Journey of an embryo...development biology

• A process where male spermatogonia develop
into mature spermatozoa.
• Occur in testes and epididymis in mammals
and takes approximately 64 days.
SPERMATOGENESIS

• Step 1 – Spermatocytosis
• Step 2 – Meiosis in Sertoli cells
• Step 3 – Spermiogenesis
STEP IN SPREMATOGENESIS

• Monospermy – fusion of a single sperm and
egg nuclei
• Polyspermy – excess of adhesion sites that
leads to fusion of a single engg with more
than 1 sperms.
MONOSPERMY & POLYSPERMY

EVENTS THAT LEADS TO
FERTILIZATION

General steps of early
embryonic development

Embryology
Morula, 8 cell stage
1 - morula, 2 - blastula
1 - blastula, 2 - gastrula with blastopore;
orange - ectoderm, red - endoderm.

There are 4 stages of embryonic
development:
Cleavage
Patterning
Differentiation
Growth

Cleavage
• Mitosis and cytokinesis of the zygote, an unusually large cell,
produces an increasing number of smaller cells
• the genes of the zygote are not expressed at first.
• The early activities of cleavage are controlled by the
mother's genome; that is, by mRNAs and proteins she
deposited in the unfertilized egg.
• In humans, the switch-over occurs after 4—8 cells have been
produced; in frogs not until thousands of cells have been
produced.
• Cleavage ends with the formation of a blastula.

Patterning
During this phase, the cells produced by
cleavage organize themselves in layers and
masses, a process called gastrulation. The
pattern of the future animal appears:
 front and rear (the anterior-posterior axis)
 back side and belly side (its dorsal-ventral
axis)
 left and right sides.

Gastrulation forms three major "germ layers":
 ectoderm
Mesoderm
endoderm
By gastrulation, the genes of the zygote
genome are being expressed

Differentiation
• Is the process by which cells or other parts of
organisms become different from one another and
different from what they were previously
• In time, the cells of the embryo differentiate to
form the specialized structures and functions that
they will have in the adult.
• They form neurons, blood cells, skin cells, muscle
cells, etc.
• These are organized into tissues, the tissues into
organs, the organs into systems.

Growth
• Is an increase of size and mass, the
enlargement of a tissues or organism
• After all the systems are formed, most
animals go through a period of growth.
• Growth occurs by the formation of new cells
and more extracellular matrix.

CLEAVAGE PRODUCES A BALL OF
CELLS FROM ZYGOTE

• cleavage is the division of cells in the early
embryo
• The zygotes of many species undergo rapid
cell cycles with no significant growth,
producing a cluster of cells the same size as
the original zygote.
MEANING

PATTERN OF CLEAVAGE
1. Amount and distribution of yolks in their eggs
• Isolechital
• Mesolechital
• Telolechital
• Centrolechital
2. Polarity of eggs
• Animal pole
• Vegetal pole

Types of cleavage
HOLOBLASTIC
In the absence of a large concentration of yolk. In
holoblastic eggs the first cleavage always occurs along
the vegetal-animal axis of the egg, the second
cleavage is perpendicular to the first.
MEROBLASTIC
In the presence of a large amount of yolk in the
fertilized egg cell, the cell can undergo partial, or
meroblastic, cleavage.

Cleavage patterns followed by holoblastic and
meroblastic eggs
Holoblastic Meroblastic
•Bilateral (tunicates,
amphibians)
Discoidal (fish, birds,reptile)
Radial (sea urchin, amphioxus) •Superficial (insects)
Rotational (mammals)
Spiral (annelids, mollusks)

Bilateral
• The first cleavage results in bisection of the zygote into
left and right halves. The following cleavage planes are
centered on this axis and result in the two halves being
mirror images of one another. In bilateral holoblastic
cleavage, the divisions of the blastomeres are
complete and separate.
Radial
• Radial cleavage is characteristic of the deuterostomes,
which include some vertebrates and echinoderms, in
which the spindle axes are parallel or at right angles to
the polar axis of the oocyte.
HOLOBLASTIC

Rotational
• Mammals display rotational cleavage, and an isolecithal
distribution of yolk (sparsely and evenly distributed).
Because the cells have only a small amount of yolk, they
require immediate implantation onto the uterine wall in
order to receive nutrients. Rotational cleavage involves a
normal first division along the meridional axis, giving rise to
two daughter cells.
Spiral
• In spiral cleavage, the cleavage planes are oriented
obliquely to the polar axis of the oocyte. At the third
cleavage the halves are oblique to the polar axis and
typically produce an upper quartet of smaller cells that
come to be set between the furrows of the lower quartet.

Meroblastic cleavage
Discoidal
• In discoida cleavage, the cleavage furrows do not penetrate the
yolk. The embryo forms a disc of cells, called a blastodis, on top of
the yolk. Discoidal cleavage is commonly found in birds, reptiles,
and fish which have telolecithal egg cells (egg cells with the yolk
concentrated at one end).
Superficial
• In superficial cleavage, mitosis occurs but not cytokinesis, resulting
in a polynuclear cell. With the yolk positioned in the center of the
egg cell, the nuclei migrate to the periphery of the egg, and the
plasma membrane grows inward, partitioning the nuclei into
individual cells. Superficial cleavage occurs in arthropods which
have centrolecithal egg cells (egg cells with the yolk located in the
center of the cell).

- a phase early in the development of most
animal embryo, during which the morphology
of the embryo is dramatically restructured by
cell migration.
GASTRULATION
- a phase early in the development of most
animal embryos, during which the
morphology of the embryo is dramatically
restructured by cell migration.

• The purpose of gastrulation is :
- to position the three embryonic germ layers,
the endoderm, ectoderm and mesoderm.
• gastrulation occurs after implantation,
around days 14-16 after fertilization in human
embryogenesis. (in human )

• The process gastrulation (in human) are :
- The formation of the primitive streak and Hensen's node and
the ingression of cells through the primitive groove to form
the endoderm and the mesoderm.
- Thus, gastrulation creates all three germ layers of the
embryo: ectoderm, mesoderm, and endoderm
- Extraembryonic mesoderm forms within the hypoblast or
embryonic mesoderm and migrates out to form the blood
vessels of the chorion and connect the chorion to the embryo
through the umbilical cord.

• The archenteron is elongated by three mechanisms :
- First, the initial invagination is caused by a
differential expansion of the inner layer made of
fibropellins and outer layer made of hyalin to cause
the layers to bend inward.
- Second, the archenteron is formed through
convergent extension.
-Third, secondary mesenchyme pull the tip of the
archenteron towards the animal pole.

• The process of gastrulation in amphibian is at
higher density of yolk in the vegetal half of the
embryo results in the blastocoel cavity being
placed asymmetrically in the animal half of
the embryo.

• four kinds of tissue movements that drive gastrulation in Xenopus, that
are :
- At the vegetal edge of the dorsal marginal zone, cells change from
a columnar shape to become a bottle cell and drive invagination.
- At this invagination, cells begin to involute into the embryo. This
initial site of involution is called the dorsal lip.
- Directed cell intercalation within the dorsal mesoderm drives
convergent extension. The dorsal cells become the first to migrate along
the roof of the blastocoel cavity and form the anterior/posterior axis of
the embryo.
- Both prior to and during the involution, the animal cap undergoes
epiboly and spread toward the vegetal pole.

EVENTS IN DEVELOPMENT THAT
INVOLVED THE MIGRATION OF
CELLS WITHIN THE EMBRYO
DEVELOPMENT

About 1th week
•After fertilization,embryo reaches two-cell stage
•The blastula implants into the uterus
2th week
•Within 2 weeks,many thousand of cells formed(embryo)
About 5th
week
•A gestational sac on ultrasound
•Embryo at 4 weeks after fertilization.
6th
week
•In the beginning of the 6th
week,a small ring called yolk sac on
ultrasound
•At the end of the 6th
week the fetal pole and perhaps cardiac
activity in the embryo
Embryo at 4 weeks after
fertilization.

At 7th
week
A well defined fetal pole and deinite cardiac cavity
By 9th
week
A baby is called fetus. At this time the heartbeat with a doptone device about
50% of the time can heard
is about five weeks old (or from the
seventh week of menstrual age).
Fetus at 8 weeks
after fertilization.
Fetus at 8 weeks after fertilization.
This embryo is also from an ectopic
pregnancy, this one in the cornu (the part
of the uterus to which the Fallopian tube
is attached). The features are consistent
with a developmental age of seven
weeks (reckoned as the ninth week of
pregnancy

At end of first trimester(12th
week)
Placenta formed and supply the baby with oxygen from mother’s blood supply,and
ridding wastes tthrough mom’s blood system
At 13th
week
Baby growing very quickly
Week 16th
– 20th
Mother may feel a fluttering that is baby’s movement(quickening)
Fetus at 18 weeks after fertilization
20th
week
Baby is half-way fully formed
Baby is quite active and moving often
21th week
Baby’s eyes still closed,movement is stronger,skin is pink
As baby and uterus grow,they are displacing organs that reside normally in the lower
abdomen and pelvis
Fetus at 18 weeks after fertilization

By 24th
week
Uterus having intermitent contractions
Baby weights over one and one half pound
Baby is consider viable(half babies born is survive at this stage)
26th
to 28th
week
Lungs matured
Baby starting to store part in the subcutaneous layer of skin and hair growing
Baby eyes is open
32th to 33th week
Baby weight about 4 ½ pounds and about 16-17 inches along
About 34th
week
Baby lung start to work well
From 36th
week
Baby consider fully develop Fetus at 38 weeks after
fertilization

The process involved in the formation of the neural plate
and neural folds and closure of the folds to form the neural
tube constitute neurulation.
Neurulation is complete by the end of the fourth week.
During neurulation, the embryo may be referred to as
neurula.
Neurulation in vertebrates results in the formation of the
neural tube, which gives rise to both the spinal cord and the
brain.
Neural crest cells are also created during neurulation.
Neural crest cells migrate away from the neural tube and
give rise to a variety of cell types, including pigment cells
and neurons.

PROCESS OF NEURULATION
Neurulation begins
with the formation
of a neural plate, a
thickening of the
ectoderm caused
when cuboidal
epithelial cells
become columnar.

 Changes in cell shape
and cell adhesion
cause the edges of the
plate fold and rise,
meeting in the midline
to form a neural tube.

The cells at the tips of
the neural folds come
to lie between the
neural tube and the
overlying epidermis.
These cells become the
neural crest cells.
Both epidermis and
neural plate are
capable of giving rise to
neural crest cells.

Organogeneis is the period of animal
development during which the embryo is
becoming a fully functional organism capable of
independent survivial.
Organogenesis is the process by which specific
organs and structures are formed, and involves
both cell movements and cell differentiation.
Organogenesis requires interactions between
different tissues. These are often reciprocal
interactions between epithelial sheets and
mesenchymal.

ORGAN PRODUCED BY THE 3 GERM LAYERS

ENDODERM
The endoderm produces tissue within the
lungs, thyroid and pancreas.

MESODERM
The mesoderm aids in the production of
cardiac muscle, skeletal muscle, smooth
muscle, tissues within the kidneys, and red
blood cells.

ECTODERM
The ectoderm produces tissues within the
epidermis and aids in the formation of
neurons within the brain, and melanocytes.

Journey of an embryo...development biology

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