Lecture 3 ESS_2nd semester

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Lecture 3 ESS_2nd semester

• Embryology - definition, the object and
  significance of embryology
• Phases of the human ontogeny
• Gametes - their structure, physiology and origin
• (gametogenesis). Differences between
  spermatogenesis
• and oogenesis
• Transportation of gametes. Sperm capacitation and
  acrosome
• reaction
• Fertilization and cleavage, the morula and
  blastocyst
• Implantation

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• development of plant as well as animal organisms
  begins when an ovum is fertilized and ends at the
  moment of the death of individual
• the time period from fertilization to death is
  called as the ontogeny
• (also as life cycle)
• in placental mammals, including the human,
  ontogeny divides 2 periods of different length
• prenatal period (prenatal development) - from
  fertilization of an ovum to delivery of fetus
• postnatal period (postnatal development) - from
  the delivery to death of the individual
• in mammals, goal of embryology consists first of
  all in study and description of events occuring
  during the prenatal period (phase) of development
• similar goals and orientation has also medical
  embryology to describe and explain events and
  processes from the fertilization to the birth of
  the human

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• Significance of embryology
• the basic theoretical medical branch
• good knowledge of embryology is important in
  many medical disciplines
• anatomy - knowledge of the human development
  helps to understand complicated relationships
  between many organs of the human body
• embryology shows more closely relationships and
  bounds to anatomy than to histology (reasons for
  teaching of embryology together with histology
  are rather practical then logical as study of the
  pre-embryonic and early embryonic stages needs to
  use and studied only with the use of histological
  slides)
• pathology - knowledge of embryology is explored
  namely in study of restorative (healing)
  processes and in description and classification
  of neoplasms (tumours)
• obstetrics, pediatrics and surgery - knowledge of
  normal development is essential to understanding
  the mechanisms, symptomatology and correction of
  congenital malformations
• a special medical line -prenatal medicine- was
  constituted in last years, it deals with
  diagnosis, treatment and prevention of congenital
  malformations

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• Phases of prenatal development in the human
• pre-embryonic phase
• involves the first 3 weeks from fertilization of
  the ovum
• several well-defined stages are distinguished
• - zygote (fertilized ovum) the cell standing
  at the beginning of development of a human being
  there is a result of a fusion of the male and
  female sex cells or gametes
• - morula (from latin morus mulberry)
• the solid ball consisting of 12 to 16
  blastomeres
• blastomeres cells that result in process of
  cleavage
• of the zygote, blastomeres are genetic equal
  and
• they do not substantially differ each other,
• prospective properties or developmental potency
• of blastomeres are the same, each of them can
• give rise to a new individuum
• - blastocyst the stage with a cavity inside
  that is filled with fluid, 2 different cell
  populations differentiate at the first time in
  development trophoblast cells inner cell mass
  or embryoblast cells, during the day 5

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• - gastrula the bilaminar pre-embryonic stage,
  in which the main body axis and polarity begin to
  become apparent, the bilaminar stage develops on
  day 7 and lasts to the end of the second week
  after fertilization
• embryonic phase
• lasts from the beginning of the 4th week to the
  end of the 8th week
• in the phase anlages of the all body organs are
  constituted
• by the end of it, the embryo is recognizable
  human in appearance and measures cca 2-3 cm from
  head to tail
• fetal phase
• it lasts from the 9 week to delivery of fetus
• during the fetal phase, development of organs
  rapidly continue, some of them enter their
  function, the fetus grows quickly in length and
  increases its weight gain, in particular during
  the terminal months

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• Phases of the postnatal development in the human
• infancy from the birth to the end of the first
  year of the age (the first two weeks of infancy
  are called as the newborn or neonatal period)
• childhood - from 13 month until 12 to 13 years
  the primary teeth appear and are replaced by the
  secondary or permanent teeth
• puberty - is the period between 12-15 years in
  girls and 13-16 years in boys, during which
  secondary sexual characteristics develop
• adolescence - follows the puberty and ends 3-4
  years after it the organism reaches sexual but
  also physical, mental and emotional maturity
• adulthood - a period in which developmental
  changes occur very slowly and are mostly
  characterized by selective loss of highly
  specialized cells and tissues
• postnatal development of the human is studied and
  treated with various
• medical branches

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• Structure and origin of gametes
• gametes generative (germ or sex) cells
• in comparison with somatic cells, gametes are
  highly specialized and contain only half the
  usual number of chromosomes
• the sperm or spermatozoon (the male germ cell)
• the ovum or egg cell (the female germ cell)
• human gametes similar to gametes of another
  species substantially differ each other in size,
  structure, motility, and viability
• The sperm, spermatozoon
• is a motile cell that is cca 60 ?m long
• a head, a neck (connecting piece) and a tail
• the head has a flattened pyriform shape ,
  approximately 4 ?m in length, consists of the
  sperm plasma membrane, condensed nucleus and
  acrosomal cap covering the anterior two-thirds of
  the nucleus

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• the tail - cca 55 ?m in length
• middle piece
• about 5 to 9 ?m in length,
• 1-2 ?m in diameter, consists of
• the axoneme that is inserted
• in the distal centriole located
• in the neck,
• is surrounded by 9 outer
• dense fibers
• (dense columns) and externally
• to them by
• a mitochondrial sheath
• principal piece - 40-45 ?m in length
• is narrower than the middle piece
• contains the axoneme, 9 outer dense
• fibers and fibrous sheath

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• the semen (ejaculate)
• viscid fluid expelled from the urethra during the
  orgasm
• volume of one ejaculate is 3 - 6 ml
• it consists of
• the seminal plasma secretions of accessory sex
  glands (seminal vesicles and prostate)
• formed elements
• spermatozoa, exfoliated epithelial cells form
  genital and urinary passages, lymfocytes,
  prostatic concretions
• average sperm density 60 - 100 million/ml -
  normospermy
• (density lesser than 20 million/ml infertility)
• average sperm velocity is 2 - 3 mm/min under
  alkaline pH
• spermatozoa retain ability to fertilize ovum not
  longer than 16 to 24 hours

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• The ovum
• a large spherical nonmotile cell with diameter of
  over 100 ?m (130-150 ?m)
• eccentrically located large and pale nucleus with
  a well-developed nucleolus
• numerous free ribosomes and smaller mitochondria
  with arch-like oriented cristae
• voluminous Golgi apparatus located near the
  nucleus, lately near the cell surface
• few endoplasmic reticulum
• absence of centrioles
• cortical granules under cell membrane, contain
  material of mucopolysaccharide character
• (presence of granules is the sign of maturation
  of an ovum)
• during the sperm penetration into ovum, the
  content of cortical granules is released and
  prevents the polyspermia basis of the cortical
  reaction
• yolk cell inclusions
• chemically, yolk involves saccharides,
• especially of glycogen, lipids and proteins
• that mostly occur in the form of proteinaceous
• lattices
• amount of the yolk in human ova is low

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• surface of the ovum is limited by the cell
  membrane, from which short microvilli project
  into the zona pellucida
• the zona pellucida is a clear envelope 10 to 12
  ?m thick
• it is composed of highly sulfated mucoproteins
• perivitelline space space between the cell
  membrane and zona pellucida

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zona pellucida
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• Gametogenesis
• process of formation and development of sex cells
  
• the aim is to prepare sex cells to their
  assimilation or fusion that is a basis of
  fertilization
• during gametogenesis, the chromosome number is
  reduced by half and shape of cells is altered

Reduction of the chromosome number is achieved by
special type of cell division called meiosis
there are two successive meiotic divisions the
first meiotic division homologous chromosomes
pair during prophase and separate during
anaphase, with one representative of each pair
going to the respective pole by the end of the
first meiotic division, each new cell formed
contains the haploid chromosome number, i.e. half
the original number of chromosomes of proceeding
cell (homologous chromosomes go well together and
two always form a "matched pair" whose members
are derived from each parents) - the reduction
division disjunction of paired homologous
chromosomes is the physical basis of segregation
(the separation) of allelic genes. the second
meiotic division follows immediately the first
division without DNA replication and without a
normal interphase, each chromosome divides in two
chromatids that are then drawn to opposite poles,
the haploid number of chromosomes is retained,
and daughter cells formed during the second
division really contain reduced chromosome
number, with one representative of every
chromosome pair.
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Spermatogenesis maturation process of male
gametes includes the series of events leading
to transformation of spermatogonia into
haploid spermatids these then develop into ripe
(mature) sperms by gradual process known as
spermiogenesis ( histological differentiation of
spermatids) 3
phases
spermatocytogenesis
meiosis
spermiogenesis
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the site where spermatozoa undergo their
development and maturation are seminiferous
tubules of the testis development from the
spermatogonium to fully differentiated
spermatozoon lasts 64 74 days in avarrage
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spermatocytogenesis
meiosis
spermiogenesis
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spermiogenesis
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• Oogenesis
• maturation process of female gametes
• is bound to the ovary (ovarian cortex) - ovarian
  follicles
• 2 phases oocytogenesis
• meiosis

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• meiosis
• the first meiotic division is completed shortly
  before the ovulation - the primary oocyte divides
  unequal and gives rise to large secondary oocyte
  and the first polar body (polar body is
  nonfunctional and soon degenerates)
• in time of ovulation the secondary oocyte enters
  the second meiotic division that progresses only
  to metaphase, in which division is arrested
• completion of the second meiotic division is
  closely connected with penetration of the sperm
  into the oocyte the division becomes again to
  continue and developed daughter cells are the
  mature ovum and the secondary polar body

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Comparison of spermatogenesis and ogenesis

• number of produced gametes 4 spermatids vs 1
  ovum (polar bodies are nonfunctional cells that
  degenerate)
• course of process continuous vs cyclic and
  limited only on period of sexual maturity (400
  ova)
• motility of gametes motile with only small
  amount of the cytoplasm vs immotile, the
  cytoplasm is abundat and contains stored
  material for the first week of development

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• Transportation of gametes
• the site of fertilisation is the uterine tube
  its dilated part termed the ampula
• nearly to the ovary
• Oocyte transport
• to release the secondary oocyte from the
  graaphian follicle occurs during the ovulation
• the oocyte surrounded by some granulosa cells is
  expelled from the follicle
• and is directed via stream of follicular fluid
  under contraction of fimbriae
• (finger-like processes of the tube) into the
  lumen of the infudibulum
• the oocyte then passes into the ampulla by waves
  of peristalsis of muscle cells in the wall of
  oviduct
• it is supposed that oocyte reaches the
  fertilization site in cca 25 min
• Sperm transport
• after cohabitation, the spermatozoa deposited
  against the cervix
• and the posterior fornix of the vagina, pass by
  movements of their tails through the
• cervical canal into the uterus and subsequently
  in the respective uterine tube
• the ascent is caused by contractions of smooth
  muscle cells in the wall of the uterus and
  uterine tube

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• Capacitation of sperm
• an activation process of sperms that takes about
  7 hours
• during this process, a glycoprotein coat and
  seminal plasma proteins are removed from the
  plasma membrane over the acrosome
• the process does not connect with morphological
  aterations
• spermatozoa undergo their capacitation in the
  uterus or uterine tube by influence of substances
  secreted by these parts
• Important sperms must be capacitated also in
  cases of in vitro fertilization
• (as capacitation media are used gamma-globulin,
  free serum, serum dialysate, follicular fluid,
  albumin, dextran etc. - artifitial capacitation
  media)
• The acrosome reaction
• it follows the capacitation
• involves a series of events that are induced by
• penetration of spermatozoa through the corona
• radiata

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• Fertilization
• is a process of fusion of male and female gametes
• in average, it needs several hours ( 24 hours)
• fertilization site - the ampulla of the uterine
  tube - is the widest segment of the tube

• germ cell viability
• spermatozoa can stay alive in the
• female reproductive tract for 24
• hours
• (there is suggestive evidence that some sperms
  may fertilize an ovum for as long as three days
• after insemination)
• if the ovum (the secondary oocyte)
• was not fertilized, it degenerates 12 to 24 hours
  after ovulation

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• phases of fertilization
• penetration of corona radiata
• penetration of zona pellucida
• fusion of oocyte and sperm cell membranes
• constitution of the male pronucleus
• fusion of the pronuclei

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• 1. Penetration of corona radiata
• the corona radiata is envelope from follicular
  cells that encloses the ovulated oocyte
• the main goal of this phase is dispersal of
  corona radiata cells by enzyme hyaluronidase
  released from the acrosomal caps of spermatozoa
• 2. Penetration of zona pellucida
• is facilitated by the action of enzymes released
  from the acrosome
• primary role plays acrosin, enzyme causes lysis
  of the zone (inhibition of acrosin prevents
  passage of sperms through the zona pellucida)
• once the first sperm passes through the zona
  pellucida, so-called a zona reaction starts,
  during which the zona pellucida becomes
  impermeable to further spermatozoa
• is supposed that the zona reaction is mediated
  by cortical granules of the secondary oocyte
• 3. Fusion of oocyte-sperm cell membranes
• this phase is characterised by actual fusion of
  the oocyte membrane and membrane covering the
  posterior region of the sperm head, the head and
  tail of the sperm enter the cytoplasm of the
  oocyte
• after fusion of membranes, the secondary
  oocyte can complete the second meiotic division
  to give rise to the mature ovum with haploid
  number of chromosomes and the second polar body
• chromosomes in the egg nucleus decondense to
  form the female pronucleus
• 4. Constitution of the male pronucleus
• the penetrated sperm moves in close vicinity to
  the female pronucleus
• its nucleus becomes rapidly swollen and forms
  the male pronucleus

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• 5. Fusion of the pronuclei
• the male and female pronuclei approach each other
  in the centre of the ovum and come into contact
• they lose then their nuclear membranes and
  maternal and paternal chromosomes intermingle
  (mix together) and they organize in an equatorial
  plane
• the first mitotic division of the zygote starts
  /cleavage of the zygote/
• Results of fertilization
• restoration of diploid number of chromosomes
  (half from the mother and half from the father)
• determination of the sex of the new individual
• (XX -female, XY-male)
• initiation of cleavage
• (not fertilized oocyte degenerates in 12-24 hours
  after ovulation)

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• Cleavage of the zygote
• the phase of development characterized by mitotic
  divisions of the zygote resulting in formation of
  the blastocyst
• cells are called blastomeres and become smaller
  with each division
• the cleavage takes place partly in the uterine
  tube (during the first three days), partly in the
  uterus (from day 4 to 6)
• the first division resulting in 2-cell stage
  needs about 24 to 30 hours, subsequent divisions
  follow in shorter intervals because blastomeres
  become progressively smaller

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TEM
SEM
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• after the eight-cell stage (three mitotic
  divisions), the morula stage develops
• the morula - a solid ball consisting of 12 to 16
  cells that lay on tightly each other
• (it shows resemblance to the fruit of mulberry
  tree)
• the morula enters the uterine cavity where is
  transformed into the blastocyst

• Formation of the blastocyst
• shortly after the morula enters the uterus (on
  day 4), fluid passes from the uterine cavity
  through the zona pellucida to form a single
  fluid-filled space blastocystic cavity
  (primitive chorionic cavity or blastocoele)
• Consequently, the morula cells separate into two
  cell lines
• an outer cell layer, the trophoblast (greek
  trophé nutrition) - gives rise to chorion and
  finally fetal part of the placenta
• a group of centrally located cells, called as
  the inner cell mass or embryoblast - gives rise
  to the embryo
• the blastocyst is enclosed with the zona
  pellucida and lies free in the uterus cavity

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during the day 6, the zona pellucida becomes to
digest by enzymes contained in the uterine fluid
and soon disappears the denuded blastocyst then
expands up to diameter of 400 ?m or more and is
prepared to start the implantation
Early blastocyst Late (mature)
blastocyst (with the zona) (denuded)
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• Implantation
• an adaptive process, during which the blastocyst
  establishes very close contact with the uterine
  mucosa (endometrium) of the maternal organism
• main goal is to provide nutrition of the
  embryoblast because yolk material has been
  completely utilized during cleavage and
  blastocyst formation
• the implantation begins on day 7 and ends on day
  13
• the trophoblast cells play key role in this
  proces, they differentiate in the cytotrophoblast
  and the syncytiotrophoblast invading the uterine
  mucosa
• Staging of the implantation
• Attaching of the blastocyst to the endometrial
  epithelium
• Invasion of the trophoblast cells in the
  endometrium
• Reparation of implantation defect in endometrium
• Formation of uteroplacental circulation
• Manifestation of decidual reaction

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• 1. Attaching of the blastocyst to the endometrial
  epithelium
• it follows the zona pellucida lysis the
  blastocyst attaches to the endometrium with
  embryonic pole (the pole, by which the
  embryoblast is placed)
• 2. Invasion of the trophoblast cells in the
  endometrium
• follows as a result of contact of trophoblast
  with the endometrium
• trophoblast cells proliferate and differentiate
  into 2 populations
• the syncytiotrophoblast located peripherally
• invades and erodes quickly the endometrial
• stroma and uterine glands
• it forms around the blastocyst a large, thick and
  
• multinucleated mass without obvious cell
  boundaries
• the erosion of the uterine mucosa is caused
• by proteolytic enzymes produced by
• syncytiotrophoblast cells
• the cytotrophoblast (secondary trophoblast)
• that gradually replaces the original trophoblast
• the cytotrophoblast cells are mitotic active and
  form

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• 3. Reparation of implantation defect in
  endometrium
• after penetration of the blastocyst, the defect
  in the epithelium is filled with a closing plug
  (operculum) a blood clot and cellular debris
• by day 12, the site of implanted conceptus
  appears as a minute elevation of the endometrial
  surface and is covered with regenerated
  epithelium
• 4. Formation of uteroplacental circulation
• around day 9, isolated spaces (cavities) called
  lacunae are formed in the syncytiotrophoblast
• they become filled with nutrition secretions
  from eroded endometrial glands and with maternal
  blood
• from ruptured maternal capillaries histiotroph
• later, between day 11 and 13, adjacent lacunae
  fuse to form lacunar network in which maternal
  vessels
• open and through which maternal blood flows

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• 5. Manifestation of decidual reaction
• the conceptus evokes reaction of the entire
  endometrium
• consists in endometrial stromal cell changes
  that enlarge and accumulate glycogen and lipid
  droplets, in vascular alterations (capillary
  network around the conceptus becomes denser), and
  glandular alterations uterine glands increase
  their activity
• changes are referred to as the decidual reaction
  
• and the decidual transformed endometrium to as
  the decidua
• The decidua involves 4 distinct regions as
  follows
• basal decidua - under the conceptus
• marginal decidua - at the side of the conceptus
• capsular decidua - above the conceptus
• parietal decidua - has no contact with the
  conceptus

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• Implantation sites of the blastocyst
• Intrauterine sites
• normally, the blastocyst implants on the
  posterior wall of the midportion of the uterus
  body
• important nidation near the internal ostium
  the developing placenta covers the internal
  ostium as placenta previa
• it causes severe bleeding during pregnancy (it
  usually occurs in the 28th week) or during the
  labor