Human Reproduction PDF

## Human Reproduction ### 2.3 Oogenesis - The process of formation of mature female gametes (ova) is called **oogenesis**. - In the female fetus, the primordial germ cells multiply by mitosis to form **oogonia**. - The oogonia undergo meiosis and differentiate into **primary oocytes**. These oocyt...

## Human Reproduction ### 2.3 Oogenesis - The process of formation of mature female gametes (ova) is called **oogenesis**. - In the female fetus, the primordial germ cells multiply by mitosis to form **oogonia**. - The oogonia undergo meiosis and differentiate into **primary oocytes**. These oocytes are surrounded by layers of follicular cells. - In the primary oocyte, the first meiotic division is completed but it is arrested in prophase I of meiosis I. - **At puberty**, some primary oocytes are activated to continue their development. They complete meiosis I to form a **secondary oocyte** and a **first polar body**. The secondary oocyte retains bulk of the nutrient rich cytoplasm of the primary oocyte. - The first polar body born out of the first meiotic division doesn't divide further or degenerate. - The tertiary follicle further changes into the mature follicle or **Graafian follicle**. - The secondary oocyte forms a new membrane called **zona pellucida** surrounding it. - The Graafian follicle ruptures to release the secondary oocyte (ovum) from the ovary by the process called **ovulation**. ### 2.4 Menstrual Cycle - The reproductive cycle in female primates is called the **menstrual cycle**. - The first menstruation begins at puberty and is called **menarche**. - In humans, menstruation is repeated on average every 28-29 days. - The events starting from one menstruation to the next is called the **menstrual cycle**. - One ovum is released during the **middle** of each menstrual cycle. #### Events during a menstrual cycle - **Menstrual phase:** This phase lasts for 3-5 days. Menstrual flow occurs due to breakdown of the endometrium lining of the uterus and its blood vessels which forms liquid that comes out through the vagina. This phase occurs if the released ovum is not fertilized. - **Follicular phase:** This phase starts after the menstrual phase and continues for about 10-14 days. The primary follicles in the ovary grow to become a fully mature **Graafian follicle**. Changes in the levels of pituitary and ovarian hormones induce changes in the endometrium of the uterus. - **Ovulatory phase:** This phase is the middle of the menstrual cycle, occurring around the 14th day. The LH surge induces the rupture of the Graafian follicle and the release of the ovum. - **Luteal phase:** During this phase, the remaining parts of the Graafian follicle transform into the **corpus luteum**. The corpus luteum secretes large amounts of progesterone, which is essential for the maintenance of the endometrium. ### 2.5 Fertilization and Implantation - During copulation, semen is released by the penis into the vagina. - The motile sperm swims rapidly, pass through the cervix, enters the uterus and finally reach the ampullary region of the fallopian tube. - The ovum released by the ovary is also transported to the ampullary region. - Fertilization can only occur when the ovum and sperms are transported simultaneously to the ampullary region. - The process of fusion of a sperm with an ovum is called **fertilization**. - During fertilization, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. - The secretions of the acrosome help the sperm 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. - The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). - The haploid nucleus of the sperm and that of the ovum fuse together to form a diploid zygote. - The sex of the baby is determined by the father and not by the mother. - The mitotic division starts as the zygote moves through the Isthmus of the oviduct towards the uterus and forms 2, 4, 8, 16 daughter cells called **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. - The blastomeres in the blastocyst are arranged into an outer layer called **trophoblast** and an inner group of cells called **inner cell mass**. - The trophoblast layer gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. - The blastocyst becomes embedded in the endometrium of the uterus. This is called **implantation**. ### 2.6 Pregnancy and Embryonic Development - After implantation, finger-like projections appear on the trophoblast called **chorionic villi**. - Chorionic villi and the uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (fetus) and maternal body called **placenta**. - The placenta facilitates the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and excretory/waste materials produced by the embryo. - The placenta is connected to the embryo through an **umbilical cord**. - Placenta acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens, etc. Later in pregnancy, a hormone called **relaxin** is secreted by the ovary. - hCG, hPL and relaxin are produced in women only during pregnancy. - Levels of other hormones like estrogens, progestogens, cortisol, prolactin, thyroxine, etc. are increased several-folds in the maternal blood. - Increased production of these hormones is essential for supporting the fetal growth, metabolic changes in the mother and maintenance of pregnancy. - Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer called **ectoderm** and an inner layer called **endoderm**. - A **mesoderm** soon appears between the ectoderm and the endoderm. - The three layers give rise to all tissues (organs) in adults. - The inner cell mass contains certain cells called **stem cells**. - Stem cells have the potency to give rise to all the tissues and organs. #### Embryonic Development - Human pregnancy lasts 9 months. - By the end of the first month of pregnancy, the embryo's heart is formed. - By the end of the second month of pregnancy, the foetus develops limbs and digits. - By the end of 12 weeks (first trimester), most of the major organ systems are well-developed. - By the end of 24 weeks (end of the second trimester), the body is covered with fine hair, eye-lids separate, and eyelashes are formed. - By the end of nine months of pregnancy, the foetus is fully developed and is ready for delivery. ### 2.7 Parturition and Lactation - The average duration of human pregnancy is about 9 months. - This is called the **gestation period**. - The process of delivery of the fetus (childbirth) is called **parturition**. - Parturition is induced by a complex neuroendocrine mechanism. - Signals for parturition originate from the fully developed fetus and the placenta. - This triggers release of oxytocin from the maternal pituitary. - Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin. - This leads to expulsion of the baby out of the uterus through the birth canal - parturition. - Soon after the infant is delivered, the placenta is also expelled out of the uterus. - The mammary glands of the female undergo differentiation during pregnancy and starts producing milk towards the end of pregnancy by the process called **lactation**. - This helps the mother in feeding the newborn. - The milk produced during the initial few days of lactation is called **colostrum**. - Colostrum contains several antibodies absolutely essential to develop resistance for the newborn babies. - Breastfeeding during the initial period of infant growth is recommended by doctors for bringing up a healthy baby.