FERTILIZATION AND IMPLANTATION

FERTILIZATION AND IMPLANTATION:

·         During copulation (coitus) semen is released by the penis into the vagina is called insemination.

·         The motile sperm swim rapidly, pass through cervix, uterus and finally reach the junction of isthmus and ampulla (ampullary-isthmic junction).

·         The ovum released from the ovary also transported to ampullary isthmic junction where fertilization takes place.

·         Fertilization only takes place if both sperm and ovum reach ampullary – isthmic junction simultaneously.

·         The process of fusion of a sperm and ovum is called fertilization.

·         Acrosome of sperm secretes enzymes helps in penetration into the ovum.

·         Once a sperm comes contact with the zona pellucida of ovum and induces the changes in the membrane that blocks the entry of additional sperms.

·         That ensures monospermy and prevents polyspermy.

·         Only one sperm fertilize with one ovum.

·         Entry of sperm into the ovum induces the ovum to complete its second meiotic division of secondary oocyte.

·         Meiosis-II is also unequal cytokinesis resulting production of one large ovum (ootid) and one small second polar body.

·         Haploid nucleus of sperm fused with the haploid nucleus of ovum to form a diploid zygote.

Sex determination:                                            

·         Sex of a baby has been decided during fertilization and in the zygote.

·         Sex is determined by the sex-chromosomes present in gametes.

·         Human female contain two XX chromosomes.

·         Human male contain XY chromosomes.

·         All the female gametes produced with only ‘X’ chromosome.

·         Sperms produced by male, 50% with ‘X’ and 50 % with ‘Y’ chromosome.

·         After fertilization zygote either carries XX or XY chromosomes.

·         Zygote with XX chromosomes develop into female and with XY chromosome develops into male.

Cleavage:

·         Repeated mitotic division of the zygote without growth resulting a multicellular ball like embryo is called cleavage.

·         Cleavage starts soon after fertilization.

·         Daughter cells produced during cleavage are called blastomeres.

·         The product of cleavage is called Morula, which is 8 to 16 celled.

·         The morula continues to divide and grow and transformed into blastocyst.

·         The blastomeres in blastocyst arranged into an outer layer called trophoblast and an inner mass of cells attached to trophoblast called inner cell mass.

·         Trophoblast cells attached to the endometrium helps development of placenta.

·         Inner cell mass gets differentiated into the embryo.

·         After attachment the uterine cells divide rapidly and cover the blastocyst.

·         Blastocyst completely embedded in the uterine endometrium. This is called implantation.

 

Pregnancy and embryonic development:

·         After implantation, finger like projections appears on the trophoblast called chorionic villi.

·         Chorionic villi surrounded by uterine tissue and maternal blood.

·         Temporary association between the fetal tissue (chorionic villi) and maternal tissue (uterine endometrium) is called placenta.

Function of placenta:

·         The embryo connected to the placenta by umbilical cord, which transports substances to and from the embryo.

·         Facilitate transport of oxygen and nutrient from mother to embryo.

·         Removes CO2 and waste material from the embryo.

·         Acts as endocrine gland and produces several hormones like:

o    Human chorionic gonadotrophins (hCG)

o    Human placental lactogen (hPL)

o    Estrogen.

o    Progesterone

o    Relaxin produced from the ovary in the later stage of pregnancy.

Embryonic development:

·         After implantation the inner cell mass of blastocyst differentiated into an outer layer called ectoderm and an inner layer called endoderm.

·         Mesoderm differentiated in-between ectoderm and endoderm.

·         The inner cell mass thus called stem cells, having potency to produce all types of cell, tissues and organs by differentiation.

Organogenesis:

·         Formation of different organs in the embryo is called organogenesis.

·         Human pregnancy lasts for 9 months.

·         After one month of pregnancy heart is formed in the embryo.

·         By the end of 2nd month the foetus develops limbs and digits.

·         By the end of 12 weeks (first trimester) most of organ system is formed (limbs and external genitalia are well developed).

·         First movement of foetus and appearance of hairs observed in 5th month.

·         By the end of 24th week (2nd trimesters) the body is covered with fine hairs, eye-lids separate, and eyelashes are formed.

·         By the end of 9 months the foetus is fully developed and is ready for delivery.

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