CBSE Class 12 Biology Human Reproduction Notes Set B

• Reproductive system is a collection of internal and external organs in both males and females that work together for purpose of producing a new generation of living organisms similar to their parents.
• Human beings reproduce sexually and are viviparous i.e., they give birth to young ones.
• Reproductive units are specialised cells called gametes, which are of two types: male gamete (spermatozoa) and female gamete (ova).
• These sex cells or gametes are formed in separate, paired organs of mesodermal origin, called gonads.
• The gonads that produce male gametes or sperms are called testes, while those producing female gametes or ova are termed as ovaries.

• Secondary sex characters are external features which provide distinctiveness to the two sexes. They have no direct role in sexual reproduction phenomenon when male and female individuals are differentiated externally is called sexual dimorphism.
• Puberty is the period when primary sex organs become functional and starts secreting sex hormones which bring about development of secondary sex organs and appearance of secondary sex characters. These changes occur in response to rising levels of gonadal hormones. It occurs approximately between 10 and 15 years in both males and females.
• Adolescence is the period between puberty to complete sexual maturity. Generally, it is considered to start with the development of secondary sexual characteristics and ends when physical growth of body slows down. In humans, age 11 to 20 years for boys and 10 to 18 years for girls is the adolescence period.

THE MALE REPRODUCTIVE SYSTEM

• Male reproductive system is located in the pelvis region. It includes a pair of testes along with accessory ducts, glands and the external genitalia.
• Testes are paired structures which lie outside the abdominal cavity in a thin pouch of skin called scrotum. The scrotum helps in maintaining the low temperature of the testes (2–2.5°C lower than the normal internal body temperature ) necessary for spermatogenesis.
• Each testis has about 250 testicular lobules, each containing 1 to 3 highly coiled seminiferous tubules. Wall of each seminiferous tubule is formed of a single layered germinal epithelium and few tall Sertoli or sustentacular cells are also present. Sertoli cells provide nutrition to the developing sperms. In between the seminiferous tubules interstitial or Leydig’s cells are present which secrete androgens i.e., male sex hormones (e.g., testosterone).
• The seminiferous tubules are closed at one end but on the other side they join to form a network, the rete testis, from where fine ciliated ductules, the vasa efferentia arise. The vasa efferentia leave the testis and open into epididymis located along the posterior surface of each testis. Vas deferens is connected with epididymis at the tail end. The union of the canals from seminal vesicles and vas deferens forms the ejaculatory duct.
• Rete testis, vasa efferentia, epididymes and vasa deferentia (or vas deferens) are called the male accessory ducts. These ducts store and transport the sperms from the testis to the outside through urethra.
• The penis is a cylindrical and highly vascularised copulatory organ. The enlarged end of penis, called the glans penis, is covered by a loose fold of the skin called prepuce or foreskin.
• Semen is a collection of secretion from epididymis, seminal vesicles, prostate glands and cowper’s gland.
• Prostate cancer, impotence, sterility, inguinal hernia, cryptorchidism are the common disorders of male reproductive system.

THE FEMALE REPRODUCTIVE SYSTEM

• The female reproductive system consists of a pair of ovaries, a pair of Fallopian tubes (oviducts), uterus, vagina, external genitalia (vulva) and breasts. A pair of breasts (mammary glands) are present for child care.

• The two ovaries are small bodies each 2 to 4 cm in length which remain attached to the abdominal wall by an ovarian ligament called mesovarium (or fold of peritoneum). It is covered by a layer of cubical epithelium called the germinal epithelium and under it is ovarian stroma.
• The oviduct (Fallopian tubes), uterus and vagina constitute the female accessory ducts.
• Each Fallopian tube is about 10 to 12 cm long muscular tube. It conveys the egg from the ovary to the uterus, and also provides the appropriate environment for its fertilisation. The oviduct consist of 3 regions–infundibulum, ampulla and isthmus. Infundibulum is the broad, funnel-shaped proximal part lying closer to the ovary. Its margin bears motile, finger-like processes called fimbriae which help in the collection of the ovum after ovulation. Ampulla is the widest and longest part of the oviduct. Isthmus is a very short, narrow, thick-walled, straight part that follows the ampulla.
• The uterus (womb) is a large, highly elastic medium specialized for the development of the embryo. Its shape is like an inverted pear. It is attached to the body wall by a double fold of peritoneum, the mesometrium (broad ligament). The uterus has a thick, highly vascular wall composed of three tissues: outer peritoneal covering called perimetrium, middle smooth muscle layer termed myometrium and inner mucous membrane known as endometrium. Endometrium undergoes cyclic changes during different phases of menstrual cycle.
The uterus opens into vagina through a narrow cervix. The cavity of the cervix is called cervical canal.
• The female external genitalia include the mons pubis, labia majora, labia minora, clitoris, vestibule of the vagina and vestibular glands. External genetalia are collectively called vulva.
• The sides of the vulva have two small fleshy folds, the labia minora (lesser lips) which are surrounded by larger hairy folds called the labia majora (greater lips). A fleshy elevation above the labia majora is known as mons pubis. In the uppermost angle of the vulva, in front of the urethral opening, is located a small erectile clitoris which is highly sensitive as it contains numerous sensory nerve endings for touch and pressure.
• Mammary glands are modied sweat glands that lie over the pectoralis major muscle. Externally, each breast has a projection, the nipple, surrounded by a circular pigmented area of skin (deep pink to light brown) called areola. The glandular tissue comprises about 15-20 mammary lobes in each breast. Each lobe is made up of a number of lobules. Each lobule is composed of grape-like clusters of milk secreting glands termed alveoli. When milk is produced it passes from the alveoli into the mammary tubules and then into the mammary ducts. Near the nipple, mammary ducts expand to form mammary ampullae (= lactiferous sinuses) where some milk may be stored before going to lactiferous ducts. Each lactiferous duct typically carries milk from one of the lobes to exterior.
• Breast cancer, cervical cancer, ovarian cysts, ectopic pregnancy, menstrual disorders, infertility are the common disorders of female reproductive system.

GAMETOGENESIS
• Gametogenesis is the process by which male and female sex cells or gametes i.e., sperm and ova are formed respectively in the male and female gonads (testes and ovaries). It is the major reproductive event in sexual reproduction.
• Gametogenesis for the formation of sperms is termed spermatogenesis while that of ova is called oogenesis.
• Spermiogenesis is the transformation of the spermatids into spermatozoa (sperms). Spermiation is the process of release of sperms from Sertoli cell.
• Ovulation is the release of secondary oocyte, after puberty, once every month from Graafian follicle, by one of the ovary. A brief account of gametogenesis is given on the next page.

MENSTRUAL CYCLE
• Menstrual cycle is the cyclic change in the reproductive tract of primate females. The reproductive period of the human female continues from about the age of about 12-15 years to 45-55 years.
• First menstruation begins at puberty and is called menarch. Menopause is a phase in woman’s life when ovulation and menstruation stop. It occurs between 45 to 55 years of age.
• The period of a menstrual cycle is counted from the day of the onset of the flow to the next onset aer 28 days. This period is divided into four main phases explained in the table given below.

FERTILISATION AND IMPLANTATION
• Fertilisation is the process of fusion of haploid male and female gametes (spermatozoon and ovum respectively) so as to form a diploid zygote.
• In human beings, fertilisation is internal and takes place mostly in the ampullary-isthmic junction of the oviduct (Fallopian tube).
• The secretions of the female genital tract remove coating substances deposited on the surface of the sperms particularly those on the acrosome, exposing its receptor site and thus sperm become active to penetrate the egg. This phenomenon is known as capacitation.
• The capacitated sperm undergoes acrosomal reaction and releases various chemicals contained in the acrosome. THese chemicals are collectively called sperm lysins.
• Due to acrosomal reaction, plasma membrane of the sperm fuses with the plasma membrane of the secondary oocyte so that the sperm contents enter the oocyte. Changes in the membrane of the oocyte block the entry of additional sperms. thus, it ensures that only one sperm can fertilise an ovum.

• Entry of the sperm induces the completion of the second meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of second polar body and a haploid ovum (ootid). Soon the haploid nucleus of the sperm and that of the ovum fuse together to form a diploid zygote.
• Mixing up of the chromosomes of a spermatozoon and ovum is called karyogamy or amphimixis.
• Fertilisation restores the diploid number of chromosomes of the species, initiates cleavage and further development and results in determination of sex in the embryo.
• The fertilised egg undergoes a series of repeated mitotic cell divisions, which occur in rapid succession to produce a many celled blastula. This phase is known as cleavage.
• Implantation is embedding of the blastocyst into the endometrium of the uterus. It takes place about seven days after fertilisation. The function of the zona pellucida is to prevent the implantation of the blastocyst at an abnormal site. Implantation leads to pregnancy.

EMBRYONIC DEVELOPMENT

• Embryonic development or embryogenesis is the development of embryo from fertilized ovum and its subsequent development into a young organism.
• After implantation, finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (foetus) and maternal body called placenta.
• Placenta acts as a barrier as well as ultrafilter between foetus and mother. It is connected to foetus by a rope-like umbilical cord which helps in the transport of substances to and from the embryo.
• It also acts as an endocrine tissue and produces hormones like hCG (human chorionic gonadotropin), chorionic thyrotropin, chorionic corticotropin, hCS (human chorionic somatomammotropin), estrogen and progesterone. hCG keeps the corpus luteum active. THe latter secretes progesterone and relaxin. In addition, during pregnancy, the levels of other hormones like estrogens, progestogens, cortisol, prolactin, thyroxine, etc., are increased several folds in the maternal blood.
• Transformation of the blastocyst into gastrula with primary germ layers by rearrangement of the cells is called gastrulation.

• Gastrulation involves the rearrangement and migration of cells from the epiblast. A primitive streak which is a faint groove on the dorsal surface of the epiblast is formed.
• In all the triploblastic animals, three germ layers – ectoderm, mesoderm and endoderm are formed by characteristic movements of some of the cells called morphogenetic movements.
• After the formation of the primitive streak, cells of the epiblast move inward below the primitive streak and detach from the epiblast. This inverting movement is called invagination. Once the cells have invaginated, some of them displace the hypoblast forming the endoderm. Endoderm develops rst during embryonic development. Other cells remain between the epiblast and newly formed endoderm forms the mesoderm. Cells remaining in the epiblast form ectoderm.
• The growing embryo/foetus develops extraembryonic or foetal membranes. These are chorion, amnion, allantois and yolk sac. 8 Organogenesis involves differentiation and specialisation of groups of cells in the individual germ layers.
• Each germ layer gives rise to the specific tissues, organs and organ – systems. The germ layers have the same fate in all animals.

• In human beings, after one 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 formed. The first movements of the foetus and appearance of hair on the head are usually observed during the fifth month. By the end of 24 weeks (second trimester), the body is covered with Line 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.

PARTURITION AND LACTATION

• Parturition is the act of expelling the full-term young one from the mother’s uterus at the end of gestation. Gestation (pregnancy) is completed in about 280 days from the start of mother’s last menstruation.
• Parturition is induced by a complex neuroendocrine mechanism. The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex. This triggers release of oxytocin from the maternal pituitary.
• Oxytocin stimulates the uterine contractions. This provides force to expel the baby from the uterus, causing birth. Oxytocin also stimulates the placenta to secrete prostaglandins which stimulate more contractions of uterus. Uterine contractions, by positive feedback induce continuation of oxytocin and prostaglandins production.
• The forceful muscular contractions of the uterine wall leading to parturition are called labour. After birth of the baby, placenta and remains of umbilical cord are expelled by another series of uterine contraction. These structures are referred to as the “after birth”.
• The mammary glands of the female undergo differentiation during pregnancy and start producing milk towards the end of pregnancy by the process called lactation. This helps the mother in feeding the new born. The milk produced during the initial few days of lactation is called colostrum which contains several antibodies absolutely essential to develop resistance for the new-born babies.