Sexual Reproduction in Humans

• Breasts are rounded structures present on the ventral thoracic wall.
• Each breast consists of fatty tissue, connective tissue and mammary glands.
• Mammary glands contain 15 to 20 mammary lobes which contain cluster of called alveoli.
• The cells of alveoli secret milk into mammary tubules which unit to form a mammary duct.
• Mammary ducts join to form a mammary ampulla which is connected to the lactiferous duct through which milk is sucked out.

• The process by which male and female sex cells or gametes (sperms and ova) are formed in the male and female gonads (testes and ovaries) is called gametogenesis.

• The process of the formation of spermatozoa or sperms from spore mother cells or spermatogonia of the germinal epithelium lining the seminiferous tubules is called spermatogenesis.
• It starts in puberty.
• Some of the spermatogonia called primary spermatocytes undergo first meiotic division and produce secondary spermatocytes.
• Primary spermatocytes contain 46 chromosomes while the secondary spermatocytes contain 23 chromosomes, hence; the secondary spermatocytes are haploid.
• The secondary spermatocytes under second meiotic division and form four haploid spermatids.
• Hence, from one diploid primary spermatocyte four haploid spermatids are produced.
• Spermatids are transformed to spermatozoa (sperms) by spermiogenesis.
• After spermiogenesis, sperm heads become embedded in the Sertoli cells. They are released from the seminiferous tubules by spermiation.

• The process of the formation of a mature female gamete (ovum) is called oogenesis.
• It occurs in the ovaries of females.
• During embryonic development, few oogonia are formed within each fetal ovary. 
• No more oogonia are formed after birth.
• Oogonia undergo meiotic division and enter prophase-I and get temporarily arrested at this stage. They are called primary oocytes.
• Each primary oocyte is then gets surrounded by a layer of granulose cells and is called primary follicle.
• The primary follicle gets surrounded by more layers of granulosa cells and a new theca. It is now called secondary follicle.
• Secondary follicle is transformed into a tertiary follicle which has fluid filled cavity called antrum.
• The primary oocyte present in the tertiary follicle completes the first meiotic division and forms the secondary oocyte and the first polar body.
• The tertiary follicle transformed into a Graafian follicle.
• The Graafian follicle ruptures and releases the secondary oocyte.
• The secondary oocyte enters the second meiotic division but gets arrested at metaphase –II until the sperm enters the secondary oocyte.
• When the sperm enters the secondary oocyte, it continues its unequal second meiotic division and forms an ootid which is an ovum and a second polar body. (The sperm fertilises the ovum which is a transformed ootid).
  
  
  
  

• The human ovum is devoid of a yolk.
• It has abundant cytoplasm called ooplasm and a centrally located nucleus.
• The ovum is surrounded by a transparent, non-cellular layer called zona pellucida.
• In human beings, the ovum is released from the ovary as a secondary oocyte.
  
  

Menstrual Cycle

• In human females, menstruation occurs at an average interval of 28–29 days. The cycle of events starting from menstruation till the next one is called the menstrual cycle.
• One ovum is released during the middle of each menstrual cycle.
• Menstruation starts between 12 and 15 years of age and continues until 45–50 years.
  
  

Menstrual Phase

• Menses takes place on the 3^rd–5^th day of the menstrual cycle of 28 days.
• The reduced production of LH causes degeneration of the corpus luteum.
• The uterine endometrium breaks down and menstruation starts.
• Cells of the endometrium, secretions, blood and the unfertilised ovum constitute menstrual flow.

Follicular/Proliferative Phase

• The follicular phase begins on the 6^th and lasts up to the 13^th or 14^th day in a 28-day menstrual cycle.
• FSH stimulates the ovarian follicle to secrete oestrogen.
• Oestrogen stimulates the proliferation of cells of the uterine endometrium.
• The endometrium becomes thicker because of rapid cell multiplication. 

Ovulatory Phase

• At about the 14^th day of the menstrual cycle, both FSH and LH attain a peak level.
• Rapid secretion of LH induces rupturing of the Graafian follicle and the release of the ovum (ovulation).

Luteal/Secretory Phase

• The luteal phase begins on the 15^th and lasts up to the 28^th day in a 28-day menstrual cycle.
• LH causes ovulation. The remaining cells of the ovarian follicles are stimulated by LH to develop the corpus luteum.
• In the absence of fertilisation, the corpus luteum degenerates which causes disintegration of the endothelium, leading to menstruation and the start of a new cycle.
• A girl’s first menstrual flow on attaining puberty at the age of 11–13 years is called menarche.
• The permanent cessation or stoppage of ovulation and menstruation is called menopause.

Fertilisation

• The fusion of a haploid male gamete (sperm) and a haploid female gamete (ovum) to form a diploid zygote is called fertilisation.
• The secretions of the acrosome help the sperm to enter the ovum through the zona pellucida.
• When a sperm comes in contact with the layer of zona pellucida, it brings changes in the layer which blocks the entry of other sperms in the ovum.
• The nuclei of a haploid sperm and an ovum fuse to form a diploid zygote.

Implantation

• Cleavage is a unique embryological process which involves a series of rapid mitotic divisions of the zygote which transforms the single-celled zygote into a multicellular structure called the blastula.
• The morula consists of a group of 16–32 centrally located cells from the inner cell mass and the surrounding cell mass.
• The embryo is called the blastocyst, which is composed of an outer envelope of cells, the trophoblast and an inner cell mass called the embryoblast.
• The attachment of the blastocyst to the uterine wall is called implantation. It occurs after 7 days of fertilisation.

Embryonic Development 

• Transformation of the blastocyst into the gastrula with primary germ layers by the rearrangement of cells is called gastrulation.
• It involves cell movements which help to attain a new shape and morphology of the embryo.
• The three germ layers are an ectoderm, mesoderm and endoderm.
• Each layer differentiates to form different body parts of the foetus.
  
  

Pregnancy

• Pregnancy is the time period from conception to birth.
• It is approximately 9 months and 7 days. 
• This period is called the gestation period.

Consequences of Over Population

Population Control

Birth Control