GENERAL EMBRYOLOGY pdf - Prof.Dr. Alhubaity

EMBRYOLOGY

Lecture 1 & 2 Prof.Dr.Alhubaity

GENERAL EMBRYOLOGY

The development (formation) of a new individual begins by the union of 2 cells,one
from the male (Sperm) & one from the female ( Ovum).The union of the 2 cells is by
fertilization tformingZygot ( fertilized ovum).Each of the sperm & ovum has haploid
number chromosomes & after fertilization the Zygot will have Diploid number of
chromosomes.

Pregnancy last for 280 days(i.e ten lunar months from the time of fertilization)or 266
days from the time of implantation of the Blastocyst.The length of pregnancy
includes 3 stages:

1-The zygot stage ( fertilized ovum) lasts for 2 weeks after fertilization.

2-Embryonic stage ,extends from beginning of 3rd week to end of 8th week.

3-Fetal stage,extends from beginning of 9th week to the end of pregnancy.

The general features of each stage are as follows:

1-Zygot stage.All the cells look like each other.

2-Embryonic stage,here the tissues,organs& systems are formed( Organogenesis)

3-Fetal stage.Here the body grows in length ,weight and tissues & organs develops
further.

Gametogenesis

It means the formations of sperms (in the testes) by a process known as
Spermatogenesis

And mature ovum(in the ovary) by a process known as Oogenesis.Each of the sperm
& ovum contains Haploid number of chromosomes.

Spermatogenesis(formation of sperms)includes
spermatocytogenesis&spermiogenesis.

In spermatocytogenesis ,the primitive male sex cells (spermatogonium) with 46
chromosomes divides by Mitotic divisions to give rise type A & type B
spermatogonium.

Type A spermatogonium remains in contact with the basement membrane of
seminiferous tubules of of the testes,while type  B spermatogonium continues
maturation and is  transformed in to Primary spermatocytes( contains 44 + xy),then
it undergoes the first meiosis division to give 2  secondary spermatocytes( 23
chromosomes).Each secondary spermatocye inters the second meiosis division to
give 2 spermatids(each has 23 chromosomes).The spermatid is not capable to move
and not capable to penetrate &furtilzes  the ovum unless change its shape in
process known as Speriogenesis ,where it undergoes some morphological  changes
to become capable to move &follws the mature ovum to penetrates & fertilizes it
to form Zygot.

The SPERMIOGENESIS includes the following changes:

A-The nucleus of the spermatid form the head of the sperm(contains genetic
material)

B-The Golgi apparatus forms the Head cap ( Acrosome ) of the sperm.

C-The Mitochondria will form a sheath around the middle piece of the
sperm(energy).

D-The proximal centriole come to lie in the neck & gives rise to the axial
filament,which forms the central core of the middle piece& tail of the
sperm,while the distal centriole becomes ring shaped forming the terminal ring.

The head of the sperm attaches itself to the Sertoli cell to get nourishment,then
discharged during Ejaculation in the seminal fluid(from sex glands as
prostate,seminal vesicles & bulbourethral glands).The volume of seminal fluid at
Ejaculation is 3-5 cc which contains about 100 million of sperms in each cc.The
sperms are genetically of 2 types ,one carries X-  chromosomes,while the other
carries Y- chromosome.On the other hand the Ovum are almost always of one type
carries X-  chromosomes.

OOGENESIS

It is a cyclic process which takes place every lunar month(28 days)under the
influence of follicle stimulating hormone (F.S.H)from anterior lobe of pituitary
gland.In Oogenesis the following events takes place:

1-Formation of Graafian follicle
2-Ovulation.
3-Formation of Corpus Luteum.

It starts from puberty(11-14) years old(Menarch) and ends at Menopause(45-50)
years old..The Graafian follicle is formed by maturation of the Primordial

follicle(consists of a primary oocyte surrounded by a layer of follicle  cells).The
Primary Oocyte undergoes the first Meiosis division(not completed unless
ovulation takes place) giving  secondary Oocyte plus first polar body.The
follicular cells increases in number &aquires granules in the follicular cytoplasm
,and thus is called Granulosa cells(Zona  granulosa),then a cavity appears between
the granulosa cells known as follicular  antrum and filled with fluid.This fluid
separates the Granulosa  cells into two parts:

A-Stratum Granulosum (outer layer)
B-Cumulus Oophorous on the inner side of the Graafian follicle &
surrounding the oocyte(the oocyte is separated from them by  Zona
Pellucida).

All the follicle then becomes surrounded by Theca ,which consists of 2 layers:

1-Theca externa of fibrous layer.
2-Theca interna which secretes Estrogen hormone.
The follicular stage is under the influence of the F.S.H from pituitary
gland.

OVULATION

Means rupture of the Mature Graafian follicle& the release of mature ovum.The
ovulation takes place at day 14+1 of the ovarian cycle.It is under the influence of
Luitinizing hormone (L.H) from pituiray  gland too to induces ovulation ,which
causes by the following  factors:

1-Increases in the intrafollicular pressure.
2-The influence of L.H
On ovulation ,the oocyte starts the second meiosis division,which is not
completed  until fertilization of the Ovum is completed.
3-The walls of the  G.F   becomes very thin .
4-The cells of Cumulous oopharous become loosened.

Thus the G.F rupture and the ovum is released(now the first meiosis division is
completed ) and secondary oocyte will inter immediately the second meiosis
division,which is not completed until fertilization of the Ovum takes place.Once
fertilization is  completed ,the second meiosis division is completed.The result of
oogenesis is to get a single mature Ovum & 3 polar bodies.

The Luteal Phase

Uppon the rupture of the mature G.F and the release of the ovum,what remains
from the GF becomes known as the Corpus Luteum(in the ovary) due to :

1-The wall of the ruptured GF collapses & becomes folded.

2-The  granulosa cells becomes filled with yellow pigment known as
Lutein.
3-The Theca Externa forms the capsule of the Corpus Luteum.
4-The ThecaInterna provides the Corpus Luteum with vascular C.T   .

The corpus luteum secretes Progesterone which is responsible for the Secretory
stage ( post ovulatory) in the Endometrium to prepare it for implantation

The fate of Corpus Luteum:
A-If the Corpus Ovum is not fertilized ,the Corpus Luteum degenerates &
is converted into fibrous tissue called Corpus Albicans after 14 days after
ovulation.
B-If the Ovum is being fertilized ,then the corpus luteum persists & increases
in size to increase the production of progesterone which is essential for the
maintenance of pregnancy.This is known as Corpus Luteum of
pregnancy,which persists until the 4

month of pregnancy,when it is going

to regress & the secretion of progesterone is going to be the function of the
placenta.

Fertilization

It takes place in the ampullary part of the fallopian tube,where fusion of the male
gamete( Sperm) & the female gamete ( Ovum) takes place as follows:

The head of the sperm only passes  through the follicular cells of the Corona
radiate (that surrounds the ovum ) ,then it penetrates the  Zona pellucida with the
aid of proteolytic  enzyme secreted by the acrosomal cap of the sperm& finally the
head becomes in the cytoplasm of the Ovum.Then the head of the sperm ( male
pronucleus) fuses with the nucleus of the Ovum ( female pronucleus)  to form the
nucleus of the Zygot ( Fertilized ovum ).Thus the zygot is now becomes
DIPLOID .The  fertilization results in the followings:

1-Restoration of the Diploid number of chromosomes.
2-Determination of the  Sex of the new organism ( going to be male or
female).
3-Initiation of repeated mitotic divisions ( Cleavage).
4-Formation of the Zygot.

THE MENSTRUAL CYCLE

Is the cyclic changes in the endometrium of the uterus which occurs every lunar
month(28 days) in response to cyclic changes in the ovary.It starts from Menarch
(Puberty at 11-13 years)& ends at Menapause(45-50 years).It includes 3 stages:

1-Menstrual stage: Where bleeding of about 30-60 ml of blood is discharged
per vagina due to lowering of blood progesterone level caused by
degeneration of Corpus Luteum.It last for 3-5  days& the discharge consists
of :
a-Clotted blood.
b-Dead( not fertilized)   ovum.
c-Sheded pieces of endometrium( includes stratum compactum
&spongiosum)
2-Proliferative stage,which last for 7-10 days after bleeding has stopped,it
coincides with the follicular stage of the Ovarian cycle& is due to Estrogen
secretion from follicular cells of the growing GF.During this stage the
stratum basalis of endometrium which remains after menstruation will start to
reform the stratum compactum &spongiosum that sheded with the
bleeding,thus:
a-Endometrium may reach 2-3 mm in thickness just before ovulation.
b-The arterioles grow  & become coiled.
c-The  glands increases in length & becomes tortous ( coiled).
3-  Secretory   stage,which starts immediately after ovulation& is due to
progesterone secretion from the Corpus luteum.It lasts for 10-14 days&
coincides with the luteal  phase of the ovary,thus:
a-Endometrium becomes 5-8 mm in thickness & consists of 3 stratum
,compactum ,spobgiosum and basalis.Thus the Endometrium is now ready to
receive the fertilized ovum (Blastocyst ) for implantation.
b-The  glands becomes more tortous& secretes mucin& glycogen ( uterine
milk).
c-The  arteries that supply compactum &spongiosum are called spiral
arteries,which becomes very tortous&tall,while  arteries supplying the basal
layer iare called as Basal arteries( short & straight).
4-Premenstrual stage: if  by the end of secretory  stage no fertilized ovum
reaches the endometrium,then progesterone secretion will reduce &stop.Thus
the spiral arteries&tortous glands will regress& endometrial mucosa starts
to reduce in thickness,hence stratum compactum &spongiosum prepare
themselves for sheding.This stage lasts about 2  days ,just prior to the
menstruation phase.

CLEAVAGE

A term which means repeated mitotic divisions of the Zygot,it occurs rapid & in
succession.It starts immediately after fertilization(also known as segmentation).

The human Zygot becomes 2 cell stage 24-36 hours pos-fertilization, 4 cell
stage 40-48 hous post-fertilization,then early Morula ( 16 cell stage )at 3

day.

looks like a mulberry,which is a solid ball(still surrounded by  Zona  pellucida.At
day 4 it becomes late Morula ( 32  cell stage).It is still  solid & reaches the uterine
cavity& obtains its nutrition now from endometrial secretion ( uterine milk).It then
become 64 cell stage ( early  Blastula) which starts to loose its Zona pellucida&
try to stick itself to endometrial mucosa of the posterior surface of the  fundic
region& continue divisions till it becomes of 107 cells stage ( late Blastocyst).It
has the following  features:

1-Bigger  in size with relatively large cavity due to fluid collection.
2-It is 107 cell which arrange as inner  cell mass of 8 cells known as
Embryoblast
& outer  cells on the circumference of the  Blastocyst of 99 cells known
as Trophoblast layer ( future placenta).
3-Lost the  Zona pellucida completely to facilitate implantation.
4-It has 2 p0les ,an Embryonic pole ( where inner cell mass is
present),while the other side is known as Ab-Embryonic pole.
5-Starts to open a very  tiny  gap in the endometrial mucosa in order to
inter through it & implants itself there.

As it loses completely  the Zona pellucida ,the  trophoblast becomes
exposed,thus becomes able to stick& eat up other cells ( Endometrial cells) to
invade itself in to the mucosa with the aid of proteolytic enzyme secreted by the
trophoblastic cells at the site of the embryonic  pole.

IMPLANTATION

Is the embedding of the Blastula into the endometrial mucosa,usually in the upper
part of posterior  wall of the fundicregion.It starts at day  6 post-fertilization& is
completed at day 12.The late Blastocyst loses zona pellucida,then stick itself to
the endometrium & with the aid of proteolytic  enzyme secreted by the outer layer
of the trophoblast,a tiny  opening is created and  the blastocyst starts to invade &
burrow itself into the endometrium.This process takes about a week ( from day 6-
12 postfertilization)&once embedding is completed,the tiny opening in the
endometrium is closed by a fibrin coagulum,

Which is finally replaced by epithelium of the endometrial mucosa.

Abnormal Implantation sites

A-Intrauterine which is  known as placenta praevia,where implantation is
near the  cervix & according to the exact site of implantation,we can see
3 named placenta praevia,these are:
1-Placenta praevialateralis,where implantation is just above the internal oss.
2-Placenta praeviacentralis,implanta around the internal oss&closes it.
3-Placenta praeviamarginalis,where margin of placenta reaches   above the
Internal oss of the  cervix.
B-Extrauterine implantation sites: includes the followings:
1-Tubal pregnancy,where implantation takes place in the ampullary region
of the uterine tube& is terminated at about 6 weeks of pregnancy as
ABORTION.
2-Interstitial implantationi.e in the interstitial part of the uterine tube.
3-Ovarian implantation I .e on the surface of the Ovary.
4-Intraabdominal implantation on the peritoneum of the abdomen.
5-Pelvic implantation,where implantation in rectouterine pouch ( of
Douglass)

THE THIRD WEEK OF DEVELOPMENT

1-The Bilaminar Embryonic disc is converted in to Trilaminar disc by the
appearance of Mesoderm between the Ectoderm & Endoderm.
2-The Trophoblast shows 3 changes during 3

week as

primary,secondary& tertiary velli.The  primary villous orms by  a ciore of
cytotrophoblast covers by syncytrophoblastlayer.Secondaryvelli forms by
a core of Mesoderm covers by cytotrophoblast ,then
syncytiotrophoblast.The Tertiary velli  shows fetal capillaries in the core
of extraembryonic mesoderm.
3-The Embryonic Disc shows the appearance of 3 important structures
as:
a-The primitive streak     b-Notochord       c-Neural tube
The primitive streak
Is a structure  starts to appear in the mid line of the embryonic disc,starts
to appear at day 14(end of 2

wk)& becomes very clear at days 14-15.It is a

narrow groove having a slightly bulging areas on either side.The cells of
the streak are in fact a modified ectodermal cells
The appearance  & formation of primitive streak proceeds the formation of
the intraembryonic mesoderm,where the ectodermal cells in the region of
primitive streak starts to migrate in between the already present ectoderm
& endoderm& creeps in all directions except in 2 areas( the buccopharyngeal
cloacal membranes).
The movement of the newly formed mesoderm( i.e modified ectodermal
cells) anteriorly occurs in characteristic way,where they migrate anteriorly
on either sides of the  mid line & meets at the Prochordalplate,thus the  mid

line area between the Hensen's node of the primitive streak &prochordal
plate remains as a bilaminar in nature( i.e doesn’t contain any mesoderm
layer)After its formation mesoderm will differentiate into 3 named parts as
follows:
A-Paraxial mesoderm,is most medially located & becomes segmented
into blocks( Somites) of about 42-44 segments.This will form the skeleton
,skeletal muscles & the skin.
B-Lateral plate mesoderm, it crosses from side  to side cranial to
buccopharyngeal membrane to form the  cardiogenic area.In this part of
the mesoderm ,a U shaped structure  cavity appears to form the future
pleural & peritoneal cavities
C-Intermediate mesoderm,lies between paraxial & lateral plate mesoderm ,it
is cord klike known as Nephrogenic  cord which gives rise to urogenital
system.At day 19 the Embryonic Disc becomes completely as
Trilaminar embryonic disc..

THE NOTOCHORD

It is an important structure which develops during the 3

wk& is considered as a

supporting element for the growing fleshy Embryo and is being used as an Axial
skeleton(Temporary) until the vertebral colum is formed.

It starts formation as a notochordal(Head) process,that develops by invagination &
proliferation of some Ectodermal cells at the sides of the Henson's (primitive)
node.It extends as a middle structure between the ectoderm & endoderm extending
from Henson's node to the prochordalplate.Thenotochordal process can be seen
very clealy when the Embryo is 17 days old& on day 18 a canal appear

In the notochordal process known as neurentericcanal.This canal will form
temporary  a connection between the Amniotic cavity & primary  yolk sac& when
the notochordal process fuses with the endoderm,thenotochordal-endodermal plate
is seen.Finally the roof of the notochordal process is folded around its axis to form
the notochord  after the  canal loses its connection with the 2 cavities & the
amnion becomes completely separated from the yolk sac again.

THE NEURAL TUBE

At first , a neural plate appears dorsal to the already formed Notochord.This
neural plate is a thickened part of the Ectoderm that lies behid the notochord &
along  each side of the neural plate lies the Neural crest.On day 18 the plate
becomes modified to form a median grooved part & 2 lateral raised parts
known as Neural folds.Theedial ends of the 2 folds are approximated gradually
& the groove becomes deeper and deeper &  eventually the 2 Neural folds fused
together forming the Neural Tube,which becomes just dorsal to the Notochord.

Just on the dorsolateral sides of the Tube  we can see the Neural Crest,the cephalic
part of the Neural tube  becomes expanded & dilated forming the Brain
Vesicle(the future brain),while the caudal part   differentiate to form spinal cord

THE DERIVATIVES OF THE NEURAL CREST

1-Neurons of  Dorsal root ganglion of spinal nerves.
2-Neurons of Sympathetic ganglion.
3-Neurons of  sensory  ganglia  OF CRANIAL  NERVES
5

,&10

4-Shwan cells of the peripheral nerves.
5-Melanoblast of the skin.
6-Chromaffin pigmented tissue.
7-The cells of the Suprarenal medulla( Adrenal medulla).
8-The Pia &  Arachnoid layers of the Central Nervous System.

By : Ody Drd 