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The gametes and their formation

Page history last edited by Mª Eugenia Alba Torreiro 12 years, 1 month ago

Human gametes are called spermatozoa (singular: spermatozoon) and ova (singular: ovum). They can be called sperm and ovule or egg. Ova is the Latin word for egg.

 

 

Spermatozoa and their formation

 

Spermatozoa are the male gametes. They are small and motile cells. As you can see in the picture below, mature sperm have three distinct parts in their structure: the head, which contains the cell nucleus; the intermediate part or body, with abundant mitochondria to produce the energy necessary for movement; and the tail, which has a long flagellum for moving.

 

 

Spermatogenesis, the formation of spermatozoa or sperm cells, takes about 65 - 75 days in the human males. It begins at puberty, due to the action of male hormones and continues throughout life.

The normal human male usually produces several hundred million sperm per day, assuring an adequate number for fertlization to take place. Fewer than 100 ever reach the vicinity of the egg and only one sperm enters an egg.

 

 

 

Ova and their formation

 

The ova or female gametes are large, spherical and immobile cells. In a mature egg we can differentiate: the nucleus; the cytoplasme, which contains food reserves and three membranes (vitelline membrane, pellucic membrane and corona radiata) that nourish and protect the egg. Ova develop from cells called oocytes (immature ovules), which are located in follicles in the outer layer of the ovary.

 

 

The formation of the ova, or oogenesis, occurs within the ovaries of females in two stages:

  1. First step. When the female is still an embryo and she is inside her mother's womb, ovules (primary oocytes) are formed in her ovaries. These primary oocytes are surrounded by cells and their development begins when the girl reaches puberty (i.e. she matures sexually).

          At birth a female's ovaries contain some 2 million oocytes, all of which have iniciated the first meiotic division. Meiosis is arrested, however, in prophase of the first meiotic division.

 

  1. Second step. From puberty on, approximately every 28 days, an immature ovule (oocyte) matures and leaves the ovary; this process is called ovulation. Ovulation is repeated until the woman reaches menopause, which marks the end of her ability to reproduce.

          It is rare for more than about 400 of the approximately 2 million oocytes with which a female was born to mature during her lifetime. 

 

 

Spermatogenesis and oogenesis

 

We can observe the differences and the similarities between spermatogenesis, the formation of male gametes, and oogenesis, the formation of female gametes, in the next figure:

 

 

Meiosis is a type of cell division which reduces by half the number of chromosomes. By means of meiosis, diploid cells become haploid cells.

Gametes ( eggs and sperm) are haploid cells, they contains a single basic complement of chromosomes.

The zygote produced by fusion of egg and sperm (and the adult tha the zygote becomes) contains in each of its cells two copies of each chromosome.

The fusion of haploid gametes to form a new diploid cell is called fertilization

Reproduction that involves this alternation of fertilisation and meiosis is called sexual reproduction.

 

You can do this QUIZ to check your knowledge about the reproduction system, the formation of gametes, the uterine and ovarian cycles, and fertilization and pregnancy.

 

 

 

 

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