Riassunto esame Embriologia, Prof. Castagneti Lara, libro consigliato Carlson, Bruce M. Carlson

Phases of Human Embryology

The phases are:

1. Gametogenesis - formation of gametes
2. Fertilization - fusion of male and female gametes (=zygote)
3. Cleavage - proliferation of cells in zygote
4. Gastrulation - formation of the 3 basic layers from which all embryo tissue develops
5. Formation of body plan (morphogenesis) - from a flat object (initial embryo), it folds to form tubular structures (tubes-within-tubes), eventually becoming a complex curved object with rudimental organs
6. Organogenesis - formation of internal organs from the 3 embryological layers

Introduction to Gametogenesis

Cells that give rise to gametes (= primordial germ cells, PGCs) reside in yolk sac (extraembryonic membrane) and, probably, arise during the phase of gastrulation. Their lineage constitutes the germline, and they can be clearly identified for their characteristics (pale cytoplasm, rounded shape and other molecular markers).

• Primordial cells migrate into dorsal body

They migrate by ameboid movement (they move through cytoplasmic adhesion=lamellipodia, actin filaments) from the yolk sac to the gut tube (the primitive precursor of both the respiratory and digestive systems) and populate the region of the body wall at the level that will form the gonads.

They continue to multiply by mitosis, some may become stranded (= teratoma). Tumors of tissues derived from all germ layers (extragonadal or gonadal). Most common tumors in newborns and are formed by 3 pluripotent cells (able to form a variety of anatomic structures ex. eyes).

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PGCs arrive in the gonad region (or genital ridge) and stimulate the epithelial cells to form somatic support cells they create swellings which will become gonads.

Somatic support cells give rise to:

• Ovarian follicles (female)
• Sertoli cells of the germinal epithelium of the seminiferous tubules (male), which will assemble into testis cords.

GAMETOGENESIS, FERTILIZATION, AND FIRST

WEEKGAMETOGENESIS (1)The timing is different in men and women. thIn males: PGCs remain dormant from the 6 week of embryonic development until puberty(seminiferous tubules mature and host cells that start will differentiate through spermiosis,spermatozoa are then produced until death)In females: PGCs undergo more mitotic divisions after becoming invested by somatic support cells;ththen they differentiate into oogonia. By the 5 month of fetal development all oogonia beginundergoing meiosis (àthey become primary oocytes). After, all sex cells enter a state of dormancyand remain in mitotic arrest until sexual maturity. At puberty each month a few ovarian folliclesresume development in response to the monthly surge of pituitary gonadotropic hormones, but usuallyonly one primary oocyte matures into secondary oocyte and is ovulated. This oocyte enters a secondphase of meiotic arrest and doesn’t complete meiosis unless it is fertilized.Function PGCs contain 23 pair of chromosomes

or 46 chromosomes; one pair comes from the maternal gamete and the other from the paternal. The first 22 pairs are called autosomes while the remaining two sex chromosomes. Somatic cells and PGCs are diploid (2n) while mature gametes have only one copy of each kind of chromosome and are called haploid (n); after duplication (before mitosis and meiosis) the cells are 4n. In meiosis a diploid cell (2n) replicates its DNA (becoming 4n) and undergoes two successive, qualitatively different nuclear and cell divisions to yield four haploid (1n) offspring. Meiosis with germ cells In the female the oogonium is now a primary oocyte while in the male the spermatogonium is now a primary spermatocyte. Once the DNA replicates, each chromosome consists of two parallel strands (=chromatids) joined together in a region called centromere. Stages meiosis I: 1. Preparatory phase: DNA replication yields double-stranded chromosomes 2. Prophase 1: chromosomes condense – two chromosomes of each homologous pair

Stages meiosis I:

1. Prophase 1: chromosomes condense and pair up in homologous pairs
2. Metaphase 1: chromosomes align along the equator and centromeres form a chiasma (eventual crossing over)
3. Anaphase and telophase 1: each double stranded chromosome of each homologous pair is distributed to each daughter cell
4. Cytokinesis: cell divides (RESULT: two secondary spermatocytes, one secondary oocyte and one polar body Haploid 2n)

Stages meiosis II:

1. Prophase: double-stranded chromosomes condense
2. Metaphase: chromosomes align along the equator and centromeres replicate
3. Anaphase and Telophase: each chromosome splits into single-stranded chromosomes, one of which is distributed to each daughter nucleus
4. Cytokinesis: cell divides (RESULT: four spermatids, one definitive oocyte and three polar bodies Haploid 1n)

SPERMATOGENESIS:

• Puberty: Testes begin to secrete great amounts of testosterone (steroid hormone), which has the following functions: development of sex characteristics, triggering testes growth, maturation of

seminiferous tubules and commencement of spermatogenesis. Sertoli cells then differentiate into a system of seminiferous tubules – PGCs resume growth and differentiate into spermatogonia, they are located under the basement membrane surrounding the seminiferous tubules (pockets between Sertoli cells). They also are interconnected by tight junctions which help establish a blood-testis barrier developing an immune privileged site during their development in the testes. Translocation from the basal to the luminal side of seminiferous epithelium while spermatogenesis take place + final process of sperm cell differentiation (maturation of spermatids into spermatozoa) called spermiogenesis. Spermatozoon = head (condensed nucleus and cap of apical vesicle, acrosome, containing hydrolytic enzymes) + midpiece (large mitochondria for swimming) + tail (microtubules that form the propulsion system). Male hormones they are the same as the female's: FSH (follicle-stimulating hormone) and

LH (luteinizing hormone).

1. FSH acts on Sertoli cells (wall of seminiferous tubules)
2. LH stimulates Leydig cells, which then produce testosterone.

Function of Sertoli cells:

• They are somatic cells of the testes and participate in the process of spermatogenesis in many ways:
• They form a blood-testis barrier: creation of a basal compartment (separated from the luminal) where spermatocytes can grow, as they are antigenically different haploid germ cells.
• They secrete the tubular fluid inside the seminiferous tubules.
• They produce an androgen-binding protein, and many other proteins, such as the retinal-binding proteins, Mullerian-inhibiting factors, and inhibin for feed-back loop to hypothalamus (it goes back to the hypothalamus so that less hypothalamic-releasing factor is produced, stimulating the production of the follicle-stimulating hormone).
• Maintenance and coordination of spermatogenesis.

Their cytoplasmic processes embrace spermatocytes as they go.

through the release of mature spermatozoa from the Sertoli cells into the lumen of the seminiferous tubules. This process is regulated by hormonal signals and is essential for the production of functional sperm. The seminiferous tubules are lined with a layer of cells called the germinal epithelium, which contains the spermatogonia, spermatocytes, and spermatids at different stages of maturation. The Sertoli cells, which are also part of the germinal epithelium, play a crucial role in supporting and nourishing the developing sperm cells. During spermatogenesis, the germ cells undergo a series of divisions and differentiations to produce mature spermatozoa. This process occurs in a synchronized manner, with waves of spermatogenesis occurring in the seminiferous tubules. However, within each local area of the germinal epithelium, the process is not synchronized throughout the tubules. Approximately four waves of spermatogenesis can be observed in any region of the tubule at any given time. Each spermatogenic wave, or cycle, represents a homogeneity in the stage of maturation of the developing sperm cells. It takes approximately 64 days for a new wave to appear in the same segment of the tubule. The Sertoli cells play a fundamental role in spermiogenesis, the final stage of sperm cell maturation. Maturing spermatocytes and spermatids are connected to the surrounding Sertoli cells through intercellular junctions and cytoplasmic processes called tubulobulbar complexes. These complexes help transfer the excess cytoplasm from the developing gametes to the Sertoli cells, as the cytoplasm of the sperm cells shrinks significantly during spermiogenesis. Finally, the process of spermiation occurs, which involves the release of mature spermatozoa from the Sertoli cells into the lumen of the seminiferous tubules. This process is essential for the production of functional sperm and is regulated by hormonal signals.quicker energy production during sperm motility.energyproductionResults The membrane of the head of the spermatozoa becomes antigenically different and is divided into different antigenic domains (changing during maturation in the male and female genital tract). The total length of the head and tail combined is 60 micrometers. ⁶

Letizia Hassan

The cytoskeleton of the tail is the axonema, made of microtubules (in particular, 9 pairs of microtubules disposed in a circle and 1 central pair; each pair is made of an A-microtubule and a B-microtubule). The protein dynein helps spermatozoa with their overall motility: when microtubules are bending, dynein is active and attached to them.

The cilia in the ductus deferens help the circulation of spermatozoa; sometimes they lose their ability to be motile (ciliopathy), or have deficiency in the axonemal structure, leading to infertility.

Functional maturation of spermatozoa

When released in the lumen of the seminiferous tubules, spermatozoa are pushed by the fluid's pressure.

nd eventually expelled from the body. This process is facilitated by muscle contractions in the vas deferens and ciliary movements in the rete testis and efferent ducts. Sperm cells are stored in the lower part of the epididymis, which is a coiled duct connected to the vas deferens. They remain in the epididymis for about 12 days. In the epididymis, sperm cells acquire motility. This is facilitated by the production of a factor called motility protein by the cells in the epididymis. This motility protein induces several changes within the spermatozoa, including tubulin phosphorylation, an increase in intracellular concentration of calcium, and an increase in cyclic AMP (cAMP), which is a second messenger that easily passes through cell membranes. In addition to acquiring motility, spermatozoa also undergo some biochemical changes in the epididymis. The head of the sperm is coated by glycoproteins, which are necessary for protection during the journey towards the egg. During ejaculation, sperm are propelled through the vas deferens and urethra before being expelled from the body.