Embryogenesis

21Jan08

HUMAN EMBRYOGENESIS

[male GAMETES]

• sperm à mobile elongated cell + small amount of cytoplasm. Nearly 60 μm long.
  
  • 2 main parts à head + tail.
    
• sperm tail is divided into:
  
• neck <connecting piece>
  
• middle piece
  
• principle piece
  
• end piece
  
• head à mostly occupied by nucleus! à
  þ 23 chromosomes of haploid no. in front of nucleus à ACROSOME is located.
  
• ACROSOME à special membrane organelle serving for the production of LYTIC ENZYMES à PENETRATE TH EGG N ITS SURROUNDING DURING FERTILIZATION!
  
• NECK à connects the head with tail. It has 2 CENTRIOLES one of which is usually fragmented. They are modified to form the connecting piece of the mature sperm.
  
• basic contents of tail à AXONEME <AXONEMAL COMPLEX> à consist of 9 outer doublet microtubules à composed of PROTEIN TUBULIN.
  
• the sperm mobility is based on the movement of the movement of the doublet microtubules in relation to one another
  
• each doublet exhibits a pair of arms that contain thymine.
  
• moreover, 9 course fibers develop from the centrioles. They are attached to the nucleus n extend into the tail as the outer dense fibers, peripheral to the microtubules of the axoneme
  
• MIDDLE PIECE à btw neck n principle piece! Characterized by the mito. Sheath which encircles the outer dense fibers n center most axoneme.
  
• the middle piece stops at annulus. ANNULUSà ring-like dense structure to which the plasma membrane adheres. It SEPARATES the junction of the middle piece with principle piece to prevent the mito. Sheath from moving caudally into the tail.
  
• 2 of the 9 outer dense fibers terminate at the annulus. The remaining 7 dense fibers continue into the principle piece. It extends from annulus à end piece.
  
• the centrally disposed AXONEME is surrounded with the 7 outer dense fibers from the middle piece. They are covered in turns by rips à series of ring-like dense structures known as the fibrous sheaths.
  
• the end piece is composed of axoneme surrounded by PLASMOLEMMA. The axoneme is gradually disorganized in the last, about 1 μm.
  

[female GAMETES]

B4 fertilization, à represented by immature cell à 2ndary OOCYTE!!! à IMMOBILE, SPHERICAL CELL WITH A LARGE CYTOPLASM surrounding the nucleus, ~150 μm in d.

• cytoplasm à
  
  • mito
    
  • cysterna of RER
    
  • free ribosomes
    
  • many GA!
    
  • lipid droplets n masses of pigments
    
  • specialized secretory vesicles à CORTICAL
    
• GRANULES <located under plasma
  
  • membrane!> à have lysosomal enzymes,
    
  • released by EXOCYTOSIS in the process of
    
  • fertilization.
    
• there are NO CENTROSOME in 2ndary oocyte!!!
  
• on the outside of plasma membrane, there are additional envelops à zona pellucida + corona radiate! à play protective role!
  
• 2ndary oocyte à DIPLOID nucleus! After gamete fusion, the 2nd division of meiosis occurs to form mature sex cells à OVUM!
  
• ovum à HAPLOID no. of chromosomes n DNA à 1n.
  

[developmentAL stages]

embryogenesis is divided into:

i. fertilization

ii. cleavage

iii. gastrulation

iv. histogenesis n organogenesis

fertilization

• male n female gametes fuse.
  
• occurs in ampulla of oviduct < widest part of uterine tube n located close to the ovary.
  
• spermatozoa n oocyte remain viable in the female reproductive tract for about 24 hours. B4 fertilization, the male n female gametes draw together.
  
• spermatozoa pass rapidly from vagina à uterus à uterine tube è caused by the contraction of the muscular of the uterus n the tube.
  
• N/B!!! à spermatozoa on arrival into female genital tract are not capable of fertilizing the oocyte! They must undergo à CAPACITATION!
  
• CAPACITATION à period of conditioning in the female genital tract – last about 7 hours!!
  
• during this time, a GLYCOPROTEIN coat n SEMINAL PLASMA MEMBRANE are removed from the plasma membrane that overlies the acrosomal region of the spermatozoa.
  
• only capacitated sperm can pass thru corona radiate n undergo the acrosome reaction.
  
• period of fertilization à 3 phases:
  
  • penetration of corona radiate
    
  • penetration of zona pellucida
    
  • fusion of the oocyte n sperm cell membranes.
    

1ST PHASE à PENETRATION OF CORONA RADIATA

• Is added by action of sperm n uterine tube mucosal enzymes.
  
• over 200k – 300k million spermatozoa are deposited in the female genital tract but only some hundreds reach the site of fertilization <AMPULLA> n only one of those is need for fertilization!
  

2ND PHASE à PENETRATION OF ZONA PELLUCIDA

• begins from sperm-binding that possesses by i/action of sperm n zonal receptors à provokes acrosomal reaction which occurs after binding of the zona pellucida. This reaction culminates in the release of enzymes needed to penetrate the zona pellucida.
  
• acrosomal enzymes à ACROSIN N TRYPSIN-LIKE SUBSTANCES. When the 1st sperm contacts with oocyte membrane, the cortical granules w/I the oocyte cytoplasm release their contents. This rxtn is named cortical rxtn. à results in the RELEASE OF LYSOSOMAL ENZYMES FROM THE CORTICAL GRANULES INTO PERICELLULAR SPACE.
  
• in turn, the enzyme cause an alteration in props of the zona pellucida to INACTIVATE THE ZONAL RECEPTORS N PREVENT NEW SPERM PENETRATION. This rxtn is named ZONAL RXTN. à RESULTS IN IMPERMEABILITY OF THE ZONA PELLUCIDA TO OTHER SPERMS!
  

3RD PHASE à FUSION OF OOCYTE N SPERM CELLS

• begins as soon as spermatozoa comes in contact with the oocyte cell membrane.
  
• the 2 plasma membranes fuse.
  
• both head n tail of the spermatozoon enter cytoplasm of oocyte but the plasma membrane is left behind of the oocyte surface.
  
• after that, the oocyte finishes its 2nd meiotic division n becomes haploid matured cell à OVUM!
  
• then both nuclei fuse n new cell becomes a germ à ZYGOTE!
  
• the developing organism from zygote up to the early gastrula b4 the 3rd week of pregnancy is named CONCEPTESS.
  
• zygte nucleus contain 46 chromosomes n 2n amount of DNA = somatic cells!
  

MAIN RESULTS OF FERTILIZATION:

1. diploid no. of chromosome
   
2. determination of sex of the new individual
   
3. initiation of cleavage n the next development
   

CLEAVAGE

• a series of mitotic division of zygote resulting in the increase of cell’s no.
  
• these cells become smaller with each cleavage division. They are known as BLASTOMERES n the whole germ is named BLASTULA.
  
• the 1st blastomeres form the compact accumulation of the LIGHT N DARK CELLS à COMPACTION.
  
• the DARK CELLS constitute the INNER CELL MASS.
  
• LIGHT CELLS à OUTER CELL MASS.
  
• thru the end of cleavage, the blastula is known as BLASTOCYST! à contain large cavity + fluid.
  
• the OUTER CELL MASS FORM à epithelial wall of the blastocyst à TROPHOBLAST! à later transformed into parts of placenta!
  
• INNER CELL MASS located on the internal surface of the trophoblast n forms EMBRYOBLAST à later become EMBRYO!!
  
• thru the end of cleavage, ZONA PELLUCIDA DEGENERATES n the trophoblast has the ability for PROTEOLYSIS N ADHESION.
  
• in 7th embryonic day, the trophoblast begins implantation into the uterus wall.
  
• IMPLANTATION à blastocyst settles into the endometrium.
  

GaSTRULATION

the embryo is named gastrula!!

2 phases:

[1ST PHASE]

• embryoblast differentiate into 2 cellular layers à procees of DILAMINATION <2 laminar formation!>
  
• dorsal/ outer layer à EPIBLAST
  
• ventral layer à hypoblast
  
• if look down, the epi n hypoblast form a flat-wide, oval-shaped disc à BILAMINAR EMBRYONIC DISC!
  
• w/I the epiblast, a vesicle with fluid appear à GERM OF THE AMNION <4 pregnancy!>
  
• during gastrulation, no. of cells migrate from the epiblast outer of the embryonic disc to form extra embryonic MESODERM. In the epiblast along the embryonic disc, cells compose the compact accumulation called PRIMITIVE STREAK.
  
• the cranial end of primitive streak have the thickening named PRIMITIVE NODE.
  

[2ND PHASE]

• last during the 3rd week of pregnancy.
  
• the primitive streak cells migrate under the epiblast on both sides from the middle axis.
  
• the form the new layer àMESODERM!
  
• at the same time, some of the migrated cells replace the hypoblast to form ENDODERM.
  
• After the formation of endoderm n mesoderm, the remaining of epiblast is called ECTODERM.
  
• ECTODERM, MESODERM N ENDODERM à are called GERM LAYERS!!!!
  
• at the same time, the primitive node cells migrate under the epiblast along the middle axis.
  
• this chordal cells are named NOTACHORD!
  
• The neighboring with notochord area at the ectoderm firstly becomes thicker to form NEURAL PLATE à NEURAL GROOVE à NEURAL TUBE.
  
• some of ectodermal cells migrate on both sides of the neural tube to form symmetrical cell aggregates called NEURAL CREST.
  
• no of cells form germ layers especially from mesoderm migrate btw germ layers n axial germs to form MESENCHYME.
  
• the main result of gastrulation is the GERM LAYERS N AXIAL COMPLEX FORMATION.
  
• THE AXIAL COMPLEX à NOTACHORD
  
  • NEURAL TUBE
    

histogenesis n organogenesis

• all the major organ systems begin to develop shortly after gastrulation.
  
• these processes compose the final 4th stage of embryogenesis.
  
• but processes of tissues n organs maturation continue till adult.
  
• during HnO, ECTODERM gives rise to organs n structures that maintain contact with the outside world:
  
  • skin epithelium <epidermis>
    
  • hair
    
  • nails
    
  • sweat glands
    
  • sebaceous glands
    
  • mamillary glands
    
  • sensory cells of ear, nose, tongue n eyes
    
  • epithelium of the oral cavity
    
  • enamel of the teeth
    
• NEURAL TUBE gives rise to:
  
  • brain
    
  • spinal cord
    
  • retina of the eye
    
• on both sides of neural tube, the separated aggregates of neural cells called the NEURAL CREST give rise to:
  
  • sensory n autonomic ganglia
    
  • nerve plexuses
    
  • neuroendocrine cells of the digestive tract
    
  • chromaffin cells of the adrenal medulla
    
• MESODERM 1stly divide into PARAXIAL <MEDIAL>, I/MEDIATE N LATERAL MESODERM.
  
• LATERAL MESODERM à 2 LAYERS!
  
  • dorsal <parietal/outer>
    
  • ventral <visceral/inner>
    
  • they are separated by cavity à COELOM.
    
• PARAXIAL <MEDIAL> MESODERM differentiate into 3 departments:
  
  • DERMATOME à CT of skin/ DERMIS
    
  • MYOTOME à SKELETAL MUSCLE ONLY!!
    
  • SCLEROTOME à SKELETAL TISSUE, CARTILAGE, BONE
    
• the INTERMEDIATE MESODERM forms NEPHROTOME àgives rise to UROGENITAL SYSTEM.
  
• LATERAL mesoderm takes part in forming of VISCERA <INTERNAL ORGANS>
  
• the mesenchyme is the precursor of all types of CT, vascular system, smooth muscle tissue n haemopoeisis.
  
• the ENDODERM gives rise to:
  
  • epithelium of digestive tract
    
  • parenchyma of the liver n pancreas
    
  • epithelial parts of the trachea, bronchi n lungs
    

lining epithelium of urinary bladder

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