First Week of Embryological Development PDF

FERTILIZATION The inception of life… Lecturer, Ceren ERDEM ALTUN [email protected] 12.21.2023 FERTILIZATION ??? The process that the sperm and oocyte comes together to form a new zygote  A new organism is produced  Chromosome number is stabilized among species  Diploid chromosome number(2n)...

FERTILIZATION The inception of life… Lecturer, Ceren ERDEM ALTUN [email protected] 12.21.2023 FERTILIZATION ??? The process that the sperm and oocyte comes together to form a new zygote  A new organism is produced  Chromosome number is stabilized among species  Diploid chromosome number(2n) is obtained by the combination of haploid gametes (n)  Variations among species occur because of homolog recombination  Natural selection Fertilization occurs in the ampulla of the fallopian tube after a mature oocyte is ovulated from the ovary and enter the fallopian tube ~300.000.000 mil sperm is ejaculated ~200 reach the site of fertilization  The viability of the sperm is ~72 hours  The fertilization potential of the sperm is ~48 hours  The viability of the oocyte is ~12-24 hours 2) OOCYTE ACTIVATION - ZONA REACTION 1) SPERM AND OOCYTE BINDING - CAPACITATION - THE COMPLETION OF II. MEIOSIS - HYPERACTIVATION - THE EXTERNALIZATION OF THE 2. POLAR BODY - CORONA RADIATA-SPERM INTERACTION - PRONUCLEUS FORMATION - PENETRATION OF ZONA PELLUCIDA (ACROSOMAL REACTION) 3) CELL DIVISION - PENETRATION WITH OOLEMMA - DNA REPLICATION HOMOLOGUOUS CHROMOSOME PAIRING OOCYTE MEIOTIC SPINDLE ACTIVATION THE FIRST CELL DIVISION  Sperm goes through the vagina, Fallopian tubes and cells surrounding the oocyte  Mechanic, enzymatic and physiologic procedures; - Mechanically by motility - Enzymatically by the acrosomal enzymes (eg Hyaluronidase) and tubal mucosal enzymes  Sperm and oocyte interaction occurs by the help of sperm receptors on oocyte and sperm.  Sperm cells enter a maturation process called capacitation in the female genital tract after ejaculation.  These changes in the sperm enable the oocyte to penetrate the zona pellucida, that is, fertilization. MALE FEMALE  Lasts about 7 hours  Sperm do not show morphological changes in this process.  During capacitation, the glycoprotein coat and seminal plasma proteins covering the plasma membrane of sperm in the acrosomal region are removed  Only capacitated sperm undergoes acrosome reaction and fertilizes the oocyte  Capacitation is characterized by a series of biochemical and biophysical alterations to the cell; - Includes changes in intracellular pH, - remodeling of the cell surface architecture, - changes in motility patterns and initiation of complex signal transduction pathways  It occurs when sperm enters female genital tract (mucosal secretions)  After capacitation the assymmetry in flagellum motility increases, helix motility is seen  It is a pattern of motility that is required for spermatozoa to penetrate through the cumulus cell layer and the zona pellucida in order to reach the inner membrane of the oocyte Activated (A) and hyperactivated (B) spermatozoa. The numbers in the center of the sperm heads indicate successive positions of the sperm heads at 20ms intervals  The capacitated sperm comes into contact with the granulosa cells (corona radiata) around the secondary oocyte with its hyperactivation ability and passes easily between them.  Between the oocyte and the first layer of granulosa cells extracellular material accumulates called the zona pellucida  Contains three glycoproteins secreted by the oocyte  The zona pellucida is important for sperm receptors, binding specific proteins on the sperm surface and inducing acrosomal activation.  It is composed of three glycoproteins. These are produced by the growing oocyte  ZP2 and ZP3 assemble into long filaments. The sperm head first interacts with the ZP3 protein.  ZP1 cross-links the filaments into a three dimensional network Penetration of sperm with glycoproteins of the zona pellucida triggers the acrosomal reaction The head of the sperm is capped by an organelle called acrosome. The acrosome contains a trypsin-like protein (digesting enzyme) and hyaluronidase, which digests the hyaluronic acid When the head of sperm comes in contact with zona pellucida, an acrosome reaction is induced by the zona proteins.  Holes are formed in the acrosome wall of the sperm, hyaluronidase, trypsin-like substances and acrosine are released.  Hyaluronidase helps bypass the zona pellucida barrier.  Trypsin-like substances function in the digestion of zona pellucida.  Acrosine is attached to the inner acrosomal membrane. It helps the sperm to pass through the zona pellucida. The oolemma of the oocyte fuses with the sperm that has passed through the zona of the oocyte. Actin and myosin filaments in oocyte subcortical region helps the incorporation  When sperm enters the zona pellucida, a change in the properties of the zona pellucida makes it impermeable to other sperm (zona reaction).  The first answer of the oocyte to the incoming sperm is cortical reaction  The cortical granulles are released to the perivitellin space to avoid polyspermy  Fast blockage to polyspermy !!!!  It creates a barrier by the help of cortical granules (specialized secretory vesicles) located within the egg's cortex (region below the plasma membrane)  They modify an existing extracellular matrix to make it impenetrable to sperm entry  The sperm binding activity of ZP3 is lost  With the zona reaction, the binding of another sperm to ZP3 is prevented. STRUCTURES OF SPERM IN OOLEMMA : 1: oolemma 2: cell membrane of the spermatozoon 3: tail o sperm (kinocilium) 4: nucleus of the spermatozoon 5: centrosome of the spermatozoon Women are born with ~400,000 primary oocytes Oocytes experience a arrest in Meiosis I prophase at this stage. When puberty is entered, Meiosis I is completed 612 hours before ovulation, the first polar body is expelled and it is called the secondary oocyte. With the completion of Meiosis I, it continues Meiosis II and arrest at Metaphase II 2-3 hours before ovulation. If fertilization occurs, the secondary oocyte completes Meiosis II and the second polar body is expelled. 1. and 2. POLAR BODY  Nucleus materials of the gametes reorganize and form pronucleus.  The first to form is the female pronucleus, then the male pronucleus is formed.  They carry haploid genome (n) nuclei  The oocyte is called zygote from now on. Female and male pronuclei  Replication starts during the formation and growing of the pronucleuses. The main markers of the fertilization process;  2 pronuclei are formed (genetic material of the mother and father)  Excretion of 2. polar body The maternal chromosomes (22 + X) of mature oocyte condense and arrange themselves in a vesicular pattern to form the female pronucleus. Chromoses of the oocyte scatter to the cytoplasm in anaphase II. Small vesicles start to come together around the scattering chromosomes They combine to form the female pronucleus membrane. Sperm nucleus membrane dissolves Sperm chromatine dissolves in cytoplasm of oocyte (decondensation) Male pronucleus memrane is reformed and forms the male pronucleus.  The male and female pronuclei loose their cell membrane and chromosomes of two nuclei (23 in each) mix together to form diploid (i.e., 46 chromosomes) zygote. PRONUKLEUS PRONUKLEUS  The chromosomes in zygote become arranged on a cleavage spindle in preparation for cleavage of zygote.  Centriole is important for cell divisions (forming the spindle) in the zygote.  The centriole is brought by the sperm to the zygote. Dişi ve erkek prunukleuslarına ait nukleuslar METAPHASE II (MII) METAFASE I (MI) THANK YOU…