Reproduction and Development

• reproduction-production of eggs and sperm and the process leading to fertilization
• development-describes the sequence of events that transform a fertilized egg to a multicellular organism

• structures directly involved in reproduction
• uterus and ovaries in females and the testes in males

• human features
• used to indicate sexual maturity or sexual readiness and to attract or locate mates, or are used by males to compete for females
• ex. body hair, distribution of muscle fat, voice quality, and breasts

• ovary-organ where ova or eggs are produced
• oviduct-eggs move from the ovary to the uterus through the oviduct. 2 oviducts for each ovary
• uterus-fertilized ovum implants. where development of the embryo occurs until birth
• vagina-where the fetus passes through

meiotic cell division that produced eggs in females (oogenesis) and sperm in males (spermatogenesis)

begins during embryonic development

• fetal cells
• divide by mitosis to produce primary oocytes which then begin meiosis
• primary oocytes progress only to prophase I. they remain at this stage until they are mature
• only one primary oocyte during each menstrual cycle (~28 days) continues its development through the remainder of meiosis I.
• development occurs within an envelope of encircling cells (follicle) which protects and nourishes the developing oocyte
• during the remainder of meiosis I, cytoplasm is concentrated in only one of the daughter cells (unequal cytokinesis)
• at the end of meiosis I, one daughter cell is the secondary oocyte containing most of the cytoplasm and the other daughter cell is a polar body with very little cytoplasm
• polar body may continue with meiosis II and divide into two daughter polar bodies (ultimately disintegrate)
• concentrating the cytoplasm in one viable secondary oocyte assures that adequate amounts of store food, as well as mitochondria, ribosomes, and other cytoplasmic organelles
• sperm contributes little cytoplasm at fertilization

• marks the release of the secondary oocyte from the follicle
• if fertilized by the sperm it will move through the oviduct, the secondary oocyte will begin meiosis II and produce an egg that will combine with the chromosomes by the sperm
• second daughter cell of meiosis II (polar) disintegrates

• begins at puberty within the seminiferous tubules of the testes
• cells called spermatogonia divide by mitosis repeatedly to produce primary spermatocytes that begin meiosis
• meiosis I produces two secondary spermatocytes (after meiosis II=4 spermatids)
• sertoli cells (in seminiferous tubules) provide nourishment to the spermatids as they differentiate into mature sperm
• sperm complete their development in the epididymis, where they are store until needed

• hypothalamus and anterior pituitary initiate the reproductive cycle-hypothalamus monitors the levels of estrogen and progesterone in the blood. negative feedback-low levels of these hormones stimulate the hypothalamus to secrete GnRH which in turn stimulates the anterior pituitary to secrete FSH and LH
• follicle develops-FSH stimulates the development of the follicle and the oocyte
• follicle secretes estrogen-FSH stimulates the secretion of estrogen from the follicle
• ovulation occurs-positive feedback from rising levels of estrogen stimulate the anterior pituitary to produce a sudden midcycle surge of LH
• corpus luteum secretes estrogen and progesterone-after ovulation, the follicle (corpus luteum) continues to develop under the influence of LH and secretes both estrogen and progesterone
• endometrium thickens-estrogen and progesterone stimulate the decelopment of the endometrium, the inside lining of the uterus. thickens in preparation for implantation
• hypothalamus and anterior pituitary terminate the reproductive cycle-if no implantation negative feedback form high levels of estrogen and progesterone cause the anterior pituitary to abate production of FSH and LH
• endometrium disintegrates-in absence of FSH and LH, corpus luteum deteriorates. menstrual cycle
• implanted embryo sustains the endometrium-if there is implantation embryo secretes HCG to sustain corpus luteum. corpus luteum will continue to produce estrogen and progesterone to maintain the endometrium. w/o HCG menstruation would begin and the embryo would abort. later in development, HCG is replaced by progesterone secreted by the placenta

consists of thickening of the endometrium of the uterus in preparation for implantation of a fertilized egg and the shedding of the endometrium if implantation does not occur

• follicular phase-development of the egg and the secretion of estrogen from the follicle
• ovulation-midcycle release of the egg
• luteal phase-the secretion of estrogen and progesterone from the corpus luteum after ovulation

• gametogenesis-formation of sperm and eggs
• embryonic development-begins with fertilization of the egg and continues to birth
• process leading to reproductive maturity
• aging process to death

• 1.fertilization
• 2. cleavage
• 3.morula
• 4.blastula
• 5.gastrula
• 6.extraembryonic membrane development
• 7.organogenesis

• occurs when the sperm penetrates the plasma membrane of the secondary oocyte
• steps:
• recognition-sperm secretes a protein that binds with special receptor molecules that reside on a glycoprotein layer surrounding the plasma membrane of the oocyte (vitelline layer-ensures the fertilization occurs only between egg and sperm of the same species)
• penetration-plasma membranes of the sperm and oocyte fuse, and sperm nucleus enters the oocyte
• formation of the fertilization membrane-vitelline layer forms a fertilization membrane which blocks the entrance of additional sperm
• completion of meiosis II in the secondary oocyte-sperm penetration triggers meiosis II in the oocyte producing a ovum and polar body. polar body is discharged through the plasma membrane
• fusion of nuclei and replication of DNA-sperm and ovum nuclei fuse, forming a zygote nucleus consisting of 23 pairs of chromosomes. each chromosome replicates so that it consists of two identical chromatids

• rapid cell divisions without cell growth
• each of the resulting cells (blastomeres) contains substantially less cytoplasm than the original zygote
• steps:
• embryo polarity-egg has an upper animal pole and a lower vegetal pole. cells formed at the vegetal pole contain more yolk or store food
• polar and equatorial cleavages-early cleavages are polar, dividing the egg into segments that stretch from pole to pole. other cleavages are parallel with the equator
• radial and spiral cleavages-in deuterostomes, early cleavages are radial, forming cells at the animal and vegetal poles that are aligned together. protostomes, cleavages are spiral forming cells on top that are shifted with respect to those below them
• indeterminate and determinate cleavages-cleavage is indeterminate f it produces blastomeres that, if separatedd can individually complete normal development. blastomeres produced by a determinate cleavage cannot develop into a complete embryo if separated from other blastomeres. radial cleavages of deuterostomes are usually indeterminate, while spiral cleavages of protostomes are often determinate

successive cleavage divisions result in a solid ball of cells

• cell divisions continue, liquid fills the morula and pushes the cells out to form a circular cavity surrounded by asingle layers of cells
• hollow sphere of cells

cavity of blastule

• formation of the gastrula (gastrulation) occurs when a group of cells invaginate (move inward) into the blastula
• forms two-layered embryo with an opening from the outside into a center cavity
• steps:
• 3 germ layers-3 cell layers forms between the outer and inner layers of the invaginated embryo. 3 cell layers: ectoderm (out), mesoderm (middle), endoderm (inside)
• archenteron-center cavity formed by gastrulation. completely surrounded by endoderm cells
• blastopore-opening into the archenteron. it becomes the mouth (protostomes) or the anus (deuterostome)

• chorion-outer membrane. for birds and reptiles this membrane acts as a gas exchange. mammals chorion implants into the endoterium. chorion forms the placenta. placenta is a blend of maternal and emryonic tissues across which gases nutrients, and wastes are exchanged
• allantois-begins as a sac that buds off from the archenteron. stores waste products (look more)
• amnion-encloses amniotic cavity-fluid filled cavity that cushions the developing embryo much like the coelom cushions internal organs in coelomate animals
• yolk sac-bids and reptiles- digests the enclosed yolk. blood vessels transfer the nutrients to the developing embryo. in placental mammals-yolk sac is empty. nutrition is obrained through the placenta