The process of giving birth to young ones identical to parents is called reproduction. Reproduction is the most crucial biological process that must take place for the continuity of species.
In humans, reproduction is always by sexual means, that is, humans can only reproduce sexually. And the process of reproduction in humans involves internal fertilization by sexual intercourse. In the process of human reproduction fusion of gametes from each of the parents results in the formation of offspring. The mix up of gametes in this process produces genetically as well as physically different individuals. That means every human individual has a unique genetic make-up, unique eye colour, and a unique identity of its own.
We will study all about reproduction in human beings in this article
Male and female reproductive systems are different in human beings. As a result, humans possess sexual dimorphism. It is the difference in characteristics of individuals of the same species beyond the differences in their sexual organs. And the reproduction process involves the fusion of male and female gametes that are produced in their respective reproductive system. Both the male and female reproductive systems are different from each other structurally and physiologically.
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Male gametes are sperms. Sperms are produced within the male reproductive system. About 200-300 million sperms are ejaculated at once by a healthy male individual. Sperms are unicellular structures that consist of the following three parts-
Head - It contains acrosome apically, which contains enzymes that facilitate entry of sperm into the ovum.
Middle Piece - that has multiple mitochondria, and its main function is to provide energy for the movement of sperms.
Tail - It is a flagellum that protrudes out of the cell body and it provides vigorous motility to sperms. The tail helps sperms to swim with great speed so that they can reach towards the ovum.
Testicles or Testes: These are a pair of oval-shaped organs masked in a pouch called the scrotum and are responsible for maintaining a healthy reproductive system. The pair of testes produce sperms and the male sex hormone testosterone.
Scrotum: Scrotum is a sac-like organ hanging below the penis and behind it. It is where the testicles are placed. It maintains the optimum temperature necessary for the production of sperms.
Vas Deferens: The sperms produced in testes are stored epididymis, which is a tube-like structure. Here the sperms mature and pass to urethra through the muscular tube. This muscular tube is known as Vas deferens.
Accessory Glands: This includes three glands- seminal vesicles, prostate gland, and Cowper’s gland. All of the three glands secrete fluid. These secretions mix to form a fluid called semen. Semen is produced to nourish the sperm, increase the volume and help in lubrication.
Penis: Penis is the external organ, which is a cylindrical tube. It serves as both a reproductive organ as well as an excretory organ in human males. It delivers sperms into the vagina along in the fluid semen during sexual intercourse.
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The female reproductive system consists of internal and external sex organs structurally and functionally working together to complete the process of reproduction right from ovulation to birth of the young one. Various parts of the female reproductive system are-
a) Ovaries: Every female individual has a pair of ovaries located internally. The main function of ovaries is to produce ovum and female reproductive hormones estrogen and progesterone. Peripheral cortex is present at the periphery of the ovarian stroma and inner medullary region in the ovarian structure.
b) Fallopian Tubes: Fallopian tubes are a pair of tubes connecting the ovaries and the uterus. They serve as a pathway for sperms to travel to the egg. It has a funnel-shaped part called the infundibulum present near the ovary. The infundibulum terminates into finger-like projections called fimbriae. Their main function is to collect the ovum from the ovaries. The Infundibulum connects to the uterus through the ampulla and isthmus.
c) Uterus or Womb: Uterus is the pear-shaped part where the embryo development takes place. The uterine wall has three differentiated layers-
Perimetrium - It is the membranous outermost layer.
Myometrium - It is made up of smooth muscles and forms the middle layer. Myometrium causes strong contraction during child delivery.
Endometrium - It is the lining that undergoes cyclic changes during the menstrual cycle. The endometrium is a glandular lining. During the developmental stages, blastocyst gets embedded in the endometrium.
Cervix - It is a narrow part that connects the uterus to the vagina. The vagina and the cervical canal together form the birth canal during childbirth.
d) External Genitalia: Mons pubis (fatty tissue), labia majora and labia minora (tissue folds) and clitoris form external female genitalia and the hymen membrane cover the vaginal opening.
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The fertilization takes place when one of the million sperms fuses an egg. It occurs at the ampullary region of the fallopian tube.
Only one sperm can fertilize an egg at a time during fertilization. This is because when one of the sperms comes in contact with the zona pellucida of the ovum, no more sperms can enter further the same ovum.
On the entry of the sperm, Second meiotic division of the secondary oocyte initiates. As a result, a large ootid (haploid) and the secondary polar body is formed.
Sperm and ovum fuse and form the zygote.
The developing fetus’s sex is determined at this stage. There are two types of male gametes in spermatogenesis. One is with the X chromosome and one with the Y chromosome. If the sperm containing X chromosome fertilizes the ovum, the zygote develops into a female and when sperm with the Y chromosome fertilizes the ovum, the zygote develops into a male.
The zygote forms blastomeres by dividing mitotically. Then it moves towards the uterus through the oviduct.
Fertilization is followed by implantation. Implantation is the beginning of pregnancy.
1. What is the Gestation Period of Humans?
In humans, the gestation period is 9 months. These 9 months are divided into three trimesters. Various developmental stages through the three trimesters are-
First Trimester - The first three months from the day of implantation is the first trimester. During this trimester, the baby’s heart, limbs, digits and all the major organs are developed and the fetus is recognizable as a human being. It is at this trimester that the sex of the fetus can also be determined.
Second Trimester - The fetus starts to move from 5th month and hairs on the head appear. During the second trimester, itself eyelashes and eyelids develop, and the body gets covered with fine hair.
Third Trimester - During this trimester, the development of most of the organs is in the final stage and the body fat accumulates on the body of the baby.
By the end of the third trimester, the child is fully developed and the body prepares for the delivery of the baby.
2. How Does Embryonic Development Take Place? Explain in Short.
Cells of the embryo start differentiating into outer ectoderm, inner endoderm and then the middle mesoderm following implantation. These three layers are the primary germinal layers. The adult tissues develop from these three layers. Stem cells are also formed and are found in the inner cell mass.
Development of placenta takes place which is the link between the growing fetus and the mother. Interdigitation of uterine cells and chorionic villi (finger-like projections), protruding from trophoblasts lead to the formation of the placenta.
Umbilical Cord is formed which connects the placenta to the fetus and transports nutrients.
Placenta produces hormones necessary for maintaining pregnancy and for fetal growth.
Hormones HCG human gonadotropic hormone and HPL human placental hormone are only produced during pregnancy which are also detected in urine for the pregnancy test.
Levels of many hormones like progesterone, estrogen, prolactin, cortisol and thyroxine, etc. increase to support the fetal growth.