What is Spermatogenesis? | Process of Spermatogenesis

Spermatogenesis is a complicated process that occurs in the seminiferous tubules of the testis of animals.

Image below shows a typical sperm of a frog and a rabbit.

Sperms of Frog and Rabbit

Also Read: What is Gametogenesis and it’s significance?

Process of Spermatogenesis

It happens in two steps:

  1. In the first step, spermatids are produced.
  2. In the second step, spermatids undergo metamorphosis and spermatozoa are formed.

The process of formation of spermatozoa or sperms from spermatids is called spermatoleosis or spermiogenesis.

Step 1: Formation of Spermatids

The spermatids are produced from the primary germ cells or primordial germ cells of the testis. This occurs in three phases.

A: Multiplication Phase

The testis are composed of seminiferous tubules that are lined with germinal epithelium. Some cell becomes differentiated as primary germ cells or primordial germ cell, while other function as the nutritive cells (for example Sertoli cells in mammal).

They become comparatively larger in size with a prominent nucleus.

During the phase of multiplication, the primordial germ cells divide mitotically and produce a large number of cells called spermatogonia or sperm mother cells.

Each spermatogonium represents a diploid cell containing 2n number of chromosomes.

B. Growth Phase

In this phase the spermatogonia increase in size by absorbing nutrients from other germinal cells. These enlarged cells are known as primary spermatocytes.

These cell become ready to undergo maturation division or reduction division.

C. Maturation Phase

It is a very important phase. Primary spermatocytes divides twice.

The first division is meiotic and two halpoid cells are formed which are known as secondary spermatocytes.

The second division is mitotic type and no futher change occurs in the chromosome number.

Thus, from two secondary spermatocytes, four spermatids are formed which are haploid with n numbers of chromosomes.

In other words, each primary spermatocytes gives rise to four spermatids

gametogenesis | Spermatogenesis
Gametogenesis | Spermatogenesis

Step 2: Spermatoleosis or Spermiogenesis

The spermatid formed as a result of maturation division is a typical cell containing mitochondria, golgi bodies and centrioles. But with one haploid set of chromosomes.

It undergoes gradual differentiation to form a motile sperm and this process in known as spermatoleosis.

The changes involve reduction of weight by losing superfluous material and development of such structure that help in movement.

different stages of spermiogenesis
Different Stages of Spermiogenesis

Structure of Spermatozoon

A human spermatozoon or sperm consists of four parts — the head, neck, middle piece and a tail or flagellum.

structure of spermatozoan
Structure of Spermatozoan or Sperm

1. Head

  • The head is almost conical in the human spermatozoa. It is formed of acrosome and nucleus enclosed in a thin membrane.
    1. The acrosome forms a cap-like structure (head cap) at the anterior end of the nucleus. It is formed from the Golgi complex and helps the spermatozoon to penetrate through egg membranes and enter the egg cytoplasm. It secretes tissue-dissolving (lytic) enzymes to facilitate this function.
    2. The sperm nucleus contains densely packed DNA and protein. The DNA is present in the quiescent state. Its posterior margin is depressed to accommodate the proximal centriole.

2. Neck

In some spermatozoa the head if followed by a short neck. It consists of just two centrioles. These are called proximal centriole and distal centriole and lie in a depression of the nucleus.

It is introduced into egg at the time of fertilization along with sperm nucleus. It is necessary to initiate cleavage or division in the zygote.

The distal centriole lies posterior to the proximal centriole and acts as a proximal granules. It provides attachment to the axial filament of the sperm tail or flagellum.

3. Middle

The middle piece consists of apical part of the axial filament surrounded by a tightly coiled spiral sheath of elongated mitochondria.

The mitochondria contain oxidative enzymes and provide energy for sperm motility.

A thin sheath of a cytoplasm present around the mitochondria and plasma membrane is called manchette.

4. Tail or Flagellus

It consists of a central axial filament, a thin layer of cytoplasm and an outer smooth plasma membrane.

The axial filament is formed of nine pairs of longitudinal fibers that extends upto the tip of axial filament.

In mammalian sperm another set of nine much thicker or band-shaped fibers is present outside the longitudinal fibers.

These fibers do not reach the tip of sperm tail. The free end of sperm tail without additional fibers is called end piece.

The sperm moves by the undulating movement of the tail.Propulsion of sperm through the female genital ducts need s a liquid medium.

The sperm virtually swims through this liquid medium for some distance to reach the ovum.

Comparison of Spermatogenesis and Oogenesis

The process of spermatogenesis and oogenesis are similar in several ways although they differ from each other in many respects. These are described below.

Similarities Spermatogenesis and Oogenesis

  1. Both the processes start from primordial germ cells or gonocytes.
  2. Both passes through the same phases, namely, multiplication, growth and maturation.
  3. Both processes involve two maturation divisions. The first of which is a reduction division (meiotic) and second one is a equational division (mitotic).
  4. At the end, a reduction in the number of chromosomes in the gametes is observed in both the processes.
  5. In both the processes, pairing of homologous chromosomes or synapsis takes place during the growth phase.

Differences Between Spermatogenesis and Oogenesis

1. It takes place in the testes.1. It takes place in the ovaries.
2. This process results in the formation of four sperms from each primary spermatocytes and all the sperms are functional.2. This process results in the formation of only one large ovum and three polar bodies from each primary oocytes. The ovum is functional but polar bodies are non-functional.
3. Sperms are very small in size, motile and active adapted to swimming by the lashing movement of the tail.3. The ovum is large, on-motile and contain large amount of yolk. Due to yolk it is much passive and receptive.
4. Numerous sperms are produced by this process.4. Only few ova are produced in this process.
5. This process is completed within the testes.5. The formation of ovum is completed outside the ovary.
6. No polar body is formed.6. Polar bodies are formed in this process.

Difference between Spermatogonia and Spermatocytes

1. Spermatogonia are produced from sperm mother cell by mitotic division during spermatogenesis. 1. During growth phase of spermatogenesis each spermatogonium enlarges to form primary spermatocytes.
2. These are formed during multiplication phase.2. During maturation phase each primary spermatocytes undergoes reduction division to form two haploid secondary spermatocytes.
3. These are diploid.3. These are haploid.

Difference between Spermatid and Spermatozoa

1. During spermatogenesis each secondary spermatocyte divides mitotically into two structures called spermatids.1. These are formed by the process called spermatogenesis.
2. From a single spermatocytes four haploid spermatids are formed. 2. From one spermatid only one spermatozoa is formed.
3. It can not move.3. It can move freely.

Now as you have finished reading Spermatogenesis, you should now read What is Oogenesis?

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