FIRST YEAR EMBRYOLOGY: FERTILIZATION

BY MUNEEB HASAN KHAN

 

FERTILIZATION

TRANSPORT OF MALE GAMETE:

May be divided into 2 phases:

1)      Emission

•      Involves passage of semen to prostatic part of urethra.

•      Sperms stored in epididymis rapidly pass through ductus deferens to urethra by peristalsis.

•      During their passage they receive following glandular secretions:

a)      Prostate – Produces vesiculase enzyme that reduces coagulability of semen, contributes most of seminal volume.

b)      Seminal Vesicles – provides fructose (energy source for sperms)

c)      Bulbourethral Glands – provides lubrication for sperm passage

 

2)      Ejaculation

•      Involves expulsion of sperms from external urethral orifice by contraction of bulbospongiosus & urethral muscle sand closure of vesical sphincter of bladder.

•      Around 2-6ml of ejaculate having 200-600 million sperms is deposited in female reproductive tract; only about 200 reach the Fallopian tubes while the rest are resorbed by female genital tract.

•      Passage of sperms to site of fertilization is assisted by:

i.        Prostaglandins in semen

ii.      Muscular contractions of uterus

iii.    Chemotactic attraction towards oocyte

iv.    Increased and less viscid cervical mucus

v.      Alkaline pH vi. Beating of flagella

CAPACITATION:

•      A conditioning period for sperms lasting 7 hours that makes them capable of fertilizing oocyte.

•      Occurs in response to epithelial interactions in uterus & uterine tubes.

•      Involves following events:

1)      Alteration of membrane components& increased activity

2)      Removal of seminal proteins & glycoprotein coat from acrosome

 

TRANSPORT OF FEMALE GAMETES:

•      The ovulated oocyte along with its follicular fluid is taken up into infundibulum of uterine tubes by:

Ø  Brush-like motions of the fimbriae over the ovary.

Ø  Contraction of cilia present in the luminal epithelium.

•      From here it passes to ampulla (where fertilization occurs) and then to uterus as a result of peristaltic movements.

  

PHASES OF FERTILIZATION

1. Penetration of Corona Radiata:

•      Granulosa cells withdraw their cytoplasmic processes from oocyte.

•      Capacitated sperms move chemotactically towards oocyte.

•      Penetration of corona radiata& dispersal of its cell occurs by: o Hyaluronidase enzyme secreted from acrosome. o Tubal mucosal enzymes

o Movements of tail of sperm

 

2. Penetration of Zona Pellucida:

 Acrosome of sperm binds to glycoprotein ZP3 on zona pellucida, inducing 2 reactions i.e. a) Acrosome reaction i.e. o Multiple point fusions between acrosomal& plasma membrane occur which later breakdown to form pores (requires prostaglandins, progesterone and Ca²⁺ ions)

o   Enzymes are released from these pores e.g. acrosin, esterase, neuroaminidase.

o   These cause lysis of ZP, creating pathway for sperm to enter oocyte. b) Zona reaction i.e. o Cortical granules in oocyte fuse with oolemma and release lysozomal enzymes into perivitelline space.

o   These change the properties of ZP, making it hard and impermeable to further sperm penetration as well as entrapping any other bound sperms (prevents polyspermy)

 

3. Fusion of Oocyte & Sperm Cell Membranes:

•      Integrins on oocyte and disintegrins on sperm cause both to adhere to each other.

•      Afterwards, fusion of plasma membranes in area of contact occurs; head & tail of sperm enter the cytoplasm of oocyte, but plasma membrane and mitochondria are left behind.

•      Following responses then occur in oocyte:

a)      Metabolic Activation – triggered by an activating factor on sperm, causes initial cellular and molecular events associated with early embryogenesis.

b)      Resumption of 2^nd meiotic division – immediately after entry of sperm, the secondary oocyte (arrested in metaphase) divides into a mature oocyte and 2^nd polar body (which is extruded into perivitelline space).

c)      Formation of Ootid – haploid nuclei of both gametes become enlarged, morphologically indistinguishable pronuclei that come close to each other and replicate their DNA.

d)      Formation of Zygote – diploid totipotent cell formed by fusion of male and female pronucleus that immediately prepares for cleavage by arranging its chromosomes along mitotic spindle.

RESULTS OF FERTILIZATION

1. Restores normal diploid number of chromosomes in embryo (half each from father and mother) 2. Causes variation of human species by mingling of maternal & paternal chromosomes

3.      Determines sex of embryo

4.      Causes metabolic activation of Ootid and initiates cleavage of zygote, without which oocyte would degenerate in 24 hours.