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This article provides information about the important phases in the development of animals: Gametogenesis, Fertilization, Cleavage or segmentation, Gastrulation, Organogenesis, Growth and Differentiation!
The development of all organisms is a slow and continuous process in which changes from simple condition lead to more complicated and organized embryo.
In general, development of animals includes the following phases.
Gametogenesis:
The prerequisite for the formation of an embryo is gametogenesis, a process which occurs in the reproductive organs of mature adults, by which sex cells or gametes of male and female individuals are produced. Formation of male gametes or spermatozoa in the testes is known as spermatogenesis and female gametes or ova as oogenesis.
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By spermatogenesis, motile, small, haploid spermatozoa with a head, middle piece and long tail or flagellum are produced. The head consists of a large nucleus with very little cytoplasm but with a pointed anterior acrosome or formed of Golgi complex.
The acrosome is useful in penetration into the ovum at the time of the fertilization. The middle piece lodges the mitochondria and provides energy for the movement. The flagellum or tail is the locomotor organ that helps in propulsion in the female genital cavities or movement in water. In contrast, the ova are larger in size, contain nutrient rich yolk in large volume of cytoplasm and non motile. Gametogenesis involves meiotic division where the diploid number of chromosomes is reduced to half or haploid condition.
Fertilization:
The union of male and female gametes to restore the diploid number of chromosomes in the zygote is called as fertilization. The prerequisite for fertilization to occur is the encounter of the male and female gametes.
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Fertilization occurs in several steps:
(i) encounter of the sex cells which usually occurs at the time of copulation in most animals or externally in water,
(ii) fertilizin-antifertilizin reaction in which the male and female gametes approach each other due to chemical attraction and formation of a chemical lock by the release of a mucopolysaccharide fertilizin from the ovum and a protein, antifertilizin from the sperm,
(iii) acrosome reaction in which the acrosome of sperm head penetrates through the protective layers of the ovum. The acrosome derived from Golgi forms a tubule which penetrates through the protective layers of the egg by releasing certain enzymes that dissolve the membranes surrounding the egg. The tubule fuses with the plasma membrane of the ovum and establishies a clear passage between the sperm head and the cytoplasm of ovum,
(iv) Migration of the sperm head and middle piece through the acrosomal tubule into the ovum,
(v) fertilization cone formation in which the cortical granules of the ooplasm form an elevated cone with the plasma membrane of the ovum at the site of penetration of the sperm so that further entry of sperm is prevented. The process of cell division by mitosis is initiated by replication of DNA and metabolic rate is accelerated
(vi) Sperm nucleus is transformed into a pronucleus and the male and female pronuclei migrate to the site of Amphimixis to fuse so that a zygote nucleus is formed. The act of fertilization is a very important stage in sexual reproduction as diploid number of chromosomes is restored.
Cleavage or segmentation:
Repeated mitotic divisions of the fertilized egg to form more and more daughter cell called as blastomeres is known as cleavage or segmentation. During this process, the single called zygote is converted into a compact spherical structure called as morula. The blastomeres soon get organized themselves into a hollow, sperical structure known as blastula enclosing a central cavity or the blastocoel. The single layer of cells surrounding the blastocoel is the blastoderm. The formation of the blastula is the process of blastulation during which no fresh synthesis of the chemical substances other than DNA and proteins occurs. Cell divisions proceed at the expense of the reserve food materials like glycogen and lipids in yolk. The process of cleavage and blastulation is important as the presumptive organ forming areas which are closely packed in ovum are isolated and distributed in the blastomeres.
Gastrulation:
Gastrulation is the process in which the blastula is converted into a three layered embryo, the gastrula. This stage is an intense phase of cellular movement through the blastopore to establish the three primitive germ layers namely ectoderm, endoderm and mesoderm. The movements are of two types, epiboly and emboly.
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Epiboly involves movements of the cell over the surface of the embryo resulting in antero-pasterior elongation and lateral expansion of the gastrula. Emboly, on the other-hand, involves movement of the cell into the interior of the embryo to form the endoderm and mesoderm, through the blast pore.
Important types of cellular movements include involution and convergence invagination and divergence, delamination, etc. The gastrula is an elongated structure enclosing a central cavity or archenteron lined on all sides by endoderm. The archenteron communicates with the exterior through a mintute opening, the blast pore.
During gastrulation, the presumptive organ forming areas are all transferred to their final destination, through mass movement of cells. Cell divisions during this stage become reduced but the process of the transcription to synthesize fresh RNA required for synthesis of proteins and other chemical substances needed for further development of the gastrula.
Organogenesis:
The primitive germ layers formed during gastrulation spilt into groups of cells called as primary organ rudiments and the process formation of organs from the three germ layers is known as organogenesis. The primary organ rudiments further subdivide into secondary organ rudiments which are the initial stages in the formation of organs and their parts. At this stage the embryo acquires resemblances with the adult or a larva.
Growth:
The embryo upto the formation of organ rudiments is very small in size. Growth is the increase in size. This is achieved by increase in the rate of cell divisions involving fresh synthesis of nuclear material and protein synthesis. All the organ rudiments increase in mass and the embryo attains the size and the shape of the adult.
Differentiation:
Differentiation occurs simultaneously with growth and the two processes cannot be separated. Differentiation is the process by which cells and tissues acquire certain characteristic features a and become different from each other. Development always involves growth and differentiation. Differentiation is of several types:
1. Morphological differentiation:
By this process cell and tissues attain their characteristic shape and structure so that they may be distinguished from each other by their physical appearance and internal structure. For example certain ectodermal cells become modified into nerve cells, others into epidermal cells or sensory cells. By morphological differentiation each part of the body takes its characteristic shape, size and structure.
2. Physiological differentiation:
During development, we find that certain groups of cells from the same stock of cells start functioning differently. For example, from ectoderm, some cells become specialized to carry nerve impulses white others become modified to receive external stimuli and get convered into sensory cells or receptor cells. Still others start to secrete sebums and sweat. Thus, the process by which cells acquire their characteristic fuctions is known as physiological differentiation.
3. Chemo differentiation:
The morphological and physiological differentiation of cells is dependent upon the chemical substances contained in them or produced by them. The process, by which cells become different due to their chemical characteristic features is known as chemo differentiation. It’s well known that chemical reactions in cells are catalyzed by enzymes.
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Enzymes are chemically proteins, and proteins are synthesized in the cells. Therefore, the process of chemo differentiation can ultimately be attributed to differences in the protein pattern of the cells.
Some of the proteins synthesized are structural proteins like actin, myosin, collagen, elastin, albumin, etc. while others are enzymes. The type of the protein synthesized is dependent on the DNA molecule in the chromosomes and the mRNA molecules. Thus, the entire pattern of chemo differentiation by which cells differ chemically in their composition and secretary product is ultimatelly dependent upon the DNA and gene expression.
4. Metamorphosis:
Metamorphosis is the last and final stage of embryonic development. We observe in nature that the young ones produced from the eggs resemble the adults in all respects expect for their small size. In other cases, there is no resemblance between the young ones that hatch out from the eggs and the parents. In fact, it becomes very difficult to identify that the young ones belong to a particular parent.
In such cases the young ones do not grow directly to resemble the adults but some intermediate stages intervene between the egg stage and young ones. There are drastic differences between the young and adult. For instance, in hutterflies, houseflies and cockroaches, the eggs hatch out into elongated worm like structures called as larvae.
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The larval stage is followed by a pupa stage and finally a young one resembling the parent emerges from the puparium. In some cases, the larva does not grow into a pupa but undergoes a series of moultings (casting of outer skin) to become a young one. This is the case in crustanceans like prawn, crab, etc.
The process by which the larva is converted into the young is known as metamorphosis. It includes a series of drastic changes in which many morphological and physiological changes occur and the process is controlled by hormones. Certain structures and organs are added while other organs undergo degeneration. In frog tadpole, tail and external gills are lost while limbs are added when the tadpole metamorphoses into young frog.
