Fertilization and Implantation | Biology Class 12 - NEET PDF Download

Introduction

From Ovulation to ImplantationDuring copulation (coitus), semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly, passing through the cervix, entering the uterus, and finally reaching the ampullary region of the fallopian tube . The ovum released by the ovary is also transported to the ampullary region, where fertilisation occurs. Fertilisation can only take place if the ovum and sperms are transported simultaneously to the ampullary region. This is why not all instances of copulation result in fertilisation and pregnancy.

Fertilisation Process

• The process of fusion between a sperm and an ovum is known as fertilisation. When a sperm comes into contact with the zona pellucida layer of the ovum , it induces changes in the membrane that block the entry of additional sperms. This ensures that only one sperm can fertilise an ovum.
• The secretions of the acrosome help the sperm enter the cytoplasm of the ovum through the zona pellucida and the plasma membrane. This action triggers the completion of the meiotic division of the secondary oocyte.

• The second meiotic division is also unequal, resulting in the formation of a second polar body and a haploid ovum (ootid). Shortly after, the haploid nuclei of the sperm and the ovum fuse together to form a diploid zygote.

Sex Determination 

• The zygote inherits 46 chromosomes—23 from the sperm and 23 from the ovum. The sex of the baby is determined at this stage based on the sex chromosomes contributed by the parents.
• Human females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). All female gametes (ova) carry an X chromosome, while male gametes (sperms) carry either an X or a Y chromosome. Thus, 50% of sperms carry X and 50% carry Y.
• If a sperm carrying an X chromosome fertilises the ovum, the zygote will be XX (female). If a sperm carrying a Y chromosome fertilises the ovum, the zygote will be XY (male).
• Therefore, the sex of the baby is determined by the father, not the mother.

Cleavage and Development of Zygote

• As the zygote travels through the isthmus of the oviduct towards the uterus, it undergoes mitotic divisions, creating 2, 4, 8, 16 daughter cells known as blastomeres.
• The embryo with 8 to 16 blastomeres is called a morula.
• The morula continues to divide and transforms into a blastocyst as it moves further into the uterus.
• In the blastocyst, the blastomeres are arranged into an outer layer called trophoblast and an inner group of cells called the inner cell mass.

Transport of ovum, fertilisation and passage of growing embryo through fallopian tube

Attachment and Implantation

• The trophoblast layer attaches to the endometrium, and the inner cell mass differentiates into the embryo.
• After attachment, the uterine cells divide rapidly and cover the blastocyst, leading to its embedding in the endometrium of the uterus. This process is called implantation, which marks the beginning of pregnancy.