Department of Histology, cytology and
embryology
EMBRYOLOGY
DR. MAKARCHUK IRYNA
The embryology is a science, which studies lows of
formation of an embryos and process of his development.
The individual development of living organisms is an
ontogenesis.
In individual development are two basic stages:
 Prenatal ontogenesis is development till birth
 Postnatal ontogenesis is development from birth up
to death of an individual
So, the embryogenes is a part of ontogenesis.
In development of embryos some stages characterized by
certain quantitative and qualitative changes are observed:
1. Fertilization is fusion of a female and male
gamete.
2. Cleavage is the series of rapid cell divisions of the
zygote with the formation of blastula
3. Gastrulation is the formative process by which the
three germ embryonic layers are established in
embryos (ectoderm, mesoderm and endoderm).
4. Histogenesis-development of tissues
5. Organogenesis development of organs
6. Systemogenesis development of systems
Two types of sex cells are distinguished:
 Male cell is spermatozoon or sperm
 Female cell is ovum or ovocyte
The spermatozoon
The spermatozoon consists of 3 main components:
 -the head
 -the neck
 -the tail
The tail is subdivided into three segments:
 -the middle piece
 -the principal piece
 -the end piece
There is a nucleus in the head. The nucleus
involves a very condensed chromatin.
The acrosomal cap surrounds the anterior twothirds of the nucleus.
Acrosome contains different hydrolytic and
mucolytic enzymes. The enzymes disaggregate
the cells of the corona radiata and dissolve the
zona pellucida during fertilization.
There are two centrioles in the neck. The tail
consists of axoneme and a lot of mitochondrias.
Ovum or ovocyte
The ovum is the female haploid gamete which fuses with the sperm and develop into an organism
after fertilization.
As a rule they have a round form, larger than spermatozoons volume of cytoplasm and nucleus, and
do not have the ability to move actively.
The ovum is characterized by the presence of protein-lipid inclusion of yolk in cytoplasm.
The ovocyte is coated by ovolemme or an initial envelope.
The human ovocyte is secondary olygolecithal, what means that the amount of a yolk in
cytoplasm little, and isolecithal what means that yolk distributed evenly on cytoplasm.
The mammalian ovum, except of ovolemme is coated with two more envelopes: jelly-like zona
pellucida and a radiate crown corona radiata.
Composition of zona pellucida enters
glycosaminoglycans (GAG) and glycoproteins.
There are some cortical granules in the
periphery of cytoplasm.
Fertilization and Cleavage
The fertilization represents penetration of a spermatozoon into an
ovum as a result of which it is restored number of chromosomes and
the single-celled embryo a zygote is formed.
The fertilization consists of 3 phases:
 Distant interaction and coming together of gametes
 Contact of gametes
 Infiltration of sperm cell in an ovum and gametic syngamy
The zygote undergoes a number of ordinary mitotic divisions that
increase the number of cells in the zygote but not its overall size. Each
cycle of division takes about 24 hours. The individual cells are known
as blastomeres.
At the 32-cell stage the embryo is known as a morula (L. “mulberry”),
a solid ball consisting of an inner cell mass and an outer cell
mass. The inner cell mass will eventually become the embryo
and fetus, while the outer cell mass will eventually become part of
the placenta.
Fertilization
Here is a pictorial representation of the main
landmarks in human prenatal development:
Day 0 - Development of the new
individual begins when a spermatozoon
penetrates the jellylike zona pellucida
and contacts the plasma membrane of
the secondary oocyte. Note the smaller
first polar body alongside the secondary
oocyte within the zona pellucida. This
was formed during the first meiotic
division. Several follicular cells are
shown still attached to the outer surface
of the zona pellucida. After the arrival of
the fertilizing sperm, no further
spermatozoa are allowed to enter.
Day 0
In response to the fertilizing
sperm, the oocyte completes its
second meiotic division. The
resulting cell division is unequal
- the larger daughter cell
becomes an ovum, and the other
much smaller daughter cell
becomes the second polar body.
While the oocyte is dividing, the
first polar body may also divide.
The head of the spermatozoon
enters the cytoplasm of the
oocyte and swells, forming the
male pronucleus. (The mid-piece
and tail of the spermatozoon do
not enter the ovum.)
Day 0
 After completing its second meiotic
division, the nucleus of the larger ovum
becomes the female pronucleus. The
male pronucleus swells, and the two
pronuclei approach each other and
merge. This establishes the diploid
genome and also the genetic sex of the
new individual. The fertilised ovum is
called a zygote.
 The nuclear DNA is replicated and after
several hours the zygote begins its first
mitotic cell division. This illustration
shows metaphase, with the
chromosomal pairs arranged at the
equator of the mitotic spindle ready to
be separated and moved to opposite
ends of the spindle.
Day 0
The nuclear DNA is
replicated and after several
hours the zygote begins its
first mitotic cell division.
This illustration shows
metaphase, with the
chromosomal pairs
arranged at the equator of
the mitotic spindle ready to
be separated and moved to
opposite ends of the spindle.
Day 1
The two-cell stage. The cytoplasm of the
original zygote has been subdivided
to form two smaller cells - there is
little synthesis of new cytoplasmic
material at this time. This also
applies to subsequent cell divisions
during the first few days, so they are
often called ‘cleavage divisions’. The
cells remain enclosed by the zona
pellucida. The polar bodies are less
prominent, and probably degenerate.
As cleavage continues, a closely-packed
ball of cells is produced - the morula.
Each one of these cells is still very
high in developmental potential, and
could each give rise to a new
individual, although usually they will
co-operate in the development of just
one baby.
Day 4
The morula drifts along the
fallopian tube as cleavage
continues. When it reaches
the uterus, the zona pellucida
dissolves and fragments,
releasing the morula.
Fluid is drawn into the centre
of the morula, creating a
hollow blastocyst.
Within the hollow, fluid-filled
blastocyst there is a special
groups of cells called the
inner cell mass. The cells
forming the outer surface of
the blastocyst are called
trophoblast cells.
Day 7
 The blastocyst makes contact with the lining of
the uterus - the endometrium. This has
prepared for such a possibility by thickening,
becoming more glandular, and developing a
rich blood supply. The blastocyst begins to
implant, a process that will take several days.
 Implantation brings the outer cells of the
blastocyst into direct contact with the
maternal cells of the endometrium. The
conceptus is genetically different from the
mother since half its chromosomes have come
from the father. Usually, genetically different
cells would be rejected by the mother’s
immune system, but this does not occur
during normal pregnancy.
 During implantation, the trophoblast thickens
and forms two layers - an outer
syncytiotrophoblast (grey) and an inner
cytotrophoblast. Meanwhile, the inner cell
mass becomes organised into a two-layered
plate of cells called the embryonic disc, with
the amniotic cavity above and the yolk sac
below. The two layers are called ectoderm and
endoderm.
The conceptus implants itself
completely within the
endometrium. The enlarging
syncytiotrophoblast develops fluidfilled lacunae within it and comes
into contact with the maternal
blood vessels. Extra-embryonic
mesodermal cells derived from the
cytotrophoblast form a layer
around the external surfaces of the
amnion and yolk sac.
The embryonic disc will give rise to
the baby itself. The first sign of the
midline axis of the baby is the
formation of the primitive streak in
the ectodermal layer facing the
amniotic cavity.
In this diagram, part of the
embryonic disc has been
removed to show the primitive
streak in cross-section.
Ectodermal cells migrate
towards the primitive streak and
then tuck inwards to form a new
layer of cells in between the
ectoderm and endoderm. The
new layer is called the
mesoderm. These three layers of
cells - ectoderm, mesoderm, and
endoderm -then give rise to all
parts of the body by a process
called morphogenesis.
 The primitive streak is situated in the
midline towards the tail end of the
future embryo. Further towards the
future head end, the ectoderm
thickens to form the neural plate. This
begins to fold, initially forming a
groove and then closing over to form
the neural tube. The neural tube will
give rise to the brain and spinal cord.
 Closure of the neural tube begins in
the future hindbrain region and then
continues for several more days,
extending both forwards and caudally
until closure is complete. Clusters of
mesodermal cells produce pairs of
somites alongside the developing
neural tube. The number of somites
increases steadily as neural closure
continues.
 In the future brain region, the
neural folds enlarge as they close
and begin to overshadow the part
of the embryonic disc that lies
further forwards. Within the
mesoderm of this part the heart
begins to develop.
 As the heart develops, blood
vessels form in the wall the yolk
sac, the embryo itself, and in the
stalk that connects the embryo to
the trophoblast. Blood cells are
also formed in the yolk sac wall.
On day 23 after fertilisation, the
heart begins to beat and the
circulation of blood is established.
 As the embryo develops, the amniotic
sac enlarges and gradually the yolk
sac begins to diminish in relative
size. The surrounding trophoblastic
tissue develops finger-like processes
which extend outwards into the
endometrium. These extensions are
best-developed across the most
deeply-implanted part of the
conceptus, and will contribute to
formation of the placenta.
 Limb buds begin to form alongside
the neural tube and somites. The
developing eyes are visible, and the
beating heart is now tucked ventrally
in the future thoracic region.
 The fingers and toes are becoming
apparent, and the external ear is visible
on the side of the neck region. The
connection between the embryo and
surrounding trophoblast is becoming the
umbilical cord, and passing through this
is a narrow duct connecting the yolk sac
with the developing digestive tract in the
embryo.
 By the end of the second month, all the
different parts of the new individual have
formed. Morphogenesis is complete.
However, the embryo is only about 3 cms
long from the top of its head to its rump.
Most of the body organs and systems are
only partially functional.
 In the third month, the fetal
period begins. It is a time of
rapid growth in size and weight,
the further maturation of
function in organs and body
systems, and the rehearsal of
increasingly complex activities
that must be perfected before
birth - breathing movements,
swallowing, production of urine,
and digestion, for example.
 The uterus and placenta
continue to enlarge to
accommodate the growing fetus
and meet its increasing needs.
 Towards the end of pregnancy,
space is at a premium, and the
placenta is finding it
increasingly difficult to meet the
needs of the fetus.
 Approximately 9 months after
conception, the process of birth
begins. A difficult transition
must be achieved with the
baby’s systems taking over
many of the responsibilities that
were met previousy by the
placenta and mother.
Gastrulation
Gastrulation is the formative process by which the three
germ embryonic layers are established in embryos
(ectoderm, mesoderm and endoderm).
Human gastrulation includes two main processes:
 Delamination, which establishes bilaminar disk
composed of two layers, the epiblast and hypoblast
 Migration, which establishes three-laminar embryonic
composed of ectoderm, mesoderm and endoderm. These
three layers give rise to all tissues and organs of the
adult.
Gastrulation begins in the same time as implantation
Derivations of the ectoderm
1) Surface ectoderm
Give rise to:
 Epidermis and its appendages
 Enamel of teeth
 Lens of eye
 Internal ear
2) Neural crests
Give rise to:
 Ganglion cells
 Pigment cells
3) Neural tube
Derivations of the mesoderm
1) PARAXIAL MESODERM
Give rise to:
 Sclerotome gives rise to skeleton (vertebral column) except cranium.
 Myotome gives rise to striated skeletal muscle (trunk limbs), muscles of head.
 Dermatome gives rise to dermis and subcutaneous tissue of the skin.
2) INTERMEDIARE MESODERM
Give rise to:
 Urinary system (pronephros, mesonephros, metanephros,)
including ducts.
 Gonads and accessory glands.
3) LATERAL PLATE of MESODERM
Give rise to:
 Serous membranes of pleura, pericardium, and peritoneum.
 Suprarenal gland cortex.
 Germinal epithelium of gonads.
 Myocardium, endocardium of heart.
 Connective tissue and muscle of viscera.
4) HEAD MESODERM
Give rise to:
 Cranium.
 Dentin.
 Connective tissue of head.
Derivations of the endoderm
Give rise to:
 Epithelium lining gastrointestinal tract (stomach,
small intestine, most part of large intestine except
caudal portion of rectum).
 The parenchyma of liver and pancreas.
 Epithelium lining respiratory tract
 The reticular stroma of thymus
Human developmental periods
Human development is continuous process that
includes three main periods:
 -progenesis
 -prenatal period
 -postnatal period
Human developmental periods
Progenesis - Progenesis is a period of maturation of specialized generative cells- gametes. This maturation process is
called spermatogenesis in males and oogenesis in female.
Prenatal period - Prenatal period begins when an oocyte from female is fertilized by a sperm from male with the
formation of zygote.
Main stages of prenatal period:

-Fertilization is fusion of a female and male gamete.

-Cleavage is the series of rapid cell divisions of the zygote with the formation of blastula

-Gastrulation is the formative process by which the three germ embryonic layers are established in embryos (ectoderm,
mesoderm and endoderm).

-Formation of axial organs: notochord, neural tube, and primordial gut.

-Histogenesis

-Organogenesis

-Systemogenesis
Postnatal period - This period occurs after the birth.

Cleavage
Human cleavage is called:

-complete

-asynchronous

-nearly equal
As a result of cleavage of zygote is formed:

-morula

-blastula

-blactocyste
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