Human Reproduction

Think of male body as having a small factory outside main body that makes sperms. Factory is testis. It is kept in a bag called scrotum outside because sperm making needs 2-2.5°C less than body temperature, like ice cream needs freezer.

Inside testis there are many tiny coiled tubes called seminiferous tubules, like thin pipes. Inside pipes there are cells that become sperm and helper cells Sertoli that feed them. Outside pipes there are Leydig cells that make testosterone hormone which makes voice deep, beard, muscles.

Sperm made in tubules does not go out directly. It goes to a net rete testis, then small tubes vasa efferentia, then to a big coiled storage epididymis where it learns swimming and matures for 18 hours. Then muscular tube vas deferens carries it upward. On way there are three glands adding food and protection: seminal vesicle adds 60% liquid with fructose sugar for energy, prostate adds 20-30% milky alkaline fluid with enzymes to help move, bulbourethral adds clear mucus to make path slippery and remove acid. All together plus sperm is called semen. Semen plus sperm goes out through urethra which is common path for urine and semen but never both at same time.

Female system is fully inside, like a nest protected. Two ovaries small almond size make eggs and hormones estrogen progesterone. Each ovary has many tiny bubbles follicles, each bubble holds one egg.

From ovary egg is caught by finger like fimbriae of fallopian tube, then travels through tube: infundibulum funnel, ampulla wide where fertilization happens, isthmus narrow, then reaches uterus. Uterus is muscular bag with three layers inner endometrium soft blood rich where baby implants, middle myometrium strong muscle that pushes baby during birth, outer perimetrium. Lower neck is cervix that opens during birth, below is vagina canal to outside.

Now making of sperm and egg is different. Spermatogenesis is like continuous factory: stem cells spermatogonia 2n divide by mitosis, some become primary spermatocyte 2n which does meiosis I → two secondary spermatocytes n, meiosis II → four spermatids n, then spermiogenesis change shape to sperm with head acrosome with enzymes to break egg cover, nucleus with DNA, midpiece with mitochondria power house, tail for swimming. Sertoli cells nourish, FSH LH testosterone control.

Oogenesis is like saving for long time: girl at birth already has 2 million primary oocytes arrested in meiosis I, by puberty 60-80k remain. One per month starts growing inside Graafian follicle large bubble with fluid. Primary oocyte 2n completes meiosis I but unequal division one large secondary oocyte n with almost all cytoplasm and one small polar body wasted. Secondary oocyte arrested in meiosis II and released at ovulation day 14. If sperm comes it completes meiosis II making second polar body and mature ovum. So from one mother cell only one egg, three polar bodies waste, because egg needs lots of food for baby. Egg has 3 layers corona radiata outer cells, zona pellucida middle hard glycoprotein that blocks extra sperm after one enters, and vitelline membrane inside.

Sperm looks like a tadpole with big head. Head has acrosome cap like helmet filled with enzymes that dissolve egg cover, and nucleus with 23 chromosomes. Middle piece has many mitochondria like battery factory to give energy to tail. Tail is long and whips to swim, needs fructose energy.

Egg is biggest cell in female, round with lots of cytoplasm and 23 chromosomes, non-motile like a queen waiting.

Every month female body prepares bed for baby. Cycle 28 days has four phases. Menstrual phase day 1-5 inner lining endometrium sheds with blood because no baby, like cleaning old bed. Follicular phase day 6-13 FSH from pituitary makes Graafian follicle grow and makes estrogen which repairs endometrium thickness blood supply. Ovulation day 14 LH surge peak makes bubble burst and egg released, this is most fertile day, slight pain some feel. Luteal phase day 15-28 leftover bubble becomes yellow body corpus luteum which makes progesterone which makes endometrium more thick spongy with glands for implantation. If no pregnancy corpus luteum dies progesterone falls endometrium breaks next period. If pregnancy, hCG hormone from embryo keeps corpus luteum alive progesterone stays no period.

During intercourse 200-300 million sperms released but only few hundred reach ampulla, others die in acidic vagina. Egg lives 24 hours sperm 48-72 hours, so timing important.

Fertilization happens in ampulla of fallopian tube. Sperm first touches corona radiata then zona pellucida, acrosome releases enzymes to make hole, one sperm enters zona triggers cortical reaction zona becomes hard to block other sperms polyspermy block. Sperm and egg nuclei fuse 23+23=46 zygote diploid, this is like two halves making full.

Zygote divides quickly mitosis cleavage as it travels to uterus. First 2 cells then 4 then 8 then morula 8-16 cells like mulberry then blastocyst with outer trophoblast which will make placenta and inner cell mass which will make baby. Blastocyst reaches uterus day 5-6 and implants day 6-7 by burrowing into endometrium. Trophoblast makes finger like chorionic villi and hCG hormone which keeps pregnancy. Implantation gives signal no period.

Placenta is connection between mother and baby, like WiFi and food pipe together. Made of chorionic villi from baby and endometrium from mother, but blood never mixes, exchange by diffusion. Functions: food oxygen from mother to baby, waste CO2 from baby to mother, hormones hCG hPL estrogen progesterone to maintain pregnancy, barrier to some germs but not all, antibody IgG passes giving first protection.

Embryo develops: heart beats day 28, limbs form week 8 now called fetus, by 3 months organs formed, by 5 months first movement mother feels quickening, by 6 months hair eyelashes, by 9 months ready head down.

Parturition birth: baby 9 months fully grown signals through placenta, oxytocin from pituitary and prostaglandins make uterus muscles myometrium contract strongly, cervix dilates opens 10 cm, baby pushed out, then placenta out as afterbirth. Hormones estrogen progesterone ratio changes, cortisol from baby also helps.

Lactation milk making: after birth progesterone falls prolactin from pituitary makes milk in alveoli of mammary gland, oxytocin makes milk ejection let down when baby sucks. First milk colostrum yellow rich in IgA antibodies very important first immunity. Exclusive breastfeeding 6 months recommended. Milk has casein protein lactose sugar fat vitamins antibodies.