Human reproduction

for A2 Biology

Part 1

DJH

Male reproductive system

Bladder

Pubic bone

Vas deferens

Erectile tissueUrethra

Penis

Glans

Prepuce

Seminal vesicle

Ejaculatory duct

Prostate gland

Cowper’s glandAnus

Epididymis

Testis

Scrotum

Spongy tissue

Female reproductive system

Pubic bone

Oviduct

OvaryFimbria

Uterus

Bladder

Urethra

Clitoris

Labium minorum

Labium majorumAnus

Vagina

Rectum

Cervix

Vertebra

Vaginal orifice Perineum

Male system memory check

Write down the name and one function

for as many of structures 1 to 19 as you

can.

Female system memory check

Write down the name and one function

for as many of structures 1 to 16 as you

can.

Quick test

Write down the name and one function of each of structures 1 – 11.

Quick test

Write down the name and one function of each of structures 1 – 14.

Gametogenesis

gametes are haploid in humans, n = 23 to produce gametes, diploid (2n) cells in

the ovary or testis must divide by meiosis

meiosis consists of two consecutive divisions: the first (reduction division) is the one that separates homologous chromosomes from each other

A reminder of mitosisIn mitosis,

• chromosomes are replicated in interphase (S phase of the cell cycle), to form sister chromatids joined at the centromere

• in prophase the replicated chromosomes condense by spiralisation, and become visible in the light microscope

• chromosomes line up on the equatorial plane, where spindle fibres attach to the centromeres (metaphase)

• the centromeres divide and the sister chromatids are pulled toward opposite poles (anaphase)

• the daughter chromosomes re-disperse, nuclear envelopes form around them, and cytokinesis occurs (telophase)

The animation shows mitosis in an animal cell where 2n = 4

Meiosis

At the beginning of prophase I chromosomes condense and become visible, as in mitosis.

As they continue to condense the homologous chromosomes pair up: this never happens in mitosis.

At this point the paired chromosomes are seen to consist of sister chromatids, joined at a centromere.

The pairs of chromosomes (each called a bivalent) move to the equatorial plane and attach to spindle fibres.

This is metaphase I

As the spindle fibres contract the homologous chromosomes are pulled apart.

This is anaphase IThis is late anaphase I, leading to telophase I

Telophase I usually leads straight into prophase II: the nuclear envelope may or may nor re-form.

This is late telophase I, leading to prophase II

Meiosis II is mechanically identical to mitosis: the separated chromosomes line up on the equatorial plane ...

This is metaphase II

... and the chromatids are separated by centromere division and spindle contraction

This is anaphase II

At telophase II four haploid and genetically different daughter cells have been produced

This is telophase II leading into interphase

This was the parent cell

… and this is how it divides

Meiosis: chiasmata and crossing over

During prophase I the chromatids of the paired homologous chromosomes become intricately entwined. At this stage they randomly break and re-join at points called chiasmata.

Chiasmata

When they separate at anaphase I, the homologous chromosomes now contain both maternal and paternal portions …

… leading to even greater genetic variation in the daughter cells (gametes).

Meiosis: how much have you understood?

These diagrams show four stages in meiosis prophase I.

1 What is the diploid number of this cell?

2 Put the diagrams into the correct order.

Meiosis: how much have you understood?

These diagrams show stages in meiosis.

1 Which division of meiosis is shown?

2 How would the diagrams differ if they showed the equivalent stages of division II?

Meiosis: how much have you understood?

These ten photographs show stages in meiosis in sea urchins. Put them into the correct order and identify each stage as precisely as you can. Each mouse click will move you on one step.

Done. If you got the sequence wrong, click the return arrow to try again. Otherwise, click the forward arrow to move on.

What happens next?

What happens next?

Outcomes of meiosis Four haploid daughter cells … which are genetically different

due to … independent assortment of

maternal and paternal chromosomes at metaphase/anaphase I …

and chiasma formation and crossing over during prophase I

SpermatogenesisSpermatogenesis takes place in the wall of the seminiferous tubules of the testes.

Spermatogenesis

A single seminiferous

tubule

Stages in spermatogenesis

1

2

3

4

5

Early spermatid

Spermatogonia

Late spermatids

Secondary spermatocytes

Primary spermatocytes undergoing meiosis 1

Sertoli cells

The cells undergoing spermatogenesis are embedded in and protected by Sertoli cells (nurse cells).

Mitosis

Meiosis I

Meiosis II

Spermiogenesis

Leydig cells (interstitial cells)

Leydig cells are found in the spaces between

seminiferous tubules: they

secrete testosterone

and other androgens.

Oogenesis

Objectives:* To be able to outline gametogenesis in the human male and female**To be able to explain the role of hormones in maintenance of the human menstrual cycle and link this to changes in the ovary and uterus during the Cycle

Oogenesis

Oogenesis starts in the foetal ovaries and continues from 5 to 25

weeks of foetal life.

At birth the ovaries contain about 3 000 000 primary oocytes, all

arrested in prophase of meiosis I.

At puberty, about 400 000 primary oocytes remain in the ovaries. In each menstrual cycle one or two of these are

stimulated to complete meiosis I: the division is unequal, forming a secondary oocyte and a small polar body.

The secondary oocyte starts meiosis II, but halts at metaphase. Meiosis II is completed only after fertilisation,

forming a second polar body and an ovum.

Comparison of spermatogenesis and oogenesis

Starts at puberty

Starts during foetal life: suspended until puberty

Continuous from puberty into old age

Four functional gametes produced from every

primary spermatocyte

Only one functional gamete produced from every primary oocyte

Re-starts at menarche: completed during menstrual cycles by only a small percentage of primary oocytes

Oogenesis in the ovary

In the foetal ovary diploid oogonia are produced by mitosis

Some of these divide by mitosis to produce primary oocytes, which start meiosis I

As they do so, some of the cells around them form a covering layer called a primordial follicle

During childhood some of the primordial follicles develop into primary follicles, in which the cells surrounding the oocyte form several layers called granulosa cells.

Cells around the granulosa cells form an additional layer around the follicle called the theca.

The granulosa cells themselves secrete a protective glycoprotein layer around the primary oocyte called the zona pellucida.

Oogenesis in the ovaryHormones secreted at puberty

stimulate primary follicles to develop into secondary

follicles: the primary oocyte (still stuck in prophase of

meiosis I) grows in size, and a fluid-filled space develops in

the follicle.

During each menstrual cycle one (usually) secondary follicle

is stimulated by the hormone FSH to develop into a

Graafian follicle: in the Graafian follicle the primary oocyte completes meiosis I, forming a small first polar

body and a secondary oocyte, which is expelled in

ovulation

The menstrual cyclePituitary secretes FSH; this triggers follicle development.Granulosa cells secrete oestrogens.

Oestrogens stimulate repair of endometrium …

… and secretion of LH by pituitary.

LH triggers ovulation …

… and development of remaining follicles cells into

corpus luteum.

Corpus luteum secretes progesterone …

…which stimulates vascularisation of endometrium and inhibits FSH

secretion

Corpus luteum degenerates, progesterone level falls;

endometrium breaks down, FSH secretion re-starts, a new cycle

begins

The ovarian cycle

Follicular phase: days 1-14

Secretory (luteal) phase:

days 14-28

Ovulation: around day 14

The uterine cycleIn the follicular (proliferative) phase the endometrium is repaired and thickened under the influence of FSH.In the secretory phase the endometrium becomes vascular and spongy under the influence of progesterone.

In the menstrual phase the endometrium breaks down as the corpus luteum degenerates and progesterone secretion fails.

How well have you understood the menstrual cycle?

What is breaking down here? Why?

What are these?What has caused this repair? Where was it made?

What has caused this development?What triggers this event?What is this? What does it produce?What has caused

this change? Where was it made?

What is this? How does it get here?

End of Part 1

Fertilisation, implantation, pregnancy, birth and lactation are covered in Part 2