EMBRYOLOGY It deals with the origin

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EMBRYOLOGYIt deals with the origin development
of single individual.

• PRENATAL PERIOD
• (38 WEEKS)
• Embryonic period Fetal period
• 1st 8 weeks Remaining 30 weeks

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GAMETOGENESIS

• It is the formation of the male female gametes
  (the sperm the ovum).
• It includes
• spermatogenesis (formation of sperms)
• oogenesis (formation of ova).

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• OOGENESIS
• Oogonia divide by mitotic divisions to give 1ry
  oocytes.
• The 1ry oocytes give 2ry oocytes 1st polar body
  by 1st meiotic division.
• The 2ry oocyte enters meiosis II which is
  arrested in metaphase approximately 3 hours
  before ovulation.
• Meiosis II is completed only if the oocyte is
  fertilized giving mature oocyte 2nd polar body.

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• SPERMATOGENESIS
• 1) Formation of spermatids
• Spermatogonia give primary spermatocytes by
  mitotic division.
• Maturation of sperms occurs at puberty
• Primary spermatocyte gives 2 secondary
  spermatocytes by 1st meiotic division.
• 2 secondary spermatocytes give 4 spermatids by
  the 2nd meiotic division.
• Spermatids differentiate into sperms by
  spermiogenesis.

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• 2) Spermiogenesis
• It is the process through which spermatids are
  transformed into sperms (lasts about 64 days)
  includes the following steps
• A) Formation of acrosome.
• B) Condensation of the nucleus (forming the head
• of the sperm).
• C) Formation of neck, middle piece a tail.
• D) Shedding of most of the cytoplasm.
• Sperms enter the lumen of the seminiferous
  tubules of testis then to epididymis.
• Sperms are produced continually during the
  reproductive life, accumulate mature in the
  epididymis.

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Spermiogenesis
A
B
C
D
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FERTILIZATION

• It is the union of the male gamete (the sperm)
  with the female gamete (the ovum) to form
  fertilized ovum or zygote. It occurs in the
  ampulla of the uterine tube.

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• Phases of fertilization
• 1) Phase I Penetration of corona radiata
• 2) Phase II Penetration of zona pellucida
• 3) Phase III Fusion of the oocyte sperm cell
  membrane.

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• Results of fertilization
• 1) Stimulation of the oocyte to complete the 2nd
  meiotic division.
• 2) Restoration of the diploid number of
  chromosomes.
• 3) Determination of the sex of the embryo
• X carrying sperm X carrying ovum female (XX)
  embryo.
• Y carrying sperm X carrying ovum male (XY)
  embryo.
• 4) Initiation of cleavage division.
• How can fertilization go away?
• Too many sperms ? dispermy or triploidy ?
  spontaneous abortion.
• Infertility.
• Bad timing
• - The sperm can survive only 72 hours.
• - The ovum can survive for 24 hours.

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• CLEAVAGE
• The head of the sperm forms the male pronucleus
  while that of the ovum forms the female
  pronucleus of the zygote (fertilized ovum).
• Membranes of the pronuclei break down,
  chromosomes condense arrange themselves for
  mitotic division.
• The first cleavage follows shortly giving 2 cells
  (called blastomeres) each with 46 chromosomes.

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• The 2 cells undergo series of mitotic division
  giving 8-cells stage (in day 2) 16 cell stage
  or morula (in day 3).
• The cells of the morula differentiate into
• Inner cell mass ? embryoblast (embryo proper).
• Outer cell mass ? trophoblast.
• 2 blastomeres 4 blastomeres
  Morula

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• Blastocyst formation
• Fluid enters inside the morula transforming it
  into cystic structure known as the blastocyst
  with one cavity (called blastocele).
• Blastocyst has outer cell mass (trophoblast)
  inner cell mass (embryoblast).
• The blastocyst reaches the uterine cavity,
  hatches from zona pellucida starts implantation.

Trophoblast
blastocele
Inner cell mass
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• IMPLANTATION
• It is the penetration of the blastocyst to the
  endometrium of the uterus where it is buried.
• The trophoblast secretes proteolytic enzymes that
  erode the endometrium, and then the cyst enters
  by amoeboid movement into it.
• It occurs normally in the fundus of the uterus.

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• Abnormal implantation
• a) Extra uterine (usually ends by abortion)
• In the uterine tube ? tubal pregnancy.
• In Douglas pouch ? Abdominal pregnancy.
• In the ovary ? Ovarian pregnancy.
• b) Intrauterine
• Occurs in the body or cervix of the uterus
  resulting in lower attachment of placenta ?
  placenta previa.

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• THE CHROMOSOME THEORY OF INHERITANCE
• Human cell contains 46 chromosomes which carry
  about 100000 genes.
• The 46 chromosomes are arranged in 23 pairs
  (diploid number), 22 pairs of matching
  chromosomes (Autosomes) one pair of sex
  chromosomes (XX in females XY in males).
• Gametes (sperms ova) contain 23 single
  chromosomes (haploid number of chromosomes).
• Union of the sperm ovum (forming the zygote)
  results in restoration of the characteristic
  diploid number of chromosomes in the human cell.

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• MITOSIS
• One cell divides to give 2 daughter cells (with
  46 chromosomes) identical to parent cell.
• MEIOSIS
• Takes place in germ cells to give sperms ova
  with haploid number of chromosomes.
• It requires 2 cell divisions meiosis I II to
  reduce the chromosomes to the haploid number.
• Meiosis I Homologous chromosomes align in pairs
  separate into 2 cells.
• Cross over In meiosis I there is an interchange
  of chromatid segments between paired chromosomes.
• Meiosis II Sister chromatids separate into 2
  cells.

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MULTIPLE BIRTHS

• Sometimes more than on ovum mature at the same
  time pass to the oviduct where they are
  fertilized ? multiple births.
• 2 people born from the same pregnancy are called
  twins.
• Types of twins
• Monozygotic twins
• A fertilized ovum splits into 2 developing
  zygotes at a very early stage.
• Twins are identical Same sex features.
• Dizygotic twins
• Result from the fertilization of 2 separate ova
  by 2 sperms.
• Non-identical twins May be of different sex
  features.
• Incidence increases with age of the mother.

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MULTIPLE BIRTHS
Monozygotic twins monozygotic twins
Dizygotic twins Monozygotic twins
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• THE MENSTRUAL (UTERINE) CYCLE
• It is monthly changes that occur in the uterus.
  It occurs as a response to the changes which
  occur in the ovary.

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• It is divided into 3 phases
• 1) The menstrual phase
• It lasts from 3 to 5 days the first day of
  bleeding is considered the first day of the
  cycle.
• During this period the superficial layer of the
  endometrium is shed associated with bleeding.
• It results from decreased level of estrogen
  progesterone as a result of degeneration of
  corpus luteum, resulting in marked
  vasoconstriction of endometrial vessels
  necrosis of the superficial layers of
  endometrium.
• The average amount of blood lost is 50 to 60 cc

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• The proliferative phase
• This phase lasts about 10 days occurs under the
  influence of the rise in the level of estrogenic
  hormones.
• The epithelial cells from the stumps of the
  glands multiply to repair the endometrium.

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• The secretory phase
• It lasts about 14 days the endometrial changes
  occur due to secretion of progesterone hormone
  produced by the corpus luteum in the ovary.
• The endometrium increases in thickness the
  glands become more tortuous filled with
  secretions rich in glycogen mucus.

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• THE OVARIAN CYCLE
• It is the monthly changes that occur in the ovary
  beginning at puberty (11-14 years) ending at
  the menopause (40-45 years).
• It is divided into 3 phases
• 1) Follicular (preovulatory) phase
• Low level of estrogen stimulates the pituitary
  gland to secrete follicle stimulating hormone
  (FSH).
• FSH stimulates 5-15 primordial follicles to grow
  but one of these follicles reach full maturity
  only one oocyte is discharged.
• It lasts about 13 days (the first half of the
  cycle).

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• Granulosa theca cells of the growing follicle
  secrete estrogen that cause
• Uterine endometrium to enter the proliferative
  phase.
• Thinning of the cervical mucus to allow passage
  of the sperms.
• Stimulates the pituitary gland to secrete
  luteinizing hormone (LH) stop secretion of FSH.
  
• 2) Ovulation
• It is the release of the ovum from the mature
  Graafian follicle that occurs about the middle of
  the ovarian cycle.
• Few days before ovulation, FSH LH enhance the
  growth of Graafian follicle which forms a local
  bulge avascular spot (stigma) in the ovary.

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• High level of LH leads to
• Increased collagenase activity digesting the
  collagen around the follicle.
• Increased prostaglandin level that stimulates
  contraction of smooth muscle in the ovary leading
  to rupture of the follicle release of the ovum
  surrounded by granulosa cells forming what is
  called corona radiata.

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• 3) Luteal (postovulatory) phase
• It represents the second half of the cycle.
• Under the effect of LH the ruptured Graafian
  follicle changes into a yellow body known as
  corpus luteum.

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• Corpus luteum secretes progesterone which induces
  the secretory phase of the menstrual cycle.
• The fate of the corpus luteum depends on whether
  fertilization occurs or not
• If fertilization does not occur The corpus
  luteum remains for 9 days after ovulation then
  decreases rapidly in size degenerates forming a
  fibrous body called corpus albicans. Degeneration
  of corpus luteum results in drop in the level of
  estrogen progesterone that results in the
  menstrual phase of the menstrual cycle.
• If fertilization occurs, the corpus luteum (under
  the effect of chorionic gonadotrophic hormone
  secreted by the growing embryo) continues to grow
  forming corpus luteum of pregnancy.
• Corpus luteum of pregnancy secretes progesterone
  for 4 months after which it starts to decrease
  slowly in size.

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• HORMONAL CONTROL OF FEMALE REPRODUCTIVE FUNCTIONS
• Hormones secreted from
• Hypothalamus Gonadotropin releasing hormones
  (GnRH).
• Anterior pituitary FSH LH.
• Ovaries Progesterone estrogens (mainly
  estradiol).
• Other tissue that secrete estrogens
  progesterone Adrenal cortex placenta.

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THE EMBRYONIC PERIOD

• FIRST WEEK OF DEVELOPMENT (from zygote to
  blastocyst)
• Fertilization In the lateral 1/3 (ampulla) of
  the uterine tube.
• Zygote (fertilized ovum) moves towards uterine
  cavity.
• Blastomeres Daughter cells formed from the
  zygote.
• Morula Cluster of 12-16 blastomeres.
• Blastocyst Fluid filled structure about 60 cells.

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SECOND WEEK OF DEVELOPMENT

• Day 8
• The blastocyst becomes partially embedded in the
  endometrium.
• The trophoblast differentiates into 2 layers
• Syncytiotrophoblast outer multinucleated layer.
• Cytotrophoblast inner mononucleated cell layer.
• Embryoblast (inner cell mass) differentiates into
  2 layers
• Hypoblast layer a layer of small cuboidal cells.
• Epiblast layer a layer of tall columnar cells.
• The 2 layers are called "Bilaminar germ disc".

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• Day 9
• Blastocyst is deeply embedded in the endometrium.
• Amniotic cavity appears within the epiblast
  layer.
• Flat cells from the hypoblast form exocoelomic
  (Heuser's) membrane that lines the cavity of the
  blastocyst which becomes known as exocoelomic
  cavity or primitive yolk sac.
• The defect in the endometrium is closed by fibrin
  clot.

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• Days 11 12
• The blastocyst is completely embedded in the
  endometrium.
• Small spaces or lacunae appear in the
  syncytiotrophoblast.
• Syncytial lacunae become continuous with maternal
  capillaries or sinusoids thus establishing
  "utero-placental circulation".
• Extra embryonic mesoderm appears between the
  cytotrophoblast the primitive yolk sac.
• Syncytiotrophoblast cytotrophoblast extra
  embryonic mesoderm chorion. The blastocyst
  becomes known as "Chorionic sac (vesicle)".
• Large space forms inside the extra embryonic
  mesoderm is called extra embryonic coelom or
  chorionic cavity.

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• The chorionic cavity splits the extra embryonic
  mesoderm into 2 layers
• Extra embryonic somatopleuric mesoderm lining the
  cytotrophoblast.
• Extra embryonic splanchnopleuric mesoderm
  covering the yolk sac.
• Extra embryonic mesoderm between the amniotic
  cavity the cytotrophoblast remains unsplit
  forming "Connecting stalk" which shares in the
  formation of the umbilical cord.

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• Day 13
• The surface defect in the endometrium is healed.
• Primary chorionic villi project into the blood
  spaces. They consist of columns of
  cytotrophoblast covered with syncytiotrophoblast.
• Hypoblast produces cells that form a new cavity
  within the primitive yolk sac known as "Secondary
  (definitive) yolk sac".

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THIRD WEEK OF DEVELOPMENT

• GASTRULATION (Bilaminar germ disc ?
  Trilaminar germ disc)
• Gastrulation starts by formation of primitive
  streak on the surface of the epiblast.
• Epiblast cells migrate toward the primitive
  streak, detach pass deep to it. This inward
  movement of the cells is called invagination.
• Some invaginated cells displace the hypoblast
  forming the endoderm.
• Other cells remain between epiblast endoderm
  form mesoderm.
• Cells remained in the epiblast form ectoderm.

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• Formation of the notochord
• Prenotochordal cells become intercalated in the
  hypoblast forming Notochordal Plate.
• Cells of the notochordal plate detach from the
  endoderm forming a solid cord (definitive
  notochord) which forms the basis of the axial
  skeleton.
• The notochord extends cranially to the prechordal
  plate caudally to the primitive pit.
• Neurenteric canal Connects the amniotic cavity
  to the yolk sac.

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• Development of villi
• Primary villi Columns of cytotrophoblast covered
  by syncytiotrophoblast.
• Secondary villi Primary villi obtain a core of
  extra embryonic mesoderm.
• Tertiary villi Secondary villi with blood
  capillaries.

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• By the end of the 3rd week the embryo becomes
  attached to the trophoblast by only the
  connecting stalk (future umbilical cord).
• The bilaminar germ disc becomes trilaminar except
  at the buccopharyngeal membrane (prechordal
  plate) the cloacal membrane caudally where the
  ectoderm endoderm are adherent together.

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3rd TO 8th WEEK OF DEVELOPMENT

• During this period organogenesis (development of
  organs) starts from the 3 germ layers.
• DERIVATIVES OF ECTODERM
• The nervous system (CNS PNS).
• Sensory epithelium of eye, ear nose.
• Epidermis of skin, hair, nails skin glands.
• Mammary gland.
• Pituitary gland.
• Enamel of teeth.

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• Development of the neural tube
• Neural plate expands towards the primitive
  streak.
• Neural folds are formed by the end of the 3rd
  week transform the neural plate into neural
  groove.
• Neural folds approach each other in the midline
  fuse (at the 5th somite) transforming the neural
  groove into neural tube.
• The 2 ends of the neural tube form cranial
  caudal neuropores.
• Cranial neuropore closes on day 25 (18-20 somite
  stage) while caudal neuropore closes on day 27
  (25 somite period).

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• When the neural tube is closed 2 ectodermal
  thickening are formed
• Otic placode ? Otic vesicles.
• Lens placodes ? Eye lenses.

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• The cranial part of the neural tube enlarges
  develops to form the brain. While the narrow
  caudal portion forms the spinal cord.
• Lateral border (crest) of neuroectoderm
  dissociates lies on either side of the neural
  tube forming Neural Crest.

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• Derivatives of the neural crest
• Connective tissue bones of the face skull.
• Odontoblasts.
• Dermis of the face neck.
• Cranial, autonomic dorsal root ganglia.
• Schwann cells.
• Arachnoid pia mater (leptomeninges)
• Adrenal medulla, Glial cells.
• Melanocytes.
• C cells of the thyroid gland.
• Conotruncal septum in the heart.

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• DERIVATIVES OF MESODERM
• Mesoderm layer differentiates into
• a) Paraxial Mesoderm
• It is organized into segments (somatomeres) which
  later become somites.

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• By the end of the 5th week there will be 42-44
  pairs
• 4 occipital.
• 8 cervical.
• 12 thoracic.
• 5 lumbar.
• 5 sacral.
• 8-10 coccygeal.

Somites
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• Each somite is divided into
• Sclerotome ? Bones cartilage.
• Dermomyotome
• divides into
• - Myotome ? Skeletal muscles.
• - Dermatome ? Dermis of the skin.

Dermomyotome
Sclerotome
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• b) Intermediate mesoderm
• Differentiates into urogenital structures.
• Cervical thoracic mesoderm form Nephrotome.
• Caudal part forms Nephrogenic cord.

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• c) Lateral Mesoderm
• Splits by intraembryonic coelom into parietal
  visceral layers.
• The parietal layer with the overlying ectoderm
  form lateral ventral body wall.
• The visceral layer with the endoderm layer forms
  the wall of the gut.
• Both layers form serous membranes.

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• Derivatives of the mesoderm as a whole
• Connective tissue, Cartilage Bone.
• Striated Smooth muscles.
• Blood Lymph cells.
• Heart, Blood Lymph vessels.
• Kidney, gonads their ducts.
• Spleen suprarenal cortex.

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• DERIVATIVES OF ENDODERM
• Epithelial lining of Primitive gut, Respiratory
  tract, Urinary bladder, Urethra, Tympanic cavity
  tympanic tube.
• Parenchyma of Thyroid gland, Parathyroid gland,
  liver pancreas.
• Reticular stroma of tonsils thymus gland.