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 (ammpullary-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:
- Human chorionic gonadotrophins (hCG)
- Human placental lactogen (hPL)
- Estrogen.
- Progesterone
- 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.
PARTURATION AND LACTATION:
- The period of pregnancy is called gestation period. (9 months).
- Ejection or expulsion or delivery of foetus is called parturition.
- Parturition is due to vigorous contraction of uterine Myometrium.
- The signal of parturition is originated from the fully developed foetus and the placenta which induces mild contraction of uterus called fetal ejection reflex.
- Fetal ejection reflex triggers the release of Oxytocin from pituitary.
- Oxytocin induces stronger contraction of uterine endometrium.
- Stimulatory reflex continues stronger contraction leads to expulsion.
- After delivery the placenta is also expelled out of the uterus.
Lactation:
- The mammary gland of the female more differentiated during pregnancy,
- Mammary gland starts producing milk towards the end of the pregnancy.
- Process of milk production in mammary gland is called lactation.
- Milk produced during initial days of lactation is called colostrum.
- Colostrum contains several antibodies which provide immunity to the new born baby.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
Email Id : harekrushnagiri@yahoo.com
