embryology part 2

1. The Human ( medical ) Embryology
Is the branch of biology and medicine
concerned with the study of embryos and their
development.

2. The Beginning of Human Development
Life begins with fertilization . A procces
by which the spermatozoon contacting the
cells surrounding the oocyte and end with
the mixing of the 23 male and 23s female
chromosomes. The result is a single-cell
embryo called a zygote, meaning "yoked or
joined together," and it is the first cell of the
human body.

3. As recently as the 18th century, the prevailing
notion in western human embryology was
preformation: the idea that semen contains an
embryo — a preformed, miniature infant, or
"homunculus" — that simply becomes larger during
development. The competing explanation of
embryonic development was epigenesis, originally
proposed 2,000 years earlier by Aristotle. Much
early embryology came from the work of the great
Italian anatomists.

4. According to epigenesis, the form of
an animal emerges gradually from a
relatively formless egg. As microscopy
improved during the 19th century,
biologists could see that embryos took
shape in a series of progressive steps,
and epigenesis displaced preformation
as the favoured explanation among
embryologists.

5. Gonads are sexual organs:
1. The testis, in the male, producing male
gametes named sperms or spermatozoa.

6. 2. The ovary, in the female, producing
female gametes named oocytes, ova or
eggs.

7. The male reproductive system
produce the spermatozoa and
transfers them to the female
through sexual intercourse (coitus)
or copulation.

8. The ovum
The formation of primary oocytes, from
which ova are developed, is complete
before birth. About 2,000,000 primary
oocytes are present at birth, but by
adolescence only about 40,000 remain.
Over the reproductive period about 400 of
these pass through maturation to ovulation

9. The spermatozoon is a microscopic
organism approximately 0.05mm long. It has
an oval head and an elongated tail (10 times
the head size), the flagellum. The junction
between the head and the tail is called the
neck and include the centrioles. The head
contains an haploid nucleus( 23
chromossomes of type X or of type Y).

10. Fertilization – Forming a Single Cell Embryo
Biologically speaking, fertilization (or
conception) is the beginning of human
development. Fertilization normally occurs
within several hours of ovulation (some
authors report up to 24 hours) when a man’s
sperm, or spermatozoon, combines with a
woman’s ovum, or secondary oocyte, inside a
woman’s uterine tube (usually in the outer
third of the uterine tube called the ampulla).

13. Modern embryology research
Currently, embryology has become an
important research area for studying the
genetic control of the development process
(e.g. morphogens), its link to cell signalling,
its importance for the study of certain
diseases and mutations, and in links to stem
cell research.

16. Lecture two

17. Although about 200-500 million spermatozoa are
deposited in the female genital tract, only one
sperm is needed to achieve successful
fertilization. Spermatozoa and oocytes remain
viable in the female reproductive tract for
approximately 24 hours. The reason why such
large number of sperms deposited in the female
genital tract ‫؛‬ is to provide adequate sperms
( usually 400 sperms ) to reach the site of
probable fertilization:

18. 1.High acidity of the vagina will kill
50% total number of the sperms
deposited in the female genital
tract within the semen.

19. 2-Narrow cervical canal leads to
destruction of another 50% of
the remaining sperms.

20. 3- 50% of the remaining sperms
are lost within the uterine
cavity.

21. 4- Very limited number of sperms
succeeds to find their way to the
right fallopian tube housing ( at a
proper time ) the freshly
ovulated ovum.
about 400 sperms only
will reach the site of
possible fertilization.

23. Gametogenesis
In preparation for possible fertilization
both male and female gamets undergo
certain changes. This is achieved by process
known as Gametogenesis. In female this
process is termed oogenesis, while in male it
is termed as spermatogenesis.

25. The purpose of this process ( Gametogenesis )
is twofold:
is a process of formation and development of the
germ cells or gametes by a process named
meiosis, which causes the sexual cells to be
haploid cells because they only have 23
chromosomes instead of the 46 found in other
body cell. The development of a human begins
with fertilization, a process by which the
spermatozoon from the male and oocyte from
female unite to give rise to a new organism, the
zygote. In preparation for fertilization, both
male and female germ cells undergo meiosis and
cytodifferntiation. The purpose of these process
is twofold:

26. To reduce the number of the chromosomes from
diploid number of 46,observed in somatic cells, to the
haploid number of 23, observed in the gametes.
( ploidy refers to the number of copies of each
chromosome ). This is achieved by meiotic or
maturation division and necessary, since fusion of a
male and a female germ cell would otherwise result in
an individual with twice the number of chromosomes
of the parent cells. Simply to keep the species
constant .

27. 2-To alter the shape of the germ cells in
preparation for fertilization. The male germ
cell initially large and round, loses practically
all of its cytoplasm and develops a head, neck,
and tail. The female germ cell, on contrast,
gradually becomed larger as a result of an
increase in the amount of cytoplasm. At
maturity, the oocyte has a diameter of about
120 μm.

28. At conception One very lucky spermatozoon
out of hundreds of millions ejaculated by the
man will penetrate the outside layer of the
ovum and fertilize it. This happens typically in
the outer third of one of the woman's Fallopian
tubes. The surface of the ovum changes its
electrical characteristics and normally
prevents additional sperm from entering.

29. . A genetically unique entity is
formed shortly thereafter, called a
zygote. This is commonly referred
to as a "fertilized ovum." However
that term is not really valid
because the ovum ceases to exist
after conception.

30. The zygote is "...is biologically
alive. It fulfills the four criteria
needed to establish biological
life:
1- metabolism,
2-growth,
3-reaction to stimuli, and
4-reproduction.

31. Lecture Three

32. The main results of fertilization:
1- Restoration of normal diploid number of
chromosomes ( 46 ) in the zygote which contains
a new combination of chromosomes different
from both parents.

33. 2- The sex of the zygote is
determined ( either male (XY)
or female (XX).

34. 3- Initiation of cleavage of the zygote
( always almost mitotin division ). About
30 hours after fertilization, the zygote
begins its repeated mitotic divisions,
Without fertilization the oocyte usually
degenewrates 24 hours after ovulation.

35. 4- Exchange of genetic blocks
during fusion of the male and
female pronuclei.

36. 5 -Determination of the polarity
of the future position of the
embryo.

38. The phases of fertilization
include:

39. Phase 1: Penetration
A single sperm penetrates the cell membrane of the
oocyte. To reach the oocyte, the sperm must pass
through the corona radiata and the zona pellucida; two
layers covering and protecting the oocyte from
fertilization by more than one sperm.
Corona radiata
The sperm passes through the corona radiata, a layer
of follicle cells on the outside of the secondary oocyte.

40. Phase 2 :Zona pellucida
The sperm then reaches the zona pellucida,
which is an extra-cellular matrix of glyco-
proteins. A special complementary molecule on
the surface of the sperm head then binds to a ZP3
glyco-protein in the zona pellucida. This binding
triggers the acrosomal reaction, where a special
vesicle of enzymes at the sperm head, the
acrosome, bursts, releasing enzymes that help the
sperm get through the zona pellucida. The most
important of these enzymes is acrosin, for which
the acrosome is named.

41. Phase 2 :Zona pellucida

42. Phase 3:
Cortical reaction and fusion of the
oocyte and sperm membrane:
When the sperm penetrates the zona pellucida, the
cortical reaction occurs: cortical granules inside the
secondary oocyte fuses with the plasma membrane of the
cell, causing enzymes inside these granules to be expelled
by exocytosis to the zona pellucida. This in turn causes the
glyco-proteins in the zona pellucida to cross-link with each
other, making the whole matrix hard and impermeable to
sperm. This prevents fertilization of an egg by more than
one sperm.

43. Fusion: The sperm fuses with the
oocyte, enabling fusion of their
genetic material, in turn.
Cell membranes :The cell
membranes of the secondary
oocyte and sperm fuse together.

45. Thank you

46. Lecture Four
The first week of development

47. Embryo Transport in the Uterine Tube
As cell division proceeds, the embryo is on
the move. The uterine tube push the embryo
toward the uterus by cilia motion of epithelial
cells lining the uterine tube wall and continued
mild muscle contractions. The cells of the corona
radiata disappear within about 2 days while the
zona persists in protecting the embryo and
preventing premature implantation in the uterine
tube.

48. Formation of the Zygote:
The male and female pronuclei don't fuse,
although their genetic material do so. Instead, their
membranes dissolve, leaving no barriers between
the male and female chromosomes. During this
dissolution, a mitotic spindle forms around them to
catch the chromosomes before they get lost in the
ovum cytoplasm. By subsequently performing a
mitosis (which includes pulling of chromatids
towards centrosomes in anaphase) the cell gathers
genetic material from the male and female together.
Thus, the first mitosis of the union of sperm and
oocyte is the actual fusion of their chromosomes.

49. Each of the two daughter cells ( The blastomeres )
resulting from that mitosis have one replica of each
chromatid that was replicated in the previous stage.
Thus, they are genetically identical. In other words,
the sperm and oocyte don't fuse into one cell, but into
two identical cells. Cleavage is initiated as a result of
fertilization.

50. Cleavage, Morula and Blastula
development

51. With fertilization of the ovum by a sperm, usually
within the Fallopian tube, human development
begins. Coupling of the ovum, which contributes
half the chromosomes, and the sperm, which
contributes the other half of the chromosomes,
creates a single cell called the zygote, which
carries all of the chromosomes.

52. About 30 hours after fertilization, the zygote
begins its repeated mitotic divisions ( the
cleavage ), first into 2 daughter cells called the
blastomeres, and then into progressively
smaller blastomeres, until at about 3 days after
fertilization a 16-32 blastomere solid ball
contained within the zona pellucida called the
morula, enters the uterine cavity. A morula
(Latin "morus", mulberry) is an embryo at an early
stage of embryonic development.

53. In mammals the morula travels to the uterus around 3-4
days after fertilization, and at about 4 days after
fertilization , the zona pellucida shows some fenestrations (
openings ) allowing passage of fluid from the uterine cavity
leading to the appearance of a fluid-filled space called the
blastocoel cavity and the morula becomes a blastula
or blastocyst. By the time some 30 blastomeres have been
produced, pools of clear fluid accumulate between some of
the internal cells ( blastomeres ), and these spaces soon
coalesce into a clear space filled with fluid.

54. Cleavage

55. Morula
16 - 32 cell stage
‘Berry’ - appearance

56. Compaction
As this process begins, the blastomeres
change their shape and tightly align
themselves against each other to form a
compact ball of blastomeres. This is
called compaction and is likely mediated
by cell surface adhesion glycoproteins.

58. Embryo pole
Inner cell mass
(embryoblast)
Blastocoel
Outer cell mass
(trophoblast)
abembryonic pole
Blastocyst
At the same time the zona pellucida
disappear 5 days after fertilization
prior to implantation of the
blastocyst into the uterine wall.

59. Cells of the inner cell mass or
The embryoblast also
differentiate into two layers:
(a) a layer of small cuboidal
cells adjacent to the blastocyst
cavity, known as the hypoblast
layer, and (b) a layer of high
columnar cells adjacent to the
amniotic cavity, the epiblast
layer

60. Lecture Five
Second Week of Development:
Bilaminar Germ Disc
( Week of two 's )

61. Functions Zona Pellucida
1- Species-specific sperm penetration
2- Permanent block to polyspermy
3- Acts as a porous selective filter –
4- uterine tube signals
5- Immunological barrier -
( no HLA (histocompatibility antigens)
6- Keeps blastomeres together (loosely
adherent)
7- Prevents premature implantation

62. Implantation Sites
Normal site is the uterus
Upper popsterior wall

63. Abnormal Implantation
( Ectopic pregnancy )
( Implantation out side the uterus )
Any type of pregnancy occurring outside the
uterine cavity is called an ectopic pregnancy
and, if not attended to, can endanger the
mother's life.
1-About 95% of the time,
the ectopic location is
within the uterine tube and
is referred as tubal
pregnancy,

64. 2-the other sites include the
mesentery of the intestines.

65. 3- even the ovary itself.

66. At the eighth day of development, the
blastocyst is partially embedded in the
endometrial stroma of the uterus.

67. No zona pellucida
In the area over the inner cell
mass ( The embryoblast ), the
trophoblast has differentiated
into two layers:
a- an inner layer of
mononucleated cells, the
cytotrophoblast, and
(b) an outer multinucleated
zone without distinct cell
boundaries, called
Syncytiotrophoblast.
a
b

68. Lecture Six

70. Implantation
Days 6 –12
Adhesion, blastocyst to
endometrium
Trophoblast
proliferation
Syncytiotrophoblast
Secretion of hydrolytic
enzymes
Breakdown of
endometrium

71. Together, the layers form a flat disc. At the same
time, a small cavity appears within the epiblast.
This cavity enlarges to become the amniotic
cavity. Epiblast cells adjacent to the
cytotrophoblast are called amnioblasts; together
with the rest of the epiblast, they line the
amniotic cavity

72. Cells of the inner cell mass or embryoblasts also
differentiate into two layers:
(a) a layer of small cuboidal cells adjacent to the
blastocyst cavity, known as the hypoblast layer,
and
(b) a layer of high columnar cells adjacent to the
amniotic cavity, the epiblast layer (Figs
).Together, the layers form a flat disc.
Epiblast
Hypoblast

73. At the same time, a small cavity appears within the
epiblast. This cavity enlarges to become the amniotic
cavity. Epiblast cells adjacent to the cytotrophoblast
are called amnioblasts; together with the rest of the
epiblast, they line the amniotic cavity (Fig.).

74. Day 9
1- The blastocyst is more deeply
embedded in the endometrium, and the
penetration defect in the surface
epithelium is closed by a fibrin coagulum
(Fig.).
2-The trophoblast shows considerable
progress in development, particularly at
the embryonic pole, where vacuoles
appear in the syncytium. When these
vacuoles fuse, they form large lacunae,
and this phase of trophoblast development
is thus known as the lacunar stage (Fig.).
.

75. 3-At the abembryonic pole, meanwhile,
flattened cells probably originating from
the hypoblast form a thin membrane,
the exocoelomic (Heuser’s) membrane,
that lines the inner surface of the
cytotrophoblast (Fig. ). This membrane,
together with the hypoblast, forms the
lining of the exocoelomic cavity, or
primitive yolk sac

76. The endometrial stroma adjacent to the
implantation site is edematous and highly
vascular.
The large, tortuous glands secrete
abundant glycogen and mucus.

77. Days 11 and 12
By the 11th to 12th day of development, the
blastocyst is completely embedded in the endometrial
stroma, and the surface epithelium almost entirely
covers the original defect in the uterine wall (Figs).
The blastocyst now produces a slight protrusion into
the lumen of the uterus. The trophoblast is
characterized by lacunar spaces in the syncytium that
form an intercommunicating network. This network is
particularly evident at the embryonic pole; at the
abembryonic pole, the trophoblast still consists mainly
of cytotrophoblastic

78. Thank you

79. Lecture Seven
The Third week of Development
Week of Three~S
From day 15 to day 21
Part one

81. During the third week of development
conceptus implantion in the uterus wall is
complete and trophoblast cells continue to
invade uterine wall in the process of early
placentation (villi formation).

82. Within the conceptus, gastrulation
converts the bilaminar embryo
into the trilaminar embryo
(ectoderm, mesoderm, endoderm)

83. Early in the third week ( day 15 ) the embryonic
disc has enlarged and become pear-shaped or
slipper-shaped in outline, and a well-formed
primitive streak occupies the midline of its caudal
(hind) half, which is narrower.

84. 1- At the end of the second week ( beginning of
third week ,
A- three new structures appear :
1- Formation of the primitive streak .
2- Formation of the notochord
3- The appearance of the Allantois

85. 1- Formation of the primitive streak :
appears at the dorsal aspect of the Epiblast germ layer. It
consists of the following:
A- Primitive groove - shallow groove ( valley )
extending along the length of the streak.
B- Primitive knot - a raised circle of cells at
the rostral tip of the primitive streak.
(Hensen's node)
C- Primitive pit - a depression in the center of
the primitive node.

86. 2- Formation of the notochord
Rod of mesoderm that forms from a specialized group of epiblast
cells at the onset of gastrulation. The notochord is a flexible, rod-
shaped body found in embryos of all chordates. It is composed of
cells derived from the mesoderm pass within the Primitive pit of
the primitive streak and defines the primitive axis of the embryo.
In lower vertebrates, it persists throughout life as the main axial
support of the body, while in higher vertebrates it is replaced by
the vertebral column. The notochord is found on the ventral
surface of the neural tube.

87. 3- The appearance of the Allantois
It is diverticulum at the coudal part of yolk sac which appears
at about the 16th day of development, that extends into the
connecting stalk. It helps the embryo in respiration
( exchange gases ) and excretion ( handle liquid waste as
reservoir for urine ). The allantois remains rudimentary but
may be involveed in abnormalities of the bladder
development. later it becomes the urachus, a vestigial
structure with an unknown function>

88. Thank you

89. The Third week of Development
Week of Three~S
From day 15 to day 21
Part two – Lecture Eight
II - Gastrulation: Appearance of the Third
germ layer

90. Gastrulation: Appearance of the Third germ
layer:
The bilaminar germ disc differentiates itself further
into a trilaminar embryo, in that the cells flow in over the
primitive streak between the two already existing
germinal layers ( The Epiblast and the Hypoblast germ
layers ) and so form the third embryonic germinal layer
( the mesoderm). This phenomenon is also termed
epithelio-mesenchymal transition or gastrulation in
lower vertebrates. The Epiblast and hypoblast are now
termed as the ectoderm and endoderm germ layers
collectively.

91. The intraembryonic mesoderm is the third
germinal layer. It arises in the 3rd week via the
immigration of cells at the primitive streak. Out
of this develop the various tissues and organs of
the embryo. In the beginning, the cells of the
mesoblast (mesodermal cells) build a thin,
widely meshed layer on both sides of the
median line, between the ectoderm and the
endoderm.

93. While the notochord is forming - it grows to the same
extent that the primitive streak recedes - the
intraembryonic mesoderm cells multiply on both sides of
the median line and so form 3 structures in the shape of
longitudinal columns.
This process begins at the cranial pole and continues up to
the 4th week in the caudal direction. The intraembryonic
mesoderm differentiates itself into three subdivisions on
both sides of the primitive streak as it recedes:
1-Paraxial mesoderm 2-Intermediate mesoderm 3-Lateral
plate mesoderm

94. Further development of trophoblast
1-Primary Villus
2-Secondary Villus
3-Tertiary Villus
Stem or anchoring Villus
Free (terminal Villus)
Capillaries of tertiary villi establish contact with
capillaries in mesoderm of chorionic plate and
connecting stalk, inturn contact with
intraembryonic circulation

95. III -Further Development of the trophoblast: As the
syncytiotrophoblast expands, spaces appear within it, filled
with secretions from the endometrial glands and blood from
the eroded vessels. Expansion of the syncytiotrophoblast and
continued erosion of vessels results in the development of a
sponge-like syncytiotrophoblast. Since erosion of the vessels in
the endometrium has occurred in both arterioles and
vennules, a primitive circulation develops.

96. There is now a large surface area of syncytiotrophoblast
bathed in circulating maternal blood - a primitive
placenta. Access to this source of nutrition allows for
further growth of the embryo. As the trophoblast is
growing, a separation occurs between the
cytotrophoblast and the amnion and yolk sac

97. The cytotrophoblast grows rapidly and pushes out
towards the blood spaces in the syncytiotrophoblast.
The arrangement of syncytiotrophoblast and
cytotrophoblast forms primary chorionic villi, present
by the end of the second week. These villi will
eventually be invaded by fetal blood vessels and will
become the site of gas, nutrient and metabolite
exchange between the embryo and mother. A third type
of chorionic villi appears known as tertiary villi.In

98. The cytotrophoblast grows rapidly and pushes out
towards the blood spaces in the syncytiotrophoblast. The
arrangement of syncytiotrophoblast and cytotrophoblast
forms primary chorionic villi, present by the end of the
second week. These villi will eventually be invaded by
fetal blood vessels and will become the site of gas,
nutrient and metabolite exchange between the embryo
and mother.
A third type of chorionic villi appears known as tertiary

99. villi.In addition to that a third layer appears
, derived from the cytotrophoblast called
Extra-embryonic mesoderm surrounding
the yolk sac.

100. Chorionic cavity
The fluid-filled extraembryonic coelom (cavity) formed initially
from trophoblast and extraembryonic mesoderm that forms
placenta. Chorion and amnion are made by the somatopleure.
The chorion becomes incorporated into placental
development. The avian and reptilian chorion lies beside the
egg shell and allows gas exchange. In humans, this cavity is lost
during week 8 when the amniotic cavity expands and fuses
with the chorion.

102. Thank you

103. ‫الثاني‬ ‫الدراسي‬ ‫الفصل‬
Lecture Nine
The Folding of the
Embryo
28-2-2016

104. Folding of the Embryo
Folding occurs by differential growth of
tissues. Neural ectoderm grows faster
than the surrounding skin ectoderm and
consequently fold to form a neural tube.
Similarly, skin ectoderm grows faster
than the underlying mesoderm and
endoderm, and this differential growth
causes folding of the trialminar disc and
gives shape to the embryo.

112. Folding occurs mainly at the edges of the
embryonic disc and forms three main
folds:
1. Head fold
2. Tail fold
3. Lateral folds - convert the embryo
into a tubular structure.
These are not three separate folds but occur
simultaneously and merge into one another.
The notochord, neural tube and somites stiffen
the dorsal axis of the embryo.

114. Transverse folding of the embryo:
1-Converts the endoderm into a
primitive gut tube
2-The intra-embryonic coelom
surrounds the gut tube
3-The communication between the
intra- and extra- embryonic
coeloms becomes constricted and
eventually obliterated.

115. As a result of the formation of the tail
fold:
1-The hindgut is formed
2-The cloaca is an invagination of
ectoderm and has the cloacal membrane
separating it from the hindgut.
3-The connecting stalk is shifted ventrally
4-The allantoic diverticulum is shifted
ventrally. It is an invagination of
hindgut endoderm into the yolk sac.
5- The amniotic cavity extends ventral to
the caudal end of the embryo.
6- The yolk sac is constricted from the
caudal

116. As a result of the formation of the head fold:
1- The foregut is formed by folding of the endoderm
2- The stomodaeum is an invagination of ectoderm, and has
the buccopharyngeal membrane separating it from the
foregut It opens into the amniotic cavity.
3- The pericardial cavity and cardiogenic mesoderm are
shifted to the ventral aspect of the embryo and lie ventral
to the foregut.
4-The part of the transverse mesoderm between the
pericardial cavity and the yolk sac is the septum
transversum proper. In it the liver will develop.
5- The amniotic cavity extends ventral to the cranial end of
the embryo.
6- The yolk sac is constricted fro the cranial aspect.

118. Conclusions

119. Drastic and important changes occur in the
embryonic cavities as a consequence of folding:
1. The amniotic cavity surrounds the embryo
completely on all aspects and becomes the
predominant cavity. It enlarges progressively.
2. The yolk sac becomes constricted on all
sides, and becomes a small sac connected to
the midgut by a narrow vitelline duct. It
becomes progressively smaller.
3. The extra-embryonic coelom is gradually
obliterated by the expanding amnion and
eventually disappears completely

121. ‫الثاني‬ ‫الدراسي‬ ‫الفصل‬
Lecture Nine
The Fourth Week of Life:
Formation of the Embryo
9-3-2016

122. Some facts :
The period from the 3rd to 8th week of the development is
known as the embryonic period. The embryonic period is very
important because this is the time when all internal and
external structures develop in the embryo.
In the period from the 4th to the 8th week, each of the three
germ layers gives rise to different tissues and organs.
As the result of the organogenesis, the shape of an embryo
greatly changes
the exposure of an embryo to certain agents may cause major
congenital malformations.

123. The period from the beginning of the 3rd month (
12 weeks ) to the birth is known as the foetal
period. The main characteristic of the foetus in this
period is the rapid growth of the body and the
maturation of the tissues.
The fetal stage officially begins after the tenth
week of pregnancy. Prior to this, the developing
child is considered an embryo. Before the tenth
week is reached, many physiological developments
have already happened. By this point, the embryo.
The foetal growth can be monitored with
ultrasound.

124. Development of Somites
At the end of week 3, the intra-
embryonic mesoderm differentiates into
three loose aggregate pairs of mesoderm
on each side of the neural tube :
1- Medially, the paraxial mesoderm differentiates into the
future dermatome (dorsal surface), myotome (middle
layer), and sclerotome (ventral layer), forming dermis,
muscle, and connective tissue respectively.
2- Moving laterally, the second aggregate pair, called the
intermediate mesoderm, will form the future urogenital
system.
3- Most laterally, the lateral plate mesoderm will develop
into future body cavities (intraembryonic coelom) and
parts of the body wall.

125. The paraxial mesoderm will develop into paired
cuboidal bodies, or somites (Gr. Soma means
body). These will eventually develop into:
1- the bones (sclerotome),
2- muscles (myotome), and
3- dermis (dermatome) of and surrounding the
axial skeleton.
Somites appear as bumps on the dorsal surface
of the embryo. At the end of week 3, 4-12
somites are present (visible on the dorsal
surface of the embryo). By the end of week 5,
42-44 can be counted. However, most appear
between days 20-30, giving this period the title
of the somite period of development.

126. The first pair of somites arises in the
cervical region of the embryo at
approximately the 20th day of
development. From here somites appear
in craniocaudal sequence, at a rate of
approximately 3 pairs / day. Until at the
end of the 5th week, 42-44 pairs are
present. There are 4 occipital,
7 cervical, 12 thoracic , 5 lumbar, 5
sacral, and 8-10 coccygeal pairs. The
first occipital and last 5-7 coccygeal
somites later disappear, while the
remaining somites ( 31 pairs ) form the
axial skeleton ( the vertebral
column ).

127. A somite is divided into two parts:
a. The sclerotome is the ventro-medial part of the
somite. It contains a “cavity” of loose cells. Cells
from the sclerotome migrate medially to surround the
notochord and neural tube and form the axial
skeleton.
b. The dermomyotome is the dorso-lateral part of
the somite. Cells from the dermomyotome migrate
laterally and, as its name implies, gives rise to (i)
skeletal muscle and (ii) the dermis of the skin. The
concept of the myotome in gross anatomy is an
embryological concept. Each anatomical myotome is
derived from the embryological dermomyotome that is
innervated by a segmental nerve and forms a goroup
of skeletal muscle cells and the dermis of the
corresponding segment of ectoderm.

128. Law of Original Innervation: The myoblasts (future
muscle cells) form concurrently with the spinal nerves
and they migrate out from the notochord together.
This results in the formation of 31 spinal nerves with
associated skin, muscle, and connective tissue During
the fourth week the embryo is segmented. Each
segment consists of a. somite innervated by a
segmental nerve derived from a segment of the neural
tube.

129. Derivatives of Mesoderm II
Somites
Axial skeleton
Skeletal muscle
sclerotome
myotome
Intermediate cell mass Urogenital system
somatopleure
splanchnopleure
Dermis of skin;
arrectores pilorum
subcutaneous fat
Visceral smooth muscle
Lateral plate