Human reproduction

DJH Human reproduction for A2 Biology Part 1

Male reproductive system Bladder Pubic bone Seminal vesicle Vas deferens Ejaculatory duct Prostate gland Urethra Cowper’s gland Erectile tissue Anus Spongy tissue Penis Epididymis Glans Testis Prepuce Scrotum

Female reproductive system Vertebra Oviduct Ovary Fimbria Uterus Cervix Bladder Rectum Pubic bone Urethra Vagina Clitoris Labium minorum Anus Labium majorum 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 mitosis • In 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 This is telophase II leading into interphase This is anaphase II This is metaphase II This is late anaphase I, leading to telophase I This is late telophase I, leading to prophase II This is metaphase I This is anaphase I This was the parent cell 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. Telophase I usually leads straight into prophase II: the nuclear envelope may or may nor re-form. As the spindle fibres contract the homologous chromosomes are pulled apart. The pairs of chromosomes (each called a bivalent) move to the equatorial plane and attach to spindle fibres. At this point the paired chromosomes are seen to consist of sister chromatids, joined at a centromere. ... and the chromatids are separated by centromere division and spindle contraction Meiosis II is mechanically identical to mitosis: the separated chromosomes line up on the equatorial plane ... At telophase II four haploid and genetically different daughter cells have been produced … and this is how it divides

Meiosis: chiasmata and crossing over When they separate at anaphase I, the homologous chromosomes now contain both maternal and paternal portions … 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. … leading to even greater genetic variation in the daughter cells (gametes). Chiasmata

Meiosis: how much have you understood? • These diagrams show four stages in meiosis prophase I. • What is the diploid number of this cell? • Put the diagrams into the correct order.

Meiosis: how much have you understood? • These diagrams show stages in meiosis. • Which division of meiosis is shown? • 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?

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

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

Spermatogenesis A single seminiferous tubule

Stages in spermatogenesis Late spermatids 5 1 Spermatogonia Secondary spermatocytes 3 Primary spermatocytes undergoing meiosis 1 2 4 Early spermatid

Sertoli cells Spermiogenesis Meiosis II Meiosis I Mitosis The cells undergoing spermatogenesis are embedded in and protected by Sertoli cells (nurse cells).

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 during foetal life: suspended until puberty Starts at puberty Re-starts at menarche: completed during menstrual cycles by only a small percentage of primary oocytes Continuous from puberty into old age Four functional gametes produced from every primary spermatocyte Only one functional gamete produced from every primary oocyte

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

How well have you understood the menstrual cycle? What is this? How does it get here? What are these? What has caused this development? What triggers this event? What is this? What does it produce? What is breaking down here? Why? What has caused this repair? Where was it made? What has caused this change? Where was it made?

End of Part 1 • Fertilisation, implantation, pregnancy, birth and lactation are covered in Part 2

Human reproduction