Detailed Notes : 2 – Chapter2-Human Reproduction

2.4 MENSTRUAL CYCLE

1. Definition and Menarche:

   • Reproductive cycle in female primates, including humans, called menstrual cycle.
   • Begins at puberty with the first menstruation, termed menarche.

2. Cycle Length and Ovulation:

   • Menstruation occurs at an average interval of about 28/29 days, from one menstruation to the next.
   • One ovum is released (ovulation) during the middle of each menstrual cycle.

3. Major Events:

   • Menstrual Phase: Lasts 3-5 days, characterized by menstrual flow due to breakdown of endometrial lining of the uterus and its blood vessels. Occurs if released ovum is not fertilized.
   • Follicular Phase: Primary follicles in the ovary grow into mature Graafian follicles. Simultaneously, the endometrium of the uterus regenerates through proliferation. Stimulated by changes in pituitary and ovarian hormones (LH and FSH).
   • Ovulation: Occurs around the 14th day of the cycle, triggered by a rapid secretion of LH (LH surge), inducing rupture of Graafian follicle and release of ovum.
   • Luteal Phase: Remaining parts of the Graafian follicle transform into the corpus luteum, which secretes progesterone essential for maintenance of the endometrium. Necessary for implantation of the fertilized ovum and pregnancy events.

4. Pregnancy and Menopause:

   • During pregnancy, all events of the menstrual cycle cease, and there is no menstruation.
   • In absence of fertilization, the corpus luteum degenerates, leading to disintegration of the endometrium and menstruation, marking a new cycle.
   • Menstrual cycles cease around 50 years of age, termed menopause.

Menstrual Hygiene

1. Regular Cleaning:

   • Take regular baths and clean yourself thoroughly, especially during menstruation.

2. Use of Sanitary Products:

   • Use sanitary napkins or clean homemade pads to absorb menstrual blood.

3. Regular Changes:

   • Change sanitary napkins or homemade pads every 4–5 hours or as per individual requirement to prevent bacterial growth and odor.

4. Proper Disposal:

   • Dispose of used sanitary napkins properly by wrapping them in used paper or a disposal bag.
   • Do not throw used napkins in drainpipes or open areas, as they can clog pipes and pose environmental hazards.

5. Hand Hygiene:

   • After handling sanitary products, wash your hands thoroughly with soap and water to maintain hygiene and prevent the spread of germs.

2.5 FERTILISATION AND IMPLANTATION

1. Insemination and Transport of Sperms:

   • During copulation (coitus), semen is released into the vagina, and motile sperms swim through the cervix into the uterus, reaching the ampullary region of the fallopian tube.
   • The ovum released by the ovary is also transported to the ampullary region, where fertilisation takes place.

2. Fertilisation Process:

   • Fertilisation occurs when a sperm fuses with an ovum.
   • Sperm contacts the zona pellucida layer of the ovum, inducing changes that block entry of additional sperms, ensuring only one sperm fertilises the ovum.
   • Secretions of the acrosome help the sperm enter the cytoplasm of the ovum, inducing completion of meiotic division of the secondary oocyte.
   • Second meiotic division results in formation of a second polar body and a haploid ovum (ootid).
   • Haploid nucleus of the sperm and ovum fuse to form a diploid zygote.

3. Determining Sex of the Baby:

   • Zygote carries either XX or XY chromosomes, determining the sex of the baby.
   • Female gametes (ova) carry X chromosome, while male gametes (sperms) carry either X or Y chromosome.
   • Zygote carrying XX chromosomes develops into a female baby, and XY chromosomes develop into a male baby.

4. Embryonic Development:

   • Mitotic division starts as the zygote moves through the isthmus of the oviduct towards the uterus, forming daughter cells called blastomeres.
   • Embryo with 8 to 16 blastomeres is called a morula.
   • Morula continues to divide and transforms into a blastocyst as it moves further into the uterus.
   • Blastocyst consists of an outer layer called trophoblast and an inner group of cells called the inner cell mass.

5. Implantation and Pregnancy:

   • Trophoblast layer attaches to the endometrium, and inner cell mass differentiates into the embryo.
   • Blastocyst becomes embedded in the endometrium of the uterus, leading to implantation and pregnancy.

2.6 PREGNANCY AND EMBRYONIC DEVELOPMENT

1. Placental Development:

   • After implantation, chorionic villi appear on the trophoblast, interdigitating with uterine tissue to form the placenta.
   • Placenta facilitates exchange of oxygen, nutrients, and waste materials between maternal and embryonic blood.
   • Connected to the embryo through an umbilical cord, which transports substances to and from the embryo.
   • Acts as an endocrine tissue, producing hormones like hCG, hPL, estrogens, progestogens, and relaxin.

2. Embryonic Differentiation:

   • After implantation, the inner cell mass (embryo) differentiates into three layers: ectoderm, endoderm, and mesoderm.
   • These layers give rise to all tissues and organs in adults.
   • Inner cell mass contains stem cells capable of giving rise to all tissues and organs.

3. Features of Embryonic Development:

   • Human pregnancy lasts 9 months, while duration varies in other animals like dogs, elephants, and cats.
   • During human pregnancy:
     • By one month, embryo’s heart is formed, and heartbeat may be detected.
     • By end of second month, foetus develops limbs and digits.
     • By 12 weeks (first trimester), major organ systems are formed, and limbs and external genital organs are well-developed.
     • First movements of foetus and appearance of hair on the head observed during fifth month.
     • By 24 weeks (end of second trimester), body covered with fine hair, eyelids separate, and eyelashes form.
     • By nine months, foetus fully developed and ready for delivery.

2.7 PARTURITION AND LACTATION

1. Parturition (Childbirth):

   • Human pregnancy lasts about 9 months, known as the gestation period.
   • Parturition is the process of delivery of the foetus, induced by vigorous uterine contractions.
   • Signals for parturition originate from the fully developed foetus and placenta, inducing mild uterine contractions (foetal ejection reflex).
   • Foetal ejection reflex triggers release of oxytocin from maternal pituitary, causing stronger uterine contractions.
   • Stimulatory reflex between uterine contraction and oxytocin secretion continues, leading to expulsion of the baby through the birth canal.
   • Placenta is expelled soon after the baby’s delivery.

2. Induction of Delivery:

   • Doctors may inject oxytocin to induce delivery, enhancing uterine contractions and facilitating childbirth.

3. Lactation:

   • Mammary glands undergo differentiation during pregnancy and start producing milk towards the end of pregnancy, a process called lactation.
   • Milk produced during initial days of lactation is called colostrum, containing essential antibodies for newborn’s immunity.
   • Breastfeeding during infancy is recommended by doctors for optimal infant growth and development.