Chapter 15: Sexual Reproduction, Development and Growth in Humans and Animals

Learning Objectives

By the end of this chapter, you should be able to:

Describe the structure and function of the male and female human reproductive systems
Explain spermatogenesis and oogenesis
Describe the menstrual cycle and the hormones involved
Explain the development of the human foetus from fertilisation to implantation
Distinguish between identical and fraternal twins
Describe health issues related to the human reproductive system
Explain the pattern of growth in humans and animals

Overview

Sexual reproduction involves the fusion of two gametes (sex cells) to form a genetically unique offspring. In humans, this process involves a complex interplay of reproductive organs, hormones, and cellular processes. Understanding reproduction is fundamental to understanding development, genetics, health, and the continuation of life.

15.1 Reproductive System of Humans

Male Reproductive System

Organ	Function
Testes	Produce sperm (spermatogenesis) and the hormone testosterone in the seminiferous tubules
Epididymis	Site of sperm maturation and storage
Vas deferens	Transports sperm from epididymis to urethra
Seminal vesicles	Secrete fructose-rich fluid to nourish sperm
Prostate gland	Secretes alkaline fluid to protect sperm in the acidic vagina
Urethra	Common passage for semen and urine (not simultaneously)
Penis	Organ of copulation; deposits semen into the vagina

Female Reproductive System

Organ	Function
Ovaries	Produce ova (oogenesis); secrete oestrogen and progesterone
Fallopian tubes (oviducts)	Transport ovum from ovary to uterus; site of fertilisation
Uterus (womb)	Site of implantation and development of embryo/foetus; inner lining = endometrium
Cervix	Lower part of uterus connecting to vagina
Vagina	Birth canal; receives semen during copulation

15.2 Gametogenesis in Humans

Gametogenesis is the process of producing gametes (sex cells) in the gonads.

Spermatogenesis (Sperm Formation)

Occurs in the seminiferous tubules of the testes
Begins at puberty and continues throughout a man's life
Produces 4 functional sperm from each primary spermatocyte

Pathway:

\text{Spermatogonium (2n)} \xrightarrow{\text{Mitosis}} \text{Primary Spermatocyte (2n)} \xrightarrow{\text{Meiosis I}} \text{Secondary Spermatocyte (n)} \xrightarrow{\text{Meiosis II}} \text{Spermatid (n)} \xrightarrow{\text{Differentiation}} \text{Sperm (n)}

Structure of a mature sperm:

Head: contains the nucleus (haploid, n) and acrosome (enzymes to penetrate the ovum)
Middle piece: packed with mitochondria (energy for movement)
Tail (flagellum): propels the sperm

Oogenesis (Ovum Formation)

Occurs in the ovaries
Begins before birth (foetal stage), pauses, resumes at puberty
Produces 1 functional ovum and 3 non-functional polar bodies from each primary oocyte

Pathway:

\text{Oogonium (2n)} \xrightarrow{\text{Mitosis}} \text{Primary Oocyte (2n)} \xrightarrow{\text{Meiosis I}} \text{Secondary Oocyte (n)} \xrightarrow{\text{Meiosis II (after fertilisation)}} \text{Ovum (n)} + \text{3 polar bodies}

Comparison: Spermatogenesis vs Oogenesis

Feature	Spermatogenesis	Oogenesis
Location	Seminiferous tubules (testes)	Ovaries
When starts	Puberty	Before birth (foetal stage)
Products	4 functional sperm	1 ovum + 3 polar bodies
Continuous?	Yes, throughout life	No — one ovum per cycle
Size of product	Small (motile)	Large (contains nutrients)

15.3 Menstrual Cycle

The menstrual cycle is a roughly 28-day cycle of hormonal changes preparing the female body for possible pregnancy.

Hormones Involved

Hormone	Source	Role
FSH (Follicle-Stimulating Hormone)	Anterior pituitary	Stimulates follicle maturation in the ovary
Oestrogen	Maturing follicle	Stimulates endometrium thickening; triggers LH surge
LH (Luteinising Hormone)	Anterior pituitary	Triggers ovulation; stimulates corpus luteum formation
Progesterone	Corpus luteum	Maintains endometrium; inhibits FSH and LH

Phases of the Menstrual Cycle

Phase	Days	Key Events
Menstruation	1–5	Endometrium sheds (if no fertilisation); bleeding occurs
Follicular phase	1–13	FSH stimulates follicle development; oestrogen rises; endometrium rebuilds
Ovulation	~Day 14	LH surge triggers release of secondary oocyte from follicle
Luteal phase	15–28	Corpus luteum secretes progesterone; endometrium thickens; if no fertilisation, corpus luteum degenerates → hormone levels drop → cycle restarts

SPM Tip: Draw a labelled diagram of the 4 phases showing which hormones are high in each phase. The LH surge on day 14 triggering ovulation is a very common exam question.

15.4 Development of the Human Foetus

From Fertilisation to Implantation

Fertilisation occurs in the fallopian tube when one sperm penetrates the secondary oocyte, completing meiosis II and forming a diploid zygote (2n).

The Placenta

The placenta develops from the trophoblast cells of the blastocyst and connects the foetus to the mother via the umbilical cord (1 umbilical vein + 2 umbilical arteries).

Functions of the placenta:

Function	Details
Gas exchange	O₂ diffuses from mother's blood to foetal blood; CO₂ diffuses in reverse
Nutrient supply	Glucose, amino acids, vitamins pass from mother to foetus
Waste removal	Urea and CO₂ pass from foetus to mother
Hormone secretion	Produces oestrogen and progesterone to maintain pregnancy
Barrier	Prevents mixing of maternal and foetal blood

Amniotic fluid surrounds the foetus in the amniotic sac — it cushions the foetus from physical shocks, allows movement, and maintains a stable temperature.

15.5 Formation of Twins

Feature	Identical Twins (Monozygotic)	Fraternal Twins (Dizygotic)
Origin	1 fertilised egg splits into 2 embryos	2 separate eggs fertilised by 2 different sperm
Genotype	Identical (same DNA)	Different (like normal siblings)
Sex	Always the same	Can be different
Placenta	Usually shared (or 2 if split very early)	Always separate placentas
Frequency	~1 in 250 pregnancies	More common; influenced by genetics, age

Conjoined Twins

Form when a monozygotic embryo fails to separate completely
Physically joined at the chest, abdomen, or other body parts
Very rare (~1 in 50,000–200,000 births)

Infertility

Infertility is the inability to conceive after one year of regular unprotected intercourse.

Cause	Male	Female
Hormonal	Low testosterone	Ovulation disorders (PCOS)
Structural	Blocked vas deferens	Blocked fallopian tubes, endometriosis
Sperm/Egg quality	Low count, poor motility	Poor egg quality

Treatment options: hormonal therapy, intrauterine insemination (IUI), in vitro fertilisation (IVF), surgery to remove blockages.

Sexually Transmitted Diseases (STDs)

Disease	Causative Agent	Key Features
HIV/AIDS	HIV (virus)	Destroys immune system (CD4 cells); managed with antiretroviral drugs; no cure
Syphilis	Treponema pallidum (bacteria)	Progresses in stages; treatable with penicillin
Gonorrhoea	Neisseria gonorrhoeae (bacteria)	Discharge, pain; treatable with antibiotics
Chlamydia	Chlamydia trachomatis (bacteria)	Often asymptomatic; treatable with antibiotics

Prevention: safe sexual practices, faithful partnerships, correct condom use, regular health screening, HPV vaccination (prevents cervical cancer).

Reproductive System Cancers

Cervical cancer: caused by HPV; preventable with HPV vaccine; detected by Pap smear
Ovarian cancer: often detected late; treatment involves surgery and chemotherapy
Prostate cancer: common in older men; detected by PSA blood test

15.7 Growth in Humans and Animals

Definition of Growth

Growth is a permanent and irreversible process involving:

Increase in cell number (through mitosis)
Increase in cell size, volume and body mass
Cell differentiation and specialisation

Measuring Growth in Humans

Common parameters measured and plotted on growth charts:

Height (standing height)
Body mass (weight)
Head circumference (critical in infants; reflects brain development)

The Sigmoid Growth Curve

Growth in most organisms follows an S-shaped (sigmoid) curve:

Phase	Description
Slow initial	Cells differentiating; slow increase in size
Rapid growth	Exponential increase; in humans this corresponds to puberty (adolescence)
Decelerating	Growth rate slows as the organism approaches maturity
Plateau	Growth ceases; adult size reached

Growth in Different Animals

Animal	Growth Pattern
Mammals (including humans)	Sigmoid curve; continuous growth until adulthood
Insects	Discontinuous / stepped — growth occurs in bursts between moults; exoskeleton shed at each instar stage
Metamorphosis (complete)	Egg → Larva → Pupa → Adult (e.g., butterfly, mosquito)
Metamorphosis (incomplete)	Egg → Nymph → Adult (e.g., grasshopper, cockroach)

Factors Affecting Growth

Factor	Effect
Genetics	Determines maximum potential height and body type
Nutrition	Adequate protein, calcium, vitamins essential for bone and tissue growth
Hormones	Growth hormone (GH), thyroid hormone, insulin regulate growth rate
Health	Chronic illness or disease can stunt growth
Environment	Stress, pollution, access to healthcare

SPM Exam Tip: Be able to draw and label the sigmoid growth curve and identify the 4 phases. Also know the difference between insects (discontinuous, moulting) and mammals (continuous sigmoid) growth patterns.

Practice Questions

State the functions of the fallopian tube and the uterus in the female reproductive system.
Explain the difference between spermatogenesis and oogenesis in terms of the number of functional cells produced.
Name the FOUR hormones involved in the menstrual cycle and state the function of each.
Describe the sequence of events from fertilisation to implantation of the blastocyst.
Distinguish between identical and fraternal twins with regard to: origin, genotype, sex, and number of placentas.
Explain why insects show discontinuous growth while mammals show a sigmoid growth curve.

Summary

Male system: testes (sperm + testosterone), epididymis, vas deferens, seminal glands, penis
Female system: ovaries (ovum + oestrogen + progesterone), fallopian tubes (fertilisation), uterus (implantation), vagina
Spermatogenesis → 4 functional sperm per primary spermatocyte; Oogenesis → 1 ovum + 3 polar bodies
Menstrual cycle (28 days): menstruation → follicular (FSH, oestrogen) → ovulation (LH surge, day 14) → luteal (progesterone)
Fertilisation in fallopian tube → zygote → morula → blastocyst → implantation day 7 in endometrium
Placenta: exchanges gases, nutrients, waste between mother and foetus; secretes hormones
Identical twins: 1 zygote splits; same DNA, same sex; Fraternal twins: 2 zygotes; different DNA, can differ in sex
Growth: permanent, irreversible; sigmoid curve — slow → rapid (puberty) → plateau; insects show discontinuous growth (moulting)