Embryogenesis first week ovulation, fertilization and implantation

Embryology
DR. SABRIN SAMI
LEC.1

EMBRYOLOGY
IS THE STUDY OF INTRAUTERINE FORMATION
AND DEVELOPMENT OF AN EMBRYO FROM THE
STAGE OF OVUM FERTILIZATION THROUGH TO THE
BIRTH.

Zygote
fertilized egg cell, consists of genes from two parents, and thus it
is diploid (carrying two sets of chromosomes).
Embryo
this term is applied to the unborn child just after fertilization of the female
egg cell until the end of the 8th
week.
Fetus
From the beginning of the 9th
week the embryo is called fetus. In
comparison to the embryo, the fetus has more recognizable external
features and a more complete set of developing organs.

The gestational period (pregnancy)
length of time that a fetus grows inside the mother’s uterus which
is 9 months, divided into two periods: embryonic and fetal.
The embryogenesis period (organogenesis period)
is a complicated process by which the zygote develops into an
embryo (the period of establishing organ primordia) extends to the
end of the first 8 weeks of human development.

The fetal period
the period from the end point of embryogenesis until birth. It begins when the
embryo has a recognizable external feature.
It is characterized by
continues growth.
morphologic differentiation.
size increasing by the full development of the organ systems.
Prenatal development
The process encompassing the period from the formation of an embryo,
through the development of a fetus, to birth (parturition).

Female reproductive system
• production and transportation of gametes.
• production of sex hormones.
• facilitates the fertilization of ova by sperm .
• supports the development of offspring (infant) during pregnancy and infancy.
1-Ovaries
2-fallopian tubes
3- uterus

Each ovary is an almond-shaped structure and consists of
1. an outer cortex.
2. an inner medulla.
The cortex contains the ovarian follicles at different stages of
maturation.
Each follicle
1- primary oocytes surrounded by a layer of squamous cells
called
2- follicular cells (granulosa).
This mass called the primordial ovarian follicles.

The ovarian follicles consists of
1- primary oocyte
(ssurrounded by a layer of
squamous cells called)
2- follicular cells(granulosa)
At this stage , its called a
primordial ovarian follicles

First week of development
The ovulation

The female monthly cycles are controlled by gonadotropins :
1- follicle-stimulating hormone (FSH)
2- luteinizing hormone (LH)
 both synthesized and secreted by the pituitary gland.
 both hormones are stimulate and control cyclic
changes in the ovary.

mature ovarian follicle (graafian follicle)
Under the influence of FSH the primordial ovarian follicles are stimulated to
grow rapidly to become a mature ovarian follicle, consists of:
 cavity filled with follicular fluid.
 periphery attached oocyte surrounded by:
- a protective membrane called the zona pellucida and
- a layer of follicular cells called cumulus oophorus.

As the surface of the ovary begins to bulge locally,
an avascular spot at the apex appears called the
stigma.

Under the influence of LH:
 the levels of prostaglandin increase
 cause local muscular contractions in the
ovarian wall.
 lead to Graafian follicle rupture
 the ovum released together with its
surrounding cumulus oophorus out of
the ovary in the process called ovulation.
 the cumulus oophorus cells rearrange
around the zona pellucida to form the
corona radiata.

The ovum that is released at the ovulation is picked
up by the fimbriated end (finger like projections at
the end of fallopian tubes) of the uterine tube and
enters the ampulla.

Corpus luteum (yellow body )
It is formed after ovulation, with rupture of
the follicle:
Its wall collapses, crumbles and gets
folded
The follicular cells become large conical.
Under the influence of LH, these cells
change into lutein cells contains
yellowish pigment, form the corpus
luteum.

The corpus luteum serves as a temporary endocrine gland,
secretes female sex hormones the estrogens and progesterone.
It can be easily recognized as a yellowish projection on the
surface of the ovary.
In the absence of fertilization, the corpus luteum has a short life of
14 days, it undergoes degeneration and gets converted into the
mass of fibrous tissue known as the corpus albicans.

FERTILIZATION
Fusion of the two mature germ cells (an ovum and the
spermatozoon), takes place in the ampullary part of
the uterine tube, resulting in formation of the zygote.

PHASES OF FERTILIZATION
■ Phase 1 (penetration of the corona radiate).
■ Phase 2 (penetration of the zona pellucida): this
contact results in release of lysosomal enzymes
from cortical granules lining the plasma membrane
of the oocyte. In turn, these enzymes alter
properties of the zona pellucida to become
impenetrable to other spermatozoa.
■ Phase 3 (fusion of the oocyte and sperm cell
membranes): after adhesion, the plasma
membranes of the sperm and egg fuse.

The main results of fertilization
 Restoration of the diploid number of chromosomes:
Half from the father and half from the mother.
 Determination of the sex of the new individual:
An x-carrying sperm produces a female (xx) embryo, and a y-
carrying sperm produces a male (xy) embryo.
 Initiation of cleavage:
Without fertilization, the oocyte usually degenerates 24 hours after
ovulation.

 Within 24 hours after fertilization, the zygote initiates a rapid series of cell divisions called
cleavage.
 These divisions are not accompanied by cell growth, they subdivide the large zygote into
many smaller daughter cells called blastomeres.
 The embryo as a whole does not increase in size during cleavage and remains enclosed in the
zona pellucida.
 By three days, the embryo consists of 16 to 32 cells. At this stage the embryo is called a
morula (latin word morum: mulberry).

 Some of blastomeres segregate to the center of the morula and others to the
outside.
 The centrally placed blastomeres are now called the inner cell mass, which
gives rise to the embryo proper called the embryoblast.
 The outside blastomeres are called the outer cell mass constitute the
trophoblast (trophe – means nutrition) which surround the embryoblast and
supplies nutrition to the them. Trophoblast considered as the primary source of
the fetal component of the placenta.

 Between the 4th
and 5th
day after the fertilization, the morula
reaches the uterine cavity and begins to absorb fluid from the
uterine cavity.
 As the pressure of the fluid increases the embryoblast
separated from the trophoblast, and a large cavity formed within
the morula called the blastocyst cavity (blastocoel).
 The embryoblast cells then form a compact mass at one side of
this cavity, and the trophoblast organizes into thin, single outer
layer. The embryo is now called a blastocyst.

The implantation occurs at the 6th
day of the first
week through the second week.
As the blastocyst increased in size, it hatches out
of the zona pellucida and the trophoblast layer
become exposed, thus enabling it to attach to the
endometrium.
The trophoblast gets implanted on the wall of the
uterus by eroding the endometrium with
proteolytic enzymes.

The embryoblast begins to
differentiate into two layers:
 A distinct external (or upper) layer
of columnar cells, called the
epiblast layer (primitive ectoderm).
 An internal (or lower) layer of small
cuboidal cells adjacent to the
blastocyst cavity, known as the
hypoblast layer (primitive
endoderm).

An extracellular basement membrane
is laid down between the two layers as
they become distinct.
The resulting two layered embryoblast
is called the bilaminar embryonic disc.
Around 8th
day, the amniotic cavity
appears as fluid begins to collect
between the epiblast and trophoblast.

As the embryo begins to implants,
the uterine endometrium induces
the trophoblast to differentiate into
two sub-layers:
1.An outer layer called the
syncytiotrophoblast, form
syncytium.
2.An inner layer called the
cytotrophoblast.


The syncytiotrophoblast facilitates implantation by
secreting proteolytic enzyme to digest the extracellular
matrix of the uterine endometrium.

By day 9th
, the syncytiotrophoblast ultimately comes to
envelop the entire embryo.

The blastocyst is more deeply embedded in the
endometrium, and the penetration defect in the surface
epithelium is closed by a fibrin coagulum.

The placenta develops once the blastocyst is
implanted, connecting the embryo to the
uterine wall.

The vacuoles appear in the syncytium, and fuse to form
large lacunae syncytial lacunae, and this phase of
trophoblast development is known as the lacunar stage.
The syncytial lacunae become continuous with the
sinusoids, and maternal blood enters the lacunae and
flow through the trophoblastic system, establishing the
uteroplacental circulation.

By the 11th
to the 12th
day of development, the blastocyst is
completely embedded inside the endometrium.
The blastocyst cavity gradually grows to form the yolk sac.
 Around the end of the 2nd
week:
1) The embryonic disc with its yolk sac is suspended within a
large cavity called chorionic cavity.
2) Attached to the trophoblastic shell by a narrow connecting
stalk, that develops lately into the umbilical cord to attach the
placenta to the embryo.

THIRD WEEK OF
DEVELOPMENT
GASTRULATION STAGE

The most characteristic event occurring during the 3rd
week of gestation is gastrulation:
The process that establishes all three germ layers in the
embryo:
1.Definitive endoderm.
2.Intraembryonic mesoderm.
3.Ectoderm.


In a 15- 16 day, the gastrulation stage begins
with formation of a longitudinal midline
thickening structure called primitive streak
in the epiblast near the caudal end of the
bilaminar embryonic disc.

This streak elongates to occupy about half
the length of the embryonic disc.

Its continuous growth changes the
embryonic disk shape from circular to
ovoid-shaped.

Formation of the primitive streak defines all major body axes:
1.The cranial-caudal (head-tail) axis.
2.The dorsal-ventral (back-belly) axis.
3.The medial-lateral axis.
4.The left-right axis.

The primitive streak contains a midline
narrow groove called the primitive
groove surrounded by slightly bulging
regions on either side.
The cranial end of the primitive streak
is expanded forming the primitive
node, which consists of a slightly
elevated area surrounding a small
circular depression called the
primitive pit.

 The primitive pit and groove represent
areas where cells are migrating and
moving from the epiblast into the
interior of the embryonic disc.
 This inward movement is known as
invagination.
 The collective movement of cells
through the primitive streak into the
interior of the embryo to form the three
primary germ layers constitutes
gastrulation.

The process of gastrulation is complete when the
formation of all three definitive germ layers of the
trilaminar embryonic disc are complete throughout the
disc.
The epiblast cells are the source of all of the germ
layers during the process of gastrulation. They will give
rise to all of the tissues and organs in the embryo.

On day 16, the epiblast cells lateral to the primitive streak elongate and
become flask or bottle shaped, detaching from their neighbors and
migrating through the primitive streak:

By the middle of 3rd
week, the intraembryonic
mesoderm separates the ectoderm and
endoderm everywhere except at:
1. Oropharyngeal membrane cranially.
2. Prechordal plate.
3. Cloacal membrane caudally.

The endoderm derivatives:
1.Epithelial lining of the gastrointestinal and respiratory
tracts.
2.Parenchyma of the tonsils, liver, thymus, thyroid,
parathyroid, and the pancreas.
3.The epithelium lining the auditory tube and tympanic
cavity.
4.The epithelium of the urinary bladder and urethra.

Intraembryonic mesoderm gives rise to:
1. Teeth except the enamel.
2. The connective tissue, cartilage and bone.
3. Striated and smooth muscles, except the iris.
4. The blood and the blood vascular and lymphatic systems.
5. Kidneys, gonads (testes and ovaries).
6. The cortex of the suprarenal glands.
7. The mesothelial linings of the pericardial, pleural and peritoneal cavities.
8. Spleen

The embryonic ectoderm gives rise to:
1. The enamel of teeth.
2. The salivary glands.
3. The epithelium lining the nose and paranasal sinuses, the roof of the
mouth, the gums and the cheeks.
4. Skin and its appendages (hair and nails).
5. Subcutaneous glands.
6. The central and peripheral nervous system.
7. The epithelium of the cornea, conjunctiva and lacrimal glands.
8. The muscle of the iris.
9. The neuroepithelium of the sense organs (eye, ear, and nose).

The embryonic disc, initially is flat and
round, gradually becomes elongated, with a
broad cranial end and narrow caudal end,
due to the continuous migration of cells from
the primitive streak region in a cranial
direction.
The primitive streak shrinks, and disappears
at the end of the fourth week, when the
invagination and migration of surface cells
into the primitive streak are stopped.

In the cephalic part, germ layers
begin their specific differentiation by
the middle of the third week, whereas
in the caudal part, differentiation
begins by the end of the fourth week.
The gastrulation continues in caudal
segments while cranial structures are
differentiating, causing the embryo to
develop cephalocaudally.

The prenotochordal cells are
derived from epiblasts.
Start invaginating in the
primitive node.
Migrate in a midline, toward
the anterior end of the embryo
to form a solid rod the
notochord

The notochord grows forward
cranially and impeded by the tightly
adherent endoderm and ectoderm of
the prechordal plate.
By day 16, the notochord is visible
throughout the length of the embryo
surrounded by layered
concentrations of cells, which are
representing the primordia of the
future vertebral bodies.

NOTOCHORD FUNCTIONS:

The notochord defines the axis of the embryo and gives it some rigidity.

It interacts with the overlying ectoderm to induce differentiation of the
neural tube which becomes the central nervous system.

Its signaling protein influences organization of somites and development
of the vertebral column.

Provides axial support for the embryo.

In an adult, the notochord persists as the gelatinous central
Nucleus pulposus of the intervertebral disks.

Occurs at or near the end of gastrulation during the 3rd
through
4th
week after fertilization.
It is the process by which the embryo develops structures that
will eventually become the nervous system, and transforms the
gastrula into a neurula.

NEURULATION OCCURS IN FOUR STAGES:
1.Transformation of the central portion of the embryonic
ectoderm into a thickened neural plate.
2.Shaping and elongation of the neural plate.
3.Bending of the neural plate around a medial groove followed
by elevation of the lateral folds.
4. Closure of the neural tube.

FORMATION OF NEURAL PLATE:
1. On day 18th
notochord induces the overlying
ectoderm to thicken and form neural plate.
2. Cells of the plate make up the pseudostratified,
columnar neuroepithelial cells neuroectoderm.
3. The neural plate forms first at the cranial end
of the embryo and then differentiates in a
cranial to caudal direction.
4. The neural plate is broad cranially and gives
rise to brain, and is tapered caudally gives rise
to the spinal cord.

1. During the 4th
week, the neural plate
increased in length and its lateral edges
elevate to form neural folds consisting of
both neuroectoderm and adjacent surface
ectoderm.
2. The neural groove appears on the neural
plate with the formation of neural folds.
3. Neural folds start fusing in the middle at the
cervical region and the process of fusion
proceeds cranially and caudally. As a
result, the neural tube is formed.

4. There will be two separate epithelial layers; the
roof plate of the neural tube and the overlying
surface ectoderm.
5. Until fusion is complete, the cephalic and caudal
ends of the neural tube communicate with the
amniotic cavity by the cranial neuropore and
caudal neuropore.
6. Closure of the cranial neuropore occurs at
approximately day 25, whereas the posterior
neuropore closes at day 28.
7. Neurulation is then complete, and the central
nervous system is represented by a closed
tubular structure with a narrow caudal portion, the
spinal cord, and a much broader cephalic portion.

They are a unique population of cells
that originating from the crest of the
neural fold as it closes during neural
tube formation.
These cells undergo an epithelial to
mesenchymal transformation and
become free mesenchymal cells.
They leave the neuroectoderm by active
migration to the distant parts of the
body, and displacement to enter the
underlying mesoderm.

The neural crest cells are so important and contributed to so many organs and tissues that they are sometimes referred to as the fourth germ
layer.
1. CONNECTIVE TISSUE AND BONES OF THE FACE AND SKULL.
2. CRANIAL NERVE GANGLIA .
3. C CELLS OF THE THYROID GLAND .
4. ODONTOBLASTS .
5. PARASYMPATHETIC GANGLIA OF THE GASTROINTESTINAL TRACT.
6. ADRENAL MEDULLA .
7. SCHWANN CELLS.
8. GLIAL CELLS.
9. MENINGES .
10. MELANOCYTES.
11. SMOOTH MUSCLE CELLS TO BLOOD VESSELS OF THE FACE AND FOREBRAIN.

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