Anatomy & Physiology
Lecture Outline: Week #7
- Reproduction and Development
LSc 107 Anatomy and Physiology Spring 1999
Reproduction and Development: Chapters31, 32, 33
Organs of reproduction - gonads (ovary and testis)
- produce gametes (sex cells, sperm and egg)
- produce sex hormones
all other organs are accessory
Formation of Gametes
- Male gamete is a sperm, female gamete is an egg (ovum)
- All cell in humans except the gametes (and red blood cells
which do ot contain nuclei) have 23 pairs of chromosomes (one
from mother, one from father) in the nucleus and are considered
diploid
- Fertilization combines a sperm and an egg; in order to maintain
the number of chromosomes at 46, the sperm and egg which join
to form the new organism must each have only 23 chromosomes (haploid),
rather than pairs
- Gametes are formed during meiosis, a reduction division
of the cell, which reduces the chromosome number by half.
- 2n is the normal number of chromosome pairs, and the cell
is called diploid
- 1n means single chromosomes (not pairs), and the cell is
called haploid
Spermatogenesis Fig. 31-5 & 33-3
- In testis – seminiferous tubules – lined with immature
cells called SPERMATOGONIA (diploid)
- After 2 meiotic divisions, spermatids and specialized spermatazoa
located near the center of the tubule are haploid, 4 sperm are
produced from one spermatogonium
Anatomy of sperm:
- HEAD - acrosome - contains enzymes which help penetrate the
egg, so that one sperm can fuse with the egg, and nucleus containing
the genetic material (23 chromosomes)
- MIDPIECE - mitochondria - energy
- TAIL - locomotion
- Maturation of sperm takes several weeks in all. About 300
million are ready each day, and once ejaculated have a life of
48 hours
Oogenesis Fig. 32-3 & 33-4
- oogonium is the diploid stem cell
- A mitotic division produces a diploid primary oocyte, occurs
in the female embryo before birth, and the egg remains in this
stage of development (meiosis I) until the child reaches puberty.
(Meiosis not complete until after fertilization).
- Each month a few follicles continue maturation and develop
into secondary oocytes (2n)
- Only one mature ovum is produced from each primary oocyte
- other material become polar bodies
- Only one or two ova are released from the ovary each month
- Remaining follicle becomes corpus luteum
- hormonal control of menstrual cycle Fig.32-16
Comparison of spermatogenesis and oogenesis
- only 1 ovum produced (and 3 polar bodies), four sperm produced
for each stem cell
- new sperm production continues throughout life after puberty,
females are born with all the immature eggs they will have
- sperm are specialized for motility, egg specialized for maintenance
of developing embryo
READ - STD's - sexually transmitted diseases Table 32-1 p.
926
Fertilization & Implantation Fig. 33-6, Fig. 33-9
- ovum secretes factors which attract sperm
- only one sperm can penetrate egg, after sperm penetrates
outer covering of ovum, the surface of the oocyte is changed
blocking further sperm from entering
- Egg 23 chromosomes + sperm 23 chromosomes = zygote 23 pairs
of chromosomes
- Cleavage (rapid mitotic divisions)
- Implants in wall of uterus
Embryonic layers p.945
- Ectoderm - epidermis, brain spinal cord, some endocrine glands
- Mesoderm - dermis, skeletal muscle and bone, blood, bone
marrow, kidneys
- Endoderm - Lining of GI tract, respiratory tract, urinary
tract, endocrine glands
Embryological development
- amniotic cavity extends and fills with fluid
- chorionic villi develop in week 3 (facilitate diffusion of
nutrients and oxygen & removal of wastes) sampling at week
10
- placenta formed by week 10
- primitive streak at week 3 followed by rapid development
of CNS, heart, limbs, digits, facial features, other organ rudiments
- by week 8, all organs that will be present in a newborn will
have begun to develop
- Differentiation into male or female 6th week of embryonic
life
If Y chromosome is present then male develops. In the absence
of this female develops.
- by 3rd month of pregnancy fetus resembles newborn
- bone replaces cartilage
- head becomes more proportional
- 3rd trimester fetus grows and moves actively,
fat fills out skin, digestive and respiratory systems mature
Developmental Problems
- Critical Period: time in development when a particular structure
is sensitive to damage from a drug or gene's action
- neural tube defects (NTD) if fold housing the CNS does not
close by day 28 (can often be detected by levels of alpha fetoprotein
(AFP) in maternal blood
- Fetal Alcohol Syndrome: critical period unknown, different
effects in different women due to differences in alcohol metabolism
- Cocaine: can cause strokes in fetus, newborns unable to make
sense of surroundings
- cigarettes: CO build-up in fetus, decreased growth factors,
low birth weight
- Down's syndrome: trisomy 21
Infertility
- inability to conceive a child after 1 year of trying
- 1/6 families
- physiological cause ~ 90% (male ~30%, female ~60 %)
Causes in Male
- low sperm count
- malformed sperm
- immune reaction against sperm
- hormone imbalance
- varicose veins in scrotum
Causes in female
- irregular cycle
- tumors
- underactive thyroid
- blocked Fallopian tubes
- endometriosis
- anti-sperm secretions
- misshaped uterus
- lack of sperm attractant molecules
Reproductive Technologies
- AI (artificial insemination)
- surrogate (gestational or genetic)
- IVF (in vitro fertilization)
- GIFT (gamete intrafallopian transfer)
- ZIFT (zygote intrafallopian transfer)
- embryo adoption
- pre-embryo screening
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