Question

How does a single-celled fertilized egg grow into an adult in human beings?

Answer 1

Hint: As we all know, fertilization in human beings is the union of male and female gametes. Fertilization occurs in the ampullary-isthmic region of fallopian tubes in females and results in a zygote. It was discovered by scientists in the nineteenth century.

Complete answer:
When a single celled ovum (female gamete) fuses with a single celled sperm (male gamete), the formation of diploid zygote takes place. This process is called fertilization.
The process with male partner ejaculating semen into the vagina of a female during copulation. Semen is a greyish white fluid which contains the spermatozoa, fructose and other enzymes that help for its survival. A sperm has a head, neck, middle piece, and tail. The head is covered with a cap-like structure called acrosome, it has an enzyme called hyaluronidase, it helps in fertilization.
The motile sperms from semen swim and pass through the cervix, enter the uterus and reach the ampulla and ovary released ovum is also transported to the ampulla and hence the fertilization takes place, and the zygote is formed.
Embryo formation: Then the mitotic division of the zygote starts, which is called cleavage. The formation of 2,4,8,16 daughter cells called blastomere takes place. The embryo with 8-16 blastomeres is called morula. The morula divides and forms into a blastocyst and moves further in the uterus.
Implantation: The blastomeres in the blastocyst start arranging into an outer layer called trophoblast and the inner group of cells get attached to trophoblast and hence the formation of inner cell mass. The trophoblast layer then gets attached to endometrium and inner cell mass gets differentiated as an embryo. As blastocyst gets embedded in the endometrium, implantation takes place followed by pregnancy.
After implantation, chorionic villi appear on trophoblast. The chorionic villi and uterine tissues together form placenta. It supplies oxygen and nutrients to the embryo and also carries out the waste material. It is connected to the embryo by umbilical cord. Placenta also acts like an endocrine tissue and produces various pregnancy hormones such as hPL, hCG, estrogen, progesterone. Later during the pregnancy, a hormone relaxin is released by the ovaries.
 After implantation, the embryo differentiates into three layers ectoderm (outer), mesoderm (middle) and endoderm (inner). These layers give rise to tissues, organs, etc.
The average duration of human pregnancy is 9 months which is generally called gestation period. Then comes the childbirth which is termed as parturition. Oxytocin is released during childbirth and acts on uterine muscles. The strong uterine contractions cause the birth of the baby.
Later, the mother undergoes various bodily changes and starts producing milk which is called lactation.

Note: Humans reproduce sexually and are viviparous (internal fertilization).
Also, the sex of the baby is determined by the father as he carries XY chromosome and female carries XX chromosome.
Sometimes, during pregnancy the fetus develops outside and such pregnancies are painful and not successful. It is a very rare condition.