Reproductive cycles of females
All women have reproduction processess which depends on certain changes in reproductive organs. There are sexual cycles in females.There are two cycles exhibited by mammals specially by woman.
Menstrual cycle
In woman menstrual cycle completes in about 26 to 28 days. After these days, ova are released from the ovary about 10 days after the onset of M.C. During this period many changes are observed in the ovary, uterus and mammary glands. During this period reproductive hormonal level also changes. All these changes are termed as menstruation.
Some important facts about menstrual cycle.
(a) The menustreal cycle starts at the age between 11 to 14 years. Prior to this is known as pre-menstrual age.
(b) The age of 11 to 14 in which menstrual cycle starts and continues to an age of about 50 years. Age of 11 to 14 is known an puberity age. In this period secondary sex characters develop i.e. growth of pubic hair takes on pubic parts and development of breasts. This is also known as puberity age.
(c) After the age of 50 the menstrual cycle stops. This is known as menopause. After menopause no pregnancy can take place.
Phases of Menstrual cycle
It can be studied under the following five phases :
(a) Menstrual Phase : During this period uterine epithelium, epithelial glands and connective tissues breakdown. During this phase progesterone hormone is nearly absent. Due to all these actions bleeding in woman starts and lasts for about 3 to 8 days. The ovum is discharged by ovary.
(b) Post Menstrual Phase : During this phase estrogen level in the blood increases and progestrone is almost absent. It lasts for about 6 to 8 days. This is also known as follicular phase.
(c) Proliferative phase : It lasts for about 14 days. During this period maturation of ovarian follicles is completed. It is controlled by F.S.H.
(d) Ovulation : On 14th day the graafian follicle ruptures and ovulation is seen.
(e) Pre-menstrual Phase : After 14th day when the ovulation takes place the corpus luteum is formed in the ovary. At this time progestrone hormone is secreted. This continues from 15th to the days of mestrual cycle onset.
2. Oestrous Cycle : This cycle is present in other than the primates but menstural cycle is absent.
At the end of the cycle there is no bleeding. Estrogen in blood is high which causes a strong sexual urge. During this female has a strong feeling of receiving male. It is called mating season or heat period.
Reproductive cycles of some females:
1. Man, Chimpanzee, Monkey—Menstrual cycle
2. Sheep, cow, pig, horse—Poly-oestrous
3. Dog—Mono-oestrous
GAMETOGENESIS
Gametogenesis refers to a process of gamete formation for the sexual reproduction. These are of two types-male gamate called sperms and female gamates calld ovum or egg. The process of sperm formation is called spermatogenesis and process of ovum formation is called oogenesis.
Now we shall discus spermatogenesis and oogenesis.
SPERMATOGENESIS
Spermatogensis takes place in male testes. Germinal epithelium is responsible for sperm formation. Sperm formation gets completed in three phases namely multiplication phase, growth phase and maturation phase.
OOGENESIS
Oogenesis is the process of ovum formation. It also gets completed in three phases namely multiplication phase, growth phase and maturation phase.
STRUCTURE OF SPERM OF MAN
1. It contains head, middle part and a tail.
2. Head contains a large nucleus.
3. Middle part is a cylindrical structure with axial filament and fused mitochondria.
4. The tail is provided with the thick and thin fibres which helps in the locomotion of sperms.
5. Acrosome is made up of golgi body. It helps in penetration of ovum.
6. In the head proximal centriole is present.
7. Distal centriols is present in the middle piece.
STRUCTURE OF MATURED EGG
It has the following sturctures :
1. Plasma membrane : Eggs is enveloped by plasma membrane. It is a unit membrane of two protein layers and a middle layer.
2. Primary egg membrane : It is fat. The primary and the secondary (two) eggs membranes are present. The primary egg membrane is present around the plasma membrane. In mammals it is known as zona pellucida.
3. Secondary egg membrane : S.E.M. is secreted by the ovarian tissues around the above mentioned membrane.
4. Tertiary egg membrane : It is secreted by the oviduct.
5. The ooplasm : It is the cytoplasm of egg. It consists a varied amount of yolk in different types of eggs. The peripheral layer is cortex.
Types of egg : Eggs are divided,
(a) On the basis of amount of yolk present in the egg.
1. Alecithal : Eggs without yolk, e.g. eutherian mammals.
2. Microlecithal : Eggs with a small amount of yolk, e.g. amphioxus.
3. Macrolecithal : Eggs with a large amount of yolk, e.g. echinoderms, amphibians, fishes, reptiles, birds and egg laying mammals.
(b) On the basis of how yolk is distributed.
1. Homolecithal : In this type, yolk is evenly distributed in the egg. e.g., echinoderms.
2. Heterolecithal : In this yolk is large in the egg. e.g., amphibians, fishes, reptiles, birds, egg laying mammals and insects.
(c) On the basis of protective covering.
1. Cleidoic : The egg is covered with calcareous hard and protective covering. e.g., birds.
2. Non-cleidoic : The egg is not covered by protective covering. They are generally found in animals in which development is in uterus.
FERTILIZATION
Fertilization is defined as the fusion of male and female nuclei. It results in the formation of zygote. It is of two types—
1. External fertilization : Sea water and fresh water animal carry it outside the body in the surrounding water, then it is external fertilization.
2. Internal fertilization : This takes place inside the body. It is also in the aquatic medium but in reproductive tract.
Process of fertilization : It is in two stages—
(1) Activation of egg : It is completed as follows—
(i) Movement of sperms towards the egg : The sperms swim towards the ovum. The egg contains a chemical known as fertilizin and the surface layer of sperms contains antifertilizin. The fertilizin attracts the sperms which contain specific type of anitifertilizin.
(ii) Activation of sperms : The acrosome of sperms contains lysin enzymes. On touching the egg layer, it bursts and these enzymes dissolve in the water medium. Then the central portion of acrosome forms a thin tube known as acrosomal filament. Its structure resembles a tube structure which is produced from the sperms. On touching the egg, it penetrates the egg jelly and vitelline membrane due to the lysin enzymes.
(iii) Activation of egg and insemination : At the touching point of acrosomal filament the surface of ooplasm is produced. This makes the fertilization cone. This cone engulfs the sperm. The penetration of the sperm into the ovum is known as insemination.
(2) Amphimixis : When the insemination has taken place the male pronuclei swell up by absorbing the water of ooplasm and become vesicular. It then move towards the female pronuclei. When the nuclear envelops of both the pronuclei come in contact, the dissolution of membranes at contact point takes place and then the transfer of chromatin material along with nucloplasm of male pronucleus takes place. This process is amphimixis.
CLEAVAGE
The sub-divisions of the zygote by miotic cell divisions into increasing number of cells of decreasing size is known as cleavage. Cleavage is of 4 types:
1. Radial : The successive cleavage furrows pass straight through the egg.
2. Bilateral : Blastomers are arranged in a bilateral symmetry.
3. Spiral : Cell parts move rationally around the egg axis.
4. Determinate : The fate of blastomeres is fixed.
Embryonic Development : When fertilization has taken place, the zygote divides mitotically and finally forms the fully developed embryo. The embryo develops in the uterus. After 10 weeks of the development, the zygote is known as embryo. After 10 weeks till birth. the developing embryo is known as foetus. For one year from birth, it is called infant.
(1) Cleavage or formation of morula : After 15 hours of fertilization, the cleavage starts in the fallopian tube. The zygote also travels towards the uterus.
(2) Blastula stage : It is defined as the divison and arrangement of blastomeres in morula with the development of blastocoel. The development of the morula into blastula is known as blastulation. In the blastula stage after 24 days of fertilization, the embryo gets embedded in the uterine endometrium.
(3) Gastrulation and formation of three germlayers : Gastrulation is defined as rearrangement of blastomeres of blastula around the embryo to form three germinal layers ectoderm, mesoderm and endoderm.
FORMATION OF THREE GERMINAL LAYERS
Endoderm formation : In this embroblast and separate and then aggregate in the blastocoel. After several divisions these cells increase in number rapidly, this is known as delamination. Thus a layer below the blastomere is formed which is known as endoderm.
Formation of mesoderm : When the endoderm is formed, it develops and arranges in a thick disc like structure which is known as embryonic disc. The embryo elongates. The cells from the pesteris part of blastodisc separate and form a middle layer which is known as mesoderm.
Formation of ectoderm : When the mesoderm cells are separated, they form the ectoderm layer.
Foetal Membrane in mammals
These membranes are known as extra embryonic membranes. These membranes protect the embryo. These are of four types :
(a) Aminion (b) Chorion
(c) Yolk sac (d) Allaxtois
Amniocentesis : This technique by which pre-natal diagnosis and disease and determination of sex of a child is possible.
A sample of amniotic fluid is taken out using a special surgical needle without harming the foetus. This fluid contains certain foetal cells. These cells are separated by the centrifugation of the fluid and are cultured for chromosomal analysis. Thus abnormalities are detected. Metabolic disorders can also be detected in an early embryonic stage. If it is detected that foetus has serious congenital and incurable diseases, the termination of pregnancy is made which is legally and clinically allowed.
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