The Human Reproductive System

Meiosis and gamete formation take place in special reproductive organs called testes in males and ovaries in females.

The Human Male Reproductive System In early development, the testes move from the abdomen down, to the outside of the body. Why is this necessary? Testes are housed in the scrotum. Each testis has 1000s of packed coils of tiny tubules in which meiosis occurs and sperm form.

Anatomy of the Testes Seminiferous Tubules: packed, tightly coiled tubules where sperm cells are produced. They are about 80cm long. Sperm are made continuously. Epididymis: sperm are stored in this long, coiled tube. Sperm mature here and become capable of swimming. Vas Deferens: sperm enter here from the epididymis. Sperm are stored here until they move into the urethra.

Spermatogenesis: the process by which sperm are produced in males. -It occurs in the testes (male reproductive organ). The human male produces over 100 million sperm a day! The ram produces over 1 billion sperm a day! A human male is considered sterile when the sperm count is less than 20 million sperm/ ml of semen.

Sperm formation Meiosis and sperm formation take about nine weeks. New sperm are formed as mature sperm care absorbed or released. As sperm are absorbed or released, others move to the epididymis and to the vans deferens. Sperm formation begins at puberty in males.

The Mature Sperm Cell The head contains the haploid nucleus with the chromosomes. The acrosome contains enzymes that help the sperm penetrate the egg. The midpiece contains many mitochondria that provide energy (ATP) to move the tail. The tail is a flagellum. Only the head of a sperm enters the egg during fertilization

Vasectomy A minor operation performed on adult males as a method of birth control. The vas deferens are cut to short- circuit the path of the sperm. A vasectomy does not effect normal sexual function or the secondary sex characteristics.

Hormones Control the Reproductive Cycles The onset of puberty and gamete formation are controlled by body chemicals called hormones. Hormones are chemical signals that travel through the blood as messengers to other organs. Sex hormones trigger sexual maturity at puberty.

Hormones Control the Reproductive Cycles Hormones in the pituitary gland of the brain stimulate the testes to secrete distinctive sex hormones. Gender is determined at fertilization but the ultimate development of male parts is determined by hormones. Sex hormones also control voice, growth of hair and body proportions.

Hormones Control the Reproductive Cycles Two hormones released by the anterior pituitary gland regulate the function of the testes. FSH (Follicle Stimulating Hormone) stimulates the sperm production in the seminiferous tubules. LH (Luteinizing Hormone) stimulates secretion of the sex hormone testosterone in the cells between the seminiferous tubules. Testosterone is the important male hormone responsible for the development of secondary sex characteristics.

It’s All About the Egg Oogenesis: the process by which eggs (oocytes) are produced in the female. -It occurs in the ovary (female reproductive organ). Meiosis begins before birth. The 1 st meiotic division stops so that millions of potential ova stop development in prophase I until puberty.

Cytoplasm divides unequally. One cell, the egg cell gets all the cytoplasm. The polar bodies disintegrate. Oogenesis

The primary oocyte is formed in utero. All the egg cells that a female will have in her lifetime are present at birth. In a female at birth, there are 2 million immature eggs. These eggs are stalled in the prophase of the first meiosis. At puberty, the sex hormones develop and this enables meiosis to resume. Only about 500 eggs are used in a woman’s reproductive cycle. The second meiosis only finishes as the sperm penetrates the egg.

Anatomy of the Female Reproductive System

Puberty in the Female Meiosis begins before birth. The 1 st meiotic division stops so that millions of potential ova stop development in prophase I until puberty. Meiosis in the ovary does not resume until puberty. Once a month, one cell resumes meiosis. The potential ovum continues into the 2 nd meiotic division and the process stops again. The cell is released from the surface of the ovary in a process called ovulation.

Ovulation Ovulation: one egg is released each month, generally. The final stage of meiosis is completed only when the egg is penetrated by the sperm. Only ova are released during an entire lifetime, from puberty to age 50. Some ova may take as long as ½ a century to complete meiosis!

Hormones Control the Reproductive Cycles Sex hormone interactions in the female are more complex. The nervous system, glands and organs and a variety of hormones interact to regulate the monthly ovum-releasing cycle called the menstrual cycle. The egg is discharged from the ovary, to the oviduct or fallopian tube, to the uterus.

Hormones and Menstruation If the egg is fertilized by a sperm, the fertilized egg develops in the uterus until birth. During the menstrual cycle, the inner lining of the uterus builds up in preparation for a fertilized egg.

Hormones and Menstruation If the egg is not fertilized, the lining which develops a rich blood supply, disintegrates and passes from the uterus through the vagina to the outside of the body. The first day of menstrual flow is the start of the menstrual period.

Hormones and the Menstrual Cycle Hypothalamus is like the thermostat: monitors and adjusts the levels of hormones in the body At the start of the menstrual cycle (actual menstruation), the levels of estrogen and progesterone (both made by the ovary) are very low. This causes the hypothalamus to secrete a releasing hormone that turns on the pituitary gland. Pituitary gland secretes FSH (follicle stimulating hormone) and LH (luteinizing hormone).

Hormones of the Human Reproductive Cycle

Hypothalamus Pituitary Hormones Ovarian Hormones Menstrual Cycle Ovarian Cycle

Hormones and the Ovary FSH causes the ovum to begin maturing into a follicle on the surface of the ovary. The follicle begins to secrete estrogen. Estrogen affects the uterus and the uterine lining builds up. The effects of estrogen and the two pituitary hormones build for about 14 days.

Hormones and the Ovary Just before day 14, a sudden increase in LH causes the follicle to burst and release the ovum. This is called ovulation. The ruptured follicle becomes the corpus luteum. The corpus luteum releases estrogen and progesterone. Both hormones build up the uterus lining.

Hormones and the Ovary The high estrogen and progesterone turn off the hypothalamus and the pituitary secretion of FSH and LH. If the egg is not fertilized in the oviduct, the corpus luteum begins to degenerate and estrogen and progesterone levels decrease. The blood supply to the uterus decreases. Menstruation (the shedding of the uterine lining) begins.

Menstruation and Fertility Menstruation alone is an unreliable indicator of fertility. A female may not have had her first menstrual period but may have released an ovum that can be fertilized. Menopause: the cessation of the menstrual cycle when a female can no longer reproduce.

Fertilization and the Zygote Fertilization: the formation of a zygote Sperm+Egg ► ► ► Zygote (1n) (1n) (2n) Fertilization takes place in the oviduct (fallopian tube) of the female reproductive system.

One Egg and One Sperm Only one sperm fertilizes each ovum and only the head of the sperm (with the genetic material) enters the egg. The membranes of the ovum immediately react to fertilization and act as a barrier to the entry of other sperm. Other sperm secrete an enzyme that helps to break down the layer of cells that surround an ovum. Many sperm are needed to do this.