Animal Sexual Reproduction and Embryonic Development

Sexual Reproduction in Animals

Sexual reproduction in animals

Sexual reproduction is carried out through the participation of reproductive cells, called gametes, which are produced by the reproductive organs. In animals, the gametes are generally produced in organs called gonads, where male gametes (spermatozoa) and female gametes (ova) are formed. In most animals, the reproductive system consists of gonads and accessory sexual organs, such as the genital ducts, uterus, vagina and copulatory organs. The male gonads are the testicles and the female gonads are the ovaries.

In some invertebrates there exists a seminal receptacle where spermatozoa received after intercourse are stored and released in a controlled manner according to the production of ova to be fertilized. In vertebrates, the reproductive organs and excretory organs are structurally related and have a common external opening. Some marine animals, such as sea urchins, have gonads whose sexual organs release gametes into the external environment after the breaking of the parent organism.

Gametogenesis

The formation of gametes is called gametogenesis. In vertebrates, it takes place in several developmental stages:

1. Proliferation or multiplication: primordial germ cells divide by successive mitoses, forming diploid cells that will become spermatogonia and oogonia.
2. Growth: spermatogonia and oogonia transform into spermatocytes and oocytes, respectively, in preparation for meiosis.
3. Meiotic maturation: oocytes and spermatocytes undergo meiosis, and all lead to the stage of differentiation that forms the definitive ova and spermatozoa.

Fertilization

The fusion of gametes, known as fertilization, is the process by which gametes unite to form the zygote. Depending on where it occurs, two types are distinguished:

• External fertilization: the most frequent type in aquatic animals, in which the female releases unfertilized ova and the male releases spermatozoa into the external aqueous environment, where fertilization takes place.
• Internal fertilization: the characteristic type in terrestrial animals. Mating is necessary so that the male introduces spermatozoa into the female genital tract, through a copulatory organ, where fertilization occurs.

When a spermatozoon makes contact with the egg, it perforates the external membranes of the egg and both gametes fuse. The nucleus of the spermatozoon enters the interior of the egg, which produces egg activation and causes the cortical granules to release their contents, forming the fertilization membrane that prevents other spermatozoa from fertilizing the egg. The nucleus of the sperm and the nucleus of the egg then merge, forming the zygote.

Embryonic Development

Depending on where the development of the zygote takes place, there may be:

• Oviparous: the zygote is expelled to the external environment and develops there.
• Ovoviviparous: the zygote develops inside the female reproductive tract, without maternal nutrition.
• Viviparous: the zygote develops inside the maternal reproductive tract, where maternal nutrition contributes to the embryo during its development.

Embryonic Development (I)

The embryonic development is the process by which a new individual is formed from the zygote. The following stages can be distinguished:

Cleavage (segmentation): These are the first cell divisions of the zygote. In most animals, the first division involves DNA synthesis and mitosis, but there is no cell growth. The embryo becomes a sphere of 16 to 32 cells, each called a morula. The resulting cells, called blastomeres, form a first layer on the surface of the embryo, enclosing the first cavity called the blastocoel. This structure is called the blastula.

Cleavage and the formation of the blastula are strongly influenced by the amount of yolk in the egg. Eggs with little yolk divide completely, while those with a lot of yolk divide only partially.

Cleavage in mammals: In mammals, the zygote is highly reduced. The placental mammal embryo and extraembryonic structures, including the placenta, develop inside the maternal uterus. Because the yolk contained in the zygote is scarce, its cleavage is complete, producing divisions and the first morula. At the 16 to 32-cell stage, these are divided into two groups: the inner cells form the embryo proper, and the outer cells form the trophoblast, which will participate in the formation of the placenta. Trophoblast cells secrete fluid and form a cavity with the inner cells placed at one end.

Gastrulation (formation of the three primary layers): In this process, the blastula cells are rearranged into layered embryonic sheets. The number of embryonic sheets depends on the animal. Diploblastic animals have two layers in the gastrula: the outer ectoderm and the inner endoderm, which delimits the primitive cavity called the archenteron, which communicates with the exterior through an opening called the blastopore. Triploblastic animals form a third embryonic layer, the mesoderm, between the ectoderm and endoderm. Triploblastic animals are divided into protostomes, in which the mouth forms from the blastopore, and deuterostomes, in which the mouth forms later at a different place from the blastopore.

Organogenesis: In this process, embryonic cells adopt specific structures and functions, organizing themselves into tissues that build organs. The ectoderm gives rise to the epidermis, which forms the skin and its appendages, the nervous tissue and the receptors of the sensory organs. The endoderm forms the epithelial lining of the digestive and respiratory tubes, the digestive glands and the urinary bladder. The mesoderm forms the dermis of the skin, the heart and blood vessels, the circulatory organs, the kidneys, the skeletal system, the gonads and the muscles.

Extraembryonic membranes: These originate from the embryo but are not part of it, thanks to the contribution of all embryonic layers. The yolk sac forms the first extraembryonic membrane that surrounds the embryo, and the amniotic membrane secretes a fluid that provides an aqueous environment. The amnion and chorion envelop the yolk bag, limiting water loss from the embryo and controlling gas exchange with the external environment. The allantois forms a bag that stores the waste products of the embryo's metabolism. In most mammals, the allantois and yolk sac join with the placenta to form the umbilical cord.

Postembryonic Development

Postembryonic development begins with birth in viviparous animals or hatching of the young in oviparous animals. Direct development is typical of oviparous animals with abundant yolk reserves, in which the young consist only of a miniature version of the adult that grows until it reaches adult size and sexual maturity. In mammals, young receive food through the placenta from their mother. Indirect development occurs when the young have no yolk reserves; the embryo cannot complete its development and gives rise to a free-living larva, which undergoes a series of profound changes called metamorphosis.

• Incomplete or gradual metamorphosis: occurs in amphibians, echinoderms, etc., in which the larva gradually changes into the adult form without passing through a stage of inactivity.
• Complete metamorphosis: the larva, after development, reaches maximum activity and then enters a stage called the nymph or pupa. The nymph is immobile and undergoes various internal processes that transform it into an adult.