Structure of sperm

Sperm

( 1 ) Sperm is microscopic , elongated haploid motile male gamete produced by spermatogenesis . 

( 2 ) It measures to about 0.055 mm or 60μ in length . 

( 3 ) The sperm consists of head , neck , middle piece and tail .

(1) Head : 

(1) Head is the main part which is flat and oval and has a large nucleus and an acrosome. 

 (ii) Acrosome is formed from Golgi complex. It secretes enzyme hyaluronidase which helps in penetration of the egg during fertilization. 

 (iii) The acrosome and anterior half of nucleus is covered by a fibrillar sheath. 

(2) Neck : Neck is short region having two centrioles. 

 (1) The proximal centriole plays a role in first cleavage of zygote. 

 (ii) The axial filament of the sperm is formed by the distal centriole. 

 ( 3 ) Middle piece : 

( 1 ) Middle piece acts as a power house for sperm . 

2) It bears many spirally coiled mitochondria or Nebenkern around the axial filament. 

(iii) The mitochondria supply energy for the sperm to swim in the female genital tract. with a speed of about 1.5 to 3 mm per minute . 

 ( iv ) Posterior half of nucleus , neck and middle piece of sperm are covered by a sheath .. 

 4 ) Tail : 

( 1 ) The tail is formed of cytoplasm and is long , slender and tapering structure . ( 2) The axial filament is a fine thread - like

structure that arises from the distal centriole and traverses the middle piece and tail . Fagibbeu are present surrounding the two central longitudinal axial filaments . ( iii ) Nine accessory fibres ( iv ) Tail lashes and helps the spermatozoa to swim .

Structure of Sperm

Each day about 300 million sperm complete the process of spermatogenesis. A sperm is about 60micro m long and contains

several structures that are highly adapted for reaching and penetrating a secondary oocyte (Figure 28.6). The major parts of a sperm are the head and the tail. The flattened, pointed head of

the sperm is about 4–5 -m long. It contains a nucleus with 23 highly

condensed chromosomes. Covering the anterior two-thirds of

the nucleus is the acrosome (AK-roˉ-soˉm; acro- - atop; -some- body), a caplike vesicle filled with enzymes that help a sperm to penetrate a secondary oocyte to bring about fertilization.

Among the enzymes are hyaluronidase and proteases. The tail of a sperm is subdivided into four parts: neck, middle piece,principal piece, and end piece. The neck is the constricted region just behind the head that contains centrioles. The centrioles form the microtubules that comprise the remainder of the tail. The middle piece contains mitochondria arranged in a

spiral, which provide the energy (ATP) for locomotion of sperm

to the site of fertilization and for sperm metabolism. The principal piece is the longest portion of the tail, and the end piece is the terminal, tapering portion of the tail. Once ejaculated, most sperm do not survive more than 48 hours within the female reproductive tract.

What is sspermatogenesis?

Spermatogenesis

 In humans, spermatogenesis takes 65–75 days. It begins

with the spermatogonia, which contain the diploid (2n) number

of chromosomes . Spermatogonia are types of stem cells; when they undergo mitosis, some spermatogonia remain near the basement membrane of the seminiferous tubule in an undifferentiated state to serve as a reservoir of cells for

future cell division and subsequent sperm production. The rest

of the spermatogonia lose contact with the basement membrane, squeeze through the tight junctions of the blood–testis barrier, undergo developmental changes, and differentiate into

primary spermatocytes (SPER-ma-toˉ-sı¯tz). Primary spermatocytes, like spermatogonia, are diploid (2n); that is, they have 46 chromosomes.

 Shortly after it forms, each primary spermatocyte replicates

its DNA and then meiosis begins . In meiosis I,

homologous pairs of chromosomes line up at the metaphasep Plate, and crossing-over occurs. Then, the meiotic spindle pulls one (duplicated) chromosome of each pair to an opposite

pole of the dividing cell. The two cells formed by meiosis I are called secondary spermatocytes. Each secondary spermatocyte has 23 chromosomes, the haploid number (n). Each chromosome within a secondary spermatocyte, however, is

made up of two chromatids (two copies of the DNA) still at-

tached by a centromere. No replication of DNA occurs in the

secondary spermatocytes. In meiosis II, the chromosomes line up in single file along the metaphase plate, and the two chromatids of each chromosome

separate. The four haploid cells resulting from meiosis II are

called spermatids (SPER-ma-tids). A single primary spermatocyte therefore produces four spermatids via two rounds of cell division (meiosis I and meiosis II).

 A unique process occurs during spermatogenesis. As spermatogenic cells proliferate, they fail to complete cytoplasmic separation (cytokinesis). The cells remain in contact via cyto-

plasmic bridges through their entire development This pattern of development most likely

accounts for the synchronized production of sperm in any given

area of the seminiferous tubule. It may also have survival value

in that half of the sperm contain an X chromosome and half

contain a Y chromosome. The larger X chromosome may carry

genes needed for spermatogenesis that are lacking on the

smaller Y chromosome.

 The final stage of spermatogenesis, spermiogenesis (sper-

me¯-oˉ-JEN-e-sis), is the development of haploid spermatids

into sperm. No cell division occurs in spermiogenesis; each

spermatid becomes a single sperm cell. During this process,

spherical spermatids transform into elongated, slender sperm.

An acrosome (described shortly) forms atop the nucleus, which

condenses and elongates, a flagellum develops, and mitochondria

multiply. Sustentacular cells dispose of the excess cytoplasm

that sloughs off. Finally, sperm are released from their connec-

tions to sustentacular cells, an event known as spermiation

(sper-me¯-Aˉ-shun). Sperm then enter the lumen of the

seminiferous tubule. Fluid secreted by sustentacular cells

pushes sperm along their way, toward the ducts of the testes.

At this point, sperm are not yet able to swim.

Sperm Each day about 300 million sperm complete the process of

spermatogenesis. A sperm is about 60 -

m long and contains

several structures that are highly adapted for reaching and penetrating a secondary oocyte . The major parts of a

sperm are the head and the tail. The flattened, pointed head of

the sperm is about 4–5 -m long. It contains a nucleus with 23 highly

condensed chromosomes. Covering the anterior two-thirds of

the nucleus is the acrosome (AK-roˉ-soˉm; acro- -atop; -some-body), a caplike vesicle filled with enzymes that help a sperm to penetrate a secondary oocyte to bring about fertilization.

Among the enzymes are hyaluronidase and proteases. The tail

of a sperm is subdivided into four parts: neck, middle piece,

principal piece, and end piece. The neck is the constricted region just behind the head that contains centrioles. The centrioles form the microtubules that comprise the remainder of the

tail. The middle piece contains mitochondria arranged in a

spiral, which provide the energy (ATP) for locomotion of sperm

to the site of fertilization and for sperm metabolism. The principal piece is the longest portion of the tail, and the end piece is the terminal, tapering portion of the tail. Once ejaculated most sperm do not survive more than 48 hours within the female reproductive tract.