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 then function as an ascus, with karyogamy followed by meiosis. The resulting ascospores germinate into new asexually reproducing haploid yeast cells.
Bacteria, archea, protists and multicellular animals including cnidarioans and tunicates, as well as many other types of animals reproduce asexually. In asexual reproduction, genetically identical cells are produced from a single parent cell through mitosis. In single-celled organisms, an individual organism divides, a process called fission, adn then each part becomes a separate but idnetical organism. Cnidarians commonly reproudce by budding, wehreby a part of the parent’s body becomes separated form teh rest and differentiates into a new individual. The new individual may beocme an independent animal or may remain attached to the parent, forming a colony.
Another form of asexaul reproduction, parthenogenesis, is common in many species of arthropods. In parthenogenesis, feamles produce offspring from unfertizlied eggs. some species are exclusively parthenogenic (and all female), whereas others switch between sexual reporduciton and parthenogenesis, producing progeny that are both diploid and haploid, respecitvely. In honey bees, for example, a queen bee mates only once and stores the sperm. She then can control the release of the sperm. If no sperm are released, the eggs develop parthenogenetically into haploid drones, which are males. If sperm are allowed to fertizie the eggs, the fertilized eggs develop into diploid worder bees, which are female. However, when fertilized eggs are exposed to the appropriate hormone, they will develop into queens.
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.
The wall of the seminiferous tubules consists of spermatogonia, or germ cells, and supporting Sertoli cells. The germ cells near the otter surface of the seminiferous tubule are diploid and are the only cells that will indergo meiosis to produce gametes. The developing gamete cells located closer to the lumen of the tubule, are haploid. A spermatogonium cell divides by mitosis to produce two diploid cells. One of these two cells that undergoes meiotic division to produce four haploid cells that will beomce sperm while the other remains as a spermatogenium. In that way, the male never runs out of spermatogonia to produce sperm. Adult males produce on average of 100-200 million sperm each day and can continue to do so throughout most of their lives.
The anterior-pituitary gland secretes two gonadotropic hormones: follicle stimulating hormone (FSH) and luteinizing hormone (LH). Although these hormones are named for their actions in the female, they are also involved in regulating male reproductive function. In males, FSH stimulates the Sertoli cells to facilitate sperm development, and LH stimulates the Leydig cells to secrete testosterone. In a negative feedback, FSH causes the Sertoli cells to release a peptide hormone called inhibin, which specifically inhibits FSH secretion. Similary, LH stimulates testosterone secretion, and testosterone feeds back to inhibit the release of LH, both directly at the anterior pituitary gland and indirectly by reducing GnRH release form the hypothalamus.
The ovaries contain microscopic structures called ovarian folicles, each of which contains a potential egg cell called a primary oocyte and smaller ganulosa cells. At puberty, the granulosa cells begin to secrete estrogin, triggering the onset of menstrual cycling. Estogren also stimualtes the formation of the female secondary sexual characteristics including breast development and the proudciton of pubic hair. In addition, estrogen and another steroid hormone, progesterone, help maintin the female accessory sex organs: the Fallopian tubes, uterus and vagina. At birth, a female’s ovaries contain about 1 million follicles, each containing a primary oocyte that has begun meioiss but is arrested in prophase of teh first meiotic devision. During eahc menstrual cycle a group of follicles is recruited to reinitiate development, but he process of follicle maturation takes many months, such that at any given time there are follicles at many different atages of maturation in the ovaries. The human menstrual cycle last about 1 month. At the beginning of each menstrual cycel, a single dominant follicle emerges from the group that was recruited many months earlier and cotninues its development while the other follicles in that group enter a pathway for destruction. The increasing level of stragen in the blood during the follicular phase stimulates the anterior pituitary gland to secrete LH about midcycle which causes the fully developed Graafian follicle to burst in the process of ovoluation, releasing its secondary oocyte. The released oocyte enters the abdominal cavity near the fimbriae. If it is not fertilized, the oocyte disintegrtes within a day following ovulation. If it is fertilized, the stimulus of fertilizaiotn prompts it to complete the seocnd meiotic devision, forming a fully mature ovum and a seocnd polar body. Fusion of teh nuclei from teh ovum and the sperm produces a diploid zygote.
Sexual Differentiation in humans:
The reproductive systems of humans appear similar for the first 40 days after conception. During this time, the cells that will give rise to ova or sperm migrate form the yolk sac to the embryonic gonads, which have the potential to become either ovaries in females or testes in males. The gonads are said to be “indifferent”. If teh embryo is a male, a gene on the Y chromosome converts the indifferent gonads into testes. In feamles, which lack a Y chromosome, this gene and the protein it encodes are absent, and the gonads become ovaries.
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. translocation of the part of the Y chromosome bearing SRY to the X chromosome can cause otehrwise XX individuals to develop as males.
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 spermatocytes each 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.