Fertilisation | Implantation | Changes during pregnancy | Embryonic development | lactation |

     Fertilization    

The process of fusion of a sperm with an ovum is called fertilization: - 

✔ Semen of a male is transferred by the penis to the vagina of female during copulation (coitus) and this process is called insemination. 

✔ Spermatozoa move through the cervix, enter the uterus and reach the ampullary-isthmic junction of Fallopian tube, where fertilization takes place. 

✔ Fertilization occurs only if the ovum released by the ovaries and sperms are transported simultaneously to the ampullary-isthmic junction of the Fallopian tube. 

✔ During fertilization, a sperm comes in contact with zona pellucida layer of the ovum and induces changes in the membrane that block the entry of other sperms ensuring that only one sperm fertilizes an ovum. 

✔ The secretions of the acrosome help the sperms enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. 

✔This induces the completion of the meiotic division of the secondary oocyte which results into the formation of a haploid ovum (ootid) and a small second polar body. 

✔ The haploid nucleus of the sperms and that of ovum fuse together to form a diploid zygote. 

✔ After fusion of the male and female gametes, the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilized the ovum. The Zygote carrying XX would develop into a female baby and XY would form a male baby. Hence, the sex of the baby is determined by the father and not by the mother. 

✅Implantation: -

 The zygote implantation in uterus occurs in the following steps: - 

• The mitotic division within the zygote termed cleavage starts as the zygote moves Towards the uterus through the isthmus of the oviduct. 
•  It forms 2, 4, 8 and 16 daughter cells called blastomeres. 
• The embryo with 8-16 blastomeres is called a morula. 
•  Morula continues to divide and transforms into blastocyst as it moves further into the uterus. 
•  Blastomeres in the blastocyst are arranged into an outer layer called trophoblast. 

 The inner group of cells attached to trophoblast constitute the inner cell mass.

• Trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. 
• After attachment, the uterine cells divide rapidly and cover the blastocyst. This leads to embedding of blastocyst in the endometrium of the uterus. This is called implantation.

✒ Changes During Pregnancy 

After implantation, following changes occur during pregnancy. 

✅The finger-like projections appear on the trophoblast called chorionic villi, surrounded by the uterine tissue and maternal blood. 

✅ Both the uterine tissue and chorionic villi become interdigitated with each other and jointly form a structural and functional unit between developing embryo and internal body called placenta.

✅ Placenta transports oxygen and nutrients to the fetus and removes carbon dioxide and excretory waste materials produced by the embryo.

✅ Placenta also acts as an endocrine tissue and secretes hormones like human Chorionic Gonadotropin (hCG), human Placental Lactogen (hPL), oestrogen, progesterone, etc. 

✅ A hormone called relaxing is secreted by the ovary, in later phase of pregnancy. 

✅Umbilical cord connects the placenta with fetus and helps in transport of substances to and from embryo. 

✅ During pregnancy, the levels of other hormones like oestrogen, progesterone, cortisol, prolactin, thyroxine, etc., are increased many times in the maternal blood. These hormones support the foetal growth, maintenance of pregnancy and metabolic changes in mother.

✒Embryonic Development: -

It starts after pregnancy and involves the following changes: -

☑ The inner cell mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm,

☑ Middle layer called mesoderm appears between the ectoderm and endoderm. 

☑ Primary germ layers give rise to all the tissues and organs of the adult. The inner cell mass contains certain cells called stem cells, which have the potency to give rise to all the tissues and organs.

☑ In humans, after one month of pregnancy the embryo's heart is formed. The sign of growing foetus can be ensured by listening to the heartbeat. 

☑ By the end of second month, limbs and digits develop 

☑ By the end of third month (first trimester). most of the major organ systems are formed. 

☑ During the fifth month, the first movement of fetus and appearance of hair on the head are observed. 

☑ By the end of sixth month (second trimester the body is covered with fine hair. eyelids separate and eyelashes are formed. 

☑ By the end of eight months, the testes in male fetus descend into the scrotum. 

☑ By the end of nine months, the fetus is fully developed and ready for birth.

 ✒ Parturition: -

The process of delivery of fetus (childbirth) is called parturition. 

✔ The average duration of human pregnancy is about 9 months, which is called gestation period. 

✔Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the fetus. 

✔ Parturition is induced by complex neuro endocrine mechanism. 

✔ Relaxin hormone is secreted by the ovary to facilitate parturition by softening the connective tissue of symphysis pubica. 

✔ Fetus and the placenta induce mild uterine contractions called fetal-ejection reflex. This initiates the release of oxytocin hormone from the posterior pituitary.

✔ Oxytocin acts on uterine muscle to cause stronger contractions, which in turn stimulate further secretion of oxytocin. This causes more stronger contractions leading 1o the expulsion of the baby out of the uterus through the birth canal. 

✔ After the baby is delivered, the placenta is also expelled out of the uterus.

✒Lactation: - 

Parturition is followed by the lactation which requires certain changes in the mother's body. 

•  Mammary glands of the female undergo differentiation during pregnancy 
•  By the end of pregnancy, this start producing milk by the process called lactation. 
•  During the initial few days of lactation, the milk produced is called colostrum.

 It contains several antibodies (IgA) and nutrients essential to develop resistance in the newborn baby.