1 Reproduction and development 2 Zanichelli editore 2016

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Reproduction and development 2 © Zanichelli editore 2016

Different reproductive strategies Vertebrates have three types of reproductive strategies: • in oviparous species, the mother lays eggs wherein the embryo develops; • in ovoviviparous species, the embryo develops inside an egg which remains inside the mother’s body; • in viviparous species, the embryo develops inside the mother’s body. 3 © Zanichelli editore 2016

Human reproduction has three characteristics: egg cell sperm cell FERTILIZATION zygote © Zanichelli editore 2016 1. it is a sexual reproduction – one egg cell and one sperm cell unite during fertilization, thereby mixing their genetic material; 2. fertilization occurs inside the female genital tract; 3. it is a viviparous reproductive strategy. 4

Male reproductive system The external male genitals include: • the testicles, that secrete androgens and stimulate the production of sperm cells; • the penis, the organ that facilitates sexual intercourse. The corpora cavernosa are formed by erectile tissue that fills with blood during an erection. 5 © Zanichelli editore 2016

Spermatogenesis is the production of sperm cells in men which occurs inside seminiferous tubules in the testicles. It is controlled by androgens. spermatogonium primary spermatocyte secondary spermatocyte Diploid spermatogonia divide by mitosis, producing primary spermatocytes. The primary spermatocyte divides by meiosis to produce haploid spermatids which, in turn, develop into mature sperm cells. spermatid 6 © Zanichelli editore 2016

The structure of the sperm cell Sperm cells consist of: • a head containing the nucleus of the cell and the acrosome; • a midpiece; • a tail, formed by a flagellum. 7 © Zanichelli editore 2016

Female reproductive system The female reproductive system consists of • ovaries (the gonads); • internal genital organs (uterus, fallopian tubes, vagina) • external genital organs (clitoris and labia). 8 © Zanichelli editore 2016

Oogenesis oogonium primary oocyte polar body secondary oocyte polar body Oogenesis is the production of female gametes – the egg cells. The process starts from oogonia which divide by meiosis to produce primary and secondary oocytes. If the secondary oocyte is fertilized, meiosis is then completed with the production of an egg cell 9 © Zanichelli editore 2016

Differences between spermatogenesis and oogenesis Spermatogenesis Oogenesis 4 gametes from one primary spermatocyte 1 gamete from 1 primary oocyte continous production cyclic production from puberty, all life long limited in time 10 © Zanichelli editore 2016

Ovulation and fertilization Ovarian follicles in ovaries are the structures in which the primary oocyte develops. During ovulation, the follicle releases an egg cell. If fertilization occurs (in the fallopian tubes), the zygote implants into the internal wall of the uterus – the endometrium. 11 © Zanichelli editore 2016

The effect of estrogens Estrogens are sexual hormones that control: • the development of reproductive organs in females; • the development of secondary sexual characteristics; • the menarche (first menstruation in puberty); • the ovarian cycle; • the uterine cycle. FSH and LH stimulate the primary oocyte to develop into a mature egg cell. Progesterone induces the thickening of the endometrium. 12 © Zanichelli editore 2016

Fertilization and the first phases of development During fertilization, the sperm cells enter the oocyte, thereby activating it. The two haploid nuclei of the gamete fuse together, generating a diploid zygote. A few hours after fertilization, the cleavage takes place: a series of mitotic divisions that lead to the formation of the embryonic stage called the morula, and then to blastocyst, which implants in the uterine endometrium. 13 © Zanichelli editore 2016

Gastrulation and neurulation Gastrulation is the second phase of embryonic development. In this phase, three different embryonic layers are formed: • endoderm; • mesoderm; • ectoderm. During neurulation, the embryo forms the notochord and the neural tube. The coelom is the main body cavity in which many organs will develop. 14 © Zanichelli editore 2016

Placenta The placenta is an organ formed by a spongy tissue which allows communication between the mother and the embryo through the umbilical cord. Its functions are to: • favor the exchange of respiratory gases and the transfer of nutrients between the mother and the embryo; • allow the embryo to eliminate waste materials; • produce hormones. 15 © Zanichelli editore 2016

Pregnancy in humans lasts 9 months and is divided into three trimesters. Tissues differentiate in the first trimester. At the end of the first trimester, the embryo becomes a fetus. In the second trimester, the fetus grows and starts moving. In the third trimester, organs start working autonomously. 16 © Zanichelli editore 2016

Delivery and lactation Pregnancy ends with the delivery of the child – a process consisting of three phases: • contractions of the uterus and opening of the cervix; • descent of the baby through the vaginal channel and birth; • expulsion of the placenta. After birth, the prolactin hormone stimulates milk production which is necessary for lactation. 17 © Zanichelli editore 2016