STRUCTURE OF OVUM

 

Structure Of Ovum: After the process of oogenesis -  an egg or the ovum is formed . Egg is released during the ovulation phase.

 

 Ovum is rounded, non motile cell. It is almost  free of  yolk and is called as alecithal. It has abundant cytoplasm , called ooplasm. It has a nucleus called as Germinal vesicle and there is the prominent occurrence of nucleolus. The nucleolus is surrounded by plasma membrane called  oolemma.  

There are no centrioles in the ovum. The ovum shows polarity. Its side which extrudes polar bodies called animal pole. The opposite side is called vegetal  pole. The ovum is enclosed by two additional egg coats: inner thin, transparent , non cellular zona pellucid and outer thick corona radiate.  

Zona  Pellucida is secreted by ovum itself and is called primary egg membrane. It is formed of three different glycol proteins. The corona radiate is formed of radically elongated follicular cells.

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GAMETOGENESIS : OOGENESIS

 

Gametogenesis –Oogenesis:  Gametogenesis is the process of formation of gametes. It is divided into two parts - Spermatogenesis and Oogenesis. Spermatogenesis is the formation of sperms and Oogenesis is the formation of egg or ova.

The process of formation of mature female gamete is called oogenesis.   It is divided into following stages ;

a)Multiplication Phase

b)Growth Phase

c) Maturation phase

Multiplication Phase –At the time of fetus development inside the mother’s womb there is the occurrence of another development in female that is the development of Egg mother cells or oogonia. No more oogonia are formed and added after birth. The egg mother cells or oogonia are larger cells with large nuclei and other constituents. These cells undergo mitosis division and increases in number. Oogonia are diploid or 2n having 46 chromosomes. Oogonial cells after the formation in huge number project into the stroma as a cord then becomes a round mass called the egg nest. One of the cell of the egg nest grows and becomes the primary oocytes. It is the future ovum. Other oogonia surrounds around the primary oocytes   to protect and give nourishment. Primary oocytes cells starts division and enters into the prophase I of the meiotic division but get temporarily arrested at this stage.

Growth Phase- In the growth phase development of primary oocytes takes place. Here along with the development of primary oocytes , follicle development also takes place. So this phase shows the development of Graulosa cells, Primary follicles, theca cells , secondary follicles etc.  It is a very long phase.

Maturation phase : In this phase, Primary oocytes completes its first meiosis and then second meiosis. In meiosis cell division there is the occurrence of 4 cells, here also it occurs but are all the cells that are formed not same. Out the four cells that develop here, 3 are small and is called as polar bodies and one is only big that is the egg or ootid.

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GAMETOGENESIS : SPERMATOGENESIS

Gametogenesis is the process of formation of gametes. It is divided into two parts - Spermatogenesis and Oogenesis. Spermatogenesis is the formation of sperms and Oogenesis is the formation of egg or ova.

Spermatogenesis is the process of formation of  sperms. A sperm is haploid and the cells from where it develops are diploid, so the process of formation of sperms need to pass through number of stages.The process of sperm formation starts when a boy reaches his puberty, generally around 15 years of age.

 The stages can be divided as follows:

1) Multiplication Phase

2)Growth Phase

3)Maturation Phase

4) Formation of Spermatozoa

In Multiplicative phase, the cells that are present in the seminiferous tubule under go division. This division is mitotic division and helps in increasing the number of cells without undergoing reduction in the constituents of the cell. It is Mitosis division. The male germ cells are also called as Spermatogonia.

After the multiplicative phase occurs the growth phase. Here in this phase Spermatogonia start growing. It accumulates huge amount of cytoplasm, DNA undergo replication and its size increases. They are now referred to as primary spermatocytes. The growth phase is also called as spermatocytogenesis. 

Next phase is the maturation phase, in this phase the spermatocytes undergo division. This division is meiotic division. Meiotic division is a reduction  division as from diploid condition the cells become haploid. Here from 23pairs of chromosome ie 46 number of chromosomes  become 23, so haploid. This leads to the formation of Spermatids.

Spermatids finally get transformed into spermatozoa. This process of formation of spermatozoa from spermatids is called Spermiogenesis.

MENSTRUAL CYCLE:

 MENSTRUAL CYCLE: The gametes formation in females takes about 28 days( it varies from female to female but average taken to be 28 days).In this cycle, rhythmic changes occur in the reproductive organ of female primates ( Monkeys, apes and human being). The first menstruation begins at puberty   ( on set of reproductive phase, in female it generally starts from the age of 12 years) and it is called menarche. Human female's fertility period extend from around 12 years to Menopause at about 45 - 50 years. Menopause is the term given when in woman's life ovulation and menstruation stops. During the reproductive period say from around 12 years to around 45 - 50 years , generally one egg is released every 28 days or so alternately from two ovaries. In female there is the occurrence of one pair of ovary - right and left ovary. Suppose if in the first  menstrual cycle, right ovary produces the egg than left ovary at that moment remains silent and in the next  menstrual cycle left ovary will release the egg where as the right ovary remains silent. Generally female remain fertile for around 32 years and about 13 eggs mature each year.

The menstrual cycle in human being lasts 28 days. The days are numbered from the first day of the blood flow in the menstrual period. So these cycle events from the first day is called the Menstrual cycle The menstrual cycle in humane female is divided into three phases: 

 1)Follicular or Proliferating Phase 

 2)Luteal or Secretory Phase

 3)Menstrual or Bleeding Phase

In all these above phases Endometrium layer lining the uterus develops, Huge number of hormone secretion occurs, Ovary becomes ready to release an  egg and if the egg is not fertilised then the endometrium starts breaking down.  

SICKEL CELL ANAEMIA

 Sickle Cell Anaemia : Sickle cell anaemia is an autosome linked recessive disease. Here in this disease both male and female can become diseased or both may be carrier depending upon the presence of  infected gene/s .Here in this context we need to know two words homozygous and heterozygous. Homozygous individual are those when two  chromosomes have the infected gene ie. HbS and HbS and in case of Hetrozygous individual their two chromosomes are arranged as HbS and HbN.                                           Here HbS is characterised by sickle condition and HbN is normal. 

Human blood cells are Biconcave disc shape, which maximises the surface area of the cell membrane for oxygen . But in case of sickle cell Anaemia cell take the shape of a Sickle and it become difficult for the blood capillaries to transport those cells. In such condition the person suffers from oxygen tension. Genetically it is a point mutation where a change of single base pair in the gene for beta globin chain that results in the change of amino acid residue glutamate to valine.

HAEMOPHILIA

 Haemophilia: Haemophilia is a genetical disease. Expression of this disease occurs in Male while female remain as carrier. This is because human being has one pair of sex chromosome and in that pair it may comprise XY  or XX chromosome and if it is XY then it is Male in Humans and if it is XX then it is Female in Humans. Haemophilia is a sex linked recessive disease. It is due to the recessive gene on X chromosome. In case of human female there is the occurrence of two X chromosome, so if in one of the X chromosome the gene for haemophilia is there, than in female there is the occurrence of another X which gives its expression. But in case of Male there is the occurrence of one X chromosome and if that X chromosome has the gene for Haemophilia than the expression of Haemophilia occurs in the male. Therefore, females are carrier and if males have the gene then they are infected and diseased. 

In this disease , a single protein that is part of the cascade of proteins involved in the clotting of blood is affected. Due to this, if there is a minor cut then blood keep on coming out. The disease is therefore transmitted to a son from a carrier mother. It is also known by another name as Bleeding disease or Royal Disease. Royal Disease named because males of Queen Victoria in different generations suffered from this disease.  

REPRODUCTION IN PLANTS

 Reproduction in Plants : Reproduction in plants can be divided into two main parts, one is Asexual and other is Sexual. In case of Asexual reproduction one parent is sufficient to produce an organism. In case of Asexual reproduction there is no gamete formation or neither zygote is formed. But in case of Sexual reproduction there is the need of two parents. Each of the parent produces a gamete and two gametes from two parents unite together to form zygote. Zygote later on forms the organism. In case of Sexual reproduction as two parents are involved so there is the occurrence of variation in the next generation or off spring. But in case of Asexual reproduction as one parent take part in the formation of an organism so there variation does not takes place.Variation plays a great role in evolution