spermatogenesis and Spermiogenesis

Spermatogenesis

Spermatogenesis is the process that occurs in the male reproductive system in which the haploid spermatozoa are formed from the primordial germ cells in the coiled tube structures called the seminiferous tubules of the testicles. The testicle is an extern male reproductive organ. The process of spermatogenesis starts with the mitotic division of the stem cells located near the basement membrane of the testicles seminiferous tubules. The stem cells from which the process of spermatogenesis start are called the spermatogonial stem cells.

In spermatogenesis, the gradual transformation of the germ cells into the spermatozoa occurs. The process of spermatogenesis can be divided into three phases that are
1. Proliferative phase
2.  Meiotic phase
3.  Differentiation phase.

As we know that female gamete is formed before birth and matures after puberty.  The production of gametes in male start from puberty and continuous for the rest of their life.

Seminiferous tubes are the sites where the process of spermatogenesis takes place. The two main cell types in the tubules that are involved in the spermatogenesis are germ cells and Sertoli cells.  The germ cell develops into the sperm while the Sertoli cells act on the germ cells.

Stages of spermatogenesis

There are three stages of spermatogenesis that are

1. Proliferative phase
In this phase type, A spermatogonial cell undergoes mitosis. After the divisions, these cells maintain their number and also give rise to B spermatogonia. The B spermatogonia give rise to primary spermatocytes on division.

2. meiotic phase
In this phase, the primary spermatocytes divided and give rise to the secondary spermatocytes. The secondary spermatocyte then divides and gives rise to spermatids. The spermatids are the cells that contain a haploid number of chromosomes.

3. Differentiation phase or spermiogenesis
The differentiation phase also called spermiogenesis is the process in which the spermatids undergo transformation into spermatozoa.  In the differentiation phase, many changes occur in the spermatids some of them are

1. formation of the acrosome
2.  condensation of the nuclear chromatin
3.  Growth of the tail.

The acrosome covers the cranial part of the head and contains hydrolytic enzymes. The hydrolytic enzymes present in the acrosome allows the fusion of male and female gametes during fertilization.

Hormonal control of spermatogenesis

The process of spermatogenesis is controlled and regulated by a complex feedback mechanism that involves the hypothalamus, pituitary gland, and testicles.

Firstly hypothalamus releases gonadotropin-releasing hormones that travel through the portal vessels into the anterior pituitary and act on gonadotrophic cells. The gonadotrophic cells then release follicle stimulating hormones or luteinizing hormones depending on the pattern of the gonadotropin-releasing hormones.

The luteinizing and follicle stimulating hormone then travels through the blood into the testicles. These hormones on the testicles act on the Leydig cells and stimulate them. The stimulated Leydig cells then convert the steroids into testosterone that contribute to the stimulation of the Sertoli cells.

Follicle stimulating hormones and testosterone act on the Sertoli cells and stimulates many functions. The functions that are stimulated or the synthesis of oestrogen, inhibin, meiosis, spermatocytes maturation, etc.

What is the structure of spermatozoa?

The spermatozoon is 50 micrometers long and contains the head, neck, and tail. The tail is further divided into three parts that are the middle piece, primary piece, and end piece.

Head of the spermatozoa
The head of the sperm contains the nucleus that has the condensed chromatin material. Anterior two third of the nucleus is included in an acrosomal cap. The cranial part of the head is covered by the acrosome. Acrosome contains various enzymes including hyaluronidase and acrosin.

The neck of the spermatozoa
The neck of the spermatozoon is narrow and contains a funnel-shaped basal plate and a centriole. The centriole gives rise to the axial filament that extends throughout the tail.

The tail of the spermatozoa
The tail of the sperm can be divided into three parts.

1. Primary piece. The principal piece is made of the axial filament included in seven outer dense fibers.

2. Middle piece. The middle piece contains mitochondria and axial filament. The axial filament in the middle piece is surrounded by a spirally arranged mitochondrial sheath. On the distal end of the tail, there is a ring-like structure called annulus through which axial filament passes.

3. End piece. It is made up of only axial filament.

The complete spermatozoon is included in a plasma membrane. The axial filament is responsible for the motion while mitochondria provide energy.

Abnormal sperms

The abnormalities in sperms are common as compared to the oocytes.

Types of abnormalities

1. Morphological abnormalities. In morphological abnormalities, the head and tail of the sperm may be abnormal or the sperm may be large or dwarf.

2. Immotility. For fertility, it is necessary that 50% of the sperms must be motile after 2 hours of ejaculation.

3. Genetic abnormalities. Sperms having unusual chromosomes content.

What is spermiogenesis?

The process in which the spermatids are transformed into the older spermatozoa is called spermiogenesis.

Procedure of spermiogenesis

1. Nuclear material gets condensed and the nucleus movement starts.
2.  Golgi apparatus forms the acrosome that covers the anterior two third of the nucleus.
3.  Centrosome divide into two centrioles. One centriole goes to the neck and gives rise to the axial filament. The other centriole goes to the distal end of the tail and forms a ring-shaped structure called annulus. Axial filament passes through the annulus.
4. The areas between the neck and annulus are surrounded by the mitochondria.

Summary

The spermatogenesis is the procedure of formation of spermatozoa from the primordial germ cells. The process occurs in the seminiferous tubules of the testicles. The process of spermatogenesis is divided into three phases that are the proliferative phase, meiotic phase, and cytodifferentiation phase. Spermiogenesis is the process in which the spermatids are transformed into spermatozoa. The structure of spermatozoa includes the head, neck, and tail. The tail is then divided into three parts that are a primary piece, middle piece, and end piece. the process of spermatogenesis and spermiogenesis takes place in the postnatal development at puberty. puberty is a period in the postnatal development in which secondary sexual characters such as hairs on face, muscle mass increases, and bones mass increases, etc are developed.