Embryology

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Embryology
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Textbooks

• The Developing Human by Kieth L.more
• Human Embryology by William Larsen
• Clinical Embryology by Richard Snell
• Medical Embryology Langmans 10th edition by T.W
  Sadler

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Terminology

• Embryology is the study of development from
  fertilized egg through the eighth week in utero
• Developmental Anatomy is the study of development
  from fertilized egg to adult form
• Development is a process begins with an oocyte
  (ovum) is fertilized by a spermatozoa (sperm).and
  ends at death.
• Teratology is the study abnormal development
  (congenital malformations)

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• Pre-embryonic period first 3 weeks after
  fertilization
• Embryonic period begins 4 weeks after
  fertilization to the 8th week.
• Fetal period from the 9th week until birth

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Prenatal period

• in which the important changes occur before birth
  
• Oocyte female germ cell (ovum)
• Zygote the resultant cell of fertilization and
  the beginning of the human being.
• Cleavage Mitotic divisions of the zygote.
• Morula solid ball of cells (16 or more
  blastomers)
• Morula turns into Blastocyst in the uterus.
• Embryo composed of embryoblasts that forms the
  Bilaminar disc and extends to the 8th week.
• Fetus from the 9th week to birth (fetal period)

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Postnatal period

• Changes occur after birth
• Infancy
• from after birth till the first year
• Newborn (neonatal) is in the first 2 weeks
• The body grows rapidly during infancy
• Length increase by about 50
• Weight is tripled

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• Childhood
• The period from 15 months to 12-13 years
• Teeth are replaced by permanent ones
• Active ossification of the bones
• Puberty
• In girls between 12-15
• In boys between 13-16
• Secondary sexual characteristics develop

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• Adolescence
• The period of 3-4 years after puberty
• After sexual maturity until the attainment of
  physical, mental, and emotional maturity.
• Adulthood
• Early adulthood 18-25 years in which
  ossifications and growth is virtually completed
• After that developmental changes occur very
  slowly leading to Senility (old age).

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Human Development
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• Development begins with fertilization
• the process by which the male gamete, the sperm,
  and the female gamete, the oocyte, unite to give
  rise to a zygote.
• Gametes are derived from primordial germ cells
• are formed in the epiblast during the second week
  and move to the wall of the yolk sac

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• During the fourth week these cells begin to
  migrate from the yolk sac toward the developing
  gonads and arrive at the fifth week
• Mitotic divisions increase their number during
  their migration and also when they arrive in the
  gonad.
• In preparation for fertilization, germ cells
  undergo gametogenesis
• which includes meiosis, to reduce the number of
  chromosomes and cytodifferentiation to complete
  their maturation

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Overview of the Reproductive System
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Reproductive system

• In men
• The reproductive system in men has components in
  the abdomen, pelvis, and perineum
• The major components are a testis, epididymis,
  ductus deferens, and ejaculatory duct on each
  side
• and the urethra and penis in the midline
• In addition, three types of accessory glands are
  associated with the system
• a single prostate
• a pair of seminal vesicles and
• a pair of bulbourethral glands.
• The design of the reproductive system in men is
  basically a series of ducts and tubules

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Testes

• The testes originally develop high on the
  posterior abdominal wall and then descend,
  normally before birth, through the inguinal canal
• The spermatic cord is the tube-shaped connection
  between the pouch in the scrotum and the
  abdominal wall.
• Each testis is composed of seminiferous tubules
  (400-600) and interstitial tissue surrounded by a
  thick connective tissue capsule (the tunica
  albuginea).
• Spermatozoa are produced by the seminiferous
  tubules

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Epididymis

• The epididymis is a single, long coiled duct that
  courses along the posterolateral side of the
  testis
• During passage through the epididymis,
  spermatozoa acquire the ability to move and
  fertilize an egg
• The epididymis also stores spermatozoa until
  ejaculation. The end of the epididymis is
  continuous with the ductus deferens.

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• The ductus deferens is a long muscular duct that
  transports spermatozoa from the tail of the
  epididymis in the scrotum to the ejaculatory duct
  in the pelvic cavity
• Each seminal vesicle is an accessory gland of the
  male reproductive system
• Secretions from the seminal vesicle contribute
  significantly to the volume of the ejaculate
  (semen).

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• The prostate is an unpaired accessory structure
  of the male reproductive system that surrounds
  the urethra in the pelvic cavity
• Secretions from the prostate, together with
  secretions from the seminal vesicles, contribute
  to the formation of semen during ejaculation.
• The bulbourethral glands one on each side, are
  small, pea-shaped mucous glands
• the bulbourethral glands contribute to
  lubrication of the urethra and the
  pre-ejaculatory emission from the penis.

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Urethra

• The urethra begins at the base of the bladder and
  ends with an external opening in the perineum
• In men, the urethra is long, about 20 cm, and
  bends twice along its course
• The urethra in men is divided into preprostatic,
  prostatic, membranous, and spongy parts.
• Preprostatic part is about 1 cm long, extends
  from the base of the bladder to the prostate

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• The prostatic part of the urethra is 3-4 cm long
  and is surrounded by the prostate
• On each side of the prostatic urethra is the
  opening of the ejaculatory duct of the male
  reproductive system.
• Therefore, the connection between the urinary and
  reproductive tracts in men occurs in the
  prostatic part of the urethra.
• .

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• The membranous part of the urethra is narrow and
  passes through the deep perineal pouch
• During its transit through this pouch, the
  urethra, in both men and women, is surrounded by
  skeletal muscle of the external urethral
  sphincter
• The spongy urethra is surrounded by erectile
  tissue (the corpus spongiosum) of the penis
• The two bulbourethral glands in the deep perineal
  pouch are part of the male reproductive system
  and open into the bulb of the spongy urethra

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Reproductive system

• In women
• The reproductive tract in women is contained
  mainly in the pelvic cavity and perineum,
  although, during pregnancy, the uterus expands
  into the abdomen
• Major components of the system consist of
• an ovary on each side and
• a uterus, vagina, and clitoris in the midline
• In addition, a pair of accessory glands (the
  greater vestibular glands) are associated with
  the tract.

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Ovaries

• Like the testes in men, the ovaries develop high
  on the posterior abdominal wall and then descend
  before birth
• Unlike the testes, the ovaries do not migrate
  through the inguinal canal into the perineum, but
  stop short and assume a position on the lateral
  wall of the pelvic cavity
• The ovaries are the sites of egg production
  (oogenesis).
• Mature eggs are ovulated into the peritoneal
  cavity and normally directed into the adjacent
  openings of the uterine tubes by cilia on the
  ends of the uterine tubes.
• Each of the two almond-shaped ovaries is about 3
  cm long and is suspended by a mesentery (the
  mesovarium) from the posterior aspect of the
  broad ligament.
• The broad ligament is a sheet-like fold of
  peritoneum

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Uterus

• The uterus is a thick-walled muscular organ in
  the midline between the bladder and rectum
• It consists of a body and a cervix, and
  inferiorly it joins the vagina
• Superiorly, uterine tubes project laterally from
  the uterus and open into the peritoneal cavity
  immediately adjacent to the ovaries.
• has a rounded superior end (fundus of uterus).
• Implantation of the blastocyst normally occurs in
  the body of the uterus.
• During pregnancy, the uterus dramatically expands
  superiorly into the abdomen.

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Endometrium

• It is the inner layer of the uterus
• Composed of
• Compact layer
• Thick spongy layer
• Basal layer ( has its own blood supply)
• The compact and spongy layers are the functional
  ones and are shed during menses.
• Menarche it is the first time menses occurs
  (age of puberty)
• Menopause is a variable period in which the
  cyclic changes become irregular and disappear ,
  the age of menopause is around (45-55 years )
• Between the menarche and menopause the genital
  system undergoes cyclic changes in structre and
  functional activity controlled by neurohormonal
  mechanisms.

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Uterine tubes (Fallopian)

• The uterine tubes extend from each side of the
  superior end of the body of the uterus
• Because the ovaries are suspended from the
  posterior aspect of the broad ligaments, the
  uterine tubes pass superiorly over, and terminate
  laterally to, the ovaries.
• Each uterine tube has an expanded trumpet-shaped
  end (the infundibulum),
• .

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• The margin of the infundibulum is rimmed with
  small finger-like projections termed fimbriae
• Medial to the infundibulum, the tube expands to
  form the ampulla and then narrows to form the
  isthmus, before joining with the body of the
  uterus.
• The fimbriated infundibulum facilitates the
  collection of ovulated eggs from the ovary.
  Fertilization normally occurs in the ampulla

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urethra

• In women, the urethra is short, being about 4 cm
  long
• The inferior aspect of the urethra is bound to
  the anterior surface of the vagina
• Two small paraurethral mucous glands (Skene's
  glands) are associated with the lower end of the
  urethra

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The Chromosomes

• Traits of a new individual are determined by
  specific genes on chromosomes inherited from the
  father and the mother.
• Humans have approximately 35,000 genes on 46
  chromosomes
• Genes on the same chromosome tend to be inherited
  together and so are known as linked genes
• In somatic cells, chromosomes appear as 23
  homologous pairs to form the diploid number of 46
  (2n)

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• There are 22 pairs of matching chromosomes, the
  autosomes, and one pair of sex chromosomes.
• If the sex pair is XX, the individual is
  genetically female
• if the pair is XY, the individual is genetically
  male
• One chromosome of each pair is derived from the
  maternal gamete, the oocyte and one from the
  paternal gamete, the sperm
• Thus each gamete contains a haploid number of 23
  chromosomes (1n)
• the union of the gametes at fertilization
  restores the diploid number of 46.

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MITOSIS

• Mitosis is the process whereby one cell divides,
  giving rise to two daughter cells that are
  genetically identical to the parent cell
• Each daughter cell receives the complete
  complement of 46 chromosomes
• Before a cell enters mitosis, each chromosome
  replicates its deoxyribonucleic acid (DNA).
• During this replication phase the chromosomes are
  extremely long
• they are spread diffusely through the nucleus,
  and they cannot be recognized with the light
  microscope

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Prophase

• With the onset of mitosis the chromosomes begin
  to coil, contract, and condense
• these events mark the beginning of prophase.
• Each chromosome now consists of two parallel
  subunits, chromatids
• that are joined at a narrow region common to both
  called the centromere.
• Throughout prophase the chromosomes continue to
  condense, shorten, and thicken
• only at prometaphase do the chromatids become
  distinguishable

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Metaphase

• During metaphase the chromosomes line up in the
  equatorial plane, and their doubled structure is
  clearly visible
• Each is attached by microtubules extending from
  the centromere to the centriole, forming the
  mitotic spindle

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Anaphase and Telophase

• Soon the centromere of each chromosome divides,
  marking the beginning of anaphase
• followed by migration of chromatids to opposite
  poles of the spindle
• Finally, during telophase, chromosomes uncoil and
  lengthen,
• the nuclear envelope reforms, and the cytoplasm
  divides
• Each daughter cell receives half of all doubled
  chromosome material and thus maintains the same
  number of chromosomes as the mother cell (2n).

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Mitosis
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MEIOSIS

• Meiosis is the cell division that takes place in
  the germ cells to generate male and female
  gametes, sperm and egg cells
• Meiosis requires two cell divisions, meiosis I
  and meiosis II, to reduce the number of
  chromosomes to the haploid number of 23 (1n)

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• As in mitosis, male and female germ cells
  (spermatocytes and primary oocytes) at the
  beginning of meiosis I replicate their DNA
• so that each of the 46 chromosomes is duplicated
  into sister chromatids
• In contrast to mitosis, however, homologous
  chromosomes then align themselves in pairs, a
  process called synapsis
• The pairing is exact and point for point except
  for the XY combination

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Crossover

• Crossovers, critical events in meiosis I, are the
  interchange of chromatid segments between paired
  homologous chromosomes
• Segments of chromatids break and are exchanged as
  homologous chromosomes separate.
• As separation occurs, points of interchange are
  temporarily united and form an X-like structure,
  a chiasma
• The approximately 30 to 40 crossovers (one or two
  per chromosome) with each meiotic I division are
  most frequent between genes that are far apart on
  a chromosome.

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• Homologous pairs then separate into two daughter
  cells
• Shortly thereafter meiosis II separates sister
  chromatids.
• Each gamete then contains 23 chromosomes 1n.

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Meiosis
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Meiosis

• As a result of meiotic divisions,
• genetic variability is enhanced through
  crossover, which redistributes genetic material
• genetic variability is also enhanced through
  random distribution of homologous chromosomes to
  the daughter cells
• each germ cell contains a haploid number of
  chromosomes
• so that at fertilization the diploid number of 46
  is restored