Developmental Biology

Developmental biology deals with gametogenesis (formation of gametes), fertilization, and the embryonic development of an organism, here illustrated mainly through the frog.

Gametogenesis

The process of formation of gametes in the male and female gonads (testes and ovaries respectively) is called gametogenesis.

The cells of the germinal epithelium undergo three essential stages, multiplication phase, growth phase, and maturation phase, to give rise to haploid gametes.

• Spermatogenesis: gametogenesis for the formation of sperm.
• Oogenesis: gametogenesis for the formation of the female gamete (ovum).

Spermatogenesis

Spermatogenesis occurs in the seminiferous tubules of the testes. Each tubule contains two types of cells:

1. Germinal cells: the germinal epithelial cells are called primary germ cells. They are diploid (2n) and undergo a number of stages to give rise to haploid spermatids.
2. Sertoli cells: provide nourishment to the developing sperm.

Stages of Spermatogenesis

There are four important stages of spermatogenesis:

1. Multiplication phase: the germinal epithelium cells (primary germ cells) of the seminiferous tubule divide by mitotic division to produce a large number of diploid cells called spermatogonia.
2. Growth phase: the spermatogonia increase in size (as stated in the source, by absorbing nutrients from the Sertoli cells) to become large, round, diploid structures called primary spermatocytes.
3. Maturation phase: each diploid primary spermatocyte undergoes the first meiotic division to give rise to two haploid secondary spermatocytes. These, in turn, undergo the second meiotic division (equatorial division) to produce four haploid spermatids. The spermatids are unspecialized cells and remain connected by cytoplasm to the Sertoli cells; they become specialized only during spermiogenesis.
4. Spermiogenesis: the transformation of unspecialized spermatids into spermatozoa (sperm). The spermatids are transformed into sperm by losing a great amount of cytoplasm, condensation of the nucleus into the head region, and formation of a flagellated tail.

The mature spermatozoon finally gets attached to and is released into the lumen of the seminiferous tubule by a process called spermiation.

Structure of Sperm

The human spermatozoon is an elongated, microscopic, flagellated, and motile cell. It consists of the following parts:

• Head: slightly oval and contains a large nucleus (DNA) with the haploid number of chromosomes. Over the anterior half of the nucleus is a cap-like structure called the acrosome (a modified Golgi body). The acrosome contains enzyme(s) necessary for fertilization.
• Neck: has a pair of centrioles, the proximal and distal centrioles, placed at right angles to each other.
• Middle piece: contains spirally arranged mitochondria, which also surrounds the microtubules of the centrioles.
• Tail (flagellum): the posterior, long, thread-like part of the sperm. It is made up of an axial filament of microtubules arranged in a 9+2 pattern. It is a modified flagellum that provides motility to the sperm.

Oogenesis

Oogenesis refers to the formation of the mature female gamete. The stages during oogenesis are:

1. Multiplication phase: the cells of the germinal epithelium of the ovary undergo a number of mitotic divisions to increase their number. These cells are called oogonia and contain the diploid number of chromosomes.
2. Growth phase: the oogonia increase in size to form the primary oocyte (2n).
3. Maturation phase: the primary oocyte undergoes the first meiotic division to produce two haploid cells. Since the division of cytoplasm is unequal, one cell (the secondary oocyte) is large and receives most of the cytoplasm, while the other, very small cell is called the first polar body. In the second meiotic division, the secondary oocyte divides to form the ovum and a smaller (second) polar body. The first polar body also divides to form two small polar bodies. As a result, one ovum and three polar bodies (nutritionally non-functional bodies that degenerate) are produced from one primary oocyte.

Structure and Types of the Frog Egg

The egg of the frog is non-motile, spherical, and large sized. It is externally covered by a double membrane, the outer vitelline membrane and the inner plasma membrane. The vitelline membrane secretes a glue-like substance called albumin that swells up and binds the egg mass (not legible in full in the source). The albumin appears dark in colour in the frog egg (as stated in the source: blue) as it contains melanin pigment that protects the egg from the sun's UV radiation.

The egg of the frog is about 18 mm in diameter (as stated in the source) and is described as mesolecithal and telolecithal. It is divisible into two parts: the upper pigmented portion is called the animal pole, with the polar body outside it, and the lower, yolk-containing, non-pigmented area is called the vegetal pole (vegetal hemisphere). The vegetal pole faces downward and remains submerged in water.