Human Reproduction – part II

1. Human Reproduction – part II In part I the processes underlying formation and maturation of gametes in both males and females was covered. In this part we’ll follow the processes of fertilization, development, and birth. Fertilization Successful fertilization depends on the structure, adaptations, and performance of sperm. Structure first:

2. The sperm absorbs fructose, ascorbic acid (vitamin C) and amino acids from semen, and the long mitochondrion makes metabolic energy (ATP) available for the flagellum to power movement through the upper vagina, the uterus, and the oviduct. When sperm reach the egg (there must be many), the acrosome at the head releases enzymes that digest a path through a jelly coat surrounding the egg. Then they reach the vitelline layer, on which there are species-specific binding sites. After binding, sperm can penetrate this layer. One sperm’s plasma membrane fuses with the egg plasma membrane, and its nucleus enters the egg cell.

3. This is what the egg cell and supporting cells look like in humans as they move into the oviduct. You can see why potent enzymatic digestion is necessary for the sperm to reach the egg cell membrane. (Jelly coat)

4. Plasma membrane fusion causes the egg cell plasma membrane to become impermeable; only one sperm gets to fertilize the egg and create a diploid embryo. The fusion also brings the egg out of metabolic dormancy, and begins the development process. The vitelline layer separates from the plasma membrane and becomes a fertilization envelope within a few minutes. The intervening space fills with fluid. Here’s the process diagrammatically: (note that human fertilization occurs in an inaccessible place, so this diagram reflects what happens in an externally fertilized species)

6. The fertilized egg is called a zygote. Activation of the metabolic machinery of the zygote also begins a process of repeated cell division called cleavage. About 3 days after fertilization the embryo is a ball of ~8 cells. This stage is called a morula. Cleavage continues, forming a ball of cells (> 1000) that is initially solid, but then becomes hollow with a fluid filled center. This stage is called a blastula. It is at this stage that the embryo implants in the wall of the uterus. Essentially as soon as implantation, the embryo begins secreting Human Chorionic Gonadotropin.

7. HCG causes the corpus luteum to enlarge and produce more progesterone, which prevents menstruation. It is also soon detectable in urine and blood, indicating pregnancy. Later, the placenta produces Human Chorionic Somatomammotropin. This is the baby’s way of influencing its food source. Here’ a diagram of the implantation process and the establishment of a circulatory link between mother and embryo:

8. More detail than you need is incorporated into this diagram. What is important? The trophoblast grows into the endometrium, and induces the expansion of the mother’s circulatory system in the vicinity of implantation. The chorion develops from the trophoblast. It produces HCG. The amnion also comes from embryonic cells.

9. The next phase of embryonic development is called gastrulation. During gastrulation cells migrate. Initially an opening forms from the surface into the fluid filled blastocoel. A smaller, separate fluid-filled chamber forms, that is called the archenteron. It is the digestive cavity. The cells migrating inward continue dividing, and the three basic tissue layers form: ectoderm, mesoderm, and endoderm.

10. The gastrulation diagrammed on the previous slide was for a frog embryo. Ectoderm forms the outer layer of the embryo. Endoderm forms the inner layer and a yolk plug that fills the blastopore. Mesoderm differentiates between those other two layers. Gastrulation is deemed to have finished when the 3 layers are all formed. What was the blastopore will eventually be open again, as the organism’s anus.

11. The next phase of development is neurulation. In this phase a notochord forms from a portion of the mesoderm, and the ectoderm above it thickens to become the neural plate. Then continued growth of the neural plate leads to it ‘folding over on itself’. The folded over part becomes a tube that separates from the ectoderm. It is the neural tube, which is the beginning of the nervous system.

12. The neural tube will become the brain and spinal cord of the organism. The neural tube is located just above the notochord, which will eventually be replaced by the vertebral column; it will enclose the spinal cord. Somites form as segmented portions of mesoderm. They become vertebrae and muscles

13. In vertebrates the mesoderm ‘splits’ leaving a cavity, which is the body cavity still evident in you. It’s called the coelom, and this kind of development is called schizocoelous.

14. Very early on at least primitive versions of most mature tissues and organs are formed. From ectoderm: epidermis, epithelial lining of mouth and rectum, sense receptors, nervous system, adrenal medulla From mesoderm: skeleton, muscles, circulatory system, excretory system, reproductive system (except germ cells, adrenal cortex From endoderm: lining of the respiratory system, liver, pancreas, thyroid & parathyroid glands, linings of the reproductive system

15. After organs have been formed growth dominates. In humans, that’s all that occurs in the third trimester. There are some important basic processes that go on along the way: Apoptosis – programmed cell death. For example, your hands and feet begin as pads. It is cell death that separates your fingers and toes. It’s also important in development of the nervous system. Induction – An enormous amount of signaling goes on between cells, where one cell causes another to differentiate in a particular way. Development of the eye results from signals between optic vesicle and overlying ectoderm.

16. Formation of the optic vesicle and the shape change that occurs as it develops towards becoming the retina was induced during gastrulation. The optic vesicle in turn induces overlying ectoderm to differentiate into lens, then it induces surface ectoderm to differentiate into the cornea.

17. Signals and the differentiation process show very strong positional effects. Development itself also has an ‘axis’. The head end of the organism tends to develop earlier than the ‘tail’ or lower end. Timing is also very critically integrated. Altered development can result from exposure to various kinds of teratogens. Measles, thalidomide, etc. alter the expression of genes, producing various physical abnormalities, but don’t alter the genes themselves. Depending on when a pregnant woman took thalidomide, her child could have ‘seal-like’ arms or legs. The effect on legs would have come from a slightly later exposure to the drug.

18. It is typical to divide pregnancy in humans into 3 month blocks or trimesters. Within the first trimester the fetus looks like a very miniature human. It has all body parts – a beating heart, arms, legs, eyes, … and, with a little luck an ultrasound image can detect its sex. During the second trimester features and organs are refined. Fingers and toes become fully distinct; eyebrows, eyelids, and eyelashes develop, the placenta takes over HCG production to maintain the level of progesterone. During the third trimester everything grows and strengthens.

19. It is now time for birth (formally: parturition. Hormones at birth and beyond Approaching parturition (fancy word for birth) the Mother’s blood increases its estrogen concentration. That increases the number of oxytocin receptors in the uterus. The fetus and pituitary release oxytocin. This hormone stimulates uterine contraction and causes fetal membranes to release prostaglandins that increase the strength of contractions.

20. When the fetus reaches “birth position” and presses against the cervix, this mechanical stimulation increases oxytocin release. More pressure…more oxytocin - a positive feedback cycle. Labor occurs, the cervix dilates (full dilation = ~10cm.) Uterine contractions force the baby through the birth canal, then parturition. Hormones are also important after birth…

21. After birth, hormones are critical to nursing. The baby’s suckling stimulates the mother’s pituitary to release prolactin. Prolactin stimulates the milk glands to produce milk. The suckling also stimulates the pituitary to produce oxytocin. Oxytocin again acts as a muscle stimulant. This time it is muscles around the milk glands, causing them to eject milk out the nipple into the baby’s mouth.