Menstrual cycle

                    Menstrual cycle

What happened when the egg is not fertilized ?

Definition

                 Menstrual cycle involves the series of cycle changes occurring in the ovaries and female reproductive trach during the reproductive period of female . 

Introduction. 

Reproductive period of female extends from puberty to menopause during this period, every month a menstrual cycle occurs whose peculiar feature is period ' _ the vaginal bleeding ( called menstruation ) that lasts for about four days . primary aim of menstrual cycle is to prepare the female for fertilization and pregnancy. 

 Period. 

            On average, a  menstrual cycle is completed in 28 days (mensem means a month) though it may very by 1 or 2 days. 

Phases. 

             Menstrual cycle is divided into four phases: 

                            

1 follicular or proliferative phase.  

                  It lasts for about 10 -12 days ( from 5th to 14th day of cycle ) and involves following changes: 

a). F s H stimulates transformation of a primary follicle of ovary into graafian follicle. 

b) Estrogens of graafian follicle regulate the growth and functioning of secondary sex organs; enlargement of endometrium of uterus and secretion of L H from the anterior pituitary. 

 2 ovulatory phase. 

                              It involves ovulation ( release of ovum from the ovary) and occurs on 14th  day of menstrual cycle and is controlled by increased level of LH in the blood. 

3. Luteal or secretary phase .  

                           It tasts for about 12 -14 days (from 15th to 28th day of menstrual cycle). It involves following changes: 

a) high level of LH stimulates transformation of empty follicle into a yellow and conical endocrine gland called corpus luteum. 

b) LH stimulates corpus luteum to secrete progesterone hormone 

C) progesterone stimulates further enlargement of endometrium of uterus and secretion of nutritive uterine milk from the uterine glands . So endometrium prepares itself for implantation and further development of embryo.

4 .  Menstrual phase . 

                      It lasts for about 3-5 days (1st to 4th day of menstrual cycle ) .  It occurs when ovum in the fallopian tube remains undertilized .in the phase , corpus luteum degenerates so the level of progesterone in the blood decreases. The uterine endometrium starts rupturing and the undertilized ovum , ruptured endometrium and about 50 -100 ml of blood menstruation or menstrual flow the monthly bleeding communly called period.

         

   EMBRYONIC DEVELOPMENT

( Steps of reproductive mechanism) 

Definition -

                       Embryonic  development includes a series of phases which transform a one - called zygote to a multicullular and fully formed embryo till birth. 

    Phases of embryonic development .

It involves following processes:. 

                          

1 Gametogenesis- 

                               It is process of formation of haploid gametes from the diploid primary germ cells , called gametogonia , of the gonads ( primary sex organs) . It is of two types; 

    a) spermatogenesis  involves formation of haploid sperms from diploid spermatogonia present in the testes of male organism. In male, it is a continuous process. It start at the puberty and occurs throughout life. 

   b) Oogenesis   

                           It involves formation of haploid female gametes , called ova, from the diploid  , egg  - mother calls , called oogonia, of ovary of female organism . It is a discontinuous and  highly wasteful process. It starts from puberty but stops after menopausal period . 

2 copulation  and insemination. 

                            Copulation involves sexual intercourse during which male inserts the 

  erect penis in vagina of female and  releases about 3.5 ml of seminal fluid called insemination. 

3  fertilization (syngamy) 

                           It involves fusion of haploid male gamete (sperm ) and female gamete (ovum) to form diploid zygote. In human female , fertilization is internal and coours in fallopian tube. Penetration of sperm into ovum is a chemical process and is aided by the sperm lysins hyaluronidase enzyme secreted by the acrosome.

4   cleavage or  Blastulation.  

                                It is rapid mitotic division of zygote to form a hollow, spherical and multicellular developmental stage called blastula so is called Blastulation. In humans, cleavage is holoblastic and unequal  so produces two types of blastomeres : small sized and peripheral micromeres and a few large sized and central macromeres. Blastula of human is called blastocyst having an outer nutritive trophoblast, a  multicullular embryonal knob and a cavity called blastocoel.

5.  Implantation .

                           Is the process of attachment of the blastocyst on the endometrium of uterus. It occurs on 7 th day after fertilization. In man , implantation is of interstitial types in which blastocyst gets buried into the endometrium. Implantation in human is controlled by estrogens and progesterone hormones of the ovaries. 

6 placentation. 

                       Is the process of formation of a temporary but intimate mechanical and physiological connection , called placenta , between the foetal and maternal tissues. During the placentation, the embryo gives  rise finger - like processes , called villi , in the endometrial tissues and gets surrounded by blood lacunae . Villi increase the surface area for exchange of materials. The placenta helps in nutrition, respiration, excretion, etc, of the foetus . In man , it fully formed in 10weeks and lasts throughout pregnancy. 

 7  Gastrulation-. 

                           It is the process by which blastocyst is changed into a gastrula with three primary germ layers . In this, the prospective areas of blastocyst move to their final and predetermined position . Gastrula has a cavity, called archenteron , and three primary germs  layers i.e . ectoderm, mesoderm and endoderm. Each primary germ layer is determined to form specific organs of the organism by the process called organogenesis. 

8 Gestation. 

                  It is the pregnancy period and extends from the fertilization to the childbirth it varies in different organisms . It is of 15 days in rat ( with minimum gestation period) . Of 30 days in rabbit ; of about 280  days in human female : while it is of about 22 months in elephants (652 - 670  days _ the longest gestation period). During this period, the foetus grows and become a baby . 

                      PARTURITION 

It is phenomenon of expelling of the fully formed young from the mother's uterus after the gestation period. It is characterized by labour pains which occur due to involuntary forceful contraction of smooth muscles of uterine wall under the influence of oxytocin hormone of posterior pituitary and voluntary contraction of abdominal muscles . It is also aided by relaxin hormone of the ovary which causes widening of pelvis and vagina for easy childbirth.