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Medical Academy named after S.I. Georgievsky
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Main page | Structure | Departments | Department of Histology and Embryology | Materials for students | Modul 1 | COMPARATIVE EMBRYOLOGY.

                                                       COMPARATIVE EMBRYOLOGY.
Embryogenesis (development of embryo) includes next stages: a) FERTILISATION (fusing of
 male  and female gamets, resulting in ZYGOT), b) CLEAVAGE – mitotic division of zygote, resulting in BLASTULA formation, c) GASTRULATION – differentiation of blastula wall on 3 germ layers (ectoderm, mesoderm, endoderm) and 3 axial organs (notochord, neural tube, gut), d) HISTOGENESIS -  differentiation of germ layers in tissues, e) ORGANOGENESIS – differentiation of tissues in organs.
OOCYTE   – is  immature female sex cell. OVUM –is MATURE gamet. Both  contain YOLK inclusions  in cytoplasm.(yolk= lecithos).     Classification  of oocytes/ovums  according yolk AMOUNT:
   a) OLIGOlecithal oocyte –it contains SMALL amount of yolk (in classes : 1.mammals, including humans, primitive chordate), b) MESOlecithal oocytes- it contains moderate amount of yolk 
  (in class amphibia),    c) POLYlecithal ovum-  it contains big amount of yolk ( in class birds).
               Classification  of oocytes/ovums  according yolk DISTRIBUTION in ooplasm:
 a) ISOlecithal oocytes – yolk droplets are distributed equally, NO poles, b)TELOlecithal ovums are POLAR gamets : yolk locates on vegetative pole, but nucleus and organells are on animal pole. Telolecithal ovums may be:  - -  SHARPLY telolecithal and   - -  MODERATELY  telolecithal.
   Mammalin/HUMAN ovum is  :secondary OLIGOlecithal ISOlecithal.    Amphibian ovum is: MESOlecithal, moderately TELOlecithal.   Bird ovum is : POLYlecithal, sharply TELOlecithal.     
         Mammalian/HUMAN ovum has 3 tunics and contains in ooplasm 2 types of granules:  
           1) YOLK granules,       2) CORTICAL granules (it contain GAGs and lytic enzymes).    
                                      3 TUNICS,  surrounding HUMAN ovum: 
   1) OOLEMMA (plasma membrane),      2) ZONA PELLUCIDA (contains:GAGs, including 
                                   hyaluronic acid,     and  glycoproteins  ZP1-, ZP2-, ZP3-fractions). 
      3) CORONA RADIATA.  It is composed of cells of follicular epithelium..
          Abbrev. “GAGs”  means:  “GlycosAminoGlycans”  (groop of compound carbohydrates).
         FERTILIZATION in mammalian/human happens in upper 1/3 of fallopian tube and includes 
 3 phases: 1) DISTANT interaction. During this CAPACITATION happens – it is removal of mask 
  proteins (glycocalyx) from sperm head and sperm becomes is able to recognize ZPs of zona pellucida.
       2) CONTACT interaction. It includes :ACROSOME reaction – acrosome releases lytic enzymes, destroying 3 ovum tunics. Sperm head enters in ooplasm and initiates CORTICAL reaction – cortical granules release it contant, resulting in transformation of  zona pellucida into  nonpermeable FERTILISATION MEMBRANE.  It prevents polyspermia (entrance of more than 1 spermatozoon).
    3) INFILTRATION.  Nuclei of sperm and ovum are fused, diploid set is restored, resulting in 
       ZYGOT formation.
   Zygot divides by mitosis (CLEAVAGE stage), resulting in BLASTULA formation. BLASTULA is vesicle-shaped multicellular embryo with:1) cavity, named BLASTOCELE, containing embryonic
 fluid, 2) wall, named BLASTODERM, consisting of embryonic cells, named BLASTOMERS.     
                                  Types of BLASTULAS in different  classes of  CHORDATA:
1) COELOBLASTULA  (in primitive chordate).  Blastomers locate in 1 layer and are equal in sizes.
2) AMPHIBLASTULA (in amphibian). Animal pole contains small-sized microblastomers,  vegetative pole contains big-sized macroblastomers.  Both poles are multi-layered.
3) DISCOBLASTULA (in birds). Microblastomers of animal pole associate into disc, floating on the surface of yolk-containing  vegetative pole.  Blastocele is very narrow.
4) BLASTOCYST (in mammalian/HUMAN).   Blastoderm is composed 2 cellular masses:  
        a)inner cell mass, or EMBRYOBLAST,    and  b) outer cell mass, or TROPHOBLAST.
                         Mechanisms of GASTRULATION in different  classes of  CHORDATA:
 1) INVAGINATION  (in primitive chordate),      2) EPIBOLY (in amphibian), 
 3) DELAMINATION (it is early gastrulation in birds and in mammalian/HUMAN). Result of it : embryo becomes BILAMINAR  and composed of 2 germ layers : a)ectoderm and  b)endoderm. 4)IMMIGRATION (late gastrulation   in birds and in mammalian/HUMAN). 2 results of immigration: 
 1st - embryo becomes TRILAMINAR  and is composed of 3 germ layers : a) ectoderm +  b) endoderm +c) MESODERM. 2nd - 3 AXIAL organs are formed: a) NOTOCHORD, 
                                    b) NEURAL TUBE (from ECTOderm),    
                                    c) GUT, or primitive alimentary canal, ( from embryonic ENDOderm)
Составитель – доцент В.В. Бондаренко.
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