Direct Development in Animals

The developmental study of development in animals or any organism comes under the branch of developmental biology. Developmental biology is a branch of biology that studies the trajectory of change that an organism undergoes from its fertilized egg state to a fully developed organism. During this development, the change in morphology, physiology, and activation, and inactivation of genes all come under the developmental biology of the organism. The process of fertilization is also studied under this branch. 

Development can be referred to as growth and changes associated with growth patterns from fertilization till the formation of an adult organism. Development can be broadly divided into two categories: direct development and indirect development.

Direct Development

Direct development can be defined as the process of development whereas an organism is born quite similar to its parent population except that they lack fully developed sexual organs. The organism born does not undergo drastic transition or metamorphosis. One of the major characteristics is the lack of metamorphosis. Nonetheless, it can not be considered that the progeny of species that follow direct development does not change, the major change marking the difference between a juvenile or young progeny and an adult organism is the fully developed sex organ. 

Maturity among the species that follow direct development patterns is marked by the onset of their sexual maturity. One of the more obvious morphological features is the growth of the organism. An increase in cell number, cell mass, and gain of function are some of the markers of growth on the embryonic stage, whereas an increase in height, weight, change in body structure, and appearance of secondary sexual characters are the predominant markers of growth in the morphology of an organism. 

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Growth in Organism Following Direct Development Pattern

Growth in simple words can be defined as the irreversible change in number and size of the cell, due to cell division and enlargement of the cell size. The growth rate is maximum at the early stages of life, it is specifically highest during the early stages of embryonic development and post-embryonic development. The growth rate of an individual gradually decreases with time, generally, with the onset of sexual maturity, most of the energy is allocated toward reproductive growth rather than the physiological development of an organism. The main reason behind it is that reproduction is energetically expensive and ensures the survival of the population.

Growth can be studied by classifying it into two categories: growth is during embryonic development and growth of newborn till maturity.

Embryonic Development- A very precise study begins with the fertilization process, it can be defined as the process of fusion of male and female gametes to produce a viable zygote. Fertilization in humans occurs in the ampulla region of the fallopian tube in humans. It is a common pattern that is observed that an organism that performs internal fertilization undergoes direct development. But it is not true in every case as it has some exceptions. After fertilization development starts, successful fertilization leads to the formation of the single-celled zygote, the zygote undergoes several mitotic divisions to form the embryo. Zygote follows mitotic division following the pattern of the single-cell stage (n) to the two-cell stage to four-cell stage and then morula, which is followed by blastula.

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Morula-  It is the 16 to 32 cell stage, all the cells of the embryo at this stage are totipotent. Among humans, the morula stage is reached after 4 days of fertilization, they do not have any cavities. A significant characteristic is the presence of epithelial cadherin, (these are a surface adhesion molecule of the cell which are vital for development). 

Blastula-  It is referred to as the 33-100 cell stage of the embryo. During this cavitation that takes place that is the formation of a cavity, this fluid-filled cavity is known as blastocoel, the membrane around it is known as blastoderm. It is the specific feature of the blastula.

Fetus- After completing gastrulation, the embryo increases in cell number and size and is now termed a fetus. Organ development starts during this period. After the complete development, the fetus undergoes parturition (birth of the baby). The time of the development from a zygote to a newborn is the gastrulation period.

Growth of Organism

After parturition, the organism grows to attain maturity. For almost all the organism that undergoes direct development attainment of sexual maturity is the most important change. Growth rate decreases over time, and it depends upon various external and internal factors among which growth factor (growth hormone) is the most important internal factor. Growth factor determines the overall development of the organism including its sexual maturity.

Growth Hormone- Growth hormone is secreted by the hypophysis, under low concentration dwarfism is developed, under high concentration of growth hormone gigantism develops. Growth factors are responsible for the cell division, as every cell has a receptor to which it binds, binding of growth hormone starts a signalling cascade that leads to transcription and translation of protein required for cell divisions Follicle-stimulating. 

Sexual Maturity- Several hormones work to attain this, the reproductive organs remain in the rudimentary stage till puberty. Follicle-stimulating hormone and luteinizing hormone are the main hormones that help in achieving sexual maturity. The hypothalamus releases GnRH (gonadotropin releasing hormone) which acts on the anterior pituitary which releases FSH (follicle stimulating hormone)  and LH (luteinizing hormone). Both of them stimulates the production of sex hormones i.e. estrogen, progesterone, and testosterone in males and females.

1. In Females-  

• FSH (follicle stimulating hormone) promotes the development of follicles in the ovary. It stimulates the production of estrogen during the follicular phase, it assists in the development of follicle to Graafian molecules.

• LH (luteinizing hormone) concentration increases during the luteal phase. It promotes the formation of the corpus luteum and stimulates progesterone secretion. Both the hormones and the sex hormone are now completely regulated by a feedback loop and show a cyclical decrease and increased concentration.

2. In Males- 

• LH acts on the interstitial cells of the male reproductive system and stimulates the production of testosterone. It is responsible for the secondary sexual characters of males. FSH acts on Sertoli cells and stimulates the production of inhibin and activin, which are involved in the regulation of FSH and LH.