Reproduction: Sexual Reproduction in Flowering Plants and Humans

Reproduction is a biological process by which living organisms produce offspring of their own kind. It is an essential life process that ensures the continuity of species from one generation to the next. This process helps maintain the population of a species and allows for the transfer of genetic information (traits) from parents to offspring. Although it is not essential for the survival of an individual, it is crucial for the survival of a species.

1.0Sexual Reproduction in Flowering Plants

• All flowering plants show reproduction.  
• A flower is a modified shoot.  
• A flower is a site of sexual reproduction.  
• Flowers are morphological and embryological marvels.
• Unisexual Male flower – Staminate  
• Unisexual Female flower – Pistillate  
• Both Male and Female flowers are present on the same plant – Monoecious, e.g. Cucurbits, Coconut.  
• Male and Female flowers present on separate plants – Dioecious, e.g. Papaya, Date palm.

Events of sexual reproduction

• Pre fertilization
• Fertilization
• Post fertilization

Pre-fertilisation (Structures & events)

(A) Male reproductive whorl – Androecium : Whorl of stamens

• A typical angiosperm anther is bilobed and dithecous.
• The proximal end of the tamen filament is attached to the thalamus or petals.
• Epidermis, Endothecium & Middle layers are protective layers and help in dehiscence of anther.
• Tapetum provides nutrition to developing pollen grains and generally has more than one nucleus.   

Microsporogenesis

• Each cell of sporogenous tissue → Pollen mother cell or microspore mother cell
• Pollen mother cell (2n) ^Meiosis→ Microspore tetrad (n)

Female reproductive whorl – Gynoecium 

• Ovary with one ovule - wheat, paddy, mango 
• Ovary with many ovules – papaya, watermelon, orchids

Megasporogenesis

• Megaspore mother cell (2n) → Megaspore

Pollination

• Transfer of pollen grains from the anther to the stigma. 
• Autogamy – Transfer of pollen grain from anther to the stigma of the same flower. In a normal flower which opens and exposes the anthers and stigma, complete autogamy is rare. Autogamy in such flowers requires anthers and stigma to lie close to each other, synchrony in pollen release and stigma receptivity. 
• Geitonogamy – Transfer of pollen grains from anther to stigma of another flower of the same plant. It is functionally cross pollination but genetically self pollination. 
• Xenogamy – Transfer of pollen grains from anther to stigma of flower of different plant. It is the only type of pollination which brings genetically different types of pollen.

Fertilization

• Pollen pistil interaction: All events from pollen deposition on the stigma until the pollen tube enters the ovule.  
• Biochemicals of pollen and pistil are involved.  
• Biochemical interaction  
• Dynamic and continuous process  Pollen recognition followed by acceptance and rejection of pollen grain by pistil.
• Germination of a pollen grain
• Entry of the pollen tube into the ovule and the embryo sac
• Double fertilization

Post – Fertilization : Structures and Events

• Development of Endosperm
• Development of Embryo

2.0Human Reproduction 

• Humans are sexually reproducing and viviparous.
• The Male Reproductive System - Located in pelvic region
• Testes
• Penis (Male external genitalia)
• Sperm formation and Synthesis of testosterone

Pathway of transport of sperms in the the male reproductive system :

Seminiferous tubules → Straight tubule → Rete testis → Vasa efferentia → Epididymis → Vas deferens → Ampulla → Ejaculatory duct → Urethra (Prostatic urethra → Membranous urethra → Penile urethra) → Out (through urethral meatus)

The Female Reproductive System

• Located in the pelvic region
• Ovary
• Fallopian tube
• Uterus (womb)
• Vagina
• Mammary glands

Gametogenesis

• In males – Spermatogenesis – Occurs in seminiferous tubules
• In females – Oogenesis Occurs in ovarian follicles 

Menstrual Cycle

• A cycle of events starting from one menstruation till the next one is called a menstrual cycle.
• Represents changes in the ovary and uterus, which are induced by changes in the level of pituitary and ovarian hormones  
• Menstruation only occurs if the released ovum is not fertilised.  
• Lack of menstruation is indicative of pregnancy.  
• During pregnancy, all events of the menstrual cycle.

Fertilisation and Implantation

• The process of fusion of sperm with an ovum to form a diploid zygote.
• Fertilisation can only occur if the ovum and sperm are transported simultaneously to the ampullary region.  
• All copulations do not lead to fertilisations and pregnancy.  
• During fertilisation, 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 sperm.  
• The secretions of the acrosome help the sperm enter the secondary oocyte/ovum.

Cleavage

• Mitotic division  
• Starts as a zygote moves through the isthmus of the oviduct

Major features of embryonic development at various months of pregnancy

Parturition 

• Process of delivery of the foetus (childbirth) 
• Complex neuroendocrine mechanism.
• Soon after the infant is delivered, the placenta is also expelled from the uterus.

Lactation

• The mammary glands of the female undergo differentiation during pregnancy and start producing milk towards the end of pregnancy by the process called lactation. 
• 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.

3.0Reproductive Health

• Reproductive organs, adolescence and related change, safe and hygienic sexual practices, sexually transmitted diseases (STD), AIDS, available birth control options, care of pregnant mothers, post natal care of the mother and child, importance of breast feeding, equal opportunities for the male and the female child, problem due to uncontrolled population growth, social evils like sex abuse and sex related crimes, etc.

Amniocentesis

• In it, some of the amniotic fluid of the developing fetus is taken to analyse the fetal cells and dissolved substances. 
• Used to test for the presence of certain genetic disorders such as, down syndrome, haemophilia, sickle-cell anemia, etc. determine the survivability of the foetus

Reasons for the increase in population

• Decline in death rate 
• Decline in maternal mortality rate (MMR)
• Decline in infant mortality rate (IMR) 
• Increase in number of people in reproductive age

Contraceptive methods -Methods, which help to prevent unwanted pregnancies.

Natural/Traditional methods

• Periodic abstinence
• Withdrawal/Coitus interruptus
• Lactational amenorrhea

Artificial methods

• Barriers – Condoms, Diaphragms, Cervical caps, Vaults.
• Intra Uterine Devices (IUDs) –Non-medicated IUDs, Copper releasing IUDs, Hormone releasing IUDs.
• Oral Contraceptives (Contraceptive pills)- Progestogens or Progestogen - estrogen combinations pills, 'Saheli' pills
• Injectables and Implants.
• Surgical methods - Vasectomy (In males), Tubectomy (In females), Medical Termination of Pregnancy.

4.0Sexually Transmitted Infections (STIs)

• Gonorrhoea
• Syphilis
• HIV infection leading to AIDS
• Chlamydiasis
• Genital herpes
• Hepatitis-B

Infertility

• Inability to produce children in spite of unprotected sexual co-habitation 
• In India, often the female is blamed for the couple being childless, but more often than not, the problem lies in the male partner.

Assisted reproductive technologies

• In-vivo fertilisation - Gamete intrafallopian transfer (GIFT), Artificial insemination (AI)
• Test tube baby programme/ In-vitro fertilisation (IVF)
• Embryo transfer 

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