An ovum is spherical and is non-motile. The sex cells are different from the normal cells and differ in their appearance and structure. Also, the parts of egg cells have distinctive names.
The cell substance present in the egg is called the yolk or ooplasm, the nucleus is known as the germinal vesicle and the germinal spot refers to the nucleolus.
The yolk or ooplasm consists of cytoplasm and nutritive yolk which contains albuminoid substances and fat granules. A centrosome and a centriole are present in the cytoplasm. The ovum is microlecithal (contains only a small amount of yolk).
The germinal vesicle is large, eccentric, and is enveloped by a delicate nuclear membrane. It also contains the germinal spot.
The human ovum is enclosed by layers of envelope such as Corona radiata, zona pellucida, and vitelline membrane. Corona radiata is the outermost coat that is thicker and is produced by the radially elongated follicular cells. Zona pellucida or zona striata is a thick and transparent membrane and can be observed under the microscope as radially striated. The innermost membrane or the vitelline membrane is secreted by the ovum and is thin and transparent.
Oogenesis is the process in which a mature female gamete is formed. It gets initiated when the mother cells or oogonia are formed in the fetal ovary. These cells divide and enter prophase I of meiosis where they get arrested temporarily and are known as the primary oocytes.
The primary oocytes get enveloped by a tier of granulosa cells and now are referred to as primary follicles; their count keeps on decreasing till puberty is attained. These follicles get enveloped by additional tiers of granulosa cells and a thecal layer to form the secondary follicle.
Secondary follicles that are formed get transformed into a tertiary follicle containing a cavity filled with a fluid (antrum) and contains two coatings of inner theca and outertheca.
The primary oocyte present inside the tertiary follicle grows and accomplishes its first meiotic division which results in the formation of a secondary oocyte along with a polar body.
These follicles mature to form the Graafian follicle. The secondary follicle gets coated with a layer called zona pellucida; the Graafian follicle ruptures at this point and releases the ovum by the process of ovulation. This follicle now known as the Corpus luteum starts to degenerate. The secondary oocyte from the ovary is guided into the fallopian tubes from where it travels to the uterus.
Fertilization is the process in which the sperm which is the male gamete in humans fuses with the ovum which is the female gamete to form a zygote.
The sperm and ovum contain only a single set of chromosomes and are referred to as haploid. The zygote formed contains two sets of chromosomes each obtained from its parents and is referred to as diploid.
Ovum that was swept into the fallopian tubes comes in contact with the sperm which has crossed the zona pellucida layer of the ovum at the ampullary-isthmic junction of the fallopian tubes where fertilization occurs. The entry of the sperm causes some alterations in the membrane hence preventing another sperm from entering. This makes sure that only a single sperm fuses with the ovum.
The acrosomal secretions of the sperm facilitate its entry via the zona pellucida and plasma membrane into the ovum. The second meiotic division completes followed by the acrosomal induction and results in the production of a second polar body and an ootid (haploid ovum).Then, the haploid nucleus of the sperm and that of the ovum fuse together to form the zygote.
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