Human Reproduction


Male Reproductive System

Testes - Contain seminiferous tubules and epididymis. 

Scrotum - An external sac that protects the testes and keeps ut at 37C, the optimum temperature. 

Penis - Organ which introduces sperm to the uterus. 

Seminiferous Tubule - Produce spermatozoa and contain interstitial cells which produce testosterone. 

Epididymis - Sperm are stored here before ejacu lation. 

Vas Deferens - Sperm pass along here before ejacu lation

Seminal Vesicle - Produces mucus to help motility

Prostate Gland - Produces an alkaline secretion that neutralises the urine and aids sperm motility. 

Urethra - Passageway between the bladder and external part of the body. 

Ureter - Connect the kidney to the bladder. 

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Femlae Reproductive System

Ovaries - There are 2 of these and they produce ova. 

Oviducts - Ova pass through these to reach the uterus. 

Uterus - Womb, has muscular walls lined by endometrium, 

Endometrium - Mucus membrane well supplied with blood. Embryo plants itself during implantation. This membrane sheds during menstruation. 

Cervix - Ring of muscle and connective tissue and is the entrance to the uterus. 

Vagina - Provides a passageway for blood and mucosa tissue from the uterus during menstraution. Provides a passageway for childbirth. 

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Sperm are made in the semi niferous tubules. 

They are made from germinal epithelial cells. 

Fsh and testosterone are required for the development. 

Germinal Epithelial cells form diploid spermatogonia. Mitosis forms a primary spermatocyte and another spermatogonium. Meiosis 1 then forms a secondary spermatocyte which is a haploid. Meiosis 2 then takes place and forms 4 haploid spermatids. The spermatids mature and differentiate to form sperm. 

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Semi Niferous Tubules

Sertoli cells secrete a fluid that nourishes the spermatids and protects them from the immune system of the male. They also provide oxygen and remove waste. 

Interstitial cells are visible by light micrographs. They are stimulated by the LH hormone. They also secrete testosterone which stimulates spermatogenesis and causes male sexual characteristics to develop. 

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Sperm Structure

Head - Contains a haploid nucleus and is covered by a lysosome called acrosome which contains enzymes used in fertilisation. 

Mid Piece - Packed with mitochondria which provide atp fir movement. 

Tail - The flagellum makes lashing movements that move the sperm and allow motility. 

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Starts in the embryo where the germinal epithelial cells undergo mitosis to form oogonia. This also undergoes mitosis to form primary oocytes and oogonia. The primary oocytes start meiosis 1 but stop in prophase. This happens before a woman is born so she has millions of primary oocytes before birth. 

Once a month, the oocytes mature and complete meiotic division to form a secondary oocyte. This stops at metaphase 2.  A polar body is formed with half the chromosomes. 

In fertilisation meiosis 2 is completed and an ovum and a 2nd polar body are formed.

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Follice Production

During oogenesis, epithelial cells divide to form follicle cells which surround primary oocytes to form primary follicles. 

At puberty, hormones stimulate the follicles to develop further. Each month follicle cells develop further into a mature Graafian follicle containing a secondary oocyte. 

The mature follicle migrates to the surface of the ovary where it hurts and the secondary oocyte is released. 

This is ovulation. The Graafian follicle then becomes the corpus luteum which secretes hormones for pregnancy. If fertilisation doesn't occur it becomes the corpus Albicans

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Secondary oocyte after ovulation

A clear glycoprotein layer called the zona pellicuda surrounds the cell membrane of the secondary oocyte. The chromosomes of the secondary oocyte are at metaphase 2. 

Corona Radiata cells surround the secondary oocyte and provide nutrients to the secondary oocyte. 

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Oogenesis Summary

Before Birth - Germinal epithelial cells divide to form oogonia which then undergo mitosis to form primary oocytes. These undergo meiosis 1 but stop at prophase 1.  This oocyte is in the primary folicle. 

Puberty - The primary oocyte completes meiosis 1 to form a secondary oocyte which is haploid. This undergoes meiosis 2 but stops at metaphase 2. The follicle becomes Graafian and contains a secondary oocyte. 

fertilization - The secondary oocyte completed meiosis 2 to become an ovum. 

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Horomes in the mesntrual cycle

FSH is secreted by the anterior pituitary gland. It triggers the development of a follicle and stimulates the production of oestrogen. 

Oestrogen inhibits FSH production so no follicles develop. It also stimulates LH production. It helps the growth and repair of the endometrium. 

A high level of LH causes ovulation to occur and allows the Graafian follicle to mature into the corpus luteum. 

The corpus luteum secretes progesterone which further develops the endometrium it also inhibits LH production and increases mucus levels in the cervix to block the entry of sperm. 

If implantation does not occur low FSH and LH levels mean that the Corpus Luteum can degenerate. Progesterone levels fall and the endometrium breaks down and is lost during menstruation. FSH is no longer inhibited and another menstrual cycle is initiated. 

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Arterioles entering the penis dilate and veins leave the penis constricted. The build-up of blood causes the **** to be erect. Sex happens. *********** occurs in the vagina and sperm goes from the cervix to the uterus. 

The sperm are attracted by chemoattractants released from the secondary oocytes and swim to the oviducts. They can remain viable for 2-5 days but are most fertile 12-14 hours after ejacutlation. Secondary oocytes are only viable for 24hrs until fertelised. 

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Sperm can only fertilise a secondary oocyte after a process called capacitation has occurred. It can happen several hours after copulation. 

Capacitation involves changes in the membrane covering the acrosome where cholesterol and glycoproteins are removed. This increases the permeability of the acrosome membrane to ca2+ ions and improves the sperms motility. 

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Acrosome Reaction

The sperm push their way through the cells of the corona radiata. 

When contacted with the zona pellicuda the acrosome membrane ruptures and releases hydrolysed enzymes which digest a path through the zona pellicuda. 

The membranes of the sperm and secondary oocyte fuse and the genetic material of the sperm enter the secondary oocyte. 

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Cortical Reaction

Once a sperm successfully penetrates the secondary oocyte the oocyte's endoplasmic reticulum releases calcium ions to the cytoplasm.m

The arrival of these ions causes granules to fuse with the cell membrane and release their contents to helo modify the zona pellicuda

The pellicuda hardens to format fertilisation membrane. Thsi prevents other sperm from pentrating the egg. 

Entry of the genetic material triggers meiosis 2 to continue forming the ovum and a polar body. The nuclei and the ovum fuse to form a zygotic nucleus. The ovum is now called a zygote. 

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The zygote undergoes repeated mitotic divisions called cleavage to form a ball of cells. 

Cleavage continues and by 7 days the ball of cells becomes hollow and is now a blastocyst. 

The blastocyst is moved into the uterus where it attaches and embeds into the endometrium. This is known as implantation. 

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Amniotic Fluid

The embryo grows and develops into the uterus enclosed by the amnion. 

The amnion is a membrane derived from the blastocyst. 

Amniotic fluid helps maintain fetal temperature. 

It also acts as a shock absorber protecting the fetus from injury outside the uterus. 

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The placenta forms between the tissue of the mum and fetus and are connected with the umbilical cord. 

The cord contains the baby's veins and arteries. 

Arteries carry deoxygenated blood and co2 away from the fetus. 

The veins carry oxygenated blood and oxygen towards the fetus. 

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Fetal Cappilaries.

Found in the chronic villi which form the boundary between mother and fetal blood. 

Blood from the maternal arterioles empties into an intervillous space. There are no capillaries between the mum veins and arteries so there is one less layer for diffusion to occur across. 

Foetal capillaries bathe in this space so that material exchange can occur. Blood containing waste materials can then be removed to the mum's venules from the intervillous space. 

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Placenta Roles

Material exchange across a thin barrier which separates maternal and foetal blood. 

Provides protection from the mothers immune system. 

Protects the fetus from differences in blood pressure as the mums blood pressure would be too high for the fetus.

Allows the passage of maternal antibodies into the foetus for short term immunity. This is known as passive immunity. 

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Hormones in Pregnancy

Before and following implantation, HCG is secreted to maintain the corpus luteum for the first 16 weeks of pregnancy. 

The placenta then secretes progesterone and oestrogen which reach high levels in the blood. 

These hormones suppress FSH and LH production. 

Progesterone suppresses the uterine walls' ability to contract and maintains the endometrium. 

Oestrogen stimulates the growth of the uterus to accommodate the growing fetus and the development of mammary glands.

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Just before birth oestrogen and progesterone levels decrease which means the uterine wall contracts. 

Oxytocin stimulates the contraction of the myometrium. These contractions have positive feedback no oxytocin therefore more is produced. So as oxytocin levels increase contractions become more and more frequent. 

Prolactin is released from the anterior pituitary gland during and after birth and stimulates the production of milk from mammary glands. 

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