid mature
ovum (containing one polar body).
In female the 1
st
meiotic division starts in utero but not completed till puberty producing
secondary oocyte and 1st polar body.
Then 2nd meiotic division occur and not completed till fertilization. During the 2
nd
meiosis,
the primary oocyte remains in meiotic arrest. Meiosis will resume once fertilization has
occurred to produce mature ovum and 2
nd
polar body.
In male the spermatogonia divide mitotically at the onset of puberty to produce primary
spermatocytes. These undergo a 1
st
meiotic division to produce the secondary
spermatocytes. The diploid secondary spermatocytes further undergo the 2
nd
meiotic
division to produce haploid spermatids.
Female ovum, which contain only X chromosomes while the sperm contain either X or Y
chromosomes. Therefore a normal sex combination will be either XX (female) or XY (male).
The ovulated egg is picked up by the fimbria of the fallopian tube and then swept by ciliary
action towards the ampulla, where fertilization occurs.
The basic PH of the seminal fluid protects the spermatozoa from the acidity of the vagina.
Within minutes after ejaculation, sperm may be found in the cervix and are will move with
great speed towards the ampulla, where fertilization of the mature ovulated oocyte usually
occurs. To achieve this, the sperm must undergo capacitation.
During this process the inner membrane beneath the acrosome cap becomes primed for
fusion with the inner membrane of the ovum. The acrosomal reaction exposes the inner
membrane of the sperm. This portion will fuse with the membrane of the ovum. Following
2
penetration of the oocyte, a series of events must occur to form the male pronucleus
(decondensation of sperm chromosomes).
The egg must complete its 2
nd
meiotic division to form the haploid female pronucleus and 2
nd
polar body is extruded. So fertilization is completed by the fusion of male and female
pronuclei and this completed within 20 hours resulting in single cell with diploid genetic
constitution called the zygote.
Following fertilization, cleavage occur in form of a rapid successive mitotic divisions that
produce the morula stage and then fluid filled cavity is formed and an inner cell mass
identified attached eccentrically to the outer layer of flattened cells (trophoblast) the embryo
at this stage is called the blastocyst.
Implantation:
The embryo remain in the fallopian tube for 3-4 days. It float freely for up to 72 hours in the
uterine cavity. The zona is shed and the blastocyst adhere to the endometrium.
Succcessful
implantation requires a receptive endometrium that has been appropriately primed with
oestrogen and progesterone.
A variety of proteolytic enzymes may play a role in separating the endometrial cell and
digesting the inter cellular matrix. The cells of blastocyst are differentiated into outer layer
mass called trophoblasts give rise to the placenta and inner cell mass give rise to the embryo.
The trophoblast differentiated into cytotrophoblast and invasive syncytiotrophpblast.
Following ovulation the corpus lulteum produces estrogen and progesterone to influence the
receptivity for implantation leading to decidual changes.
The decidua basalis is the decidual layer directly beneath the site of implantation.
The decidua capsularis is the layer overlying the developing embryo and separating it
from the rest of uterine cavity.
The decidua vera (parietalis) is the remaining lining the uterine cavity and it is fused
with decidua capsularis by the 4
th
month. The decidua basalis enter in the formation
of the basal plate of the placenta and it is invaded extensively by trophoblastic gait cell.
Nitabuch's layer is a zone of fibrinoid degeneration where the trophoblast meets the
decidua.
Embryology:
Following successful fertilization, the differentiation of cells into specialized tissues to form
interrelated organ system, is known as the embryonic period. It starts with the generation of
the embryonic disc during the 2
nd
week post fertilization (4
th
weeks after LMP) and ends at
the last day of 8
th
weeks (10
th
weeks after LMP).
3
In the 3
rd
week
:
1. The primitive streak, appears on the dorsal aspect of the germ disc.
2. The mesodermal layer is generated and migrated between the ecto and endodermal
layer.
3. The primary yolk sac grows rapidly, it is an important organ for exchanging metabolites
between the mother and the embryo at the time when there is no placenta (full
development by day 32 and degeneration by the end of 6
th
week).
In the 4
th
week:
1. The embryo folds into an embryonic cylinder, within which is a craniocaudal, blind
ending tube which has 3 segments (forgut, midgut, hindgut).
2. Primitive heart (cardiac activity is evident by 22 day post fertilization).
3. Neurolation (development of the nervous system).
4. Towards the end of the 4
th
wk, the forgut septates along the mid line into respiratory
and digestive primitive elements.
5. By day 26 the mesonephric duct and mesonephros differentiate and by day 28 the
ureteric buds are defined.
6. By the end of the 4
th
wk almost all organ systems, albeit immature, can be readily
identified.
The external appearance of the embryo change dramatically.