Structure & Function Of Gametes

Structure of Sperm

It is microscopic, consists of a head, a neck, a middle piece, and a tail. It is enveloped by a plasma membrane.

Head

The head of the nucleus contains an enlarged haploid nucleus, the frontward part of which is enveloped by a cap-like structure, acrosome.

Function of Acrosome

The enzymes form the most of acrosome which helps in the fertilization of the ovum.

Function of mitochondria

Numerous mitochondria are present in the middle piece which creates energy for the movement of tail or flagellum.

Function of Tail

It is responsible for the vigorous movement of sperm towards ovum.

Hormones responsible for the formation of sperms

Gonadotropin-releasing hormone (GnRH) is responsible for the initiation of spermatogenesis at puberty. GnRH is a hypothalamic hormone. The rise in the levels of this hormone triggers the anterior pituitary gland to release two gonadotropins.

  • Luteinising Hormone (LH) – It acts at the Leydig cells and triggers the synthesis and secretion of androgens
  • Androgens – They trigger the process of spermatogenesis.
  • Follicle stimulating hormones (FSH) – These acts on the Sertoli cells and trigger some factors that further enhance the process of spermiogenesis.

The male body ejaculates around 200 to 300 million sperms during intercourse. Out of this sixty percent of the sperms have normal size and shape and a minimum forty percent of them show intensive motility.

Seminiferous tubules release sperms that are transported by the accessory ducts. Secretions of vas deferens, epididymis, prostate and vas deferens are important for the motility and maturation of sperms. Testicular hormones (androgens) regulated the functions of male sex accessory ducts and glands.

Fertilisation

During coitus (copulation) semen is liberated by the penis into the vagina (insemination). The mobile sperms swim swiftly and pass through the cervix, and then enter into the uterus and finally reach the ampulla of the fallopian tube. In this region, the fertilization takes place which occurs only when ovum and sperms are transported simultaneously to the ampulla. This is the reason that all copulations do not lead to fertilization and pregnancy.

The process of fusion of a sperm with an ovum is called fertilization. During fertilization following events occur:

  • Sperm comes in contact with the zona pellucida layer of the ovum and prompt changes in the membrane that restrict the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum.
  • The acrosomal secretions help the sperm to enter the cytoplasm of the ovum through the zona pellucida and the plasma membrane. This directs the completion of meiotic division of the secondary oocyte.
  • The second polar body and an ootid (haploid ovum) are formed due to the second meiotic division.

How the sex of the baby is decided?

The pattern of the chromosome in the human female is XX, and that of the male is XY. Thus, all the haploid ova produced by the female have sex chromosome X whereas the sperms have X or Y sex chromosome. Hence, there are fifty percent chances for both X and Y chromosome to join the X chromosome of the female. So, the male and female gamete would carry either XX or XY depending on whether the sperm carrying X or Y fertilized the ovum. The zygote carrying XX would develop into a female and XY would form a male. This is why sex of the baby is determined by the father and not by the mother.