Embryology

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EMBRYOLOGY
PRESENTOR-VAISHNAVI KASARPATIL
1ST YEAR MDS ORTHODONTICS

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• Embryology- is the study of the formation and development of
the embryo (or fetus) from the moment of its inception up to the
time when it is born as an infant.
• Karl Ernst von Baer: Father of embryology
• Panchanan Maheswari - Indian Father of embryology
INTRODUCTION

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GENETICS
• Genetics is study of DNA
• Concept of a gene as a unit of recombination refers to a single
deoxyribonucleic acid (DNA) nucleotide base pair.(A=T, C=G)
• Genes are conceived as structural, operator, or regulatory genes
• The embryologically significant gene as a unit of function is a
sequence of hundreds or thousands of nucleotides.

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• In females, there is inactivation of one of the two X
chromosomes and failure of expression of its genes (the Lyon
hypothesis).
• In males, the presence of the Y chromosome and only a single
X chromosome accounts for the sex linkage of certain inherite
traits.

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• The developmental ontogeny of the craniofacial complex depends primarily on
the following three elements:
1. Genetic factors: inherited genotype; expression of genetic
mechanisms
2. Environmental factors: nutritional and biochemical interactions;
physical phenomena—temperature, pressures, hydration, etc.
3. Functional factors: extrinsic and intrinsic forces of muscle actions, space-
occupying cavities and organs, growth expansion, and atrophic
attenuation

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SIGNAL TRANSDUCTION
• A signaling center or node (eg. Hensen’s node) is a group of cell
that regulates the behavior of surrounding cells by producing
positive and negative intercellular signaling molecules.
• Also called as Chordoneural hinge, organizer of embryo.
• During gastrulation and neurulation, it undergoes a rostral to
caudal movement of embryo.

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• Growth factors stimulate cell proliferation and differentiation
• Growth factors play roles in embryogenesis, in the immune
system, and during inflammation and wound repair.

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SIGNALING & GROWTH FACTORS

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EMBRYOGENESIS
PREIMPLANTATION
PERIOD
(the first 7 days),
EMBRYONIC PERIOD(upto 8
weeks)
A. PRESOMITE
B. SOMITE
C. POST-SOMITE
FETAL
PERIOD
(9WEEKS –
BIRTH)

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SPERMATOGENESIS
• Spermatozoa is formed in seminiferous
tubules.
• As spermatozoa pass trough epididymis
they undergo maturation.
• Current of fluid carries to epididymis.
• Spermatid changes its shape-
spermatozoon.
• Tranformation of a circular spermatid to
sperm- SPERMIOGENESIS

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OOGENESIS
• Growth process in which the primary
egg cell (or ovum) becomes a mature
ovum
• Formed in ovary.
• Primary oocyte remains in prophase of 1st
meiotic division(since birth) until they
begin to maturate and ovulate.
• Viable for 24hrs.
ovulation

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EMBRYONIC PERIOD v/s FETAL PERIOD
• EMBRYONIC period- first 8 weeks of development of 3
primary germ layers give rise to all structures and basic
body plan takes shapes.
• FETAL period- remaining 30 weeks. Structures and organs
continue to grow and develop.
EMBRYO AT 8 WEEKS FETUS

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EARLY EMBRYONIC DEVELOPMENT
• The mating of male and female gametes in the maternal uterine
tube initiates the development of a zygote.
• All the inherited characteristics and sex are established at the
time of union of the gametes.
• The differentiation of these cells into specialized forms
depends on genetic, cytoplasmic, and environmental factors.

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• Not only are mitosis and cell growth essential for embryonic
development, but even cell death( useless or crippled cells are
eliminated)—genetically and hormonally controlled—forms a
significant part of normal embryogenesis.

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STAGES OF DEVELOPMENT
1. Fertilization
2. Cleavage
3. Gastrulation
4. Organogenesis
5. Maturation
Embryogenesis

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(A)PRE-IMPLANTATION
Conception to 7-8 days of intrauterine
life

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•Day 1 Fertilization • The process of fusion or union of
spermatozoon with the mature ovum in
ampulla-fertilization
• Which produced the fertilized single cell
call the zygote.
• The 23 chromosomes of female nucleus
and 23 chromosomes of male nucleus
gets mixed up and form 46 pairs.

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Stages in the maturation of the ovum
(A) Ovum just
before ovulation
(B) Ovum at the
time of ovulation
(C)Ovum at the time
of fertilization
(D) Ovum just
after fertilization

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Stages in penetration of a spermatozoon into an ovum

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Chromosomes during fertilization.
(A) Male (22 + X/22 + y) and
female (22 + X) pronuclei
(B)Duplication of chromosomes
and arrangement at equatorial
plate
(C) Forming two cells

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•Day 2 Two-cell stage
• The cleavage division lasts for 6 days, i.e. up to 7th
day after fertilization.
• At the first cleavage divides the zygote forms one
large cell and one small cell.
• In the next cleavage the larger cell divides first
followed by smaller one

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• After 30 hours of fertilization, cell divides into two.
• Subdivisions of cleavage: three subdivisions in cleavage. They are:
1. Stage of compaction
2. Morula
3. Blastocyst

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•Day 3 Morula
• After 15 hours the two cell becomes four cell.
• And at the end of 3 days fertilized egg cell has
become a berry like structure made up of 16
cells(hyperplasia).
• So that the 16-cell stage, known as the MORULA

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• Differentiation creates an approximately 100-
cell fluid-filled blastocyst.
• Some fluid passes into morula from uterine
cavity.
• This separates the inner cell mass from
trophoblast .
• Cavity formed is called blastocoele.
Embryonic pole
Day 4 Blastocyst
Formation of blastocyst

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• Trophoblast becomes flattened(outer
sphere)
• The inner cell mass-Embryoblast will form
the embryo.
• Inner mass cell attached to one side of
trophoblast.(eccentric)
• During this period, the conceptus passes
along the uterine tube to enter the uterus

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• It implants into the uterine endometrium on the 7th
postconception day.
• The trophoblast converts into the chorion- the
outermost fetal membrane around the embryo.
• Chorionic implantation establishes the placenta, the
organ of fetomaternal exchange of nutrition and
waste disposal.

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Day 6-7 IMPLANTATION
epiblast hypoblast
• Zona pellucida prevents from invading to
the epithelium from uterine tube to
uppermost part of uterine cavity.
• Zona pellucida degenerates and
decomposes before implantation.
• Trophoblast cells have the capacity to
invade and burrow into tissue which they
come in contact with.

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• Endometrium provides necessary to support implantation of egg
after conception
The stages of development of blastocyst as the fertilized ovum
traverses through the fallopian tube

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(B) PERIOD OF EMBRYO
-PRESOMITE PERIOD
Period is from 8th-20th day of intra uterine
life
Formation of fetal membranes, amnion,
chorion and germ layers.

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•Day 9 Trophoblast with lacunae
Trophobalstic
lacunae
Enlarged
blood
vessels
Fibrin
coagulum
Exocoelomic
membrane
• Spaces in the early syncytiotrophoblast
• The syncytiotrophoblast(outerlayer of
trophoblast)-releases hCG.
• The cytotrophoblast is a layer of
mononucleated cells
• Cytrophoblast(inner trophoblast layer)
2 layered tissue-epiblast (columnar)
-hypoblast(cuboidal)

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• Some cells of inner cell mass differentiate into
flattened cells- called hypoblast.
• Remaining inner cell mass become columnar- called
epiblast
• Embryo in two layers
DAY 8 BILAMINAR DISC
The blastocyst with amniotic cavity developed. Inner cell mass has differentiated into epiblast and hypoblast
layers.Syncytiotrophoblast and cytotrophoblast layers are seen. The inner cell mass is a bilaminar disk made of
epiblast and hypoblast

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• A space appears between epiblast and
trophoblast-amniotic cavity filled with amniotic
fluid.
• Roof is amniogenic cells derived from trophoblast
• Floor is epiblast
• Cells of trophoblast give rise to mass cells-
mesoderm

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2 important structures develop- CHORION (on inside) and
trophoblast(on outside)
-AMNION from amniogenic cells forming wall
of amniotic cavity
Yolk sac becomes smaller-secondary yolk sac(cells become flattened)
(end of 2 weeks)

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DAY 14 PROCHORDAL PLATE
• At one circular end, hypoblast becomes
columnar, called as PROCHORDAL PLATE.
• Appearance of prochordal plate determines
central axis of embryo(distinguish head and tail)
• Formation of prochordal plate is achieved before
the formation of mesoderm
Embryonic disc after
establishment of a central
axis. “B” represents a
section along the central
axis

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Day 15 PRIMITIVE STREAK APPEARS
• Epiblast cells near the tail end begin to
proliferate and appear by formation of
bulging of epiblast cells into amniotic
sac- PRIMITIVE STREAKS
• PRIMITIVE streak is rounded, with
elongation of embryonic disc-linear
structure in central axis.
Formation of primitive streak. Note the expansion
of cranial end of the streak to form primitive node

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•Day 16 Gastrulation:Formation of germ layers
• Gastrulation is defined as an early
developmental process in which an embryo
transforms from a one-dimensional layer of
epithelial cells (blastula) and reorganizes into a
multilayered structure called the gastrula
-Jeremy Muhr; Kristin M. Ackerman
• 3 layers are formed-endoderm
-ectoderm
-mesoderm
Differentiation of endoderm
and ectoderm, and the
formation of the amniotic
cavity and the yolk sac

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• Cells proliferating at primitive streak pass sideways pushing b/w epiblast and
hypoblast-Forms intra-embryonic mesoderm
• Some cells displace hypoblast-endoderm
• Remaning cells of epiblast-forms ectoderm
• Both endoderm and mesoderm derived from epiblast.

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Embryonic disk at bilaminar (above) and trilaminar (below) stages.
Enlarged view of the center
The early development of a primitive streak (a rapidly proliferating
elongating mass of cells in the embryonic germ disk) demarcates the
initial distinction of embryonic tissues.

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Day 16 INTRA-EMBRYONIC MESODERM
(TRILAMINAR DISC)
• Cells of inner cell mass become columnar-forms epiblast
• Remaining cells of epiblast-ectoderm
• Intraembryonic mesoderm spread throughout the discs
• A edges of embryonic disc, intra-embryonic mesoderm is continuous with
extraembryonic mesoderm.

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• Subdivided into 3 parts of Intra-embryonic mesoderm
1. Paraxial mesoderm-either side of notochord
2. Lateral plate-more lateral, thinner layer
3. Intermediate-b/w paraxial and
lateral

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FATE OF GERM LAYERS
• Ectodermal cells -Nervous system; the
epidermis and its appendages (hair, nails,
sebaceous and sweat glands); the epithelium
lining the oral cavity, nasal cavities and
sinuses; a part of the intraoral glands, and
the enamel of the teeth.
• Endodermal cells-Epithelial lining of the
gastrointestinal tract and the respiratory
system.
• Mesodermal Cells–Connective
tissue,cartilage,bone,striated and smooth
muscles,heart ,blood and lymphatic vessels.

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Flow chart depicting the conversion of zygote into germ
layers and prochordal plate

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Day 17 Formation of NOTOCHORD
embryonic disc showing formation.
Note that the notochordal process is
deep to ectoderm and that its position is
(A) Primitive
knot
(B)blastopore
(C) notochordal
process
(D) notochordal
canal
• The notochord is a midline structure
that develops in the region lying
between the cranial end of the
primitive streak and the caudal end of
the prochordal plate
• During its development, the notochord
passes through several stages that are
as follows:
1. KNOT, PRIMITIVE NODE OR
HENSON’S NODE
2. BLASTOPORE
3. PROCESS OR HEAD PROCESS ends in
formation of NOTOCHORD

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DAY 19 CONNECTING STALK
• The embryo remains attached to trophoblast by extraembryonic mesoderm.
• This extra embryonic mesoder forms-connecting stalk
• This trophoblast and the tissues of uterus forms-PLACENTA
• As embryo grows the connecting stalk becomes smaller and seen near the caudal
end.
• At first 2 arteries an 2veins are present
• Later, right vein disappears.

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DAY 21 NEURAL CREST AND NEURAL FOLD
• Development of the ectoderm into its
cutaneous and neural portions, by
fibroblast growth factor (FGF) signaling,
occurs at 20 to 22 days by the infolding
of the neural plate ectoderm along the
midline forming the neural folds.
• Group of cell differentiate-forms neural
crest cell.
• Otic pit forms

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Day 22 NEURAL TUBE AND PERICARDIAL BULGE
• Ectoderm overlying the notochord extending from prochordal plate to primitive
knot-NEURAL TUBE
• The neural fold encompasses midline depression-
neural groove
• Neural tube divides into-cranial enlarged part-brain
-Caudal tubular portion-
spinal cord
• This process of formation of neural tube-
NEURULATION.
• Future position of heart-pericardial bulge is seen.

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• Ectoderm above notochord-
thickening-NEURAL PLATE
• Midline of neural plate deepers-
forms-groove with elevated
margins-NEURAL FOLDS
• Neural folds grow towards each
other-NEURAL TUBE-which form-
CNS
• Edges of neural groove-NEURAL
CREST CELLS
Formation of neural tube
Folds of ectoderm forms neural
folds that deepens to form groove
and then edges of groove fuses to
form neural tube

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DAY 22 FOLDING OF EMBRYO(DAY22-DAY30)
• The embryonic disc becomes folded in head and tail ends-head and
tail folds
• On lateral aspect same folds are seen-lateral folds
• Due to this, a part of yolk sac becomes enclosed within
embryo
• Embryonic disc folds in both directions-amniotic cavity
expands and surrounds embryo

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-SOMITE PERIOD
Period of organogenesis from 21st-31st day
intrauterine
Developmental anamolies occur in this period

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AROUND 4TH WEEK OF INTRAUTERINE
(FORMATION OF BRACHIAL ARCH)
• At this stage, the head is represented by the bulging caused by the
developing brain
• The pericardium occupying the region of the future thorax.
• The two are separated by the stomatodeum
• A mesodermal thickenings are seen in the wall of cranial
most part of foregut-BRACHIAL ARCHES or
PHARYNGEAL POUCHES-6 cyclindrical thickenings

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• The pharyngeal arches are separated by pharyngeal grooves on the
external aspect of the embryo-PHARYNEAL POUCHES
• The 5th arch disappears soon after it formation. Hence, only 5 arches
remain.

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• Each of the five pairs of arches contains the following basic set of
structures
1. A central cartilage rod that forms the skeleton of the arch
2. A muscular component
3. A vascular component (an aortic arch artery)
4. A nervous element consisting of sensory and special visceral motor
fibers

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ARCH
NUMBER
ARCH NAME CARTILAGE
DERIVATIVE
MUSCLE NERVE ARTERY
1 MANDIBULAR INCUS
MALLEUS
SPINE OF SPHENOID
SPHENOMANDIBULAR
LIGAMENT
MYLOHYOID
TENSOR VELI PALATINI
TENSOR TYMPANI
TENSOR PALATINI
MUSLES OF
MASTICATION
MYLOHYOID
ANT. BELLLY OF
DIGASTRIC
TRIGEMINAL
V
MANDIBULAR
DIVISION
FIRST
AORTIS
ARTERY
2 HYOID STAPES
STYLOHYOID
LIGAMENT
LESSER AND UPPER
PART OF BODY OF
HYOID BONE
STAPEDIUS
STYLOHYOID
BUCCINATOR
PLATYSMA
POST. BELL OF
DIGASTRC
FACIAL VII STAPPEDIAL

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3 THIRD STYLOPHARYNGEU
S
GLOSSOPHARY
NGEAL
COMMON CAROTID
4 FOURTH PHARYNGEAL AND
EXTRINSIC
LARYNGEAL
MUSCLE
TENSOR VELI
PALATINI
VAGUS X
SUPERIOR
LARYNGEAL
PROXIMAL PART OF
SUBCLAVIAN ARTERY ON
RIGHT
ARCH OF AORTA B/W
LEFT COMMOON
CAROTID AND LEFT
SUBCLAVIAN
6 SIXTH INTRINSIC
LARYNGEAL
MUSCLE
VAGUS X
RECURRENT
LARYNGEAL
PART B/W PULMONARY
TRUNK AND DORSAL
AORTA
BECOMES DUCTUS
ARTERIOSUS ON LLEFT,
DIAPPEARS ON RIGHT

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Nerves of pharyngeal arches

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POUCH DERIVATIVES
1ST PHARYNGEAL POUCH Tympani cavity
Auditory tube
Inner surface of the eardrum
2ND PHARYNGEAL POUCH Palatine tonsil
Tonsillar fossa
3RD PHARYNGEAL POUCH Thymus
Inferior parathyroid glands
4TH PHARYNGEAL POUCH Superior parathyroid glands
Thyroid gland
5TH PHARYNGEAL POUCH Ultimobranchial body
Derivatives of endodermal pouches

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Schematic representation of general embryology

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REFERENCES
1. Geoffrey H. Sperber-Craniofacial Development
2. Langman- Medical Embryology
2. Inderbir Singh-Human Embryology
3. Sridhar Premkumar- Textbook of Craniofacial book

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Thank you