Eggs and sperm form through gametogenesis

Gametogenesis occurs in the gonads, which are testes (singular, testis) in males and ovaries in females. The tiny gametes of males, called sperm, move by beating their flagella. The larger gametes of females, called eggs or ova (singular, ovum), are nonmotile (see Figure 20.1).

Gametes are produced from germ cells, which have their origin in the earliest cell divisions of the embryo and remain distinct from the rest of the body. All other cells of the embryo are called somatic cells. Germ cells are sequestered in the body of the embryo until its gonads begin to form. The germ cells then migrate to the gonads, where they take up residence and proliferate by mitosis, producing oogonia (singular, oogonium) in females and spermatogonia (singular, sper-matogonium) in males. Oogonia and spermatogonia, which are diploid, also multiply by mitosis, eventually producing primary oocytes and primary spermatocytes, which are still diploid cells.

Meiosis, the next step in gametogenesis, reduces the chromosomes to the haploid number, and the resulting haploid cells eventually mature into sperm and ova. (You may want to review the discussion of meiosis in Chapter 9 before reading further.) Although the steps of meiosis are very similar in males and females, there are some significant differences in gametogenesis between the sexes.

spermatogenesis produces sperm. Primary spermatocytes undergo the first meiotic division to form secondary sper-matocytes, which are haploid. The second meiotic division produces four haploid spermatids for each primary sper-matocyte that entered meiosis (Figure 43.3a). In mammals, these cells remain connected by cross-bridges of cytoplasm after each division.

The reason that mammalian spermatocytes remain in cy-toplasmic contact throughout their development probably is the asymmetry of sex chromosomes in males. Half the sec ondary spermatocytes receive an X chromosome, the other half a Y chromosome. The Y chromosome contains fewer genes than the X chromosome, and apparently some of the products of genes found only on the X chromosome are essential for spermatocyte development. By remaining in cy-toplasmic contact, all four spermatocytes can share the gene products of the X chromosomes, even though only half of them have an X chromosome.

A spermatid bears little resemblance to a mature sperm. Through further differentiation, however, it will become compact, streamlined, and motile. We will look at the differentiation of human sperm in more detail below.

oogenesis produces eggs. Oogonia, like spermatogonia, proliferate through mitosis. The resulting primary oocytes immediately enter prophase of the first meiotic division. In many species, including humans, the development of the oocyte is arrested at this point and may remain so for days,

43.3 Gametogenesis (a) Mitosis in diploid spermatogonia produces haploid spermatids, which differentiate into sperm. (b) Mitosis in diploid oogonia produces haploid secondary oocytes, which mature into ova.

Differentiation and maturation into gametes

Spermatids (n) Sperm cells (n)

(a) Spermatogenesis

Male germ cell (2n)

Spermatogonium

Primary spermatocyte (2 n)

Secondary spermatocytes (n)

Male germ cell (2n)

Spermatogonium

Primary spermatocyte (2 n)

First meiotic division

First meiotic division

Secondary spermatocytes (n)

Cytoplasmic bridge

Cytoplasmic bridge

The first meiotic division produces haploid cells.

Second meiotic division

Spermatids, each of which is different genetically, will differentiate into individual sperm.

(b) Oogenesis

Female germ cell Oogonium (2n)

Primary oocyte (2n)

Differentiation and growth

Secondary oocyte (n)

Female germ cell Oogonium (2n)

Primary oocyte (2n)

Differentiation and growth

Secondary oocyte (n)

Spermatogenesis And Oogenesis

The first meiotic division produces a haploid secondary oocyte and a small adjacent, nucleus-containing polar body.

The second meiotic division produces another polar body and the haploid egg (the first polar body may also divide at this time).

The first meiotic division produces a haploid secondary oocyte and a small adjacent, nucleus-containing polar body.

Second polar body

The second meiotic division produces another polar body and the haploid egg (the first polar body may also divide at this time).

months, or years. In contrast, there is no arrest during male gametogenesis, which goes steadily to completion once the primary spermatocyte has differentiated. In the human female, as we will see, some primary oocytes may remain in arrested prophase I for up to 50 years!

During this prolonged prophase I, or shortly before it ends, the primary oocyte undergoes its major growth phase. It grows larger due to increased production of ribosomes, RNA, cytoplasmic organelles, and energy stores. At this time, the primary oocyte acquires all the energy, raw materials, and RNA that the egg will need to survive its first cell divisions after fertilization. In fact, the nutrients in the egg will have to maintain the embryo until it is either nourished by the maternal system or can feed on its own.

When a primary oocyte resumes meiosis, its nucleus completes the first meiotic division near the surface of the cell. The daughter cells of this division receive grossly unequal shares of cytoplasm. This asymmetry represents another major difference from spermatogenesis, in which cytoplasm is apportioned equally. The daughter cell that receives almost all the cytoplasm becomes the secondary oocyte, and the one that receives almost none forms the first polar body (Figure 43.3b).

The second meiotic division of the large secondary oocyte is also accompanied by an asymmetrical division of the cytoplasm. One daughter cell forms the large, haploid ootid, which eventually differentiates into a mature ovum, and the other forms the second polar body. Polar bodies degenerate, so the end result of oogenesis is only one mature egg for each primary oocyte that entered meiosis. However, that egg is a very large, well-provisioned cell.

A second period of arrested development occurs after the first meiotic division forms the secondary oocyte. The egg may be expelled from the ovary in this condition. In many species, including humans, the second meiotic division is not completed until the egg is fertilized by a sperm.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • carol
    How and where sperm and eggs are formed gametogenesis?
    7 years ago
  • Askalu
    How the sperm and egg develop in gametogenesis ?
    3 years ago

Post a comment