Embryology: Ovulation And Fertilization

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Fertilization general

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-fusion of sperm and ovum to initiate the development of new individual
-preceeded by: 1.ovulation: release of primary or secondary oocytes from the surface of the ovary
2. insemination: deposition of sperm into the female genital tract during coitus or artificial insemination

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Hormones of ovulation

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- FSH promotes growth
-LH (leutenizing hormone) promotes ovulation
- produced by adenohypophysis

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Corpus luteum

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-ruptured follicles are converted into the corpus luteum under hormonal influence
- corpus luteum is an important endocrine structure that secretes progesterone which induces uterine changes facilitating the implantation and maintenence of the conceptus

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Induced ovulation

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Cat and rabbit ovulate when stimulated by coitus
-eg cat's barbed penis
- other mammals sponaneously ovulate

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polytocous/ monotocous

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-bitch, cow, sow, cat and small ruminants are polytocous, meaning more than one oocyte is set free (usually from separate follicles)
-monotocous animals only set one oocyte free during each ovulation period

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Travel of egg during ovulation

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- ovulated egg received at the fimbriated end (abdominal ostium) of the oviduct through a constant stream of peritoneal fluid which enters the uterine tube
- it is then transported towards the uterus through ciliary action and muscular contraction of the uterine tube

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insemination

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-millions of spermatozoa are deposited into the vagina
- transport of the sperm is achieved not through their motility, but muscular contraction of the cervix and uterus

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sperm/ovum viability

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-most species: sperm remains viable within the female genital tract for 1-2 days
-sperm of horse and dogs may be viable for up to 6-7 days
- sperm host glands are in the avian vagina so sperm viability is 32 days for fowl and 70 days for turkey
- ovum viability is generally 24 hours

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site for fertilization

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-normally occurs in the ampullary region of the uterine tube, 12-24 hours after ovulation
- infundibulum of the uterine tube in dogs
-spermatazoa must undergo process of capicitation and acrosomal reaction and are therefore unable to fertilize the oocyte upon arrival in vagina

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capacitation of sperm

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-removal of glycoprotein coat and seminal plasma protein from the plasma membranes that overlie the acrosomal region of the spermatozoa

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acrosomal reaction of sperm

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-following capacitation, enzymes such as hyaluronidase and trypsin like substance are released in the acrosome
- these enzymes facilitate the penetration of the oocyte barrier, including the corona radiata, zona pellucida, and the oocyte cell membrane

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results of sperm penetration

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- the 2nd meiotic division of the oocytes if completed producing ovum and 2nd polar body
- fertilization membrane/ reaction develops which prevents further entry of sperm into oocyte (polyspermy), probably as a result of chemical changes in the zona pellucida (zona reaction)
-nuclei of the female and male germ cells now swell are are termed male and female pronuclei
- the pronuclei fuse together reconstructing the diploid chromosomal number and creating a zygote

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result of fertilization

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-prior to fusion, the pronuclei duplicate their DNA content
- immediately on fusion chromosomes split in the course of a normal mitotic division, resulting in the formation of a 2 cell zygote
-restoration of the diploid
-determination of sex, XX female (reverse in birds)
-initiation of clevage

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twinning

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1. monozygous: one ovum is fertilized but subsequently divides at the blastocyst stage into two genetically identical masses with the same sex and blood group
2. dizygous: two separate ova are fertilized

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ectopic implantation

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-fertilized ovum may be lost in the peritoneal cavity and become implanted into a peritoneal surface
- early embryonic death usually follows

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freemartin

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-in dizygous twinning cattle the development of a common circulation by placental anastomis usually occurs
- if opposite sex, development of female's reproductive tract is retarded under the influence of testosterone of the male
- females are then infertile

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polyspermy

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-occasionally more than one sperm may penetrate the female germ cell
- common in pigs
- majority of these conceptuses die at a very early stage of development

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superfecundation

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-impregnation by successive acts of coitus
- two or more ovum liberated at approximately the same time
-fertilization by two or more males
-cats, dogs

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superfetation

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-when pregnant female ovulates, conceive and produce a second younger fetus
-pigs

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mare gestation period

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335-345 days

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cow gestation

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279-282 days

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ewe gestation

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148-150 days

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sow gestation

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114-120 days

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dog gestation

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60-65 days

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cat (queen) gestation

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60 days