See also cell development.

Asexual Reproduction

Organisms can reproduce without sex. For example, there are species of lizards that consist only of females and reproduce without matting. Such asexual reproduction gives rise to offspring that are genetically identical to the parent.

Most yeast reproduce asexually be cell fission or budding, when a smaller cell forms form a larger one. Soemtimes two yeast cells of opposite mating type fuse to form a dikaryon. This cell may tehn function as an ascus, with karyogamy followed by meiosis. The resulting ascospores germinate into new asexually reproducing haploid yeast cells.

Sexual Reproduction:

In contrast, sexual reproduction leads to offspring that differ genetically from both their parents. Most animals reproduce sexually. Animal egss, which are not mobile, are much larger than the small, usually flgellated sperm. In animals, cells formed in meiosis funciton as gametes. These haploid cells do not divide by mitosis first, as they do in plants and fungi, but rather fuse directly with each other to form the zygote. 

In sexual reproduction, genomes mix when two haploid cells (each carrying a single set of chromosomes) fuse to form diploid cells (each carrying a double set of chromosomes). Later, new haploid cells are generated when a descendant of this diploid cell divides by the process of . During meoisis, the chromosomes of the double chromosome set exchange DNA by genetic recombination before being sorted out in new combinations into single chromosome sets. 

Haploid cells that are specialized for sexual fusion are called gametes. (Gametes are produced by meiosis.) Typically two types of gametes are formed: one is large and nonmotile and is called the egg (or ovum) and the other is small and motile and is called the sperm (or spermatozoon). During the diploid phase that follows the fusion of gametes, the cells proliferate and diversify to form an embryo. 

In all vertebrate embryos, certain cells are singled out early in development as progenitors of the gametes. These primordia germ cells migrate to the developing gonads, which will form the ovaries in females and the testes in males. The determination as to whether these primordial germ cells will develop into eggs or sperm depends not on their own makeup but rather on whether the organ to which they have migrated develops into an ovary or a testes. This in turn depends on the sperm that fertilized the egg. Eggs have a single X chromosome, whereas the sperm can have either an X or a Y. A sperm that carries a Y chromosome instead of an X chromosome will induce somatic cells of the developing gonads into a testes.

Developmental Pathway to Testes

The critical gene on the Y chromosome that has a testes determining function is called Sry (“sex-determining region of Y). The  Sry gene on the Y chromosome contains a TDF region which results in differentiation of the testes. Without this region, the female gonad will form instead.

Sry is expressed only in a subset of the somatic cells of the developing gonad, and it causes these cells to differentiate into sertoli cells which are the main type of supporting cells found in the testes. Sertolli cells have a number of functions such as the following:

• support developing germ cells as they differentiate from spermatogonia to sperm such as by producing lactate. This is important since the blood testis barrier prevents nutritional components from reaching the later stages of the germ cells. This need is provided by sertoli cells;
• divides the seminiferous tubules into basal and adluminal compartments by sertoli-sertoli cell tight junctions. 
• secrete anti-Mullerian hormone which suppresses the development of the female reproductive tract by causing the mullerian duct to regress;
• induce other somatic cells in the developing gonad to become leydig cells. Leydig cells secrete the male sex hormone testosterone. Testosterone, as other steroids, is derived from cholesterol and is stimulated by the action of luteinizing hormone (LH) (there is a LH receptor on the leydig cell) from the anterior pituitary. Testosterone is responsible for inducing the development of many ducts and accessory glands.

Spermatogenesis in the Male

Spermatogenesis is the process of differentiation from a diploid spermatogonia to a haploid spermatozoa through the process of meiosis and differentiation.

In spermatogenesis, immature germ cells called spermatogonia (singular, spermatogonium) proliferate continuously by mitosis around the outer edge of the seminiferous tubules next to the basal lamina which surrounds the seminiferous tubule. Some of the daughter cells stop proliferating and differentiate into primary spermatocytes. The process of developing primary spermatocytes from spermatogonia is referred to as spermatocytogenesis. 

Primary spermatocytes then proceed with  to produce two secondary spermatocyteseach containing 22 duplicated autosomal chromosomes and either a duplicated X or a duplicated Y chromosome. These two secondary spermatocytes then proceed through to produce four spermatids each with a haploid number of single chromosomes. These haploid spermatides then undergo morphological differentiation into spermatozoa which escape into the lumen of the seminiferous tubule. The process of development of spermatozoa from spermatids is referred to as spermiation.

Developmental Pathway to Ovary

In the absence of Sry, the genital ridge develops into an ovary. The supporting cells become follicle cellsinstead of Sertoli cells. Follicle cells are arranged as an epithelial layer around the oocyte to which they are connected by gap junctions which permit the exchange of small molecules. 

Other somatic cells become theca cells instead of Leydig cells and secrete at the beginning of puberty the female sex hormone estrogen instead of testosterone. The primordial germ cells develop into eggs instead of sperm. 

Oogenesis

Oogenesis is the development of an oocyte (egg). In this process, primordial germ cells migrate to the forming gonad which then proliferates by mitosis before differentiating into primary oocytes where the first  begins (usually before birth in mammals). In mammals, primary oocytes remain arrested in prophase of meiotic division I until the female becomes sexually mature where under the influence of hormones a small number of primary oocytes periodically mature to become secondary oocytes, completing division I. At ovulation, the arrested secondary oocyte is released from the ovary and undergoes a rapid maturation step that transforms it into an egg that is prepared for fertilization. If fertilization occurs, the egg is stimulated to complete meiosis. 

Prior to ovulation, follicular cells proliferate and secrete estrogen under FSH stimulation. There is also a surge in LH which induces ovulation. After ovulation, there is synthesis of progesterone as well as estrogen which inhibit LH and FSH secretion. 

Fertilization

Once released, egg and sperm will die within minutes/hours unless they find each other and fuse in the process of fertilization. To become competent to migrate through the layer of follicle cells and then bind to and cross the zona pellucida (egg coat) the sperm must become competent by conditions in the female reproductive track in a process called capacitation. This requires 5-6 hours in humans and is triggered by bicarbonate ions in the vagina which enter the sperm and activate a soluble  enzyme in the cytosol. Capacitation involves alterations in membrane characteristics and increased

On binding to the zona, the sperm is induced to undergo an acrosome reaction in which the contents (proteolytic enzymes) of the acrosomal vesicle at the head of the sperm are released which allow the sperm to penetrate the zona. This release is triggered by a glycoprotein on the oocyte called ZP3. 

When the sperm fuses with the ova, it causes an increase in calcium ions which leads to the initiation of something called the cortical reaction or the release of cortical granules from the egg by exocytosis. The contents of the cortical granules include various enzymes that change the structure of the zona pellucida which hardens it so that sperm no longer binds to it. It thus provides a block to fertilization of more than one sperm. 

Zygote: (fertilized egg): has the capability of giving rise to all the kida of cells in an animal’s body. It is accordingly totipotent. Fertilization is completed when the two haploid nuclei come together and combine their chromosomes into a single diploid nucleus. In humans the sperm contributes a centriole to the fertilized egg which the egg lacks. The centriole replicates and helps organize the assembly of the first mitotic spindle in the fertilized egg (zygote). Fertilization is the start of  in which the zygote develops into a new individual.

Fertilization in Particular types of Organisms:

Amphibians:

Amphibians’ eggs must be laid in water or a moist setting to avoid drying out.

–Frogs and toads: return to water to reproduce, laying their eggs directly in water. Their eggs lack watergith external membrane and would dry out quickly on land. Eggs are fetrilized externally and hatch into swimming larval forms called tadpoles. Tadpoles live in the water, where they geenrally feed on algae. After considerable growth, the body of the tadpole gradually undergoes metamorphosis into that of an adult frog.

Reptiles:

Reptiles are a higly successful group. There are mroe living species of snakes and lizard than there are of mammals. Reptiles occur world wide except in the coldest regions, wehre it is impossible for ectotherms to survive.

Reptiles do not practice external fertilization as most amphibians do. Sperm would be unable to penetrate the membrane barreris protecting the egg. Instead, the male places sperm inside the female, where sperm fetilizes the egg before the protective membranes are formed. This is called internal fertilization.

Although marine turtles spend their lives at sea, they must return to land to lay their eggs. Many species migrate long distances to do this. Atlantic green turtiles migrate from their feeding grounds off the coast of Brazil to the middle of the South Atlantic, a distinace of more than 2000 im, to lay their eggs on the same beaches wehre they were hatched themselves.