Human Reproduction: WJEC A Level Biology Notes

Lesson 1 of 21

Which organs make up the male reproductive system?

These notes cover topic 4.1, Sexual reproduction in humans, in WJEC A level Biology Unit 4: , Inheritance and Options. Each lesson follows the WJEC specification, and the exam tips come from BioWelsh analysis of WJEC past papers.

Sexual reproduction is the production of offspring by the fusion of two gametes. In humans the male is the and the female is the . The male reproductive system has two jobs: to make huge numbers of , and to deliver them into the female tract.

The diagram below shows the male reproductive system from the front.

Labelled diagram of the male reproductive system. A cream bladder sits at the top centre, with a purple seminal vesicle on each side below it and a pink prostate gland beneath. A pink penis runs down the middle with a gold urethra inside it. On each side, a rounded scrotum contains a pink testis drawn with wavy lines for the seminiferous tubules, a dark pink crescent for the epididymis on its outer side, and a gold vas deferens curving up from it to the seminal vesicle. Labels name the bladder, seminal vesicle, prostate gland, vas deferens, testis, epididymis, seminiferous tubules, scrotum, ejaculatory duct and urethra. — The male reproductive system seen from the front. Original BioWelsh diagram.

The two testes sit in the scrotum, a sac of skin outside the body cavity. Each testis is packed with coiled seminiferous tubules, where are made.
The is a long coiled tube on the outside of each testis. mature and are stored here.
The () carries from the up over the bladder.
The and the add fluid, making .
The two ejaculatory ducts join the urethra inside the , so and urine both leave through the urethra and the penis.

The parts of the male reproductive system and what each one does
Structure	Function
Testis	Contains seminiferous tubules that make , and that secrete
Scrotum	Holds the testes outside the body cavity, at about 34 °C, which is the best temperature for making
	mature here, gain the ability to swim, and are stored
	Muscular tube that carries from the to the urethra
	Secretes a fluid containing fructose (an energy source for the ) and mucus
	Secretes an alkaline fluid that neutralises the acidity of the vagina and of any urine in the urethra
Urethra	Carries during ejaculation and urine at other times

is suspended in the fluids from the seminal and the . A typical ejaculate is 2 to 5 cm³ and contains 40 to 300 million per cm³. Only one of them will fertilise the , but the released by the whole help to clear a path through the layers around it.

Lesson 2 of 21

What can you see in a section of a testis?

Slide up a prepared section of testis and you see dozens of circles. Each circle is one cut across, because the tubules are tightly coiled. Between the tubules there are wedges of .

The diagram below shows one in transverse section.

Labelled diagram of a circular tubule. The wall holds four concentric rings of cells: large purple circles at the outer edge, then blue, then pink, then small cream ones. Six pale green wedges run through the wall from the outer edge to the centre. A cream circle in the middle is the lumen, with dark sperm heads and curved tails pointing into it. Groups of orange oval cells lie in the corners outside the tubule. Labels name the germinal epithelium, spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids, spermatozoa, a Sertoli cell and interstitial cells. — A seminiferous tubule in transverse section. Original BioWelsh diagram.

The outer wall of the tubule is the . The cells here divide to make all the .
Moving inwards, the cells get further through their development: at the edge, then primary spermatocytes, then , then .
Sertoli cells are tall cells that stretch from the outer wall to the . They nourish and protect the developing cells.
The in the middle is where the finished collect, with their tails pointing into the space.
(Leydig cells) lie in the wedges of tissue between tubules, outside the wall. They secrete .

The light micrograph below shows the same thing in real tissue, with the stages of formation labelled.

Pink and purple stained tissue filling the frame, with many round nuclei in layers. Labels with leader lines read spermatogonia, sustenticular cells (Sertoli cells), 1° spermatocytes, 2° spermatocytes and spermatids. A paler open space lies at the lower right. — Light micrograph of part of a seminiferous tubule, with the stages of sperm formation labelled. Photo: Dusan Zivkovic, CC BY 4.0. Resized.

The layers of cells are not as neat as in a diagram, but the order is the same: the largest, darkest are at the edge and the smallest are nearest the .
The pale space in the lower right is the of the tubule.
Sustentacular cell is another name for a .

from the does three things: it stimulates , it maintains the male reproductive organs, and it causes the male secondary sexual characteristics at puberty.

Lesson 3 of 21

Which organs make up the female reproductive system?

The female reproductive system does far more than make gametes. It receives , provides the site of fertilisation, then holds and feeds an embryo for nine months.

The diagram below shows the female reproductive system from the front.

Labelled diagram. In the centre a pear-shaped uterus is drawn with three nested layers: a dark pink outer band, a mid pink band and a white cavity. Below it a purple tube narrows at the cervix and widens into the vagina. From each upper corner a gold oviduct curves outwards to a cream funnel edged with short gold fimbriae, and below each funnel is a purple oval ovary containing small white circles. Labels name the oviduct, oviducal funnel with fimbriae, ovary, myometrium, endometrium, cervix and vagina. — The female reproductive system seen from the front. Original BioWelsh diagram.

The two ovaries make and secrete and .
Beside each is the funnel of an (), edged with finger-like that sweep the released oocyte in.
The is lined with ciliated epithelium and smooth muscle, which move the oocyte along. Fertilisation normally happens here.
The two open into the uterus, a thick-walled muscular organ where the embryo implants and develops.
The is the ring of muscle at the neck of the uterus, and it opens into the vagina.

The wall of the uterus has three layers, shown below.

Diagram of four horizontal bands. The top band is pale lilac. Below it a thick dark pink band contains rows of small curved lines. Below that a mid pink band contains five upright loops. The bottom band is very pale pink. Labels on the right name the perimetrium as a thin outer covering, the myometrium as thick smooth muscle that contracts at birth, the endometrium as the lining that thickens each cycle and is shed, and the lumen of the uterus. — The three layers of the wall of the uterus. Original BioWelsh diagram.

The is a thin outer covering.
The is thick smooth muscle. It contracts during birth to push the baby out.
The is the lining. It is rebuilt and then shed every cycle, and the implants in it.

Lesson 4 of 21

What can you see in a section of an ovary?

A section of is a snapshot of one moment in the cycle, so it shows at several different stages all at once. Each is a ball of cells with one developing oocyte inside.

The diagram below follows one round a whole cycle, drawn on a single so you can compare the stages.

A large pale purple oval with a thick purple outline. Six structures are arranged clockwise round the inside, joined by purple arrows: a small pink circle with a dot, a larger pink circle with a white space inside, a large pink circle with a big white space and a small pale circle to one side, a broken pink C shape with a small round cell escaping above the outline, a yellow circle ringed with small yellow dots, and a pale yellow circle with a cross through it. Text labels name the germinal epithelium, primary follicle, the follicle growing and forming an antrum, the stroma, and the statement that it degenerates if there is no pregnancy. Labels on the right name the Graafian follicle, ovulation and the corpus luteum. — One cycle of follicle development in the ovary. Original BioWelsh diagram.

The surface of the is covered by a layer called the . Inside it is the , the of the .
A primary is small: one in a single ring of .
As it grows, the divide and a fluid-filled space called the appears, pushing the oocyte to one side.
A mature is large and bulges from the surface of the .
At the bursts and the , with its ring of cells, is released.
What is left of the becomes the , which secretes . If there is no pregnancy it degenerates.

The two light micrographs below show real tissue: first a whole , then a closer view of the mound of cells that holds the oocyte.

Densely packed small purple-stained nuclei form a mound that bulges upwards into a cream-coloured empty space at the top of the frame. Near the top of the mound is a bright pink rounded structure about twelve nuclei wide, ringed by a close layer of the purple nuclei. Below the mound the tissue becomes looser and paler pink. — Light micrograph of the cumulus oophorus, the mound of follicle cells that holds the oocyte. Photo: Ed Uthman, CC BY 2.0. Resized.

A large follicle fills the centre of the frame. In its lower half is a wide, irregular, pale pink and white space. Around the whole follicle is a dense purple layer of small crowded nuclei, several cells thick, and outside that a paler layer of spindle-shaped cells. Near the top, set in a mound of the purple cells, is a rounded structure with a thick bright pink ring and a paler granular centre. The tissue beyond the follicle is pale purple and fibrous. — Light micrograph of a Graafian follicle in a human ovary. Photo: Ed Uthman, CC BY 2.0. Resized.

In the first micrograph the pale, irregular space in the lower half of the is the .
The dense purple layer several cells thick all around the is the layer of , and the paler spindle-shaped cells outside it are the theca.
The rounded structure near the top, with a thick bright pink ring, is the oocyte. The pink ring is its .
The second micrograph is a closer view of the mound of , called the cumulus oophorus, that projects into the and holds the oocyte.
The pale tissue all around both is the of the .

Lesson 5 of 21

What is gametogenesis, and why must gametes be haploid?

Gametogenesis is the making of gametes. In the testis it is called , and in the it is called . Both happen in the same three stages: multiplication, growth and maturation.

Multiplication: cells of the divide by mitosis, making many cells.
Growth: each of those cells grows larger and takes in food reserves, becoming a or a .
Maturation: the cell divides twice by meiosis, halving the chromosome number and giving gametes.

A human body cell is , 2n = 46. A must be , n = 23, so that when two gametes fuse the is again. Without meiosis the chromosome number would double every generation. Meiosis also shuffles the , by and by , so every is genetically different and the offspring vary.

Lesson 6 of 21

How does the testis make spermatozoa?

happens in the wall of the seminiferous tubules and runs continuously from puberty. It takes about 70 days from the first division to a finished , and a healthy adult makes well over 100 million a day.

The flow diagram below follows one cell through the whole process.

Flow diagram of six rows of ring-shaped cells, each row joined to the next by downward arrows. Row 1 has one purple cell, row 2 two purple cells, row 3 two blue cells, row 4 four pink cells, row 5 four cream cells and row 6 four grey cells. Labels to the right of each row read germinal epithelium cell 2n, spermatogonium 2n, primary spermatocyte 2n, secondary spermatocyte n, spermatid n and spermatozoon n. Labels to the left of the arrows read mitosis, mitosis then growth, meiosis I, meiosis II and maturation. — Spermatogenesis: one primary spermatocyte makes four spermatozoa. Original BioWelsh diagram.

A cell of the divides by mitosis to give .
Some keep dividing by mitosis, which keeps the supply going. Others grow into primary spermatocytes, which are still .
Each divides by meiosis I into two .
Each divides by meiosis II into two , so one gives four .
The mature into : the and tail form and most of the cytoplasm is lost. All four are the same size.

All of this happens while the cells are held by Sertoli cells, which pass nutrients to them and protect them from the immune system. The finished are released into the and washed to the , where they finish maturing.

Lesson 7 of 21

How is a spermatozoon adapted to its job?

A has one job: to reach a and deliver a into it. Almost everything else has been stripped away, which is why it is one of the smallest cells in the body.

The diagram below shows a and its three regions.

Labelled side view. At the left an oval head has a cream cap over its front half and a purple oval inside it. A small grey block with two bars follows. Then a cream rectangle crossed by nine curved gold lines. From it a long dark line waves to the right, its waves growing larger. Scale bars underneath read head about 5 micrometres, middle piece about 8 micrometres and tail about 50 micrometres. Labels name the acrosome, the haploid nucleus, the centrioles, the mitochondria in the middle piece and the tail. — The structure of a human spermatozoon. Original BioWelsh diagram.

The head is about 5 µm long. The is a vesicle at the tip that holds .
Behind the is the , carrying 23 chromosomes.
The middle piece is about 8 µm long and is packed with in a . They make the that powers swimming.
The tail, or flagellum, is about 50 µm long and lashes from side to side.
Total length is about 60 µm, so the cell is far too small to see without a microscope.

The scanning electron micrograph below shows real human , with a scale bar.

Grey scanning electron micrograph of three sperm cells lying on a dark surface covered with many small round holes. Each cell has a bright, smooth, oval head and a long very thin tail that curves away across the surface. A white scale bar at the bottom left is labelled 10 micrometres. — Scanning electron micrograph of human spermatozoa. Photo: No specific author, Public domain.

The heads are flattened and oval, and the tails are long and thin.
The scale bar lets you check the size: the head is only a few micrometres across.
Nothing else is visible from outside, because all the detail is inside the cell.

How each part of a spermatozoon is adapted
Part	Adaptation
	Holds that digest a path through the and
	, so the is ; it is the only part that must reach the oocyte
Middle piece	Many release by , for the tail
Tail	Beats from side to side so the cell can swim to the oocyte
Small size and little cytoplasm	Light, so the cell moves quickly

A swimming uses faster than almost any other cell in the body, which is why the middle piece is packed with and why the seminal supply fructose as a respiratory . (If is unfamiliar, read the ATP lesson on why cells need ATP.)

Lesson 8 of 21

How does the ovary make a secondary oocyte?

follows the same three stages as , but with two big differences: it starts before birth and then pauses for years, and each division is unequal.

The flow diagram below follows one cell through .

Flow diagram of five rows of ring-shaped cells joined by downward arrows. Row 1 has one purple cell, row 2 two purple cells, row 3 two blue cells, row 4 two pink cells each with a much smaller ring beside it, and row 5 two cream cells each with a much smaller ring beside it. Labels to the right read germinal epithelium cell 2n, oogonium 2n, primary oocyte 2n stopped at prophase I, secondary oocyte n and a small first polar body, and ovum n and a small second polar body. Labels to the left of the arrows read mitosis, mitosis then growth, meiosis I unequal and meiosis II unequal. — Oogenesis: one primary oocyte makes one ovum and two polar bodies. Original BioWelsh diagram.

Before birth, cells of the divide by mitosis to give .
The grow into , which begin meiosis I but stop in prophase I. Every a person will ever have is already there at birth.
From puberty, a few finish meiosis I each month. The division is unequal: one large and one tiny first .
The begins meiosis II but stops again, at metaphase II. This is the cell released at .
Meiosis II only finishes if a enters. It gives one and a second . The have almost no cytoplasm and break down.

The number of oocytes falls steadily from before birth onwards, as the bar chart below shows.

Bar chart with six purple bars. The vertical axis is the number of oocytes in both ovaries in millions, from 0 to 7. The bars, left to right, are 5 months before birth at 6.8, birth at 1.6, puberty at 0.4, age 25 at 0.2, age 40 at about 0.02 and menopause at almost zero. Text in the upper part reads that no new oocytes are made after birth and only about 400 are ever ovulated. — Number of oocytes in both ovaries at different stages of life. Original BioWelsh graph.

The peak is about 6.8 million, reached at about 5 months of foetal life.
By birth the number has already fallen to about 1 to 2 million, and by puberty to about 400 000.
Only about 400 are ever ovulated. The rest .
By the menopause almost none are left, which is why the cycle stops.

Lesson 9 of 21

What surrounds a secondary oocyte?

A is huge compared with a , about 120 µm across, and it is not released bare. Two layers travel with it, and both matter at fertilisation.

The diagram below shows a just after .

Labelled diagram of a large circle. A ring of pale purple ovals radiates around the outside. Inside them is a pink ring, then a paler pink interior. Small dark red dots form a ring just inside the edge. Near the centre, four short purple bars sit in a line between two dashed curves that meet at points on either side. A small white circle containing a dark dot sits between the pink ring and the interior at the upper right. Labels name the corona radiata, the zona pellucida, the cortical granules, the chromosomes on the spindle at metaphase II, the first polar body and the cytoplasm. — A secondary oocyte just after ovulation. Original BioWelsh diagram.

The outer layer is the : that came from the and nourish the oocyte.
Inside that is the , a jelly-like coat of glycoprotein.
The cytoplasm is packed with food reserves that will feed the embryo for its first few days.
Just under the cell membrane lie the , small of .
The chromosomes are lined up on a spindle at metaphase II, waiting.
The first sits between the oocyte and the .

The light micrograph below shows an oocyte in a real .

A large smooth oval stained mid pink lies left of centre, with a small paler round area inside it. A narrow clear gap runs all the way around the oval. Beyond the gap, many small round purple-stained nuclei are packed together and fan outwards across the rest of the frame. A few bright red cells lie among them near the top. — Light micrograph of an oocyte in the cumulus oophorus of a human ovary. Photo: Ed Uthman, CC BY 2.0. Resized.

The oocyte is the large smooth pink oval, with its showing as a paler round area inside it.
The narrow clear gap running round the oval is where the lies.
The small purple-stained cells packed around it and fanning outwards are the . The innermost of them make up the .
Compare the size of the oocyte with the cells around it: at about 120 µm across it is one of the largest cells in the body.

Lesson 10 of 21

How do spermatogenesis and oogenesis compare?

Both processes use the same sequence of mitosis, growth and meiosis, but they are tuned for very different jobs. One makes millions of tiny mobile cells; the other makes a few large cells full of food.

Spermatogenesis and oogenesis compared
Feature	Spermatogenesis	Oogenesis
Where	Seminiferous tubules of the testis	in the of the
When it starts	At puberty	Before birth
When it stops	Continues into old age	Stops at the menopause
Pattern		One completes the cycle about every 28 days
Products of one primary cell	Four equal	One and two
Is the division equal?	Yes, all four cells are the same size	No, almost all the cytoplasm goes to one cell
Is meiosis completed?	Yes, before the is released	No, meiosis II finishes only if a enters
Size of the	About 60 µm long, almost no cytoplasm	About 120 µm across, packed with food reserves
Number made	Hundreds of millions per day	About 400 released in a lifetime

The reason for the difference is what each has to do. A only has to travel, so it is small, light and made in vast numbers to make fertilisation likely. An oocyte has to feed the embryo until it implants, so the cytoplasm must not be split between four cells.

Lesson 11 of 21

How do you work out the real size of a cell down a microscope?

Examiners ask this in almost every series, usually from a micrograph with a scale bar or from a . The mistakes that cost marks are nearly always about units, not about biology.

Two conversions do most of the damage. There are 1000 µm in 1 mm, and 1000 mm in 1 m. Convert both measurements into the same unit before you divide, and always ask yourself whether the answer is a sensible size for the structure.

In the laboratory you cannot lay a ruler on the specimen, so you use an , calibrated against a . The diagram below shows how.

Two large circles side by side, each representing the field of view. In the left circle a black scale numbered 0, 5 and 10 lies above a red scale, and text reads that 10 eyepiece units line up with 0.1 millimetres, which is 100 micrometres, so one eyepiece unit is 10 micrometres. In the right circle the same black scale lies above a pink circle with a purple centre, and a green bar beneath it is labelled the cell measures 6.4 eyepiece units, 6.4 multiplied by 10 equals 64 micrometres. — Calibrating an eyepiece graticule with a stage micrometer. Original BioWelsh diagram.

The is a scale on a glass disc in the eyepiece. Its divisions are called (epu), and on their own they nothing.
A is a slide with a scale of known length engraved on it, usually with divisions of 0.01 mm.
Line up the two scales at zero and count how many fit a known length. Here 10 epu line up with 0.1 mm, which is 100 µm, so 1 epu = 10 µm.
Swap in the real slide and measure the specimen in . Here the cell is 6.4 epu across.
Actual size = 6.4 × 10 = 64 µm.

You must calibrate again every time you change the objective lens. At a higher the specimen looks bigger but the graticule scale does not change, so one stands for a smaller real length.

The specification asks you to examine the histology of and testis. The five steps below are the whole procedure.

Five pale purple cards in a row, joined by purple arrows. The cards show an eyepiece graticule, a stage micrometer slide, a microscope slide with a coverslip, a light microscope, and a pencil with a ruler. The captions read fit the graticule, calibrate, set up the slide, low then high power, and draw and measure. — The five steps of examining ovary and testis slides. Original BioWelsh diagram.

Fit the into the eyepiece and put the on the stage.
Line the two scales up at zero, count how many fit a known length, and work out the value of one . Repeat for every objective lens you will use.
Replace the micrometer with the prepared slide and focus at low power, so that you can see how the whole section is arranged.
Switch to high power on one structure and identify its parts, focusing upwards and using the fine focus so that the lens cannot touch the slide.
Measure across the widest part of the structure in , repeat on two or three more examples, and convert the to micrometres.

What to look for on each slide
Slide	Structure	Where to look and what it looks like
T.S. testis		Circles filling the whole field; each is one coiled tubule cut across
T.S. testis		Dark, dense heads with fine tails, in the at the centre of a tubule
T.S. testis		Small groups of cells in the wedges of tissue between tubules
T.S.	Primary	Small; one oocyte in a single ring of , near the surface
T.S.		Large, with a clear fluid-filled and the oocyte to one side
T.S.		A large solid mass of pale, folded tissue, usually deeper in the

Typical measurements taken with a graticule calibrated at 1 eyepiece unit = 10 µm
Structure	Measurement / eyepiece units	Actual diameter / µm
	18.0, 20.5 and 17.5	180, 205 and 175; 187
Primary	5.0	50
	62.0	620
	12.0	120

The three tubule diameters are not the same because a coiled tubule is cut at different angles, so some sections do not pass through the widest part. That is why you measure several and give a . Dividing the two diameters shows how much a grows: 620 ÷ 50 = 12.4 times wider.

A shows the outlines and positions of tissues only, with no individual cells drawn.
A high power drawing shows a small number of individual cells in detail.
Use a sharp pencil and single lines: no shading, no colouring and no sketching.
Label lines must be ruled, must not cross and must touch the structure they name.
Add a title, the plane of section and the or a scale bar.

Lesson 12 of 21

What happens when a sperm meets a secondary oocyte?

During sexual intercourse is released at the top of the vagina, next to the . swim through the and uterus and up into the , helped by contractions of the uterus wall. Of the millions released, only a few hundred reach the .

The survives about 24 hours after . can stay alive in the female tract for up to five days. Fertilisation is therefore only possible in a window of a few days around , which is why in many mammals happens towards the end of the time the female will mate.

The six panels below show fertilisation in order.

Six panels in two rows. Each panel has a title, a drawing of a round pink oocyte and a short explanation. Panel 1, capacitation, shows a sperm approaching an oocyte ringed by pale purple ovals. Panel 2, acrosome reaction, shows small gold dots streaming from the sperm head towards the oocyte. Panel 3, sperm head entry, shows a purple circle inside the oocyte with an arrow pointing to it. Panel 4, cortical reaction, shows a thick dark red ring around the oocyte, red dots inside it and a second sperm blocked by a red cross. Panel 5, meiosis II, shows two small circles inside the oocyte and a small white circle at its edge. Panel 6, nuclei fuse, shows a single purple oval labelled 2n. — Fertilisation, from a capacitated sperm to a zygote. Original BioWelsh diagram.

: in the , cholesterol and glycoproteins are removed from the 's cell membrane. Only after this can the react, and the tail beats more strongly.
reaction: the fuses with the cell membrane and releases . These digest a path through the and then through the .
Entry: the membranes of the and the oocyte fuse. Only the head, carrying the , sinks into the cytoplasm. The tail and the are left outside.
: the empty their , which change the into a .
Meiosis II: entry of the triggers the oocyte to finish meiosis II, giving the and a second .
Fusion: the two pronuclei fuse. The cell is now a , and it begins to divide by mitosis.

Lesson 13 of 21

How does the oocyte stop a second sperm getting in?

is the entry of more than one . If two got in, the cell would have three sets of chromosomes: 23 from the oocyte and 23 from each , which is 69 in total. A embryo develops abnormally and does not survive.

The prevents this. The diagram below compares the oocyte before and after the first enters.

Two panels side by side. In the left panel, headed before a sperm arrives, a pale pink band labelled zona pellucida lies above a thick black line labelled cell membrane of the oocyte, with eight dark red circles below the line and a sperm above with an arrow pointing down. In the right panel, headed after the first sperm enters, the band is darker and thicker and labelled fertilisation membrane, eight small red circles below the membrane have arrows pointing up into it, and a sperm above is crossed out in red and labelled blocked. — The cortical reaction before and after the first sperm enters. Original BioWelsh diagram.

Before: the is soft, and the are waiting just under the cell membrane.
Entry of the first makes the fuse with the cell membrane and release their into the .
The cross-link the glycoproteins, so the zona thickens and hardens into a .
No other can digest its way through, so no second can enter.

In , are collected from the ovaries and fertilised in the laboratory. If an oocyte is collected before it is fully mature, it has fewer and cannot harden its zona properly, so the risk of is higher than in the body.

Lesson 14 of 21

How does the blastocyst reach the uterus and implant?

Fertilisation happens in the , but the embryo has to develop in the uterus. Getting there takes about a week, and the embryo divides all the way.

The diagram below follows the embryo from to .

A curved gold tube runs from a purple oval at the far left to a thick pink uterus at the right. Along the tube, seven circles show the stages in order: a single cell with a purple dot, a single cell labelled fertilisation, then balls of 2, 4, 8 and about 14 cells, then a hollow circle with a small pink patch at one side. Inside the uterus a small hollow circle has short spikes radiating into the pink wall. Text reads cleavage, mitosis with no growth between the divisions, and labels name ovulation, the blastocyst and implantation. — From fertilisation to implantation, over about seven days. Original BioWelsh diagram.

Cilia in the lining of the , and waves of muscle contraction, move the cell along towards the uterus.
The divides by mitosis. These divisions are called : the cells divide but do not grow between divisions, so the ball of cells is no bigger than the .
After 2, 4, 8 and 16 cells the solid ball is called a .
Fluid collects inside and the ball becomes hollow. This is the .
About seven days after fertilisation the reaches the uterus and sinks into the .

The has two distinct parts, shown below.

A large pale blue circle ringed by a single layer of pink oval cells. Eight purple circles are clustered together inside the ring at the upper left. Labels name the trophoblast as the outer layer of cells, the inner cell mass as the part that becomes the embryo, and the blastocoele as a fluid-filled cavity. — A blastocyst at about seven days. Original BioWelsh diagram.

The is the outer layer of cells. It will become the and then the .
The is a clump of cells at one side. It becomes the embryo itself and the amnion.
The is the fluid-filled cavity between them.

At , finger-like grow out from the into the . They anchor the , absorb nutrients and later develop into the of the . The has to be thick and well supplied with blood for this to work, which is why the timing matters: in the embryo is not transferred for a few days, so that it has reached the stage and the is ready.

Lesson 15 of 21

What happens in the ovary and the uterus over 28 days?

The is a repeating cycle of about 28 days that prepares an oocyte for release and the for . Day 1 is the first day of bleeding. In mammals other than humans the equivalent cycle is called the , and the lining is reabsorbed rather than shed.

What happens in each part of the cycle
Days	In the ovary	In the uterus
1 to 5	Several begin to grow	: the breaks down and is shed
6 to 13	One keeps growing into a ; the rest	The is rebuilt and thickens
14	: the is released	The is about 6 mm thick
15 to 28	The remains of the become the	The thickens further and its glands secrete, ready for
28 (no pregnancy)	The degenerates	The starts to break down, and the next cycle begins

The graph below shows how the thickness of the changes through one cycle.

Line graph with the day of the menstrual cycle from 0 to 28 on the horizontal axis and the thickness of the endometrium in millimetres from 0 to 10 on the vertical axis. A purple line starts at 6, falls steeply to 1 at day 5, rises steadily to 6 at day 14, rises more gently to a peak of 8 at about day 22, then drops sharply to 6 at day 28. Text along the top reads menstruation, rebuilt by oestrogen and maintained by progesterone, and ovulation is marked near day 14. — Thickness of the endometrium during one menstrual cycle. Original BioWelsh graph.

The line falls steeply from day 1 to day 5, to about 1 mm. This is .
From day 5 it rises steadily to about 6 mm by day 14, as rebuilds the lining.
It keeps rising more slowly to about 8 mm by day 22, maintained by .
It falls sharply at the very end of the cycle as the degenerates and drops.

The light micrograph below shows a thickened in the second half of the cycle.

A thick layer of pale pink and purple stained tissue. Running through it are many large, pale, deeply folded glands, two of which are marked with the letter G. The upper edge of the tissue is labelled Lumen. Two further labels read DEC and immune cell influx. A scale bar at the top left is labelled 500 micrometres. — Light micrograph of the endometrium in the second half of the menstrual cycle. Photo: Douglas A. Gibson, Ioannis Simitsidellis and Frances Collins, CC BY 4.0. Resized.

The layer is deep and packed with coiled glands, marked G, which secrete nutrients into the cavity.
The cavity of the uterus is at the top, labelled .
The other labels are features beyond this specification: DEC marks cells changed in preparation for .

Lesson 16 of 21

How do FSH, LH, oestrogen and progesterone control the cycle?

Four hormones run the cycle. Two come from the anterior in the brain: stimulating hormone () and luteinising hormone (). Two come from the : from the and from the .

What each hormone does
Hormone	Made by	Main effects
	Anterior	Stimulates in the to develop, and stimulates the to secrete
	Anterior	A surge of triggers , then makes the remains of the become the
	in the	Rebuilds and thickens the ; inhibits at low levels but stimulates a surge of when very high
	(later the )	Maintains the thickened ; inhibits and ; stops the contracting

The graph below shows all four hormones through one cycle.

Line graph with the day of the menstrual cycle from 0 to 28 on the horizontal axis and relative concentration in the blood from 0 to 11 on the vertical axis. A green solid line for FSH starts at about 3, peaks at 4 near day 3, falls to 2, rises to a smaller peak of 5.6 near day 13, drops back to 2 and then rises again to about 2.9 at day 28. A red dashed line for LH stays near 1.6 then rises to a single sharp peak of 10 just before day 14 and falls back. A blue dotted line for oestrogen rises from 1 to a peak of 9 just before day 13, falls, then has a second broad peak of 4 near day 21. An orange dot-dash line for progesterone stays at 0.4 until day 14, rises to a broad peak of 9 near day 22, then falls sharply to 0.4 by day 28. A grey dashed vertical line marks day 14. — Hormone concentrations during one menstrual cycle. Original BioWelsh graph.

is highest in the first few days, which is what starts the growing. It then falls as rises.
rises through the first half of the cycle as the grows, and peaks just before day 14.
That very high causes a sharp surge of , and follows on day 14.
After , rises to a broad peak around day 21 as the develops.
If there is no pregnancy, and both fall sharply near day 28. is no longer inhibited, so it rises and the next cycle begins.

The switch between negative and is the part students find hardest. The three panels below show it.

Three panels side by side, headed days 1 to 12, days 12 to 14 and days 15 to 28. Each panel has three boxes joined by downward arrows: a blue box for the anterior pituitary, a pink box for the ovary structure and a yellow box for the hormone. A curved arrow runs up the left side of each panel from the hormone box to the pituitary box. In panels 1 and 3 the arrow is red and marked with a minus sign, with text saying negative feedback keeps FSH low. In panel 2 the arrow is green and marked with a plus sign, with text saying positive feedback releases a surge of LH that triggers ovulation. — How oestrogen and progesterone feed back on the pituitary gland. Original BioWelsh diagram.

For most of the cycle, acts by : it inhibits , so only one keeps growing.
Just before day 14, is so high that its effect reverses: it now stimulates the , which releases a surge of . This is , and it triggers .
After , from the inhibits both and , so no new develops while there could be an embryo implanting.

Lesson 17 of 21

How is the placenta built for exchange?

The foetus cannot eat, breathe or excrete for itself. The does all three, by bringing the foetal blood very close to the mother's blood without ever letting them mix. It forms from the of the embryo and the of the mother, so it is built from two genetically different individuals. By the end of pregnancy it is a disc about 20 cm across, giving a large surface area for exchange.

The diagram below shows the in the wall of the uterus.

Cross-section diagram in horizontal bands. At the top a dark pink band is labelled myometrium and below it a mid pink band is labelled endometrium. Below them a pale pink space contains scattered red ovals and four finger-shaped projections that point upwards from a cream band labelled chorion. Each projection contains a red and a blue line. A red vessel enters the space from the top left with an arrow pointing down, and a blue vessel leaves at the top right with an arrow pointing up. Below the chorion a narrow column containing one blue and two red lines is labelled umbilical cord to the foetus. Labels name the intervillous space, a chorionic villus, a foetal capillary, the maternal artery and vein, the umbilical vein and the two umbilical arteries. — The structure of the placenta in the wall of the uterus. Original BioWelsh diagram.

Finger-like from the foetal side project into a space in the called the , or .
Maternal arteries open into that space, so the villi are bathed directly in the mother's blood. Maternal veins carry it away.
Each villus contains foetal capillaries, and its wall is only one cell thick.
The villi join the , and blood travels to and from the foetus along the .
The cord has two umbilical arteries and one umbilical vein.

Blood in the umbilical cord
Umbilical artery	Umbilical vein
Carries blood from the foetus to the	Carries blood from the to the foetus
Less oxygen	More oxygen
More carbon dioxide	Less carbon dioxide
More	Less
Fewer nutrients such as glucose	More nutrients such as glucose

Lesson 18 of 21

Why must the blood of the mother and the foetus never mix?

Everything crossing the has to pass through the wall of a . That wall is the barrier, and it has to be thin enough for fast exchange but complete enough to keep the two bloods apart.

The diagram below shows one villus and what crosses it.

A pale pink rectangle labelled maternal blood in the intervillous space contains scattered red ovals. In the centre a pale finger-shaped villus rises from the bottom edge, fringed with short pink lines along its whole outline. Inside it a narrow pale blue loop contains small blue ovals. A green arrow points from the left into the villus, labelled oxygen, glucose, amino acids and antibodies. A dark red arrow points from the villus to the right, labelled carbon dioxide, urea and water. A bracket between the villus surface and the blue loop is labelled only about 5 micrometres, and a label on the right reads microvilli increase the surface area. — Exchange across the wall of a chorionic villus. Original BioWelsh diagram.

Oxygen, glucose, amino acids, water, mineral ions and pass from the mother to the foetus.
Carbon dioxide, and water pass from the foetus to the mother.
The distance is only about 5 µm, because the wall of the villus and the wall of the foetal capillary are each one cell thick.
on the surface of the villus, and the very large number of villi, give a huge surface area.
Blood flows in opposite directions on the two sides, which keeps a along the whole length of the villus.

The light micrograph below shows one in section.

An oval mass of pink-stained tissue fills most of the frame, edged by a rim of dark purple nuclei. Inside it are pale spaces, several of which contain small bright red cells. Outside the oval, to the left, seven bright red cells of the same kind lie loose on the white background. — Light micrograph of a chorionic villus at term. Photo: Mikael Häggström, CC0.

In the middle are foetal capillaries, packed with small foetal red blood cells.
The thin outer layer of the villus is the .
The loose red cells outside the villus are the mother's, in the . The two sets of cells are close together but never mixed.

Maternal blood pressure is much higher, and would damage the delicate foetal capillaries.
The foetus is genetically different from the mother, so her immune system could recognise and attack its cells.
If the blood groups differ, mixing could cause and clotting.
The barrier keeps out many , , drugs and maternal hormones.

The barrier is not perfect. Alcohol, nicotine, some drugs and some viruses do cross it, which is why what a pregnant person takes in matters to the foetus.

Lesson 19 of 21

Which hormones maintain a pregnancy, and how is one detected?

If the degenerates on day 28 the breaks down, taking any implanted embryo with it. A pregnancy therefore has to send a chemical message to keep the alive. That message is human chorionic gonadotrophin, or .

is secreted by the of the implanting embryo, and later by the . It maintains the , so stays high and the is kept. If levels are too low, the degenerates, falls, the breaks down and the pregnancy is lost.

The graph below shows the three placental hormones through a pregnancy.

Line graph with time from fertilisation in weeks from 0 to 40 on the horizontal axis and relative concentration from 0 to 11 on the vertical axis. A red solid line for hCG rises from near zero to a peak of about 9.8 at week 9, then falls steeply and stays near zero from week 20. A blue dotted line for oestrogen and an orange dot-dash line for progesterone both rise steadily from near zero, reaching about 8.4 and 6.3 by week 40. A grey dashed vertical line marks week 38. — Hormone concentrations in the mother's blood during pregnancy. Original BioWelsh graph.

is absent before pregnancy, rises steeply from about week 3 and peaks at about week 9.
It then falls and stays low, because by then the itself is secreting enough .
and both rise steadily and stay high right up to birth.
High and inhibit and , so no new develop and there are no periods during pregnancy.

What each hormone does during pregnancy
Hormone	Effects
	Maintains the for the first few months, so stays high
	Maintains the and the ; stops the contracting; inhibits and ; inhibits
	Makes the uterus grow to hold the growing foetus; makes the develop; inhibits and

Because appears only in pregnancy and rises quickly, it is ideal for a pregnancy test. The diagram below shows how a test strip works.

Two panels. The left panel, headed inside the strip, shows a long cream rectangle with three purple Y shapes near the left end, a blue block two thirds along and a green block near the right end, labelled mobile antibodies, test line and control line. An arrow points up into the left end labelled urine soaks in here. The right panel, headed reading the result, shows three white strips labelled A, B and C. Strip A has two dark bars, at the test and control positions, and is marked positive. Strip B has one dark bar at the control position only and is marked negative. Strip C has no bars and is marked invalid. — How a pregnancy test detects hCG. Original BioWelsh diagram.

Urine soaks into one end of the strip and picks up mobile that bind to .
If is present, the carry it to the test line, where a second set of immobilised traps it and a coloured line appears.
Any that are still free carry on to the line, where they are trapped. This line appears whether or not the person is pregnant.
Strip A has both lines, so the test is positive. Strip B has only the line, so it is negative.
Strip C has no line at all, so the test is invalid: it does not show that the strip worked.

Lesson 20 of 21

What triggers birth and the release of milk?

Towards the end of pregnancy, falls while keeps rising. had been stopping the contracting, so as it falls the uterus becomes sensitive again and contractions can begin.

Birth is then driven by a , shown below.

Four pink boxes arranged in a square and joined by arrows running clockwise. The boxes read the baby's head pushes on the cervix, stretch receptors send impulses to the hypothalamus, the posterior pituitary releases more oxytocin, and the myometrium contracts more strongly. Text in the centre reads positive feedback, each turn of the loop makes the next one stronger. — Oxytocin and birth: a positive feedback loop. Original BioWelsh diagram.

The baby's head pushes on the and stretches it.
Stretch send impulses to the .
The releases into the blood.
makes the contract more strongly, which pushes the head harder against the .
Each turn of the makes the next one stronger, until the baby is born. The stretching then stops, so the breaks.

, from the anterior , stimulates the cells of the to secrete milk. During pregnancy the high level keeps low, so milk is not made too early. After birth the is lost, falls and rises.

has a second job after birth. When the baby suckles, impulses from the nipple cause release, which makes muscle around the milk ducts contract and eject the milk. This is another : more suckling gives more , which gives more milk.

also inhibits . Without , do not develop and there is no , so someone who is breastfeeding has a chance of conceiving. It is not a reliable method of contraception, but it does space out pregnancies.

Lesson 21 of 21

Which hormone does what? A summary

Nine hormones appear in this topic. Most lost marks come from mixing up where a hormone is made or what it acts on, so it is worth learning this table as a whole.

Every hormone in this topic
Hormone	Secreted by	Main effect
	of the testis	Stimulates and maintains the male reproductive system
	Anterior	Stimulates development and secretion
	Anterior	A surge causes ; forms and maintains the
	, later the	Rebuilds the ; growth of the uterus and ; feedback on and
	, later the	Maintains the ; stops the contracting; inhibits , and
	, then the	Maintains the in early pregnancy
		Contraction of the at birth, and ejection of milk
	Anterior	Secretion of milk by the ; inhibits
		Stimulates the anterior to secrete and

How the ideas in this topic connect

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Zoom in to read each branch. Swipe or scroll sideways to explore.

Sexual reproduction in humans
- Making gametes
  - in the seminiferous tubules
  - in the ovarian
- Fertilisation and
  - reaction, then the
  - to a , then
- Hormonal
  - The : , , ,
  - Pregnancy: , ,
  - Birth and lactation: and
- The
  - Short distance and large surface area
  - A barrier that keeps the two bloods apart

The cell released at is a , not an .
The scrotum keeps the testes cool; it does not just protect them.
Sertoli cells nourish developing ; secrete .
The secretes .
is mitosis with no growth in between.
The umbilical artery carries deoxygenated blood away from the foetus.
The contracts at birth, not the .
Maternal and foetal blood never mix.

Which organs make up the male reproductive system?

What can you see in a section of a testis?

Which organs make up the female reproductive system?

What can you see in a section of an ovary?

What is gametogenesis, and why must gametes be haploid?

How does the testis make spermatozoa?

How is a spermatozoon adapted to its job?

How does the ovary make a secondary oocyte?

What surrounds a secondary oocyte?

How do spermatogenesis and oogenesis compare?

How do you work out the real size of a cell down a microscope?

What happens when a sperm meets a secondary oocyte?

How does the oocyte stop a second sperm getting in?

How does the blastocyst reach the uterus and implant?

What happens in the ovary and the uterus over 28 days?

How do FSH, LH, oestrogen and progesterone control the cycle?

How is the placenta built for exchange?

Why must the blood of the mother and the foetus never mix?

Which hormones maintain a pregnancy, and how is one detected?

What triggers birth and the release of milk?

Which hormone does what? A summary

Worksheets

Worksheet 1: 4.1 Human reproduction (132 marks)

Check your understanding.

The testes are held in the scrotum, outside the body cavity. Which statement best explains why?

Sexual reproduction in humans

Which organs make up the male reproductive system?

What can you see in a section of a testis?

Which organs make up the female reproductive system?

What can you see in a section of an ovary?

What is gametogenesis, and why must gametes be haploid?

How does the testis make spermatozoa?

How is a spermatozoon adapted to its job?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 8 marks in total. This is a revision priority, not a prediction of your exam.

How does the ovary make a secondary oocyte?

What surrounds a secondary oocyte?

How do spermatogenesis and oogenesis compare?

How do you work out the real size of a cell down a microscope?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 8 marks in total. This is a revision priority, not a prediction of your exam.

What happens when a sperm meets a secondary oocyte?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 8 marks in total. This is a revision priority, not a prediction of your exam.

How does the oocyte stop a second sperm getting in?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 8 marks in total. This is a revision priority, not a prediction of your exam.

How does the blastocyst reach the uterus and implant?

What happens in the ovary and the uterus over 28 days?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 13 marks in total. This is a revision priority, not a prediction of your exam.

How do FSH, LH, oestrogen and progesterone control the cycle?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 13 marks in total. This is a revision priority, not a prediction of your exam.

How is the placenta built for exchange?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 16 marks in total. This is a revision priority, not a prediction of your exam.

Why must the blood of the mother and the foetus never mix?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 11 marks in total. This is a revision priority, not a prediction of your exam.

Which hormones maintain a pregnancy, and how is one detected?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 16 marks in total. This is a revision priority, not a prediction of your exam.

What triggers birth and the release of milk?

Which hormone does what? A summary

Worksheets

Worksheet 1: 4.1 Human reproduction (132 marks)

Check your understanding.

How is a spermatozoon adapted to its job?

How do you work out the real size of a cell down a microscope?

What happens when a sperm meets a secondary oocyte?

How does the oocyte stop a second sperm getting in?

What happens in the ovary and the uterus over 28 days?

How do FSH, LH, oestrogen and progesterone control the cycle?

How is the placenta built for exchange?

Why must the blood of the mother and the foetus never mix?

Which hormones maintain a pregnancy, and how is one detected?