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Reproduction


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Reproduction is basically the tendency or ability of an organism to procreate, reproduce, breed, produce offspring, give birth to young ones, multiply, etc.


It is a biological process by which organisms ensures the continuity of their species and ultimately their existence. Reproduction is fundamentally unique to all known forms of life on earth. If for whatever reason, should this fundamental feature seize in any population or organism, the population of such species of organisms begins to drop gradually and over a period of time, the organisms will face extinction.



Forms of Reproduction

There are basically two forms of reproduction, which are:
  • Asexual reproduction, and 
  • Sexual reproduction.


Asexual reproduction

Asexual reproduction involves the ability of an organism to reproduce WITHOUT involving another organism. In this form of reproduction, the organism regenerates or produce offspring that are genetically (morphologically, physiologically and anatomically) identical to the reproducing organism (single parent). This form of reproduction can be achieved through different methods, some of which are:

  1. Cloning, 
  2. Budding, 
  3. Layering, 
  4. Binary fission, 
  5. Cutting, 
  6. Grafting, etc.
The exact type of asexual reproduction used depends on the type of organism. Here, the reproducing organism creates a genetically identical or similar copy of itself as a progeny or offspring. In some instances, the parent organism just replicates or divides through a process called binary fission. The individual cells of some of these reproducing organisms are said to be totipotent. Totipotency refers to a phenomenon where cells retain their ability to grow into a completely new organism, which are identical to the ones they were extracted from if placed in a suitable environment or conditions.



Sexual reproduction

This is a biological process which requires the fusion of two specialized sex cells (gametes) produced by organisms of the same species to form zygote. The gametes include the female sex cell (egg or ovum) and the male sex cell (sperm or spermatozoa). The zygote is the product of fusion of the male and female sex cells. The act of this fusion is called fertilization. Each gamete is haploid (n). That is, it contains half the number of chromosomes found in a normal cell.


Each zygote inherits 23 chromosomes from its mother (ovum) and 23 chromosomes from its father (sperm). This however, does not produce 46 different chromosomes, but rather produces a unique pair of 23 homologous chromosomes, hence making it diploid (2n). That is, containing the same number of chromosomes found in normal cells. In humans, the 23rd pair of chromosomes could either be an XY or an XX. This helps in defining the sex of an individual.


A gene on the Y chromosomes causes the embryonic gonads to differentiate into testes (for males). Females however lack a Y chromosome, and the absence of this gene results in the development of ovaries.


The embryonic testes secrets the hormone called testosterone, which triggers the development of male accessory sex structures. The absence of testes in female embryo causes the development of female accessory sex structures.


In mammals, more importantly in humans, sexual reproduction typically involves sexual intercourse, copulation or coition between a sexually mature male and a female. During this process, an erect penis is inserted into an aroused and receptive vagina. The interaction between the male and the female sex structures results in the release of gametes (spermatozoa) into the female reproductive tracts (vagina), through a wave-like contraction of the pelvic muscle, forcing these secretions (semen) out through the urethra. A process called ejaculation, which marks the climax of sexual intercourse. These interactions results in fertilization of the egg within the female reproductive tracts by the spermatozoa. The fusion of the male and female sex cells results in the formation of the zygote, which clearly marks the point of genetic recombination.


Pregnancy

This is the period from fertilization (that is, formation of the zygote) to delivery. It is the period of time during which the foetus develops within the female body. During this period, the developing foetus receives all of its nutrients and oxygen from the mother through the placenta which is attached to the abdomen of the developing foetus through the umbilical cord. Waste products such as urea and Carbon dioxide are excreted also through the umbilical cord and placenta to the mother's blood.



Parturition

Parturition which is also called childbirth involves the sequence of events which are basically called LABOUR. Gestation period which is the period of prenatal development or the period of pregnancy is usually 266 days, or about 280 days from the beginning of the last menstrual period to the time of childbirth.



N/B: most foetus are born within 10 - 15 days before or after the calculated of delivery date.



Once the foetus is fully and/ or sufficiently mature or developed, chemicals signals (hormones such as oxytocin, prostaglandins, etc) begins the process of childbirth. At the end of parturition, the foetus is pushed out through the birth canal (vagina), a process which is accompanied by a sequence of physiological and physical events called labour.


The onset of labour is denoted by rhythmic and forceful contractions of the myometrial layer of the uterus. It is important to note that Labour is divided into three stages, which are:


  • Dilation stage: this is the stage where the cervix dilates to a diameter of approximately 10cm. There are regular contractions during this stage, and the amniotic sac generally ruptures.

  • Expulsion stage: this is the stage where parturition or actual childbirth occurs. This stage consists of forceful uterine contraction and abdominal compressions to expel the foetus from the uterus through the vagina.

  • Placental stage: generally, at this stage, within 10 to 15 minutes after delivery, the placenta separates from the uterine wall and is expelled as the afterbirth. This stage is characterized by forceful uterine contractions, and also constricting uterine blood vessels to prevent haemorrhage.



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